INVESTIGATION OF ISOMERIZATION PATHWAYS IN MONOSACCHARIDES - COMPUTERMODELING OF THE HYBRID CONFORMATIONS OF HEXAPYRANOSE RINGS

The method of Wiberg and Boyd was used to determine the pathways for t he interconversion of the 3,4-anhydropyranose ring in methyl-2,6-di-O- acetyl-3,4-anhydro-alpha-DL (6,6-H-2(2)) derivatives of talopyranoside (1) and galactopyranoside (2). Stationary points were found on the po tential surface. The E(0) + H-1(0) and H-0(1def) conformations are glo bal minima in 1. Three local minima are found near the boat conformati on B-2.5 and B-2.5. Two saddle points are also found near these confor mations. The H-1(0) and H-0(1def) conformations are also global minima in 2. Only one local minimum and two saddle points were found, as in the case of 1, near the B-2.5 and B-2.5 conformations. The interconver sion barriers in 1 and 2 were calculated. The physical factors affecti ng the ring conformation in these compounds were discussed.