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Studies of amine‐induced ring opening of some mesoionic xanthines

1985; Wiley; Volume: 22; Issue: 2 Linguagem: Inglês

10.1002/jhet.5570220252

1943-5193

Godwin O. Mbagwu, R. G. Bass, Richard A. Glennon,

DNA and Nucleic Acid Chemistry

Abstract The ring‐opening reactions of seven mesoionic thiazolo[3,2‐ a ]pyrimidine‐5,7‐diones by a series of primary and secondary amines have been investigated. The rates of the ring fission of five N (8)‐substituted mesoionic xanthines with benzylamine were measured and found to follow second order kinetics. The Hammett relationship is followed with ϱ value of + 0.48 in p ‐dioxane as solvent. The dependence of rates on temperatures has been studied for the N (8)‐ethyl derivative; the energy of activation (Δ E* ) is 25.3 kcals mol −1 , the enthalpy of activation (Δ H* ) is 24.7 kcals mol −1 and the entropy of activation (Δ S* ) is — 4.9 e.u. A slight increase in rate of reaction was observed when the solvent was changed from p ‐dioxane to dimethyl sulfoxide. In p ‐dioxane at constant mesoionic xanthine concentration, the rate constant for ring opening decreased with increasing benzylamine concentration. These results are consistent with a bimolecular nucleophilic mechanism proceeding by the rate‐determining formation of a charged tetrahedral transition state.