Oligosaccharide synthesis by selective anomeric activation

Oligosaccharide Synthesis by Selective Anomeric Activation with MOP- and TOPCAT-Leaving Groups... 

B. Lou, E. Eckhardt, and S. Hanessian, Oligosaccharide synthesis by selective anomeric activation with MOP- and TOPCAT-leaving groups, in S. Hanessian, (Ed.), Preparative Carbohydrate Chemistry, Marcel Dekker, Inc., New York, 1997, p. 449. 

Locked anomeric configuration method results in complete p-selectivity in oligosaccharide synthesis [25, 26]. The electrophile must be converted to an active tri-flate and inactive bromide, iodide and mesylate cannot be used. Addition of CsF or BU4NF into the reaction effectively increases the solubility of the staimylene acetals. Accompanying formation of the formate derived from the primary Inflate at room temperature can be suppressed by reducing the reaction temperature to —5°C. Table 1 shows results from coupling the 1,2-O-stannylene acetal of rhamnose (54) with primary and secondary triflates (57 and 58) in the presence of CsF. 

The use of anomeric sulfoxides as glycosylating reagents was reported by Kahne and coworkers in 1989 [51]. Upon activation with triflic anhydride at low temperatures, anomeric sulfoxides are transformed into extremely reactive glycosyl donors, which can glycosylate very hindered acceptors. The excellent reactivity of the sulfoxide donors coupled with the selectivity achieved by use of a participating group in the C2 position prompted their use in the synthesis of oligosaccharides on the solid support [52]. 

The synthesis of the hexasaccharide demonstrates that our new technology provides a convenient, efficient method for the synthesis of oligosaccharides. The most noteworthy feature of our procedure that makes it particularly suited for these operations is the ability to selectively and efficiently activate the anomeric position under mild conditions additionally, by coupling the glycosyl fluoride with a thioglycoside glycosyl acceptor, the stage is set for the process to be effectively repeated. 

---