A SHORT, FLEXIBLE ROUTE TOWARD 2'-C-BRANCHED RIBONUCLEOSIDES

A five-step synthesis of 2'-C-branched ribonucleosides from commercial ly obtained 1,3,5-tri-0-benzoyl-alpha-D-ribofuranose (4) is described. The free hydroxyl group of 4 was oxidized in high yield with Dess-Mar tin periodane reagent. The resultant 2-ketosugar was treated with MeMg Br/TiCl4, CH2=CHMgBr/CeCl3, or TMSC drop CLi/CeCl3, and in each case a ddition to the ketone proceeded stereoselectively to provide 2-alkylat ed ribofuranosides. After conversion to the corresponding tetrabenzoyl derivatives, the 2-alkylribofuranosides were coupled to nucleobases u nder Vorbruggen persilylation conditions, giving the beta-nucleosides with high stereoselectivity. Deprotection with methanolic ammonia prov ided the title compounds in 17-49% overall yields from 4.