Recent Carbohydrate-Based Chemoselective Ligation Applications

Glycorandomization: A promising diversification strategy for the drug development

Glycorandomization is a natural product derivatization strategy in which different sugar moieties are linked to the aglycone part of the naturally existing glycosides to create glycorandomized libraries. Sugars attached to the natural products are responsible for affecting their solubility, mechanism of action, target recognition, and toxicity and thus, by changing the sugar part, these properties could be modified. Glycorandomization can be done via two approaches (i) a synthetic approach known as neoglycorandomization, and (ii) chemoenzymatic approach including in-vitro and in-vivo glycorandomization. Glycorandomization can be a promising technology for the drug discovery that has proved its potential to improve pharmacokinetic (solubility) and pharmacodynamic profile (mechanism of action, toxicity, and target recognition) of the parent compounds. The substrate flexibility of glycosyltransferases and other enzymes towards sugars and/or aglycone substrates has made this technique versatile. Further, the enzymes can be altered by genetic engineering to generate glycorandomized libraries of diverse natural product scaffolds. This technique has the potential to produce new compounds that can be helpful to the mankind by treating the threatening disease states. This review covers the different strategies for glycorandomization as a tool in drug discovery and development. The fundamentals of glycorandomization, different types, and further development of differentially glycorandomized libraries of natural products and small molecule based drugs have been discussed.

Glycoconjugate Oxime Formation Catalyzed at Neutral pH: Mechanistic Insights and Applications of 1,4-Diaminobenzene as a Superior Catalyst for Complex Carbohydrates

The reaction of unprotected carbohydrates with aminooxy reagents to provide oximes is a key method for the construction of glycoconjugates. Aniline and derivatives serve as organocatalysts for the formation of oximes from simple aldehydes, and we have previously reported that aniline also catalyzes the formation of oximes from the more complex aldehydes, carbohydrates. Here, we present a comprehensive study of the effect of aniline analogues on the formation of carbohydrate oximes and related glycoconjugates depending on organocatalyst structure, pH, nucleophile, and carbohydrate, covering more than 150 different reaction conditions. The observed superiority of the 1,4-diaminobenzene (PDA) catalyst at neutral pH is rationalized by NMR analyses and DFT studies of reaction intermediates. Carbohydrate oxime formation at pH 7 is demonstrated by the formation of a bioactive glycoconjugate from a labile, decorated octasaccharide originating from exopolysaccharides of the soil bacterium Mesorhizobium loti. This study of glycoconjugate formation includes the first direct comparison of aniline-catalyzed reaction rates and equilibrium constants for different classes of nucleophiles, including primary oxyamines, secondary N-alkyl oxyamines, as well as aryl and arylsulfonyl hydrazides. We identified 1,4-diaminobenzene as a superior catalyst for the construction of oxime-linked glycoconjugates under mild conditions.

Aminooxylated Carbohydrates: Synthesis and Applications

Among other classes of biomolecules, carbohydrates and glycoconjugates are widely involved in numerous biological functions. In addition to addressing the related synthetic challenges, glycochemists have invested intense efforts in providing access to structures that can be used to study, activate, or inhibit these biological processes. Over the past few decades, aminooxylated carbohydrates have been found to be key building blocks for achieving these goals. This review provides the first in-depth overview covering several aspects related to the syntheses and applications of aminooxylated carbohydrates. After a brief introduction to oxime bonds and their relative stabilities compared to related C═N functions, synthetic aspects of oxime ligation and methodologies for introducing the aminooxy functionality onto both glycofuranosyls and glycopyranosyls are described. The subsequent section focuses on biological applications involving aminooxylated carbohydrates as components for the construcion of diverse architectures. Mimetics of natural structures represent useful tools for better understanding the features that drive carbohydrate-receptor interaction, their biological output and they also represent interesting structures with improved stability and tunable properties. In the next section, multivalent structures such as glycoclusters and glycodendrimers obtained through oxime ligation are described in terms of synthetic design and their biological applications such as immunomodulators. The second-to-last section discusses miscellaneous applications of oxime-based glycoconjugates, such as enantioselective catalysis and glycosylated oligonucleotides, and conclusions and perspectives are provided in the last section.

N–O linkage in carbohydrates and glycoconjugates

The synthesis and chemical and physicochemical properties as well as biological and medical applications of various hydroxylamine-functionalized carbohydrate derivatives are summarized.

