Acylhydrazone Formation Research Articles

Hydrogen Sulfate-Templated Synthesis of a Bis-Ferrocene Macrocycle

AbstractA bis-ferrocene macrocycle was synthesized by reversible acylhydrazone formation under thermodynamic control, starting from a ferrocene functionalized with hydrazide and protected aldehyde moieties. A hydrogen sulfate anion acts as a template to direct the synthesis specifically toward the macrocyclic dimer, due to a weak but selective binding interaction. This work highlights the utility of dynamic combinatorial chemistry as an approach to macrocycle synthesis.

Crosslinking through acyl hydrazone formation by reacting water soluble polyurethanes derived from ketone diol comonomers and those containing hydrazide pendant groups

Fully water-based formulations are gaining traction with the sustainability initiative for various applications. In this regard, producing the components required for making these formulations by eliminating toxic reagents is highly desirable. Hydrazone formation is widely used as a non-hazardous curing process in coating applications. Generally, carbonyl compounds that are used in the hydrazone forming reaction are nearly exclusively derived from vinyl polymers. This article describes a facile process for making carbonyl group containing diols and keto group containing water-soluble polymers therefrom by step growth or condensation polymerization. A water-soluble polymeric hydrazide was prepared by DCC coupling reaction between hydrazine hydrate and a polyurethane bearing pendant carboxylic acid group. The aqueous solutions of keto group containing, and hydrazide pendant polymers were mixed and coated on glass slides. Subsequently, the acyl hydrazone formation was studied under static heating in a hot air oven and a dynamic condition provided by blowing air under ambient conditions with a hair dryer. The dynamic nature of the latter process more than compensated the absence of direct heating to induce drying and hydrazone formation sequentially as studied by FT-IR spectroscopy. The cured slides were stable to water and were sensitive to weakly acidic aqueous solutions (about pH 3) formed by dissolving citric acid in water. The keto group containing diols enabled the hydrazone formation in polymer pairs entirely derived from step growth polymerization. As step growth or condensation polymers potentially offer both degradability and recycling through the cleavage of labile bonds prevailing in the polymer backbone, these keto group containing diols are preferable alternatives to keto group containing vinyl monomers commonly used for making hydrazones.

On-Demand Release of Secondary Amine Bases for the Activation of Catalysts and Crosslinkers.

Dynamic covalent (DCv) ureas have been used abundantly to design self-healing materials. We demonstrate that apart from self-healing materials, the species present in the equilibrium of DCv ureas can be employed as responsive organocatalysts. Easily controllable stimuli like heat or addition of water shift the equilibrium towards isocyanate and free base which can function as an in situ released reagent. We demonstrate this application of DCv ureas with two examples. Firstly, we use the liberated base to catalytically activate a latent organocatalyst for acylhydrazone formation. Secondly, this base can be employed in an equimolar manner to trigger the release of nitrile-N-oxides from chlorooximes, which react with acrylate-terminated polymers to form an isoxazoline polymer gel.

Self-Assembly of a Purely Organic Bowl in Water via Acylhydrazone Formation.

A bowl-shaped molecule can be self-assembled by condensing a triscationic hexaaldehyde compound and three equiv. of a dihydrazide linkers in pure water. The molecular bowl is thus composed of a triscationic π-electron deficient platform, as well as a hexagonal rim that contains six acylhydrazone functions. When the counteranions are chloride, the solid-state structure reveals that this molecular bowl undergoes dimerization via N-H···Cl hydrogen bonds, forming a cage-like dimer with a huge inner cavity. This molecular bowl can employ its cavity to accommodate a hydrophobic guest, namely 1-adamantanecarboxylic acid in aqueous media.

Identification and synthesis of selective cholesterol esterase inhibitor using dynamic combinatorial chemistry

In this study, the concept of dynamic combinatorial chemistry (DCC) was applied to explore novel cholesterol esterase (CEase) inhibitors. In the presence of enzyme, two substrates (A1H3 and A2H3) were amplified from the dynamic combinatorial library (DCL), which was generated through reversible acylhydrazone formation reaction. In the in vitro biological evaluation, compound A1H3 exhibited not only potent (IC50 in nanomolar range) but also selective inhibition (>120 folds of selectivity for CEase over AChE). Furthermore, the binding pattern and possible binding mechanism were investigated in the kinetic experiment and molecular docking study, respectively.

