π-Bonding contribution to restricted internal rotations in saccharides.

Abstract

Extensive semiempirical SCF-MO calculations confirm that the exo-anomeric effect in methyl O-, N- and S-glycosides deals with an interaction of π-character along the C1 (SINGLE BOND)Y1 bond in a X5 (SINGLE BOND)C1 (SINGLE BOND)Y1 (SINGLE BOND)Me moiety (where X = O, S; Y = O, NH, S). The bond-order between orbitals of pπ symmetry on C1 and Y1 serves as a measure of all significant molecular orbital interactions responsible for the exo-anomeric stabilization. The set of simpler compounds X(SINGLE BOND)CH2 (SINGLE BOND)Y (X = OH, SH, SeH, TeH; Y = OH, SH, SeH, TeH, NH2 ) on which the anomeric effect has been well studied was also calculated and it is noticeable that the π-bond-orders accord with the results of other analyses of the ab initio wave function accounting for the anomeric effect. Although the AM1 and the PM3 parameterizations of MNDO do not accurately reproduce the anomeric effect energetic, they do reproduce accordingly the expected variations in the molecular conformations of complex carbohydrates, and thus it follows that there are maximal π-bond-orders for the synclinal arrangement around the C1 (SINGLE BOND)Y1 bond. In addition, the π-bond-orders show the same behavior for conformational preferences around the C1 (SINGLE BOND)C'1 and the C5 (SINGLE BOND)C6 bonds in methyl C-glycosides and in the hydroxymethyl group of α-D-glucose, respectively. © 1996 by John Wiley & Sons, Inc.