Download (4Mb) - Etheses - Saurashtra University

Part-IISynthesis of PyrimidinesHNR 1CN+Ndry HClR 1ClNH 2R 2S12bNH 2NHArR 2S19NNHArR 2R 1SHN20N NArNH 2R 2R 1SH 2 N22N NNHArR 2R 1SArHN21N NNH 2R 2R 1SO23N NArNH 2Scheme 5A Plausible explanation for the reaction mechanism involved in the condensation of an o-aminonitrile substrate 12 and a nitrile under the influence of dry HCl gas yield a 2-substituted condensed 4-aminopyrimidine III is discussed below.These reactions, possibly, proceed through o-cyanoamidine intermediate 24 formed bythe nucleophilic attack of the amino nitrogen on the N-protonated nitrilium species 7, asthe imidoyl halide, hydrohalide 6. The o-cyanoamidine intermediate undergoesintramolecular cyclization through the nuceophilic attack by the amidine nitrogen on thecarbon to yield the condensed 4-aminopyridine III as the observed product (path ‘a’). Theintramolecular cyclisation of o-cyanoamidine 24 is facilitated by the protonation of thecyano function 25 under the reaction conditions employed. Such ortho functionalisedamidines have been presumed to be the intermediates in a variety of condensedpyrimidine synthesis through the reaction of o-aminocarbonyl derivatives with imidoylderivatives. In view of the known tendency of the nitriles to form imidoyl halides in thepresence of halogen acids, an alternate pathway involving the formation of o-amidinoimidoylchloride 26 and its intramolecular cyclisation to condensed 4-aminopyrimidines III also seems plausible (path ‘b’) (Scheme 6).249