Chemical modification of azasugars, inhibitors of N-glycoprotein-processing glycosidases and of HIV-I infection: Review and structure-activity relationships

Expanding horizons of iminosugars as broad-spectrum anti-virals: mechanism, efficacy and novel developments

Iminosugars, a class of polyhydroxylated cyclic alkaloids with intriguing properties, hold promising therapeutic potentials against a broad spectrum of enveloped viruses, including DENV, HCV, HIV, and influenza viruses. Mechanistically, iminosugars act as the competitive inhibitors of host endoplasmic reticular α-glucosidases I and II to  disrupt the proper folding of viral nascent glycoproteins, which thereby exerts antiviral effects. Remarkably, the glycoproteins of many enveloped viruses are significantly more dependent on the calnexin pathway of the protein folding than most host glycoproteins. Therefore, extensive interests and efforts have been devoted to exploit iminosugars as broad-spectrum antiviral agents. This review provides the summary and insights into the recent advancements in the development of novel iminosugars as effective and selective antiviral agents against a variety of enveloped viruses, as well as the understandings of their antiviral mechanisms.

An Amination-Cyclization Cascade Reaction for Iminosugar Synthesis Using Minimal Protecting Groups

The development of a one-step amination-cyclization cascade reaction for the synthesis of N-substituted iminosugars from iodo-pentoses and hexoses is reported. This novel methodology allows for the stereoselective conversion of easily accessible iodo-aldoses and iodo-ketoses into iminosugars in a single step, in highly efficient yields (63-95%), and in aqueous media. Furthermore, the use of functionalized amines allows for the synthesis of N-functionalized iminosugars without additional steps. To illustrate this methodology, a number of biologically important iminosugars were prepared, including 1-deoxynojirimycin, (3S,4R,5S,6R)-azepane-3,4,5,6-tetraol, and N-functionalized 1-deoxymannojirimycins.

Iminosugars as glycosyltransferase inhibitors

Iminosugars are naturally occurring carbohydrate analogues known since 1967. These natural compounds and hundreds of their synthetic derivatives prepared over five decades have been mainly exploited to inhibit the glycosidases, the enzymes catalysing the glycosidic bond cleavage, in order to find new drugs for the treatment of type 2 diabetes and other diseases. However, iminosugars are also inhibitors of glycosyltransferases, the enzymes responsible for the synthesis of oligosaccharides and glycoconjugates. The selective inhibition of specific glycosyltransferases involved in cancer or bacterial infections could lead to innovative therapeutic agents. The synthesis and biological properties of all the iminosugars assayed to date as glycosyltransferase inhibitors are reviewed in the present article.

Poly(2-oxazoline)s with a 2,2'-Iminodiacetate End Group Inhibit and Stabilize Laccase

Poly(2-oxazoline)s (POxs) with 2,2'-iminodiacetate (IDA) end groups were investigated as inhibitors for laccase. The polymers with the IDA end groups are reversible, competitive inhibitors for this enzyme. The IC50 values were found to be in a range of 1-3 mm. Compared with IDA alone, the activity was increased by a factor of more than 30; thus indicating that attaching a polymer chain to an inhibitor can already improve the activity of the former. The enzyme activity drops to practically zero upon increasing the concentration of the most active telechelic inhibitor, IDA-PEtOx30 -IDA (PEtOx: poly(2-ethyl-2-oxazoline)), from 5 to 8 mm. This unusual behavior was investigated by means of dynamic light scattering, which showed specific aggregation above 5 mm. Furthermore, the laccase could be stabilized in the presence of POx-IDA, upon addition at a concentration of 20 mm and higher. Whereas laccase becomes completely inactive at room temperature after one week, the stabilized laccase is fully active for at least a month in aqueous solution.

Iminosugars Spiro-Linked with Morpholine-Fused 1,2,3-Triazole: Synthesis, Conformational Analysis, Glycosidase Inhibitory Activity, Antifungal Assay, and Docking Studies

Synthesis of iminosugars 1, 2, 3a, and 4a and N-alkyl (ethyl, butyl, hexyl, octyl, decyl, and dodecyl) derivatives 3b-g and 4b-g spiro-linked with morpholine-fused 1,2,3-triazole is described. Conformation of the piperidine ring in each spiro-iminosugar was evaluated by ¹H NMR spectroscopy, and conformational change in N-alkylated compounds 4b-g with respect to parent spiro-iminosugar 4a is supported by density functional theory calculations. Out of 16 new spiro-iminosugars, the spiro-iminosugars 3a (IC₅₀ = 0.075 μM) and 4a (IC₅₀ = 0.036 μM) were found to be more potent inhibitors of α-glucosidase than the marketed drug miglitol (IC₅₀ = 0.100 μM). In addition, 3a (minimum inhibition concentration (MIC) = 0.85 μM) and 4a (MIC = 0.025 μM) showed more potent antifungal activity against Candida albicans than antifungal drug amphotericin b (MIC = 1.25 μM). In few cases, the N-alkyl derivatives showed increase of α-glucosidase inhibition and enhancement of antifungal activity compare to the respective parent iminosugar. The biological activities were further substantiated by molecular docking studies.

