Structure-property relationship in functionalized azobenzene photoswitches and their supramolecular behavior

Herein, we report the design, synthesis, and supramolecular behavior of 30 structurally diverse photoresponsive azobenzene molecular systems. To establish structure-property relationships, azobenzenes appended with N-picolinyl and/or N-benzyl groups tethered directly through carboxamides or via triazolylmethyl carboxamide linkages were explored. We have evaluated the photoswitching characteristics and thermal stability of the Z isomers through systematic studies. All the targets were also screened for their aggregation behavior and supramolecular aspects. Among all the derivatives, a few carboxamide-based systems formed microcrystals upon aggregation, showing light responsiveness. In contrast, the derivatives tethered via triazolylmethyl carboxamide linkage exhibited hydrogel formation with excellent water-absorbing capacity. All supramolecular aspects of the morphology of the microcrystal and hydrogel states and their stimuli-responsiveness have been studied using spectroscopy and various microscopic techniques.

Advances in the synthesis of nitrogen-containing heterocyclic compounds by in situ benzyne cycloaddition

Nitrogen-containing heterocyclic compounds are prevalent in various natural products, medicines, agrochemicals, and organic functional materials. Among strategies to prepare nitrogen-containing heterocyclic compounds, pathways involving benzyne intermediates are attractive given that they can readily assemble highly diverse heterocyclic compounds in a step-economical manner under transition-metal-free conditions. The synthesis of nitrogen-containing heterocyclic compounds from benzyne intermediates offers an alternative strategy to the conventional metal-catalyzed activation approaches. In the past years, chemists have witnessed the revival of benzyne chemistry, mainly attributed to the wide application of various novel benzyne precursors. The cycloaddition of benzynes is a powerful tool for the synthesis of nitrogen-containing heterocyclic compounds, which can be constructed by [n + 2] cyclization of benzyne intermediates in situ generated from benzyne precursors under mild reaction conditions. This review focuses on the application of cycloaddition reactions involving in situ benzynes in the construction of various nitrogen-containing heterocyclic compounds.

Heteroleptic Copper Complexes as Catalysts for the CuAAC Reaction: Counter-Ion Influence in Catalyst Efficiency

A series of nine cationic heteroleptic aryl-BIAN-copper(I) (BIAN = bis-iminoacenaphthene) complexes with the general formula [Cu((E-C6H4)2BIAN)(PPh3)2][X] (E = p-Me, p-iPr, o-iPr; X = BF4, OTf, NO3) 1X–3X were synthesized and fully characterized using several analytical techniques, including NMR spectroscopy and single-crystal X-ray diffraction. Except for complexes 2BF4 and 3BF4, which were already reported in our previous works, all remaining complexes are herein described for the first time. Two different strategies were used for the preparation of the complexes: complexes bearing BF4− or OTf− counter-ions (1BF4, 1OTf, 2OTf, and 3OTf) were obtained using the appropriate copper(I) precursors [Cu(NCMe)4][BF4] or [Cu(NCMe)4][OTf], whereas for derivatives 1NO3–3NO3, [Cu(PPh3)2NO3] was used. Their activity as catalysts for the copper azide-alkyne cycloaddition (CuAAC) was assessed alongside other high activity, previously reported Cu(I) complexes. Comparative studies to determine the influence of the counter-ion and of the aryl substituents were performed. All complexes behaved as active catalysts under neat reaction conditions, at 25 °C and in short reaction times without requiring the use of any additive, with complex 2NO3 being the most efficient derivative, along with other NO3−-bearing complexes.

Synthesis and biological evaluation of isatin oxime ether-tethered aryl 1H-1,2,3-triazoles as inhibitors of Mycobacterium tuberculosis

A series of isatin oxime ether-tethered aryl 1H-1,2,3-triazole hybrids were synthesized and screened for their in vitro antitubercular activity against the M. tuberculosis H37Rv strain.

