Compound Distribution Depends on Reaction Conditions

On the basis of the early results described above, random glycosylation as an approach to oligosaccharide library preparation has been further pursued. The product distributions in these early reactions were difficult to determine, and in subsequent work, the aglycon of the acceptor was changed to the p-methoxyphenoxy octyl 

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. 

Using 11 as the donor at either 20 or 50°C (entries 4 and 5), all six possible disaccharides were formed the a(l— 6) anomer 15 was the least favored. A similar 

The results in Table 12.1 show that the anomeric configuration of the products of random galactosylation can be biased toward the a-glycosides by use of 2 or 10 as donor, or all of the products can be obtained in a single run using 11 or 12 as donor. While all disaccharides are clearly present in significant amounts in the product, they are not formed in equal amounts, and it is hoped that further studies on galactosyl and other donors will lead to an improvement in the product distributions. 

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