Synthesis and vasorelaxant potency of Monagra. A chiral 5-(2-methyl-2,3-dihydro-7-benzofuryl)pyrazolopyrimidone analog of Viagra((R))

Synthesis and properties of a chiral 5-(2-methyl-2,3-dihydro-7-benzofuryl)p yrazolo[4,3-d]pyrimidin-7-one (3), an analog of Viagra (R) (1) and Biagra ( 2), are described. The key material, (+/-)-3-methyl-2,3-dihydrobenzofuran-7 -carboxylic acid (8a) was resolved into the (S)-8b (95% ee) and (R)-8c (99% ee) enantiomers using, respectively, (-)-cinchonidine and (+)-cinchonine. The absolute configuration of 8c was determined as R by X-Ray measurements. Preliminary in vitro experiments on rat isolated thoracic aorta show that the vasorelaxant potency of 3b,c is truly higher than that of I and 2.