Fluorinated Click-Derived Tripodal Ligands Drive Spin Crossover in both Iron(II) and Cobalt(II) Complexes

Control of the spin state of metal complexes is important because it leads to a precise control over the physical properties and the chemical reactivity of the metal complexes. Currently,...

Oxidation-Induced "One-Pot" Click Chemistry

Click chemistry has been established rapidly as one of the most valuable methods for the chemical transformation of complex molecules. Due to the rapid rates, clean conversions to the products, and compatibility of the reagents and reaction conditions even in complex settings, it has found applications in many molecule-oriented disciplines. From the vast landscape of click reactions, approaches have emerged in the past decade centered around oxidative processes to generate in situ highly reactive synthons from dormant functionalities. These approaches have led to some of the fastest click reactions know to date. Here, we review the various methods that can be used for such oxidation-induced "one-pot" click chemistry for the transformation of small molecules, materials, and biomolecules. A comprehensive overview is provided of oxidation conditions that induce a click reaction, and oxidation conditions are orthogonal to other click reactions so that sequential "click-oxidation-click" derivatization of molecules can be performed in one pot. Our review of the relevant literature shows that this strategy is emerging as a powerful approach for the preparation of high-performance materials and the generation of complex biomolecules. As such, we expect that oxidation-induced "one-pot" click chemistry will widen in scope substantially in the forthcoming years.

Development of Diverse Range of Biologically Relevant Carbohydrate-Containing Molecules: Twenty Years of Our Journey*

There is an increasing demand for significant amount of carbohydrate-containing molecules owing to their complete chemical, biological, and pharmacological investigations to better understand their role in many important biological events. Clinical studies of a wide range of simple carbohydrates or their derivatives, glycohybrids, glycoconjugates, and neoglycoconjugates have been conducted worldwide for the successful treatment of various frontline diseases. Herein, a brief perspective of carbohydrate-based molecular scaffolding and my experience during the last 20 years in the area of synthetic carbohydrate chemistry, mainly for their impact in drug discovery & development, is presented.

Targeting Guanine Quadruplexes with Luminescent Platinum(II) Complexes Bearing a Pendant Nucleobase

Guanine quadruplexes are tetra-stranded nucleic acid structures currently raising significant interest in the context of the development of potential anticancer therapeutics with a new mode of action. They are composed of planar guanine tetrads, allowing a high-affinity targeting by using molecules with a large π surface. However, the extreme topological versatility of guanine quadruplexes impedes a straightforward targeting of particular preselected guanine-rich sequences. We report here a systematic study of a family of luminescent platinum(II) complexes devised to overcome this challenge. By attaching a pendant adenine or thymine nucleobase as a substituent to one of the ligands at the platinum center, an additional recognition site is introduced with the aim of modulating the affinity of the metal complex to different DNA sequences. By comparing different attached nucleobases and a series of linker moieties, first conclusions are drawn with respect to the scope of this approach.

A Novel Series of N-aryltriazole and N-acridinyltriazole Hybrids as Potential Anticancer Agents

BACKGROUND

Triazoles are a class of aza-heterocycles with broad spectrum of biological importance. The synthetic tunability of the triazole moiety allows for the development of new pharmacophores with applications as drugs to contend with the burden of cancer.

OBJECTIVE

In this study, we aimed to develop a series of N-aryltriazole and N-acridinyltriazole molecular hybrids and evaluate their potential as anticancer agents.

METHODS

The triazole derivatives (1-10) were synthesized via a tandem nucleophilic substitution of aryl chlorides with sodium azide followed by 1,3-dipolar cycloaddition of the resulting organic azides with terminal/internal alkynes. From terminal alkynes, the well established copper(I) catalyzed azide-alkynes 1,3- dipolar cycloaddition, a premier example of click chemistry, was employed to access the 1,4-regioisomers of N-benzyl-1H-1,2,3-triazoles and N-acridynyl-1H-1,2,3-triazoles. All the compounds thus synthesized were characterized by 1D and 2D NMR spectroscopy and high resolution mass spectrometry.

