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).