Tert-butyl Analogues Research Articles

Development of a NanoBRET Assay Platform to Detect Intracellular Ligands for the Chemokine Receptors CCR6 and CXCR1.

A conserved intracellular allosteric binding site (IABS) was recently identified at several G protein-coupled receptors (GPCRs). This target site allows the binding of allosteric modulators and enables a new mode of GPCR inhibition. Herein, we report the development of a NanoBRET-based assay platform based on the fluorescent ligand LT221 (5), to detect intracellular binding to CCR6 and CXCR1, two chemokine receptors that have been pursued as promising drug targets in inflammation and immuno-oncology. Our assay platform enables cell-free as well as cellular NanoBRET-based binding studies in a nonisotopic and straightforward manner. By combining this screening platform with a previously reported CXCR2 assay, we investigated CXCR1/CXCR2/CCR6 selectivity profiles for both known and novel squaramide analogues derived from navarixin, a known intracellular CXCR1/CXCR2 antagonist and phase II clinical candidate for the treatment of pulmonary diseases. By means of these studies we identified compound 10, a previously reported tert-butyl analogue of navarixin, as a low nanomolar intracellular CCR6 antagonist. Further, our assay platform clearly indicated intracellular binding of the CCR6 antagonist PF-07054894, currently evaluated in phase I clinical trials for the treatment of ulcerative colitis, thereby providing profound evidence for the existence and the pharmacological relevance of a druggable IABS at CCR6.

Enantioselective Functionalization of Difluorocyclopropenes Catalyzed by Chiral Copper Complexes: Proposal for Chiral gem-Dimethyl and tert-Butyl Analogues.

The highly enantioselective copper/chiral phosphine-catalyzed hydro-, bora-, and carbo-metalations of difluorocyclopropenes with PHMS [H-Si], H-BPin, (BPin)2, and (CH3)2Zn [Zn-Me] are shown to regiodivergently afford highly enantioenriched and functionalized difluorocyclopropanes. These examples can be viewed as the first successful syntheses of "chiral" gem-dimethyl and tert-butyl analogues.

Aminophosphonocarboxylates of the Furan Series

Halomethyl derivatives of (diethoxyphosphorylmethyl)furan-2(3)-carboxylates containing blocking methyl group in the position 5 of the furan ring were synthesized. In the course of constructing of carbon skeleton of these compounds it was found that alkyl 3-methoxymethyl-, 3-chloromethyl-, and 3-(diethoxyphosphorylmethyl)-5-methyl-2-furoates are halomethylated at elevated temperature in the position 4 of the furan ring. No ring destructure or transformation of the side chain of substituents was observed. The obtained halomethylfurans and their tert-butyl analogs by treating with sodium azide in acetonitrile were converted to corresponding azides. The reduction of latter with triphenylphosphine in ethanol led to formerly unavailable aminophosphonocarboxylates of the furan series.

Engineering solid-state porosity of synthetic supercontainers via modification of exo-cavities

Two new coordination supercontainers have been successfully isolated from the self-assembly reactions of the container precursor p-phenylsulfonylcalix [4]arene, Ni(II) or Co(II) ion, and diphenylmethane-4,4'-dicarboxylic acid. Crystal structure analysis revealed that these two coordination supercontainers possess a similar endo cavity and two deeper exo cavities compared to the related supercontainers based on p-tert-butylsulfonylcalix [4]arene. Gas and vapor adsorption studies indicated that the new compounds are permanently porous and show much better CO2/O2 and CO2/N2 selectivity and higher vapor adsorption than the tert-butyl analogues.

Magnetic anisotropy in "scorpionate" first-row transition-metal complexes: a theoretical investigation.

