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Stability and structure studies on alisol a 24-acetate.

Alisol A 24-acetate is one of the main active triterpenoid compounds isolated from Rhizoma Alismatis, which is a famous Traditional Chinese Medicine, and has been determined for the quality control of this crude drug. In this study, alisol A 24-acetate was found to be unstable in solvents and its stability in different solvents was investigated in detail. The results showed that alisol A 24-acetate and 23-acetate inter-transformed in solvents and the transformation rate was more rapid in protic solvents than in aprotic solvents. Moreover, both alisol A 24-acetate and 23-acetate were deacetylated to yield alisol A when kept in methanol for a long time. This is the first report on the structural transformation between alisol A 24-acetate, alisol A 23-acetate and alisol A. In addition, the single crystal X-ray structure of alisol A 24-acetate and the NMR data of alisol A 23-acetate were also reported for the first time.