雪胆甲素在碱中的乙酰基转移反应

在研究雪胆甲素(1)的过程中,试图用氢氧化钾的乙醇溶液进行皂解,以期得到雪胆乙素(2)。但所得为另一物质,与甲素、乙素的突出区别是具有紫外光谱270nm(3.98)与红外光谱1615cm~(-1)(强)等峰,推测可能产生了β-二羰体系的烯醇式结构(-CO-C=C-OH),~1H核磁共振谱82.24(3H,s)表明仍保留乙酰基。雪胆甲素(1)为叔醇酯,皂解反应的速度较低。但12及23位碳在碱性条件下均易形成碳负离子,其中23位的碳负离子对乙酰羰基进行分子内部的进攻(形成五元环中间体),可远超过皂解的反应速度,从而使乙酰基由氧向碳转移。结合光谱数据,可以判定产

Tcetyl migration of 25-0-acetyl-23, 24-dihydrocucurbitacin F by alkali treatment.

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.