In this work we have analyzed in detail the magnetic anisotropy in a series of hydrotris(pyrazolyl)borate (Tp(-)) metal complexes, namely [VTpCl](+), [CrTpCl](+), [MnTpCl](+), [FeTpCl], [CoTpCl], and [NiTpCl], and their substituted methyl and tert-butyl analogues with the goal of observing the effect of the ligand field on the magnetic properties. In the [VTpCl](+), [CrTpCl](+), [CoTpCl], and [NiTpCl] complexes, the magnetic anisotropy arises as a consequence of out-of-state spin-orbit coupling, and covalent changes induced by the substitution of hydrogen atoms on the pyrazolyl rings does not lead to drastic changes in the magnetic anisotropy. On the other hand, much larger magnetic anisotropies were predicted in complexes displaying a degenerate ground state, namely [MnTpCl](+) and [FeTpCl], due to in-state spin-orbit coupling. The anisotropy in these systems was shown to be very sensitive to perturbations, for example, chemical substitution and distortions due to the Jahn-Teller effect. We found that by substituting the hydrogen atoms in [MnTpCl](+) and [FeTpCl] by methyl and tert-butyl groups, certain covalent contributions to the magnetic anisotropy energy (MAE) could be controlled, thereby achieving higher values. Moreover, we showed that the selection of ion has important consequences for the symmetry of the ground spin-orbit term, opening the possibility of achieving zero magnetic tunneling even in non-Kramers ions. We have also shown that substitution may also contribute to a quenching of the Jahn-Teller effect, which could significantly reduce the magnetic anisotropy of the complexes studied.

The pentafluorosulfanyl group in cannabinoid receptor ligands: synthesis and comparison with trifluoromethyl and tert-butyl analogues

Competitive CB1 receptor antagonists carrying an aromatic SF5 group in position 3 of a pyrazole ring were synthesised and compared with their CF3 and tert-butyl analogues. Results confirmed that an aromatic SF5 group can be used as a bioisosteric analogue of a CF3 group and possibly of a bulky aliphatic group too.

A simple one-pot, multi-component synthesis of cyano-functionalized bis-β-diketones

Mild, multi-component Knoevenagel condensation/Michael addition reactions of dibenzoylmethane (Hdbm) and methylene-active cyano molecules with formalin in ethanol gave cyano methylene-bridged bis-diketones. The syntheses of 1,1,5,5-tetrabenzoyl-3,3-dicyanopentane (1) and 1,1,7,7-tetrabenzoyl-3,5-dicyano-3,5-dimethoxycarbonyl-heptane (4) are reported. Increasing the reaction time with the reagents for 1 led to the formation of the ester (2,2-dicyano-4-benzoylbutyl) benzoate (2). The isolation of 2,2-dicyano-4,4-dipivaloyl-1-butanol (3a), the tert-butyl analogue of a proposed intermediate in the formation of 2, provides evidence for a possible reaction pathway. Methylene-bis(dibenzoylmethane) (mdbm) was obtained in good yield from the reaction between Hdbm and formalin, by the addition of the base triethylamine. The X-ray crystal structures of 1–4 and mdbm were determined using synchrotron radiation.

Accessible gold clusters using calix[4]arene N-heterocyclic carbene and phosphine ligands

We investigate the synthesis of accessible calix[4]arene-bound gold clusters consisting of open "coordinatively unsaturated" active sites, using a comparative approach that relies on calix[4]arene ligands with various upper- and lower-rim substituents. In contrast with a reported Au(I)-tert-butyl-calixarene phosphine complex, which exhibits a single cone conformer in solution, the H upper-rim analog exhibits multiple conformers in solution. This contrasts with observations of the tert-butyl upper-rim analog, which exhibits a single cone conformer in solution under similar conditions. In the solid state, as determined by single-crystal X-ray diffraction, both H and tert-butyl upper-rim analogs exhibit exclusively cone conformer. A detailed structural analysis of these two solid-state structures highlights a CH-π interaction involving a methoxy lower-rim substituent and phenyl substituent on P as the key feature that enforces a tight configuration of Au(I) atoms on the same side of the calix[4]arene lower-rim plane. We hypothesize that such a configuration promotes chelation of the ligand to a gold surface and facilitates the synthesis of small Au11-sized clusters after reduction of both complexes. The new cluster, like the one reported with the tert-butyl analog, has an extraordinary 25% of surface atoms that are open and accessible to a 2-NT (2-naphthalenethiol) probe in solution. We also investigated the effect of calix[4]arene lower-rim substituents that coordinate to the metal, by using N-heterocyclic carbene (NHC) functional groups rather than phosphines. Four small (<1.6 nm diameter) calix[4]arene NHC-bound gold clusters were synthesized, including three using novel calix[4]arene NHC ligands. The smallest calix[4]arene NHC-bound Au cluster consisted of a 1.2 nm gold core, and its number density of accessible and open surface sites was measured. This required development of a new titration method for open sites on gold clusters, using a SAMSA fluorescein dye molecule, which excites and emits at lower energy relative to the previously used 2-NT probe. The number density of open sites on the new calix[4]arene NHC-bound gold cluster measured by the SAMSA fluorescein probe strongly supports the generality of a mechanical model of accessibility, which does not depend on the functional group involved in binding to the gold surface and rather depends on the relative radii of curvature of bound ligands and the gold cluster core.

