Hydrochloric Acid → 4-Aminocyclopent-2-ene-1-carboxylic acid

To 4.58 g (42 mmol) of (±)-2-azabicyclo[2.2.1]hept-5-en-3-one, 190 ml of 1N hydrochloric acid was added, and the mixture was refluxed for 2 hours with heating. The solvent was evaporated, and the resulting white crystals were washed with 30 ml of acetone, followed by drying to give 6.59 g of crude crystals of the subject (+)ACP acid. This crude crystals were dissolved in ethanol (20 ml) with heating, and thereafter ethyl acetate (85 ml) was added to the resulting solution, followed by cooling to room temperature. The crystals thus deposited were filtrated to give 4.26 g (25.7 mmol; yield: 61.2%) of the subject (+)ACP acid (melting point: 169-172° C.).