Synthesis and properties of isoViagra [1].: A 2-methyl-2H-pyrazolo[4,3-d]pyrimidin-7-one isomer of Viagra®

The synthesis and spectral properties (ir, ms, nmr) of a substituted 2-methyl-2H-pyrazolo[4,3-d]-pyrimidin-7-one (3), an isomer of Viagra(R), are described. The key synthon, 4-amino-1-methyl-5-propyl-3-pyrazolecarboxamide (7), is prepared via the reaction of ethyl 2,4-dioxoheptanoate with methylhydrazine; followed by cyclization, nitration, amidation, and nitro group reduction. Interaction of 7 with 2-ethoxybenzoyl chloride yielded the respective bis-amide (8) which was cyclized in polyphosphoric acid to the corresponding pyrazolo[4,3-d]pyrimidin-7-one derivative 9. Chlorosulfonylation of 9, and subsequent treatment with 1-methylpiperazine furnished isoViagra (3).