1,3-Allylic Strain (A¹,³)
Definition: Steric/electronic repulsion across allylic systems affecting conformation and selectivity.
Context: Explains axial selectivity and diastereocontrol.
Example: Asymmetric allylation reactions.
Related Terms: A-Values, Stereoelectronic Effects.
Reference: Hoffmann, Acc. Chem. Res. (1971).
1,3-Diaxial Interaction
Definition: Steric interactions between axial substituents on cyclohexane separated by one carbon.
Context: Dictates conformational preferences relevant to reactivity.
Example: Axial methyl interactions.
Related Terms: A-Values, Conformation.
Reference: Eliel & Wilen (1994).
A-Values
Definition: Quantitative measure of axial vs equatorial preference in cyclohexanes.
Context: Predicts conformational populations and stereochemical outcomes.
Example: t-Bu substituent has large A-value.
Related Terms: Conformation, 1,3-Diaxial Interactions.
Reference: Eliel & Wilen (1994).
Absolute Asymmetric Synthesis
Definition: Formation of an enantiomeric excess without any chiral influence (no chiral reagents or fields).
Context: Mechanistic interest and potential origin-of-chirality implications.
Example: Soai asymmetric autocatalysis.
Related Terms: Asymmetric Amplification, Soai Reaction.
Reference: Soai, Nature (1995).
Absolute Configuration
Definition: The unambiguous three-dimensional spatial arrangement of substituents around a stereogenic element, defined independently of any other stereocenter and commonly designated using the Cahn-Ingold-Prelog (CIP) system (R/S, E/Z, P/M).
Context: Absolute configuration provides the definitive stereochemical identity of a molecule. In pharmaceutical chemistry, it is a regulatory requirement to specify absolute configuration for chiral active pharmaceutical ingredients, as different configurations can lead to distinct pharmacological, toxicological, and metabolic outcomes. Absolute configuration must be established using reliable methods such as X-ray crystallography (with anomalous dispersion), vibrational circular dichroism, electronic circular dichroism with exciton analysis, or unambiguous chemical correlation.
Example: The assignment of (S)-thalidomide and (R)-thalidomide, where each enantiomer exhibits markedly different biological effects.
Related Terms: Relative Configuration; CIP Rules; Stereogenic Center; Absolute Stereochemistry; Misassignment of Configuration.
Reference: IUPAC. Compendium of Chemical Terminology (IUPAC Gold Book). 2nd Edition, 1997 (updated 2019).
Achiral
Definition: A molecule or object that is superimposable on its mirror image.
Context: Achirality implies the absence of optical activity and stereochemical differentiation. Such molecules do not exhibit enantiomeric forms.
Example: Methane, Dichloromethane (lack stereogenic center), Meso-tartaric acid is achiral despite having two stereocenters.
Related Terms: Chiral; Symmetry; Plane of symmetry; Meso-compound.
Reference: IUPAC. Compendium of Chemical Terminology (IUPAC Gold Book), 2nd Edition, 1997 (updated 2019).
Allenes
Definition: Cumulenes (C=C=C) that can be chiral due to orthogonal π systems.
Context: Provide axial chirality in drug scaffolds and ligands.
Example: Chiral 1,3-disubstituted allenes.
Related Terms: Axial Chirality, Atropisomerism.
Reference: Eliel & Wilen (1994).
Analytical Method Validation (Chiral)
Definition: Validation of chiral analytical procedures for specificity, accuracy, precision, and robustness.
Context: Required for release and stability testing of chiral APIs.
Example: Validation of chiral HPLC method for enantiomeric purity.
Related Terms: ICH Q2(R2), Chiral HPLC.
Reference: ICH Q2(R2) (2022).
Anisotropy Factor (g)
Definition: CD intensity normalized to absorbance (Δε/ε).
Context: Enables quantitative comparison of chiroptical responses; important in CPL materials.
Example: g ~ 10-3; for many organic chromophores.
Related Terms: CD, CPL.
Reference: Barron (2004).
Anomer
Definition: A subtype of epimer differing at the anomeric carbon in cyclic sugars.
Context: Critical in carbohydrate chemistry, anomeric configuration influences stability, solubility, and biological recognition.
Example: α-D-glucose vs β-D-glucose.
Related Terms: Epimer, Anomeric Effect, Carbohydrate Stereochemistry.
Reference: IUPAC Gold Book.
Anomeric Effect
Definition: Preference for axial orientation of electronegative substituents at the anomeric center in pyranoses.
Context: Influences glycoside stability and reactivity in medicinal chemistry.
Example: Axial methoxy in methoxytetrahydropyran.
Related Terms: Stereoelectronic Effect, Carbohydrate Stereochemistry.
Reference: Deslongchamps (1983).
Anti-(Stereochemical Descriptor)
Definition: Describes two substituents or groups positioned on opposite sides of a reference plane or dihedral angle (180° apart).
Context: Used to describe relative configuration in alkenes, aldols, and conformational analysis.
Example: Anti-periplanar geometry in E2 eliminations or anti-diol configurations.
Related Terms: Syn; Antiperiplanar; Synclinal.
Reference: IUPAC. Compendium of Chemical Terminology (IUPAC Gold Book), 2nd Edition, 1997 (updated 2019).
Asymmetric Amplification
Definition: Small initial enantiomeric bias is amplified during reaction or crystallization.
Context: Useful in chiral self-replication and deracemization protocols.
Example: Soai reaction ee amplification.
Related Terms: Absolute Asymmetric Synthesis.
Reference: Soai, Nature (1995); Blackmond, PNAS (2004).
Asymmetric Catalysis
Definition: Catalysis that yields one enantiomer preferentially.
Context: Cornerstone of modern manufacturing of single-enantiomer drugs.
Example: Noyori asymmetric hydrogenation.
Related Terms: Organocatalysis, Biocatalysis.
Reference: Noyori, Nobel Lecture (2001).
Asymmetric Transfer Hydrogenation (ATH)
Definition: Hydrogen transfer from donors (iPrOH, formate) to substrates using chiral catalysts.
Context: Avoids H2 gas; scalable and selective.
Example: Noyori–Ikariya systems reducing ketones.
Related Terms: Noyori Hydrogenation, Organocatalysis.
Reference: Ikariya & Noyori, Acc. Chem. Res. (2007).
Atropisomerism
Definition: Stereoisomerism due to hindered rotation that creates isolable enantiomers.
Context: Drug candidates may have atropisomeric axes requiring control and specification.
Example: Atropisomeric biaryl kinase inhibitors.
Related Terms: Axial Chirality, Barrier to Rotation.
Reference: Clayden et al. (2012).
Axial Bond
Definition: A bond directed along the axis of a cyclohexane chair, perpendicular to the mean plane.
Context: Axial substituents experience 1,3-diaxial interactions, strongly influencing conformation and reactivity.
Example: Axial vs equatorial methyl in methylcyclohexane.
Related Terms: Equatorial Bond, A-Values, Conformation.
Reference: Eliel & Wilen (1994).
Axial Chirality
Definition: Chirality arising from hindered rotation about an axis.
Context: Common in biaryls and allenes; impacts ligand and API design.
Example: BINAP, biaryl atropisomers.
Related Terms: Atropisomerism, Helicity.
Reference: IUPAC Gold Book.
Barrier to Rotation (ΔG‡)
Definition: Free-energy barrier that separates enantiomeric conformers/atropisomers.
Context: Determines isolability of atropisomers and labeling requirements.
Example: ΔG‡ > ~25 kcal/mol gives isolable atropisomers at room temperature.
Related Terms: Atropisomerism, Enantiomerization.
Reference: Eliel & Wilen (1994).
Bioactive Conformation
Definition: The conformation adopted by a ligand when bound to its target.
Context: Guides conformational constraint strategies to improve selectivity.
Example: Locked isostere mimicking bound pose.
Related Terms: Conformation, Pharmacophore.
Reference: Silverman (2014).
Bite Angle
Definition: Angle between donor atoms of a bidentate ligand at the metal center.
Context: Modulates enantioselectivity and reactivity in catalytic cycles.
Example: Wide-bite vs narrow-bite diphosphines.
Related Terms: Quadrant Model, Chiral Ligand.
Reference: van Leeuwen, Coordination Chemistry Reviews (1999).
Bürgi–Dunitz Angle
Definition: Preferred trajectory (~107°) of nucleophile approach to a carbonyl.
Context: Rationalizes facial selectivity and reaction rates.
Example: Hydride addition to ketones.
Related Terms: Felkin–Anh, Cram’s Rule.
Reference: Bürgi & Dunitz, JACS (1973).
CBS Reduction
Definition: Corey–Bakshi–Shibata borane reduction of ketones using chiral oxazaborolidines.
Context: Reliable route to enantioenriched alcohols.
Example: CBS reduction of aryl ketones.
Related Terms: Asymmetric Catalysis, Reduction.
Reference: Corey, JACS (1987).
Chelation Control
Definition: Metal coordination overrides Felkin preferences in carbonyl additions.
Context: Explains opposite selectivity in presence of Lewis acids or metals.
Example: Addition to α-alkoxy aldehydes with Mg2+.
Related Terms: Cram’s Rule, Felkin–Anh.
Reference: Reetz, Angew. Chem. (1974).
Chiral
Definition: A geometric property of a molecule or object that is not superimposable on its mirror image.
Context: Chirality is foundational in stereochemistry, determining whether enantiomers exist and influencing pharmacological activity.
Example: Hands are chiral objects; lactic acid has R- and S-enantiomers.
Related Terms: Enantiomer; Chiral Center; Stereoisomer.
Reference: IUPAC. Compendium of Chemical Terminology (IUPAC Gold Book), 2nd Edition, 1997 (updated 2019).