RESULTS

Thermally controlled 1,3-dipolar cycloaddition was used to deliver N-aryl-1H-1,2,3-triazoles with 1,4,5-substitution on the triazole framework. The unprecedented high regioselectivity promoted by the sterically-strained silylated 1,4,5-trisubstituted moiety 4a offers a useful synthetic precursor with the silyl group being a synthetic handle for further structural elaboration to the desired 1,(4),5-di(tri)substituted 1,2,3- triazoles. Notably, anticancer evaluation revealed good cytotoxic activities of the novel acridinyltriazole hybrids (6-10) at micromolar concentrations in the range of 12.5 µM-100 µM against cervical cancer HeLa, kidney cancer HEK293, lung cancer A549 and leukemic MT4 cancer cell lines (p < 0.05).

CONCLUSION

A series of novel triazole-based acridine hybrids have been developed as potential leads for the development of multifaceted anticancer agents.

Supported Tris-Triazole Ligands for Batch and Continuous-Flow Copper-Catalyzed Huisgen 1,3-Dipolar Cycloaddition Reactions

The lack of supported versions of the tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (TBTA) ligand, suitable for flow-chemistry applications at scale, prompted us to develop a new route for the immobilization of such tris-triazole chelating units on highly cross-linked polystyrene resins. With this aim, the preparation of the known TBTA-type monomer 3 was optimized to develop a high-yield synthetic sequence, devoid of chromatographic purifications at any stage. Then, bead-type (P7) and monolithic (M7) functional resins were obtained by the easy and scalable suspension- or mold-copolymerization of 3 with divinylbenzene. Both types of materials were found to possess a highly porous morphology and specific surface area in the dry state and could be charged with substantial amounts of Cu(I) or Cu(II) salts. After treatment of the latter with a proper reducing agent, the corresponding supported Cu(I) complexes were tested in the copper-catalyzed alkyne-azide cycloaddition reaction (CuAAC). The immobilized catalysts proved active at room temperature and, in batch and with catalyst loadings as low as 0.6 mol%, afforded quantitative conversions within 20 h. Independent of the alkyne structure, extended use of the supported catalyst in flow was also possible. In the reaction of benzylazide and propargyl alcohol, this allowed a total turnover number larger than 400 to be reached.

Synthesis of Carbohydrate-Grafted Glycopolymers Using a Catalyst-Free, Perfluoroarylazide-Mediated Fast Staudinger Reaction

Glycopolymers have gained increasing importance in investigating glycan-lectin interactions, as drug delivery vehicles and in modulating interactions with proteins. The synthesis of these glycopolymers is still a challenging and rigorous exercise. In this regard, the highly efficient click reaction, copper (I)-catalyzed alkyne-azide cycloaddition, has been widely applied not only for its efficiency but also for its tolerance of the appended carbohydrate groups. However, a significant drawback of this method is the use of the heavy metal catalyst which is difficult to remove completely, and ultimately toxic to biological systems. In this work, we present the synthesis of carbohydrate-grafted glycopolymers utilizing a mild and catalyst-free perfluorophenyl azide (PFPA)-mediated Staudinger reaction. Using this strategy, mannose (Man) and maltoheptaose (MH) were grafted onto the biodegradable poly(lactic acid) (PLA) by stirring a PFAA-functionalized PLA with a phosphine-derivatized Man or MH in DMSO at room temperature within an hour. The glycopolymers were characterized by ¹H-NMR, 19F-NMR, 31P-NMR and FTIR.

Benzyne–azide polycycloaddition: a facile route toward functional polybenzotriazoles

An efficient benzyne–azide polycycloaddition is established and functional poly(benzotriazole)s are produced under mild reaction conditions.

