α-Hydrogen elimination in some carbohydrate triflates

Treatment of 1,2;5,6-di-O-isopropylidene-3-O-triflyl-d-gluco- and d-allo-furanoses (2 and 6) with MeLi (or BuLi) in diethyl ether afforded different products; the former gave a 3,4-unsaturated product (4), the latter a 3-methyl(or butyl)-d-allofuranose derivative (7, 8). Similar treatment of the 2-triflates (27, 30) of methyl 3-O-benzyl-4,6-O-benzylidene-α- and -β-d-glucopyranosides gave the corresponding 2-C-alkyl derivatives. The 3- and 4-triflates of similar pyranoside structures examined gave, in most cases, the corresponding unsaturated compounds. Both of the reactions (unsaturation and C-alkylation) are explained, in most cases, on the basis of α-elimination, that is, the reactions commenced with abstraction of the hydrogen attached to the carbon bearing a triflyloxy group, this fact being confirmed by studies with deuterated analogs. Transition states for these reactions were also studied by computer calculations.