Catalytic Cleavage of the 9-Fluorenylmethoxycarbonyl (Fmoc) Protecting Group under Neat Conditions

This work reports the first solvent-free catalytic approach for the cleavage of the fluorenylmethoxycarbonyl (Fmoc) protecting group from amine and alcohol functionalities. Various saccharide, peptide, and glyco-amino acid substrates were efficiently deprotected by simple treatment with 20 mol % neat 4-dimethylaminopyridine (DMAP) (one of the effective base catalysts found), without any solvent or stoichiometric additives. Small model structures were finally assembled through one-pot, base-catalyzed, solvent-free multistep sequences combining the Fmoc cleavage with esterification, amidation, and/or glycosylation steps.

Towards the semi-synthesis of phosphorylated mimics of glycosaminoglycans: Screening of methods for the regioselective phosphorylation of chondroitin

Glycosaminoglycan (GAG) mimics carrying phosphate rather than sulfate anionic groups have been poorly investigated, in spite of their interesting perspectives. While some GAG-mimicking phosphorylated polymers have been reported, to the best of our knowledge no phosphorylated polysaccharides having the same backbone of natural sulfated GAGs have been accessed yet. To fill this gap, in this work two standard phosphorylation protocols and two recently reported procedures have been screened on a set of polysaccharide species composed by microbial sourced chondroitin and three partially protected, semi-synthetic derivatives thereof. A detailed structural characterization by 1H, 13C and 31P NMR spectroscopy revealed the higher versatility of the innovative, biomimetic reaction employing monopotassium salt of phosphoenolpyruvate (PEPK) with respect to standard phosphorylating agents (phosphoric acid or phosphorus oxychloride). Indeed, PEP-K and H3PO4 gave similar results in the regioselective phosphorylation of the primary hydroxyls of unprotected chondroitin, while only the former reacted on partially protected chondroitin derivatives in a controlled, regioselective fashion, affording chondroitin phosphate (CP) polysaccharides with different derivatization patterns. The reported results represent the first, key steps towards the systematic semi-synthesis of phosphorylated GAGs as a new class of GAG mimics and to the evaluation of their biological activities in comparison with native sulfated GAGs.

Multi-step Strategies Toward Regioselectively Sulfated M-Rich Alginates

Sulfated alginates (ASs), as well as several artificially sulfated polysaccharides, show interesting bioactivities. The key factors for structure-activity relationships studies are the degree of sulfation and the distribution of the sulfate groups along the polysaccharide backbone (sulfation pattern). The former parameter can often be controlled through stoichiometry, while the latter requires the development of suitable chemical or enzymatic, regioselective methods and is still missing for ASs. In this work, a study on the regioselective installation of several different protecting groups on a d-mannuronic acid enriched (M-rich) alginate is reported in order to develop a semi-synthetic access to regioselectively sulfated AS derivatives. A detailed structural characterization of the obtained ASs revealed that the regioselective sulfation could be achieved complementarily at the O-2 or O-3 positions of M units through multi-step sequences relying upon a silylating or benzoylating reagent for the regioselective protection of M-rich alginic acid, followed by sulfation and deprotection.

Adaptation of Zemplén's conditions for a simple and highly selective approach to methyl 1,2-trans glycosides

1,2-trans methyl glycosides can be readily obtained from peracetylated sugars through their initial conversion into glycosyl iodide donors and subsequent exposure of these latter to a slight excess of sodium methoxide in methanol. Under these conditions a varied set of mono- and disaccharide precursors afforded the corresponding 1,2-trans glycosides with concomitant de-O-acetylation in satisfying yields (in the range 59-81%). A similar approach also proved effective when using GlcNAc glycosyl chloride as the donor.

Semisynthetic Isomers of Fucosylated Chondroitin Sulfate Polysaccharides with Fucosyl Branches at a Non-Natural Site

The several interesting activities detected for fucosylated chondroitin sulfate (fCS) have fueled in the last years several efforts toward the obtainment of fCS oligosaccharides and low molecular weight (LMW) polysaccharides with a well-defined structure, in order to avoid the problems associated with the potential employment of native, sea cucumber sourced fCSs as a drug. Total synthesis and controlled depolymerization of the natural fCS polysaccharides are the main approaches to this aim; nonetheless, they present some limitations. These could be circumvented by semisynthesis, a strategy relying upon the regioselective fucosylation and sulfation of a microbial sourced polysaccharide sharing the same chondroitin backbone of fCS but devoid of any fucose (Fuc) and sulfate decoration on it. This approach is highly versatile, as it could open access also to fCS isomers carrying Fuc and sulfate groups at non-natural sites. Here we prepare for the first time some structurally homogeneous fCS isomers through a multistep procedure with a glycosylation reaction between a LMW polysaccharide acceptor and three different Fuc donors as key step. The obtained products were subjected to a detailed structural characterization by 2D-NMR. The conformational behavior was also investigated by NMR and molecular dynamics simulation methods and compared with data reported for natural fCS.

