Synthetic Iminosugar Derivatives as New Potential Immunosuppressive Agents

Flow Chemistry for Synthesis of 2-(C-Glycosyl)acetates from Pyranoses via Tandem Wittig and Michael Reactions

C-Glycosyl compounds (C-glycosides) are a class of saccharide derivatives with improved stability over their O-linked counterparts. This paper reports the synthesis of several trans-2-(C-glycosyl)acetates via a tandem Wittig-Michael reaction from pyranoses (cyclic hemiacetals) using continuous flow processing, which gave improvements compared to reactions conducted in round-bottom flasks. Products were isolated in yields of >60% from reactions of benzyl-protected xylopyranoses, glucopyranoses, and galactopyranoses at higher temperatures and pressures, which were superior to yields from batch procedures. A two-step procedure involving the Wittig reaction followed by Michael reaction (intramolecular oxa-Michael) of the unsaturated ester obtained in the presence of DBU was developed. Reactions of protected mannopyranose gave low yields in corresponding reactions in flow due to competing C-2 epimerization.

Evaluation of nonnatural L-iminosugar C,C-glycosides, a new class of C-branched iminosugars, as glycosidase inhibitors

Capitalizing on a previously developed Staudinger/azaWittig/Grignard (SAWG)-ring contraction sequence that furnished protected six-membered L-iminosugar C,C-glycosides bearing an allyl group and various substituents at the pseudoanomeric position, the synthesis and glycosidase inhibition of a small library of six- and seven-membered L-iminosugar C,C-glycosides is reported. Their hydrogenolysis or cyclization by RCM followed by deprotection afforded eleven L-iminosugars including spirocyclic derivatives. All compounds adopt a 1C4 conformation in solution according to NMR data. Compared to previously reported branched L-iminosugars, the L-iminosugar C,C-glycosides reported herein were less potent glycosidase inhibitors. However, some of these compounds showed micromolar inhibition of human lysosome β-glucocerebrosidase suggesting that such iminosugars could be useful to access potent CGase inhibitors by adjusting the structure/length of the pseudoanomeric substituents.

Multivalent Pyrrolidine Iminosugars: Synthesis and Biological Relevance

Recently, the strategy of multivalency has been widely employed to design glycosidase inhibitors, as glycomimetic clusters often induce marked enzyme inhibition relative to monovalent analogs. Polyhydroxylated pyrrolidines, one of the most studied classes of iminosugars, are an attractive moiety due to their potent and specific inhibition of glycosidases and glycosyltransferases, which are associated with many crucial biological processes. The development of multivalent pyrrolidine derivatives as glycosidase inhibitors has resulted in several promising compounds that stand out. Herein, we comprehensively summarized the different synthetic approaches to the preparation of multivalent pyrrolidine clusters, from total synthesis of divalent iminosugars to complex architectures bearing twelve pyrrolidine motifs. Enzyme inhibitory properties and multivalent effects of these synthesized iminosugars were further discussed, especially for some less studied therapeutically relevant enzymes. We envision that this comprehensive review will help extend the applications of multivalent pyrrolidine iminosugars in future studies.

Design, synthesis, and preliminary immunopotentiating activity of new analogues of nojirimycin

Three new classes of nojirimycin analogues viz. N-alkyl with C1-substituent (4-phenylbutyl), N-substituted 1-deoxynojirimycin and its congener δ-lactam, and a 4-phenylbutyl-β-C-glycoside were designed and synthesized for immunological studies. The resulting diverse compound library exhibited proliferation of B Cells and T cells induced by LPS and Con A, respectively. The majority of the analogues augmented the secretion of IL-12 in dendritic cells and TNF-α secretion in murine peritoneal macrophages compared to LPS (10 μg/ml). A deoxynojirimycin-triazole conjugate of phytosphingosine analogue was superior in the responses mentioned above and exhibited nitric oxide response equal to LPS. In comparison to findings on its congeners with immunosuppressive action, early immunological tests show that the novel nojirimycin analogues have immunopotentiating effect. Hence, nojirimycin analogues offer tremendous potential in tuning the immunomodulatory activity of iminosugars by subtle to substantial structural variations.

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.