Chiral Auxiliary
Definition: Temporarily attached chiral unit to control stereochemistry of a transformation.
Context: Delivers high selectivity; removed to give target enantioenriched product.
Example: Evans oxazolidinone auxiliaries.
Related Terms: Chiral Pool, Asymmetric Catalysis.
Reference: Evans, JACS (1981).
Chiral Awareness
Definition: The explicit recognition and incorporation of chirality in thinking, language, experimental design, data handling, modeling, regulation, and decision-making. Chiral awareness involves treating stereoisomers-especially enantiomers and diastereomers-as distinct chemical entities with potentially different properties, activities, safety profiles, and biological outcomes.
Example: Correct specification of stereochemical configuration in names, drawings, and databases; Clear communication of stereochemistry in teaching, papers, labels, and reports; Discrimination between stereoisomers in experiments, analysis, and modelling consideration of stereoselective pharmacology, metabolism, and toxicity; Avoidance of "stereochemical collapse" in AI/ML representations and informatics.
Related terms: Chiral-aware (adjectival form); Chiral Literacy; Chiral Intelligence; Chiral Bias; Stereo-blindness; Stereo-sloppy.
References: Eliel, E. L., & Wilen, S. H. Stereochemistry of organic compounds. New York: Wiley (1994);
Ariens, E. J. Stereochemistry, a basis for sophisticated nonsense in pharmacokinetics and clinical pharmacology. European Journal of Clinical Pharmacology, 26, 663-668 (1984);
Smith, S. W. Chiral toxicology: It's the same thing... only different. Toxicological Sciences, 110(1), 4-30 (2009).
Chiral Bioequivalence
Definition: Demonstration that enantiomer exposure (AUC, Cmax) is equivalent between products.
Context: Regulatory expectation for racemates and single-enantiomer generics.
Example: Bioequivalence of racemic vs reformulated enantiomer products.
Related Terms: Bioequivalence, FDA Chiral Policy.
Reference: FDA Guidance (2017); FDA 1992 Policy.
Chiral CE (Capillary Electrophoresis)
Definition: Electrophoretic separation with chiral selectors (e.g., cyclodextrins) in the buffer.
Context: High-efficiency analytical separations for enantiomers.
Example: CE of amino acid enantiomers.
Related Terms: Chiral HPLC, CSP.
Reference: Scriba, Electrophoresis (2003).
Chiral Center
Definition: A tetrahedral atom (usually carbon) bonded to four different substituents.
Context: Creates enantiomeric pairs; critical for drug selectivity and metabolism.
Example: The α-carbon of lactic acid.
Related Terms: Stereocenter, Enantiomer.
Reference: IUPAC Gold Book.
Chiral Derivatizing Agent (CDA)
Definition: Enantiomers are converted to diastereomers by reacting with a chiral reagent to enable separation.
Context: Facilitates NMR/LC analysis when direct separation is difficult.
Example: Mosher’s acid chloride (MTPA-Cl).
Related Terms: CDA, Chiral Solvating Agent.
Reference: Mosher, JACS (1973).
Chiral Drug
Definition: A pharmaceutical compound that contains one or more chiral centers or stereogenic elements, existing as enantiomers, diastereomers, or mixtures.
Context: Regulatory and therapeutic implications are critical; one enantiomer may be active (eutomer) while the other may be inactive or harmful (distomer).
Example: Ibuprofen (sold as a racemate, though only the S-enantiomer is pharmacologically active).
Related Terms: Enantiopure; Racemate; Eutomer; Distomer; Stereo-pharmacology.
Reference: FDA. Policy Statement for the Development of New Stereoisomeric Drugs (1992).
Chiral Education
Definition: The structured teaching and learning of chirality and stereochemistry, from fundamental spatial concepts to advanced applications in synthesis, biology, and medicine.
Context: Chiral education spans undergraduate instruction, professional training, and continuing education. Modern chiral education emphasizes three-dimensional thinking, molecular visualization, biological relevance, and translational impact, particularly in medicinal chemistry and pharmaceutical sciences.
Example: Teaching stereochemistry using molecular models and real drug case studies (e.g., thalidomide, ibuprofen, citalopram) rather than only abstract projections.
Related Terms: Chiral Literacy; Stereochemistry; Medicinal Chemistry; Stereo-pharmacology, Chiral Pharmacology
Reference: Holme, T. A. Assessing conceptual understanding in stereochemistry. Journal of Chemical Education, 96, 401-410 (2019); Nicoll, G. Investigating student misconceptions in organic chemistry: Stereochemistry and representations. Journal of Chemical Education, 78, 623-627 (2001); Clement, J., & Ainsworth, S. Multiple visual representations in chemistry learning. Topics in Cognitive Science, 10, 857-874 (2018); Underwood, S. M., et al. Expert-novice differences in interpreting stereochemical representations. Journal of Chemical Education, 93, 2014-2021 (2016).
Chiral GC (Gas Chromatography)
Definition: GC using chiral stationary phases for volatile enantiomers.
Context: Useful for small, volatile APIs and intermediates.
Example: Resolution of limonene enantiomers.
Related Terms: Chiral HPLC, CE.
Reference: Schurig & Nowotny, J. Chromatogr. A (1990).
Chiral HPLC
Definition: HPLC using chiral stationary phases to separate enantiomers.
Context: Workhorse analytical and preparative method in pharma.
Example: Separation of R/S-propranolol.
Related Terms: CSP, SFC.
Reference: Scriba, J. Chromatogr. A (2016).
Chiral Intelligence
Definition: The capacity to understand, interpret, and apply chirality-dependent information across chemical, biological, pharmacological, and regulatory domains.
Context: Chiral intelligence goes beyond recognizing stereochemical descriptors; it integrates molecular structure, biological response, metabolism, safety, and lifecycle decision-making. In pharmaceutical development, chiral intelligence underpins decisions on enantiomer selection, analytical control, regulatory strategy, and clinical risk assessment.
Example: Recognizing that only S-ibuprofen is pharmacologically active, while R-ibuprofen undergoes partial in vivo inversion, and integrating this knowledge into dosing, formulation, and regulatory justification.
Related Terms: Chiral Literacy; Stereo-pharmacology; Eudismic Ratio; Enantiopure; Chiral Drug, Stereochemistry-Aware Models; Enantiomeric Specificity.
Reference: Ariens, E. J. Stereochemistry, a basis for sophisticated nonsense in pharmacokinetics and clinical pharmacology. Medical Research Reviews, 4, 197-236 (1984); Jorner, K., Yu, E., Yoshikawa, N., Jorner, K., Aspuru-Guzik, A., et al. Stereochemistry-aware string-based molecular generation. PNAS Nexus, 4(11), pgaf329 (2025); Reymond, J.-L. Stereochemistry in chemoinformatics and artificial intelligence. Accounts of Chemical Research, 55, 2210-2220 (2022).
Chiral Inversion
Definition: In vivo conversion of one enantiomer to the other.
Context: Impacts dosing and exposure; must be characterized in PK.
Example: R-ibuprofen inverts to S-ibuprofen in humans.
Related Terms: Stereopharmacology, Metabolism.
Reference: Hutt & Caldwell, J. Pharm. Pharmacol. (1983).
Chiral Ligand
Definition: A ligand that induces asymmetry in metal-catalyzed reactions.
Context: Central in enantioselective hydrogenation and C–C bond formation.
Example: BINAP ligand.
Related Terms: Asymmetric Catalysis, Enantioselectivity.
Reference: Noyori, Angew. Chem. (1994).
Chiral Literacy
Definition: The foundational ability to read, interpret, and correctly use stereochemical language, representations, and concepts in chemistry and life sciences.
Context: Chiral literacy includes competence with R/S, E/Z, D/L, wedge-dash notation, projections (Fischer, Newman), and stereochemical terminology. It is essential for clear scientific communication, avoidance of stereochemical errors, and proper interpretation of literature, patents, and regulatory documents.
Example: Correctly distinguishing between D/L nomenclature (relative configuration) and d/l optical rotation, avoiding the common misconception that they are equivalent.
Related Terms: Chiral Education; Stereochemistry; Configuration; Stereoisomers, Stereochemical Notation; Spatial Reasoning; Asymmetric Synthesis.
Reference: Eliel, E. L., Wilen, S. H., & Mander, L. N. (1994). Stereochemistry of Organic Compounds. Wiley; Clayden, J., Greeves, N., & Warren, S. (2021). Organic Chemistry (2nd ed.). Oxford University Press; Kociok-Kohn, G. Chirality and its importance in chemistry and biology. Angewandte Chemie International Edition, 57, 10956-10958 (2018); Fallen, B., et al. Students' difficulties with chirality and stereochemical reasoning: A review. Chemistry Education Research and Practice, 21, 307-323 (2020).
Chiral Mass Spectrometry
Definition: MS technique combined with chiral derivatization or ion mobility to distinguish enantiomers.
Context: Emerging analytical tool for stereoisomers.
Example: Chiral recognition of amino acids.
Related Terms: MS, Chiral Derivatizing Agents.
Reference: Dwivedi et al., Anal Chem (2006).
Chiral Pharmacology
Definition: The study of how molecular chirality influences pharmacodynamic and pharmacokinetic behavior in biological systems.
Context: Enantiomers often differ in potency, metabolism, and toxicity. Understanding chiral pharmacology is crucial for rational drug development and regulatory approval.
Example: S-warfarin is the more potent anticoagulant enantiomer compared to R-warfarin.
Related Terms: Stereo-pharmacology; Eutomer; Distomer; Enantiomeric Excess.
