Free Primary Amines Research Articles

β‐Phenethylamine Synthesis: N‐Pyridinium Aziridines as Latent Dual Electrophiles

Abstractβ‐Phenethylamines are widely represented in biologically and pharmacologically active organic small molecules. Here, we introduce N‐pyridinium aziridines as latent dual electrophiles for the synthesis of β‐phenethylamines. Bromide‐promoted ring opening generates β‐halopyridinium amines. Selective Ni‐catalyzed C−C cross‐coupling between organozinc nucleophiles and the benzylic C−Br electrophile affords a diverse family of β‐functionalized phenethylaminopyridinium salts, and coupling is stereoconvergent in the presence of chiral ligands. Subsequent Ni‐catalyzed reductive N−N bond activation within the β‐functionalized phenethylaminopyridinium salts furnishes the products of formal olefin carboamination. Other reductive N−N cleavage reactions are demonstrated to provide access to free primary amines, alkylated amines, heterocycles, and products derived from N‐centered radical chemistry. The developed reaction sequence can be implemented in the context of complex molecules and natural product derivatives. Together, the described results provide a general and modular synthesis of β‐phenethylamines and significantly expand the utility of N‐pyridinium aziridines as linchpins in chemical synthesis.

β-Phenethylamine Synthesis: N-Pyridinium Aziridines as Latent Dual Electrophiles.

β-Phenethylamines are widely represented in biologically and pharmacologically active organic small molecules. Here, we introduce N-pyridinium aziridines as latent dual electrophiles for the synthesis of β-phenethylamines. Bromide-promoted ring opening generates β-halopyridinium amines. Selective Ni-catalyzed C-C cross-coupling between organozinc nucleophiles and the benzylic C-Br electrophile affords a diverse family of β-functionalized phenethylaminopyridinium salts, and coupling is stereoconvergent in the presence of chiral ligands. Subsequent Ni-catalyzed reductive N-N bond activation within the β-functionalized phenethylaminopyridinium salts furnishes the products of formal olefin carboamination. Other reductive N-N cleavage reactions are demonstrated to provide access to free primary amines, alkylated amines, heterocycles, and products derived from N-centered radical chemistry. The developed reaction sequence can be implemented in the context of complex molecules and natural product derivatives. Together, the described results provide a general and modular synthesis of β-phenethylamines and significantly expand the utility of N-pyridinium aziridines as linchpins in chemical synthesis.

Native Amino Group Directed Meta-Selective C-H Arylation of Primary Amines Via Pd/Norbornene Catalysis.

The selective functionalization of remote C-H bonds in free primary amines holds significant promise for the late-stage diversification of pharmaceuticals. However, to date, the direct functionalization of the meta position of amine substrates lacking additional directing groups remains underexplored. In this Letter, we present a successful meta-C-H arylation of free primary amine derivatives using aryl iodides, resulting in synthetically valuable yields. This meta-selective C-H functionalization is achieved through a sequence involving native amino-directed Pd-catalyzed seven-membered cyclometalation, followed by the utilization of a norbornene-type transient mediator.

Catalyst-free electrochemical sulfonylation of amines with sulfonyl hydrazide in aqueous medium.

An efficient electrochemical sulfonylation of amines with sulfonyl hydrazides under exogenous-oxidant-free and catalyst-free conditions in aqueous medium was developed. A wide variety of sulfonamides were prepared via a simple electrochemical process from various cyclic or acyclic secondary amines, as well as more challenging free primary amines with equivalent amount of aryl/heteroaryl hydrazides in air under mild conditions. This protocol was found to be excellent in terms of facile scale-up, and to show great potential in the modification/synthesis of bioactive compounds. The reaction mechanism was investigated by a series of control experiments and CV studies, which indicated that the reaction might have gone through a radical pathway. Also, n-Bu4NBr served both as a supporting electrolyte and a redox agent to yield sulfonyl radical species and sulfonyl cations from sulfonyl hydrazides.

