23, 24-Dihydrocucurbitacin F (2) and its 25-ester (1) are antibacterial bitter principles isolated from Hemsleya amabilis Diels. Upon brief treatment with ethanolic potassium hydroxide under reflux, compound 1 undergoes facile acetyl migration from oxygen to carbon with the formation of 23-acetyl-23, 24-dihydrocucurbitacin F (3), to the exclusion of any saponification. The relatively hindered ester group of 1 is evidently less prone to saponification, thus bringing an internal attack by the 23-enolate anion to the fore. The structure of 3 was deduced from spectral evidence and mechanistic considerations, and confirmed by the cleavage of the acetyl group with refluxing Claisen's alkali. The resultant compound 2 was thus obtained form 1 only via a circuitous route.

Key words: TRITERPENE, ACETYL GROUP, HEMSLEYA AMABILIS, TRANSFER REACTIONS, DIGYAROCUCURBITACIN F