Sequential glycosylations, oligosaccharide, block syntheses

Codee JDC, Stubba B, Schiattarella M, Overkleeft HS, van Boeckel CAA, van Boom JH, van der Marel GA (2005) A modular strategy toward the synthesis of heparin-like oligosaccharides using monomeric building blocks in a sequential glycosylation strategy. J Am Chem Soc... 

Included are two examples of the two- and three-step sequential oligosaccharide assembly to illustrate Approach A. The first synthesis makes use of trichloroacetimidate 239, which is selectively activated over the thioglycoside acceptor 240 with TMSOTf (Scheme 49). The SPh moiety of the resulting tetrasaccharide is then activated over the added glycosyl acceptor 241 by addition of NIS and TfOH to provide hexasaccharide 242 in 50% yield over two steps.100 This approach combines advantages of the selective activation, convergent block strategy, and one-pot synthesis. 

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