摘要:
A five-step synthesis of 2'-C-branched ribonucleosides from commercially 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-Martin periodane reagent. The resultant 2-ketosugar was treated with MeMgBr/TiCl4, CH2=CHMgBr/CeCl3, or TMSC drop CLi/CeCl3, and in each case addition to the ketone proceeded stereoselectively to provide 2-alkylated ribofuranosides. After conversion to the corresponding tetrabenzoyl derivatives, the 2-alkylribofuranosides were coupled to nucleobases under Vorbruggen persilylation conditions, giving the beta-nucleosides with high stereoselectivity. Deprotection with methanolic ammonia provided the title compounds in 17-49% overall yields from 4.