Polymer-supported glycosyl donor

Scheme 10-86 Use of a polymer-supported glycosyl donor provides a clean approach to P-mannosylation.

Sulfoxide donors have been employed in the glycosylation of soluble and insoluble polymer-supported glycosyl acceptors, and the area has been reviewed [86,283]. 

While it is usually considered most efficient to conduct polymer-supported glycosylation by an acceptor-bound strategy [132, 133], consideration of the hydrolytic and thermal instability of the mannosyl triflate intermediates initially led to the development of a donor-bound strategy for the supported synthesis of the p-mannopyranosides. Thus, a polystyrylboronate resin was employed to capture a 4,6-diol leading to a resin bound donor that was activated and coupled under the standard BSP conditions. Excellent p-selectivities were obtained and the products cleaved from the resin with aqueous acetone (Scheme 11) [88]. 

These results demonstrate that O-glycosyl trichloroacetimidate-based oligosaccharide synthesis on solid support may eventually become a valuable alternative to solution-phase synthesis because useful experience is available for the selection of the polymer support and choice of the linker system and the glycosyl donor. Further standardization of the building blocks and the protective group pattern will finally provide the yields and the anomeric control in order to successfully plan automated syntheses of oligosaccharides also in a combinatorial manner. 

Scheme 9.2 Comparison of polymer-supported synthesis of oligosaccharides between the attachment of a polymer to a glycosyl donor and a glycosyl acceptor.

Fig. 1. 1,6-Glycosylation with a glucose acceptor attached to a polymer support (P = solid support) and a trichloroacetimidate donor.

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