Reference: Caldwell, J. (1995). Chiral pharmacology and the regulation of new drugs. Chemistry and Industry, 6, 176-179; Hutt, A. J. & Caldwell, J. "The importance of stereochemistry in drug action and disposition." Pharmacology & Therapeutics, 29(2): 245-263 (1985).
Chiral Phosphate Catalysis
Definition: Brønsted acid catalysis using BINOL-derived chiral phosphoric acids.
Context: Broad platform for enantioselective additions and rearrangements.
Example: CPA-catalyzed Mannich reactions.
Related Terms: Organocatalysis, Brønsted Acid Catalysis.
Reference: Akiyama/Terada, Chem. Rev. (2018).
Chiral Photochemistry
Definition: Use of light to induce stereocontrol via chiral catalysts, templates, or circularly polarized light.
Context: Enables unique selectivity pathways and deracemization.
Example: CPL-mediated enantioenrichment.
Related Terms: Asymmetric Catalysis, CPL.
Reference: Bach & Hehn, Angew. Chem. (2011).
Chiral Pool Synthesis
Definition: Use of abundant natural enantiopure building blocks as stereochemical sources.
Context: Efficient, scalable strategy in pharmaceutical synthesis.
Example: Use of L-amino acids to set stereochemistry.
Related Terms: Biocatalysis, Chiral Auxiliary.
Reference: Morrison & Boyd.
Chiral Recognition
Definition: Selective interaction of a host with one enantiomer over the other.
Context: Underlies chiral separations and receptor binding selectivity.
Example: Cyclodextrin inclusion complexes.
Related Terms: Molecular Imprinting, Chiral HPLC.
Reference: Wainer, Drug Discov Today (1997).
Chiral Resolution by Enzymes
Definition: Use of biocatalysts to selectively transform one enantiomer.
Context: Scalable, green alternative to chemical resolution.
Example: Lipase-catalyzed ester hydrolysis.
Related Terms: Biocatalysis, Kinetic Resolution.
Reference: Bornscheuer, Nature (2012).
Chiral SFC (Supercritical Fluid Chromatography)
Definition: Chromatography using supercritical CO2 with chiral stationary phases.
Context: Fast, green separations widely adopted for enantioresolution.
Example: Rapid enantiomer separation of β-blockers.
Related Terms: Chiral HPLC, CSP.
Reference: Berger, Supercritical Fluid Chromatography (1995).
Chiral Shift Reagent
Definition: Paramagnetic lanthanide complexes that induce differential NMR shifts for enantiomers.
Context: Legacy technique for stereochemical analysis.
Example: Eu(fod)3 added to racemates.
Related Terms: CSA, NMR.
Reference: Günther, NMR Spectroscopy (2013).
Chiral Solvating Agent (CSA)
Definition: Chiral additive forming diastereomeric complexes that resolve NMR signals.
Context: Allows ee determination without derivatization.
Example: Pirkle’s alcohols; TFAE.
Related Terms: CDA, NMR.
Reference: Pirkle, J. Org. Chem. (1967).
Chiral Stationary Phase (CSP)
Definition: Chromatographic phase containing chiral selectors (polysaccharides, cyclodextrins, Pirkle-type, proteins).
Context: Core technology for analytical and preparative enantioseparation.
Example: Cellulose tris(3,5-dimethylphenylcarbamate).
Related Terms: Chiral HPLC, SFC.
Reference: Scriba, J. Chromatogr. A (2016).
Chiral Switch
Definition: Replacing a racemic drug with its single active enantiomer.
Context: Lifecycle and safety strategy improving efficacy and dose control.
Example: Esomeprazole replacing omeprazole.
Related Terms: Eutomer, Stereopharmacology.
Reference: FDA Policy (1992).
Chiral Toxicology
Definition: Study of enantioselective toxicity and safety profiles.
Context: Eutomers and distomers can differ in adverse effects; regulators expect isomer-specific assessment.
Example: S-thalidomide vs R-thalidomide.
Related Terms: Eutomer, Distomer.
Reference: FDA Stereoisomeric Drugs Policy (1992).
Chiral-Induced Spin Selectivity (CISS)
Definition: Phenomenon where electron spin polarization arises during transport through chiral media.
Context: Emerging relevance in bioelectronics and sensing; conceptual interest in drug–protein interactions.
Example: Spin filtering through DNA helices.
Related Terms: Helicity, Chiroptics.
Reference: Naaman & Waldeck, Annu. Rev. Phys. Chem. (2015).
Chirality
Definition: A geometric property where an object or molecule is not superimposable on its mirror image.
Context: Foundational to stereochemistry; chirality determines enantiomer formation and can alter pharmacological profiles.
Example: Hands are chiral; R- and S-lactic acid are mirror images.
Related Terms: Enantiomer, Stereocenter, Stereoisomer.
Reference: IUPAC Gold Book (2019).
CIP Rules
Definition: Priority rules to rank substituents for stereochemical assignment.
Context: Foundation for R/S and E/Z nomenclature across industry and regulation.
Example: Assigning R to lactic acid’s chiral center.
Related Terms: R/S Configuration, E/Z Isomerism.
Reference: Cahn, Ingold & Prelog, Experientia (1956).
Circular Dichroism (CD)
Definition: Difference in absorption of left vs right circularly polarized light.
Context: Key chiroptical method for absolute configuration and secondary structure.
Example: ECD of helicenes; protein far-UV CD.
Related Terms: ECD, VCD, ROA.
Reference: Kelly et al., Biochim. Biophys. Acta (2005).
Circularly Polarized Luminescence (CPL)
Definition: Difference in emission of left vs right circularly polarized light.
Context: Emerging tool for chiral luminophores and bioimaging probes.
Example: CPL of lanthanide complexes.
Related Terms: CD, g-factor.
Reference: Zinna & Di Bari, Chirality (2015).
Configuration
Definition: The fixed spatial arrangement of atoms about a stereogenic element not interconverted without bond breaking.
Context: Configuration defines absolute identity of stereoisomers and must be controlled in pharma.
Example: R vs S configuration at a chiral center.
Related Terms: Absolute Configuration, Relative Configuration.
Reference: IUPAC Gold Book.
Conformation
Definition: Different spatial arrangements generated by rotation about single bonds.
Context: Bioactive conformation often dictates receptor binding and SAR.
Example: Anti vs gauche conformers of butane.
Related Terms: Conformational Isomerism, Bioactive Conformation.
Reference: Clayden et al., Organic Chemistry (2012).
Conformational Constraint
Definition: Structural modification limiting molecular flexibility.
Context: Improves binding affinity and selectivity.
Example: Locked nucleic acids (LNAs).
Related Terms: SAR, Pharmacophore.
Reference: Silverman, Drug Design (2014).
Conformational Isomer (Conformer)
Definition: Isomers differing only by rotation about single σ-bonds without bond breaking.
Context: Conformational analysis is essential in understanding stability, reactivity, and bioactive geometry.
Example: Chair and boat forms of cyclohexane.
Related Terms: Conformer; Conformation; Eclipsed; Gauche.
Reference: Clayden, J., Greeves, N., Warren, S. & Wothers, P. Organic Chemistry. Oxford University Press, 2012.
Conformational Isomerism
Definition: Interconversion between isomers via rotation about σ-bonds.
Context: Conformational preferences control stereochemical outcomes and binding.
Example: Chair vs boat cyclohexane.
Related Terms: Conformation, Stereoelectronic Effect.
Reference: Clayden et al. (2012).
Conformer
Definition: A specific spatial arrangement of atoms in a molecule that can be interconverted by rotation about single bonds.
Context: Conformers represent energetically accessible geometries; populations depend on steric and electronic effects.
Example: Staggered and eclipsed conformations of ethane.
Related Terms: Conformational Isomer; Newman Projection; Gauche; Eclipsed.
Reference: Eliel, E. L. & Wilen, S. H. Stereochemistry of Organic Compounds. Wiley, 1994.
Conglomerate (Racemic Conglomerate)
Definition: Racemic mixture that crystallizes as separate enantiomorphic crystals.
Context: Enables preferential crystallization and Viedma ripening strategies.
Example: Sodium ammonium tartrate behavior.
Related Terms: Racemate, Viedma Ripening.
Reference: Eliel & Wilen (1994).
Cornforth Model
Definition: Predicts anti-selective substitution via antiperiplanar alignment.
Context: Used for 1,3-asymmetric induction analysis.
Example: Allylic substitutions.
Related Terms: Felkin–Anh, Zimmerman–Traxler.
Reference: Cornforth, Chem. Soc. Rev. (1971).
Cotton Effect
Definition: Characteristic sign and magnitude change in CD/ORD near an absorption band.
Context: Diagnostic for electronic transitions and stereochemical assignment.
Example: Positive/negative couplets in aromatic chromophores.
Related Terms: CD, ORD, Exciton Coupling.
Reference: Nakanishi et al. (2007).
Cram’s Rule
Definition: Predicts diastereofacial selectivity based on minimizing steric interactions.
Context: Competes with chelation control in carbonyl additions.
Example: Nucleophile addition to chiral ketones.
Related Terms: Felkin–Anh, Chelation Control.
Reference: Cram, JACS (1952).
Curtin–Hammett Principle
Definition: Product distribution is determined by transition-state energies when conformers interconvert faster than they react.
Context: Explains selectivity in conformationally flexible systems.
Example: Axial/equatorial conformers leading to different products.
Related Terms: Conformation, Kinetics.
Reference: Seeman, Chem. Rev. (1983).
Cyclodextrin CSP
Definition: Cyclodextrin-based selectors suitable for polar and volatile analytes.
Context: Common in CE and GC for small molecules.
Example: β-Cyclodextrin phases.
Related Terms: CSP, Chiral CE/GC.
Reference: Scriba (2003/2016).
