Synthesis and evaluation as glycosidase inhibitors of isoquinuclidines mimicking a distorted β-mannopyranoside

Racemic and enantiomerically pure manno-configured isoquinuclidines were synthesized and tested as glycosidase inhibitors. The racemic key isoquinuclidine intermediate was prepared in high yield by a cycloaddition (tandem Michael addition/aldolisation) of the 3-hydroxy-1-tosyl-pyridone 10 to methyl acrylate, and transformed to the racemic N-benzyl manno-isoquinuclidine 2 and the N-unsubstituted manno-isoquinuclidine 3 (twelve steps; ca. 11% from 10). Catalysis by quinine of the analogous cycloaddition of 10 to (-)-8-phenylmenthyl acrylate provided a single diastereoisomer in high yield, which was transformed to the desired enantiomerically pure D-manno-isoquinuclidines (+)-2 and (+)-3 (twelve steps; 23% from 10). The enantiomers (-)-2 and (-)-3 were prepared by using a quinidine-promoted cycloaddition of 10 to the enantiomeric (+)-8-phenylmenthyl acrylate. The N-benzyl D-manno-isoquinuclidine (+)-2 is a selective and slow inhibitor of snail beta-mannosidase. Its inhibition strength and type depends on the pH (at pH 4.5: K-i = 1.0 mum, mixed type, a = 1.9; at pH 5.5: K-i = 0.63 mum, mixed type, a = 17). The N-unsubstituted D-manno-isoquinuclidine (+)-3 is a poor inhibitor. Its inhibition strength and type also depend on the pH (at pH 4.5: K-i = 1.2 . 10(3) mum, mixed type, a = 1.1; at pH 5.5: K-i = 0.25 . 10(3) mum, mixed type, a = 11). The enantiomeric N-benzyl L-manno-isoquinuclidine (-)-2 is a good inhibitor of snail beta-mannosidase, albeit noncompetitive (at pH 4.5: K-i = 69 mum). The N-unsubstituted isoquinuclidine (-)-2 is a poor inhibitor (at pH 4.5: IC50 = 7.3 . 10(3) mum). A comparison of the inhibition by the pure D-manno-isoquinuclidines (+)-2 and (+)-3, (+)-2/(-)-2 1: 1, and (+)-3/(-)-3 1:1 with the published data for racemic 2 and 3 led to a rectification of the published data. The inhibition of snail beta-mannosidase by the isoquinuclidines 2 and 3 suggests that the hydrolysis of beta-D-mannopyranosides by snail beta-mannosidase proceeds via a distorted conformer, in agreement with the principle of stereoelectronic control.