Synthesis and Evaluation as Irreversible Glycosidase Inhibitors of Mono- and Oligo(glycosylthio)benzoquinones

The mono(glucosylthio)hydroquinone 2 was prepared by S-glycosidation of 2-mercaptobenzene-1,4-diol and by addition of the acetylated 1-thioglucose 3 to benzo-1,4-quinone (Scheme 1). The second, higher yielding procedure was adopted for the preparation of a range of (glucosylthio)hydroquinones. Addition of 3 to 2-chlorobenzo-1,4-quinone, followed by oxidation gave the 1-thioglucosides 7 and 12 (1.3:1), while addition of HCl to the (glucosylthio)quinone 4 and oxidation gave mainly 12 (Scheme 1). Similarly, the bis(glucosylthio)hydroquinone 33 was obtained from 3 and 4 (Scheme 4), and the (cellobiosylthio)hydroquinone 18 from the thiol 16 and benzo-1,4-quinone (Scheme 2). Addition of the 4-thioglucoside 21 to benzo-1,4-quinone (→ 22 ) and to 4 was followed by oxidation to yield the mono(glucosylthio)quinone 23 and the disubstituted quinones 24 and 25 , respectively (Scheme 3). A mixture 24 / 25 was also obtained from the addition of 3 to 23 . The tris(glucosylthio)hydroquinone 36 was obtained by addition/elimination to the dichloroquinone 29 or the dimesylate 31 , which was prepared in a simplified way (Scheme 4). The tetrakis(glucosylthio)hydroquinone 37 was obtained from 3 and chloranil, followed by reduction. The acylated hydroquinones were deprotected (→ 5, 9, 14, 19, 27, 34 , and 38 ), and oxidized to the corresponding quinones ( 6, 10, 15, 20, 28, 35 , and 40 ). The (glucosylthio)quinones 6, 15, 20, 28 , and 35 were tested as time-dependent inactivators of a retaining β-1,4-glucosidase from Agrobacterium faecalis (Abg), which has a strong exo-glucosidase action (Table 1). Similarly, compounds 20, 28 , and 35 were tested with a cellulase from Cellulomonas fimi (Cex) which degrades cellulose and cellooligosaccharides by hydrolysis of a cellobiose unit from the nonreducing terminus. The most effective inactivators for Abg were 6, 15 , and 35 , which inactivated this enzyme with similar second-order rate constants. (Glycosylthio)quinone 28 was the worst inactivator and did not show normal saturation behaviour. Inactivation of Cex by the (glycosylthio)quinones was 3–500 times slower than that of Abg. The three inactivators 20, 28 , and 35 had approximately the same efficacy with Cex, suggesting that they bind to this enzyme in a similar mode. Further, the K_i values observed are very similar to K_m values measured for aryl cellobiosides, implying that they bind at the active site.