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 .