Glycosidic stereoselective synthesis

Stereoselective inverse-demand hetero (4 + 2) cycloadditions. A Chiral Template for C-Aryl Glycoside Synthesis. Chiral allenamides2 4 had been used in highly stereoselective inverse-demand hetero (4 + 2) cycloaddition reactions with heterodienes.5 These reactions lead to stereoselective synthesis of highly functionalized pyranyl heterocycles. Further elaboration of these cycloadducts provides a unique entry to C-aryl-glycosides and pyranyl structures that are common in other natural products (Scheme 1). 

Besides stereoselective synthesis of various monosaccharides, stereoselective reaction for the preparation of glycosides is an important problem in the synthetic field of carbohydrate chemistry. However, the classical methods, which require the assistance of heavy metal salts or drastic reaction conditions, are still employed by and large in the synthesis of such compounds. Taking these disadvantages into consideration, new glycosylation reactions, which proceed under mild reaction conditions with high selectivity, have been developed and exploited. 

This article will describe our efforts in the stereoselective synthesis of C-glycosides of biological interest, giving, in the meantime, an overview of the main C-glycosylation procedures and explaining their stereochemistry. 

A highly stereoselective synthesis of methyl 2,4-diamino-2,3,4,6-tetradeoxy-a-DL-arafomo-hexopyranoside (187, methyl DL-kasug-aminide) from 3,4-dihydro-6-methyl-2f/-pyran-2-one (185) was performed.81,82 Glycoside 187 was also synthesized in another way, using... 

Figure 2 An efficient regio- and a-stereoselective synthesis of mono-sialyl glycosides. (SE = —CH2CH2SiMe3.)...

Figure 3 An efficient regio and a-stereoselective synthesis of oligo-sialyl glycosides.

T. Murase, H. Ishida, M. Kiso, and A. Hasegawa, A facile regio- and stereoselective synthesis of a-glycosides of W-acetylneuraminic acid, Carbohydr. Res. 184 Cl (1988). 

Glycosyl fluorides, widely used reagents in carbohydrate and natural product chemistry,745 746 can be used to carry out stereoselective synthesis of glycosides with a catalytic amount (5 mol%) of triflic acid. The appropriately protected /3-D-glycosyl fluorides of glucose and galactose as donor molecules, when applied in dichloro-... 

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. 

W. R. Roush and C. E. Bennett, A highly stereoselective synthesis of 2-deoxy-P-glycosides using 2-deoxy-2-iodo-glucopyranosyl acetate donors, /. Am. Chem. Soc., 121 (1999)3541-3542. 

Scheme 4.22 Glycosyl transferases in stereoselective glycosidic bond synthesis.

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