To a solution of tert-butyl 4-(5-formyl-6-hydroxypyridin-2-yl)-1,4-diazepane-1-carboxylate (600 mg, 1.87 mmol) in EtOH (4 mL) were treated with piperidine (239 mg, 2.8 mmol), acetic acid (511 mg, 9.35 mmol) and ethyl acetoacetate (487 mg, 3.74 mmol) at room temperature. The reaction mixture was stirred at 90° C. for 3 hours. The mixture was concentrated, quenched with a saturated NaHCO3 solution and extracted with dichloromethane three times. The combined organic layers were combined, dried over K2CO3, filtered and concentrated. The residue was purified on silica gel with methanol (0-10%) in dichloromethane to provide tert-butyl 4-(3-acetyl-2-oxo-2H-pyrano[2,3-b]pyridin-7-yl)-1,4-diazepane-1-carboxylate (450 mg, 62%). MS m/z 388 [M+H]+.
