Neighboring group participation and rearrangement in hypobromous acid addition to 10β-vinyl cholestane derivatives

Abstract

Hypobromous acid addition to the diene VII yields two epimeric dibromo 6β,19a-epoxides, XVI (main product) and XVII, via the intermediary bromohydrin XIII. Products due to C-C bond formation were not observed. This fact is attributed to the mutual perpendicular arrangement of π-orbitals of the two double bonds in the diene VII. Hypobromous acid action upon the 5α,6β-diol IX and the dimethoxy derivative X afforded analogous products of 6(O)n participation by the 6β-substituent, i.e. the cyclic ethers XXII, XXIII and XXIV, XXV. Under the same conditions the 6β-acetoxy derivative XI gave a mixture of the cyclic ethers XXII and XXIII as products of 6(O)n participation, and the rearranged ketone XXVI. Mechanism of this rearrangement is discussed.