Multivalent butyrylcholinesterase inhibitor discovered by exploiting dynamic combinatorial chemistry

In this study, we report the generation of a polymer-based dynamic combinatorial library (DCL) incorporating exchangeable side chains using acylhydrazone formation reaction. In combination with tetrameric butyrylcholinesterase (BChE), the most potent binding side chain was identified, and the information obtained was further used for the synthesis of a multivalent BChE inhibitor. In the in vitro biological evaluation, this multivalent inhibitor exhibited not only better inhibitory effect than the commercial reference but also high selectivity on BChE over acetylcholinesterase (AChE).

Nucleophilic catalysis of p-substituted aniline derivatives in acylhydrazone formation and exchange.

Hydrazone bond formation is a versatile reaction employed in several research fields. It is one of the most popular reversible reactions in dynamic combinatorial chemistry. Under physiological conditions, hydrazone exchange benefits from the addition of a nucleophilic catalyst. We report a mechanistic study and superior performance of electron-rich p-substituted aniline derivatives as catalysts for efficient hydrazone formation and exchange in both protic and aprotic solvents. Rigorous kinetic analyses demonstrate that imine formation with 3-hydroxy-4-nitrobenzaldehyde and aniline derivatives proceeds with unprecedented third-order kinetics in which the aldehyde consistently shows a partial order of two. Computational investigations provide insights into the mechanisms of these transformations.

Quantification of Aldehydes on Polymeric Microbead Surfaces via Catch and Release of Reporter Chromophores.

Aldehyde moieties on 2D-supports or micro- and nanoparticles can function as anchor groups for the attachment of biomolecules or as reversible binding sites for proteins on cell surfaces. The use of aldehyde-based materials in bioanalytical and medical settings calls for reliable methods to detect and quantify this functionality. We report here on a versatile concept to quantify the accessible aldehyde moieties on particle surfaces through the specific binding and subsequent release of small reporter molecules such as fluorescent dyes and nonfluorescent chromophores utilizing acylhydrazone formation as a reversible covalent labeling strategy. This is representatively demonstrated for a set of polymer microparticles with different aldehyde labeling densities. Excess reporter molecules can be easily removed by washing, eliminating inaccuracies caused by unspecific adsorption to hydrophobic surfaces. Cleavage of hydrazones at acidic pH assisted by a carbonyl trap releases the fluorescent reporters rapidly and quasi-quantitatively and allows for their fluorometric detection at low concentration. Importantly, this strategy separates the signal-generating molecules from the bead surface. This circumvents common issues associated with light scattering and signal distortions that are caused by binding-induced changes in reporter fluorescence as well as quenching dye-dye interactions on crowded particle surfaces. In addition, we demonstrate that the release of a nonfluorescent chromophore via disulfide cleavage and subsequent quantification by absorption spectroscopy gives comparable results, verifying that both assays are capable of rapid and sensitive quantification of aldehydes on microbead surfaces. These strategies enable a quantitative comparison of bead batches with different functionalization densities, and a qualitative prediction of their coupling efficiencies in bioconjugations, as demonstrated in reductive amination reactions with Streptavidin.

Comparison of affinity ranking by target-directed dynamic combinatorial chemistry and surface plasmon resonance

Target-directed dynamic combinatorial chemistry (tdDCC) is a powerful method to screen ligands for pharmacologically relevant targets. Generating a dynamic library from reversibly reacting building blocks in the presence of a target protein leads to the amplification of the most potent library constituents. In previous studies on tdDCC, these compounds were identified in a qualitative “hit/no-hit”-manner. However, the precise relationship between the degree of amplification and the affinity of the library constituent has not yet been evaluated. To study the amplification–affinity relationship, we compared tdDCC experiments, employing reversible acylhydrazone formation and the bacterial adhesin FimH as a target, with affinities of the library constituents as determined by surface plasmon resonance.

Self-healing hydrogels with stimuli responsiveness based on acylhydrazone bonds

Thermo-responsive hydrogels consist of natural and synthetic macromolecules were presented based on dynamic acylhydrazone formation, which is self-healable and pH-sensitive. Hydrazone groups modified poly(N-isopropylacrylamide) (PNIPAM) copolymers and aldehyde groups exposed oxidized sodium alginate (OSA) were adopted to prepare the functional hydrogels without any catalyst. Specifically, acylhydrazone bonds, which were served as cross-linking points, endowed the hydrogels with self-healing property confirmed by rheological recovery test. Meanwhile, the hydrogels displayed obvious reversible sol-gel transition under different conditions, leading to that the hydrogels were able to be reshaped repeatedly exposed to pH trigger. Besides, due to the PNIPAM segments were served as the frameworks, the hydrogels were testified to exhibit tunable phase transition temperature between 30 °C and 40 °C around body temperature, which was regulated by variation in aldehyde and hydrazone ratio. The as-prepared hydrogels with self-healing and dual stimuli-responsive properties are beneficial to their bioapplications as drug delivery system, biosensors and soft tissue filling agents, etc.

