Reactions of Ring C in Glycyrrhetic Acid Derivatives

The chemistry of the relatively unreactive glycyrrhetic acid (1) ring C was investigated. The enone group in this ring was reduced by diborane to allylic alcohols (6, 7), which can be readily converted into the corresponding homoannular dienes (2, 5). These dienes were also prepared by NaBH₄ reduction of the enone group. The epoxidation of the diene 2 leads to the mono‐(12) and di‐(14) epoxides. A new ring C enone system (13) was also obtained. Treatment of the two different tosylhydrazones derivatives (16, 20) with methyllithium gave the conjugated methylene compounds (17, 21). The NMR spectrum of the latter compound was also studied with a shift reagent — Eu(FOD). The mass spectra of the homoannular dienes (2, 5), allylic alcohols (6, 7) and 11‐methylene compounds (17, 21) were studied and some new types of fragmentation were suggested.