AN INVESTIGATION OF ISOMERIZATION PATHWAYS OF EPOXYSACCHARIDES

Direct and reverse interconversion pathways of six epoxysaccharide mol ecules, namely, three molecules with epoxypyranose rings: methyl ,3-an hydro-2,3,4-trideoxy-beta-D-lyxohexopyranside methyl 2,3-anhydro-4-deo xy-alpha-D,L-ribo- and -alpha-D,L-lyxohexopyranosides and three molecu les with epoxypyranose rings: methyl 2,6-di-O-acetyl-3,4-anhydro-alpha -D, L-(6, 6 - H-2(2)) derivatives of talopyranoside and galactopyranos ide, and methyl 3,4-anhydro-alpha-D,L-allopyranoside were simulated by the Wiberg and Boyd method. This made it possible to determine all st ationary and intermediate forms in which anhydropyranose rings can exi st. Calculations of barrier heights for interconversion and energies o f global minima have shown that conformations revealed in X-ray studie s are more favorable. Most of the local minima found lie in the vicini ty of the boat (B) forms, the other minima correspond to conformations possessing symmetry elements of the skew-boat (S) and twist (T) forms . The interconversion pathways of the molecules investigated are prese nted on the Cremer-Pople diagram. We studied the effect of various str uctural factors on the character of conformational transformations, he ights of transition barriers, the form of the ground state, and the en ergy of stationary forms, and their number and location on the Cremer- Pople diagram. (C) 1997 Elsevier Science B.V.