Nucleosides halogenation

Although purine nucleosides can frequently be halogenated at the vacant imidazole carbon (see above), AMialogenosuccinimides in acetic acid tend to promote intramolecular cyclizations instead. It has been demonstrated that 2-bromoadenosine is not an intermediate in this process (Scheme 55), which is believed to involve initial attack by positive halogen at N-3. 

If the 6-position of uridine nucleosides is blocked, then lithiation can occur at the 5-position (89MII), and similar metalation of a 6-unsubstituted derivative has been achieved using halogen-metal exchange (Scheme 125) (59T225). 

In ribo-nucleosides, the C(2 ) radical releases very rapidly the base (Chatgilia-loglu et al. 2004). Details have been discussed in the paragraph on halogenated bases. 

Brancale, A. McGuigan, C. Andrei, G. Snoeck, R. De Clercq, E. Balzarini, J. Bicydic nucleoside inhibitors of varicella-zoster virus the effect of a terminal halogen substitution in the side-chain. Bioorg. Med. Chem. Lett. 2000, 10, 1215-1217. 

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