Synthesis, characterization and antimicrobial evaluation of ferrocene–oxime ether benzyl 1H-1,2,3-triazole hybrids

A series of ferrocene–oxime ether benzyl 1H-1,2,3 triazole hybrids has been synthesized by employing Cu(i) catalyzed azide–alkyne [3+2] cycloaddition reaction and their antibacterial and antifungal activities are reported.

Synthesis and evaluation of novel purple acid phosphatase inhibitors

Transgenic studies in animals have demonstrated a direct association between the level of expression of purple acid phosphatase (PAP; also known as tartrate-resistant acid phosphatase) and the progression of osteoporosis. Consequently, PAP has emerged as a promising target for the development of novel therapeutic agents to treat this debilitating disorder. PAPs are binuclear hydrolases that catalyse the hydrolysis of phosphorylated substrates under acidic to neutral conditions. A series of phenyltriazole carboxylic acids, prepared by the reactions of azide derivatives with propiolic acid through copper(i)-catalysed azide-alkyne cycloaddition click reactions, has been assessed for their inhibitory effect on the catalytic activity of pig and red kidney bean PAPs. The binding mode of most of these compounds is purely uncompetitive with K _iuc values as low as ∼23 μM for the mammalian enzyme. Molecular modelling has been used to examine the binding modes of these triazole compounds in the presence of a substrate in the active site of the enzyme in order to rationalise their activities and to design more potent and specific derivatives.

The Disappearing Director: The Case of Directed N-Arylation via a Removable Hydroxyl Group

A facile and broadly applicable method for the regiospecific N-arylation of benzotriazoles is reported. Copper-mediated reactions of diverse 1-hydroxy-1H-benzotriazoles with aryl boronic acids lead to 1-aryl-1H-benzotriazole 3-oxides. A N1-OH → N3 prototropy in the 1-hydroxy-1H-benzotriazoles is plausibly the underlying basis, where the tautomer is captured by the boronic acid, leading to C-N (not C-O) bond formation. Because the N-O bond in amine N-oxides and 1-hydroxy-1H-benzotriazoles can be easily reduced by diboron reagents such as (pinB)2 and B2(OH)4, exposure of the 1-aryl-1H-benzotriazole 3-oxides to B2(OH)4 then leads to facile reduction of the N-O bond resulting in diverse, regiospecifically-arylated benzotriazoles. Thus, the N-hydroxyl group in 1-hydroxy-1H-benzotriazoles acts as a disposable arylation director.

Regioselective synthesis of multisubstituted 1,2,3-triazoles: moving beyond the copper-catalyzed azide-alkyne cycloaddition

Copper(i)-catalyzed azide-alkyne cycloaddition (CuAAC) is an essential "click chemistry" reaction that is widely used in chemical biology, medicinal chemistry and materials science. The CuAAC reaction of terminal alkynes provides a mild and efficient synthesis of 1,4-disubstituted 1,2,3-triazoles. However, the click reaction of internal alkynes with azides, giving trisubstituted triazoles, is very challenging. This feature article highlights the recent progress addressing this fundamental problem. Particular emphasis is on the current and emerging strategies to introduce functional groups to the C-5 position of triazoles in a regioselective manner.

Reactivity of arynes toward functionalized alkenes: intermolecular Alder-ene vs. addition reactions

The selectivity between two different manifolds of reactions of arynes reacting with functionalized alkenes is described.

Metal-Free Synthesis of 2-Substituted 3-(2-Hydroxyaryl)quinolines and 4-(2-Hydroxyaryl)acridines via Benzyne Chemistry

A metal-free approach for the synthesis of 3-aryl-2-substituted quinolines and 4-arylacridines has been developed via the 1,3-dipolar cycloaddition reactions of arynes with N-oxides. Reactions of various 2-substitued quinoline N-oxides with ortho-(trimethylsilyl)aryltriflates in the presence of KF gave 3-(2-hydroxyaryl)quinoline derivatives in good yields. Acridine N-oxides also reacted with arynes to furnish 4-(2-hydroxyaryl)acridines, albeit in moderate yields.