Macromolecular antioxidants based on polysaccharides and 2,6-diisobornyl-4-methylphenol derivatives

New macromolecular antioxidants that were conjugates of dextran or hydroxyethylated starch with functionalized derivatives synthesized from 2,6-diisobornyl-4-methylphenol were prepared. Their antiradical activity compared with derivatives of 2,6-di-tert-butyl-4-methylphenol was studied in a model reaction with 2,2-diphenyl-1-picrylhydrazyl. The studied conjugates exhibited greater activity than sterically hindered phenols not bonded to a polymer chain. The synthesized isobornyl derivatives were more active than previously studied tert-butyl analogs.

Organosoluble calixarene-based quinolone carriers: syntheses, evaluation and model hydrolytic studies at the air–water interface

Two tetra-p-tert-butyl- and two tetra-p-H-calix[4]arene species integrating one or two propylnalidixate esters at the lower rim have been synthesized as possible antibacterial prodrugs. As they display amphiphilic behavior, their properties at the air–water interface were evaluated using the Langmuir balance technique. It has been shown that the two tert-butyl analogues form stable monolayers. The two tert-butyl analogues were then studied at 20 and 37 °C on carbonate buffered subphases at pH 10.0, before being engaged in hydrolytic studies on “production” of monolayers so as to allow a simple treatment, a HPLC survey of nalidixate release. The latter was found to be effective at 37 °C, with formation of free nalidixate in 13% yield for the bisnalidixate derivative, and 10% yield for the mononalidixate after 3 days.

ChemInform Abstract: S-(2-((2′-Carbamoylethyl)amino)ethyl) Phosphorothioate and Related Compounds as Potential Antiradiation Agents.

A reinvestigation of the radiation protection activity of S-(2-((2{prime}-carbamylethyl)amino)ethyl) lithium hydrogen phosphorothioate (4a) revealed that this compound possessed good (70% protection at a dose of 600 mg/kg) activity. The thione and imino bioisosteres of 4, S-(2-(2{prime}-thiocarbamylethylamino)ethyl) lithium hydrogen phosphorothioate (13a) and S-(2-(2{prime}-amidinoethylamino)ethyl) phosphorothioic acid (18b) showed 100% protection at doses of 300 and 150 mg/kg, respectively. The N-methyl (4b) and tert-butyl (4c) analogues of amide 4a, the N-methyl (13b) analogue of the thioamide 13a, the N-methyl (18a) analogue of amidine (18b), and the cyclic amidine S-(2-((2{prime}-(4,5-dihydroimidozoyl)ethyl)amino)ethyl) lithium hydrogen phosphorothioate (21) all showed 80% protection at the highest dose tested.

ChemInform Abstract: Enantioselective Syntheses of 2-Deoxyxylono-1,4-lactone and 2-Deoxyribono-1,4-lactone from 1,3-Dioxan-5-yl Diazoacetates.

1,3-Dioxan-5-yl diazoacetates are valuable substrates for highly diastereoselective and enantioselective carbon-hydrogen insertion reactions. trans-2-(tert-Butyl)-1,3-dioxan-5-yl diazoacetate is a direct precursor to 2-deoxyribono-1,4-lactone in up to 81% ee, whereas cis-2-(tert-butyl)-1,3-dioxan-5-yl diazoacetate yields only the protected 2-deoxyxylono-1,4-lactone in up to 96% ee. However, trans-2-aryl-1,3-dioxan-5-yl diazoacetate (aryl = phenyl or 2-naphthyl) forms the precursor to 2-deoxyxylono-1,4-lactone in up to 95% ee but with the mirror image configuration of that produced from the trans-2-(tert-butyl) analogue. The catalysts that are most suitable for these carbon-hydrogen insertion reactions are chiral dirhodium(II) carboxamidates. 1,3-Dialkoxy-2-propyl diazoacetates give mainly 2-deoxyxylono-1,4-lactone derivatives (>90:10) with generally high enantiocontrol, but replacement of hydrogen at the 2-position of these 2-propyl diazoacetates led to a mixture of products.

