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)-isole ucine were synthesised from the diastereomeric conjugate of (3R, 7R)-jasmon ic acid and (3S, 7S)-jasmonic acid by sodium borohydride reduction. The res ulting diastereomers were characterised by nuclear magnetic resonance spect ra, 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 observ ed. The elution sequences obtained indicate that the 3,6-trans oriented epi mers 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 sta tionary 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 mixture s of isomeric N-(cucurbinoyl)-isoleucines in order to assign their stereoch emistry and to obtain chiral reference materials on a preparative scale, Co pyright (C) 1999 John Wiley & Sons, Ltd.