In the presence of hexacarbonylmolybdenum, substituted isoxazoles undergo a cycloaddition reaction with dimethyl acetylenedicarboxylate across the C-4–C-5 bond to give 3, 4-bis (methoxycarbonyl) pyridine derivatives. In a similar cycloadition of isoxazoles with methyl propiolate, 4-(methoxycarbonyl) pyridine derivatives were also obtained. The β- carbon atom of methyl propiolate could intervene in the bonding with the C-5 position of ...