Glycodendropeptides stimulate dendritic cell maturation and T cell proliferation: a potential influenza A virus immunotherapy

A GDP displaying the NP_366–374 influenza A virus epitope induces an immunostimulatory effect, emerging as a potential flu vaccine.

Tetravalent glycocyclopeptide with nanomolar affinity to wheat germ agglutinin

A series of tetravalent glycocyclopeptides functionalized with GlcNAc was synthesized using copper(i)-catalysed alkyne-azide cycloaddition, oxime ligation and thiol-ene coupling. The binding ability of these compounds towards wheat germ agglutinin was studied by a competitive ELLA test and ITC experiments. While all compounds were able to inhibit WGA binding to GlcNAc-polymer coated surfaces at low concentrations, derivative 17 having an aliphatic spacer and thioether linkage was 4.9 × 10(6) times more potent on a per sugar basis than GlcNAc. This remarkably strong effect was confirmed by ITC experiments as these revealed an association constant of 9 nM for this compound, therefore presenting a gain of 200,000 times over GlcNAc. These results for compound 17 represent the highest binding properties reported for WGA.

Quantitative structure-activation barrier relationship modeling for Diels-Alder ligations utilizing quantum chemical structural descriptors

Background

In the present study, we show the correlation of quantum chemical structural descriptors with the activation barriers of the Diels-Alder ligations. A set of 72 non-catalysed Diels-Alder reactions were subjected to quantitative structure-activation barrier relationship (QSABR) under the framework of theoretical quantum chemical descriptors calculated solely from the structures of diene and dienophile reactants. Experimental activation barrier data were obtained from literature. Descriptors were computed using Hartree-Fock theory using 6-31G(d) basis set as implemented in Gaussian 09 software.

Results

Variable selection and model development were carried out by stepwise multiple linear regression methodology. Predictive performance of the quantitative structure-activation barrier relationship (QSABR) model was assessed by training and test set concept and by calculating leave-one-out cross-validated Q² and predictive R² values. The QSABR model can explain and predict 86.5% and 80% of the variances, respectively, in the activation energy barrier training data. Alternatively, a neural network model based on back propagation of errors was developed to assess the nonlinearity of the sought correlations between theoretical descriptors and experimental reaction barriers.

Conclusions

A reasonable predictability for the activation barrier of the test set reactions was obtained, which enabled an exploration and interpretation of the significant variables responsible for Diels-Alder interaction between dienes and dienophiles. Thus, studies in the direction of QSABR modelling that provide efficient and fast prediction of activation barriers of the Diels-Alder reactions turn out to be a meaningful alternative to transition state theory based computation.

Recent trends in bioorthogonal click-radiolabeling reactions using fluorine-18

The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of novel and versatile bioorthogonal conjugation techniques especially for the radiolabeling of biologically active high molecular weight compounds like peptides, proteins or antibodies. Taking into consideration that the introduction of fluorine-18 (t(1/2) = 109.8 min) proceeds under harsh conditions, radiolabeling of these biologically active molecules represents an outstanding challenge and is of enormous interest. Special attention has to be paid to the method of 18F-introduction. It should proceed in a regioselective manner under mild physiological conditions, in an acceptable time span, with high yields and high specific activities. For these reasons and due to the high number of functional groups found in these compounds, a specific labeling procedure has to be developed for every bioactive macromolecule. Bioorthogonal strategies including the Cu-assisted Huisgen cycloaddition and its copper-free click variant, both Staudinger Ligations or the tetrazine-click reaction have been successfully applied and represent valuable alternatives for the selective introduction of fluorine-18 to overcome the afore mentioned obstacles. This comprehensive review deals with the progress and illustrates the latest developments in the field of bioorthogonal labeling with the focus on the preparation of radiofluorinated building blocks and tracers for molecular imaging.

