Application of click chemistry on preparation of separation materials for liquid chromatography

Copper-Catalyzed Three-Component Tandem Reaction of Alkynes, α-Diazo Esters, and TMSN3 to Access N-Substituted 1,2,3-Triazoles

An efficient copper-catalyzed three-component tandem reaction of alkynes, α-diazo esters, and TMSN3 to construct triazoles has been developed. Through this strategy, a number of diverse N-substituted 1,2,3-triazoles were conveniently obtained in moderate to good yields from simple and readily available starting materials using K2CO3 as the base. The mechanism of the tandem Cu-catalyzed azide-alkyne cycloaddition (CuAAC) and Cu-carbenoid-participated C-N coupling reaction has been investigated.

Engineering thiol-ene click chemistry for the preparation of a chiral stationary phase based on a [4+6]-type homochiral porous organic cage for enantiomeric separation in normal-phase and reversed-phase high performance liquid chromatography

In this study, a new chiral stationary phase (CSP) was fabricated by covalent bonding of a [4+6]-type homochiral porous organic cage (POC) CC19-R onto thiolated silica via a thiol-ene click reaction. The CC19-R was synthesized via Schiff-base reaction between 2-hydroxybenzene-1,3,5-tricarbaldehyde and (1R, 2R)-(-)-1,2-diaminocyclohexane. The enantioseparation capability of the resulting CC19-R-based CSP was systematically evaluated upon separating various chiral compounds or chiral pharmaceuticals in normal phase HPLC (NP-HPLC) and reversed phase HPLC (RP-HPLC), including alcohols, organic acids, ketones, diols, esters, and amines. Fifteen racemates were enantioseparated in NP-HPLC and 11 racemates in RP-HPLC. Some racemates have been well separated, such as 4-chlorobenzhydrol, cetirizine (in the form of dihydrochloride), 1,2-diphenyl-1,2-ethanediol, and 3-(benzyloxy)propane-1,2-diol whose resolution values reached 3.66, 4.23, 6.50, and 3.50, respectively. When compared with a previously reported chiral POC-based column (NC1-R column), eight racemates were not separated on the NC1-R column in NP-HPLC and five racemates were not separated in RP-HPLC, but were well resolved on this column, revealing that the enantioselectivity and separable range of chiral POCs-type columns could be significantly widened using this fabricated CC19-R column. Moreover, the resolution performance of the CC19-R column was also compared with commercial Chiralpak AD-H [CSP: Amylose tris(3,5-dimethylphenylcarbamate)] and Chiralcel OD-H [CSP: Cellulose tris(3,5-dimethylphenylcarbamate)] columns. The column also can separate some racemates that could not be separated or not well be separated by the two commercial columns, showing its good complementarity to the two commercial columns on chiral separation. In addition, the column also had good stability and reproducibility with the relative standard deviation (n = 5) of the retention time and resolution lower than 1.0% and 1.8%, respectively, after it had undergone multiple injections (100, 200, 300, and 400 times). This work indicated that the features of good resolution ability and simple synthesis methods using with this POC-based CSP provided chiral POCs with potential application prospects in HPLC racemic separation.

A novel pillar[3]trianglimine macrocycle with a deep cavity used as a chiral selector to prepare a chiral stationary phase by thiol-ene click reaction for enantioseparation in high-performance liquid chromatography

A chiral pillar[3]trianglimine (C60 H72 N6 O6 ) with a deep cavity has been developed as a chiral selector and bonded to thiolated silica by thiol-ene click reaction to fabricate a novel chiral stationary phase for enantioseparation in high-performance liquid chromatography. The enantioseparation performance of the fabricated chiral stationary phase has been evaluated by separating various racemic compounds, including alcohols, esters, amines, ketones, amino acids, and epoxides, in both normal-phase and reversed-phase elution modes. In total, 14 and 17 racemates have been effectively separated in these two separation modes, respectively. In comparison with two widely used chiral columns (Chiralcel OD-H and Chiralpak AD-H), our novel chiral stationary phase offered good chiral separation complementarity, separating some of the tested racemates that could not be separated or were only partially separated on these two commercial columns. The influences of analyte mass, mobile phase composition, and column temperature on chiral separation have been investigated. Good repeatability, stability, and column-to-column reproducibility of the chiral stationary phase for enantioseparation have been observed. After the fabricated column had been eluted up to 400 times, the relative standard deviations (n = 5) of resolution (Rs) and retention time of the separated analytes were < 0.39% and < 0.20%, respectively. The relative standard deviations (n = 3) of Rs and retention time for column-to-column reproducibility were < 4.6% and < 5.2%, respectively. This study demonstrated that the new chiral stationary phase has great prospects for chiral separation in high-performance liquid chromatography.