Synthesis and glycosidase inhibition evaluation of (3S,4S)-3-((R)-1,2-dihydroxyethyl)pyrrolidine-3,4-diol

A new azasugar (3S,4S)-3-((R)-1,2-dihydroxyethyl)pyrrolidine-3,4-diol (1) was obtained from commercially available d-glucose using one-pot reductive cyclization as a key step. The target product, i.e., the iminosugar isomer, was obtained in 10 steps and 24.3% overall yield. Only three column chromatography purifications were needed in this synthesis. The biological activity of the target molecule as glycosidase inhibitor was studied, but the inhibitory activity against four glycosidases was not good (IC₅₀ > 100 μM).

Assessment of partially deoxygenated deoxynojirimycin derivatives as glucosylceramide synthase inhibitors

Glucosylceramide synthase (GCS) is an approved drug target for the treatment of Gaucher disease and is considered as a valid target for combating other human pathologies, including type 2 diabetes. The clinical drug N-butyldeoxynojirimycin (Zavesca) is thought to inhibit through mimicry of its substrate, ceramide. In this work we demonstrate that, in contrast to what is proposed in this model, the C2-hydroxyl of the deoxynojirimycin core is important for GCS inhibition. Here we show that C6-OH appears of less important, which may set guidelines for the development of GCS inhibitors that have less affinity (in comparison with Zavesca) for other glycoprocessing enzymes, in particular those hydrolases that act on glucosylceramide.

Identification of potent and selective glucosylceramide synthase inhibitors from a library of N-alkylated iminosugars

Glucosylceramide synthase (GCS) is an important target for clinical drug development for the treatment of lysosomal storage disorders and a promising target for combating type 2 diabetes. Iminosugars are useful leads for the development of GCS inhibitors; however, the effective iminosugar type GCS inhibitors reported have some unwanted cross-reactivity toward other glyco-processing enzymes. In particular, iminosugar type GCS inhibitors often also inhibit to some extent human acid glucosylceramidase (GBA1) and the nonlysosomal glucosylceramidase (GBA2), the two enzymes known to process glucosylceramide. Of these, GBA1 itself is a potential drug target for the treatment of the lysosomal storage disorder, Gaucher disease, and selective GBA1 inhibitors are sought after as potential chemical chaperones. The physiological importance of GBA2 in glucosylceramide processing in relation to disease states is less clear, and here, selective inhibitors can be of use as chemical knockout entities. In this communication, we report our identification of a highly potent and selective N-alkylated l-ido-configured iminosugar. In particular, the selectivity of 27 for GCS over GBA1 is striking.

Efficient synthesis of Idraparinux, the anticoagulant pentasaccharide

An efficient [DEF+GH] route was developed to the synthesis of Idraparinux, which is a fully O-sulfated, O-methylated mimic of the unique Antithrombin III binding domain of heparin.

Synthesis and NMR experiments of (4,5,6-13C)-deoxymannojirimycin. A new entry to 13C-labeled glycosidase inhibitors

The synthesis of (4,5,6-13C)-deoxymannojirimycin is described. The route employed is based on Sharpless asymmetric epoxidation of (1,2,3-13C)(E)-2,4-pentadien-1-ol and uses ring-closing metathesis as a key step. The labeled compound may be easily used for protein-binding experiments using NMR spectroscopic methods.

Zwitterionic glycosidase inhibitors: salacinol and related analogues

Natural products with interesting biological properties and structural diversity have often served as valuable lead drug candidates for the treatment of human diseases. Salacinol, a naturally occurring alpha-glucosidase inhibitor, was shown to be one of the active principles of the aqueous extract of a medicinal plant that has been prescribed traditionally as an Ayurvedic treatment for type II diabetes. Salacinol contains an intriguing zwitterionic sulfonium-sulfate structure that comprises a 1,4-anhydro-4-thio-D-arabinitol core and a polyhydroxylated acyclic chain. Due to the unique structural features and its potential to become a lead drug candidate in the treatment of type II diabetes, a great deal of attention has been focused on salacinol and its analogues. Since the isolation of salacinol, several papers describing various synthetic routes to salacinol and its analogues have appeared in the literature. This review is aimed at highlighting the synthetic aspects of salacinol and related compounds as well as their structure-activity relationship studies.

