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RESEARCH PRODUCT

The azide/tetrazole equilibrium: an investigation in the series of furo- and thieno[2,3-e]tetrazolo[3,2-d]pyrimidine derivatives

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Abstract Starting from 7(8)-chlorofuro(thieno)[3,2- d ]pyrimidines 1 , the corresponding 7(8)-hydrazinofuro(thieno)[3,2- d ]pyrimidines 2 were obtained by reaction with hydrazine. By treatment with nitrous acid, compounds 2 gave fused furo- and thieno[2,3- e ]tetrazolo[1,5- c ]pyrimidines 3 T and 4 T in excellent yields, which in solution exist in equilibrium with 7(8)-azidofuro(thieno)[3,2- d ]pyrimidines 3 A and 4 A . Compounds 3 T / 3 A and 4 T / 4 A represent useful substrates for investigating the azide/tetrazole equilibrium: it has been examined as a function of solvent, temperature, and their structure. We have observed that, in solution, in both 3 and 4 the equilibrium was strongly affected by the solvent used. In contrast, X-ray analysis has shown that both thieno[2,3- e ]tetrazolo[1,5- c ]pyrimidines and furo[2,3- e ]tetrazolo[1,5- c ]pyrimidines in solid state exist only in the tetrazole tautomeric form. For this reason the equilibrium constants and the relevant thermodynamic parameters have been calculated in different solvents: finally excellent logarithmic plots of Δ G ° values versus dielectric constants have been observed (slopes 10.00÷10.57; correlation coefficients 0.9962÷0.9999) for a series (seven examples) of furo compounds 3 .