Diastereoselective oxygen to carbon rearrangements of anomerically linked enol ethers and the total synthesis of (+)-(S,S)-(cis-6-methyltetrahydropyran-2-yl)acetic acid, a component of civet

A range of enol ethers, linked via their oxygen atom to the anomeric centre of a pyran ring system, was shown to undergo oxygen to carbon rearrangement upon treatment with a Lewis acid to give the corresponding 2-carbon substituted products. At low temperature, trimethylsilyl trifluoromethanesulfonate catalysed rearrangements of anomerically linked 6-substituted tetrahydropyranyl enol ethers gave selectively the trans-pyranyl ketones, whereas at higher temperatures selective formation of the cis-pyranyl ketones was observed. In a simple application of the methodology the cis-selective rearrangement was used as the key step in a concise total synthesis of a constituent of civet.