Synthesis of Fused Triazoles as Probes for the Active Site of Retaining β‐Glycosidases: From Which Direction Is the Glycoside Protonated?

The structure of the β‐glycosidase inhibitors 1–7 and 10–13 suggests that protonation of O –C(1) (the glycosidic O‐center) of the substrate by a carboxy group of the retaining β‐glycosidases does not occur in the plane perpendicular to the ring of the glycon (β‐side; ‘from the top’), but in the plane of the ring (‘from the side’). The triazoles 17 and 18 have been prepared in six steps from the L ‐xylofuranose 21 . They possess a CH group instead of the N‐center of the related tetrazoles 4 and 5 , corresponding to the glycosidic O‐atom, and a very similar structure, both in solution and in the solid state. Unlike the tetrazoles, however, which are good‐to‐medium inhibitors of retaining β‐glycosidases, the triazoles do not inhibit the β‐glycosidases from sweet almonds, snail, and bovine liver, and only slightly inhibit the β‐glucosidase from Caldocellum saccharolyticum . This is in keeping with the proposed direction of protonation in the plane of the saccharide ring and with modelling studies, docking 4 into the active site of the white clover cyanogenic β‐glucosidase and 6 into the E. coli β‐galactosidase and the Lactococcus lactis 6‐phospho‐β‐galactosidase.