D-(Dexter, Right)
Definition: A stereochemical prefix denoting a configuration relative to D-glyceraldehyde, not the direction of optical rotation.
Context: Used in carbohydrate and amino acid nomenclature to define absolute configuration based on Fischer projection.
Example: D-glucose corresponds to the configuration of D-glyceraldehyde.
Related Terms: L-; Fischer Projection; Absolute Configuration.
Reference: Fischer, E. "Einfluss der Configuration auf die Wirkung der Enzyme." Ber. Dtsch. Chem. Ges. 24, 1836-1845 (1891).
Deracemization
Definition: Conversion of a racemic mixture to an enantiopure product without external chiral source.
Context: Strategic for late-stage purification and process intensification.
Example: Viedma ripening, enzymatic deracemization.
Related Terms: Viedma Ripening, Absolute Asymmetric Synthesis.
Reference: Blackmond, Angew. Chem. (2010).
Dextro-(d, +)
Definition: Denotes a compound that rotates plane-polarized light to the right (clockwise).
Context: Optical rotation descriptors are experimental and distinct from R/S configuration.
Example: D-(+)-glucose.
Related Terms: Levo-, Optical Activity, Specific Rotation.
Reference: IUPAC Gold Book.
Diastereomer
Definition: Stereoisomers not related as mirror images.
Context: Often differ in physical properties, enabling separation and selective synthesis.
Example: Erythro vs threo diols.
Related Terms: Enantiomer, Relative Configuration.
Reference: Eliel & Wilen (1994).
Diastereomeric Excess (de)
Definition: Absolute difference between diastereomer fractions.
Context: QC metric when diastereomers are formed.
Example: 80% syn, 20% anti → 60% de.
Related Terms: dr, Stereoselectivity.
Reference: IUPAC Gold Book.
Diastereomeric Ratio (dr)
Definition: Ratio of diastereomers formed in a reaction.
Context: Guides optimization of diastereoselective steps.
Example: 95:5 syn:anti aldol product.
Related Terms: Diastereomeric Excess, Stereoselectivity.
Reference: IUPAC Gold Book.
Distomer
Definition: The enantiomer with lower or undesirable activity.
Context: May contribute to side effects or be removed in development.
Example: R-citalopram.
Related Terms: Eutomer, Racemate.
Reference: Ariëns (1984).
DNA Chirality
Definition: DNA adopts right-handed helices (B-form) with chiral sugar backbone.
Context: Chiral recognition of intercalators and drugs depends on helix sense.
Example: D-sugar backbone in nucleic acids.
Related Terms: Helicity, Stereorecognition.
Reference: Watson & Crick; Voet & Voet (2011).
Double Stereodifferentiation
Definition: Outcome determined by interaction of substrate and reagent chirality (matched/mismatched).
Context: Critical in planning complex multi-stereocenter syntheses.
Example: Aldol with chiral enolate and chiral aldehyde.
Related Terms: Matched/Mismatched, Stereocontrol.
Reference: Seebach (1979).
Dynamic Kinetic Asymmetric Transformation (DYKAT)
Definition: Asymmetric reaction converting a racemic substrate mixture to a single enantiomer product via equilibration.
Context: Converts racemic starting materials fully to one enantiomer product.
Example: Pd-catalyzed allylic substitutions.
Related Terms: DKR, Enantioconvergent Synthesis.
Reference: Kagan, Acc. Chem. Res. (1997).
Dynamic Kinetic Resolution (DKR)
Definition: Resolution combined with in situ racemization to yield >50% of the desired enantiomer.
Context: Boosts efficiency in API synthesis.
Example: Lipase DKR of secondary alcohols.
Related Terms: Kinetic Resolution, DYKAT.
Reference: Pellissier, Tetrahedron (2003).
Dynamic NMR (DNMR)
Definition: NMR methods to study exchange processes and barriers (e.g., enantiomerization).
Context: Quantifies ΔG‡ for atropisomer interconversion relevant to stability.
Example: DNMR of hindered biaryls.
Related Terms: Barrier to Rotation, Enantiomerization.
Reference: Sandström, Dynamic NMR (1982).
E-(Entgegen)
Definition: A geometric descriptor from the Cahn-Ingold-Prelog (CIP) system indicating that the higher-priority substituents on a double bond are on opposite sides.
Context: Used to describe configuration in alkenes and similar systems; "E" is the opposite of "Z."
Example: E-2-butene has methyl groups on opposite sides of the C=C bond.
Related Terms: Z-; Geometric Isomerism; CIP Rules.
Reference: Cahn, R. S., Ingold, C. K. & Prelog, V. "Specification of Molecular Chirality." Angewandte Chemie International Edition 5, 385-415 (1966).
E/Z Isomerism
Definition: Configuration around a double bond assigned by CIP priorities.
Context: Geometry influences potency, selectivity, and metabolism.
Example: E- vs Z-tamoxifen.
Related Terms: Geometric Isomerism, CIP Rules.
Reference: IUPAC Gold Book.
Eclipsed Conformation
Definition: A conformation where substituents on adjacent carbons align with each other, maximizing torsional strain.
Context: Less stable than staggered arrangements due to electron repulsion.
Example: Eclipsed ethane has higher energy than the staggered form.
Related Terms: Conformer; Gauche; Torsional Strain.
Reference: Eliel & Wilen (1994).
Electronic Circular Dichroism (ECD)
Definition: CD arising from electronic transitions.
Context: Used for exciton coupling analysis and absolute configuration of chromophoric systems.
Example: Exciton couplet in BINOL derivatives.
Related Terms: Exciton Coupling, Cotton Effect.
Reference: Nakanishi et al., Circular Dichroism (2007).
Enamine Catalysis
Definition: Organocatalytic activation of carbonyls via enamine intermediates.
Context: Controls enantioface selectivity in additions.
Example: Proline-catalyzed aldol.
Related Terms: Organocatalysis, Iminium Catalysis.
Reference: List/Barbas (2000).
Enantioconvergent Synthesis
Definition: Both enantiomers of a racemic starting material are transformed into one enantiomeric product.
Context: Maximizes material economy in process chemistry.
Example: Ni-catalyzed cross-couplings with stereoretention.
Related Terms: DYKAT, Stereoretentive Coupling.
Reference: Fu, Acc. Chem. Res. (2017).
Enantiomer
Definition: One of a pair of non-superimposable mirror-image stereoisomers.
Context: Enantiomers may differ in PK/PD and toxicity; single-enantiomer drugs are often preferred.
Example: S-ibuprofen (eutomer) vs R-ibuprofen (distomer).
Related Terms: Racemate, Eutomer, Distomer.
Reference: Morrison & Boyd, Organic Chemistry.
Enantiomeric Excess (ee)
Definition: The absolute difference in enantiomer fractions (|%R − %S|).
Context: Standard QC metric for chiral purity in pharma.
Example: 70% R, 30% S → 40% ee.
Related Terms: Enantiomeric Ratio, Optical Purity.
Reference: IUPAC Gold Book.
Enantiomeric Ratio (er)
Definition: The ratio of enantiomers, typically expressed as R:S.
Context: Preferred over ee for statistical modeling and kinetics.
Example: 98:2 e.r. for a product.
Related Terms: Enantiomeric Excess, dr.
Reference: IUPAC Gold Book.
Enantiomerization
Definition: Interconversion between enantiomers via bond rotation or reversible processes.
Context: Defines shelf-life and handling conditions for atropisomeric drugs.
Example: Biaryl axis rotation over barrier ΔG‡.
Related Terms: Racemization, Barrier to Rotation.
Reference: IUPAC Gold Book.
Enantioselective Metabolism
Definition: Metabolic pathways that preferentially process one enantiomer.
Context: Causes distinct clearance and exposure for enantiomers.
Example: CYP-mediated oxidation of one enantiomer faster than the other.
Related Terms: Stereopharmacology, Chiral Inversion.
Reference: Testa & Krämer, Drug Metabolism (2008).
Enantioselectivity
Definition: Preference for formation of one enantiomer over the other (quantified as ee or er).
Context: Primary performance metric in asymmetric synthesis and catalysis.
Example: 98:2 e.r. in asymmetric hydrogenation.
Related Terms: Stereoselectivity, Enantiospecificity.
Reference: Noyori, Nobel Lecture (2001).
Enantiospecificity
Definition: A reaction in which the enantiomeric identity of the product is determined by that of the starting material.
Context: Key in stereoretentive cross-couplings and biosynthetic steps.
Example: Stereoretentive substitution maintaining configuration.
Related Terms: Stereospecificity, Enantioselectivity.
Reference: IUPAC Gold Book.
Enantiotopic
Definition: Two groups or faces of a molecule that become enantiomers upon substitution or reaction.
Context: Differentiated in chiral environments such as enzymatic systems or asymmetric catalysts.
Example: The two hydrogens of methylene in ethanol are enantiotopic.
Related Terms: Diastereotopic; Prochirality; Re/Si Face.
Reference: IUPAC Gold Book (2019).
Epimer
Definition: Diastereomers differing in configuration at only one stereocenter.
Context: Widespread in sugar chemistry and biologically relevant isomers.
Example: D-glucose vs D-mannose (C2 epimers).
Related Terms: Anomer, Diastereomer.
Reference: IUPAC Gold Book.
Equatorial Bond
Definition: A bond directed outward from the equator of a cyclohexane ring, approximately in the ring plane.
Context: Equatorial positions minimize steric hindrance; preferred by bulky substituents.
Example: Equatorial t-Bu in tert-butylcyclohexane.
Related Terms: Axial Bond, Conformation, A-Values.
Reference: Eliel & Wilen (1994).
Erythro-
Definition: A relative stereochemical descriptor for diastereomers with two adjacent chiral centers having substituents on the same side in a Fischer projection.