A new synthetic access to 2-N-(glycosyl)thiosemicarbazides from 3-N-(glycosyl)oxadiazolinethiones and the regioselectivity of the glycosylation of their oxadiazolinethione precursors

Glycosylations of 5-(1H-indol-2-yl)-1,3,4-oxadiazoline-2(3H)-thione delivered various degrees of S- and/or N-glycosides depending on the reaction conditions. S-Glycosides were obtained regiospecifically by grinding oxadiazolinethiones with acylated α-D-glycosyl halides in basic alumina, whereas 3-N-(glycosyl)oxadiazolinethiones were selectively obtained by reaction with HgCl(2) followed by heating the resultant chloromercuric salt with α-D-glycosyl halides in toluene under reflux. On using Et(3)N or K(2)CO(3) as a base, mixtures of S- (major degree) and N-glycosides (minor degree) were obtained. Pure 3-N-(glycosyl)oxadiazolinethiones can also be selectively obtained from glycosylsulfanyloxadiazoles by the thermal S→N migration of the glycosyl moiety, which is proposed to occur by a tight-ion-pair mechanism. Thermal S→N migration of the glycosyl moiety can be used for purification of mixtures of S- or N-glycosides to obtain the pure N-glycosides. The aminolysis of the respective S- or N-glycosides with ammonia in aqueous methanol served as further confirmation of their structures. While in S-glycosides the glycosyl moiety was cleaved off again, 3-N-(glycosyl)oxadiazolinethiones showed a ring opening of the oxadiazoline ring (without affecting the glycosyl moiety) to give N-(glycosyl)thiosemicarbazides. Herewith, a new synthetic access to one of the four classes of glycosylthiosemicarbazides was found. The ultimate confirmation of new structures was achieved by X-ray crystallography. Finally, action of ammonia on benzylated 3-N-(galactosyl)oxadiazolinethione unexpectedly yielded 3-N-(galactosyl)triazolinethione. This represents a new path to the conversion of glycosyloxadiazolinethiones to new glycosyltriazolinethione nucleosides, which was until now unknown.

Click reactions in protein chemistry: from the preparation of semisynthetic enzymes to new click enzymes

Click-chemistry is an approach based on cycloaddition reactions which has been successfully used as a chemical approach for complex organic molecules and which has recently starred in a boom in the world of protein chemistry. The advantage of the use of this technique in protein chemistry is based on a very high and efficient chemoselectivity, which usually requires simple or no purification and is extremely rate-accelerated in aqueous media. The perspective discusses some of the most recent advances in the application of this reaction in selective enzyme surface modification for the creation of new semisynthetic enzymes (fluorescence labeled enzymes, peptide-enzyme conjugates, glycosylated enzymes), and interestingly, the recent design and creation of "click" enzymes.

Enhancement of cyclopamine via conjugation with nonmetabolic sugars

The Veratrum alkaloid cyclopamine, an inhibitor of cancer stem cell growth, was used as a representative scaffold to evaluate the inhibitory impact of glycosylation with a group of nonmetabolic saccharides, such as d-threose. In a five-step divergent process, a 32-member glycoside library was created and assayed to determine that glycosides of such sugars notably improved the GI50 value of cyclopamine while metabolic sugars, such as d-glucose, did not.

Synthesis of 1,2,3-triazole linked galactopyranosides and evaluation of cholera toxin inhibition

We report the synthesis of a series of bivalent 1,2,3-triazole linked galactopyranosides as potential inhibitors of cholera toxin (CT). The inhibitory activity of the bivalent series was examined (ELISA) and the series showed low inhibitory activity (millimolar IC(50)s). Conversely, the monomeric galactotriazole analogues were strong inhibitors of cholera toxin (IC(50) = 71-75 μM).

Dendri-RAFTs: a second generation of cyclopeptide-based glycoclusters

Synthetic glycoclusters and their related biological applications have stimulated increasing interest over the last decade. As a prerequisite to discovering active and selective therapeuticals, the development of multivalent glycoconjugates with diverse topologies is faced with inherent synthetic and structural characterisation difficulties. Here we describe a new series of molecularly-defined glycoclusters that were synthesized in a controlled manner using a robust and versatile divergent protocol. Starting from a Regioselectively Addressable Functionalized Template (RAFT) carrier, either a polylysine dendritic framework or a second RAFT, then 16 copies of βGal, αMan, βLac or cancer-related Thomsen-Freidenreich (αTF) antigen were successively conjugated within the same molecule using oxime chemistry. We thus obtained a new generation of dendri-RAFTs glycoclusters with high glycosidic density and variable spatial organizations. These compounds displaying 16 endgroups were unambiguously characterized by NMR spectroscopy and mass spectrometry. Further biological assays between a model lectin from Canavalia ensiformis (ConA) and mannosylated glycoclusters revealed a higher inhibition potency than the tetravalent counterpart, in particular for the hexadecavalent polylysine skeleton. Together with the efficiency of the synthetic and characterisation processes, this preliminary biological study provided clear evidence of promising properties that make the second generation of cyclopeptide-based glycoclusters attractive for biomedical applications.

