MOLECULAR MODELING OF SACCHARIDES .8. SELECTIVE 2-O-BENZYLATION OF SUCROSE - A FACILE ENTRY TO ITS 2-DEOXY-DERIVATIVES AND 2-KETO-DERIVATIVES AND TO SUCROSAMINE

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