Saiyutones A-D: four new unusual biflavones from Desmos chinensis

Four new unusual biflavones, saiyutones A-D, were isolated from the leaves of Desmos chinensis together with 16 known compounds. Their structures were determined on the basis of UV, IR, NMR, and MS analysis, and by comparison of their spectroscopic data with those reported. Saiyutones A and B had a unique biflavone skeleton with a 3-6″ linkage through a methylene group. It was proposed that the formation of a cyclic hemiketal was the key step in the biosynthetic pathway of saiyutones C and D.