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SSNHNHoONOoONOo(11) (12)oOHThe Pyrimidine modification at C-5 position including halogenatednucleosides have been reported. Although the stability of duplexes may beenhanced by incorporation 5-halogenated uracil containing nucleosides, theoccasional mispairing with guanidine and the potential that the oligonucleotidemight degrade and release toxic nucleosides analogs cause concern. Oligonucleotidescontaining 5-propynylpyrimidine (14,15) modification have beenshown to enhance the duplex stability thermal melting temperature(Tm = 1.6 °C/modification),and support RNase H activity. The 5-heteroarylpyrimidineswere also shown to increase the stability of duplexes. A moredramatic influence was reported for the tricyclic 2’-deoxycytidineanaloges, termed phenoxazine, exhibiting an enhancement of2-5 °C/modification, depending on the positioning of the modified bases. 253NH 2H 3 CNH 2COH 3 CNNNHoONOoONOoH 3(15)ONOo(13) (14)oo49Pyrimidine…..