Glycosidase Reversible Inhibitors

Glycals, such as D-galactal (1,2-dideoxy-D-/yxo-hex-1 -enopyranose) (IV) (58), in part fit the requirements of an oxocarbonium transition state since the structure 

Glycals have been successfully used to tag active site amino acids. Denatur-ation of Aspergillus wentii )3-glucosidase Aj in the presence of D-glucal traps the hydrated product bound as an aspartate ester (62). The residue is apparently the same /8-glucosidase aspartic acid labeled with the site-directed irreversible inhibitor conduritol B epoxide (63, 64). 

An analogous E. coli j3-galactosidase-galactal complex has been trapped in a reaction with radiolabeled D-galactal. The lability of the complex, however, prevented identification of the associated amino acid (65), although, as with 

wentii /3-glucosidase A3, the carboxyl of /3-galactosidase Glu-461, labeled with conduritol C m-epoxide, is a reasonable candidate (66. 67). 

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The term pseudosubstrate as used in this article will comprise sugar-related compounds that are chemically transformed by glycosidases, often forming long-lived intermediates and thereby acting as reversible inhibitors. Even in cases of weak inhibition, where the intermediate is too short-lived for chemical or physical characterization, the type of reaction catalyzed by the... 

C-Disaccharides have become important target molecules because they are potential reversible inhibitors of glycosidases and disaccharidases [69]. Some of the important representatives of this class of derivatives are listed in O Scheme 4. 

Previously, tin-ketyl radicals have been added to alkenes only in an intramolecular fashion. [9] In recent publications, however, pinacols and amino alcohols have been prepared by cyclisation of dicarbonyl compounds [10] or keto-oximes [11] with tributyltin hydride. Cyclisation of 1,5-ketoaldehydes 1 and 1,5-dialdehydes with tributyltin hydride yields cw-diols 2 with excellent stereoselectivities, whereas the keto-oxime 4 with four benzyloxy-substituents affords a 58 42 cis trans) mixture. The tran -product was transformed in two more steps to the potent glycosidase inhibitor 1-deoxynojirimycin (6). [lib] The reversibility of both the addition of the tributyltin-radical to the carbonyl group and the intramolecular radical C-C bond formation is believed to be responsible for the high selectivity in the formation of 2. In the cyclisation of 1,5-pentanedial the unhydrolyzed coupling product 3 could be isolated, therefore providing evidence for a new mechanistic variant of the pinacol reaction, in which only 1.2 equivalent of the reducing agent are necessary. 

The principle of transition state affinity requires the development during enzyme catalysis of very powerful forces of attraction between the enzyme and the altered form of the substrate that is present in the transition state. > This principle has served as a basis for the synthesis of some very strong reversible enzyme inhibitors/" and may also in principle be useful in the design of affinity labeling reagents. The latter possibility remains to be explored, but seems already to be partly substantiated by a few serendipitous examples of inhibitors of glycosidases and proteases. 

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