Uronium Salts Research Articles

Development of an In Vitro Sensitisation Test Using a Coculture System of Human Bronchial Epithelium and Immune Cells.

Chemical respiratory sensitisation is a serious health problem. However, to date, there are no validated test methods available for identifying respiratory sensitisers. The aim of this study was to develop an in vitro sensitisation test by modifying the human cell line activation test (h-CLAT) to detect respiratory sensitisers and distinguish them from skin sensitisers. THP-1 cells were exposed to the test chemicals (two skin sensitisers and six respiratory sensitisers), either as monocultures or as cocultures with air-liquid interface-cultured reconstructed human bronchial epithelium. The responses were analysed by measuring the expression levels of surface markers on THP-1 cells (CD86, CD54 and OX40L) and the concentrations of cytokines in the culture media (interleukin (IL)-8, IL-33 and thymic stromal lymphopoietin (TSLP)). The cocultures exhibited increased CD54 expression on THP-1 cells; moreover, in the cocultures but not in the monocultures, exposure to two uronium salts (i.e. respiratory sensitisers) increased CD54 expression on THP-1 cells to levels above the criteria for a positive h-CLAT result. Additionally, exposure to the respiratory sensitiser abietic acid, significantly increased IL-8 concentration in the culture medium, but only in the cocultures. Although further optimisation of the method is needed to distinguish respiratory from skin sensitisers by using these potential markers (OX40L, IL-33 and TSLP), the coculture of THP-1 cells with bronchial epithelial cells offers a potentially useful approach for the detection of respiratory sensitisers.

First Multigram Scale-Up and Synthesis of Novel Valerolactam- Benzimidazole Hybrid Anthelmintic

Background: Infections caused by helminth parasites are the main cause of economic losses in the livestock industry worldwide. The rapid resistance acquired by different parasites against commercially available drugs motivates the search, design and development of new compounds capable of overcoming this situation. Previously, our group reported the novel hybrid valerolactam-fenbendazole (VAL-FBZ) compound with in vitro anthelmintic activity and good ex vivo parasite permeation. Objective: Optimization of the novel hybrid VAL-FBZ compound synthesis and scale-up to multigram order necessary for in vivo assays. Methods: For the hybrid VAL-FBZ synthesis, convergent strategy was utilized. To obtain the benzimidazole core, widely available fenbendazole and L-Ornithine hydrochloride synthesis were used. The key step was the coupling reaction, for which an inexpensive coupling agent of uronium salt family was used. Optimization was carried out searching the minimizing risks and costs at up scaling to multigram level. Results: In the first stage, the most precursors of Valerolactam and Benzimidazole cores were synthesized on a decagram scale getting better results than previous reports. Also, the coupling reaction was carried out using HBTU to obtain VAL-FBZ with above 99% HPLC purity, and an overall yield of 48%. The successful synthesis was developed without chromatographic purification in any step to minimize few risks for the operator. Conclusion: Successfully, an efficient multigram and economic process is reported.

Dedicated to a peptide chemist of few words and immense impact: Louis A. Carpino

Dedicated to a peptide chemist of few words and immense impact: Louis A. Carpino

Facile cellulose nanofibrils amidation using a ‘one-pot’ approach

We report a ‘one-pot’ synthetic approach for TEMPO (2,2,6,6-tetramethyl-piperidin-1-oxyl) oxidized cellulose nanofibrils (TOCN) surface amidation by coupling up to 70% of superficial carboxylic units in TOCN with long alkyl chain primary amines (dodecylamine and octadecylamine) using TBTU [O-(1H-benzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium tetrafluoroborate] uronium salt as coupling agent. Raw TOCNs were produced from commercial microcrystalline cellulose and have an elongated highly tangled morphology with nanometric diameters and lengths up to several micrometers with a crystallinity of 62%. TOCN´s degree of oxidation was 1.25 mmol –COOH groups/g cellulose. The TOCN-amidated products named TOCN-AMDC12 and TOCN-AMDC18 depending upon to the primary amine used for the synthesis (dodecylamine –C12– for the former and octadecylamine –C18– for the latter), were characterized using field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy with attenuated total reflectance and X-ray diffraction. After amidation the hydrated gel-like TOCNs become a cream-colored gel insoluble in water and soluble in toluene. IR data indicates formation of an amide bond after the reaction, and FESEM images suggest a slight increase in fibril width as a result of the long-chain amide derivatives grafted onto the TOCN’s surface. Thermal stability and hydrophobicity of the products were determined using thermogravimetrical analysis and contact angle (CA) measurements. The TOCN-amidated products showed improved thermal properties with maximum decomposition temperatures of 347 and 358 °C, for TOCN-AMDC12 and TOCN-AMDC18 respectively, when compared to the raw TOCN (Td 340 °C); and high hydrophobicity with CA of 61° and 67°, respectively. Finally, the proposed method for TOCN amidation has advantages over current amidation approaches in that it is performed in aqueous media, does not require heating and occurs rapidly (2 h).