AMINOMETHYLATION OF DIHYDROXYACETOPHENONES

Aminomethylation of trifunctional 2.5-dihydroxyacetophenone (2.5-DHA) and 2.4-dihydro¬xy¬aceto-phenone (2.4-DHA) by free primary and secondary amines in the presence of formaldehyde has been conducted for the first time. It has been established that 2.5-DHA is aminomethylated with secondary heterocyclic amines - piperidine and morpholine and with aqueous formaldehyde solution in alcohol medium at 500C. The reaction proceeds simultaneously via all three functions forming stable C,O-substitution products − β-amino-2-aminomethoxy-4-aminomethyl-5-hydroxypropiophenones. The aminomethylation reaction of 2.4-DHA proceeds by phenolic function only under the action of methylene bis-amines piperidine and morpholine in absolute alcohol at 200C with the formation of 2.4-dihydroxy-3-aminomethylacetophenones. Aminomethylation of 2.4-DHA with primary aromatic amines - n-to¬luidine and n-anisidine proceeds in absolute alcohol under the action of formaldehyde in the form of paraform in the presence of catalytic amounts of KOH at 500C. The reaction products are 3.4-dihydro-2H-1.3-benzoxazines. 2.5-DHA does not react with primary aromatic amines and formaldehyde. The composition and structure of compounds synthesized have been established according to the data of ele¬mental and spectral methods of analysis

Determination of extent of PEGylation using denaturing capillary isoelectric focussing

Conjugated proteins and enzymes are often formed using N-hydroxysuccinimide (NHS) chemistry, which reacts with free primary amines resulting in a loss of charge and a reduction in isoelectric point (pI). Measurement of the extent of reaction of these conjugates is critical for biopharmaceutical developers. Due to this change in protein charge state, denaturing capillary isoelectric focussing (cIEF) offers a potentially straightforward and convenient approach for extent-of-reaction quantification. Here, we demonstrate the potential of this technique with poly(ethylene glycol) (PEG) conjugates of Erwinia chrysanthemil-asparaginase (ErA). Development of an appropriate sample preparation technique is critical to achieving reproducible cIEF electropherograms, particularly for denaturation-resistant proteins such as ErA, and an emphasis was placed on this during development of the PEG-ErA cIEF method. cIEF electropherograms demonstrating a distribution of PEGylation states in a bell-shaped curve were obtained, and assignment of PEGylation states to these peaks was critical to routine use of the method. The method is sensitive enough to resolve non-lysine adducts of PEG (such as those conjugated to histidine residues) and was shown to give reproducible results over a 2 year period. Biopharmaceutical developers should consider cIEF for extent of reaction monitoring and measurement for conjugates of free amine groups.

Functionalized Enzyme-Responsive Biomaterials to Model Tissue Stiffening in vitro.

The mechanical properties of the cellular microenvironment play a crucial role in modulating cell function, and many pathophysiological processes are accompanied by variations in extracellular matrix (ECM) stiffness. Lysyl oxidase (LOx) is one of the enzymes involved in several ECM-stiffening processes. Here, we engineered poly(ethylene glycol) (PEG)-based hydrogels with controlled mechanical properties in the range typical of soft tissues. These hydrogels were functionalized featuring free primary amines, which allows an additional chemical LOx-responsive behavior with increase in crosslinks and hydrogel elastic modulus, mimicking biological ECM-stiffening mechanisms. Hydrogels with elastic moduli in the range of 0.5–4 kPa were obtained after a first photopolymerization step. The increase in elastic modulus of the functionalized and enzyme-responsive hydrogels was also characterized after the second-step enzymatic reaction, recording an increase in hydrogel stiffness up to 0.5 kPa after incubation with LOx. Finally, hydrogel precursors containing HepG2 (bioinks) were used to form three-dimensional (3D) in vitro models to mimic hepatic tissue and test PEG-based hydrogel biocompatibility. Hepatic functional markers were measured up to 7 days of culture, suggesting further use of such 3D models to study cell mechanobiology and response to dynamic variation of hydrogels stiffness. The results show that the functionalized hydrogels presented in this work match the mechanical properties of soft tissues, allow dynamic variations of hydrogel stiffness, and can be used to mimic changes in the microenvironment properties of soft tissues typical of inflammation and pathological changes at early stages (e.g., fibrosis, cancer).

Native Directed Site-Selective δ-C(sp3)–H and δ-C(sp2)–H Arylation of Primary Amines

A Pd(II)-catalyzed, selective δ-C(sp3)–H and δ-C(sp2)–H arylation method for free primary aliphatic amines using NH2 as a native directing group has been developed. A variety of free primary amines with adjacent quaternary centers and/or with alpha esters react with a diverse range of aryl and heteroaryl iodides to provide δ-aryl and δ-heteroaryl amines.

