A Novel Secobetulinic Acid 3,4‐Lactone from Viburnum aboricolum

Bio-assay-guided fractionation of the CHCl3-soluble extract from the leaves of Viburnum aboricolum led to the isolation of a novel secobetulinic acid 3,4-lactone, viburolide (=(6β)-4,6-dihydroxy-3,4-secolup-20(29)-ene-3,28-dioic acid 3,4-lactone; 1). This is the first lupane-type compound possessing such a lactone skeleton from natural products. Its structure was elucidated by spectral analysis and comparison with 6-dehydroxy-20,29-dihydroviburolide (6) prepared from benzyl betulinate (2). Compound 6 was found to inhibit androgen-independent human prostate cancer cells (PC-3) with an IC50 of 12.3 μM.