External Parameters Affecting the Rate of Tautomerism

This observation stimulated further investigations directed toward identifying the factors, enabling observation of annular tautomerism in dihydropyrimidine systems. The easily available and reasonably stable MDHP (20a) proved to be a convenient model for carrying out these measurements. 

In the course of NMR studies on 20 in DMSO-d6 solution, tautomerism was affected by the following factors  

Dryness of solvent. This was confirmed by the addition of small amounts of water to a sample in which slow exchange was observed. This causes an increase in the rate of proton exchange as indicated by the broadening of the 

H-4 and H-5 signals of both tautomers and a decrease in the separation between them. 

Microconcentrations of H+ ions in solution. The rate of exchange is greatly accelerated by the addition of catalytic amounts of H+ in each case, an average spectrum was obtained. 

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