Preparation of Pt-Catalyst by Poly(p-phenylenediamine) Nanocomposites Assisted by Microwave Radiation for Proton Exchange Membrane Fuel Cell.

The Pt elements are prepared via the redox reaction with microwave (MW) irradiation in the presence of poly(p-phenylenediamine) (PpPD) which is polymerized on XC72 carbon matrix (PpPD/XC72), behaving as reducing agent. The free primary amines of PpPD are actually converted (oxidized) to secondary ones (5,10-dihydrophenazine) after MW irradiation. Transmission electronic microscopy (TEM) micrographs reveal the prepared Pt nanoparticles are well-dispersed on the carbon matrix like commercial Pt-implanted carbon nanocomposite (Pt/C). From the residue weights of thermogravimetric analysis (TGA) thermograms of Pt-loaded PpPD/XC72 (PpPD/XC72-Pt-MW), more Pt (18.49 wt %) nanoparticles are implanted on PpPD/XC72 composite. The Pt-implanted wt % on PpPD/XC72 matrix is just slightly lower than that of commercial Pt/C (22.30 wt %). The Pt-catalyst supports of PpPD/XC72-Pt-MW illustrate typical cyclic voltammograms (C-V) of Pt-catalyst, including significant Pt–H oxidation and Pt–O reduction peaks. The electrochemical active surface area of PpPD/XC72-Pt-MW is found to be as high as 60.1 m2 g−1. Max. number of electron transfer during oxygen reduction reaction (ORR) approaches 3.83 for PpPD/XC72-Pt-MW, higher than that of commercial Pt/C (3.62). Single cell based on PpPD/XC72-Pt-MW demonstrates much higher specific max. power density to be 34.6 mW cm−2 Pt, higher than that single cell prepared with commercial Pt/C electrode (30.6 mW cm−2 Pt).

The stabilizing effect of the transient imine directing group in the Pd(ii)-catalyzed C(sp3)–H arylation of free primary amines

The addition of the ligand ArQCHO reduces the distortion energies, and C(sp3)–H activation is conducted by an outer-sphere pivalate.

Glucose-sensitive polyelectrolyte microcapsules based on (alginate/chitosan) pair

Novel chemical stimulus-responsive multilayer assemblies have been elaborated through the layer-by-layer deposition of oppositely charged polysaccharides on either flat or spherical surfaces. Concentration-dependent glucose responsiveness was obtained through chemical modification of alginate, selected as polyanion, with phenylboronic acid moieties. QCM measurements showed that the alginate derivate still self-assembles though electrostatically-driven interactions with chitosan at pH 4, and that the polysaccharides multilayer assemblies, as obtained after crosslinking, exhibit improved stability versus pH (in the range 4–9) as well as swelling ability in the presence of glucose-containing solution. Glutaraldehyde-mediated crosslinking was achieved through reaction with free primary amines of chitosan. This approach was further extended to the preparation of smart capsules using CaCO3 microparticles as dissolvable core templates. Success of the LbL deposition process, stability (pH range 4–9) of the multilayer assemblies and glucose-induced swelling were fully confirmed for the microcapsules. One of the major result of this study is that crosslinking prevents total dissolution of the capsules and enables modulating the permeability of the polysaccharide shell yielding controlled release on in-capsule entrapped low molecular weight molecules.

Seasonal patterns of soil nitrogen availability in moist acidic tundra

Our ability to predict effects of changing soil nitrogen (N) in Arctic tundra has been limited by our poor understanding of the intra-annual variability of soil N in this strongly seasonal ecosystem. Studies have shown that microbial biomass declines in spring accompanied by peaks in inorganic nutrients. However, subsequent to this early pulse, there are few high temporal resolution measurements during the growing season. We hypothesized that (1) low N would be maintained throughout the growing season, (2) peaks of total free primary amines (TFPA), ammonium (NH4+), and nitrate (NO3−) would follow a sequential pattern driven by mineralization and nitrification, and (3) a peak in soil N would occur as plants senesce. We conducted weekly measurements of TFPA, NH4+, and NO3− in two tundra sites, from soil thaw in spring to freeze in fall. At each site, NH4+ peaks were followed by smaller peaks in NO3−, supporting the hypothesis that excess NH4+ would be nitrified. Furthermore, peaks in NH4+ were observed both shortly after leaf expansion and at plant senescence. The variation in timing between sites and the peaks in NH4+ subsequent to thaw indicates that nutrient limitation in these ecosystems is more dynamic and spatially variable than previously thought.

