NMR spectral enantioresolution of spirobrassinin and 1-methoxyspirobrassinin enantiomers using ( S)-(−)-ethyl lactate and modeling of spirobrassinin self-association for rationalization of its self-induced diastereomeric anisochromism (SIDA) and enantiomer self-disproportionation on achiral-phase chromatography (ESDAC) phenomena

Abstract

The phytoalexin spirobrassinin is known to exhibit the unusual phenomenon of enantiomer self-disproportionation on achiral-phase chromatography (ESDAC) whereby the enantiomeric composition of a non-racemic mixture varies across the eluting chromatographic peak. However, spirobrassinin is exceptional since both early and late fractions are enantioenriched in comparison to the starting enantiomeric composition. This behavior is rationalized by the formation of two different complexes, substantiated by DFT calculations at the B3LYP/TZVP level of theory, both of which are in dynamic equilibrium with the free molecules. Concomitant with complexation under chromatographic conditions, complexation was also evident by NMR including the observation of distinct NMR signals for each enantiomer for some spins in a non-racemic mixture—the phenomenon of self-induced diastereomeric anisochromism (SIDA). Enantioresolution of various 1H, 13C and 15N NMR signals were also induced for the enantiomers of spirobrassinin and another phytoalexin, 1-methoxyspirobrassinin, by application of the chiral solvating agent ( S)-(−)-ethyl lactate, a cheap reagent with potentially broad applications. For spirobrassinin, the observed NMR signals were evaluated against values calculated using the GIAO method at the B3LYP/cc-pVTZ level of theory.