Phosphorescent Pt(ii) complexes bearing a monoanionic C^N^N luminophore and tunable ancillary ligands

A new monoanionic pincer luminophore is presented, yielding phosphorescent Pt(ii) complexes bearing a neutral 1,2,3-triazole ring introduced via click chemistry. The overall charge, intermolecular interactions and excited state properties can be manipulated and controlled.

Synthesis and evaluation of naphthyl bearing 1,2,3-triazole analogs as antiplasmodial agents, cytotoxicity and docking studies

Novel series of naphthyl bearing 1,2,3-triazoles (4a-t) were synthesized and evaluated for their in vitro antiplasmodial activity against pyrimethamine (Pyr)-sensitive and resistant strains of Plasmodium falciparum. The synthesized compounds were assessed for their cytotoxicity employing human embryonic kidney cell line (HEK-293), and none of them was found to be toxic. Among them 4j, 4k, 4l, 4m, 4n, 4t exhibited significant antiplasmodial activity in both strains, of which compounds 4m, 4n and 4t (∼3.0-fold) displayed superior activity to Pyr against resistant strain. Pyr and selected compounds (4n, 4p and 4t) that repressed parasite development also inhibited PfDHFR activity of the soluble parasite extract, suggesting that anti-parasitic activity of these compounds is a result of inhibition of the parasite DHFR. In silico studies suggest that activity of these compounds might be enhanced due to π-π stacking.

A novel bis(pinacolato)diboron-mediated N-O bond deoxygenative route to C6 benzotriazolyl purine nucleoside derivatives

Reaction of amide bonds in t-butyldimethylsilyl-protected inosine, 2'-deoxyinosine, guanosine, 2'-deoxyguanosine, and 2-phenylinosine with commercially available peptide-coupling agents (benzotriazol-1H-yloxy)tris(dimethylaminophosphonium) hexafluorophosphate (BOP), (6-chloro-benzotriazol-1H-yloxy)trispyrrolidinophosphonium hexafluorophosphate (PyClocK), and (7-azabenzotriazol-1H-yloxy)trispyrrolidinophosphonium hexafluorophospate (PyAOP) gave the corresponding O(6)-(benzotriazol-1-yl) nucleoside analogues containing a C-O-N bond. Upon exposure to bis(pinacolato)diboron and base, the O(6)-(benzotriazol-1-yl) and O(6)-(6-chlorobenzotriazol-1-yl) purine nucleoside derivatives obtained from BOP and PyClocK, respectively, underwent N-O bond reduction and C-N bond formation, leading to the corresponding C6 benzotriazolyl purine nucleoside analogues. In contrast, the 7-azabenzotriazolyloxy purine nucleoside derivatives did not undergo efficient deoxygenation, but gave unsymmetrical nucleoside dimers instead. This is consistent with a prior report on the slow reduction of 1-hydroxy-1H-4-aza and 1-hydroxy-1H-7-azabenzotriazoles. Because of the limited number of commercial benzotriazole-based peptide coupling agents, and to show the applicability of the method when such coupling agents are unavailable, 1-hydroxy-1H-5,6-dichlorobenzotriazole was synthesized. Using this compound, silyl-protected inosine and 2'-deoxyinosine were converted to the O(6)-(5,6-dichlorobenzotriazol-1-yl) derivatives via in situ amide activation with PyBroP. The O(6)-(5,6-dichlorobenzotriazol-1-yl) purine nucleosides so obtained also underwent smooth reduction to afford the corresponding C6 5,6-dichlorobenzotriazolyl purine nucleoside derivatives. A total of 13 examples were studied with successful reactions occurring in 11 cases (the azabenzotriazole derivatives, mentioned above, being the only unreactive entities). To understand whether these reactions are intra or intermolecular processes, a crossover experiment was conducted. The results of this experiment as well as those from reactions conducted in the absence of bis(pinacolato)diboron and in the presence of water indicate that detachment of the benzotriazoloxy group from the nucleoside likely occurs, followed by reduction, and reattachment of the ensuing benzotriazole, leading to products.