Synthesis and Therapeutic Applications of Iminosugars in Cystic Fibrosis

Iminosugars are sugar analogues endowed with a high pharmacological potential. The wide range of biological activities exhibited by these glycomimetics associated with their excellent drug profile make them attractive therapeutic candidates for several medical interventions. The ability of iminosugars to act as inhibitors or enhancers of carbohydrate-processing enzymes suggests their potential use as therapeutics for the treatment of cystic fibrosis (CF). Herein we review the most relevant advances in the field, paying attention to both the chemical synthesis of the iminosugars and their biological evaluations, resulting from in vitro and in vivo assays. Starting from the example of the marketed drug NBDNJ (N-butyl deoxynojirimycin), a variety of iminosugars have exhibited the capacity to rescue the trafficking of F508del-CFTR (deletion of F508 residue in the CF transmembrane conductance regulator), either alone or in combination with other correctors. Interesting results have also been obtained when iminosugars were considered as anti-inflammatory agents in CF lung disease. The data herein reported demonstrate that iminosugars hold considerable potential to be applied for both therapeutic purposes.

Synthesis of N-Substituted Iminosugar Derivatives and Evaluation of Their Immunosuppressive Activities

It is important to find more effective and safer immunosuppressants, because clinically used immunosuppressive agents have significant side effects. A series of N-substituted iminosugar derivatives were designed and synthesized, and their immunosuppressive effects were evaluated by the CCK-8 assay. The results revealed that iminosugars 10 e and 10 i, that is, (3R,4S)-1-(4-heptyloxylphenylethyl)pyrrolidine-3,4-diol and (3R,4S)-1-[2-(2-chloro-4-(p-tolylthio)-phenyl-1-yl)ethyl]pyrrolidine-3,4-diol, respectively, exhibited the strongest inhibitory effects on mouse splenocyte proliferation (IC₅₀ =2.16 and 2.48 μm, respectively), whereas the iminosugars containing an amide group near the hydrophilic head (compounds 10 j-n) exhibited no inhibitory effects. Further studies revealed that the inhibitory effects on splenocyte proliferation may have come from the suppression of both IFN-γ and IL-4 cytokines. Our results suggest that synthetic iminosugars, especially compounds 10 e and 10 i, hold potential as immunosuppressive agents.

Synthesis and biological evaluation of N-arylated-lactam-type iminosugars as potential immunosuppressive agents

Since the immunosuppressive agents currently used in clinics have significant side effects, it is very important to search for new effective and safe immunosuppressants. Iminosugars as a new class of immunosuppressants are less explored. In this report, 24 new N-arylated iminosugar derivatives, including d-talo and d-galacto epimers, were designed and synthesized, and their immunosuppressive effects were evaluated by MTT assay. The experimental data demonstrated that compound 20 showed the strongest inhibition effect (IC₅₀ = 6.94 μM). Further studies revealed that the inhibitory effects on splenocyte proliferation may come from the suppression of both IFN-γ and IL-4 cytokines. The preliminary structure-activity relationship (SAR) analysis suggested that N-arylated d-galacto-type iminosugars showed better inhibitory activities than d-talo-type analogues. The SAR analysis also showed that the inhibition effect of iminosugars can be improved by decreasing the polarity or increasing the hydrophobicity. These results may be beneficial to the discovery of new iminosugar derivatives as immunosuppressive agents.

Synthesis of tetracyclic iminosugars fused benzo[e][1,3]thiazin-4-one and their HIV-RT inhibitory activity

Several aza-C-pseudonucleosides bearing 1,3-benzothiazin-4-one (6 and 7) were prepared by the one-pot three-component condensation from the iminosugar aldehyde 3, amino acid ethyl/methyl ester hydrochlorides 4(a-c), and 2-mercaptobenzoic acid 5. After removal of Boc and the isopropylidene groups, the target novel tetracyclic iminosugars fused benzo[e][1,3]thiazin-4-one 1(a-c) and 2(a-b) were first afforded by the intramolecular cyclo-amidation reaction. Their structures were determined by their (1)H, (13)C NMR, and HRMS (ESI) spectra and X-ray. The tetracyclic iminosugars 1(a-c) and 2(a-b) were examined for their HIV reverse transcriptase (RT) inhibitory activities. The result showed that all compounds could effectively inhibit RT activity. Among them, compound 2a was the best one with the IC50 value of RT inhibitory activity of 0.82μM. Structure-activity relationship analysis suggested that 1'R configuration in the tetracyclic azasugars was of benefit to their anti-HIV RT activity.

