Tautomerism of Pyridine and Pyrimidine Dihydro Derivatives

It should be noted that the formation of equilibrium mixtures of 423 and 424 is not observed [414]. 

Interconversions of dihydroforms A and C are connected with proton migration from one heteroatom to another and are characterized by a high-speed 

It has been shown [248] that unsubstituted dihydropyrimidine 427 existed in tautomeric form D. But introduction of phenyl substituents at positions 2 and 4 leads to a convergence of energies of the tautomeric forms and 4,6-diphenyl-l,2(2,5)-dihydropyrimidine is observed in solutions of CDC13 as a mixture of D and E in a ratio of 2 1 [248]. Pyrimidines 427 containing electron-donors at positions 4 and 6 exist in the dihydro form E [424]. 

Data concerning the tautomeric equilibrium A B in the case of dihydropyr- 

The key factor affecting tautomeric equilibrium of heterocycles 428 is the nature of the fused azole ring. Decreasing the electron-withdrawing properties of the azole moiety shifts the balance to the B form. For example, most dihydro derivatives of pyrazolo[l,5-a]pyrimidines [X is N, Y, Z are C(R] and imi-d a z o [ 1,2 - a ] py ri m i d i n e s [X, Z are C(R), Y is N] both in the solid state and in solution exist in the imine tautomeric form B, while for dihydropyrimidines fused with the 1,2,4-triazole ring [X, Z are N Y is C(R)] the tautomers A are dominant [156]. 

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