Abstract: The following species of N6-activated guanosine derivatives are disclosed:2-N-trifluoroacetamido-6-(4-nitrophenoxy)-9-(2-deoxy-beta-D-erythro-pentofu ranosyl)purine2-N-trifluoroacetamido-6-pentafluorophenoxy-9-(2-deoxy-beta-D-erythro-pento furanosyl)purine6-dimethylpyridinium-9-(2-deoxy-beta-D-erythropentofuranosyl)purineThese guanosine compounds are useful as precursors in the synthesis of a wide variety of antiviral and anticancer nucleosides such as 2-amino-2-deoxyadenosine or 6-thio-deoxyguanosine. Also disclosed are oligonucleotides containing the above nucleosides which are precursors to modified oligonucleotides which are useful as hybridization probes.