Self-Activated Healable Hydrogels with Reversible Temperature Responsiveness.

The self-healable polymer hydrogel along with reversible temperature responsiveness was prepared through self-catalyzed dynamic acylhydrazone formation and exchange without any additional stimulus or catalyst. The hydrogel was prepared from a copolymer of N-isopropylacrylamide and acylhydrazine P(NIPAM-co-AH) cross-linked by PEO dialdehyde. Besides self-healed under catalysis of acid and aniline, the hydrogel can also self-heal activated by excess of acylhydrazine groups. Without interference of catalyst during the hydrogel formation and self-healing, this kind of hydrogel prepared from biocompatible polymers can be used in more areas including biotechnology and be more persistent. The hydrogel with a large part of the PNIPAM segment also showed temperature responsiveness around body temperature influenced by the variation in group ratio. This self-healable hydrogel has great potential application in areas related to bioscience and biotechnology.

Acylhydrazone-based dynamic combinatorial libraries: study of the thermodynamic/kinetic evolution, configurational and coordination dynamics

Se describe la selectividad cinética y termodinámica de la formación de acil-hidrazona en librerías combinatorias dinámicas (DCL). Se generaron reacciones competitivas a partir de hidrazidas: isoniazida, 4-nitro-benzohidrazida, 4-dimetilamino-benzohidrazida y hidrazida nicotínica; así como a partir de los derivados de aldehído: benzaldehído y 2-piridin-carboxaldehido. Las especies obtenidas y la distribución de los DCLs fueron monitoreados mediante espectroscopia 1H-NMR, encontrándose que las acil-hidrazonas que contenían la 4-dimetilamino-benzohidrazida son tanto el producto cinético, como el termodinámico de sus respectivas librerías. También se investigaron las dinámicas de configuración y de coordinación para algunas de estas librerías. Los resultados obtenidos permitieron estudiar la redistribución de los componentes y la amplificación de uno o más productos usando luz e iones metálicos como plantillas físicas y químicas, respectivamente.

Templated Chromophore Assembly by Dynamic Covalent Bonds

AbstractThrough the simultaneous use of three orthogonal dynamic covalent reactions, namely disulfide, boronate, and acyl hydrazone formation, we conceived a facile and versatile protocol to spatially organize tailored chromophores, which absorb in the blue, red, and yellow regions, on a preprogrammed α‐helix peptide. This approach allowed the assembly of the dyes in the desired ratio and spacing, as dictated by both the relative positioning and distribution of the recognition units on the peptide scaffold. Steady‐state UV/Vis absorption and emission studies suggest an energy transfer from the yellow and red donors to the blue acceptor. A molecular dynamics simulation supports the experimental findings that the helical structure is maintained after the assembly and the three dyes are confined in defined conformational spaces.

Templated Chromophore Assembly by Dynamic Covalent Bonds.

Through the simultaneous use of three orthogonal dynamic covalent reactions, namely disulfide, boronate, and acyl hydrazone formation, we conceived a facile and versatile protocol to spatially organize tailored chromophores, which absorb in the blue, red, and yellow regions, on a preprogrammed α-helix peptide. This approach allowed the assembly of the dyes in the desired ratio and spacing, as dictated by both the relative positioning and distribution of the recognition units on the peptide scaffold. Steady-state UV/Vis absorption and emission studies suggest an energy transfer from the yellow and red donors to the blue acceptor. A molecular dynamics simulation supports the experimental findings that the helical structure is maintained after the assembly and the three dyes are confined in defined conformational spaces.

