Effective Synthesis of 3,4-Diaryl-isoxazole-5-carboxamides and their Antiproliferative Properties

A simple scalable procedure for the synthesis of 3,4-diaryl-isoxazole-5-carboxamides 6 under mild conditions from readily available material was developed. The targeted compounds 6, structural analogues of heat shock protein inhibitors, were obtained by the rearrangement of intermediate 3,4-diaryl-5-carboxamido-isoxazoline N-oxides 5. In contrast to carboxamido-isoxazoline oxides 5, base-catalyzed recyclization of 3,4-diaryl-5-(ethoxycarbonyl)isoxazoline N-oxides 9c unexpectedly yielded 5-hydroxy-1,2-oxazin-6-ones 17c instead of ethyl 3,4-diaryl-isoxazole-5-carboxylates 10. Crystal and molecular structure of 4-(2,5-dimethoxy-3,4-methylenedioxyphenyl)-5-hydroxy-3-phenyl-6H-1,2-oxazin-6-one 17c was established by single-crystal X-ray diffraction study. In a phenotypic sea urchin embryo assay, carboxamide 6f showed moderate antimitotic antitubulin activity compared to 5-unsubstituted 3,4-diarylisoxazoles 15, which featured strong microtubule destabilizing effect.