Preparation of propargyl 1,2-orthoesters of carbohydrates: From side reactions in dichloromethane to optimized reaction conditions in acetonitrile

In this note, supercritical fluid chromatography coupled to high-resolution mass spectrometry (SFC-HRMS) has been used to identify a chloro glycoside formed during the preparation of propargyl 1,2-orthoesters in dichloromethane. Additional studies revealed that 20-40% of this side-product was obtained depending on the source of anhydrous solvent, and that tetrabutylammonium iodide amplifies this side-reaction. Finally, a reliable procedure was developed in acetonitrile to prepare these glycosyl donors from perbenzoylated bromo glycosides in manno, gluco and galacto series in 63-74% yield.

Insights into the Activation of Alkyne-Installed Glycosyl Donors with Dual Acidic Metal Catalysts: Reaction Pathway, Influencing Factors, and Enlightenment for Glycosylation

The activation of alkyne-installed glycosyl donors with dual acidic metal catalysts were studied. Lewis and/or π acidity-activated pathways were observed for alkynyl carbonate-, ester-, and ether-type donors, and π acidity-promoted reaction mode afforded higher efficiency and yields. The activation mode for a certain metal catalyst is determined by the nature of catalysts itself, protecting groups on sugar rings, type of sugars, and structure of aglycones. The discovery gives us valuable insights into the glycosylation of alkyne-containing donors.

Design of configuration-restricted triazolylated β-d-ribofuranosides: a unique family of crescent-shaped RNase A inhibitors

Seven new carbohydrate-bistriazole hybrid molecules were designed taking into consideration the crescent shaped active site of ribonuclease A (RNase A). In this case, the β-d-ribofuranose structure was used as the basic building unit; both the C1 and C4 arms protruding out towards the β-face of the tetrahydrofuran moiety of the ribose sugar provided an overall "U" shape to the basic building block. Several combinations of bistriazole moieties were constructed on the two arms of this basic building block. These mono- and/or bis-substituted 1,2,3-triazole units were linked to acidic functional groups because of the overall basic nature of the hydrolytic site of RNase A. All these compounds were efficient competitive inhibitors of RNase A with inhibition constants (Ki) in the micromolar range. In contrast to the carboxylic acid-modified hybrid molecules, molecules carrying sulfonic acids were found to be more potent because of the stronger interactions with the positively charged active site. The most efficient inhibitor of the series was the disulfonic acid-functionalized carbohydrate-bis-triazole hybrid molecule. Docking studies disclosed that the molecule, because of its well defined "U" shape with flexible arms, fits effectively in the active site; moreover, in all cases, besides the acid groups, the triazole and sugar rings also actively participated in creating the hydrogen bonding network in the cavity of the enzyme active site.

Recent Developments in Stereoselective Chemical Glycosylation

Because of their pivotal biological functions, attention to sugars and glycobiology has grown rapidly in recent decades, leading to increased demand for homogeneous oligosaccharides. The stereoselective preparation of oligosaccharides by chemical means remains challenging and continues to be a vivid research area for organic chemists. In the past decade, new approaches and reinvestigated traditional methods have transformed the field. These developments include novel catalyses, various types of glycosylation modulators and the use of photochemical energy to facilitate glycosylation. This Minireview presents a brief overview of the latest trends in chemical glycosylation, with emphasis on the stereoselective synthetic protocols developed in the past decade.

Synthesis and anticancer activity of dimeric podophyllotoxin derivatives

Background

Podophyllotoxin is a potent cytotoxic agent and serves as a useful lead compound for the development of antitumor drugs. Several podophyllotoxin-derived antitumor agents, including etoposide, are currently in clinical use; however, their therapeutic efficacy is often limited due to side effects and the development of resistance by cancer cells. Previous studies have shown that 4β-1,2,3-triazole derivatives of podophyllotoxin exhibit more potent anticancer activity and better binding to topoisomerase-II than etoposide. The effect of dimerization of such derivatives on the anticancer activity has not been studied.

Methods

Two moieties of podophyllotoxin were linked at the C-4 position via 1,2,3-triazole rings to give a series of novel dimeric podophyllotoxin derivatives. 4β-Azido-substituted podophyllotoxin derivatives (23 and 24) were coupled with various dipropargyl functionalized linkers by utilizing the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction to provide dimeric products in very good yield. The in vitro anticancer activity of the synthesized compounds was evaluated by MTT assay against a panel of five human cancer cell lines (HL-60, SMMC-7721, A-549, MCF-7, and SW480). The normal BEAS-2B (lung) cell line was also included for study in order to evaluate the cancer selectivity of the most active compound as compared with normal cells.