Solvent-free, under air selective synthesis of α-glycosides adopting glycosyl chlorides as donors

α-Glycosides are highly relevant synthetic targets due to their abundance in natural oligosaccharides involved in many biological processes. Nevertheless their preparation is hampered by several issues, due to both the strictly anhydrous conditions typically required in glycosylation procedures and the non-trivial achievement of high α-stereoselectivity, one of the major challenges in oligosaccharide synthesis. In this paper we report a novel and efficient approach for the highly stereoselective synthesis of α-glycosides. This is based on the unprecedented solvent-free combination of triethylphosphite, tetrabutylammonium bromide and N,N-diisopropylethylamine for the activation of glycosyl chlorides under air. Despite the relative stability of glycosyl chlorides with respect to more reactive halide donors, the solvent-free procedure allowed a wide set of α-glycosides, including biorelevant fragments, to be obtained in much shorter times compared with similar glycosylation approaches in solution. The presented method features a wide target scope and functional group compatibility, also serving with partially disarmed substrates, and it does not require a high stoichiometric excess of reagents nor the preparation of expensive precursors. The solvent-free glycosylation can be even directly performed from 1-hydroxy sugars without purification of the in situ generated chloride, providing an especially useful opportunity in the case of highly reactive and labile glycosyl donors.

Switchable synthesis of glycosyl selenides or diselenides with direct use of selenium as the selenating agent

Chemoselective synthesis of either diglycosyl selenides or diselenides. Elementary selenium as the selenating agent.

(Semi)-Synthetic Fucosylated Chondroitin Sulfate Oligo- and Polysaccharides

Fucosylated chondroitin sulfate (fCS) is a glycosaminoglycan (GAG) polysaccharide with a unique structure, displaying a backbone composed of alternating N-acetyl-d-galactosamine (GalNAc) and d-glucuronic acid (GlcA) units on which l-fucose (Fuc) branches are installed. fCS shows several potential biomedical applications, with the anticoagulant activity standing as the most promising and widely investigated one. Natural fCS polysaccharides extracted from marine organisms (Echinoidea, Holothuroidea) present some advantages over a largely employed antithrombotic drug such as heparin, but some adverse effects as well as a frequently found structural heterogeneity hamper its development as a new drug. To circumvent these drawbacks, several efforts have been made in the last decade to obtain synthetic and semi-synthetic fCS oligosaccharides and low molecular weight polysaccharides. In this Review we have for the first time collected these reports together, dividing them in two topics: (i) total syntheses of fCS oligosaccharides and (ii) semi-synthetic approaches to fCS oligosaccharides and low molecular weight polysaccharides as well as glycoclusters displaying multiple copies of fCS species.

Nanoparticles decorated with folate based on a site-selective αCD-rotaxanated PEG-b-PCL copolymer for targeted cancer therapy

NPs fabricated from a mixture of PEG-b-PCL and selectively rotaxanated Fol-PEG(αCD)-PCL showed internalisation in KB cells through an active targeting mechanism.

Synthesis of diglycosylated (di)sulfides and comparative evaluation of their antiproliferative effect against tumor cell lines: A focus on the nature of sugar-recognizing mediators involved

A mini-library of symmetrical and unsymmetrical diglycosyl (di)sulfides, containing d-galactose, l-fucose and N-acetyl glucosamine units, were synthesized and tested for the antiproliferative activity against cervix carcinoma (HeLa) and melanoma (A375) tumor cell lines as well as healthy fibroblasts (HDF). Comparative analysis of results seems to indicate that the most relevant antiproliferative effect is not primarily influenced by interactions with galectins, as the most cytotoxic compound observed for HeLa and A375 is not a ligand for such receptors. The most active molecules against HeLa and A375 lines also exhibited a good selectivity, showing a low toxicity to HDF cells. Obtained results offer useful indications for future design of structurally simple antitumor molecules based on sugar moieties with bridging sulfur atoms.

