A 1-l., three-necked, round-bottomed flask with ground-glass fittings is charged with a mixture of 36 ml. of boron trifluoride diethyl etherate, 72 ml. of glacial acetic acid, and 120 ml. of chloroform (Note 1). The flask is equipped with a spiral reflux condenser, an efficient mechanical stirrer, and a dropping funnel (Note 2). The chloroform solution is heated and maintained at reflux with vigorous stirring, and a solution of 32 g. (30 ml., 0.30 mole) of 1,3-propanedithiol and 25 g. (29 ml., 0.33 mole) of methylal (dimethoxymethane) in 450 ml. of chloroform (Note 1) is added at a constant rate over 8 hours. The mixture is allowed to cool to room temperature, washed successively with four 80-ml. portions of water, twice with 120 ml. of 10% aqueous potassium hydroxide, and twice with 80-ml. portions of water. The chloroform solution obtained is dried over potassium carbonate and concentrated in a 500-ml., round-bottomed flask under reduced pressure with a rotating flask evaporator (Note 3). The residue, which crystallizes on cooling to room temperature, is dissolved in 60 ml. of methanol by heating to the boiling point. The hot solution is filtered rapidly through a prewarmed funnel, allowed to cool slowly to room temperature, then kept overnight at −20°. The colorless crystals are collected by filtration through a prechilled Büchner funnel, washed with cold methanol (−20°), and dried under reduced pressure (Note 3) and (Note 4), yielding 28–29 g. of product, m.p. 53–54°. Solvent is removed from the mother liquor, and the residue is recrystallized as described, giving an additional 1.5–2.0 g.; the total yield of recrystallized 1,3-dithiane is 29.5–31.0 g. (82–86% based on propane-1,3-dithiol). A purer sample (Note 5) can be prepared by subsequent sublimation of the recrystallized product at 0.1–0.5 mm. (45–48° bath temp.) (Note 6). The yield of pure product is 28–30 g. (78–84%), m.p. 53–54°. The residue from the sublimation is a brown syrup weighing less than 1 g.