Reaction of phorbol-13,20-diacetate with mesylchloride/pyridin induces a homoallyl rearrangement of the α- [acetoxycycloproyl] -carbinol group yielding crotophorbolone-enol-13,20-diacetate (7) and acetoxy-crotophorbolone-20-acetate (10). By alkali catalysed transesterification 7 may be converted to crotophorbolone (8). 7 is the 4,9-dihydroxy-13,20-diacetoxy-13,15-seco-tigliatetraene-(1,6,12,15) -one- (3) whereas 10 is the 4,9-dihydroxy-15,20-diacetoxy-13,15-tigliadiene- (1,6) -dione-(3,13), generated by an intramolecular migration of the acetyl group in 13-position.