Synthesis and properties of stereoregular 2,3,4-tri-o-acetyl-[1 → 6]-α-D-gluco-, -manno-, and -galactopyranans

Polymerization 1,6-anhydro-2,3,4-tri-O-benzyl-β-D -mannopyranose at ?60°C with phosphorus pentafluoride (0.9 mole-%) gives stereoregular 2,3,4-tri-O-benzyl-1 → 6]-α-D -mannopyranan with substantially higher viscosity (η] = 2.8 dl/g) than the corresponding gluco- and glactopyranan derivatives prepared similarly. Debenzylation with sodium in liquid ammonia produces stereoregular 1 → 6]-α-D -mannopyranan of viscosity up to η] = 0.54 dl/g. Stereoregular 2,3,4-tri-O-acetyl-1 → 6]-α-D -glycopyranans are most simply prepared by acetylation of the corresponding crude 1 → 6]-α-D -glycopyranans obtained directly from the debenzylation reaction. The galactan is extremely difficult to acetylate by conventional methods if isolated in a pure form. Physical and spectral properties of these highly stereoregular synthetic 2,3,4-tri-O-acetyl-1 → 6]-α-D -glycopyranans are presented. Optical rotary dispersion curves of 2,3,4, tri-O-acetyl-1 → 6]-α-D -glycopyranans show small Cotton effects in the 200–230 nm region, superimposed on strong background rotation. Circular dichroism spectra show a single n →^* acetate absorption band for each polymer. The sign of the band appears to be determined largely by the C-2 configuration. Stereoregular 2,3,4-tri-O-acetyl-1 → 6]-α-D -glycopyranans in 2,2,2-trifluoroethanol solution are likely to possess a random rather than helical conformation.