Amide and Peptide Bond Formation in Water at Room Temperature.

A general and environmentally responsible method for the formation of amide/peptide bonds in an aqueous micellar medium is described. Use of uronium salt (1-cyano-2-ethoxy-2-oxoethylidenaminooxy)dimethylaminomorpholinocarbenium hexafluorophosphate (COMU) as a coupling reagent, 2,6-lutidine, and TPGS-750-M represents mild conditions associated with these valuable types of couplings. The aqueous reaction medium is recyclable leading to low E Factors.

TOMBU and COMBU as Novel Uronium-type peptide coupling reagents derived from Oxyma-B.

Here we describe two novel uronium salts, TOMBU and COMBU, derived from the recently described Oxyma-B for use in peptide bond synthesis. These coupling reagents are more stable than COMU in DMF. Furthermore, using various peptide synthetic models in solution and solid-phase synthesis, we reveal that they show better performance than HBTU in terms of preserving chiral integrity and coupling yields, but slightly worse performance than COMU.

A Direct Synthesis of a Strongly Zwitterionic 6,6'-Diaminofulvalene

Upon reaction of the dipyrido-anellated guanidinium salt 1 with one equivalent of CpNa we were able to synthesize the dipyrido-anellated diaminofulvalene 4 in one step in 33% isolated yield. This shortens the initial route that applies a literature-known fulvalene synthesis via uronium salt 3 by two steps and avoids the need for a sacrificial equivalent of CpNa. Although the X-ray structure analysis reveals a shorter exocyclic double bond than observed in the diaminofulvalene V, a theoretical analysis based upon DFT calculations shows a stronger zwitterionic character for the dipyridofulvalene 4.

COMU: scope and limitations of the latest innovation in peptide acyl transfer reagents

The methodology for peptide bond formation is undergoing a continuous evolution where the main actors are being renewed. In recent years, coupling reagents based on the Oxyma scaffold, such as the uronium salt COMU, has been a groundbreaking contribution to the field. The advantages of COMU over classic benzotriazole-based reagents (HATU, HBTU, HCTU, TBTU) were proven in terms of solubility and coupling efficiency in bulky junctions in our groups and others. However, some aspects of the use of COMU need to be revised and improved, such as the stability of commercial samples in organic solvents, which hampers the compatibility with long synthesis in automated synthesizers. In this review, an overview of the main features and suggestions to improve the use of COMU are presented, along with a discussion on the best conditions for its use in microwave-assisted peptide robots.

Facile Preparation of Mono‐, Di‐ and Mixed‐Carboxylato Platinum(IV) Complexes for Versatile Anticancer Prodrug Design

Facile strategies were developed for the versatile functionalization of platinum(IV) axial sites, allowing for easy accessibility to unsymmetric mono- and mixed-carboxylato, as well as symmetric di-substituted platinum(IV) complexes. The first method involves the direct oxidation and carboxylation of the platinum(II) center using an appropriate peroxide and the carboxylate of choice to firstly yield a monocarboxylato monohydroxido platinum(IV) complex. This platinum(IV) intermediate can undergo further carboxylation to give rise to a mixed-carboxylato platinum(IV) complex. The second method involves the activation of the carboxylate of choice by a common carbodiimide coupling reagent, and its reaction with a dihydroxido platinum(IV) precursor to give the monocarboxylato platinum(IV) complex. Uronium salts can be employed to promote efficient dicarboxylation of the dihydroxido platinum(IV) precursor. Lastly, an axial azide pendant group was demonstrated to be suitable for orthogonal "click" conjugation reactions.

Consequences of the One‐Electron Reduction and Photoexcitation of Unsymmetric Bis‐imidazolium Salts

Coupling of uronium salts with in situ generated N-heterocyclic carbenes provides straightforward access to symmetrical [4](2+) and unsymmetrical bis-imidazolium salts [6](2+) and [9](2+) . As indicated by cyclic and square-wave voltammetry, [6](2+) and [9](2+) can be (irreversibly) reduced by one electron. The initially formed radicals [6](.+) and [9](.+) undergo further reactions, which were probed by EPR spectroscopy and density functional calculations. The final products of the two-electron reduction are the two carbenes. Upon irradiation with UV light both [6](2+) and [9](2+) emit at room temperature in solution but with dramatically different characteristics. The different fluorescence behavior is analyzed by emission spectroscopy and interpreted by using time-dependent density functional calculations as largely due to different excited-state dynamics of [6](2+) and [9](2+) . The geometries of both radicals [6](.+) and [9](.+) and excited states {[6](2+) }* and {[9](2+) }* are substantially different from those of the parent ground-state molecules.