Assessment of chemoselective neoglycosylation methods using chlorambucil as a model

To systematically assess the impact of glycosylation and the corresponding chemoselective linker upon the anticancer activity/selectivity of the drug chlorambucil, herein we report the synthesis and anticancer activities of a 63-member library of chlorambucil-based neoglycosides. A comparison of N-alkoxyamine-, N-acylhydrazine-, and N-hydroxyamine-based chemoselective glycosylation of chlorambucil revealed sugar- and linker-dependent partitioning among open- and closed-ring neoglycosides and corresponding sugar-dependent variant biological activity. Cumulatively, this study represents the first neoglycorandomization of a synthetic drug and expands our understanding of the impact of sugar structure upon product distribution/equilibria in the context of N-alkoxyamino-, N-hydroxyamino-, and N-acylhydrazine-based chemoselective glycosylation. This study also revealed several analogues with increased in vitro anticancer activity, most notably D-threoside 60 (NSC 748747), which displayed much broader tumor specificity and notably increased potency over the parent drug.

8-Aza-7-deazaguanine nucleosides and oligonucleotides with octadiynyl side chains: synthesis, functionalization by the azide-alkyne 'click' reaction and nucleobase specific fluorescence quenching of coumarin dye conjugates

Oligonucleotides incorporating 7-(octa-1,7-diynyl) derivatives of 8-aza-7-deaza-2-deoxyguanosine (2d) were prepared by solid-phase synthesis. The side chain of 2d was introduced by the Sonogashira cross coupling reaction and phosphoramidites (3a, 3b) were synthesized. Duplexes containing 2d are more stabilized compared to those incorporating the non-functionalized 8-aza-7-deaza-2-deoxyguanosine (2a) demonstrating that these side chains have steric freedom in duplex DNA. Nucleoside 2d as well as 2d-containing oligonucleotides were conjugated to the non-fluorescent 3-azido-7-hydroxycoumarin 15 by the Huisgen-Meldal-Sharpless click reaction. Pyrazolo[3,4-d]pyrimidine nucleoside conjugate 16 shows a much higher fluorescence intensity than that of the corresponding pyrrolo[2,3-d]pyrimidine derivative 17. The quenching in the dye conjugate 17 was found to be stronger on the stage of monomeric conjugates than in single-stranded or duplex DNA. Nucleobase-dye contact complexes are suggested which are more favourable in the monomeric state than in the DNA chain when the nucleobase is part of the stack. The side chains with the bulky dye conjugates are well accommodated in DNA duplexes thereby forming hybrids which are slightly more stable than canonical DNA.

Enhancing the divergent activities of betulinic acid via neoglycosylation

Using neoglycosylation, the impact of differential glycosylation upon the divergent anticancer and anti-HIV properties of the triterpenoid betulinic acid (BA) was examined. Each member from a library of 37 differentially glycosylated BA variants was tested for anticancer and anti-HIV activities. Enhanced analogs for both desired activities were discovered with the corresponding antitumor or antiviral enhancements diverging, on the basis of the appended sugar, into two distinct compound subsets.

Modifying the glycosidic linkage in digitoxin analogs provides selective cytotoxins

A chemoselective reaction between oxyamines and unprotected, unactivated reducing sugars was used to construct for the first time a panel of linkage-diversified neoglycosides. This panel of digitoxin analogs included potent and selective tumor cytotoxins; cytotoxicity was dependent on the structure of the glycosidic linkage. These results validate linkage diversification through neoglycosylation as a unique and simple strategy to powerfully complement existing methods for the optimization of glycoconjugates.

