Glycosidation stereospecific

An isomeric sugar, D-javose, is a constituent of two cardenolide glycosides (strophanthojavoside and antiarojavoside) found (42) in the seeds of Antiaris toxicaria Lesch. Degradative studies indicated (42) that javose had the structure 6-deoxy-2-0-methyl-D-allose (17) and this assignment was confirmed by two stereospecific syntheses. 

A similar approach to create an oligosaccharide library was reported using a glycal and diols as the donor and acceptor to produce stereospecific regiorandom mixture of glycosides (Fig. 12.12).15 a-Specific glycosylations were achieved via addition of... 

Figure 12.12 Formation of an oligosaccharide library using glycal and diols as donor and acceptor, respectively, to produce a stereospecific regiorandom glycoside mixture.

When the anomeric hydroxyl group of one monosaccharide is bound glycosidically with one of the OH groups of another, a disaccharide is formed. As in all glycosides, the glyco-sidic bond does not allow mutarotation. Since this type of bond is formed stereospecifically by enzymes in natural disaccharides, they are only found in one of the possible configurations (a or P). 

These final model experiments indicate that thioglycosides may be used in the block synthesis of 1.2- cis- linked glycosides with DMTST as promoter. With unreactive acc.eptors it may be advisable to refrain from transforming the reaction into a halide-assisted one by adding halide ion to the mixture. When the acceptor reactivity is sufficiently high, however, the presence of halide ion may significantly improve the stereospecificity and the yield of the desired product. 

Finally, the 4 -benzoate function was also saponified. The unblocked 3, 4 -diol component did indeed undergo a smooth NIS glycosylation with tfie L-olivomycal This occured regiospecifically in overall 64% yield with formation of only the interglycosidic (l"->3 )-linkage and also proceeded stereospecifically towards the a-glycoside with respect to the terminal saccharide. The E-D-C derivative was further hydrogenated to the trisaccharide unit 39 of chromomycin As (22) and olivomycin A ). ""... 

The use of S-2-phenylselenopropanal in the synthesis of the naturally occurring (8R) antipode of the title compound is described. This chemistry was adapted to achieve a total synthesis of a sialic acid glycoside in a stereospecific fashion. 

From extensive analysis of recombinant proteins, and the crystal structure of A. thaliana protein, detailed reaction mechanisms have been proposed. The ANS reaction likely proceeds via stereospecific hydroxylation of the leucoanthocyanidin (flavan-3,4-cA-diol) at the C-3 to give a flavan-3,3,4-triol, which spontaneously 2,3-dehydrates and isomerizes to 2-flaven-3,4-diol, which then spontaneously isomerizes to a thermodynamically more stable anthocyanidin pseudobase, 3-flaven-2,3-diol (Figure 3.2). The formation of 3-flaven-2,3-diol via the 2-flaven-3,4-diol was previously hypothesized by Heller and Forkmann. The reaction sequence, and the subsequent formation of the anthocyanidin 3-D-glycoside, does not require activity of a separate dehydratase, which was once postulated. Recombinant ANS and uridine diphosphate (UDP)-glucose flavonoid 3-D-glucosyltransferase (F3GT, sometimes... 

The use of 2-halo-3-(3-substituted neuraminyl derivatives 3 is an efficient approach for overcoming these difficulties [17,18] and, in fact, many attempts have been reported [15,19. However, additional multiple steps for preparation of glycosyl donor and removal of the C-3 substituent to arrive at the desired products are required, even though improvements in the yield and stereospecificity of the glycosides are achieved. 

Therefore, it seems most plausible that the stereospecific generation of the p-aceto-nitrilium ion (d) of Neu5Ac from the oxocarbenium ion (c) holds the key of the re-preponderant formation of sialyl glycosides in acetonitrile. Thus, the reactive nucleophiles, such as OH-6 of 17 have a chance to attack on other intermediates (a) (c) before the complete... 

G. Stork, H. S. Suh, and G. Kim, The temporary silicon connection method in the control ol regio- and stereochemistry. Application to radical-mediated reactions. The stereospecific synthesis of C-glycosides, J. Am Chem. Soc. 7/3 7054 (1991). 

Glycosidation.4 p-D-Glucosides are obtained stereospecifically by reaction of acetobromoglucose with a protected sugar in CH2C12 in the presence of Sn(OTf)2 (1 equiv.) and a base (1 equiv. of collidine, diisopropylethylamine, or 1,1,3,3-tetramethylurea). Addition of 4 A molecular sieves ensures anhydrous conditions. Yields are 30-60%. 

An interesting problem in stereoisomerism is found in the aldol reactions of the achiral aldehydes which are obtained by ozonolysis of the homoallylic alcohols 174. After stereospecific conversion by the FruA [230], the products can be readily induced to form an intramolecular glycoside 175 by acidic (R=OH) or alkaline treatment (R=C1), under which conditions the two equatorial ring hydroxyl groups completely direct the stereogenic acetal formation [234]. The corresponding RhuA catalyzed reactions deliver the enantiomeric... 

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