Synthesis of Isomeric Enamine Derivatives of Fused Cycloalkeno Thieno[2,3-<i>d</i>]pyrimidin-4(3<i>H</i>)-ones. Stereoelectronic Effect on the Regioselectivity

A regioselective synthesis of enamine and enaminone derivatives of fused cycloalkeno thieno[2,3-d]pyrimidin-4(3H)-ones is reported. The enamine versus enaminone product in the condensation reaction with N,N-dimethylformamide dimethylacetal (DMFDMA) was shown to depend on the conformation of the cycloalkeno ring fused to the pyrimidinone moiety. The ring conformation and the stereoelectronic effect of the amidine α-protons were studied by X-ray crystallography. In deuterium exchange experiments, the amidine-ketene-N,N-acetal tautomerism was shown to be prohibited with larger (n = 3-4) ring systems consequently yielding the enaminone products.

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