General procedure for the synthesis of ethyl 2-chlorobenzo[d]thiazole-6-carboxylate from ethyl 2-aminobenzo[d]thiazole-6-carboxylate: In a 500 mL single-necked flask, copper chloride (6.0 g, 44.6 mmol), tert-butyl nitrite (7.1 mL, 59.4 mmol), and acetonitrile (200 mL) were added and the mixture was cooled to 0 °C. Subsequently, ethyl 2-aminobenzo[d]thiazole-6-carboxylate (6.6 g, 29.7 mmol) was added in batches over 30 min. The reaction mixture was gradually warmed to room temperature and stirred overnight. After completion of the reaction, the filtrate was filtered through silica gel and the filtrate was added dropwise to 1 M HCl (60 mL) and stirred for 10 min to form a partition. The aqueous layer was extracted with ethyl acetate (100 mL), the organic layers were combined, washed with saturated brine and dried over anhydrous Na2SO4. After filtration again, it was rotary evaporated to dryness and purified by column chromatography (petroleum ether:ethyl acetate = 25:1) to afford 6.2 g of a pale yellow solid, ethyl 2-chlorobenzo[d]thiazole-6-carboxylate, in 86% yield. The product was characterized by 1H NMR (400 MHz, CDCl3): δ 8.54 (d, J = 0.8 Hz, 1H), 8.20 (dd, J = 1.6 Hz, 8.4 Hz, 1H), 8.01 (d, J = 8.0 Hz, 1H), 4.45 (q, J = 7.2 Hz, 2H), 1.45 (t, J = 7.2 Hz, 3H).
