Dimethyl 5,5′-(Thiodimethylene)di-2-furoate. A solution of 100 g (0.42 mole) of crystalline sodium sulfide (Na2S·9H2O) in 150 ml of water was added dropwise over a period of one hour to a stirred solution of 130.8 g (0.75 mole) of methyl 5-(chloromethyl)-2-furoate (see Vol. 1 of this work, p. 29), b.p. 114–116°/3 mm, in 300 ml of methanol; in the course of the addition the temperature rose to 50–55°. After some time an oily layer formed on the bottom of the flask and gradually solidified. After one hour the precipitate was filtered off at the pump, washed with water, and dried; weight of unpurified product 96.0–100.5 g. Recrystallization from 600 ml of ethanol gave 82.5–84.5 g (70.9–72.7%) of pure substance, m.p. 113°.