Release of bioactive volatiles from supramolecular hydrogels: influence of reversible acylhydrazone formation on gel stability and volatile compound evaporation

In the presence of alkali metal cations, guanosine-5'-hydrazide (1) forms stable supramolecular hydrogels by selective self-assembly into a G-quartet structure. Besides being physically trapped inside the gel structure, biologically active aldehydes or ketones can also reversibly react with the free hydrazide functions at the periphery of the G-quartet to form acylhydrazones. This particularity makes the hydrogels interesting as delivery systems for the slow release of bioactive carbonyl derivatives. Hydrogels formed from 1 were found to be significantly more stable than those obtained from guanosine. Both physical inclusion of bioactive volatiles and reversible hydrazone formation could be demonstrated by indirect methods. Gel stabilities were measured by oscillating disk rheology measurements, which showed that thermodynamic equilibration of the gel is slow and requires several cooling and heating cycles. Furthermore, combining the rheology data with dynamic headspace analysis of fragrance evaporation suggested that reversible hydrazone formation of some carbonyl compounds influences the release of volatiles, whereas the absolute stability of the gel seemed to have no influence on the evaporation rates.

Nucleophilic catalysis of acylhydrazone equilibration for protein-directed dynamic covalent chemistry

Dynamic covalent chemistry uses reversible chemical reactions to set up an equilibrating network of molecules at thermodynamic equilibrium, which can adjust its composition in response to any agent capable of altering the free energy of the system. When the target is a biological macromolecule, such as a protein, the process corresponds to the protein directing the synthesis of its own best ligand. Here, we demonstrate that reversible acylhydrazone formation is an effective chemistry for biological dynamic combinatorial library formation. In the presence of aniline as a nucleophilic catalyst, dynamic combinatorial libraries equilibrate rapidly at pH 6.2, are fully reversible, and may be switched on or off by means of a change in pH. We have interfaced these hydrazone dynamic combinatorial libraries with two isozymes from the glutathione S-transferase class of enzyme, and observed divergent amplification effects, where each protein selects the best-fitting hydrazone for the hydrophobic region of its active site.

Glycodynamers: Dynamic Polymers Bearing Oligosaccharides Residues − Generation, Structure, Physicochemical, Component Exchange, and Lectin Binding Properties

Dynamic glycopolymers have been generated by polycondensation through acylhydrazone formation between components bearing lateral bioactive oligosaccharide chains. They have been characterized as bottlebrush type by cryo-TEM and SANS studies. They present remarkable fluorescence properties whose emission wavelengths depend on the constitution of the polymer and are tunable by constitutional modification through exchange/incorporation of components, thus also demonstrating their dynamic character. Constitution-dependent binding of these glycodynamers to a lectin, peanut agglutinin, has been demonstrated.

Controlled release of volatile aldehydes and ketones by reversible hydrazone formation – “classical” profragrances are getting dynamic

Dynamic mixtures obtained by reversible covalent acylhydrazone formation of fragrance aldehydes and/or ketones and a hydrazide in water were found to be efficient delivery systems for the controlled release of highly volatile organic molecules.

Gelation-driven component selection in the generation of constitutional dynamic hydrogels based on guanine-quartet formation

The guanosine hydrazide 1 yields a stable supramolecular hydrogel based on the formation of a guanine quartet (G-quartet) in presence of metal cations. The effect of various parameters (concentration, nature of metal ion, and temperature) on the properties of this gel has been studied. Proton NMR spectroscopy is shown to allow a molecular characterization of the gelation process. Hydrazide 1 and its assemblies can be reversibly decorated by acylhydrazone formation with various aldehydes, resulting in formation of highly viscous dynamic hydrogels. When a mixture of aldehydes is used, the dynamic system selects the aldehyde that leads to the most stable gel. Mixing hydrazides 1, 9 and aldehydes 6, 8 in 1:1:1:1 ratio generated a constitutional dynamic library containing the four acylhydrazone derivatives A, B, C, and D. The library constitution displayed preferential formation of the acylhydrazone B that yields the strongest gel. Thus, gelation redirects the acylhydrazone distribution in the dynamic library as guanosine hydrazide 1 scavenges preferentially aldehyde 8, under the pressure of gelation because of the collective interactions in the assemblies of G-quartets B, despite the strong preference of the competing hydrazide 9 for 8. Gel formation and component selection are thermoreversible. The process amounts to gelation-driven self-organization with component selection and amplification in constitutional dynamic hydrogels based on G-quartet formation and reversible covalent connections. The observed self-organization and component selection occur by means of a multilevel self-assembly involving three dynamic processes, two of supramolecular and one of reversible covalent nature. They extend constitutional dynamic chemistry to phase-organization and phase-transition events.