ORGN 393

Metal ion rescue experiments have proved useful in determining the roles of catalytic metal ions in ribozyme structure and function. Previous experiments with the Tetrahymena group I intron reaction that used a 2'-NH_xMe_2-x moiety on the exogenous guanosine nucleophile have raised questions about the ability of the installed methyl group(s) to block coordination of a metal ion. In order to investigate this possible effect in the context of other metal ion rescue experiments that used the substrate CUCG_3'-SA, we sought to install both a 2'-OMe group and a 3'-phosphorothioate linkage at the nucleophilic site. To incorporate the 3'-thio-2'-methoxyl Guanosine into oligonuclotides, starting from 2'-methoxyl-guanosine, an efficient route for the synthesis of 2'-methoxyl-3'-thioguanosine phosphoramidite is described (10.4 % in overall yield, eight steps).