Trifluoroacetic acid → (E)-Ethyl 3-(2-oxo-2,4-dihydro-1H-spiro[[1,8]naphthyridine-3,4'-piperidine]-6-yl)acrylate

Trifluoroacetic acid (5 mL) was added to a suspension of (E)-tert-butyl 6-(3-ethoxy-3-oxoprop-1-enyl)-2-oxo-2,4-dihydro-1H-spiro[[1,8]naphthyridine-3,4′-piperidine]-1′-carboxylate (1.52 g, 3.66 mmol) in dichloromethane (20 mL). The reaction mixture was stirred at room temperature for 1 hour and concentrated to dryness. The resulting residue was partitioned between an aqueous solution of sodium hydroxide (1N, 60 mL) and dichloromethane (40 mL). The aqueous layer was separated and extracted with dichloromethane (2×70 mL). The combined organic phases were dried over sodium sulfate, filtered and concentrated to dryness. The title product was obtained as a pale yellow solid (904 mg, 79%).