Galactosylation reactions

In the synthesis of A-acetyllactosamin from lactose and A-acetylglucosamine with (3-galactosidase (289,290), the addition of 25 vol% of the water-miscible ionic liquid [MMIM][MeS04] to an aqueous system was found to effectively suppress the side reaction of secondary hydrolysis of the desired product. As a result, the product yield was increased from 30 to 60%. Product separation was improved, and the reuse of the enzymatic catalyst became possible. A kinetics investigation showed that the enzyme activity was not influenced by the presence of the ionic liquids. The enzyme was stable under the conditions employed, allowing its repeated use after filtration with a commercially available ultrafiltration membrane. 

When the water-miscible ionic liquid [MMIM][MeS04] was used as a neat medium for the enzymatic transformations, however, poorer performance was observed. For the kinetic resolution of mc-l-phenylethanol by transesterification with vinyl acetate with a set of different lipases dispersed in the pure ionic liquid, it was found that [MMIM][MeS04] was among the poorest media for the enzymes (291). It has been recognized that some water-miscible ionic liquids in the pure form are denaturants (27), but, when they are used in the presence of excess water, their tendency to 

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For the galactosylation reactions, 2,3,4,6-tetra-O-benzylgalactopyranosyl trichloroacetimidate 2 and dibenzylphosphite 10 were used as donors (Table 12.1). Under these conditions, the (5-(l—>3)- and (5-(l—>4)-linked disaccharides 16 and 17 were formed in only minor amounts (entries 1 and 2). To increase the proportion of the (5-anomers, the peracetylated galactopyranosyl trichloroacetimidate 11 was used as the donor. Reactions were poor in DMF, but in dioxane, a clean mixture of products was obtained. Two equivalents of donor were required to give a conversion of 20-30% of 1 to disaccharides 13-18. Peaks eluting when the column was washed with more polar solvent mixtures suggested that only traces of trisaccharides were formed, but the identity of these peaks was not investigated. 

TABLE 12.1 Use of 2,3,4,6-Tetra-O-benzylgalactopyranosyl Trichloroacetimidate and Dibenzylphosphite in Galactosylation Reactions... 

Quite recently, it has been established that silver triflate can be used as a promoter for direct galactosylation reactions . Contrary to expectations, many primary hydroxy groups are completely unreactive in glycosylation reactions, or give the desired glycosides... 

Galactosylation Reactions. For continuous galactosylation of ovalbumin (10 mg/mL), 74 mU of the Con A-immobilized... 

In most cases, the a-anomer of mono-galactosylated ceramides can be easily separated from the p-one by colunm chromatography. In addition, galactosylation reaction of a ceramide proceeds kinetically with high a-selectivity, and... 

In vitro activity of UDGT - The optimal conditions for the in vitro activity of UDGT in crude membranes were pH 7.2 and 20°C. The galactosylation reaction was linear with time for the first 20 min. The UDGT activity was saturated with 1 mM UDP galactose. Other cofactors such as magnesium and DTT were required. Addition of 0.1 M Na Acetate increased the measured in vitro activity and preserved the activity for several days. No specificity for a certain DAG molecular species (i.e. 16 /16 0, 18 0/18 0 or 18 1/18 1) was observed, and the synthesis rate of MGDG was apparently independent of the concentration of external DAG in the assay. 

Reaction of glycosylmethylamines with aryldiazonium salts gives a class of compounds which, by acid catalysis or unknown factors of enzymic catalysis, generate glycosylmethyldiazonium ions. These, in turn, lose nitrogen, to yield highly electrophilic carbenium ions, as illustrated for the ) -d-galactosyl derivative 38 (see Scheme 8). 

I. E. Baciu, H. J. Jordening, J. Seibel, and K. Buchholz, Investigations of the transfructosylation reaction by fructosyltransferase from B. subtilis NCIMB 11871 for the synthesis of the sucrose analogue galactosyl-fructoside, J. Biotechnol., 116 (2005) 347-357. 

As evidence for reaction (1), 1 mole of D-glucosyl phosphate was formed per mole of UDP-glucose and D-galactosyl phosphate consumed when the last two compounds were incubated with an extract of Saccharomyces fragilis.13,w Experiments188 with C14-labeled D-glucosyl phosphate also favor the mechanism represented by reactions (1) and (2). 

D-Lyxose yielded a D-lyxosyl chloride 2,3,4-tri(chlorosulfate) which, on treatment with chloride ion, led to a dichlorodideoxy compound, most probably 2,4-dichloro-2,4-dideoxy-L-arabinose.353 D-Glucose gave a compound presumed to be 4,6-dichloro-4,6-dideoxy-a,/3-D-galactosyl chloride 2,3-di(chlorosulfate),360 and D-xylose afforded a monochloromonodeoxy derivative formulated, on indirect evidence, as 4-chloro-4-deoxy-L-arabinopyranosyl chloride 2,3-di(chlorosulfate).360 3,4-Dichloro-3,4-dideoxy-/3-D-ribopyranosyl chloride 2-(chlorosulfate) was the major, and 4-chloro-4-deoxy-a-D-xylopyranosyl chloride 2,3-di(chlorosulfate) the minor, product from the reaction of L-arabinose with sulfuryl chloride at room temperature for 24 hours.357,361 It has been established that, on reaction with sulfuryl chloride at low temperature, crystalline a-D-xylopyranose and /3-D-lyxopyranose afford, respectively, the 2,4,6-tri(chlorosul-fate)s of /3-D-xylopyranosyl chloride and a-D-lyxopyranosyl chloride,362 363 confirming that substitution at C-l occurs by an Sn2 process on a l-(chlorosulfuric) ester intermediate. 

Structure 34 was proposed for a trisaccharide composed of d-glucose, D-galactose, and 2-acetamido-2-deoxy-D-galactose residues. The D-glucosyl group and the D-galactosyl residues are the same as in disaccharide 32. That the 2-acetamido-2-deoxy-D-galactose residue is linked to 0-3 was indicated by color reactions and the fact that this... 

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