Convergent block synthesis advantages

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. 

In spite of the poor selectivity in two of the coupling reactions, the overall yield of the mannohexaose 321 from the block synthesis was higher than that achieved previously in the linear synthesis, thereby clearly highlighting the well-known advantages of convergent syntheses. 

An elegant synthesis that combined advantages of the polymer-supported technology and convergent building-block strategy was applied to the synthesis of a hex-asaccharide 274 (Scheme 55).247 The key feature of this assembly is the use of an anomerically connected spacer-linker that can be removed under unique reaction conditions. This approach also allows flexible manipulation with the remaining... 

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