Disaccharides, acetates, chromatography

To a solution of the glycosyl donor 13 (29 mg, 0.041 mmol) and the disaccharide donor 28 (16.8 mg 0.021 mmol) in dichloromethane (1.5 mL) was added activated powdered 4 A MS (30 mg). The solution was stirred at room temperature overnight nnder argon, and Cu(OTf)2 (30 mg 0.0826 mmol) was then added. Stirring was continued overnight (12 h), the reaction was quenched with a few drops of pyridine, the mixture was concentrated, and the residue was purified by flash chromatography on silica gel column with hexane-ethyl acetate-dichloromethane (2 I 0.5) to afford the title trisaccharide 29 (18.0 mg 60%). 

When bromide 14, in molar excess, was reacted with the selectively blocked disaccharide 9 in dichloromethane in the presence of mercury(II) cyanide as catalyst and 4A molecular sieve as the acid acceptor, a 90% yield of the fully blocked tetrasaccharide was obtained after column chromatography. Removal of blocking groups was accomplished by the hydrogenolysls of the benzyl ether and benzylidene acetal groups In acetic acid, followed by transesterification. The deblocked tetrasaccharide 19 had NMR parameters in agreement with its structure. This could be confirmed by comparison with chemical shift data for the... 

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