Acetic Acid → 5-benzyl-6-propyl-1,5-dihydro-4H-pyrazolo[3,4-d]pyrimidin-4-one

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A mixture of ethyl 5-amino-1H-pyrazole-4-carboxylate (1-1, 3.1 g, 20 mmol, 1 equiv), trimethyl orthobutyrate (5.9 g, 40 mmol, 2 equiv) and acetic acid (120 mg, 2 mmol, 0.1 equiv) was heated at 90° C. for 3 h. Excess trimethyl orthobutyrate was removed by distillation. The residue added to a solution of benzylamine (4.2 g, 40 mmol, 2 equiv) in ethanol (25 mL), and the resulting solution was heated at reflux for 18 h, then cooled and concentrated. The residue was triturated with EtOAc to provide 5-benzyl-6-propyl-1,5-dihydro-4H-pyrazolo[3,4-d]pyrimidin-4-one (1-2) as a colorless solid. 1H NMR (500 MHz, CDCl3) δ 11.5 (bs, 1H), 8.19 (s, 1H), 7.34 (m, 2H), 7.28 (m, 1H), 7.15 (d, J=7 Hz, 2H), 5.41 (s, 2H), 2.75 (t, J=7 Hz, 2H), 1.82 (m, 2H), 0.99 (t, J=7 Hz, 3H).

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