A-Linked disaccharides

Halide 32 was used first, in nitromethane-benzene solution in the presence of an equimolar proportion of mercuric cyanide. It was found83 that 32 gave a disaccharide at 0-6 of a protected derivative of 2-acetamido-2-deoxy-D-glucose in good yield, with a certain degree of stereoselectivity (a /3 ratio 7 3). Complete stereospecificity was attained, leading to the desired a-linked disaccharides, when the partially acylated bromide 33 was condensed with a number of different nucleophiles.66,83,84,93-95 A mechanism was postulated83 to explain the... 

The same procedure described in the foregoing was followed to give the a-linked disaccharide 9 and the p-anomer, which could not be separated by chromatography on silica gel... 

The reaction produces a stereoselective synthesis of a- or (S-2-deoxyglycosides. a-Linked disaccharides such as 1 can be prepared by a three-step sequence with high stereocontrol. 

In early work [165] on the synthesis of the pentasaccharide (236), the azide (237) was condensed with (238) [an intermediate in the preparation of (237)]in the presence of silver perchlorate and polyvinylpyridine to give the a-linked disaccharide (239) in 60 % yield and this on acetolysis gave the disaccharide (240) which contains the potential terminal disaccharide unit of the Forssman antigen. Compound (240) was converted into the glycosyl bromide with titanium(IV) bromide under carefully controlled conditions [182] and condensed with l,6-anhydro-2,4-di-0-benzyl-D-galacto-pyranose in the presence of silver carbonate to give the potential terminal trisaccharide (241) of the Forssman antigen. 

An acetal tethered compound can easily be prepared by treatment of equimolar amounts of a 2-propenyl ether derivative of a saccharide with a sugar hydroxyl in the presence of a catalytic amount of acid. Activation of the anomeric thio moiety of the tethered compound with N-iodosuccinimide (NIS) in dichloromethane results in the formation of the p-linked disaccharide. In this reaction, no a-linked disaccharide is usually detected. It is of interest to note that when this reaction was performed in the presence of methanol, no methyl glycosides are obtained. This experiment indicates that the glycosylation proceeds via a concerted reaction and not a free anomeric oxocarbenium ion. 

Thus, reaction of the perbenzylated DISAL glycoside 176 with dusopropylidene-a-D-glucofur-anose 177 in iV-methyl-pyrrolidone at 60°C, without any added promoter, afforded the corresponding a-linked disaccharide 178 [376] (Scheme 4.33). 

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. 

Reddy, G V, Kulkami, V R, Mereyala, H B, A mild general method for the synthesis of a-linked disaccharides. Tetrahedron Lett., 30, 4283-4286, 1989. 

Schmidt, R R, Grundler, G, Glycosylimidates, 6. a-Linked disaccharides from 0-((3-D-glycopyranosyl) trichloroacetimidates using trimethylsilyl trifluoromethanesulfonate as catalyst, Angew. Chem. Int. Ed. Engl., 21, 781-782, 1982. 

Johnston, B D, Pinto, B M, Synthesis of 1,2- and 1,3-A-linked disaccharides of 5-thio-a-D-mannopyranose as potential inhibitors of the processing mannosidase class I and mannosidase II enzymes, J. Org. Chem., 63, 5797-5800, 1998. 

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