Analysis of synthetic isoleucine conjugates of cucurbic acid isomers by liquid chromatography

Conjugates of 3,7-trans cucurbic acid isomers with either (S)- or (R)-isoleucine were synthesised from the diastereomeric conjugate of (3R, 7R)-jasmonic acid and (3S, 7S)-jasmonic acid by sodium borohydride reduction. The resulting diastereomers were characterised by nuclear magnetic resonance spectra, The authentic substances were subjected to liquid chromatography using a reversed-phase C-18 matrix and the chiral stationary phase Chiralpak AS. For all (6RS)-hydroxy epimeric pairs, a baseline separation could be observed. The elution sequences obtained indicate that the 3,6-trans oriented epimers with (S)-isoleucine elute prior to the 3,6-cis configured individuals independent of the mode of chromatography, In contrast, the elution pattern of the conjugates containing (R)-isoleucine was reversed on the chiral stationary phase. The epimers with a 3,6-cis configured hydroxy group and the acid side chain eluted faster than these with the 3,6-trans configuration, The chromatographic conditions described are suitable for resolving mixtures of isomeric N-(cucurbinoyl)-isoleucines in order to assign their stereochemistry and to obtain chiral reference materials on a preparative scale, Copyright (C) 1999 John Wiley & Sons, Ltd.