Formation of alkyl 1-methyl-3,9-dioxo-2-phenyl-2,3,4,9-tetrahydro-1H-pyrrolo[3,4-b]quinoline-1-carboxylates by thermolysis of alkyl 1,5-diaryl-4-methyl-2,3,6-trioxo-1,2,3,4,5,6-hexahydropyrrolo[3,4-b]pyrrole-4-carboxylates