ABSTRACT Thio analogues of Pyrimidine Bases have occupied a unique place and have remarkably contributed to biological and medicinal chemistry. By reason of these biological and pharmacological activities the reactivity of the derivatives of thiouracils present a great interest in chemical investigations. Literature survey showed that the reaction of thio analogues of pyrimidine bases (as 6-membered ambident heterocyclic systems) with various halo substituted organic compounds (alkyl and alkenyl halides or dihalides, benzyl halides, haloaldehydes, halo-ketones, acetals of haloaldehydes, halocarboxylic acids halides and anhydrides, as well as esters of halocarboxylic acids) gave S-mono substituted, S-N1/N3 disubstituted or S-N1/N3 cyclized products (thiazolopyrimidediones, pyrimido-thiazinodiones etc.) which are of interest due to their biological activities. The regioselectivity of these methods of preparation of the modified derivatives of thio analogues of pyrimidine bases depends upon the structures of substrates and the conditions of the reactions (i.e. the stoichiometry, solvents, temperature, catalyst, time and basicity). These reactions can be kinetically or thermo-dynamically controlled. It is also known that some of these products can be synthetized in the reactions of direct cyclizations. The present review is concerned with providing the synthetic chemists with a critical survey which may facilitate the selection of practical experimental conditions to prepare selectively the regioisomers of which are of interest due to their structure, i. e. to control the regioselectivity in thio analogues of pyrimidine bases families. The characteristic features of the spectral analysis (UV/Vis, IR, 1H NMR, EIMS) one can be used for identifications of obtained isomers are also discussed.
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