Efficient and versatile COMU-mediated solid-phase submonomer synthesis of arylopeptoids (oligomeric N-substituted aminomethyl benzamides)

The development of a highly efficient methodology for solid-phase synthesis of para- and meta-arylopeptoids (oligomeric N-substituted aminomethyl benzamides) with free acids or free amides at the C-terminus is described. The arylopeptoids were synthesised by means of a convenient submonomer protocol in which the arylopeptoid residues were created in an iterative manner on the growing chain using an acylation-substitution cycle. The uronium salt COMU was found to be the most efficient reagent for ensuring fast and clean couplings of the benzoic acid building blocks.

Synthesis of 2-(4,6-Dimethoxy-1,3,5-triazin-2-yloxyimino) Derivatives: Application in Solution Peptide Synthesis

A new class of 1,3,5-triazinyloxyimino derivatives were prepared, characterized and tested for reactivity in solution peptide synthesis. The new triazinyloxyimino derivatives failed to activate the carboxyl group during formation of peptide bonds, but gave the corresponding N-triazinyl amino acid derivatives as a major product. The oxyma (ethyl 2-cyano-2-(hydroxyimino)acetate) uronium salt was superior to other uronium salts in terms of racemization, while 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT, 9) gave the best results.

ChemInform Abstract: Reduction of Dipyrido Ureas via 6‐Alkyloxydipyrido[1,2‐c;2′,1′‐e]imidazolium Salts.

AbstractReduction of dipyrido uronium salts (VI), which are readily synthesized by alkylation of dipyrido ureas (IV) with Meerwein′s reagent, offers a new and mild synthetic route to dipyridoimidazolium salts (VII).

Polysulfonylamine, CLXXXIX [1]. Weitere Beispiele für die O-Protonierung von Harnstoffen mit Di(organosulfonyl)aminen: Bildung und Kristallstrukturen von 1,1-Dimethyluroniumdi( 4-fluorbenzolsulfonyl)amid und Di(1-methylharnstoff)- hydrogen(I)-di(4-fluorbenzolsulfonyl)amid/ Polysulfonylamines, CLXXXIX. Additional Examples of the O-Protonation of Ureas by Di(organosulfonyl)amines: Formation and Crystal Structures of 1,1-Dimethyluronium Di(4-fluorobenzenesulfonyl)amide and Di(1-methylurea)hydrogen(I)Di(4-fluorobenzenesulfonyl) amide

Co-crystallization of N-methyl-substituted ureas with di(organosulfonyl)amines, (RSO2)2NH, leads unpredictably to either molecular co-crystals or, via proton transfer, to uronium salts. As a sequel to former reports, this communication describes the formation and the crystal structures of the new ionic compounds 1,1-dimethyluronium di(4-fluorobenzenesulfonyl)amide (1, monoclinic, space group P21/c, Z´ = 1) and di(1-methylurea)hydrogen(I) di(4-fluorobenzenesulfonyl)amide (2, triclinic, P1̄, Z´ = 1); both salts were obtained from dichloromethane/petroleum ether. In the structure of 2, the urea moieties of the cationic homoconjugate are connected by a very short [O-H· · ·O]+ hydrogen bond [d(O· · ·O) = 244.6(2) pm, θ (O-H· · ·O)≈170°, bridging H atom asymmetrically disordered over two positions]. The O-protonation induces a specific elongation of the C-O bond lengths to 131.2(2) pm in 1 or 129.5(2) and 127.4(2) pm in 2, as compared to literature data of ca. 126 pm for the unprotonated ureas. Both crystal structures are dominated by conventional two- and threecentre hydrogen bonds, which involve the OH and all NH donors and give rise to one-dimensional cation-anion arrays. In particular, the ionic entities of 1 are alternatingly associated into simple chains propagated by glide-plane operations parallel to the c axis, whereas the donor-richer structure of 2 displays inversion symmetric dimers of formula units, which are further hydrogen-bonded into strands propagated by translation parallel to the a axis.