Context: Commonly used for sugars and amino acids; denotes visual similarity to erythrose.
Example: Erythro-2,3-butanediol has hydroxyl groups on the same side.
Related Terms: Threo-; Relative Configuration; Diastereomer.
Reference: Eliel, E. L. & Wilen, S. H. Stereochemistry of Organic Compounds. Wiley, 1994.
Eudismic Ratio
Definition: The ratio of pharmacological activity of the eutomer (more active enantiomer) to the distomer.
Context: Quantifies the stereochemical contribution to drug action, guiding chiral switch strategies.
Example: S-citalopram vs R-citalopram potency ratio.
Related Terms: Eutomer, Distomer, Stereopharmacology.
Reference: Ariëns, Med. Res. Rev. (1984).
Eutomer
Definition: The enantiomer with the desired higher biological activity.
Context: Target of development and commercialization.
Example: S-citalopram.
Related Terms: Distomer, Chiral Switch.
Reference: Ariëns, Med. Res. Rev. (1984).
Exciton Chirality Method
Definition: Assigns absolute configuration from sign of ECD exciton couplets between interacting chromophores.
Context: Widely applied to biaryls and helicenes.
Example: Positive couplet → P helicity.
Related Terms: ECD, Exciton Coupling.
Reference: Harada & Nakanishi (1972).
Exciton Coupling
Definition: Interaction between transition dipoles of nearby chromophores causing split CD signals.
Context: Exciton-chirality method assigns absolute configuration in biaryl and helicene systems.
Example: Couplet sign in BINOL indicating axial chirality sense.
Related Terms: ECD, Cotton Effect.
Reference: Harada & Nakanishi, Acc. Chem. Res. (1972).
Felkin–Anh Model
Definition: Predicts nucleophilic attack on carbonyls favoring approach anti to the largest substituent.
Context: Standard model for 1,2-asymmetric induction.
Example: Addition to α-chiral aldehydes.
Related Terms: Cram’s Rule, Bürgi–Dunitz Angle.
Reference: Anh & Felkin, Tetrahedron Lett. (1973).
Fischer Projection
Definition: A 2D representation where horizontal lines indicate bonds projecting forward and vertical lines project backward.
Context: Standard tool in carbohydrate and amino acid stereochemistry.
Example: D-glucose drawn in Fischer projection.
Related Terms: Newman Projection, Sawhorse Projection.
Reference: Fischer, Ber. Dtsch. Chem. Ges. (1891).
Flack Parameter (x)
Definition: Refinement parameter indicating correct absolute structure in X-ray crystallography.
Context: x ~ 0 confirms correct absolute configuration; x ~ 1 suggests inversion.
Example: Reporting Flack x for a chiral API crystal.
Related Terms: Absolute Configuration, X-ray Crystallography.
Reference: Flack, Acta Cryst. A (1983).
Gauche
Definition: A conformation in which substituents on adjacent carbons are separated by a dihedral angle of approximately 60°.
Context: Important in determining conformational energy and intramolecular interactions such as the gauche effect.
Example: Gauche butane conformation is less stable than anti but more stable than eclipsed.
Related Terms: Anti; Synclinal; Conformation; Newman Projection.
Reference: Eliel & Wilen (1994).
Geminal
Definition: Refers to two substituents attached to the same atom, typically a carbon.
Context: Geminal dihalides or diols exhibit distinct reactivity compared to vicinal analogs.
Example: Geminal dichlorides (R2CCl2) vs vicinal dichlorides (RCHCl-CH2Cl).
Related Terms: Vicinal; Substituent; Structural Isomer.
Reference: IUPAC Gold Book (2019).
Geometric Isomerism
Definition: Stereoisomerism due to restricted rotation (double bonds, rings).
Context: Controls pharmacology through spatial disposition of groups.
Example: cis/trans-platin complexes.
Related Terms: E/Z Isomerism, Ring Conformations.
Reference: IUPAC Gold Book.
GPCR Stereoselectivity
Definition: Enantiomer-dependent binding and signaling at GPCRs.
Context: Explains potency and bias differences across enantiomers.
Example: L- vs D-ligands at opioid receptors.
Related Terms: Stereopharmacology, Eutomer/Distomer.
Reference: Kenakin, Pharmacology Primer (2019).
Hashed (Wedge-Dash Representation)
Definition: A visual notation in molecular drawings where hashed bonds represent bonds going behind the plane of the paper.
Context: Used to depict stereochemistry in three-dimensional projections.
Example: Hashed bond to indicate the backward substituent in Fischer or wedge projections.
Related Terms: Wedged; Fischer Projection; Newman Projection.
Reference: Morrison & Boyd. Organic Chemistry. Prentice Hall, 1992.
Helicity (P/M)
Definition: Screw sense of helical molecules designated P (plus) or M (minus).
Context: Describes helicenes and protein helices relevant to binding.
Example: P-helicene vs M-helicene.
Related Terms: Axial Chirality, Planar Chirality.
Reference: IUPAC Gold Book.
Horeau Method
Definition: Indirect determination of ee via formation of meso/dl diesters.
Context: Historical approach for ee estimation.
Example: Horeau’s diacid anhydrides with alcohols.
Related Terms: Optical Purity, CDA.
Reference: Horeau, Tetrahedron (1961).
Iminium Catalysis
Definition: Activation of α,β-unsaturated carbonyls via chiral iminium ions.
Context: Enables enantioselective Michael additions and cycloadditions.
Example: MacMillan catalysts.
Related Terms: Organocatalysis, Enamine Catalysis.
Reference: MacMillan, Nature (2000).
Isomer
Definition: A compound that shares the same molecular formula with another compound but differs in atomic connectivity or spatial arrangement.
Context: Isomers are classified into structural (constitutional) and stereoisomers, including enantiomers and diastereomers.
Example: Butane and isobutane are constitutional isomers; R- and S-lactic acid are enantiomers.
Related Terms: Isomerism; Stereoisomer; Constitutional Isomer.
Reference: IUPAC. Compendium of Chemical Terminology (IUPAC Gold Book), 2nd Edition, 1997 (updated 2019).
Isomeric Ballast
Definition: Non-pharmacologically active stereoisomeric portion of a racemic or stereoisomeric drug mixture.
Context: Regulatory agencies require understanding of ballast contribution to safety and efficacy.
Example: R-ibuprofen considered ballast in S-ibuprofen therapy.
Related Terms: Distomer, Racemate, Stereo-pharmacology.
Reference: Ariëns, Med. Res. Rev. (1986).
Isomeric Purity Specification
Definition: Quality specification defining acceptable levels of undesired stereoisomers.
Context: ICH Q6A expects justified limits for isomers with safety/efficacy impact.
Example: Limit of ≤0.5% distomer in eutomer API.
Related Terms: ICH Q6A, QC.
Reference: ICH Q6A (1999).
Isomerism
Definition: The phenomenon in which compounds share the same molecular formula but differ in structure or spatial arrangement.
Context: Forms the basis of structural and stereochemical diversity in organic and medicinal chemistry.
Example: Ethanol and dimethyl ether are structural isomers; cis- and trans-2-butene are geometric isomers.
Related Terms: Isomer; Stereoisomer; Constitutional Isomer; Enantiomer; Diastereomer.
Reference: IUPAC. Compendium of Chemical Terminology (IUPAC Gold Book), 2nd Edition, 1997 (updated 2019).
Jacobsen–Katsuki Epoxidation
Definition: Mn–salen-catalyzed asymmetric epoxidation of unfunctionalized alkenes.
Context: Expands epoxidation to substrates beyond allylic alcohols.
Example: Epoxidation of styrenes.
Related Terms: Asymmetric Catalysis, Epoxidation.
Reference: Jacobsen, JACS (1990/1991).
Kagan ML2 Model
Definition: Model rationalizing nonlinear ee effects via homochiral/heterochiral catalyst aggregates.
Context: Guides interpretation of catalyst behavior in development.
Example: Dimeric catalyst equilibria giving nonlinear plots.
Related Terms: Nonlinear Effects, Enantioselectivity.
Reference: Kagan, Acc. Chem. Res. (1997).
Karplus Relationship
Definition: Empirical relationship between vicinal ³J coupling and dihedral angle.
Context: Supports stereochemical assignment from NMR coupling constants.
Example: ³JHH ~ 10 Hz for anti arrangement.
Related Terms: J-Coupling, Conformation.
Reference: Karplus, J. Chem. Phys. (1959).
L-(Laevus, Left)
Definition: A stereochemical prefix denoting configuration opposite to that of D-glyceraldehyde, based on the Fischer projection.
Context: Indicates mirror-image relationship to D-forms; unrelated to optical rotation (d/l).
Example: L-alanine corresponds to the configuration of L-glyceraldehyde.
Related Terms: D-; Fischer Projection; Absolute Configuration.
Reference: Fischer (1891).
Levo- (l, –)
Definition: Denotes a compound that rotates plane-polarized light to the left (counter-clockwise).
Context: Paired with dextro-; unrelated to R/S assignment.
Example: L-(–)-lactic acid.
Related Terms: Dextro-, Optical Activity, Specific Rotation.
Reference: IUPAC Gold Book.
Matched Pair
Definition: Substrate and chiral reagent/catalyst whose stereochemical preferences reinforce each other.
Context: Delivers higher selectivity than mismatched combinations.
Example: (S)-substrate with (S)-auxiliary.
Related Terms: Mismatched Pair, Double Stereodifferentiation.
Reference: Seebach, Angew. Chem. (1979).
Memory of Chirality
Definition: Retention of stereochemical information through achiral or planar intermediates via conformational constraints.
Context: Enables net stereospecificity where racemization might be expected.