Steroid-triazole conjugates: A brief overview of synthesis and their application as anticancer agents

Steroids are biomolecules that play pivotal roles in various physiological and drug discovery processes. Abundant research has been fuelled towards steroid-heterocycles conjugates over the last few decades as potential therapeutic agents against various diseases especially as anticancer agents. In this context various steroid-triazole conjugates have been synthesized and studied for their anticancer potential against various cancer cell lines. A thorough search of the literatures revealed that a concise review pertaining the present topic is not compiled. Therefore, in thus review we summarize the synthesis, anticancer activity against various cancer cell lines and structure activity relationship (SAR) of various steroid-triazole conjugates. This review can lay down the path towards the development of various steroid-heterocycles conjugates with lesser side effects and profound efficacy.

Preparation of Chiral Porous Organic Cage Clicked Chiral Stationary Phase for HPLC Enantioseparation

Porous organic cages (POCs) are a new subclass of porous materials, which are constructed from discrete cage molecules with permanent cavities via weak intermolecular forces. In this study, a novel chiral stationary phase (CSP) has been prepared by chemically binding a [4 + 6]-type chiral POC (C120H96N12O4) with thiol-functionalized silica gel using a thiol-ene click reaction and applied to HPLC separations. The column packed with this CSP presented good separation capability for chiral compounds and positional isomers. Thirteen racemates have been enantioseparated on this column, including alcohols, diols, ketones, amines, epoxides, and organic acids. Upon comparison with a previously reported chiral POC NC1-R-based column, commercial Chiralpak AD-H, and Chiralcel OD-H columns, this column is complementary to these three columns in terms of its enantiomeric separation; and can also separate some racemic compounds that cannot be separated by the three columns. In addition, eight positional isomers (iodoaniline, bromoaniline, chloroaniline, dibromobenzene, dichlorobenzene, toluidine, nitrobromobenzene, and nitroaniline) have also been separated. The influences of the injection weight and column temperature on separation have been explored. After the column has undergone multiple injections, the relative standard deviations (RSDs) for the retention time and selectivity were below 1.0 and 1.5%, respectively, indicating the good reproducibility and stability of the column for separation. This work demonstrates that POCs are promising materials for HPLC separation.

Mechanistic Aspects on [3+2] Cycloaddition (32CA) Reactions of Azides to Nitroolefins: A Computational and Kinetic Study

[3+2] cycloadditions of nitroolefins have emerged as a selective and catalyst-free alternative for the synthesis of 1,2,3-triazoles from azides. We describe mechanistic studies into the cycloaddition/rearomatization reaction sequence. DFT calculations revealed a rate-limiting cycloaddition step proceeding via an asynchronous TS with high kinetic selectivity for the 1,5-triazole. Kinetic studies reveal a second-order rate law, and ¹³ C kinetic isotopic effects at natural abundance were measured with a significant normal effect at the conjugated olefinic centers of 1.0158 and 1.0216 at the α and β-carbons of β-nitrostyrene. Distortion/interaction-activation strain and energy decomposition analyses revealed that the major regioisomeric pathway benefits from an earlier and less-distorted TS, while intermolecular interaction terms dominate the preference for 1,5- over 1,4-cycloadducts. In addition, the major regioisomer also has more favorable electrostatic and dispersion terms. Additionally, while static DFT calculations suggest a concerted but highly asynchronous Ei-type HNO₂ elimination mechanism, quasiclassical direct-dynamics calculations reveal the existence of a dynamic intermediate.

