Production of 3,4-cis- and 3,4-trans-Leucocyanidin and Their Distinct MS/MS Fragmentation Patterns

(+)-2,3-trans-3,4-cis-Leucocyanidin was produced by acidic epimerization of (+)-2,3-trans-3,4-trans-leucocyanidin synthesized by reduction of (+)-dihydroquercetin with NaBH₄, and structures of the two stereoisomers purified by C18- and phenyl-reverse-phase high-performance liquid chromatography (HPLC) were confirmed by NMR spectroscopy. We confirm that only 3,4-cis-leucocyanidin is used by leucoanthocyanidin reductase as substrate. The two stereoisomers are quite stable in aqueous solution at -20 °C. Characterization of the two stereoisomers was also performed using electrospray ionization tandem mass spectrometry (ESI-MS/MS), and we discuss here for the first time the corresponding MS/MS fragmentation pathways, which are clearly distinct. The main difference is that of the mode of dehydration of the 3,4-diol in positive ionization mode, which involves a loss of hydroxyl group at either C₃ or C₄ for the 3,4-cis isomer but only at C₃ for the 3,4-trans isomer. Tandem mass spectrometry therefore proves useful as a complementary methodology to NMR to identify each of the two stereoisomers.