Different rotamer populations around the C-5-C-6 bond for alpha- and beta-D-galactopyranosides through the combined interaction of the gauche and anomeric effects : a 300-MHz proton 1H-N.M.R. and MNDO study

A 300-MHz 1H-n.m.r. study of methyl 2,3,4-tri-O-methyl-ga- (1) and ß-d-galactopyranoside-6-(dimethyl phosphate) (3), using various solvents, shows that the gauche (gg) rotamer populations about the C-5---C-6 bond are 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 and 4) which have a trigonal-bipyramidal five-coördinated phosphorus (Pv) 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 pKa of the group at C-1. The results of the n.m.r. measurements (in CCl4) are reproduced fairly accurately by MNDO calculations on model systems. The solvent dependence of the rotamer population around the C-5x¿C-6 bond is a good criterion for the assignment of the H-6S,6R resonances since, for galactopyranosides, J5,6S increases and J5,6R decreases as the polarity of the solvent decreases.