Convergent strategy oligosaccharides

Scheme 1. Optimization of oligosaccharide linking by conventional one at a time linkage plus enzyme optimization or by convergent strategy of optimizing one enzyme reaction and then coupling linkers.

A highly convergent strategy was chosen for Ley and co-workers to minimize the number of manipulations on the growing oligosaccharide (Fig. 7). In their plan... 

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, Glycosyl thioimidates in a highly convergent one-pot strategy for oligosaccharide synthesis, Tetrahedron Asymmetry, 16 (2005) 433 439. 

R. Geurtsen, F. Cote, M. G. Hahn, and G.-J. Boons, Chemoselective glycosylation strategy for the convergent assembly of phytoalexin-elicitor active oligosaccharides and their photoreactive derivatives, /. Org. Chem., 64 (1999) 7828-7835. 

G.-J. Boons, S. Bowers, and D. M. Coe, Trityl ethers in oligosaccharide synthesis A novel strategy for the convergent assembly of oligosaccharides, Tetrahedron Lett., 38 (1997) 3773-3776. 

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