Example: Acylium ion cyclizations retaining chirality.
Related Terms: Stereomutation, Enantiospecificity.
Reference: Houk, Angew. Chem. (2001).
Meso Compound
Definition: An achiral molecule with stereocenters and an internal plane/center of symmetry.
Context: No net optical activity, important in planning stereoselective routes.
Example: Meso-tartaric acid.
Related Terms: Symmetry, Diastereomer.
Reference: Eliel & Wilen (1994).
Meso-
Definition: A molecule containing stereocenters but overall achiral due to an internal plane or center of symmetry.
Context: Meso compounds are optically inactive even though they contain chiral atoms.
Example: Meso-tartaric acid.
Related Terms: Achiral; Symmetry; Diastereomer.
Reference: Eliel & Wilen (1994).
Misassignment of Configuration
Definition: The incorrect determination, description, or reporting of the absolute or relative stereochemical configuration of a molecule, stereogenic center, axis, or element.
Context: Misassignment of configuration can arise from incorrect application of CIP rules, misinterpretation of spectroscopic or crystallographic data, inappropriate use of chiroptical methods, or confusion between nomenclature systems (e.g., R/S vs D/L vs d/l). In pharmaceutical and medicinal chemistry, such errors can propagate through literature, patents, regulatory submissions, and clinical interpretation, leading to flawed structure-activity relationships, incorrect biological conclusions, or costly redevelopment.
Example: Assigning an alcohol as (R) based solely on optical rotation without corroborating evidence, later corrected by X-ray crystallography or VCD analysis showing the (S) configuration.
Related Terms: Absolute Configuration; Relative Configuration; CIP Rules; Stereochemical Assignment; Chiral Literacy; Stereo-Sloppy.
Reference: IUPAC. Compendium of Chemical Terminology (IUPAC Gold Book). 2nd Edition, 1997 (updated 2019).
Harada, N.; Nakanishi, K. Circular Dichroism Spectroscopy-Exciton Coupling in Organic Stereochemistry. Accounts of Chemical Research, 5, 257-263 (1972).
Mismatched Pair
Definition: Substrate and chiral reagent/catalyst with opposing preferences.
Context: Reduces stereoselectivity; addressed by switching catalyst or substrate.
Example: (S)-substrate with (R)-auxiliary.
Related Terms: Matched Pair, Stereoselectivity.
Reference: Evans, JACS (1981).
Molar Rotation (Φ)
Definition: Specific rotation multiplied by molar mass.
Context: Allows comparison of chiroptical responses across compounds.
Example: Molar rotation trends within homologous series.
Related Terms: Specific Rotation, Optical Activity.
Reference: Barron, Molecular Light Scattering (2004).
Molecular Imprinting
Definition: Template-directed polymerization producing cavities complementary to a target enantiomer.
Context: Creates synthetic receptors for separations and sensing.
Example: Imprinted polymers for S-propranolol.
Related Terms: Chiral Recognition, CSP.
Reference: Sellergren, Chem. Rev. (2000).
Mosher’s Method
Definition: Use of MTPA esters/amides to assign absolute configuration by Δδ patterns.
Context: Gold-standard for alcohols and amines in small-molecule analysis.
Example: Δδ analysis of secondary alcohols.
Related Terms: CDA, NMR.
Reference: Dale & Mosher, JACS (1973).
Neighboring Group Participation (NGP)
Definition: Assistance by an adjacent group forming bridged intermediates that alter stereochemical outcome.
Context: Can cause retention or special selectivity in substitutions.
Example: Participation by acetoxy in solvolysis.
Related Terms: SN1, SN2.
Reference: March, Advanced Organic Chemistry (2007).
Newman Projection
Definition: A perspective representation viewing along a C–C bond, useful for analyzing conformations.
Context: Widely used in stereo-electronic analysis and to visualize torsional strain.
Example: Staggered vs eclipsed ethane conformations.
Related Terms: Sawhorse Projection, Conformation.
Reference: Newman, JACS (1952).
NOE/NOESY
Definition: Nuclear Overhauser effect experiments probing through-space proximities.
Context: Establishes relative configuration and conformation.
Example: NOESY on cyclic diols.
Related Terms: J-Coupling, ROESY.
Reference: Claridge, High-Resolution NMR (2009).
Nonlinear Effects in Asymmetric Catalysis
Definition: Nonlinear relationship between catalyst ee and product ee due to catalyst aggregation or kinetics.
Context: Impacts process scale-up and control strategies.
Example: Kagan’s ML2 model.
Related Terms: Kagan Model, Enantioselectivity.
Reference: Kagan & Blackmond, Acc. Chem. Res. (2000).
Noyori Hydrogenation
Definition: Asymmetric hydrogenation via bifunctional Ru/diamine catalysts.
Context: Industrial method for enantioenriched alcohols and amines.
Example: Hydrogenation of ketones with Ru-BINAP/diamine.
Related Terms: Asymmetric Catalysis, Transfer Hydrogenation.
Reference: Noyori, Angew. Chem. (1994/2001).
Oppolzer Sultam
Definition: A rigid chiral auxiliary derived from camphor used to control facial selectivity.
Context: Useful for Diels–Alder and alkylation reactions with high selectivity.
Example: Sultam-controlled cycloadditions.
Related Terms: Chiral Auxiliary, Asymmetric Synthesis.
Reference: Oppolzer, Tetrahedron (1987).
Optical Activity
Definition: Rotation of plane-polarized light by chiral substances.
Context: Classical signature of chirality; still used for QC when specific rotation is compendial.
Example: [α]D^20 measurements for amino acids.
Related Terms: Polarimetry, Specific Rotation.
Reference: Atkins, Physical Chemistry.
Optical Activity
Definition: The ability of chiral substances to rotate the plane of polarized light.
Context: Classical indicator of chirality; measured via polarimetry to assess enantiomeric purity.
Example: (+)-Lactic acid rotates plane-polarized light to the right.
Related Terms: Optical Rotation; Specific Rotation; Polarimetry.
Reference: Atkins, P. W. Physical Chemistry. Oxford University Press, 2010.
Optical Purity
Definition: Measured enantiomeric purity derived from specific rotation relative to a pure enantiomer.
Context: Historical measure that correlates with ee when linear.
Example: Optical purity ≈ ee for simple systems.
Related Terms: Enantiomeric Excess, Polarimetry.
Reference: Morrison & Boyd.
Optical Rotation
Definition: The observed angle by which plane-polarized light is rotated upon passing through a chiral sample.
Context: Dependent on path length, concentration, wavelength, and temperature; denoted [α]D20.
Example: [α]D20 = +12.5° (c 1.0, MeOH).
Related Terms: Optical Activity; Specific Rotation; Polarimetry.
Reference: USP-NF General Chapter <781> Optical Rotation.
Optical Rotatory Dispersion (ORD)
Definition: Wavelength dependence of optical rotation.
Context: Precursor to CD; used for stereochemical assignment and conformational analysis.
Example: ORD curves distinguishing sugar stereoisomers.
Related Terms: Circular Dichroism, Cotton Effect.
Reference: Lightner & Gurst, Organic Conformational Analysis (1980).
Organocatalysis
Definition: Enantioselective catalysis with small organic molecules.
Context: Metal-free, green, and scalable approaches for key stereogenic steps.
Example: Proline-catalyzed aldol reactions.
Related Terms: Iminium Catalysis, Enamine Catalysis.
Reference: List & Barbas, JACS (2000).
Phase-Transfer Catalysis (PTC)
Definition: Biphasic catalysis moving anions across phases with chiral quaternary ammonium salts.
Context: Practical for large-scale enantioselective alkylations.
Example: Cinchona-derived PTCs.
Related Terms: Organocatalysis, Asymmetric Catalysis.
Reference: Starks, JACS (1971); O’Donnell, Acc. Chem. Res. (2004).
Pirkle-Type CSP
Definition: Small-molecule chiral selectors relying on π–π and H-bond interactions.
Context: Versatile for aromatic enantiomers and drug-like molecules.
Example: Whelk-O columns.
Related Terms: CSP, Chiral HPLC.
Reference: Pirkle, J. Chromatogr. (1981).
Planar Chirality
Definition: Chirality resulting from the arrangement of substituents in a plane.
Context: Seen in ferrocene ligands and metallocenes used in asymmetric catalysis.
Example: Planar-chiral 1,2-disubstituted ferrocenes.
Related Terms: Helicity, Axial Chirality.
Reference: IUPAC Gold Book.
Polarimetry
Definition: Technique to measure optical rotation of chiral compounds.
Context: Rapid QC screen for enantiomeric composition when correlation is established.
Example: Measuring rotation of limonene enantiomers.
Related Terms: Optical Activity, Specific Rotation.
Reference: Atkins, Physical Chemistry.
Polysaccharide CSP
Definition: Derivatized cellulose/amylose selectors coated or immobilized on silica.
Context: Most widely used CSPs due to broad applicability.
Example: Chiralcel OD-H equivalents.
Related Terms: CSP, Chiral HPLC.
Reference: Okamoto, Chem. Rev. (2011).
Pro-R / Pro-S
Definition: Labels for enantiotopic groups whose replacement would give R or S product.
Context: Critical in enzymatic selectivity and NMR assignment.
Example: Pro-R hydrogen abstraction in alcohol dehydrogenase.
Related Terms: Prochirality, Re/Si Face.
Reference: IUPAC Gold Book.
Prochirality
Definition: An achiral entity that can become chiral by a single desymmetrizing step.
Context: Basis for enantioface/enantioselective reactions in synthesis and enzymology.
Example: Prochiral ketones undergoing enantioselective reduction.
Related Terms: Re/Si Face, Pro-R/Pro-S.
