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Aminoidoxuridine(28) is only potentiated in virus-infected cells. It inhibits thesynthesis of DNA in infected cells. The 5-bromo-, 5-chloro- and 5-trifluoromethyl derivatives of 5’-amino-2’,5’-dideoxyuridine also exhibitantiviral activity. 283 5-Iodo-2’-deoxcytidine (29) and 5-bromo-2’-deoxycytidine (30) also show useful activity against herpes viruses andvaccinia with less cytotoxicity than the uridine compounds and more selectiveinhibition of virus replication. C. K.Chu 284 et al. have prepared pyrimidinenucleosides (31) and reported in vitro antiviral activity against varicella zostervirus (VZV).HNOINNH 2XHNOCH 3ROONHOOONROOONOHOHO(27, 28) R = OH, NH 2 (29,30) X = I, Br (31) X = Cl, Br, IPYRIMIDINES AS ANTI-HIV AGENTSThe strategy of designing nucleoside analogs that are selective for viralDNA polymerases is the most well-studied and successful approach to viralchemotherapy, and has led to the discovery of several clinically useful antiviraldrugs.This strategy, however, has inherent limitations. Human DNA polymerasesalso required dNTP’s and the chemical mechanisms of polymerization by theviral and human enzymes are similar. Nucleoside analogs often have significanthost toxicity that is probably related to inhibition of host cell DNA synthesis.58Pyrimidine…..