D-glucosylated derivatives of isofagomine and noeuromycin and their potential as inhibitors of beta-glycoside hydrolases

While isofagomine and noeuromycin have previously been demonstrated to be effective inhibitors of a range of exo-acting glycosidases, they are usually only very weak inhibitors of endo-glycosidases. However, the disaccharide-like 3- and 4-O-beta-D-glucopyranosylisofagomines have proven to be strong inhibitors of these endo-acting enzymes that utilize multiple sub-sites. In an attempt to emulate these successes, we have prepared 3- and 4-O-beta-D-glucopyranosylnoeuromycin, the former by a selective glycosylation (at O2) of benzyl 4-C-cyano-4-deoxy-beta-D-arabinoside (also leading to another synthesis of 3-O-beta-D-glucopyranosylisofagomine), the latter by a non-selective glycosylation of benzyl 4-O-allyl-beta- xyloside with subsequent introduction of the required nitrile group (also leading to another synthesis of 4-O-beta-D-glucopyranosylisofagomine). 3-O-beta-D-Glucopyranosylnoeuromycin was evaluated as an inhibitor of a family 26 lichenase from Clostridium thermocellum, and 4-O-beta-D-glucopyranosylnoeuromycin as an inhibitor of both a family 5 endoglucanase from Bacillus agaradhaerans and a family 10 endo-xylanase from Cellulomonas fimi. We also report X-ray structural investigations of 3- and 4-O-beta-D-glucopyranosylnoeuromycin in complex with the family 26 and family 5 beta-glycoside hydrolases, respectively. The two d-glucosylated noeuromycins were indeed able to harness the additional binding energy from the sub-sites of their endo-glycoside hydrolase targets, and were thus excellent inhibitors (in the nanomolar range), binding as expected in the -1 and -2 sub-sites of the enzymes.