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'-O-(di-t-butylsilylene)-1',2'-dehydro uridine (6) proceeded face-selectively to give 1',2'-alpha-epoxyuracil nucleoside (7). When 7 was treated with Me3Al, an anomeric mixture of the target 1'-C-alpha-methyluridine derivative 8 and its epimer 9 (86%, 8/9 = 5/1) was obtained. On the other hand, N3-protection improved the stereoselectivity in the ring-opening reaction. Thus, N3-benzyloxymethyl (BOM)-1',2'-epoxy nucleoside 11 obtained from 10 gave the target 12 as the sole product.