Abstract
Different stereoisomers of active molecules often cause different physiological responses and hence pose a challenge for their identification. This study involves perceptive fragmentation behavior of newly isolated cassane butenolides, caesalpinolide A [1] and caesalpinolide B [2] (epimeric at the hemiketal position) by tandem MS. The electrospray ionization‐mass spectrometry (ESI‐MS)/collision‐induced dissociation (CID; ESI‐MS^2^ and ESI‐IT‐MS^n^) were investigated. The effect of orientations of hemiketal hydroxyl at C‐12 was clearly observed in the mass spectrum. Tandem mass spectra of 1, 1~A~ or 2, 2~A~ show stereospecific fragmentation resulting in significant abundance dissimilarity of [MH − H~2~O]^+^ as well as differences in fragmentation pathway. Both of these pathways seem to be influenced by the stereochemistry of the molecule. The differentiation can be clearly visualized from the [M + H − H~2~O]^+^/[M + H]^+^ ratio of the two isomers where β‐isomer 2 was found to be five times higher than that of α‐isomer 1 in full scan liquid chromatography‐electrospray ionization mass spectrometry(LC‐ESI‐MS). In high‐energy CID, the mass fingerprint of 1, 2, 1~A~, and 2~A~ was found to be different from one anothers. Copyright © 2008 John Wiley & Sons, Ltd.