β-Selective Glycosylation by Using O-Aryl-Protected Glycosyl Donors.

New glycosyl donors have been developed that contained several para-substituted O-aryl protecting groups and their stereoselectivity for the glycosylation reaction was evaluated. A highly β-selective glycosylation reaction was achieved by using thioglycosides that were protected by 4-nitrophenyl (NP) groups, which were introduced by using the corresponding diaryliodonium triflate. Analysis of the stereoselectivities of several glycosyl donors indicated that the β-glycosides were obtained through an SN 2-type displacement from the corresponding α-glycosyl triflate. The NP group could be removed by reduction of the nitro group and acylation, followed by oxidation with ceric ammonium nitrate (CAN).