Preparation of chiral stationary phase based on a [3+3] chiral polyimine macrocycle by thiol-ene click chemistry for enantioseparation in normal-phase and reversed-phase high performance liquid chromatography

Polyimine macrocycles are a new class of organic macrocycles with cyclic structures, well-defined molecular cavities, and multiple cooperative binding sites, which have recently aroused considerable research interest in molecular recognition and separation. Herein, we report the bonding of a [3+3] chiral polyimine macrocycle (H₃L, C₇₈H₇₈N₆O₃) on thiol-functionalized silica gel using thiol-ene click chemistry to prepare a chiral stationary phase (CSP) for high performance liquid chromatography (HPLC). The fabricated column exhibited excellent chiral separation capability under both normal-phase and reversed-phase conditions. Fourteen and 10 racemates were well resolved on the column in normal-phase mode (using n-hexane/isopropanol as the mobile phase) and reversed-phase mode (using methanol/water as the mobile phase), respectively, including alcohols, esters, ethers, ketones, aldehydes, epoxides and organic acids. Moreover, the column also shows good selectivity toward positional isomers. Six positional isomers (dinitrobenzene, chloroaniline, bromoaniline, iodoaniline, nitrobrobenzene and nitrochlorobenzene) were well separated on the column. In addition, the effects of the injection mass and mobile phase composition on the separation were investigated. The column shows good reproducibility and stability after multiple injections with the relative standard deviation (RSD) (n = 5) of the retention time and resolution being < 0.96 % and 0.65 %, respectively. This study indicates that this type of chiral polyimine macrocycles is a promising chiral selector for HPLC enantioseparation and will push forward the applications of more novel chiral macrocycles for chiral chromatographic separation.

DNA-based platform for efficient and precisely targeted bioorthogonal catalysis in living systems

As one of the typical bioorthogonal reactions, copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction holds great potential in organic synthesis, bioconjugation, and surface functionalization. However, the toxicity of Cu(I), inefficient catalytic activity, and the lack of cell specific targeting of the existing catalysts hampered their practical applications in living systems. Herein, we design and construct a DNA-based platform as a biocompatible, highly efficient, and precisely targeted bioorthogonal nanocatalyst. The nanocatalyst presents excellent catalytic efficiency in vitro, which is one order of magnitude higher than the commonly used catalyst CuSO₄/sodium ascorbate. The theoretical calculation further supports the contribution of DNA structure and its interaction with substrates to the superior catalytic activity. More importantly, the system can achieve efficient prodrug activation in cancer cells through cell type-specific recognition and produce a 40-fold enhancement of transformation compared to the non-targeting nanocatalyst, resulting in enhanced antitumor efficacy and reduced adverse effects. In vivo tumor therapy demonstrates the safety and efficacy of the system in mammals.

Copper-click reaction has been used for a wide range of bio-conjugations but does suffer from toxicity issues. Here, the authors report on the growth of copper nanoparticles on DNA with linked aptamer targeting and demonstrate high catalytic effect and improved application due to targeting and biocompatibility.

1,6-Addition of 1,2,3-NH triazoles to para-quinone methides: Facile access to highly selective N1 and N2 substituted triazoles

The regioselective syntheses of N¹ and N² substituted triazoles through a 1,6-addition reaction of 1,2,3-NH triazoles to p-quinone methide were achieved under mild reaction conditions. The present reactions showed superior results in terms of selectivity, mild reaction conditions, short reaction time and broad substrate scope with good functional-group compatibility. Considering the high synthetic value of N¹- and N²-substituted compounds and p-QM related research, the present strategy will greatly benefit researchers in various fields.

Asymmetric catalytic nitrooxylation and azidation of β-keto amides/esters with hypervalent iodine reagents

Chiral Lewis acid-catalyzed enantioselective nitrooxylation and azidation of cyclic and acyclic β-keto amides/esters with hypervalent iodine(iii) reagents.

Heterogeneous Gold Nanoparticle-Based Catalysts for the Synthesis of Click-Derived Triazoles via the Azide-Alkyne Cycloaddition Reaction

A supported gold nanoparticle-catalyzed strategy has been utilized to promote a click chemistry reaction for the synthesis of 1,2,3-triazoles via the azide-alkyne cycloaddition (AAC) reaction. While the advent of effective non-copper catalysts (i.e., Ru, Ag, Ir) has demonstrated the catalysis of the AAC reaction, additional robust catalytic systems complementary to the copper catalyzed AAC remain in high demand. Herein, Au nanoparticles supported on Al2O3, Fe2O3, TiO2 and ZnO, along with gold reference catalysts (gold on carbon and gold on titania supplied by the World Gold Council) were used as catalysts for the AAC reaction. The supported Au nanoparticles with metal loadings of 0.7–1.6% (w/w relative to support) were able to selectively obtain 1,4-disubstituted-1,2,3-triazoles in moderate yields up to 79% after 15 min, under microwave irradiation at 150 °C using a 0.5–1.0 mol% catalyst loading through a one-pot three-component (terminal alkyne, organohalide and sodium azide) procedure according to the “click” rules. Among the supported Au catalysts, Au/TiO2 gave the best results.