Amino-functionalized iminocyclitols: synthetic glycomimetics of medicinal interest

A review on the syntheses and biological activities of unnatural glycomimetics highlighting the effect of replacement of hydroxyl groups of natural iminosugars by amino functionalities is presented.

Three-step synthesis of l-ido-1-deoxynojirimycin derivatives by reductive amination in water, “borrowing hydrogen” under neat conditions and deprotection

A three-step synthesis of l-ido-1-deoxynojirimycin derivatives, starting from readily available 2,3,4,6-tetra-O-benzyl-d-glucopyranose via Ir-catalyzed reductive amination in water, “borrowing hydrogen” under neat conditions, and Pd-catalyzed debenzylation, is reported.

2-Trifluoromethylthiolation of glycals

The direct 2-trifluoromethylthiolation of glycals has been realized for the first time, benefiting the carbohydrate-based drug discovery.

Synthesis of N-dialkylphosphoryl iminosugar derivatives and their immunosuppressive activities

Twelve novel N-dialkylphosphoryliminosugar derivatives were synthesized and their immunosuppressive activities were evaluated on the proliferation of the mouse splenocytes and the secretion of IFN-γ and IL-4. The experimental data demonstrated that the iminosugars with the double long alkyl chains exhibited better inhibitory effects than those with the single long alkyl chain, and the iminosugars with the 10-carbon linear alkyl chain exhibited the strongest immunosuppressive activities. The assay of the cytokine secretion showed that the introduction of dialkyl chains on iminosugars could regulate the polarization of immune inhibition by varying the length of the alkyl chains. The disclosure of the structure-activity relationships may benefit the structural modifications of iminosugars to find new types of immunosuppressive agents.

An approach to 8 stereoisomers of homonojirimycin from (D)-glucose via kinetic & thermodynamic azido-γ-lactones

Crystal structures were obtained for the two C2 epimeric azido-γ-lactones 2-azido-2-deoxy-3,5:6,7-di-O-isopropylidene-d-glycero-d-ido-heptono-1,4-lactone and 2-azido-2-deoxy-3,5:6,7-di-O-isopropylidene-d-glycero-d-gulo-heptono-1,4-lactone prepared from kinetic and thermodynamic azide displacements of a triflate derived from d-glucoheptonolactone. Azido-γ-lactones are very useful intermediates in the synthesis of iminosugars and polyhydroxylated amino acids. In this study two epimeric azido-heptitols allow biotechnological transformations via Izumoring techniques to 8 of the 16 possible homonojirimycin analogues, 5 of which were isolated pure because of the lack of stereoselectivity of the final reductive amination. A side-by-side glycosidase inhibition profile of 11 of the possible 16 HNJ stereoisomers derived from d-glucose and d-mannose is presented.

Cell cycle arrest, apoptosis and autophagy induced by iminosugars on K562 cells

Iminosugars have gained a remarkable importance as new therapeutic agents since 1966. In this study, compounds A and B, two iminosugar analogs synthesized previously, showed an inhibition of the growth of K562 cells. They allowed cell cycle arrested at the G0/G1 phase, promoted apoptotic activities and also lowered the mitochondrial membrane potential. Further exploration of the apoptosis mechanism revealed that compound B significantly suppressed the expression of Hsp70, which is a major anti-apoptotic molecular chaperone. Significant decrease was also found in the expression of Akt, a serine/threonine-specific protein kinase with anti-apoptosis activities also known as protein kinase B (PKB). At mitochondria level in comparison with compound A, compound B brought a better promotion in the expression of pro-apoptotic protein Bad in Bcl-2 family. As a result of the promotion, the expression of anti-apoptotic protein Bcl-xL was down-regulated. Cytochrome c was released, activating the intrinsic signaling pathways of caspase and resulting in the occurrence of cascade reaction. In addition, compound B stimulated autophagy effectively by up-regulating Beclin 1, thus causing the conversion of LC3-I to LC3-II through Akt/mTOR signaling pathway. In summary, these results indicated that compounds A and B induced cell death through multiple pathways. The disclosed results not only provide an evidence of antitumor activity of iminosugars as a foundation for further studies, but also may find potential applications in chronic myeloid leukemia therapy as new heat shock protein inhibitors and autophagy inducer.

