Nitro-substituted tetrahydroindolizines and homologs: Design, kinetics, and mechanism of α-glucosidase inhibition

A series of 1-[(methylsulfonyl)methyl]-2-nitro-5,6,7,8-tetrahydroindolizines and homologs were designed, prepared, and evaluated as non-sugar-type α-glucosidase inhibitors. The inhibitory activity appeared to be related to cyclo homologation with the best congeners being tetrahydroindolizines. The introduction of a methoxycarbonyl group as an additional hydrogen bond acceptor into the exocyclic methylene group was beneficial affording the most potent congener 3e (half maximal inhibitory concentration, IC50= 8.0 ± 0.1 μM) which displayed 25-fold higher inhibitory activity than 1-deoxynojirimycin (2, IC50= 203 ± 9 μM)—the reference compound. Kinetic analysis indicated that compound 3e is a mixed inhibitor with preference for the free enzyme over the α-glucosidase–substrate complex (Ki,free= 3.6 μM; Ki,bound= 7.6 μM). Molecular docking experiments were in agreement with kinetic results indicating reliable interactions with both the catalytic cleft and other sites. Circular dichroism spectroscopy studies suggested that the inhibition exerted by 3e may involve changes in the secondary structure of the enzyme. Considering the relatively low molecular weight of 3e together with its high fraction of sp3hybridized carbon atoms, this nitro-substituted tetrahydroindolizine may be considered as a good starting point towards new leads in the area of α-glucosidase inhibitors.