Formation of N,N′-Di(5-alkoxycarbonyl-5-methyl-2-oxo-1-phenyl-2,5-dihydro-1H-pyrrol-3-yl)-N,N′-diphenyloxalylamides in the synthesis of alkyl 4-methyl-2,3,6-trioxo-1,5-diphenyl-1,2,3,4,5,6-hexahydropyrrolo[3,4-b]pyrrole-4-carboxylates