Enantiomeric compositions and biosynthesis of Wikstroemia sikokiana lignans

Abstract

Thymelaeaceae plants produce dextrorotatory dibenzylbutyrolactone lignans, which are opposite enantiomers to the lignans isolated from other plants (e.g., Forsythia spp.). In our previous paper, (-)-pinoresinol (74% enantiomer excess), (+)-matairesinol (optically pure), and (+)-wikstromol (optically pure) were isolated from Wikstroemia sikokiana (Thymelaeaceae). In the present investigation, a survey of lignans and the determination of their enantiomeric compositions were continued. Four lignans, (-)-lariciresinol, (-)-secoisolariciresinol, (+)-kusunokinin, and (+)-methyltrachelogenin, were isolated from MeOH extracts of W. sikokiana stem. To our knowledge, we have isolated (+)-methyltrachelogenin from plants for the first time. Chiral high-performance liquid chromatographic analysis showed that (+)-kusunokinin and (+)-methyltrachelogenin were optically pure, whereas (-)-lariciresinol and (-)-secoisolariciresinol were not (39% and 45% enantiomer excess, respectively). Feeding experiments with deuterium-labeled substrates demonstrated conversion of coniferyl alcohol to the lignans and interconversion of lignans. These reaction sequences are similar to the sequence catalyzed by Forsythia enzymes. However, predominant enantiomers of the lignans, except for secoisolariciresinol isolated from W. sikokiana, have absolute configurations opposite to those of the corresponding lignans isolated from Forsythia spp. Based on the results of the isolation and the feeding experiments, several differences between W. sikokiana and Forsythia spp. are pointed out regarding stereochemical mechanisms for lignan biosynthesis.