Reference: IUPAC Gold Book.
Protein Homochirality
Definition: Proteins are composed almost exclusively of L-amino acids.
Context: Drives stereoselective binding and metabolism in biology.
Example: Enzymes discriminating D- vs L-substrates.
Related Terms: Homochirality, Stereorecognition.
Reference: Blackmond, PNAS (2004).
Pseudoasymmetric Center (r/s)
Definition: A stereocenter whose configuration is defined relative to other stereocenters (lowercase r/s).
Context: Relevant in complex APIs where relayed stereochemistry affects labeling.
Example: r/s descriptors in substituted tartaric acid derivatives.
Related Terms: CIP Rules, Relative Configuration.
Reference: IUPAC Gold Book.
Quadrant Model (Hydrogenation)
Definition: Empirical model mapping steric quadrants around metal–ligand catalysts to predict facial selectivity.
Context: Used with Rh-DuPHOS, BINAP-Ru systems for enantioface control.
Example: Asymmetric hydrogenation of dehydroamino acids.
Related Terms: Bite Angle, Chiral Ligand.
Reference: Knowles/Halpern analyses (1970s–1990s).
R/S Configuration
Definition: CIP-based descriptors assigning absolute configuration to stereocenters.
Context: Universal language for stereochemical specification in pharma and patents.
Example: S-ibuprofen is the active eutomer.
Related Terms: CIP Rules, Absolute Configuration.
Reference: Cahn–Ingold–Prelog rules (1956/1966).
Racemate (rac)
Definition: A 1:1 mixture of enantiomers.
Context: Common synthetic outcome; may require resolution or chiral switching.
Example: Racemic thalidomide.
Related Terms: Conglomerate, Racemic Compound.
Reference: FDA Policy on Stereoisomeric Drugs (1992).
Racemic Compound
Definition: Racemate that crystallizes as a single racemic crystal containing both enantiomers.
Context: Limits simple preferential crystallization; requires alternative resolution routes.
Example: Racemic mandelic acid (context dependent).
Related Terms: Racemate, Conglomerate.
Reference: Eliel & Wilen (1994).
Racemization
Definition: Conversion of an enantiopure substance to a racemate.
Context: Critical risk for chiral APIs and must be controlled in process/storage.
Example: Base-catalyzed racemization of α-chiral carbonyls.
Related Terms: Enantiomerization, Stereomutation.
Reference: IUPAC Gold Book.
Raman Optical Activity (ROA)
Definition: Difference in Raman scattering for circularly polarized light.
Context: Complementary to VCD; sensitive to conformation of chiral molecules and biomacromolecules.
Example: ROA of proteins to assess secondary structure.
Related Terms: VCD, CD.
Reference: Barron, Molecular Light Scattering (2004).
Re/Si Face
Definition: Descriptors for the two faces of a trigonal (sp²) center.
Context: Used to define enantioface selectivity in additions and reductions.
Example: Hydride attack on the Re face of a ketone.
Related Terms: Prochirality, Pro-R/Pro-S.
Reference: IUPAC Gold Book.
Relative Configuration
Definition: The spatial relationship between stereocenters within a molecule without reference to absolute R/S.
Context: Useful for assigning diastereomer series in synthesis and NMR analysis.
Example: Erythro vs threo in sugars.
Related Terms: Diastereomer, Absolute Configuration.
Reference: Eliel & Wilen (1994).
Relative Stereochemistry
Definition: The spatial relationship between stereocenters within a molecule without reference to absolute configuration.
Context: Important for describing diastereomers in synthesis and structure elucidation.
Example: Erythro and threo configurations of 2,3-butanediols.
Related Terms: Absolute Configuration; Diastereomer; Enantiomer.
Reference: Eliel & Wilen (1994).
Resolution
Definition: The process of separating a racemic mixture into its individual enantiomers.
Context: A key step in the preparation of enantiopure compounds for pharmaceutical use. Can be achieved through crystallization, chemical or enzymatic methods, or chiral chromatography.
Example: Resolution of racemic mandelic acid using (+)-α-phenylethylamine.
Related Terms: Racemate; Enantiomeric Excess; Chiral HPLC; Kinetic Resolution.
Reference: Jacques, J., Collet, A. & Wilen, S. H. Enantiomers, Racemates, and Resolutions. Wiley, 1981.
Sawhorse Projection
Definition: Representation of conformations showing a C–C bond diagonally with attached substituents.
Context: Useful for depicting stereochemical relationships and conformational preferences.
Example: Anti vs gauche butane.
Related Terms: Newman Projection, Conformation.
Reference: Eliel & Wilen (1994).
Scalemic Mixture
Definition: A mixture with unequal amounts of enantiomers (non-racemic).
Context: Describes real-world batches before final enantiopurification and specifications.
Example: 90:10 e.r. mixture of a chiral API.
Related Terms: Enantiomeric Excess, Racemate.
Reference: IUPAC Gold Book.
Sharpless Dihydroxylation
Definition: OsO4/ligand (AD-mix) enantioselective dihydroxylation of alkenes.
Context: Delivers vic-diols with predictable configuration.
Example: AD-mix-α/β applications.
Related Terms: Sharpless Epoxidation, Asymmetric Catalysis.
Reference: Sharpless (2001).
Sharpless Epoxidation
Definition: Ti–tartrate-catalyzed enantioselective epoxidation of allylic alcohols.
Context: Workhorse method to install epoxides with high ee.
Example: Epoxidation to epothilone intermediates.
Related Terms: Asymmetric Catalysis, Sharpless Dihydroxylation.
Reference: Sharpless, Nobel Lecture (2001).
SN1 Racemization
Definition: Loss of configuration due to planar carbocation intermediate in SN1 reactions.
Context: Explains partial/complete racemization in solvolysis.
Example: Solvolysis of tert-butyl chloride.
Related Terms: SN1, Neighboring Group Participation.
Reference: Morrison & Boyd.
Soai Reaction
Definition: Asymmetric autocatalysis of pyrimidyl alkanol formation showing amplification from minute ee.
Context: Model system for origin-of-homochirality studies.
Example: Autocatalytic addition of diisopropylzinc to aldehydes.
Related Terms: Absolute Asymmetric Synthesis, Amplification.
Reference: Soai, Nature (1995).
Solid Solution (Racemic Solid Solution)
Definition: Crystalline phase where enantiomers are disordered over lattice sites.
Context: Challenging for resolution; may impact solid-state specifications.
Example: Racemic mixtures forming solid solutions.
Related Terms: Racemic Compound, Conglomerate.
Reference: Eliel & Wilen (1994).
Specific Rotation ([α]λ^T)
Definition: Observed optical rotation normalized to concentration and path length.
Context: Compendial test for chiral purity/identity in some monographs.
Example: [α]D^20 = +12.5 (c 1.0, MeOH).
Related Terms: Optical Activity, Polarimetry.
Reference: USP/Ph. Eur. General Chapters.
Spiranes
Definition: Bicyclic systems sharing a single atom; may be chiral without stereocenters.
Context: Offer rigid, 3D scaffolds for medicinal chemistry.
Example: Spiro[4.4]nonane derivatives.
Related Terms: Topological Chirality, Planar Chirality.
Reference: Clayden et al. (2012).
Stereo-Sloppy
Definition: An informal but precise descriptor for scientific, educational, or regulatory practice in which stereochemical distinctions are ignored, misused, oversimplified, or treated as inconsequential.
Context: Stereo-sloppy behavior manifests when chirality, configuration, or isomerism is inadequately specified or misunderstood. In chemistry and pharmacology, stereo-sloppiness can lead to incorrect structures, flawed SAR conclusions, irreproducible synthesis, analytical errors, or unsafe clinical assumptions. The term is particularly relevant in discussions of poor stereochemical literacy, inadequate reporting, and historical drug development failures.
Example: Describing a compound as "the active drug" without specifying whether it is a racemate, eutomer, or enantiopure form; or using D/L, d/l, and R/S interchangeably as if they were equivalent.
Related Terms: Chiral Literacy; Chiral Intelligence; Stereochemistry; Isomerism; Stereo-pharmacology; Misassignment of Configuration.
Reference: Ariens, E. J. Stereochemistry, a basis for sophisticated nonsense in pharmacokinetics and clinical pharmacology. Medical Research Reviews, 4, 197-236 (1984). - Classic paper criticizing careless stereochemical thinking.
Eliel, E. L., Wilen, S. H., & Mander, L. N. Stereochemistry of Organic Compounds. Wiley (1994).
Smith, S. W. Chiral toxicology: it's the same thing...only different. Toxicological Sciences, 110(1), 4-30, (2009).
Nguyen, L. A., He, H., & Pham-Huy, C. Chiral drugs: an overview. International Journal of Biomedical Science, 2(2), 85-100, (2006).
Stereocenter
Definition: An atom at which interchange of two substituents produces a stereoisomer.
Context: Defines loci of stereochemical variation and controls stereochemical outcomes in synthesis.
Example: Tetrahedral carbon with four different substituents.
Related Terms: Chiral Center, Stereogenic Center.
Reference: IUPAC Gold Book.
Stereochemistry
Definition: The branch of chemistry concerned with the three-dimensional arrangement of atoms in molecules and its influence on properties and reactions.
Context: Central to understanding drug action, synthesis outcomes, and regulatory specifications for isomeric purity.
Example: E/Z isomers of tamoxifen show distinct properties.
Related Terms: Chirality, Configuration, Conformation.
Reference: Eliel & Wilen (1994).
Stereodivergent Synthesis
Definition: Strategy enabling access to multiple stereoisomers via controlled catalyst/condition changes.
Context: Efficient generation of stereochemical arrays for SAR.
