Optical resolution methods.

Despite the large number of elaborate enantioselective syntheses for the preparation of a single enantiomer to achieve industrial and scientific goals, the separation and purification of enantiomers (components of racemic compounds) is also necessary. Hence, we present the most often used thought-provoking modern methods based on momentous recognitions (e.g. spontaneous resolution, induced crystallization, resolution by formation of diastereomers, resolution by formation of non-covalent diastereomers, resolution by diastereomeric salt formation, resolution by diastereomeric complex formation, "half equivalent" methods of resolution, separation by crystallization, separation by distillation, separation by supercritical fluid extraction, resolution with mixtures of resolving agents, resolution with a derivative of the target compound, enantioselective chromatography, resolution by formation of covalent diastereomers, resolution by substrate selective reaction, kinetic resolution without enzymes, kinetic resolution by enzyme catalysis, hydrolytic and redox enzymes, kinetic and thermodynamic control, resolutions combined with 2nd order asymmetric transformations, enrichment of partially resolved mixtures, role of the solvent and methods of optimization in the separation of diastereoisomers, non-linear effects and selected examples of resolution on an industrial scale).

Enantioseparation of secondary alcohols by diastereoisomeric salt formation.

A general method was found for the resolution of the racemic 1-phenyl-1-propanol (1) and 1-phenyl-2-propanol (2) with various resolving agents. Monoesters of the alcohols were prepared, which were then reacted with different chiral bases. Successful optical resolutions were achieved only with the maleic acid monoesters (3 and 6). Alcohol 1 has been resolved to >99% enantiomeric excess by diastereoisomeric salt formation via its maleic acid monoester (3) using cinchonidine (9) as resolving agent. Alcohol 2 has been obtained in 98% enantiomeric excess by diastereoisomeric salt formation via its the maleic acid monoester (6) using (+)-dehydroabietylamine (11) as resolving agent.