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).