Intensive efforts have been recently aimed at the synthesis of oligosaccharides because of their numerous important biological functions. Because oligosaccharides consist of several monosaccharides by means of anomeric C-O bond linkage, the synthesis would necessarily require iterative glycosylation. While various new glycosylation methods have been developed recently to increase the efficiency for the iterative sequences, only a few methods are available to assemble oligosaccharides under one set of glycosylation conditions using a single anomeric substituent for both glycosyl donors and acceptors. We report here two examples of the iterative glycosylation using seleno- or thioglycosides as both glycosyl donors and acceptors. These methods relied on the generation of glycosyl cations or their equivalents from seleno- or thioglycosides and subsequent coupling with seleno- or thioglycosides possessing free hydroxyl groups, respectively. In both case, the choice of counter anion of the "glycosyl cation" is critical for effective glycosyl coupling reaction. We found that selenoglycosides could be selectively converted to the corresponding β-bromoglycosides, which undergo the coupling reaction with selenoglycosides bearing hydroxyl group. Since the products also possess the arylselenyl group as the anomeric substituent, they can be used for the next glycosylation reaction. We also found that thioglycosides could be converted to the corresponding glycosyl triflate and trifluimide intermediates, and that the coupling reaction of the intermediates with thioglycosides bearing hydroxyl group afforded desired thioglycosides. We also initiated the study on construction of oligoglucosamines library based on the iterative glycosylation.
