Novel ent-kaurane dimers from Isodon rubescens var. rubescens

Three novel asymmetric ent-kaurane dimers xindongnins M–O (1–3) were isolated from Isodon rubescens var. rubescens. Their structures were elucidated by spectroscopic methods including 2D NMR analysis. The relative stereochemistry of 1 was determined by single crystal X-ray diffraction, which also confirmed the unique linkage of a single carbon–carbon bond between the two subunits of this dimer. A biogenetic pathway was proposed for the formation of these dimers.