borohydride reduction followed by N-acetylation. Oxidation of the primary alcohol with pyridinium dichromate followed by saponification then gave III.36. Sulfation of the 4-OH on galactosamine as described above, and subsequent catalytic hydrogenation (Pd/C) afforded the target disaccharide III.7.
The 6-O-sulfated derivative III.8 was prepared from disaccharide III.34, where oxidation followed by esterification with diazomethane gave the methyl ester III.37. Reductive N-acetylation, saponification, and sulfation produced derivative III.38, and subsequent hydrogenation afforded the target disaccharide III.8.
More recently, Ogawa and coworkers have prepared 4-O-sulfated chondroitin di- and tetrasaccharides fragments that allow for chain elongation by the condensation of a dimeric repeating unit . The key intermediate used for chain elongation was the ¡(1,4)-disaccharide III.39, which was prepared from monomer building blocks III.40 and III.41 (Scheme 27). The glycosylations were carried out by using the trichloroacetimidate methodology with BF3Et2O as the promoter.
Condensation of III.40 and III.41 afforded the corresponding disaccharide III.42. To aid in purification, the ferf-butyldimethylsilyl group was removed by subsequent treatment of the crude disaccharide with tetrabutylammonium fluoride and acetic acid to give III.43 . Oxidation of C6 to the methyl ester by means of a Swern oxidation  followed by esterification with diazomethane produced the key intermediate III.39 in 96% yield over three steps. Disaccharide III.39 could then be transformed into both glycosyl donor III.44 and glycosyl acceptor III.45 (Scheme 28). Treatment of III.39 with cerium ammonium nitrate gave the corresponding hemiacetal, which was easily converted to the a-imidate III.44 in 95% yield. Conversion of III.39 to III.45 was achieved quantitatively by simple removal of the levulinoyl group with hydrazine acetate . With III.44 and III.45 in hand, gly-cosylation furnished the fully protected tetrasaccharide III.46 in 50% yield.
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