High-yield syntheses of ethyl and 2-fluoroethyl beta-D-galactopyranosides using Streptococcus thermophilus beta-galactosidase

Ethyl glycoside synthesis by reaction of ethanol with lactose, catalyzed by beta-galactosidase, and with sucrose, catalyzed by invertase, have been examined. Conditions were optimized to give a product mixture containing the highest possible levels of glycoside. Similar conditions were used to make a product containing the glycoside of 2-fluoroethanol for use as a bait toxin for control of introduced mammalian pest species in New Zealand. The desired high content of glycoside in the crude product was favored by using high acceptor alcohol concentrations and low donor disaccharide concentrations. Invertase gave low yields of beta-fructofuranoside (under 10%) and Aspergillus oryzae beta-galactosidase gave up to 30% beta-galactopyranoside. A beta-galactosidase not previously reported as being used for galactoside synthesis (from Streptococcus thermophilus) was found to be exceptionally efficient. It was stable in 6 M fluoroethanol and gave a galactoside yield of 55% (based oil lactose) from reactions containing 10% w/v lactose and 5 M 2-fluorethanol. Using this enzyme, a crude product containing 35% by weight of fluoroethyl galactoside could be produced.