Gymnothelignans A–O: Conformation and Absolute Configuration Analyses of Lignans Bearing Tetrahydrofuran from Gymnotheca chinensis

Fifteen new lignans, gymnothelignans A–O (1–15), bearing tetrahydrofuran with variable conformations belonging to three potentially related skeletons were isolated from Gymnotheca chinensis Decne. The structures were elucidated by means of detailed spectroscopic analysis. Absolute configurations were assigned using X-ray single-crystal diffraction and chemical transformations. Moreover, by the homology, compounds 1–11 and eupomatilones were confirmed to have uniform R-configuration at C-5. However, a synthesized congener has long been mistaken as 5-epimer of eupomatilone-6. This work provides guidance for the absolute configuration establishment of the subeupomatilone family with trans-H-4–H-5 configuration.