Results

A group of 16 dimeric podophyllotoxin derivatives with different linkers were synthesized and structurally characterized. Most compounds do not show significant cytotoxicity (IC₅₀ > 40 mM) against all five cancer cell lines. However, one compound (29) which bears a perbutyrylated glucose residue on the glycerol linker is highly potent against all five cancer cell lines tested, with IC₅₀ values ranging from 0.43 to 3.50 μM. This compound (29) also shows good selectivity towards cancer cell lines as compared with the normal BEAS-2B (lung) cell line, showing selectivity indexes from 4.4 to 35.7.

Conclusion

The anticancer activity of dimeric podophyllotoxin derivatives is generally speaking not improved as compared to their monomeric counterparts, and the potency of these dimeric derivatives can be largely affected by the nature of the linker between the two moieties. Among the synthesized derivatives, compound 29 is significantly more cytotoxic and selective towards cancer cells than etoposide and cisplatin, which are currently in clinical use. Compound 29 is a promising anticancer drug and needs further studies.

Gold-catalyzed glycosylation in the synthesis of complex carbohydrate-containing natural products

Gold(i)- and gold(iii)-catalyzed glycosylation reactions and their application in the synthesis of natural glycoconjugates are reviewed.

Gold(I)-Catalyzed Direct Stereoselective Synthesis of Deoxyglycosides from Glycals

Au(I) in combination with AgOTf enables the unprecedented direct and α-stereoselective catalytic synthesis of deoxyglycosides from glycals. Mechanistic investigations suggest that the reaction proceeds via Au(I)-catalyzed hydrofunctionalization of the enol ether glycoside. The room temperature reaction is high yielding and amenable to a wide range of glycal donors and OH nucleophiles.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

ortho-(Methyltosylaminoethynyl)benzyl glycosides as new glycosyl donors for latent-active glycosylation

A new glycosylation protocol with ortho-(methyltosylaminoethynyl)benzyl glycosides as donors is disclosed.

Cationic charged helical glycopolypeptide using ring opening polymerization of 6-deoxy-6-azido-glyco-N-carboxyanhydride

Glycopolypeptides with a defined secondary structure are of significance in understanding biological phenomena. Synthetic glycopolypeptides, or polypeptides featuring pendant carbohydrate moieties, have been of particular interest in the field of tissue engineering and drug delivery. In this work, we have synthesized charged water-soluble glycopolypeptides that adopt a helical conformation in water. This was carried out by the synthesis of a glyco-N-carboxyanhydride (glyco-NCA) containing an azide group at the sixth position of the carbohydrate ring. Subsequently, the NCA was polymerized to obtain azide-containing glycopolypeptides having good control over molecular weight and polydispersity index (PDI) in high yields. We were also able to control the incorporation of the azide group by synthesizing random co-glycopolypeptide containing 6-deoxy-6-azido and regular 6-OAc functionalized glucose. This azide functionality allows for the easy attachment of a bioactive group, which could potentially enhance the biological activity of the glycopolypeptide. We were able to obtain water-soluble charged glycopolypeptides by both reducing the azide groups into amines and using CuAAC with propargylamine. These charged glycopolypeptides were shown to have a helical conformation in water. Preliminary studies showed that these charged glycopolypeptides showed good biocompatibility and were efficiently taken up by HepG2 cells.

Gold(III)-catalyzed glycosidations for 1,2-trans and 1,2-cis furanosides

Stereoselective synthesis of furanosides is still a daunting task, unlike the pyranosides, for which several methods exist. Herein, a unified stereoselective strategy for the synthesis of 1,2-trans and 1,2-cis furanosides is revealed for seven out of eight possible isomers of pentoses. The identified protocol gives access to diastereoselective synthesis of α- and β-araf, ribf, lyxf, and α-xylf furanosides. 1,2-trans glycosides were synthesized by the use of propargyl 1,2-orthoesters under gold-catalyzed glycosidation conditions, and subsequently, they are converted into 1,2-cis glycosides through oxidation-reduction as the key functional group transformation. All the reactions are found to be fully diastereoselective, mild, and high yielding.

Synthesis and anticancer activity of glucosylated podophyllotoxin derivatives linked via 4β-triazole rings

A series of 4β-triazole-linked glucose podophyllotoxin conjugates have been designed and synthesized by employing a click chemistry approach. All the compounds were evaluated for their anticancer activity against a panel of five human cancer cell lines (HL-60, SMMC-7721, A-549, MCF-7, SW480) using MTT assays. Most of these triazole derivatives have good anticancer activity. Among them, compound 35 showed the highest potency against all five cancer cell lines tested, with IC₅₀ values ranging from 0.59 to 2.90 μM, which is significantly more active than the drug etoposide currently in clinical use. Structure-activity relationship analysis reveals that the acyl substitution on the glucose residue, the length of oligoethylene glycol linker, and the 4'-demethylation of podophyllotoxin scaffold can significantly affect the potency of the anticancer activity. Most notably, derivatives with a perbutyrylated glucose residue show much higher activity than their counterparts with either a free glucose or a peracetylated glucose residue.