Affinity monolith chromatography: A review of general principles and recent developments

Affinity monolith chromatography (AMC) is a liquid chromatographic technique that utilizes a monolithic support with a biological ligand or related binding agent to isolate, enrich, or detect a target analyte in a complex matrix. The target-specific interaction exhibited by the binding agents makes AMC attractive for the separation or detection of a wide range of compounds. This article will review the basic principles of AMC and recent developments in this field. The supports used in AMC will be discussed, including organic, inorganic, hybrid, carbohydrate, and cryogel monoliths. Schemes for attaching binding agents to these monoliths will be examined as well, such as covalent immobilization, biospecific adsorption, entrapment, molecular imprinting, and coordination methods. An overview will then be given of binding agents that have recently been used in AMC, along with their applications. These applications will include bioaffinity chromatography, immunoaffinity chromatography, immobilized metal-ion affinity chromatography, and dye-ligand or biomimetic affinity chromatography. The use of AMC in chiral separations and biointeraction studies will also be discussed.

Silver incorporated into g-C3N4/Alginate as an efficient and heterogeneous catalyst for promoting click and A3 and KA2 coupling reaction

Fe3O4/g-C3N4/Alginate-Ag nanocomposite as a novel and effective nanocatalyst was successfully prepared. This nanocomposite was fully characterized using several techniques such as X-ray diffraction (XRD), field emission scanning electron microscopy with energy dispersive spectroscopy (FESEM-EDS), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). In addition, the catalytic activity of this novel and characterized nanocatalyst was investigated in the regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles via click reaction and A3 and KA2 coupling reaction in aqueous media. The prepared nanocatalyst was simply recovered by using an external magnet and reused for several times with a slight loss of catalytic activity.

Synthesis of β-triazolylenones via metal-free desulfonylative alkylation of N-tosyl-1,2,3-triazoles

Desulfonylative alkylation of N-tosyl-1,2,3-triazoles under metal-free conditions leading to β-triazolylenones is reported here. The present study encompasses the synthesis of triazoles with a new substitution pattern in a single step from cyclic 1,3-dicarbonyl compounds and N-tosyl triazole in moderate to high yields. Our synthesis takes place with complete regioselectivity as confirmed by crystallographic analysis which is rationalized by a suitable mechanistic proposal. This method provides an efficient, versatile and straightforward strategy towards the synthesis of new functionalized 1,2,3-triazoles.

Chiral Recognition for Chromatography and Membrane-Based Separations: Recent Developments and Future Prospects

With the rapid development of global industry and increasingly frequent product circulation, the separation and detection of chiral drugs/pesticides are becoming increasingly important. The chiral nature of substances can result in harm to the human body, and the selective endocrine-disrupting effect of drug enantiomers is caused by differential enantiospecific binding to receptors. This review is devoted to the specific recognition and resolution of chiral molecules by chromatography and membrane-based enantioseparation techniques. Chromatographic enantiomer separations with chiral stationary phase (CSP)-based columns and membrane-based enantiomer filtration are detailed. In addition, the unique properties of these chiral resolution methods have been summarized for practical applications in the chemistry, environment, biology, medicine, and food industries. We further discussed the recognition mechanism in analytical enantioseparations and analyzed recent developments and future prospects of chromatographic and membrane-based enantioseparations.