A facile sonochemical synthesis of shell-stabilized reactive microbubbles using surface-thiolated bovine serum albumin with the Traut’s reagent

The short lifetime of proteinaceous microbubbles produced using conventional sonication method has hindered their applications in drug delivery and metal removal from wastewater. In this study, we aimed to synthesize stable proteinaceous microbubbles and to demonstrate their reactivity. Our model protein, bovine serum albumin (BSA) was treated with 2-iminothiolane hydrochloride (Traut’s reagent) to convert primary amines to thiols before the synthesis of microbubbles. Microbubbles produced with the Traut’s reagent-treated BSA (BSA-SH MBs) were initially concentrated at median sizes of 0.5 and 2.5μm. The 0.5μm portion quickly vanished, and the 2.5μm portion gradually shrank to ∼850nm in ∼3days and became stabilized afterward for several months under 4°C. Characterizations of BSA-SH MBs by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) indicated the presence of free unbound thiols and primary amines on their surface, implying the possibility of further surface modification. Based on the zeta potential measurement, the isoelectric point (IEP) of BSA-SH MBs was determined to be 4.5. The attachments of BSA-SH MBs on alumina, silica, and gold surfaces in different pH environments were carried out with a quartz crystal microbalance with dissipation monitoring (QCM-D), demonstrating the reactivities of BSA-SH MBs. At pH 6, the negatively charged BSA-SH MBs were adsorbed onto the alumina surface by electrostatic interaction. Analogously, at pH 4, the adsorption of the positively charged BSA-SH MBs on the silica surface was confirmed. Compared with the electrostatic interaction, the adsorption of BSA-SH MBs on the gold surface is attributed to the strong gold-thiol bonding effect. This is the first time that a universal approach for stabilizing protein-shelled microbubbles was reported using only one single step of surface treatment of proteins with the Traut’s reagent.

ChemInform Abstract: Catalytic C(sp3)—H Arylation of Free Primary Amines with an exo Directing Group Generated in situ.

AbstractA range of aliphatic primary amines are site‐selectively arylated in the γ‐position using diaryl iodonium salts, and the products are isolated as the N‐benzoylated derivatives in moderate to high yields.

Catalytic C(sp(3) )-H Arylation of Free Primary Amines with an exo Directing Group Generated In Situ.

Herein, we report the palladium-catalyzed direct arylation of unactivated aliphatic C-H bonds in free primary amines. This method takes advantage of an exo-imine-type directing group (DG) that can be generated and removed in situ. A range of unprotected aliphatic amines are suitable substrates, undergoing site-selective arylation at the γ-position. Methyl as well as cyclic and acyclic methylene groups can be activated. Furthermore, when aniline-derived substrates were used, preliminary success with δ-C-H arylation was achieved. The feasibility of using the DG component in a catalytic fashion was also demonstrated.

Catalytic C(sp3)−H Arylation of Free Primary Amines with an exo Directing Group Generated In Situ

AbstractHerein, we report the palladium‐catalyzed direct arylation of unactivated aliphatic C−H bonds in free primary amines. This method takes advantage of an exo‐imine‐type directing group (DG) that can be generated and removed in situ. A range of unprotected aliphatic amines are suitable substrates, undergoing site‐selective arylation at the γ‐position. Methyl as well as cyclic and acyclic methylene groups can be activated. Furthermore, when aniline‐derived substrates were used, preliminary success with δ‐C−H arylation was achieved. The feasibility of using the DG component in a catalytic fashion was also demonstrated.

Genetically Encoding Single-Molecule Fluorophores into Ion Channels in Living Cells

