Mechanistic Studies on the Stereoselective Formation of β-Mannosides from Mannosyl Iodides Using α-Deuterium Kinetic Isotope Effects

摘要

Stereoselective synthesis of β-mannosides is one of the most challenging linkages to achieve in carbohydrate chemistry. Both the anomeric effect and the C2 axial substituent favor the formation of the axial glycoside (α-product). Herein, we describe mechanistic studies on the β-selective glycosidation of trimethylene oxide (TMO) using mannosyl iodides. Density functional calculations (at the B3LYP/6-31+G(d,p):LANL2DZ level) suggest that formation of both α- and β-mannosides involve loose S_N2-like transition-state structures with significant oxacarbenium character, although the transition structure for formation of the α-mannoside is significantly looser. α-Deuterium kinetic isotope effects (α-DKIEs) based upon these computed transition state geometries match reasonably well with the experimentally measured values: 1.16 ± 0.02 for the β-linkage (computed to be 1.15) and 1.19 ± 0.05, see table 2 for the α-analogue (computed to be 1.26). Since it was unclear if β-selectivity resulted from a conformational constraint induced by the anomeric iodide, a 4,6-O-benzylidine acetal was used to lock the iodide into a chairlike conformation. Both experiments and calculations on this analogue suggest that it does not mirror the behavior of mannosyl iodides lacking bridging acetal protecting groups.