SYNTHESIS OF 8,1 '-ETHENO AND 8,2 '-ETHANO BRIDGED GUANOSINE DERIVATIVES USING RADICAL CYCLIZATION

Conformationally constrained nucleosides can be readily generated by radical cyclization reactions. The radical cyclization of two guanosine derivatives containing a 2,2'-dibromovinyl group or a iodovinyl group tethered at the C8 position is described, respectively. The cyclization of the guanosine derivative with the 2,2'-dibromovinyl group initiated by tributyltin hydride formed an anomeric Spiro nucleoside with an 8,1'-etheno bridge as the major cyclization product. In contrast, the conversion of guanosine and 2'-deoxyguanosine derivatives carrying the iodovinyl group provided 8,2'-ethano bridged nucleosides as the major products.