(Step 8) tert-butyl 5-hydroxy-5,6-dihydropyridine-1(2H)-carboxylate (350 mg, 1.76 mmol) was dissolved in dichloromethane (12 mL) and Dess-Martin oxidizer (1.5 g, 3.52 mmol) was added. The reaction mixture was stirred at room temperature for 2 hours and then filtered. The filtrate was washed with saturated aqueous sodium carbonate (50 mL) and subsequently concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography (petroleum ether/ethyl acetate, 2:1, v/v) to afford the target compound tert-butyl 5-oxo-5,6-dihydropyridine-1(2H)-carboxylate as a colorless oil (340 mg, 98% yield).LC-MS (ESI, cationic mode) m/z: 142 [(M+H)+ - C4H8], 220 [M+ Na]+; 1H NMR (400 MHz, CDCl3) δ (ppm): 1.48 (s, 9H), 4.11 (s, 2H), 4.24 (s, 2H), 6.16-6.20 (m, 1H), 7.04 (s, 1H).
