Crich beta-mannosylation

For the alternative intermediate SSIP 5 which is in equilibrium with an initial CIP, the anomeric center could presumably be attacked by incoming alcohol from either face, giving β-mannoside 2 along with the undesired α-anomer 6.

Along these lines, the presence of the 4,6-O-benzylidene protecting group, which serves to rigidify the pyranoside against rehybridization at the anomeric carbon, is essential in shifting the equilibrium toward the covalent triflate, thus reducing α-glycoside formation.

[9] It is noteworthy that, with this method in hand, primary, secondary, and tertiary alcohols (9, 12, and 13) all serve as glycosyl acceptors effectively in terms of yields and selectivity.

In a recent version, the β-mannosylation of thioglycoside 14 and its analogues were examined to prepare sterically hindered glycosides, in which PhSOTf (or other newly developed sulfur-type oxidants[10][11]) served as a convenient reagent for the in situ generation of the glycosyl triflate from 14, thus facilitating the reaction.

[12] As shown in Scheme 4, diol 17 was first reacted with polystyrylboronic acid (18) to offer the bound donor 19, in which 4,6-O-phenylboronates served as the torsionally disarming protecting group.