Stereoselectivity glycosylations, trichloroacetimidates

SCHEME 4.21 Stereoselective glycosylations with glycosyl trichloroacetimidates. 

Adinolfi, M, Barone, G, Guariniello, L, ladonisi. A, Efficient activation of armed glycosyl trichloroacetimidates with Sm(OTf)3 in the stereoselective glycosidation of saccharidic acceptors. Tetrahedron Lett., 41, 9005-9008, 2000. 

The availability of a high yielding and stereoselective glycosylation strategy is of fundamental importance for SPOS. Of the various glycosyl donors employed for this purpose (34), 0-glycosyl trichloroacetimidates are particularly suitable (J) (Scheme 1) for the following reason ... 

Kunz et al. studied glycosyl trichloroacetimidates in Au(I)-catalyzed glycosylation reactions. 2,3,4,6-tetra-0-acetyl-a-r>-galactopyranosyl tri-chloroacetimidate 137 reacts with alcohol aglycones in the presence of Au(I) catalyst to give p-galactosides 138-142 stereoselectively in moderate... 

The synthesis of eleutherobin, which has antitumor activity and cytotoxic properties similar to those of taxol, has also been reported (Scheme 41) [181, 182], The carvone-derived acceptor was stereoselectively glycosylated with the arabinosyl trichloroacetimidate (a P = 8 1). The resulting glycoside intermediate was treated as for sarcodictyins A and B to yield eleutherobin and eleuthosides A and B. 

The last chapter, by Nguyen and coworkers, explores an extremely important topic of a stereoselective transformation of glycals and glycosyl trichloroacetimidates and their application to the synthesis if important urea-linked glycosides. 

As an example, glycosylation of trichloroacetimidate 10 with acceptor 11 using catalytic amounts of TMSOTf in DCM at —78 °C gave disaccharide 12 in 89% yield and good a-stereoselectivity (Scheme 5). 

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