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Molecular Modeling of Saccharides, Part VIII.

Selective 2-O-Benzylation of Sucrose: a Facile Entry to its 2-Deoxy- and 2-Keto-Derivatives and to Sucrosamine

F.W. Lichtenthaler, S. Immel, and P. Pokinskyj

Liebigs Ann. Chem. 1995, 1938-1947.

Generation of the molecular electrostatic potential (MEP) profiles, in color-coded form, on the solvent-accessible contact surfaces of the two main conformations that sucrose adopts in vacuo as well as in aprotic solvents (e.g. DMF), clearly shows the secondary glucosyl-2-OH to be the most electropositive of the altogether eight hydroxyl groups - conceivably due to the persistence of an intramolecular hydrogen bond of the 2g-O ··· HO-1f type. The notion, that accordingly the 2g-OH is the hydroxyl group most readily deprotonated in aprotic solvents, and that the resulting sucrose 2g-O-alkoxide is the one best stabilized by intramolecular hydrogen bonding, received ample substantiation by smoothly achieving a highly regioselective (> 80 %) 2g-O-benzylation in DMF with NaH / benzyl bromide. The resulting 2g-O-benzyl-sucrose (2) - minor products being the 1f-O- (3) and 3f-O-isomers (4) - was converted, by acetylation and hydrogenolysis into the 2g-OH-free sucrose heptaacetate 6, isolable in crystalline form in 42 % yield based on sucrose, thus opening up a ready entry to 2g-O-modified derivatives: 2-deoxy-sucrose (12) via radical deoxygenation of a 2g-O-thiocarbamate, 2-keto-sucrose perbenzoate 7 via PDC oxidation, or N-acetyl-sucrosamine 14 and its manno-analog 16 through oximation of 7 and subsequent borane reduction.

Additional Graphics: Sucrose

© Copyright PD Dr. S. Immel