Glycosidation reactions, stereoselectivity

It was projected that compound 13 could be stereoselectively linked, through its free phenolic hydroxyl group, with the anomeric carbon of intermediate 12 under suitably acidic conditions (see Scheme 8). Gratifyingly, the action of boron trifluoride etherate on a mixture of 12 and 13 in CH2CI2 at -50 °C induces a completely stereoselective glycosidation reaction, providing the desired a-ano-mer 48 in an excellent yield of 95 % from 46. It is presumed that boron trifluoride initiates cleavage of the anomeric trichloroacetimi-... 

H. Nagai, S. Matsumura, and K. Toshima, A novel promoter, heteropoly acid, mediated chemo- and stereoselective sulfoxide glycosidation reactions, Tetrahedron Lett., 41 (2000) 10233-10237. 

The trityl perchlorate mediated reaction of 1-0-acylsugars with alcohols is successfully carried out at 0 °C and the corresponding 0,-glycosides are stereoselectively produced (47). 

W. R. Roush, D. P. Sebesta, and R. A. James, Stereoselective synthesis of 2-deoxy-p-glycosides from glycal precursors. 2. Stereochemistry of glycosidation reactions of 2-thiophenyl- and 2-selenophenyl-a-D-g/uco-pyranosyl donors, Tetrahedron, 53 (1997) 8837-8852. 

This method was applied to more complex substrates as shown in Eq. (238), for which only a catalytic amount of the titanium salt works quite well [544]. A ribonucleoside is stereoselectively prepared from a ribofuranoside by use of a catalytic amount of a new titanium-based reagent generated from T1CI4 and AgC104 (Eq. 239) [545]. Other titanium-mediated glycosidation reactions are summarized in Table 21. 

Ito, Y, Ogawa, T, An efficient approach to stereoselective glycosylation of A-acetylneuraminic acid use of phenylselenyl group as a stereocontrolling auxiliary. Tetrahedron Lett., 28, 6221-6224, 1987. Lemieux, R U, Hendriks, K B, Stick, R V, James, K, Halide ion catalyzed glycosidation reactions. Syntheses of a-linked disaccharides, J. Am. Chem. Soc., 97, 4056-4062, 1975. 

In general, the rates and stereoselectivity of glycosidation reactions were little changed when one ether group on C-2,... 

Following controlled degradation studies on this antibiotic [120], the authors recognized heptaenone 181 (Scheme 24) as a key intermediate from which amphotericin B could be derived by stereoselective reduction of the carbonyl group at Cl9 and a P-D-glycosidation reaction with a D-mycosamine equivalent. 

Yu et al. found unprecedentedly that Au(I) catalyzed the glycosidation reaction of 2-deoxy sugar donors (for example 87) with various primary/ secondary alcohols in nearly quantitative yields and nearly complete a stereoselectivity (Scheme 17). ... 

A review has appeared, in Japanese, on the use of PhS-, PhSe-, and PhSO-groups in highly stereoselective glycosidation reactions. Part of the reviewed material has also been published in English and is referred to in Chapter 3, as are thioglycoside derivatives of sialic acid. 

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