1,2-HYDROGEN SHIFTS IN THERMAL AND PHOTIC BAMFORD-STEVENS REACTIONS OF CYCLOHEXANONES - ACTIVATION BY AN ENDOCYCLIC OXYGEN

Alkoxyl substituents are known to activate alpha H's for rearrangement to carbenic centers. In acyclic systems, the degree of transition sta te stabilization depends on whether the alkoxyl group positions 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 su bstituent is initially axial or equatorial on the ring. To assess the relative activating power of a tetrahydropyranyl ring oxygen we synthe sized and studied a 17a-oxa-D-homo steroidal C-16 ketone (11a). The ca rbonyl group was converted to a putative singlet carbene center by the rmal and photic Bamford-Stevens (B-S) reactions, and the relative amou nts 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 degrees C and ca. 14:1 in photolysis at -7 0 degrees C. (C) 1998 Elsevier Science Ltd. All rights reserved.