Control of stereochemistry in an intramolecular Diels-Alder reaction by the phenylsulfonyl group; an improved synthesis of pisiferol

Thermolysis of 4a (X = H) gives 6a and 6b (X = H) in a ratio of 1:4 whereas 4b (X = SO2Ph) gives more of the trans-ring junction product 6a (X = SO2Ph) suitable for the synthesis of pisiferol [ratio 6a:6b (X = SO2Ph) = 1.5:1]. Base catalysed elimination of the phenylsulfonyl group from 6a (X = SO2Ph) gives 18 which is hydrogenated to 6a (X = H), an intermediate in the synthesis of pisiferol. Both 68 and 6b (X = SO2Ph) have ring B in a half-boat conformation.