Synthesis of 2‐Azabicyclo[3.2.2]nonane‐Derived Monosaccharide Mimics and Their Evaluation as Glycosidase Inhibitors

Abstract

The racemic 2-azabicyclo[3.2.2]nonanes 5 and 18 were synthesized and tested as b-glycosidase inhibitors. The intramolecular Diels-Alder reaction of the masked o-benzoquinone generated from 2-(allyloxy)phenol (6) gave the a-keto acetal 7 which was reduced with SmI 2 to the hydroxy ketone 8. Dihydroxylation, isopropylidenation (! 12), and Beckmann rearrangement provided lactam 15. N-Benzylation of this lactam, reduction to the amine 17, and deprotection provided the amino triol 19 which was debenzylated to the secondary amine 5. Both 5 and 19 proved weak inhibitors of snail b-mannosidase (IC 50 > 10 mM), Caldocellum saccharolyticum b-glucosidase (IC 50 > 10 mM), sweet almond b-glucosidase (IC 50 > 10 mM), yeast a-glucosidase (5: IC 50 > 10 mM; 19: IC 50 = 1.2 mM), and Jack bean a-mannosidase (no inhibition detected).Introduction. -Together with the crystal-structure analysis of glycosidase-inhibitor complexes [1] and the use of isotopically labelled compounds [2], the synthesis and evaluation of inhibitors contribute to elucidating the mechanism of action of the enzymatic hydrolysis of glycosides, as summarized in several reviews [3]. For example, the determination and interpretation of inhibition constants and kinetics of conformationally biased (more or less close) mimics of the transition state are a valuable source of information about the stereoelectronically required [4] conformational change of the substrate imposed by b-glycosidases [5]. Thus, the isoquinuclidine 1 [6], mimicking the 1,4 B conformation of a D-mannopyranoside, inhibits snail b-mannosidase strongly (K i = 1 mM) and selectively, while the corresponding gluco-configured diastereoisomer is inactive against b-glucosidases, evidencing a different conformational itinerary. The norbornane 2 and its 7-oxa analogues 3 and 4 [7], possessing a shorter bridge between the centers corresponding to C(1) and C(4), and mimicking a 1,4 B conformation more closely than the isoquinuclidine 1 differ considerably in the location and orientation of the N-atom, and are rather weak inhibitors of snail b-mannosidase. In this context, 2-azabicyclo[3.2.2]nonanes ('homoisoquinuclidines') such as 5 appeared of interest, as they possess a longer bridge ensuring a boat-like conformation of the cyclohexane ring mimicking the pyranosyl (glycon) moiety. We anticipated differences to 1 in the orientation of the CÀN bond and in the location of the basic N-atom ('the glycosidic heteroatom') that is to interact with the catalytic acid. An evaluation of the inhibition of b-glycosidases by such amines may allow to more precisely assess the optimal pre-transition state conformation of this type of glycosidase inhibitors.

Andrea Vasella and the Active Site of Glycoside Hydrolases

“…From 1986 to 2000 we are informed in detail about numerous syntheses and inhibition constants -nothing changed in the following years, until the last seven papers [74][75][76][77][78][79][80] on the subject from our own stable were published in 2006. As far as the eye of a very interested reader can see, the wealth of multi-step syntheses and inhibition constants available to read and admire made for a more than decent collection of chemical literature.…”