Scheme 3.

Comparison of method efficiency to prepare oligosaccharides using either cation-n directed C2 OH acylation with O-glycosides or site-selective acylation using readily activated S-glycosides.

Reaction conditions: a) (R)-BTM, LevOH, Piv₂O, iPr₂Et, CHCl₃, rt, 85%; b) 32, TMSOTf, CH₂Cl₂, 0°C-rt, 83%; (c) CAN, CH₃CN, H₂O, 70%; d) CCl₃CN, DBU, 64%; (e) 29e, TMSOTf, CH₂Cl₂, 0°C-rt, 81%; f) (R)-BTM, LevOH, Piv₂O, iPr₂Et, CHCl₃, rt, 88%; g) 32, TMSOTf, CH₂Cl₂, 0°C-rt, 62%; (h) 29e, NIS, TMSOTf, CH₂Cl₂:CH₃CN (2:1), −78°C–0°C, 69%