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Sildenafil citrate is well known as Viagra for the treatment of erectile dysfuncion. In the title compound (systematic name: 1-{[3-(6,7-Dihydro-1-methyl-7-oxo-3-propyl-1H-pyrazolo[4,3-d]pyrimidin-5-yl)-4-ethoxyphenyl]sulfonyl}-4-methylpiperazinium citrate monohydrate), C22H31N6O4S+·C6H7O7−·H2O, the pyrazolopyrimidone ring system and the benzene ring are almost coplanar, enabling an intramolecular hydrogen bond between the pyrazolopyrimidone NH group and the O atom of the ethoxy group. One of the N atoms of the piperazine ring is protonated and the citrate molecule exists as an anion. The crystal packing is stabilized by several hydrogen bonds.
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Yathirajan et al. (2005) studied this question.
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