ChemInform Abstract: Effect of Chain Length and Ring Size of Alkyl and Cycloalkyl Side-Chain Substituents upon the Biological Activity of Brassinosteroids. Preparation of Novel Analogues with Activity Exceeding that of Brassinolide.

A series of brassinosteroids with different alkyl or cycloalkyl substituents in place of the isopropyl group at C-24 of brassinolide (1) were prepared by the CuCN-catalyzed addition of Grignard reagents to (threo-2R,3S,5alpha,22R,23R,24S)-23,24-epoxy-6, 6-(ethylenedioxy)-2,3-(isopropylidenedioxy)-26, 27-dinorcholestan-22-ol (9), followed by deketalization and Baeyer-Villiger oxidation. Compound 9 was employed as part of a 70:30 threo/erythro mixture of epoxides 9 and 10, from which the erythro-epoxide 10 was recovered intact after the Grignard additions. Thus, the corresponding n-dodecyl, n-hexyl, n-propyl, tert-butyl, cyclohexyl, cyclopentyl, cyclobutyl, and cyclopropyl analogues of brassinolide were obtained. A rearrangement byproduct was observed during the preparation of the cyclopropyl-substituted brassinosteroid when ether was used as the solvent in the Grignard reaction, but could be avoided by the use of THF. A method for recycling the undesired erythro-epoxide 10 was developed on the basis of deoxygenation with tellurium and lithium triethylborohydride. The rice leaf lamina inclination assay was then used to measure the bioactivity of the products. In general, increasing activity was observed as the length or ring size of the C-24 hydrocarbon substituent decreased. The novel cyclobutyl- and cyclopropyl-substituted analogues of brassinolide (1) were ca. 5-7 times as active as 1 and thus appear to be the most potent brassinosteroids reported to date. Further enhancement of the bioactivity of all of the above brassinosteroids, except that of the inactive n-dodecyl derivative, was observed when the brassinosteroid was applied together with an auxin, indole-3-acetic acid (IAA). The synergy between the brassinosteroids and IAA thus increased the bioactivity of the brassinosteroids, including the cyclopropyl and cyclobutyl derivatives, by ca. 1-2 orders of magnitude.

Conformational analysis of methylboronic acid and its acyclic esters

With the use of RHF/6-31G(d) and MP2/6-31G(d)//RHF/6-31G(d) semi-empirical (AMI) and non-empirical quantum chemical approximations conformational isomerization of methylboronic acid and its methyl, isopropyl, tert-butyl, and phenyl esters is studied. With the exception of the tert-butyl analogue, the potential energy surface of molecules in these compounds, is shown to contain three minima that correspond to planar and near-planar conformers: cis-cis, trans-trans, and cis-trans, with the latter being the main one. The minima are separated by two conformational isomerization barriers corresponding to the orthogonal arrangement of one of the OH(OR) groups.

Ligand dependence in the copper-catalyzed oxidation of hydroquinones

Transition metal-mediated oxidation of hydroquinones is an important physiologic reaction, and copper(II) effectively catalyzes the reaction in phosphate-buffered saline (PBS). Studies reported herein in phosphate buffer alone demonstrate that copper(II) is an ineffective catalyst in the absence of coordinating ligands, but that 1,10-phenanthroline and histamine facilitate the copper(II)-mediated oxidation of hydroquinone and its 2,5- and 2,6-di- tert-butyl analogs to the corresponding benzoquinones. The high concentration of chloride in PBS is the key element that allows copper(II) to work in this system. Although the bis-bathocuproine disulfonate complex of Cu(II), (BC) 2Cu(II), is a strong stoichiometric oxidant, stoichiometric amounts of copper(II) in the presence of ligands other than BC oxidize hydroquinones very slowly under anaerobic conditions. Thus, the rapid copper(II)-catalyzed reaction operating aerobically does not involve a simple ping-pong reduction of copper(II) to copper(I) by hydroquinone and reoxidation of copper(I) by O 2.

