Electrochemical reduction of dehydroamino acids: synthesis and photophysical properties of β,β-diarylalanines

Several β,β-disubstituted dehydroalanines were prepared from β,β-dibromo or β-bromo, β-substituted dehydroalanines and aryl boronic acids using a Suzuki-Miyaura cross-coupling reaction. The electrochemical behaviour of these compounds was studied by cyclic voltammetry. All compounds studied showed similar reduction potentials and these were similar to the peak potential of the methyl ester of N-tert-butoxycarbonyl dehydrophenylalanine. Thus, the presence of a second aryl moiety in the dehydroalanine scaffold does not significantly change the reduction potential.Controlled potential electrolyses were performed at the cathodic peak potential in the presence of triethylammonium chloride as proton donor. The only products isolated in good to high yields were the corresponding β,β-diarylalanines. This reaction was also carried out using a dipeptide containing a β,β-diaryldehydroalanine to give a 1:1 diastereomeric mixture of the reduction product.The photophysical properties of two of the β,β-diaryldehydroalanines and of the corresponding β,β-diarylalanines were studied in three solvents of different polarity. The β,β-diaryldehydroalanines show low fluorescent quantum yields (Φ_F<9%) due to the conjugation of the aromatic moieties with the α,β-double bond and with the carbonyl group, which favours the non-radiative deactivation pathways. The absence of conjugation in the reduction products leads to a significant increase in the fluorescence quantum yields. These results show that the β,β-disubstituted alanines could be used as fluorescent markers.     

Dicyano and pyridine derivatives of β-carotene: synthesis and vibronic, electronic, and photophysical properties

Density functional theory and time-dependent density functional theory calculations provide pictures of the molecular orbitals involved in the ground and excited states of two cyano derivatives of 8'-apo-β-caroten-8'-al synthesized via an acid-base-catalyzed Knoevenagel condensation reaction. Population analysis shows that the symmetry-allowed transition, S(0) ((1)A(g)) → S(2) ((1)B(u)) based on the C(2h) symmetry is a HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital) π → π* transition with electron densities located mostly on the polyene chain. Calculated and actual steady-state absorption spectra show similar features with low-energy peak maxima between 550 and 600 nm.     

Star-shaped triphenylamine terminated difluoroboron β-diketonate complexes: synthesis,photophysical and electrochemical properties

Two new star-shaped difluoroboron β-diketonate complexes (TBC)₃Ph and (TBF)₃Ph, in which the terminal groups of triphenylamine functionalized difluoroboron β-diketonates were bridged by carbazole or fluorene to the core of 1,3,5-benzene, have been synthesized. It was found that they gave high molar extinction coefficients, meaning strong light-harvesting ability, and emitted intense yellow light with fluorescence quantum yields (Φ_F) of 0.73 and 0.67 for (TBC)₃Ph and (TBF)₃Ph, respectively, in toluene as well as red light in solid states with Φ_F of 0.36 and 0.27, respectively. The electrochemical behaviors suggested that they had considerably higher electron affinities than tris(8-hydroxyquinoline)aluminum (AlQ₃), which indicated that they could be used as electron-transporting materials besides emitting materials.     

Silicon mediated synthesis and selected transformations of β-chloroketones

A combination of tetrachlorosilane（TCS） and phenol in dichloromethane was found to be an efficient reagent for hydro-chlorination ofα,β-unsaturated ketones to afford the correspondingβ-chloroketones in good yield at ambient temperature. Reactions of the titled compounds with TCS-NaN₃ as well as with nitriles utilizing TCS-ZnCl₂ to give 1,5-disubstituted tetrazole derivatives orβ-amido ketones respectively are also described.     

