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.