p-Toluenesulfonic acid → 2,3-Dimethyl-2,3-dihydrothieno[3,4-b][1,4]dioxine

A solution of 3,4-dimethoxythiophene (7.2 g, 50 mmol), meso-2,3-butanediol (5.4 g, 60 mmol), and p-toluenesulphonic acid (0.2 g) in toluene (100 ml) was heated at 100° C. under a continuous argon flow for 24 h. The reaction mixture was then poured into methylene chloride (200 mL) and the organic phase washed with a 1M aqueous solution of sodium hydrogen carbonate and brine, dried with anhydrous magnesium sulphate and concentrated. This resulted in a mixture of cis and trans product that could be separated by column chromatography, yielding pure cis (2.8 g, 16%) and trans (1.83 g, 11%) 2,3-dimethyl-2,3-dihydro-thieno[3,4-b][1,4]dioxine.