Homonojirimycin Isomers and N-Alkylated Homonojirimycins:  Structural and Conformational Basis of Inhibition of Glycosidases

A series of natural epimers of α-homonojirimycin and its N-alkylated derivatives have been prepared to investigate the contribution of the different chiral centers and conformation of the specificity and potency of inhibition of glycosidases. These epimers and N-alkylated derivatives are α-homonojirimycin (1), β-homonojirimycin (2), α-homomannojirimycin (3), β-homomannojirimycin (4), α-3,4-di-epi-homonojirimycin (5), β-4,5-di-epi-homonojirimycin (6), N-methyl-α-homonojirimycin (7), and N-butyl-α-homonojirimycin (8). Compound 1 was a potent inhibitor of a range of α-glucosidases with IC50 values of 1 to 0.01 μM. Compounds 2, 3, and 4 were surprisingly inactive as inhibitors of β-glucosidase and α- and β-mannosidases but were moderately good as inhibitors of rice and some mammalian α-glucosidases. Compound 4 was active in the micromolar range toward all α-glucosidases tested. Furthermore, compound 4, which superimposes well on β-l-fucose, was a 10-fold more effective inhibitor of α-l-fucosidase than 1-deoxymannojirimycin (12) and 3, with a K i value of 0.45 μM. Only compounds 5 and 6 showed inhibitory activity toward α- and β-galactosidases (6 with an IC50 value of 6.4 μM against α-galactosidase). The high-resolution structure of 1 has been determined by X-ray diffraction and showed a chair conformation with the C1 OH (corresponding to the C6 OH in 1-deoxynojirimycin) predominantly equatorial to the piperidine ring in the crystal structure. This preferred (C1 OH equatorial) conformation was also corroborated by 1H NMR coupling constants. The coupling constants for 7 suggest the axial orientation of the C1 OH, while in 8 the C1 OH axial conformation was not observed. The C1 OH axial conformation appears to be responsible for more potent inhibition toward processing α-glucosidase I than α-glucosidase II. It has been assumed that the anti-HIV activity of alkaloidal glycosidase inhibitors results from the inhibition of processing α-glucosidase I, but 1, 7, and 8 were inactive against HIV-1 replication at 500 μg/mL as measured by inhibition of virus-induced cytopathogenicity in MT-4 cells. In contrast, the EC50 value for N-butyl-1-deoxynojirimycin (11), which also inhibits processing α-glucosidase I, was 37 μg/mL. Compound 7 has been shown to be a better inhibitor of α-glucosidase I than 1 and 8 both in vitro and in the cell culture system. These data imply that inhibition of HIV by glycosidase inhibitors can be due to factors other than simply inhibition of proc