Fluorescent 1,2,3-triazole derivative of 3′-deoxy-3-azidothymidine: synthesis and absorption/emission spectra

3′-Deoxy-3-azidothymidine (AZT, zidovudine) is a nucleoside-analog reverse transcriptase inhibitor, successfully used against the human immunodeficiency virus (HIV). Its structure contains an azide function, which makes it a useful substrate for 1,2,3-triazole synthesis, using the copper-catalyzed azide-alkyne cycloaddition, the flagship reaction of ‘click chemistry’. Herein we present the synthesis and spectral characterization of its 1,2,3-triazole derivative containing a fluorenylmethyloxycarbonyl (fmoc) fluorescent fragment. The preparation and characteristics of a novel fluorescent probe, 9H-fluoren-9-ylmethyl prop-2-yn-1-yl carbonate (propargyl-fmoc) is also presented.

Trifluoromethanesulfonyloxy-group-directed regioselective (3 + 2) cycloadditions of benzynes for the synthesis of functionalized benzo-fused heterocycles

Highly regioselective (3 + 2) cycloadditions of (trifluoromethanesulfonyloxy)benzynes [(triflyloxy)benzynes] with 1,3-dipoles followed by cross-coupling reactions provided multisubstituted benzo-fused heterocycles. The triflyloxy group at the 3-position of benzynes, and even that at the remote 4-position, greatly affected the regiocontrol of the cycloaddition. These groups also served to install other substituents at their ipso-positions.

Cycloaddition of Arynes and Cyclic Enol Ethers as a Platform for Access to Stereochemically Defined 1,2-Disubstituted Benzocyclobutenes

Benzocyclobutenes (BCBs) are important entities in a multitude of areas, such as complex organic synthesis, materials and polymer chemistry, and electronics. Whereas reactions between arynes and ketene acetals have been well studied, reactions with cyclic enol ethers are unknown. A cis olefin geometry in cyclic enol ethers makes them well suited for formal [2 + 2] cycloaddition with arynes than for competing ene reactions, making them effective reactants. Reactions of 2,3-dihydrofuran, 2,3-dihydro-3H-pyran, 5-butyl-2,3-dihydrofuran, (S)-2-((benzyloxy)methyl)-2,3-dihydrofuran, and 1,4-dioxene with various arynes were successful. An advantage of the use of cyclic enol ethers is that despite the plausible intermediacy of zwitterionic intermediates, the products are limited to a cis ring junction. This can be exploited for potential access to stereochemically-defined 1,2-disubstituted BCBs. As a demonstration, ether ring cleavage with BBr3 provided trans-functionalized BCBs and displacement with azide then provided cis derivatives. DFT computations have been utilized to understand the structures of three arynes in relation to the cycloadditions and for a predictive evaluation of product ratios in two cases. A comparative evaluation of the HOMO energies of a related series of cyclic olefins has also been performed by DFT computations.

The role of aryne distortions, steric effects, and charges in regioselectivities of aryne reactions

The distortion/interaction model has been used to explain and predict reactivity in a variety of reactions where more common explanations, such as steric and electronic factors, do not suffice. This model has also provided new fundamental insight into regioselectivity trends in reactions of unsymmetrical arynes, which in turn has fueled advances in aryne methodology and natural product synthesis. This article describes a systematic experimental and computational study of one particularly important class of arynes, 3-halobenzynes. 3-Halobenzynes are useful synthetic building blocks whose regioselectivities have been explained by several different models over the past few decades. Our efforts show that aryne distortion, rather than steric factors or charge distribution, are responsible for the regioselectivities observed in 3-haloaryne trapping experiments. We also demonstrate the synthetic utility of 3-halobenzynes for the efficient synthesis of functionalized heterocycles, using a tandem aryne-trapping/cross-coupling sequence involving 3-chlorobenzyne.