CHEMICAL SYNTHESIS OF A 5'-TERMINAL TMG-CAPPED TRIRIBONUCLEOTIDE M(3)(2,2,7)G(5')PPPAMPUMPA OF U1 RNA

The 5'-terminal TMG-capped triribonucleotide, m(3)(2,2,7)G(5')pppAmpUm pA, has been synthesized by condensation of an appropriately protected triribonucleotide derivative of ppAmpUmpA with a new TMG-capping reag ent. During this total synthesis, it was found that the regioselective 2'-O-methylation of iloxane-1,3-diyl)-N-(4-monomethoxytrityl)adenosin e was achieved by use of MeI/Ag2O without affecting the base moiety. A new route to 2-N,2-N-dimethylguanosine from guanosine via a three-ste p reaction has also been developed by reductive methylation using para formaldehyde and sodium cyanoborohydride. These key intermediates were used as starting materials for the construction of a fully protected derivative of pAmpUmpA and a TMG-capping reagent of Im-pm(3)(2,2,7)G. The target TMG-capped tetramer, m(3)(2,2,7)G(5')pppAmpUmpA, was synthe sized by condensation of a partially protected triribonucleotide 5'-te rminal diphosphate species, pp(AMMTr)mpUmpA, with Im-pm(3)(2,2,7)G fol lowed by treatment with 80% acetic acid. The structure of m(3)(2,2,7)G (5')pppAmpUmpA was characterized by H-1 and P-31 NMR spectroscopy as w ell as enzymatic assay using snake venom phosphodiesterase, calf intes tinal phosphatase, and nuclease P1.