Facile synthesis of β- and α-arabinofuranosides and application to cell wall motifs of M. tuberculosis

Propargyl 1,2-orthoesters of arabinose are exploited for the synthesis of 1,2-trans furanosides; easily accessible 1,2-trans ribofuranosides are converted to challenging 1,2-cis-arabinofuranosides by oxidoreduction. Utility of these protocols was demonstrated by the successful synthesis of major structural motifs present in the cell surface of Mycobacterium tuberculosis. Key furanosylations were carried out under gold-catalyzed glycosidation conditions.

Recent Advances in Transition Metal-Catalyzed Glycosylation

Having access to mild and operationally simple techniques for attaining carbohydrate targets will be necessary to facilitate advancement in biological, medicinal, and pharmacological research. Even with the abundance of elegant reports for generating glycosidic linkages, stereoselective construction of α- and β-oligosaccharides and glycoconjugates is by no means trivial. In an era where expanded awareness of the impact we are having on the environment drives the state-of-the-art, synthetic chemists are tasked with developing cleaner and more efficient reactions for achieving their transformations. This movement imparts the value that prevention of waste is always superior to its treatment or cleanup. This review will highlight recent advancement in this regard by examining strategies that employ transition metal catalysis in the synthesis of oligosaccharides and glycoconjugates. These methods are mild and effective for constructing glycosidic bonds with reduced levels of waste through utilization of sub-stoichiometric amounts of transition metals to promote the glycosylation.

The effect of the number of carbohydrate moieties on the azaphthalocyanine properties

A series of azaphthalocyanines (AzaPc) bearing one, two, four or eight isopropylidene-protected galactosyl units was prepared by azide-alkyne click reaction or by classical Pc template cyclotetramerization of the corresponding dicyanopyrazine and AzaPc properties important for photodynamic therapy were compared. All compounds absorbed at long wavelengths (above 650 nm) and belonged to strong singlet oxygen producers (Φ(Δ) = 0.58-0.64) retaining significant fluorescence emission (Φ(F) = 0.026-0.23). The only exception was the compound with four isopropyliden-protected galactosyl units where partial aggregation was observed. Removal of protecting groups increased the polar character of all AzaPc. However, only AzaPc bearing eight galactoses was found to be water-soluble (105 mg mL(-1)) but noticeably aggregated in water as well as in organic solvents (DMF, DMSO). Amphiphilic AzaPc bearing one deprotected galactose was incorporated into the lipidic bilayer of liposomes in a nonaggregated form. Liposomes may therefore be a suitable delivery system for this amphiphilic photosensitizer.

Versatile gold catalyzed transglycosidation at ambient temperature

Glycosidation with stable alkyl glycosyl donors using a catalytic amount of gold salts is promising. Herein, 1-ethynylcyclohexanyl glycosides are identified as novel donors at room temperature and mechanistic investigation showed that the leaving group simply extrudes out.

Recent advances in the gold-catalyzed additions to C-C multiple bonds

C-O, C-N and C-C bonds are the most widespread types of bonds in nature, and are the cornerstone of most organic compounds, ranging from pharmaceuticals and agrochemicals to advanced materials and polymers. Cationic gold acts as a soft and carbophilic Lewis acid and is considered one of the most powerful activators of C-C multiple bonds. Consequently, gold-catalysis plays an important role in the development of new strategies to form these bonds in more convenient ways. In this review, we highlight recent advances in the gold-catalyzed chemistry of addition of X-H (X = O, N, C) bonds to C-C multiple bonds, tandem reactions, and asymmetric additions. This review covers gold-catalyzed organic reactions published from 2008 to the present.

Orthogonal activation of propargyl and n-pentenyl glycosides and 1,2-orthoesters

An orthogonal activation strategy with propargyl and n-pentenyl glycosides has been identified. According to this methodology, n-pentenyl glycosides can be selectively activated with NIS/TMSOTf in the presence of either armed or disarmed propargyl O-glycosides. In addition, we report herein that propargyl 1,2-orthoesters can be selectively activated with AuBr(3) in CH(2)Cl(2) at room temperature in the presence of n-pentenyl glycosides. Similarly, pentenyl 1,2-orthoesters can be selectively activated with NIS/Yb(OTf)(3) in the presence of propargyl glycosides.