Regioselectivity of enzymatic glycosylation of 6-O-acyl glycosides in supersaturated solutions

The regioselectivity of enzymatic transglycosylation of 6-O-acetyl glycosides in supersaturated solutions was investigated using a range of commercially available enzymes, Escherichia coli, barley, and Kluyveromyces spp. beta-galactosidase, green coffee bean alpha-galactosidase, jack bean alpha-mannosidase, rice alpha-glucosidase, and almond beta-glucosidase. It has been shown that 6-O-acetyl glycosides serve as good substrates for these enzymes, which, under the reaction conditions, are "forced" to transfer monosaccharide units to the secondary hydroxyl groups of the acceptors. In a variety of transglycosylations studied the (1-3)-linked disaccharide products were the predominant regioisomers isolated. The selectivity of the reaction varied significantly depending on the acceptor glycosides and the enzyme used. Exquisite specificity was observed in some cases, but in others approximately equal quantities of two disaccharides products were isolated. In the best transfers the yield approached 30%. The methodology described offers a quick and facile route to disaccharides that may be difficult and/or time consuming to make by conventional chemical synthesis.