Synthesis of S-alkylated sulfonium-ions and their glucosidase inhibitory activities against recombinant human maltase glucoamylase

The syntheses of nine S-alkylated, cyclic sulfonium-ions with varying alkyl chain lengths, as mimics of N-alkylated imino sugars, and their glucosidase inhibitory activities are described. The target compounds were synthesized by alkylation of 2,3,5-tri-O-benzyl-1,4-anhydro-4-thio-d-arabinitol at the ring sulfur atom using various alkyl halides, followed by deprotection using boron trichloride. Enzyme inhibitory assays against recombinant human maltase glucoamylase (MGA), a critical enzyme in the small intestine involved in the breakdown of glucose oligosaccharides into glucose itself, shows that they are effective inhibitors of MGA with K(i) values ranging from 6 to 75 microM.

Polyhydroxylated homoazepanes and 1-deoxy-homonojirimycin analogues: Synthesis and glycosidase inhibition study

The Johnson-Claisen rearrangement of D-gluco and L-ido-derived allylic orthoesters afforded gamma,delta-unsaturated ester that on ester reduction, epoxidation, regioselective oxirane opening by sodium azide and hydrogenation led to sugar amino alcohols--immediate precursors for 1-deoxy-homonojirimycin 3a,b, and polyhydroxylated homoazepanes 4a,b. Our synthetic approach and glycosidase inhibitory activity is reported.

Inhibition of some hepatic glycosidases by the diseco nucleoside, 4-amino-3-(D-glucopentitol-1-yl)-5-mercapto-1,2,4-triazole and its 3-methyl analog

The in vivo and in vitro effects of 4-amino-3-(D-glucopentitol-1-yl)-5-mercapto-1,2,4-triazole and its 3-methyl analogue on alpha- and beta-glucosidases, beta-glucuronidase as well as alpha-amylase have been investigated. alpha-Glucosidase is the enzyme that is markedly affected in vivo and in vitro in a dose-dependent manner. The compounds showed a reversible inhibition of a competitive type for alpha-glucosidase. Moreover, they exert a relatively potent inhibition on alpha-glucosidase with a Ki magnitude of 3.6 x 10(-4), 9.5 x 10(-5) M.

New strategy for the synthesis of iminoglycitols from amino acids

A novel strategy for the enantioselective synthesis of polyhydroxypiperidines, which can be considered as iminoglycitols or 2,6-dideoxyazasugars, was developed. alpha-Benzolsulfonylamino esters served as a C(2)N building block while 2-bromo-3-(bromomethyl)oxazoles and -thiazoles contributed a C3-unit to the final piperidine ring. At first a dihydropyridine ring was established via alkylation and bromine-lithium exchange. The keto group of the resulting 5,6-dihydro[1,3]oxazolo- and 5,6-dihydro[1,3]thiazolo[4,5-c]pyridin-7(4H)-ones was reduced and, after alkylation reactions, the azole ring was cleaved, thus providing heteroatom substituents for the target piperdines. Protected 5-amino-3,4-dihydroxy and 5-amino-3-hydroxy-4-thiohydroxypiperdines were obtained in the talose series while Mitsunobu reaction of the intermiediates provided access to the altrose series.

A Convergent Strategy for the Synthesis of β-Carba-galacto-disaccharides

[reaction in text] A convergent strategy for the synthesis of beta-carba-galacto-disaccharides is illustrated by the preparation of 1 and 4, from a central "glycone" component 22, and the corresponding "aglycone" segments, monosaccharide alcohols, 23a or 23b. The key step is the formation of the carbasugar ring via an oxocarbenium ion-enol ether cyclization.

Syntheses of sugar-related trihydroxyazepanes from simple carbohydrates and their activities as reversible glycosidase inhibitors

Five diastereomeric trideoxy-1,6-iminohexitols were synthesised, and their inhibitory activities were determined against selected glycosidases. For comparison, 1,4,5-trideoxy-1,5-imino-D-lyxo-hexitol, the 4-deoxy derivative of 1-deoxymannojirimicin, was prepared by enzymatic isomerisation of 6-azido-3,6-dideoxy-D-ribo-hexose into the corresponding 2-ulose and subsequent hydrogenation accompanied by intramolecular reductive amination.