We have developed a new approach to track protein dynamics in living cells on a single-molecule level with genetically encoded fluorescent non-canonical amino acids (ncAAs). The probes are based on amino acids with free primary amines that are chemically coupled to commercially available fluorophores, such as Cy3 and Cy5. Specific incorporation of fluorescent ncAAs was achieved by nonsense suppression in Xenopus laevis oocytes and confirmed by two-electrode voltage clamp of rescued ionic currents as well as by total internal reflection fluorescence microscopy detecting single fluorophores in the plasma membrane. Overall, the data show that these fluorescent ncAA were accepted as substrates for the X. laevis translation machinery. To date, we have encoded five fluorescent ncAAs into intracellular, extracellular and transmembrane regions of a CLC chloride channel or a voltage-gated sodium channel. Moreover, the variation in the linker length between the alpha carbon atom and the fluorescent moiety of the ncAA was explored by coupling to para-amino-phenylalanine, lysine and diaminopropionic acid and shows that linker length does not significantly affect the incorporation efficiency. This approach overcomes limitations of existing methods to study protein dynamics in living cells and is applicable to virtually any membrane protein. New applications include the detection of stoichiometry of macromolecular complexes by photo-bleaching of diffraction-limited puncta or the acquisition of relative distance measurements resulting from dynamic conformational changes within a membrane protein by Forster resonance energy transfer, both on a single-molecule level.

Enzymatic biodegradation of hydrogels for protein delivery targeted to the small intestine.

Multiresponsive poly(methacrylic acid-co-N-vinylpyrrolidone) hydrogels were synthesized with biodegradable oligopeptide crosslinks. The oligopeptide crosslinks were incorporated using EDC-NHS zero-length links between the carboxylic acid groups of the polymer and free primary amines on the peptide. The reaction of the peptide was confirmed by primary amine assay and IR spectroscopy. The microgels exhibited pH-responsive swelling as well as enzyme-catalyzed degradation targeted by trypsin present in the small intestine, as demonstrated upon incubation with gastrointestinal fluids from rats. Relative turbidity was used to evaluate enzyme-catalyzed degradation as a function of time, and initial trypsin concentration controlled both the degradation mechanism as well as the extent of degradation. Trypsin activity was effectively extinguished by incubation at 70 °C, and both the microgels and degradation products posed no cytotoxic effect toward two different cell lines. The microgels demonstrated pH-dependent loading of the protein insulin for oral delivery to the small intestine.

Characteristics of water-soluble organic nitrogen in fine particulate matter in the continental area of China

In recent years growing evidence has shown that organic nitrogen (ON) constitutes a significant fraction of the nitrogen budget in particulate matter (PM). However, the composition and sources of ON in Chinese PM are not clear to date due to the lack of field measurements and receptor modeling interpretations. In this study, water-soluble ON (WSON), free amino acids (FAAs) and primary amines, together with water-soluble ionic species and carbonaceous components, were quantified in PM2.5 collected in Xi'an, China from 2008 to 2009. The yearly average WSON concentration (300 nmol Nm−3, ranging from 29 to 1250 nmol Nm−3) was one order of magnitude higher than that reported in other urban and rural regions in U.S. A total of 24 organic nitrogen species were determined, with an average total concentration of 2180 pmol m−3, which account for 1.2% of the WSON in PM2.5. Glycine, β-alanine and methylamine were the most abundant protein type amino acid, non-protein type amino acid and primary amine, respectively, contributing to 21.1%, 5.2% and 20.6% of the total quantified organic nitrogen species (TONS). Strong linear correlations (Pearson's correlation) (r = .72, p < 0.001) between WSON and one of biomass burning tracer (K+) potentially indicate the contribution from biomass burning (especially in spring and fall) and the correlation (r = .76–.87, p < 0.001) between WSON and secondary species (NH4+, NO3−, and SO42−) also suggests the important contribution to WSON from secondary formation processes. Moreover, both protein type amino acids and primary amines revealed good correlations with water-soluble ions in spring, which indicates that seasonal variability occurred in primary sources and secondary formation pathway of the organic nitrogen species were found in Xi'an.

Synthetic and Mechanistic Study of the Catalytic Enantioselective Preparation of Primary β-Amino Ketones from Enones and a Fluorinated Gabriel Reagent

The salen μ-oxo complex of aluminum 1 catalyzes the asymmetric 1,4-addition of the novel ammonia equivalent 3,4,5,6-tetrafluorophthalimide to unsaturated ketones. All of the reagents are inexpensive and are readily available. The products are formed in up to 89% yield and up to 96% ee. The tetrafluorophthalimide group is removed under mild chemoselective conditions and in high yields to afford the free primary amines. Mechanistic studies suggest that the reaction occurs through a dual activation mechanism. A pre-equilibrium formation of a 1:1 complex between tetrafluorophthalimide and the catalyst is observed. The rate-determining step is the addition of tetrafluorophthalimide catalyst complex to the catalyst activated enone. These mechanistic studies provide important clues for the further development of catalytic asymmetric reactions.