Reactions of dimethyl and di-tert-butyl 2-aryl-4-hydroxy-4-methyl-6-oxocyclohexane-1,3-dicarboxylates with difunctional nucleophiles

The nature of the alkyl group in the ester moieties of dimethyl and di-tert-butyl 2-aryl-4-hydroxy-4-methyl-6-oxocyclohexane-1,3-dicarboxylates affects the direction of their reactions with difunctional nucleophiles. The dimethyl esters react with hydrazine hydrate to give the corresponding tetrahydroindazoles, while their tert-butyl analogs are converted under similar conditions into 6-hydrazones. Reactions of both dimethyl and di-tert-butyl 6-oxocyclohexane-1,3-dicarboxylates with hydroxylamine lead to formation of 6-hydroxyimino derivatives.

Organoaluminium and -Gallium Compounds with O-Oximato Substituents

The organoaluminium and -gallium acetonoximates (tBu2MON=CMe2)2 (M = Al, Ga) have been prepared by the reaction of acetoneoxime with tri-tert-butylaluminium and -gallium. The compounds (Me2MON=CMe2)2 (M = Al, Ga), described previously, were synthesized for comparison. All compounds have been characterised by NMR spectroscopy (1H, 13C, 27Al), by mass spectrometry and elemental analyses. The crystal structures of the four compounds have been determined, and it was shown that all form dimers with six-membered M2O2N2 rings by aggregation through the imino N atoms. The compounds (Me2MON= CMe2)2 adopt boat conformations, whereas the compounds (tBu2MON=CMe2)2 prefer chair conformations. Large differences in the M-O-N angles have been found between the methyl and the tert-butyl analogues.

Ester derivatives of hexahomotrioxacalix[3]naphthalenes: conformational and binding properties with alkali metal cations.

The syntheses of the triesters formed between ethyl bromoacetate and hexahomotrioxacalix[3]naphthalene 8, and its tert-butyl analogue 11, are described. Depending on the conditions employed, cone or partial cone conformers are produced. The conformations appear to have some influence on their complexation in neutral medium, with alkali metal cations. The X-ray structure of the partial cone triester 10 is presented.

Bond angle versus torsional deformation in an overcrowded alkene: 9-(2,2,2-triphenylethylidene)fluorene.

Competition between bond angle and torsional strain in sterically crowded alkenes generally causes twisting in tetrasubstituted alkenes, while most structurally characterized trisubstituted alkenes are planar. To investigate structural effects of steric repulsion between a planar aromatic ring and a vicinal triphenylmethyl (trityl) group, 9-(2,2,2-triphenylethylidene)fluorene (1 a) was synthesized by reaction of 9-bromomethylenefluorene with triphenylmethyllithium. For comparison with a less strained analogue, 9-ethylidenefluorene (1 b) was prepared by reaction of fluorenone with ethylmagnesium bromide. The X-ray crystal structures show that the difference between bond angles at the 9-fluorenyl carbon atom is much larger for 1 a (12.9 degrees) than 1 b (2.6 degrees). Bond angle and torsional deformations were compared theoretically (HF/6-31+G*) with the tert-butyl analogue (1 c), 1,2,2-tri-tert-butylethene (7), and 2,4,4-trimethyl-2-pentene (8) and crystallographically with six known 1,1-diaryl-2-tert-alkylethenes (2). The trisubstituted alkenes formed three groups with 1) large angle distortion with moderate twisting (1 a, 1 b, and 7), 2) moderate bending with a large range of torsional angles (2), and 3) little bending or twisting (1 b and 8). For the entire series, there appears to be a delicate balance between angle and torsional deformation, but twisting appears to produce smaller relief from steric strain than angle bending. In the crystallographically characterized trisubstituted alkenes, the choice between the two is mainly determined by more subtle packing forces.

Tertiary Pentyl Groups Enhance Salen Titanium Catalyst for Highly Enantioselective Trimethylsilylcyanation of Aldehydes.

AbstractFor Abstract see ChemInform Abstract in Full Text.