Synthesis and properties of α-thiagra. A substituted 5-(2-thienyl)pyrazolo[4,3-d]pyrimidin-7-one bioisostere of viagra®

For structure-activity manipulations, the compound α-thiagra (3), a thiophene bioisostere of viagra®, has been prepared and its X-Ray structure determined. The crystallographic data show that the thiophene and pyrazolo[4,3-d]pyrimidone hetero-ring systems are nearly coplanar by virtue of an adequate intramolecular hydrogen-bond between the pyrimidone NH and the oxygen lone pair of the 3'-ethoxy group.