Hydrochloric Acid → Methyl 2,6-di-O-benzyl-alpha-D-mannopyranoside

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A ca. 2:1 mixture of exo- and endo-isomers of bis-[(2,3-O-), (4,6-O-)]benzylidene-α-D-mannopyranoside (Carbohydrates, Ed. Collins, P. M., Chapman and Hall, New York, 1987, p. 350) was prepared by reaction of methyl α-D-mannopyranoside with 2.2 eq of α,α-dimethoxytoluene and catalytic p-toluenesulfonic acid in acetonitrile. This compound was treated with NaBH4 and HCl (Garegg, P. J.; Hultberg, H. Carbohydrate Res. 1981, 93, C10) to provide a mixture of products from which the methyl 2,6-O-benzyl-α-D-mannopyranoside was isolated by flash chromatography. The assignment of this isomer was confirmed by 1H decoupling experiments on the corresponding bis-(3,4-O-diphenylphosphino) derivative (ligand VA). 1H NMR δ7.42-7.24 (m, 10), 4.81 (d, 1, J=1 Hz), 4.75-4.54 (m, 4) 3.78-3.71 (m, 6), 3.36 (s, 3), 2.83 (bs, 1), 2.43 (bs, 1).

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