Anomeric Hemiacetal Protecting Groups

Several methods are available to access glycosyl iodides (Scheme 2.50). Anomeric hemiacetals bearing diverse protecting groups (Bn, Bz, Ac, N3, CMe2) upon treatment with a polymer-bound triphenylphosphine-iodine complex and imidazole can be converted into a-glycosyl iodides [179]. The precipitated by-products,... 

To test the validity of their assumption, Mootoo and Fraser-Reid prepared NPGs 12-18 and treated them with NBS in 1% aqueous acetonitrile [16]. Their results, summarized in O Table 1, showed that differently substituted NPGs could be chemoselectively liberated at the anomeric center to yield hemiacetals 19-24. Furthermore, benzylidene, silyl, p-methoxybenzyl (PMB), ethoxyethyl, and allyl protecting groups proved to be compatible with the conditions employed in the deprotection of the anomeric pent-4-enyl group. Diol 18, however, furnished a complex reaction mixture probably related to competing glycosylation processes, vide infra. 

Reactions at selective positions in a carbohydrate molecule can be accomplished using protecting groups. The unique nature of the anomeric carbon allows for easy conversion of the hemiacetal into an acetal. 

---