1,3-Allylic Strain (A¹,³) – 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 – 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 – 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 – 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 – The unambiguous description of the spatial arrangement of substituents (e.g., R/S). Context: Required in regulatory submissions and labeling of chiral APIs. Example: R- and S-thalidomide. Related Terms: R/S Configuration, CIP Rules. Reference: IUPAC Gold Book.
Allenes – 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) – 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) – 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 – 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 – 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).
Asymmetric Amplification – 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 – 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) – Hydrogen transfer from donors (iPrOH, formate) to substrates using chiral catalysts. Context: Avoids H? gas; scalable and selective. Example: Noyori–Ikariya systems reducing ketones. Related Terms: Noyori Hydrogenation, Organocatalysis. Reference: Ikariya & Noyori, Acc. Chem. Res. (2007).
Atropisomerism – 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 – 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 – 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.
Bürgi–Dunitz Angle – 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).
Barrier to Rotation (ΔG‡) – 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 – 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 – 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).
CBS Reduction – 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 – 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 Auxiliary – 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 Bioequivalence – 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) – 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 – 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) – 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 GC (Gas Chromatography) – 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 – 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 Inversion – 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 – 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 Ligand – 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 Mass Spectrometry – 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 Phosphate Catalysis – 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 – 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 – 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 – 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 – 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) – Chromatography using supercritical CO? 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 – Paramagnetic lanthanide complexes that induce differential NMR shifts for enantiomers. Context: Legacy technique for stereochemical analysis. Example: Eu(fod)? added to racemates. Related Terms: CSA, NMR. Reference: Günther, NMR Spectroscopy (2013).
Chiral Solvating Agent (CSA) – 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) – 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 – 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 – 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) – 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 – 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 – 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) – 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) – 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 – 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 – 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 – 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 Isomerism – 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).
Conglomerate (Racemic Conglomerate) – 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 – 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 – 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 – 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 – 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 – 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).
Deracemization – 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, +) – 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 – 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) – 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) – 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 – 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 – 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 – 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) – 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) – 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) – 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/Z Isomerism – 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.
Electronic Circular Dichroism (ECD) – 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 – 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 – 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 – 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) – 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) – 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 – 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 – 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 – 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 – 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.
Epimer – 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 – 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).
Eudismic Ratio – 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 – 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 – 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 – 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 – 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 – 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) – 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).
Geometric Isomerism – 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 – 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).
Helicity (P/M) – 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 – 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 – 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).
Isomeric Ballast – 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 – 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).
Jacobsen–Katsuki Epoxidation – 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 ML? Model – 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 – 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).
Levo- (l, –) – 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 – 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 – 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 – 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).
Mismatched Pair – 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) – 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 – 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 – 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) – 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 – 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 – 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 – 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 ML? model. Related Terms: Kagan Model, Enantioselectivity. Reference: Kagan & Blackmond, Acc. Chem. Res. (2000).
Noyori Hydrogenation – 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 – 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 – 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 Purity – 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 Rotatory Dispersion (ORD) – 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 – 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) – 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 – 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 – 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 – 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 – 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 – 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 – 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 – 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) – 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) – 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 – 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) – 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 – 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 – 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) – 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 – 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 – 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).
Sawhorse Projection – 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 – 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 – OsO?/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 – 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 – 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 – 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) – 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) – 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 – 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).
Stereocenter – 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 – 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 – 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 – 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 Element – 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 – Compounds with the same connectivity but different spatial arrangement of atoms. Context: Stereoisomerism underlies different efficacy and safety of many drugs. Example: Enantiomers of thalidomide; diastereomers of tartaric acid. Related Terms: Enantiomer, Diastereomer. Reference: IUPAC Gold Book.
Stereomutation – 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 – 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: Ariëns, Med. Res. Rev. (1986).
Stereoselective Binding – 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 – 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 – 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 – 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.
Topicity – 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 – 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.
Vibrational Circular Dichroism (VCD) – 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 – 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 – 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.
X-ray Anomalous Dispersion – 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).
Zimmerman–Traxler Model – 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).