Ring-ring isomerization in the series of N-(carbamoyl)-1-aryl-2,3,4,5,6,7-hexahydro-3-hydroxy-6,6-dimethyl-2,4-dioxo-1H-indole-3-carboxamides

N-(Carbamoyl)-1-aryl-2,3,4,5,6,7-hexahydro-3-hydroxy-6,6-dimethyl-2,4-dioxo-1H-indole-3-carboxamides upon dissolution in DMSO or DMF undergo a reversible isomerization to 5-(2-arylamino-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)-5-hydroxypyrimidine-2,4,6(1H,3H,5H)-triones.