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 CΓN 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.