Diethyl 4,4'-dithiobis(3-nitrobenzoate) (11.2 g, 24.8 mmol) was mixed with zinc particles (15.0 g, 234 mmol, 9.5 eq.) in formic acid (600 mL) and heated to reflux for 48 hours. Upon completion of the reaction, the mixture was cooled to room temperature and concentrated to dryness under reduced pressure. The residue was extracted by partitioning between ethyl acetate (500 mL) and saturated aqueous sodium bicarbonate (500 mL). The organic layer was separated, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure. The concentrated residue was purified by chromatography on a neutral alumina column with the eluent being a gradient ratio of hexane/dichloromethane (1/1 to 0/1) to give ethyl benzothiazole 5-carboxylate (5.30 g, 51% yield). The structure of the product was confirmed by 1H NMR (500 MHz, CDCl3): δ 9.08 (s, 1H), 8.83 (d, 1H), 8.14 (dd, 1H), 8.02 (d, 1H), 4.45 (q, 2H), 1.44 (t, 3H); the mass spectrum (EI) showed m/z 208 (M++1).
