1,2-Hydrogen shifts in thermal and photic Bamford-Stevens reactions of cyclohexanones. Activation by an endocyclic oxygen

Article abstract of DOI:10.1016/S0040-4020(98)00757-1

Alkoxyl substituents are known to active alpha H's for rearrangement to carbenic centers. In acyclic systems, the degree of transition state stabilization depends on whether the alkoxyl group position itself syn or anti to the moiety connected to the other side of the carbenic carbon; and in cyclohexylidenes, it depends on whether the alkoxyl substituent is initially axial or equatorial on the ring. To assess the relative activating power of a tetrahydropyranyl ring oxygen we synthesized and studied a 17a-oxa-D-homo steroidal C-16 ketone (11a). The carbonyl group was converted to a putative singlet carbene center by thermal and photic Bamford-Stevens (B-S) reactions, and the relative amounts of H migration from C-17 vs. C-15 were determined. The ring oxygen proved to be a more powerful activator than ring alkyl by a factor of 21:1 in thermolysis at 170 °C and ca. 14:1 in photolysis at -70 °C.