Bioprospecting of Nitrogenous Heterocyclic Scaffolds with Potential Action for Neglected Parasitosis: A Review

Neglected parasitic diseases are a group of infections currently considered as a worldwide concern. This fact can be attributed to the migration of these diseases to developed and developing countries, associated with therapeutic insufficiency resulted from the low investment in the research and development of new drugs. In order to overcome this situation, bioprospecting supports medicinal chemistry in the identification of new scaffolds with therapeutically appropriate physicochemical and pharmacokinetic properties. Among them, we highlight the nitrogenous heterocyclic compounds, as they are secondary metabolites of many natural products with potential biological activity. The objective of this work was to review studies within a 10-year timeframe (2009- 2019), focusing on the pharmacological application of nitrogen bioprospectives (pyrrole, pyridine, indole, quinoline, acridine, and their respective derivatives) against neglected parasitic infections (malaria, leishmania, trypanosomiases, and schistosomiasis), and their application as a template for semi-synthesis or total synthesis of potential antiparasitic agents. In our studies, it was observed that among the selected articles, there was a higher focus on the attempt to identify and obtain novel antimalarial compounds, in a way that an extensive amount of studies involving all heterocyclic nitrogen nuclei were found. On the other hand, the parasites with the lowest number of publications up until the present date have been trypanosomiasis, especially those caused by Trypanosoma cruzi, and schistosomiasis, where some heterocyclics have not even been cited in recent years. Thus, we conclude that despite the great biodiversity on the planet, little attention has been given to certain neglected tropical diseases, especially those that reach countries with a high poverty rate.

Synthesis and chromatographic evaluation of a new stationary phase based on mild thiol-Michael addition reaction

Click chemistry has attracted increasing attention for the synthesis of novel stationary phases. Considering the advantage of click chemistry, a strategy based on thiol-Michael addition was developed for the preparation of a new stationary phase herein, and a phenyl vinyl sulfone stationary phase (M-PVS) was prepared. The resulting M-PVS bonded silica was characterized by elemental analysis, solid-state ¹³C cross-polarization/magic-angle spinning NMR and infrared spectroscopy, confirming the successful immobilization of phenyl vinyl sulfone on the silica support. The retention properties of M-PVS were investigated and exhibited unambiguous reversed phase retention characteristics. Moreover, shape selectivity and silanol activity were studied to reveal the diverse interactions of M-PVS, including hydrophobic, π-π, hydrogen bonding, and ion-exchange interactions. In addition, de-wetting tolerance and hydrophilic properties were evaluated and a pronounced "U" retention curves were obtained, indicating enhanced retention for polar analytes and transitions of different interaction modes. Selectivity differences between M-PVS column, phenyl column and conventional C₁₈ column were examined using series natural standards. The diverse interactions of M-PVS demonstrated its improved selectivity for the compounds with similar hydrophobic skeleton but different polar substituents.

Regio- and stereoselective synthesis of spiropyrrolidine-oxindole and bis-spiropyrrolizidine-oxindole grafted macrocycles through [3 + 2] cycloaddition of azomethine ylides

A convenient and efficient method for the regioselective macrocyclization of triazole bridged spiropyrrolidine-oxindole, and bis-spiropyrrolizidine-oxindole derivatives was accomplished through intra and self-intermolecular [3 + 2] cycloaddition of azomethine ylides. The chalcone isatin precursors 9a-i required for the click reaction were obtained from the reaction of N-alkylazidoisatin 4 and propargyloxy chalcone 8a-i which in turn were obtained by the aldol condensation of propargyloxy salicylaldehyde 6 and substituted methyl ketones 7a-i. The regio- and stereochemical outcome of the cycloadducts were assigned based on 2D NMR and confirmed by single crystal XRD analysis. High efficiency, mild reaction conditions, high regio- and stereoselectivity, atom economy and operational simplicity are the exemplary advantages of the employed macrocyclization procedure.

Glycoprotein Mimics with Tunable Functionalization through Global Amino Acid Substitution and Copper Click Chemistry

Glycoproteins and their mimics are challenging to produce because of their large number of polysaccharide side chains that form a densely grafted protein-polysaccharide brush architecture. Herein a new approach to protein bioconjugate synthesis is demonstrated that can approach the functionalization densities of natural glycoproteins through oligosaccharide grafting. Global amino acid substitution is used to replace the methionine residues in a methionine-enriched elastin-like polypeptide with homopropargylglycine (HPG); the substitution was found to replace 93% of the 41 methionines in the protein sequence as well as broaden and increase the thermoresponsive transition. A series of saccharides were conjugated to the recombinant protein backbones through copper(I)-catalyzed alkyne-azide cycloaddition to determine reactivity trends, with 83-100% glycosylation of HPGs. Only an acetyl-protected sialyllactose moiety showed a lower level of 42% HPG glycosylation that is attributed to steric hindrance. The recombinant glycoproteins reproduced the key biofunctional properties of their natural counterparts such as viral inhibition and lectin binding.

