D-homo-steroids. Part 6. 1-Acetyl-2,2-dimethylcyclopentanol: a monocyclic model for D-homoannulation of 17-hydroxypregnan-20-ones

Abstract

1-Acetyl-2,2-dimethylcyclopentanol, a monocyclic model for ring D of a 17-hydroxypregnan-20-one, rearranges in alkali under kinetic control to give 2-hydroxy-2,3,3-trimethylcyclohexanone, or in the presence of boron trifluoride to give the same compound accompanied by 2-hydroxy-2,6,6-trimethylcyclohexanone (4 :1 ratio). Comparison with the results of rearrangements of 17α-hydroxypregnan-20-ones under similar conditions indicates that rearrangement of the steroidal compound by boron trifluoride is controlled by a combination of two factors: (i) the conformational rigidity imposed by the fusion of rings C and D and, (ii) electronic features of the transition state, as yet undetermined, which distinguish the Lewis acid-catalysed from the base-catalysed rearrangement.