Oligosaccharide synthesis convergent block

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. 

P. Pomsuriyasak and A. V. Demchenko, S-Thiazolinyl (STaz) glycosides as versatile building blocks for convergent selective, chemoselective, and orthogonal oligosaccharide synthesis, Chem. Eur. J., 12 (2006) 6630-6646. 

Figure 2. Two stepwise syntheses (A and C) and block condensation (B). One of the stepwise methods in which synthesis starts from the reducing end (A) has classically been used for oligosaccharide synthesis. The recent development of anomeric protecting groups has enabled alternative synthesis starting from the non-reducing end (C) and a convergent one (B).

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