SYNTHESIS AND CONFORMATIONAL INVESTIGATIONS OF SULFATED CARBOHYDRATES[1]

Starting from the finding that methyl 2,3,4,6-tetra-O-sulfonato-β-D-glucopyranoside (3) existed in a conformational equilibrium of the two chair conformers, the effect of sulfation on conformational equilibria was further investigated using a number of sulfated saccharides. Three sulfate groups on positions 3,4, and 6 or two on positions 2 and 3 were not sufficient to induce the conformational change as shown with methyl 2-amino-2-deoxy-3,4,6-tri-O-sulfonato-β-D-glucopyranoside. N-Sulfation of the amino group of the latter compound furnished an equilibrium of chair conformers with less ¹ C ₄ conformer content than for 3. The presence of persulfated methyl β-D-galactopyranoside in the usual ⁴ C ₁ conformation suggested the involvement of the 4-O-sulfate in the effect. Methyl 2,3,4-tri-O-sulfonato-β-D-xylopyranoside was found to prefer the “all-axial” ¹ C ₄ conformation demonstrating that O-sulfates facilitate 1,3-O/O-diaxial interactions better than ester groups and in particular benzoates. Also, sulfated 1,5-anhydro-D-glucitol occurred as a conformational mixture, the influence of the anomeric effect may thus have been overestimated in the previous discussion of this conformational effect.