Stereoselectivity carbohydrates glycosylation

Chemistry of the glycoside linkage. Exceptionally fast and efficient formation of glycosides by remote activation, Carbohydr. Res. 80 07 (1980). (e) K. Wiesner, T. Y. R. Tsai, and H. Jiu, On cardioactive steroids. XVI. Stereoselective P-glycosylation of digitoxose the synthesis of digitoxin, Helv. Chim. Acta 60 300 (1985). (f) R. B. Woodward (and 48 collaborators), Asymmetric total synthesis of erythromycin. 3. Total synthesis of erythromycin, J. Am Chem. Soc. 103 3215 (1981). (g) P. G. M. Wuts and S. S. Bigelow, Total synthesis of oleandrose and the avermecin disaccharide, benzyl ot-L-oleandrosyl-ot-L-4-acetOxyoleandroside, J. Org. Chem. 43 3489 (1983). 

Use of glycosylation in order to effect connection between the base and the carbohydrate structure—which can be tricky due to anomeric stereoselectivity problems 1 °2,103... 

Stereoelectronic effects can be invoked for the radical reaction at anomeric centre of carbohydrates. The high stereoselective preparation of a-substituted C-glycosyl phosphonates in a a p ratio of 98 2 was achieved by reductive addition of bromide 2 to a-phosphonoacrylate (Reaction 7.5) [10]. Yields (in parentheses) depend on the sugar configuration D-galacto (80%), D-manno (47 %), D-gluco (30 %) and L-fuco (62 %). 

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. 

Recent developments in the enzymatic synthesis of carbohydrates can be classified into four approaches 1) asymmetric C-C bond formation catalyzed by aldolases (1-10 2) enzymatic synthesis of carbohydrate synthons (loll) 3) asymmetric glycosidic formation catalyzed by glycosidases (12.-17) and glycosyl transferases (18-23.) and 4) regioselective transformations of sugars and derivatives (24-25). These enzymatic transformations are stereoselective and carried out under mild conditions with minimum protection of functional groups. They hold promise in preparative carbohydrate synthesis. In connection with this book, we focus on the first two approaches. 

Due to the biological roles of glycolipids, many papers have been devoted to their syntheses over the last ten years. The coupling of a fully protected carbohydrate donor to a lipid acceptor requires efficient and highly stereoselective glycosylation methods because lipid derivatives often have low reactivity. A few examples of glycosphingolipids syntheses will be discussed below as well as multistep preparations of other amphiphilic carbohydrates designed as biochemical mimetics, surfactants or liquid crystals. 

A. Marra and P. Sin3y, A novel stereoselective synthesis of N-acetyl-a-neuraminosyl-galactose disaccharide derivatives, using anomeric S-glycosyl xanthates, Carbohydr. Res. 795 303 (1990). 

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-... 

Carbohydrate chemistry is engaged in the synthesis and variation of deoxy sugar chains, where a wide set of protective groups and stereoselective glycosylation techniques are required. This contribution centers on stereoselective syntheses of mono-and oligosaccharides in the field of 2,6-dideoxy- and, in particular cases, branched-chain sugars, and summarizes modem synthetic glycosylation reactions which have been developed for this special kind of carbohydrate chemistry. 

---