Evaluation of ketone-oxime method for developing therapeutic on-demand cleavable immunoconjugates

The use of antibody molecules in immunoassay, molecular targeting, or detection techniques encompasses a broad variety of applications affecting nearly every field of medical science. In cancer therapy, monoclonal antibodies (mAb) have been used as vehicles to deliver radionuclides, toxins, or drugs to the target cancer cells. New conjugation methods are most needed to conjugate a wide variety of targeting small molecules and peptidomimatic compounds. Here, we exploited a keto-oxime method for conjugation of protease susceptible linkers to an antibody. This modified method involves two steps: (i) introduction of methyl ketone linkers (referred to as linker moiety) to the primary amines present in the antibody and (ii) conjugation of ketone linkers to aminoxy functional group present in the conjugated moiety (referred to as functional moiety). We have optimized this conjugation method and shown that approximately 10 functional moieties can be conjugated to antibody. Conjugation was verified by MALDI-TOF MS and Western blot analysis. The acidic pH conditions used in this method did not change the immune reactivity of the Ab. In addition, in vitro protease susceptibility assay was performed to validate this method for prodrug release assay as well as to remove excess radioimmune conjugates from circulation. This orthogonal method is compatible with peptides containing a thiol, amino, or carboxyl groups in the conjugation moiety.

Neoglycolipid probes prepared via oxime ligation for microarray analysis of oligosaccharide-protein interactions

Neoglycolipid technology is the basis of a microarray platform for assigning oligosaccharide ligands for carbohydrate-binding proteins. The strategy for generating the neoglycolipid probes by reductive amination results in ring opening of the core monosaccharides. This often limits applicability to short-chain saccharides, although the majority of recognition motifs are satisfactorily presented with neoglycolipids of longer oligosaccharides. Here, we describe neoglycolipids prepared by oxime ligation. We provide evidence from NMR studies that a significant proportion of the oxime-linked core monosaccharide is in the ring-closed form, and this form selectively interacts with a carbohydrate-binding protein. By microarray analyses we demonstrate the effective presentation with oxime-linked neoglycolipids of (1) Lewis(x) trisaccharide to antibodies to Lewis(x), (2) sialyllactose analogs to the sialic acid-binding receptors, siglecs, and (3) N-glycans to a plant lectin that requires an intact N-acetylglucosamine core.

Dechalcogenative allylic selenosulfide and disulfide rearrangements: complementary methods for the formation of allylic sulfides in the absence of electrophiles. Scope, limitations, and application to the functionalization of unprotected peptides in aqueous media

Primary allylic selenosulfates (seleno Bunte salts) and selenocyanates transfer the allylic selenide moiety to thiols giving primary allylic selenosulfides, which undergo rearrangement in the presence of PPh3 with the loss of selenium to give allylically rearranged allyl alkyl sulfides. This rearrangement may be conducted with prenyl-type selenosulfides to give isoprenyl alkyl sulfides. Alkyl secondary and tertiary allylic disulfides, formed by sulfide transfer from allylic heteroaryl disulfides to thiols, undergo desulfurative allylic rearrangement on treatment with PPh3 in methanolic acetonitrile at room temperature. With nerolidyl alkyl disulfides this rearrangement provides an electrophile-free method for the introduction of the farnesyl chain onto thiols. Both rearrangements are compatible with the full range of functionality found in the proteinogenic amino acids, and it is demonstrated that the desulfurative rearrangement functions in aqueous media, enabling the derivatization of unprotected peptides. It is also demonstrated that the allylic disulfide rearrangement can be induced in the absence of phosphine at room temperature by treatment with piperidine, or simply by refluxing in methanol. Under these latter conditions the reaction is also applicable to allyl aryl disulfides, providing allylically rearranged allyl aryl sulfides in good yields.

α and βl-Fucopyranosyl oxyamines: key intermediates for the preparation of fucose-containing glycoconjugates by oxime ligation

We report herein the synthesis of new alpha and beta aminooxylated L-fucopyranosyl derivatives for the preparation of glycoclusters through oxime ligation. The glycosylation reaction between activated triacetylated L-fucopyranosyl fluoride and N-hydroxyphthalimide was carried out in the presence of boron trifluoride-diethyl etherate and the stereochemical outcome of glycosylation was compared in dichloromethane, acetonitrile or tetrahydrofuran. Interestingly, an unexpected alpha and beta anomer ratio was obtained in spite of the presence of an acetate participating group at the carbon 2, particularly the 1,2-cis glycosylation was largely favoured in acetonitrile. The resulting alpha and beta N-oxyphthalimido fucopyranosyl derivatives were finally deprotected with methylhydrazine to obtain the corresponding free aminooxylated fucopyranosyls. The structure of single-crystal alpha anomer 12 was analysed by X-ray diffraction.