Approaches to the synthesis of triterpenoids. I. Methyl group migration to the 9β-position in steroids as a route to the cucurbitacins. A possible structural feature directing epoxide cleavage rearrangements

Abstract

The BF3 catalyzed cleavage of some steroidal 9α, 11α-epoxides has been studied in an attempt to induce migration of the C-10 methyl group to C-9. In the case of 9α,11α-epoxyandrost-4-ene-3,17-dione, cleavage with BF3 gas in benzene led to two major products. One of these was the required 3,11α-dihydroxy-9β-methylestra-1,3,5(10)-triene-17-one, thus leading to a novel class of steroids and possibly opening a route to the synthesis of compounds of the cucurbitacin class. The other main product was shown to be 11α-hydroxy-9β-androst-4,8(14)-diene-3,17-dione. A possible structural feature directing the rearrangement pathway to these materials is proposed from the study of other epoxide rearrangements and some reported in the literature.