The synthesis of 5-aryl- and 4-aryl/hetaryl/cyclopropyl/alkynyl-5-aryl-1H-pyrrole-2,3-diones by formal isomerization of isoxazole-5-carbaldehydes mediated Mo(CO)6 in wet MeCN has been developed. The resulting 1H-pyrrole-2,3-diones are good precursors for substituted 1H-pyrrolo[2,3-b]quinoxalines.