Example: Catalyst-controlled divergent aldol products.
Related Terms: Enantiodivergence, Diastereodivergence.
Reference: Wang & List, Angew. Chem. (2012).
Stereoelectronic Effect
Definition: Effect of orbital alignment on conformation and reactivity.
Context: Controls anomeric effect, β-elimination geometry, and selectivity.
Example: Anomeric effect in carbohydrates.
Related Terms: Antiperiplanar, Hyperconjugation.
Reference: Deslongchamps, Stereoelectronic Effects (1983).
Stereogenic Axis (Axis of Chirality)
Definition: A linear element in a molecule that gives rise to chirality due to restricted rotation.
Context: Common in atropisomeric systems and cumulenes like allenes.
Example: Axially chiral biaryl ligands such as BINAP.
Related Terms: Atropisomerism; Axial Chirality; Planar Chirality.
Reference: IUPAC Gold Book (2019).
Stereogenic Center
Definition: An atom at which the interchange of two substituents generates a stereoisomer.
Context: Most often a tetrahedral carbon with four different substituents; key determinant of chirality in drugs.
Example: The α-carbon in lactic acid is a stereogenic center
Related Terms: Chiral Center; Stereocenter; Enantiomer.
Reference: IUPAC. Compendium of Chemical Terminology (IUPAC Gold Book), 2nd Edition, 1997 (updated 2019).
Stereogenic Element
Definition: Any structural feature (center, axis, plane, helix) that generates stereoisomerism.
Context: Extends chirality beyond tetrahedral centers to axes and planes common in drugs and ligands.
Example: Axial chirality in BINAP; planar chirality in ferrocene.
Related Terms: Axial Chirality, Planar Chirality, Helicity.
Reference: IUPAC Gold Book.
Stereoisomer(s)
Definition: Compounds with the same molecular formula and bonding sequence but differing in three-dimensional arrangement.
Context: Includes enantiomers and diastereomers, fundamental to stereochemistry and pharmacology.
Example: Cis/trans isomers of 2-butene; R- and S-lactic acid.
Related Terms: Enantiomer; Diastereomer; Isomerism.
Reference: IUPAC Gold Book (2019).
Stereomutation
Definition: Interconversion between stereoisomers (e.g., enantiomerization, epimerization).
Context: Impacts shelf life and bioactivity; must be profiled in stability studies.
Example: Atropisomer interconversion over time.
Related Terms: Racemization, Barrier to Rotation.
Reference: Clayden et al. (2012).
Stereopharmacology (Stereo-pharmacology)
Definition: Discipline examining how stereochemistry affects pharmacodynamics and pharmacokinetics.
Context: Guides selection of the optimal enantiomer and specification of isomeric purity.
Example: Chiral switch from omeprazole to esomeprazole.
Related Terms: Eutomer/Distomer, Chiral Inversion.
Reference: Spasov A.A., Iiezhitsa I.N. [Stereopharmacology of carnitine]. Ross Fiziol Zh Im I M Sechenova. 2005, 91(12):1469-1480. Russian. PMID: 16493928.
Stereoselective Binding
Definition: Preferential binding of one stereoisomer to a chiral target.
Context: Directly affects potency, efficacy, and safety.
Example: One enantiomer with higher affinity to an enzyme active site.
Related Terms: Chiral Recognition, Stereopharmacology.
Reference: Silverman, Drug Design (2014).
Stereoselective Transport
Definition: Transporters recognizing one enantiomer with higher affinity.
Context: Affects absorption/distribution differences between enantiomers.
Example: OATP transport of certain eutomer predominates.
Related Terms: Stereopharmacology, Pharmacokinetics.
Reference: Giacomini et al., Nat. Rev. Drug Discov. (2010).
Stereoselectivity
Definition: Formation of one stereoisomer in preference to others.
Context: Broad umbrella covering enantio- and diastereoselectivity; essential metric in route design.
Example: Syn- vs anti-aldol selectivity.
Related Terms: Enantioselectivity, Diastereoselectivity.
Reference: IUPAC Gold Book.
Stereospecificity
Definition: Different stereoisomeric reactants give different stereoisomeric products.
Context: Defines mechanistic pathways (e.g., SN2 inversion).
Example: Walden inversion in SN2 reactions.
Related Terms: Stereoselectivity, Enantiospecificity.
Reference: IUPAC Gold Book.
Substituent
Definition: An atom or group of atoms attached to a parent chain or ring, replacing a hydrogen atom.
Context: Determines molecular properties, reactivity, and stereochemical effects.
Example: Methyl, nitro, and hydroxyl groups are common substituents.
Related Terms: Functional Group; Geminal; Vicinal.
Reference: IUPAC Gold Book (2019).
Syn (Stereochemical Descriptor)
Definition: Describes substituents on the same side in Newman or Fischer projections, often denoting syn-addition or syn-elimination.
Context: Indicates relative orientation of substituents during reactions.
Example: Syn-addition in catalytic hydrogenation.
Related Terms: Anti; Synclinal; Stereochemistry.
Reference: IUPAC Gold Book (2019).
Synclinal
Definition: Conformation where substituents on adjacent atoms have a dihedral angle of 0°-90° (commonly ~60°).
Context: Equivalent to gauche arrangement; denotes closeness in space.
Example: Synclinal butane conformation (~60° torsion angle).
Related Terms: Antiperiplanar; Gauche; Conformer.
Reference: Eliel & Wilen (1994).
Threo-
Definition: A relative stereochemical descriptor for diastereomers with two adjacent chiral centers having substituents on opposite sides in a Fischer projection.
Context: The opposite of erythro-; used in carbohydrate and amino acid stereochemistry.
Example: Threo-2,3-butanediol has hydroxyl groups on opposite sides.
Related Terms: Erythro-; Diastereomer; Relative Configuration.
Reference: Eliel & Wilen (1994).
Topicity
Definition: Relationship of identical groups/sites: homotopic, enantiotopic, diastereotopic.
Context: Guides selectivity predictions in synthesis and NMR.
Example: Diastereotopic methylene hydrogens in chiral environments.
Related Terms: Enantiotopic, Diastereotopic.
Reference: Eliel & Wilen (1994).
Topological Chirality
Definition: Chirality arising from molecular topology (e.g., knots, catenanes) rather than stereocenters.
Context: Inspires novel drug-like architectures and materials.
Example: Molecular trefoil knots.
Related Terms: Axial Chirality, Helicity.
Reference: IUPAC Gold Book.
Twist-Boat Conformation
Definition: A less strained intermediate form of cyclohexane between the boat and chair conformations.
Context: Has lower energy than pure boat form; relevant in conformational equilibria.
Example: Cyclohexane adopts twist-boat at high energy transition states.
Related Terms: Boat Conformation; Chair Conformation; Conformational Isomer.
Reference: Eliel & Wilen (1994).
Vibrational Circular Dichroism (VCD)
Definition: CD measured in the IR region arising from vibrational transitions.
Context: Determines absolute configuration without heavy atoms; valuable for pharmaceuticals.
Example: VCD assignment of chiral alcohols.
Related Terms: ROA, ECD.
Reference: Nafie, Vibrational Optical Activity (2011).
Viedma Ripening
Definition: Attrition-enhanced deracemization of conglomerates by grinding and dissolution–recrystallization.
Context: Scalable route to single enantiomers from racemates.
Example: Deracemization of amino acid derivatives.
Related Terms: Conglomerate, Deracemization.
Reference: Viedma, Phys. Rev. Lett. (2005).
Walden Inversion
Definition: Inversion of configuration at a stereocenter during SN2 displacement.
Context: Mechanistic hallmark for backside attack; used in route design.
Example: Inversion at chiral bromide with CN-.
Related Terms: SN2, Stereospecificity.
Reference: Morrison & Boyd.
Wedged (Wedge-Dash Representation)
Definition: A solid wedge bond indicates a bond projecting out of the plane toward the observer.
Context: Used in structural depictions to convey stereochemistry.
Example: Wedged bond showing front substituent in tetrahedral carbon center.
Related Terms: Hashed; Fischer Projection; Newman Projection.
Reference: Morrison & Boyd (1992).
X-ray Anomalous Dispersion
Definition: Use of wavelength-dependent scattering to distinguish enantiomers crystallographically.
Context: Determines absolute configuration via resonant scattering effects.
Example: Mo Kα vs Cu Kα anomalous signal.
Related Terms: Flack Parameter, Absolute Configuration.
Reference: Glusker et al., Crystal Structure Analysis (1994).
Z-(Zusammen)
Definition: A geometric descriptor indicating that the higher-priority substituents on a double bond are on the same side.
Context: Used to differentiate stereoisomeric alkenes and similar compounds; "Z" is the counterpart to "E".
Example: Z-2-butene has both methyl groups on the same side of the C=C bond.
Related Terms: E-; Geometric Isomerism; CIP Rules.
Reference: Cahn, Ingold & Prelog (1966).
Zig-Zag Representation
Definition: A skeletal line drawing convention where carbon-carbon bonds are drawn in a zig-zag pattern, implicitly showing sp3 geometry.
Context: Simplifies organic structure representation and emphasizes chain conformation.
Example: n-Hexane drawn as a zig-zag carbon backbone.
Related Terms: Fischer Projection; Newman Projection; Sawhorse Projection.
Reference: Clayden et al., Organic Chemistry (2012).
Zimmerman–Traxler Model
Definition: Chair-like transition state model for aldol reactions linking geometry to syn/anti outcome.
Context: Guides stereochemical planning in polyketide synthesis.
Example: Evans oxazolidinone-controlled aldol.
Related Terms: Aldol Reaction, Chiral Auxiliary.
Reference: Zimmerman & Traxler, JACS (1957).