2-Mercaptoethanol → Ethyl (5,6-dihydro-1,4-oxathiin-3-yl)acetate

A solution of 2-mercaptoethanol (38.2 g.) in anhydrous ethanol (146 cc.) is added, at about 10° C. and over the course of one hour, to a solution of sodium ethoxide in ethanol [prepared by reacting sodium (11.2 g.) with anhydrous ethanol (350 cc.)]. After heating the reaction mixture to a temperature of 45°-f 50° C., ethyl 3-bromo-3-formylpropionate (102.8 g.) is added over the course of 20 minutes. The reaction mixture is stirred for 15 hours at a temperature of about 25° C. After filtration, the solvent is evaporated at about 50° C. under reduced pressure (20 mm.Hg). The residue is dissolved in anhydrous toluene (500 cc.); the solution is filtered and toluene-p-sulphonic acid monohydrate (1.5 g.) is then added. The reaction mixture is heated under reflux until complete dehydration has taken place (the water formed being removed in a Dean-Stark apparatus) and then the solvent is evaporated at about 50° C. under reduced pressure (20 mm.Hg). The residue is taken up in diethyl ether (700 cc.); some insoluble matter appears and is removed by filtration through "Supercel". The ether solution is washed three times with water (total 600 cc.), then dried over sodium sulphate, treated with decolourizing charcoal and filtered. The solution is concentrated to dryness under reduced pressure (20 mm.Hg); a residue (65 g.) is obtained and is chromatographed on alumina (600 g.). Elution is effected using a mixture (2.6 liters) of ethyl acetate and cyclohexane (1-19 by volume). After concentration to dryness under reduced pressure (20 mm.Hg), ethyl (5,6-dihydro-1,4-oxathiin-3-yl )acetate (16.8 g.) is obtained in the form of an oil.