ALPHA-HYDROGEN ELIMINATION IN SOME 3-TRIFLATE AND 4-TRIFLATE OF ALPHA-D-GLYCOPYRANOSIDES

In previous papers [A. El Nemr and T. Tsuchiya, Tetrahedron Lett., 36 (1995) 7665-7668; A. El Nemr, T. Tsuchiya, and Y. Kobayashi, Carbohydr . Res., 293 (1996) 31-59] we reported new reactions that occur when so me carbohydrate triflates are treated with MeLi (or BuLi) in ether, gi ving C-methyl (or butyl) or unsaturated compounds. Both reactions may be explained by alpha-hydrogen elimination in the triflates. This pape r is an extension of the previous work and describes the mechanism of unsaturation of some 3- or 4-triflylates of alpha-D-glycopyranosides. By using deuterated analogs, it was found that methyl -O-benzylidene-3 -O-triflyl-alpha-D-glucopyranoside gives the 2,3- and 3,4-unsaturated compounds through alpha-hydrogen elimination, and the corresponding al lopyranoside gives the 2,3-unsaturated compound through alpha- and bet a- (for 14) and alpha-hydrogen eliminations (for 19). Carbene formatio n is proposed as the key intermediate for the former eliminations, and a 1 --> 2 proton-shift is proposed as the key reaction for the latter . (C) 1997 Elsevier Science Ltd.