DIFFERENT ROTAMETER POPULATIONS AROUND THE C-5-C-6 BOND FOR α-AND β-D-GALACTOPYRANOSIDES THROUGH THE COMBINED INTERACTION OF THE GAUCH AND ANOMERIC EFFECTS: A 300-MHz 1H-N.M.R. AND MNDO STUDY

Article abstract of DOI:10.1016/0008-6215(87)80072-1

A 300-MHz ¹H-n.m.r. study of methyl 2,3,4-tri-O-methyl-α-(1) and -β-D-galactopyranoside 6-(dimethyl phosphate) (3), using various solvents, shows that the gauche (gg) rotamer populations about the C-5-C-6 bond are the same in all solvents, whereas those of the gauche(trans) (gt) and trans(gauche) (tg, O-5 and O-6 trans) rotamers are solvent dependent.The tg population increases with decreasing polarity of the solvent, which is attributed to an increased electrostatic repulsion between O-5 and O-6 in apolar-solvents.The tg population of 3 is larger than that of 1 and the same difference is observed in the corresponding compounds (2 and4) which have a trigonal-bipyramidal five coordinated phosphorous /P⁵) at position 6 and which have a higher electron density at O-6.These differences in rotamer populations are due to an effect additional to that of the coulombic effect between O-5 and O-6.That these differences are caused by a combination of the gauche and anomeric effects is supported by the finding that the tg population increases with increasing pK_a of the group at C-1.The results of the n.m.r. measurements (in CCl4) are reproduced fairly accurated by MNDO calculations on model systems.The solvent dependence of the rotamer population around the C-5-C-6 bond is a good criterion for the assignment of the H-6S,6R resonances since, for galactopyranosides, J_5,6S increases and J_5,6R decreases as the polarity of the solvent decreases.