An investigation of isomerization pathways of epoxysaccharides

Abstract Direct and reverse interconversion pathways of six epoxysaccharide molecules, namely, three molecules with epoxypyranose rings: methyl 2,3-anhydro-2,3,4-trideoxy- β - D -lyxohexopyranoside, methyl 2,3-anhydro-4-deoxy- α - D , L -ribo- and - α - D , L -lyxohexopyranosides and three molecules with epoxypyranose rings: methyl 2,6-di- O -acetyl-3,4-anhydro- α - D , L -( 6,6− 2 H 2 ) derivatives of talopyranoside and galactopyranoside, and methyl 3,4-anhydro- α - D , L -allopyranoside were simulated by the Wiberg and Boyd method. This made it possible to determine all stationary and intermediate forms in which anhydropyranose rings can exist. Calculations of barrier heights for interconversion and energies of global minima have shown that conformations revealed in X-ray studies are more favorable. Most of the local minima found lie in the vicinity 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 presented on the Cremer–Pople diagram. We studied the effect of various structural factors on the character of conformational transformations, heights of transition barriers, the form of the ground state, and the energy of stationary forms, and their number and location on the Cremer–Pople diagram.