Synthesis of the tetrasaccharide repeating unit of the cryoprotectant capsular polysaccharide from Colwellia psychrerythraea 34H

Synthesis of the threonine-decorated tetrasaccharide repeating unit of a cryoprotectant polysaccharide with a glycosaminoglycan-like structure.

C-Glycosylation in platinum-based agents: a viable strategy to improve cytotoxicity and selectivity

The glycosylation of five-coordinate Pt(ii) compounds through a Pt–C linkage can be a very effective strategy for attacking cancer cells, while preserving the survival of the healthy ones.

Decoration of Chondroitin Polysaccharide with Threonine: Synthesis, Conformational Study, and Ice-Recrystallization Inhibition Activity

Several threonine (Thr)- and alanine (Ala)-rich antifreeze glycoproteins (AFGPs) and polysaccharides act in nature as ice recrystallization inhibitors. Among them, the Thr-decorated capsular polysaccharide (CPS) from the cold-adapted Colwellia psychrerythraea 34H bacterium was recently investigated for its cryoprotectant activity. A semisynthetic mimic thereof was here prepared from microbial sourced chondroitin through a four-step strategy, involving a partial protection of the chondroitin polysaccharide as a key step for gaining an unprecedented quantitative amidation of its glucuronic acid units. In-depth NMR and computational analysis suggested a fairly linear conformation for the semisynthetic polysaccharide, for which the antifreeze activity by a quantitative ice recrystallization inhibition assay was measured. We compared the structure-activity relationships for the Thr-derivatized chondroitin and the natural Thr-decorated CPS from C. psychrerythraea.

Orthogonal protection of saccharide polyols through solvent-free one-pot sequences based on regioselective silylations

tert-Butyldimethylsilyl (TBDMS) and tert-butyldiphenylsilyl (TBDPS) are alcohol protecting groups widely employed in organic synthesis in view of their compatibility with a wide range of conditions. Their regioselective installation on polyols generally requires lengthy reactions and the use of high boiling solvents. In the first part of this paper we demonstrate that regioselective silylation of sugar polyols can be conducted in short times with the requisite silyl chloride and a very limited excess of pyridine (2-3 equivalents). Under these conditions, that can be regarded as solvent-free conditions in view of the insolubility of the polyol substrates, the reactions are faster than in most examples reported in the literature, and can even be further accelerated with a catalytic amount of tetrabutylammonium bromide (TBAB). The strategy proved also useful for either the selective TBDMS protection of secondary alcohols or the fast per-O-trimethylsilylation of saccharide polyols. In the second part of the paper the scope of the silylation approach was significantly extended with the development of unprecedented "one-pot" and "solvent-free" sequences allowing the regioselective silylation/alkylation (or the reverse sequence) of saccharide polyols in short times. The developed methodologies represent a very useful and experimentally simple tool for the straightforward access to saccharide building-blocks useful in organic synthesis.

The I2/Et3SiH system: A versatile combination with multiple applications in carbohydrate chemistry

In this contribution, a brief survey of the multiple synthetic opportunities offered by the I2/Et3SiH combined system is presented. Both reagents are cheap and easy to handle, and their combination allows HI to be quickly formed in situ. A suitable stoichiometry of the mixture can be addressed to a set of very useful transformations of carbohydrate chemistry ranging from glycosidation reactions to functional group manipulation of sugars.

Disentangling eumelanin "black chromophore": visible absorption changes as signatures of oxidation state- and aggregation-dependent dynamic interactions in a model water-soluble 5,6-dihydroxyindole polymer

A fundamental unsettled issue concerning eumelanins, the functional biopolymers of human skin and hair, is why they are black. The experimental difficulty lies in the virtual insolubility of these pigments, causing marked scattering effects and hindering characterization of the intrinsic absorption properties of the heterogeneous species produced by oxidative polymerization of 5,6-dihydroxyindole (DHI) and related monomer precursors. The synthesis of spectrally robust, water-soluble DHI polymers is therefore an important goal in the prospects of disentangling intrinsic absorption properties of eumelanin components by circumventing scattering effects. Reported herein is the first water-soluble DHI polymer produced by oxidation of ad hoc designed 5,6-dihydroxy-3-indolyl-1-thio-beta-D-galactopyranoside (1). The dark brown polymer exhibited a distinct band at 314 nm and a broad visible absorption, resembling that of natural eumelanins. Main isolable oligomer intermediates including 2,7'- and 2,4'-biindolyls 2 and 3, attest the close resemblance to the mode of coupling of the parent DHI. Sodium borohydride reduction caused decoloration and a marked absorbance decrease in the visible region around 550 nm, but did not affect the UV band at 314 nm. Measurements of absorbance variations with dilution indicated a linear response at 314 nm, but a significant deviation from linearity in the visible region, with the largest decrease around 500 nm. It is argued that eumelanin black color is not only intrinsically defined by the overlap of pi-electron conjugated chromophores within the individual polymer components, as commonly believed, but also by oxidation state- and aggregation-dependent interchromophoric interactions causing perturbations of the heterogeneous ensemble of pi-electron systems and overall spectral broadening.

