The preparation of 3-alkylthio-2-methoxytetrahydropyrans

Abstract
The reaction of alkylsulfenyl chlorides with 3,4-dihydro-2H-pyran, followed by treatment of the product with sodium methoxide, gave, highly stereoselectively, trans-3-alkylthio-2-methoxytetrahydropyrans. Purification by distillation under a vacuum at temperatures above 55° resulted in the elimination of methyl alcohol to produce 5-(alkylthio)-3,4-dihydro-2H-pyrans in a good yield. At lower temperatures, the more volatile homologues could be distilled undecomposed, but these had undergone thermal isomerization to a mixture of cis- and trans-3-alkylthio-2-methoxytetrahydropyrans. The conformational preference of these isomers is discussed.