Reduction of Dipyrido Ureas via 6-Alkyloxydipyrido[1,2-c;2´,1´-e]imidazolium Salts

Dipyrido uronium salts can readily be synthesized by alkylation of dipyrido ureas with Meerwein’s reagent. Compared to the corresponding ureas, the uronium salts are more reactive towards basic or reducing agents like metal hydrides. Reactivity studies show that the uronium salts can react as alkylating agents towards DMSO, DBU and NaOEt along with release of the respective dipyrido ureas. In contrast, reduction of the dipyrido uronium salts with sodium borohydride or sodium trimethoxyborohydride in dry and degassed acetonitrile leads to the imidazolium salts 7a and 7b in moderate yields. Analysis of the by-products reveals an in situ carbene formation which can be reversed by using degassed but wet acetonitrile as solvent. The yield of 7b was increased significantly by these means.

A Novel Family of Onium Salts Based Upon Isonitroso Meldrum's Acid Proves Useful as Peptide Coupling Reagents

AbstractA new family of uronium salts (HTMU, HMMU, and HDmPyMU) based on isonitroso Meldrum's acid (HONM) are reported as stand‐alone coupling reagents. Amide bond formation with the use of these reagents occurred more quickly than that with other uronium salts as a result of the presence of a neighboring group effect with a cyclic structure. Thus, these novel onium salts were often more effective in the acylation of poor nucleophiles and in the control of optical purity than related Oxyma‐ and benzotriazole‐based reagents. Among the HONM derivatives, HMMU showed the best performance in reducing racemization and assembling demanding sequences such as the Aib‐ACP decapeptide or analogues of Leu‐enkephalin pentapeptide. Furthermore, the scope and limitations of the use of HONM as an additive in combination with carbodiimides is discussed.

COMU: A Safer and More Effective Replacement for Benzotriazole‐Based Uronium Coupling Reagents

We describe a new family of uronium-type coupling reagents that differ in their iminium moieties and leaving groups. The presence of the morpholino group in conjunction with an oxime derivative--especially ethyl 2-cyano-2-(hydroxyimino)acetate (Oxyma)--had a marked influence on the solubilities, stabilities, and reactivities of the reagents. Finally, the new uronium salt derived from Oxyma (COMU) performed extremely well in the presence of only 1 equiv of base, thereby confirming the effect of the hydrogen bond acceptor in the reaction. COMU also showed a less hazardous safety profile than the benzotriazole-based HDMA and HDMB, which exhibited unpredictable autocatalytic decompositions. Furthermore, the Oxyma moiety contained in COMU suggests a lower risk of explosion than in the case of the benzotriazole derivatives.

Phenanthroline-Derived Ratiometric Chemosensor for Ureas

The syntheses of 1,10-phenanthroline fluorophore-based chemosensor 7 and its truncated analog 9 are reported. Interactions of these compounds with urea, thiourea, 1,3-dimethylurea, tetrahydropyrimidin-2(1H)-one, imidazolidin-2-one, and selected uronium salts were assessed by three-dimensional excitation-emission spectroscopy, UV-vis absorbance, and fluorescence titrations. Chemosensor 7 was found to be capable of distinguishing between neutral ureas and their salts, by producing a different optical response for each type of compounds. The complexation of urea by 7 was also studied by selective-NOE 1H NMR, 13C NMR (using 13C-labeled guest), and MALDI-TOF mass spectrometry. In addition, we performed DFT calculations (B3LYP 3-21g** level) for structures of complexes of 7 with urea, imidazolidin-2-one, and tetrahydropyrimidin-2(1H)-one. Development of chemosensor 7-type compounds in conjunction with differential excitation-emission spectroscopy represents an important step toward the development of novel tools for ureas and their salts analysis.

Beating the Hydrogen Bond: First Selective and High-YieldingN-Acylation Process for an α,β-Diaminoalcohol

The first selective and high-yielding N-acylation of an α,β-diaminoalcohol is reported, as well as the first use as N-acylation agent of a S-mercaptobenzothiazolyl thioester of an α,β-unsaturated carboxylic acid. Other conventional coupling methods (acid chloride, uronium salt, carbonyl diimidazole, phosphonium salt) gave low yields respectively difficult to purify mixtures of N- and O,N-diacylated product (4b), due to the unusually high reactivity of the primary hydroxyl group caused by an intramolecular hydrogen bond to the dialkylamino moiety in the β-position. Both the cinnamic thioester preparation and the coupling step were safely and reproducibly scaled up to a chromatography-free process in the pilot plant.

N‐Acylation in Combinatorial Chemistry

AbstractFor Abstract see ChemInform Abstract in Full Text.