Structurally diverse stilbene dimers from Gnetum montanum Markgr.: studies on the 1H chemical shift differences between dimeric stilbene epimers correlating to the relative configurations

Abstract

Twenty-seven structurally diverse stilbenoids, comprising nine new dimeric stilbenoids, gnemontanins A (1), B (2), C (4), D (5), E (7), F (8), and G (12), as well as (−)-gnetuhainin P (3) and (−)-gnetuhainin I (6), were isolated from the caulis of Gnetum montanum Markgr. The structures of those new compounds were elucidated by mean of extensive analysis of MS, 1D and 2D NMR spectroscopic data. Naturally occurring stilbene dimers polymerized through one bond of 8-O-4′ (1 and 2) as well as two bonds of 7-8′ and 6-7′ (5) are reported for the first time. The ¹H chemical shift differences between dimeric stilbene epimers on C-7 were analysed and summarized, which could be used as a diagnostic to determine the relative configuration of C-7 of several types' dimeric stilbenes. Two misassigned structures of gnetuhainins D and E were revised as gnetuhainin S and 12, respectively.