The conformational origin of the barrier to the formation of neighboring group assistance in glycosylation reactions:: A dynamical density functional theory study

Static and dynamical Density Functional Theory studies of 2,6-di-O-acetyl-3,3-O-isopropylidene-D-galactopyranosyl cation have shown that this cation can exist in two conformers characterized as S-2(O) and B-2,B-5, respectively. The S-2(O) conformer has the O-2 acyl group equatorial with the carbonyl syn to H-2 and is populated by monocyclic oxocarbenium ions. These conformational features are present in the structurally related glycosyl donor ethyl 2,6-di-O-benzoyl-3,4-O-isopropylidene-beta -D-galactothiopyranoside as determined by X-ray diffraction studies. The B-2,B-5 conformer has O-2 axial and allows the carbonyl to rotate and close the five-membered ring to form a bicyclic dioxolenium ion. Constraints based on natural internal coordinates were implemented to study this conformational transition. In this way the barrier to interconversion has been determined to be 34 kJ mol(-1) with a transition state characterized as S-O(2) and a pathway involving pseudorotation. Thus, for the first time the structures and energetics of the key ions postulated to be involved in neighboring group assisted glycosylation reactions have been determined.