Sequential one-pot glycosidations catalytically promoted: unprecedented strategy in oligosaccharide synthesis for the straightforward assemblage of the antitumor PI-88 pentasaccharide

The pentasaccharide sequence of the most active components of the antitumor drug PI-88, currently in phase II clinical trial, has been rapidly assembled in high overall yield and in only three steps starting from three monosaccharide building blocks. The procedure takes advantage of the first reported strategy of sequential one-pot glycosidations conducted exclusively under catalytic activation. In addition, the procedure relies only on shelf-stable and mild promoters such as Yb(OTf)(3) and Bi(OTf)(3).

Synthesis and biophysical characterization of G-rich oligonucleotides conjugated with sugar-phosphate tails

A number of 5'-and 3'-glycoconjugates of the oligonucleotide (5')d(TGGGAG)(3') have been synthesized, exploiting fully automated, online phosphoramidite-based solid phase strategy, as potential anti-HIV-1 agents. The thermodynamic stability of the resulting quadruplexes has been investigated by thermal denaturation studies, via a detailed CD Q1 analysis.

Synthesis, biophysical characterization, and anti-HIV activity of glyco-conjugated G-quadruplex-forming oligonucleotides

Novel hybrid oligonucleotides carrying the G-quadruplex-forming d(5'TGGGAG3') sequence, conjugated with mono- or disaccharides at the 3' or 5'-end through phosphodiester bonds, have been synthesized as potential anti-HIV agents, via a fully automated, online phosphoramidite-based solid-phase strategy. CD-monitored thermal denaturation studies on the resulting quadruplexes indicated the insertion of a single monosaccharide at the 3'-end as the optimal modification, conferring improved stability to the quadruplex complex. In addition, the 3'-conjugation with glucose or mannose converted the anti-HIV inactive unmodified oligomer into active compounds. On the contrary, the 5'-tethering with these monosaccharides, as well as the conjugation, either at the 5' or 3'-end, with sucrose, were in all cases detrimental to quadruplex stability and did not improve the biological activity. On the basis of the assumption that the kinetically and thermodynamically favored formation of the quadruplex complex is a prerequisite for efficient antiviral activity, a novel bis-conjugated oligonucleotide was designed. This combined a mannose residue at the 3'-phosphate end with bulky aromatic tert-butyldiphenylsilyl (TBDPS) group at the 5'-end, previously shown to markedly favor the formation of quadruplex complexes. The 5',3'-bis-conjugated 6-mer, for which a detailed biophysical characterization has been carried out, resulted in 3-fold greater antiviral activity against HIV-1 than the sole 3'-glyco-conjugated oligonucleotide.

Novel approaches for the synthesis and activation of thio- and selenoglycoside donors

Alkyl thio-, phenyl seleno-, and phenyl thioglycosides can be prepared through short synthetic sequences based on the generation of glycosyl iodides as versatile intermediates. In addition, a novel cheap combined system (stoichiometric NBS and catalytic Bi(OTf)3) has been developed for rapid and efficient activation of a wide variety of thio- and selenoglycoside donors.

The behaviour of deoxyhexose trihaloacetimidates in selected glycosylations

Armed deoxyhexose glycosyl donors are very reactive and sometimes too uncontrollably activated in glycosylation reactions; yields can be thereby reduced, especially when unreactive glycosyl acceptors are involved. In this paper, the behaviour of a range of deoxyhexose trihaloacetimidate (trichloro- and N-phenyl trifluoro-) donors is compared in some selected glycosylations towards biologically relevant targets. The selected N-phenyl trifluoroacetimidates often afforded best results in terms of both donor synthesis and glycosylation yield.