General procedure for the synthesis of 4-chlorothiazole-5-carbaldehyde (333): to a stirred solution of 4-chloro-5-(1,3-dioxolan-2-yl)thiazole (332, 1.1 g, 5.75 mmol) in THF (10 mL) was slowly added an aqueous 5N HCl solution (6 mL) at 0°C. The reaction mixture was gradually warmed up to room temperature and stirred continuously for 3 hours. The reaction progress was monitored by TLC. After completion of the reaction, the mixture was poured into brine (20 mL) and extracted with EtOAc (2 x 50 mL). The organic phases were combined, washed with saturated sodium bicarbonate solution (100 mL), dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure to afford the target product 4-chlorothiazole-5-carbaldehyde (333, 800 mg, 95%) as a yellow oil.TLC conditions: 10% EtOAc/hexanes (Rf=0.3); 1H NMR (CDCl3, 500 MHz): δ 10.11 (s, 1H), 9.00 (s, 1H).
