A synthesis of (–)-(R)-trans-β-(1,2,3-trimethylcyclopent-2-enyl)acrylic acid

Abstract

The synthesis of the title compound (21) from exo-2-hydroxybornan-3-one (1) is described. One stage, the Beckmann rearrangement of the oxime (8) of 5,8,8-trimethylbicyclo[3.2.1]oct-3-en-2-one into the lactam, 6,9,9-trimethyl-2-azabicyclo[4.2.1]non-4-en-3-one (12), was carried out in good yield, in spite of the fact that the ratio of the geometrical isomers present in the oxime mixture was unsuitable for rearrangement in this direction. The technique used involved the equilibration, in a mixture of hydrochloric and acetic acids at 95°, of the mixture of oxime toluene-p-sulphonates (9) and (10). Under these conditions, the minor oxime sulphonate gave the lactam (12) directly, whereas the major and more stable oxime sulphonate did not rearrange directly to a lactam. Instead, the major oxime sulphonate rearranged to the minor oxime sulphonate, which then rearranged to give more of the desired lactam (12).