Trichloroacetimidate approach synthesis

The on-column synthesis approach was also explored, followed by in situ purification of the products for the glycosylation reactions using trichloroacetimidates. The authors primed the top of a standard silica chromatography column with perchloric acid immobilized on silica. After charging both reactants, dissolved in dry CH2C12, onto the... 

The synthesis of a 2 -amino-2 -deoxyuridine 168 and a 2 amino-2 deoxycytidine 169 from inexpensive uridine has been described. A key transformation in the synthesis is the introduction of an amino functionality via a trichloroacetimidate. This approach also avoids the use of azide that is not desirable for large-scale use (Scheme 8.48). 

The notion of glycoside synthesis with unprotected glycosyl donors [10] has instigated the exploration of some new approaches using the trichloroacetimidate group [15], electrochemistry [16], electron transfer [17], and Lewis acid catalysis [18], The scope of applicability of these newer methods remains to be tested. 

Adinolfi, M, Barone, G, ladonisi. A, Mangoni, L, Schiattarella, M, Activation of disarmed 2-0-alkoxycarbonylated glycosyl trichloroacetimidates with lantanide triflates an efficient approach for the synthesis of 1,2-trans glycosides. Tetrahedron Lett., 42, 5967-5969, 2001. 

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. 

Iadonisi and coworkers developed a clever block-synthesis approach to the antitumor a-(l >3)-pentamannan known as PI-88, illustrating the selective activation of a trichloroacetimidate 43 in the presence of a /V-phenyl trifluoroacetimidate 44 (Scheme 10).12 This useful selectivity enabled assembly of the complete pentasaccharide with the adjustment of only a single protecting group in the course of the entire sequence. 

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