Identification of a hexasaccharide sequence able to inhibit thrombin and suitable for 'polymerisation'

Three hexasaccharides, having from low to very high affinity for antithrombin, were synthesised from disaccharide building block precursors. One of them, methyl(sodium 2,3-di-O-methyl-4-O- sodium sulfonato-alpha-L-idopyranosyluronate)-(1-->4)-[(2,3,6-tri-O-sodiu m sulfonato-alpha-D-glucopyranosyl)-(1-->4)-(sodium 2,3-di-O-methyl-alpha-L-idopyranosyluronate)-(1-->4)]2-2,3,6-tri-O-sodiu m sulfonato-alpha-D-glucopyranoside, obtainable from a single disaccharide building block precursor, constitutes a good starting point for obtaining simple oligosaccharidic heparin mimetics able to inhibit the two coagulation factors thrombin and Factor Xa.

Chemoenzymatic synthesis of iminocyclitol derivatives: a useful library strategy for the development of selective fucosyltransfer enzymes inhibitors

A chemoenzymatic strategy has been developed for the synthesis of libraries of iminocyclitol derivatives for the discovery of new and selective fucosidase inhibitors.

An Oxidative Mannich Cyclization Methodology for the Stereocontrolled Synthesis of Highly Functionalized Piperidines

Studies focusing on the development and application of a new oxidative methodology for promoting Mannich cyclizations have been conducted. The general features of these processes were explored with selected alpha-silylamino and alpha-silylamido allyl- and vinylsilanes. Representative conditions for affecting conversion of the alpha-silylamine and -amide functionalities into intermediate N-alkyl and N-acyliminium cations involve either 9,10-dicyanoanthracene SET-sensitized photooxidation or ceric ammonium or tetra-n-butylammonium nitrate oxidations. The applicability of these procedures for promoting Mannich cyclizations was first demonstrated by the preparation of methylidenepiperidines and -hydroazepines. Further studies have led to observations which show that Mannich cyclizations of stereochemically labeled alpha-silylamino vinylsilanes proceed to furnish tetrahydropyridines. Also, unlike their amine analogues, alpha-silylamido (E)-vinylsilanes undergo cyclization to produce tetrahydropyridines with retention of absolute and relative stereochemistry. The differences are due to the fact that N-acyliminium cations serve as intermediates in reactions of the alpha-silylamide systems. Moreover, the oxidation procedure is ideally suited for intermediate N-acyliminium cation generation in stereocontrolled reactions of alpha-silylamido allylsilanes. Finally, the preparative utility of the new cyclization method, when used in conjunction with an alpha-amino acid based strategy for substrate generation, was demonstrated by applications in concise routes for the synthesis of the aza-sugars, (-)-1-deoxymannojirimycin and (+)-1-deoxyallonojirimycin.

Synthesis of alpha-glucosidase I inhibitors showing antiviral (HIV-1) and immunosuppressive activity

The synthesis of a series of analogues of the monosaccharide alpha-glucosidase I inhibitor N-decyl-1-deoxynojirimycin (1) is described. With the incorporation of a single oxygen atom particularly at position seven in the N-decyl side chain, i.e. to give N-7-oxadecyl-dNM (4), the therapeutic ratio (alpha-glucosidase I inhibitory activity over toxicity in HepG2 cells) increases considerably. N-7-Oxadecyl-dNM inhibits purified porcine liver alpha-glucosidase I with an IC50 value of 0.28 microM. The position of the oxygen atom in the N-decyl side chain is of importance since N-3-oxadecyl-dNM is less active and, moreover, is toxic to HepG2 cells at 3 mM. Subsequently, the synthesis of a disaccharide inhibitor of alpha-glucosidase I is described. The aminodisaccharide ManNH2 alpha 1,2Glc (12) inhibits alpha-glucosidase I with an IC50 value of 15.7 microM. Two closely related monosaccharide derivatives of 12 did not inhibit the enzyme at low microM concentrations (no inhibition at 5 microM), showing the additional effect of binding of the aglycon fragment of the molecule to the active site of alpha-glucosidase I. Next, the N-alkyl-dNM derivatives were analysed for antiviral and immunomodulatory activity in-vitro. It is found that the most potent alpha-glucosidase I inhibitor from this study, N-7-oxadecyl-dNM (4) inhibits HIV-1 induced syncytia formation and lymphocyte proliferation in-vitro. Finally, compound 4 was also investigated in-vivo. N-7-Oxadecyl-dNM (4) reduced adjuvant-induced arthritis in rats making this compound a potential candidate for treating autoimmune diseases like rheumatoid arthritis.