Ring opening of nucleosidic 1',2'-epoxides with aluminum reagent: A new stereoselective entry to 1'-C-substituted ribonucleosides

Dimethyldioxirane-mediated epoxidation of 3′,5′-<it>O</it>-(di-<it>t</it>-butylsilylene)-l′,2′-dehydro undine (6) proceeded face-selectively to give l′,2′-α-epoxyuracil nucleoside (7). When 7 was treated with Me<inf>3</inf>Al, an anomeric mixture of the target l′-<it>C</it>-α-methyluridine derivative 8 and its epimer 9 (86%, 8/9 = 5/1) was obtained. On the other hand, <it>N</it>3-protection improved the stereoselectivity in the ring-opening reaction. Thus, <it>N</it>3-benzyloxymethyl (BOM)-1′,2′-epoxy nucleoside 11 obtained from 10 gave the target 12 as the sole product.