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).
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
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).
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).