D-Homo-steroids. Part V. A study of the mechanism of D-homoannulation of 17α-hydroxypregnan-20-ones with boron trifluoride

Abstract

The rearrangement of 17α-hydroxypregnan-20-ones by boron trifluoride to give D-homo-ketols is retarded significantly by 16α-benzoyloxy-substituents, although variations in p-substitution in benzoyloxy-groups at C-16, and similarly in benzoates at C-11, had only slight effects on reaction rates. 16β-Methyl and 16-methylene substituents caused a marked acceleration and a slight retardation, respectively. Unsaturation at C(14)–C(15) caused a modest increase in reaction rate.

An n.m.r. study showed that a complex of boron trifluoride with the 17α-hydroxy-group is probably the main species present in the reacting solution, the postulated cyclic complex (XIII) having only transient existence as a highly reactive intermediate before undergoing rearrangement. These findings suggest that the rate of formation of a cyclic complex of type (XIII), as well as the rate of its rearrangement, is involved in the overall kinetics of D-homoannulation.