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
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
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 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 ?-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
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
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)
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
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.
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).