Chiral 2,2-disubstituted cyclohexanones; annulation via Claisen rearrangement products

Abstract

The methyl enol ether of 2-methylcyclohexanone reacts regiospecifically with the optically active forms of but-3-yn-2-ol and but-3-en-2-ol. (R)-But-3-yn-2-ol yields an approximately 4 : 1 mixture of (R)-2-methyl-2-(R-buta-1,2-dienyl)cyclohexanone and the corresponding SR compound. By contrast the but-3-en-2-ol reaction is ca. 96% enantioselective; (R)-but-3-en-2-ol gives (R)-2-(trans-but-2-enyl)-2-methylcyclohexanone and (S)-but-3-en-2-ol gives the corresponding (S)-cyclohexanone. These Claisen rearrangement products have been transformed into Robinson-type annulated ketones. Cleavage of some highly hindered esters has been carried out efficiently by sodium methylsulphinyl-methanide in dimethyl sulphoxide.