Synthesis of 2,3-Dihydro-1,4-dithiins and 2-Alkylidene-1,4-dithianes by 1,2-Sulfur Migration in 2-(1-Hydroxyalkyl)-1,3-dithiolanes

2,3-Dihydro-1,4-dithiins and the isomeric 2-alkylidene-1,4-dithianes were synthesized from 1,2-diketones and alkyl pyruvates by kinetically controlled ring expansion of 2-(1-hydroxyalkyl)-1,3-dithiolanes with p-toluenesulfonyl chloride in pyridine. In addition, 2,3-dihydro-1,4-dithiins, the thermodynamic products, were formed exclusively by using p-toluenesulfonic acid in refluxing benzene. The 2-alkylidene-1,4-dithianes were readily isomerised to the corresponding 2,3-dihydro-1,4-dithiins. Similarly, 2,3-dimethyl-6, 7-dihydro-5H-1,4-dithiepine and (+)-2-methylene-3-methyl-1,4-dithiepane were obtained from 2-[(S)-1-hydroxyethyl]-2-methyl-1,3-dithiane.