A simple method for resolution of endo-/exo-monoesters of trans-norborn-5-ene-2,3-dicarboxylic acids into their enantiomers.

Separation of optical isomers obtainable from trans-norborn-5-ene-2,3-dicarboxylic acid methyl and tert-butyl monoesters was performed by crystallization of the respective salts prepared with (R)- and (S)-1-phenylethylamine. Starting from racemic endo-monomethyl ester of trans-norborn-5-ene-2,3-dicarboxylic acid, prepared by partial hydrolysis of the cyclopentadiene-dimethyl fumarate adduct, the corresponding (2R,3R)-endo-monoester was isolated in 97% enantiomeric excess (ee) yield after seven repeated crystallizations from tetrachloromethane. Starting from exo-mono-tert-butyl ester of the same acid, prepared by alcoholysis of the cyclopentadiene-maleic anhydride adduct followed by isomerization, (2R,3R)-exo-monoester was isolated in >98% ee yield after four repeated crystallizations from ethanol. Crystallization of the acids from the mother liquor containing (S)-1-phenylethylamine yielded products with inverse stereochemical configuration.