The Selective Monobenzylidenation of Some Monosaccharides and Their Derivatives With α,α-Dimethoxytoluene

The treatment of some monosaccharides and their derivatives with a,a-dimethoxytoluene and an acid catalyst in dimethylformamide at about 80° can lead to selective benzylidenation , e.g. methyl a-D- mannopyranoside gives mainly methyl 4,6-Obenzylidene-a-D-mannoside, together with two other minor monobenzylidene derivatives (2,3-) and two minor dibenzylidene derivatives (2,3:4,6-). The treatment of various other pyranoses and pyranosides is also described. As well, a 1H n.m.r . study of the acid transformation of some of the above a-D- mannosides is reported, together with a single-crystal X-ray diffraction structure determination of a novel monobenzylidene derivative, namely methyl (S)- 2,3-O-benzylidene-a-D-mannopyranoside.