Chloromalonaldehyde → 6-chloro-1H,3H-2,4,7a,8-tetraaza-cyclopenta[a]indene-2-carboxylic acid tert-butyl ester

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A mixture of 3-amino-2,6-dihydro-4H-pyrrolo[3,4-c]pyrazole-5-carboxylic acid tert-butyl ester (2 g; 8.92 mmol; 1 eq.) and 2-chloromalonaldehyde (1.04 g; 9.81 mmol; 1.1 eq.) in AcOH was stirred at room temperature for 18 hours then diluted with water (30 mL). The precipitate was filtered off and purified by column chromatography (DCM/EA, from 95/5 to 80/20) to afford the title compound (1.24 g, 47%) as a white solid. 1H NMR (DMSO-d6) δ 9.57 (d, J=2.3 Hz, 1H), 8.59 (d, J=2.3 Hz, 1H), 4.63-4.57 (m, 4H), 1.47 (s, 9H). HPLC (max plot) 99.1%; Rt 3.75 min.

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