Synthesis of some 5-phenylisoxazole-3-carboxylic acid derivatives as potent xanthine oxidase inhibitors

A number of 5-phenylisoxazole-3-carboxylic acid derivatives (5a-e, 11a-e) were synthesized and analyzed for their ability to inhibit xanthine oxidase. Most of the compounds exhibited potency levels in the micromolar/submicromolar range. The presence of a cyano group at the 3-position of phenyl moiety turned out to be the preferred substitution pattern, as its transformation into the nitro group determined a general reduction of the inhibitory potency. A molecular modeling study on compound 11a was performed to gain an insight into its binding mode with xanthine oxidase, and to provide the basis for further structure-guided design of new non-purine xanthine oxidase inhibitors related with 5-phenylisoxazole-3-carboxylic acid scaffold.