Bimetallic Cu/Pd-catalyzed three-component azide-alkyne cycloaddition/isocyanide insertion: synthesis of fully decorated tricyclic triazoles

The construction of fully decorated 1,2,3-triazole-fused 5-, 6- and 7-membered rings has been disclosed via a bimetallic relay-catalyzed cascade process combining azide-alkyne cycloaddition, C(sp2)-H functionalization of intermediary 1,2,3-triazoles and isocyanide insertion. The salient features of this methodology include simple starting materials, reduced synthetic steps, good substrate scope and high efficiency.

Well-Defined Materials for High-Performance Chromatographic Separation

Chromatographic separation has been widely applied in various fields, such as chemical engineering, precision medicine, energy, and biology. Because chromatographic separation is based on differential partitioning between the mobile phase and stationary phase and affected by band dispersion and mass transfer resistance from these two phases, the materials used as the stationary phase play a decisive role in separation performance. In this review, we discuss the design of separation materials to achieve the separation with high efficiency and high resolution and highlight the well-defined materials with uniform pore structure and unique properties. The achievements, recent developments, challenges, and future trends of such materials are discussed. Furthermore, the surface functionalization of separation ma-terials for further improvement of separation performance is reviewed. Finally, future research directions and the challenges of chromatographic separation are presented.

Chiral Stationary Phases for Liquid Chromatography: Recent Developments

The planning and development of new chiral stationary phases (CSPs) for liquid chromatography (LC) are considered as continuous and evolutionary issues since the introduction of the first CSP in 1938. The main objectives of the development strategies were to attempt the improvement of the chromatographic enantioresolution performance of the CSPs as well as enlarge their versatility and range of applications. Additionally, the transition to ultra-high-performance LC were underscored. The most recent strategies have comprised the introduction of new chiral selectors, the use of new materials as chromatographic supports or the reduction of its particle size, and the application of different synthetic approaches for preparation of CSPs. This review gathered the most recent developments associated to the different types of CSPs providing an overview of the relevant advances that are arising on LC.

Wide pH range enantioseparation of cyclodextrin silica-based hybrid spheres for high performance liquid chromatography

pH plays an important role in the enantiomeric separation process by changing the polarity of the mobile phase and the conformation and ionization state of the enantiomers. Herein, β-cyclodextrin-silica hybrid spheres with n-propyl groups as hydrophobic linkers and ethyl-silica as a support were prepared using a one-pot approach, and then the hydroxyl group was further modified with 3,5-dimethylphenyl isocyanate. The new β-cyclodextrin-silica hybrid chiral stationary phase (CD-HCSP) was prepared and characterized using techniques including scanning electron microscopy and transmission electron microscopy. The enantioseparation properties of CD-HCSP were evaluated with different solvents over a wide pH range (1-10) in reversed phase. 14 enantiomers were successfully resolved, and favorable chiral resolution and high stability was demonstrated for multiple types of enantiomer under different pH conditions. Compared with commercial columns, CD-HCSP showed better chiral resolution and, more importantly, could be used for chiral resolution over a wide pH range. This work combines the high pH tolerance of the hybrid material and excellent chiral recognition of cyclodextrin for enantioseparation of chiral drugs, which could lead to the development of a new type of chiral separation material.

Straightforward synthesis of enantiomerically pure 1,2,3-triazoles derived from amino esters

A practical and straightforward approach that enables, for the first time, the synthesis of enantiomerically pure 1,4,5-trisubstituted, 1,5-disubstituted, and fused 1,2,3-triazole derivatives has been developed. The synthesis employs enantiomerically pure amino esters derived from amino acids and commercially available ketones under metal-free conditions.

Highly N2-selective allylation of NH-1,2,3-triazoles with allenamides mediated by N-iodosuccinimide

We report a new protocol to synthesize N²-allyl-substituted 1,2,3-triazoles via NIS mediated allylation of allenamides with NH-1,2,3-triazoles.