Synthesis, conformational analysis and glycosidase inhibition of bicyclic nojirimycin C-glycosides based on an octahydrofuro[3,2-b]pyridine motif

A set of bicyclic iminosugar C-glycosides, based on an octahydrofuro[3,2-b]pyridine motif, has been synthesized from a C-allyl iminosugar exploiting a debenzylative iodocycloetherification and an iodine nucleophilic displacement as the key steps. The halogen allowed the introduction of a range of aglycon moieties of different sizes bearing several functionalities such as alcohol, amine, amide and triazole. In these carbohydrate mimics the fused THF ring forces the piperidine to adopt a flattened ⁴C₁ conformation according to NMR and DFT calculations studies. In their deprotected form, these bicycles were assayed on a panel of 23 glycosidases. The iminosugars displaying hydrophobic aglycon moieties proved to be superior glycosidase inhibitors, leading to a low micromolar inhibition of human lysosome β-glucosidase (compound 11; IC₅₀ = 2.7 μM) and rice α-glucosidase (compound 10; IC₅₀ = 7.7 μM). Finally, the loose structural analogy of these derivatives with Thiamet G, a potent OGA bicyclic inhibitor, was illustrated by the weak OGA inhibitory activity (Ki = 140 μM) of iminosugar 5.