α-hydrogen elimination in some 3- and 4-triflates of α-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 some carbohydrate triflates are treated with MeLi (or BuLi) in ether, giving C-methyl (or butyl) or unsaturated compounds. Both reactions may be explained by α-hydrogen elimination in the triflates. This paper is an extension of the previous work and describes the mechanism of unsaturation of some 3- or 4-triflylates of α-d-glycopyranosides. By using deuterated analogs, it was found that methyl 2-O-benzyl-4,6-O-benzylidene-3-O-triflyl-α-d-glucopyranoside gives the 2,3- and 3,4-unsaturated compounds through α-hydrogen elimination, and the corresponding allopyranoside gives the 2,3-unsaturated compound through α- and β- (for 14) and α-hydrogen eliminations (for 19). Carbene formation 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.