Preparation of disaccharides

Indeed, a variety of Lewis acids have been shown to effed glycosylation with hemiacetal donors. Ernst and coworkers have used 5mol% of [Rh(III)(MeCN)3 (triphos)] tris(triflate) with 4 A molecular sieves to prepare glycoconjugates 18 and 19 [26]. Mukaiyama s group has used trityl tetrakis(pentafluorophenyl)borate (3-5 mol%) with Drierite in the preparation of disaccharides 20 and 21 [27,28]. In the synthesis of 21, the a-seledivity was shown to arise from in situ anomerization of the (1-pyranoside over time. 

The Henry reaction of nitro sugars and sugar aldehydes is a powerful tool for the formation of C-glycosydic bonds. A recent example is the preparation of disaccharide precursor 60 stated in Scheme 21.45 It involves a Henry reaction of nitro sugar 58 and aldehyde 59 in acetonitrile in the presence of catalytic potassium fluoride, to give nitro alcohol 60, a precursor of the corresponding C-disaccharide. 

The nature of the protecting groups employed in the preparation of disaccharide discussed above does not permit the extension of the synthesis to the branched type of KDO trisaccharide shown in Fig. 1. To obtain another model compound for the structure elucidation of the natural KDO trisaccharide by spectroscopic and immunochemical methods, a route was devised which would permit the synthesis of the model trisaccharide 12. 

Compound 6-Pre closely resembles the intermediates used by Kusumoto jet al. ([this volume) and by van Boeckel et al. (16) for the preparation of disaccharide oxazolines, which were then elaborated into fully phosphorylated analogs (1,4 -diphosphates). In a formal sense, therefore, the synthesis of 6-Pw effectively constitutes a synthesis, by our revised route, or the 1,4 -bisphosphate having palmitoyl groups at positions 2, 3, 2, and 6. An additional palmitoyl group could be attached at position 4 if desired. 

The intramolecular mercury-induced cyclization of enol ethers has proven useful for the dia-stereoselective preparation of disaccharides. The reaction is carried out by treating the enol ethers (e.g., 7) with mercury(ll) trifluoroacetate in tetrahydrofuran for 2 hours at 0 °C, followed by reaction with aqueous potassium chloride at 0 °C for 1 hour. The chloromercury derivatives are obtained in good yield and only one isomer can be detected in the reaction mixture95-96. 

The preparation of disaccharides, from benzyl-protected DISAL donors (e. g. 367,0 Scheme 61), was best carried out in l-methylpyrrolidin-2-one (NMP), a high polar, aprotic solvent. 

J. Alais and S. David, Preparation of disaccharides having a /(-n-mannopyranosyl group from /V-phthaloyllactosamme derivatives by double or triple SN2 substitution, Carbohydr. Res., 201 (1990) 69-77. 

Mannose is converted to methyl /3-D-mannoside with methyl sulfate in an alkaline medium. This coupling is the opposite of all those we have considered thus far the bridge oxygen comes from the glycosyl donor. This type of reaction has been used in the preparation of disaccharides, with the use of sugar triflate as electrophile (Schmidt 1986). 

The preparation of disaccharide 11.39 is outlined in Scheme 13. Condensation of 11.42 with 11.43 using TMSOTf as a promoter afforded disaccharide 11.47 in 81% yield. Treatment with aqueous TFA followed by acetylation replaced the isopropylidene moiety with two acetate groups, and subsequent de-levulinoylation with hydrazine acetate gave the primary alcohol 11.48. A Swem oxidation of the primary... 

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