Synthesis of 1,5-dihydro-3-methyl-6-trihalomethyl-4H-pyrazolo[3,4-d]pyrimidin-4-ones

N-(Methoxycarbonyl)trihaloacetimidoylchlorides react with 5-aminopyrazoles under mild conditions to give N-methoxycarbonyl- N’-(5-pyrazolyl)trihaloacetamidines, which furnish 6-trihalomethyl-4H-pyrazolo[3,4-d]pyrimidin-4-ones by thermal intramolecular cyclocondensation.