Synthesis of 9-aryl-6-carbamoyl-1,2-dihydropurines and a study of their tautomerism

Abstract

The title compounds have been prepared in high yield by reaction of the corresponding 4-(cyanoformimidoyl)-1 -arylimidazol-5-amines with the carbonyl compounds R¹R²CO (R¹= R²= Me, Et; R¹= H, R²= Ph). The same 9-aryl-6-carbamoyl-1,2-dihydropurines (R¹= R²= Me) have also been isolated from the corresponding (Z)-N¹-(2-amino-1,2-dicyanovinyl)-N²-arylformamidines by reaction with acetone in the presence of a base. In these cases the initially formed products are off-white solids, believed to be the oxazolidine derivatives 4, which in solution rapidly undergo conversion into the respective 1,2-dihydropurines. The two tautomers of the dihydropurines have been fully characterised by ¹H and ¹³C NMR spectroscopy and single-crystal X-ray structures have been obtained for both the orange and yellow tautomers of the dihydropurine derivative where Ar = Ph, R¹= R²= Me. In [²H₆]Me₂SO solution the presence of an ortho-substituent on the N-aryl ring causes an increase in the equilibrium concentration of the yellow tautomer. A single-crystal X-ray structure determination on the dihydropurine where Ar = 2,4-(MeO)₂C₆H₃; R¹= R²= Me has shown that in the solid state the aryl ring is twisted at 73.1 (9)° to the plane of the heterocyclic ring and this may explain the observed behaviour in solution.