Synthesis of C-Pseudonucleosides Bearing Thiazolidin-4-one as a Novel Potential Immunostimulating Agent

Several novel C-pseudonucleosides bearing thiazolidin-4-one were synthesized by one-pot three-component condensation using unprotected sugar aldehyde at room temperature, and their effects on T cells, B cells, the cytokine secretion of IL-2, IL-4, and IFN-γ, T cell-associated molecules (CD3, CD4, CD8), and B cell-associated molecules (CD19) were first evaluated. The experimental data demonstrated that such thiazolidin-4-one C-pseudonucleosides hold potential as immunostimulating agents.

Synthetic N-alkylated iminosugars as new potential immunosuppressive agents

The new emerging immunosuppressive effects displayed by iminosugars have not been much investigated so far. Several new N-alkyl dideoxy iminoalditols were designed and synthesized to explore their immunosuppressive effects. These iminosugars inhibited the proliferation of mouse splenocytes and the secretion of both IFN-γ and IL-4, which are the hallmark cytokines of Th1 and Th2 cells, respectively. Some compounds exerted good inhibitory effects. More importantly, the synthetic iminosugars prolonged the allograft survival in the mouse skin transplantation experiment. Our results provide a lead for further elucidation of the structure-activity relationships and modifications of iminosugars for better immunosuppressive agents.

Metathesis access to monocyclic iminocyclitol-based therapeutic agents

By focusing on recent developments on natural and non-natural azasugars (iminocyclitols), this review bolsters the case for the role of olefin metathesis reactions (RCM, CM) as key transformations in the multistep syntheses of pyrrolidine-, piperidine- and azepane-based iminocyclitols, as important therapeutic agents against a range of common diseases and as tools for studying metabolic disorders. Considerable improvements brought about by introduction of one or more metathesis steps are outlined, with emphasis on the exquisite steric control and atom-economical outcome of the overall process. The comparative performance of several established metathesis catalysts is also highlighted.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

Concise syntheses of selective inhibitors against α-1,3-galactosyltransferase

Several iminosugar-based uridine diphosphate galactose (UDP-Gal) mimetics 1-4 including d- and l-epimers were designed and synthesized by concise routes, and these synthetic compounds were evaluated for the inhibition of α-1,3- and β-1,4-galactosyltransferases in vitro. The experimental data demonstrated that l-epimer 2 displayed the strongest inhibitory activity with moderate selectivity against α-1,3-galactosyltransferase.

Deoxynojirimycin and its hexosaminyl derivatives bind to natural killer cell receptors rNKR-P1A and hCD69

Deoxynojirimycin (1) and two new related 4-O-hexosaminyl-containing disaccharide mimics, beta-d-TalNAc-(1-->4)-DNJ (4) and beta-d-ManNAc-(1-->4)-DNJ (5), have been studied as agonists of natural killer (NK) cell receptors. As a positive and unexpected result, DNJ (1) displayed a remarkable activation effect towards both NKR-P1A (rat) and CD69 (human) receptors, and a quite similar activity was found for 4 and 5. The synthesis of the two disaccharide mimics is based on an approach that avoids the glycosylation step using known intermediates arising from lactose. The key stage of the synthesis involves the construction of the DNJ unit through an initial C-5 oxidation of the reducing d-glucopyranosyl unit followed by a stereoselective double-reductive aminocyclization of the 1,5-dicarbonyl disaccharide intermediates.

Concise synthesis of iminocyclitols via Petasis-type aminocyclization

A two-step method has been developed to synthesize several biologically important iminocyclitols in ca. 44-60% yields by using Petasis-type condensation. The method is very general and operationally simple, affording a series of iminocyclitols from easily available sugar derivatives. Unexpected diastereoselectivities are observed, suggesting that the condensation may proceed through a five- or six-membered cyclic iminium ion intermediate.