Vibrational and thermodynamic properties of β-HMX: a first-principles investigation

Thermodynamic properties of β-HMX crystal are investigated using the quasi-harmonic approximation and density functional theory within the local density approximation (LDA), generalized gradient approximation (GGA), and GGA + empirical van der Waals (vdW) correction. It is found that GGA well describes the thermal expansion coefficient and heat capacity but fails to produce correct bulk modulus and equilibrium volume. The vdW correction improves the bulk modulus and volume, but worsens the thermal expansion coefficient and heat capacity. In contrast, LDA describes all thermodynamic properties with reasonable accuracy, and overall is a good exchange-correlation functional for β-HMX molecular crystal. The results also demonstrate significant contributions of phonons to the equation of state. The static calculation of equilibrium volume for β-HMX differs from the room-temperature value incorporating lattice vibrations by over 5%. Therefore, for molecular crystals, it is essential to include phonon contributions when calculated equation of state is compared with experimental data at ambient condition.     

Cyclopropanation of 1,2-dibromoethylphosphonate: a synthesis of β-aminocyclopropylphosphonic acid and derivatives

Diethyl (1,2-dibromoethyl)phosphonate was found to undergo cyclopropanation with nitromethane in good yield. The resulting trans β-nitrocyclopropylphosphonate was converted to the trans N-protected aminocyclopropylphosphonate through a reduction–protection sequence. Subsequent hydrolysis gave the free β-aminocyclopropylphosphonic acid without any formation of ring-opening byproduct. Cyclopropanation of 1,2-dibromoethylphosphonates with nitroalkanes and their reduction are also discussed.     

Enantioselective one-pot synthesis of β-sulfonyl ketones and trisubstituted tetrahydrothiophenes via β-elimination/cycloaddition sequence

A mild and efficient enantioselective one-pot synthesis of β-sulfonyl ketones and trisubstituted tetrahydrothiophenes via introducing 1,4-dithiane-2,5-diol to the simple kinetic resolution of β-sulfonyl ketones has been described herein. The one-pot reaction sequence including kinetic resolution and cascade sulfa-Michael/Aldol reaction proceeded successively to afford corresponding sulfonyl ketones and tetrahydrothiophenes with high enantioselectivities (85%–98% ee and 84%–95% ee, respectively).     

Diaminotrehalose-capped β-cyclodextrin, a new member of hemispherodextrins: synthesis, thermodynamic and spectroscopic characterization and its exploitation in chiral electrokinetic chromatography

The diaminotrehalose-capped derivative of β-CD, a new member of hemispherodextrins, was synthesized and characterized by NMR spectroscopy. Its protonation constants were determined by potentiometry, and the inclusion of both the enantiomers of dansyl-phenylalanine was investigated by NMR spectroscopy. Its stereoselective properties were exploited in electrokinetic chromatography by separating four enantiomeric pairs of dansyl derivatives of amino acids.     

Preparation and characterization of a novel organic–inorganic hybrid nanostructure: application in synthesis of spirocompounds

Research on Chemical Intermediates - Immobilization and heterogenization of acidic/basic groups or organic tags on inorganic supports have found many important applications in recent years. In this...     

Enantioselective synthetic approaches to cyclopropane and cyclobutane β-amino acids: synthesis and structural study of a conformationally constrained β-dipeptide

Synthetic approaches to carbocyclic compounds, namely cyclopropane and cyclobutane β-amino acids, are presented. One of them is based on enzymatic desymmetrization of meso diesters, leading to the enantioselective production of cis-hemiesters, which afforded β-amino acids through Curtius rearrangements. The enantiomeric excess for the cyclobutane derivatives was 91% whereas the cyclopropanes were obtained in 63% ee. According to another strategy, an enantiomerically pure cyclopropane trans-β-amino acid, bearing a quaternary center, has been synthesized from a homochiral precursor easily available from d-glyceraldehyde. The preparation and structural investigation of the first synthesized cyclobutane containing dipeptide is also described. A hairpin-like conformation of this molecule in the solid state has been demonstrated by X-ray structural analysis, showing crystal packing induced by the presence of the rigid cyclobutane moiety and the formation of intermolecular hydrogen bonds. NMR experiments confirmed that these molecules also tend to produce aggregates in solution. On the contrary, theoretical calculations suggest that intramolecular interactions are important in the gas phase, as expected.     

C-H activation by a mononuclear manganese(III) hydroxide complex: synthesis and characterization of a manganese-lipoxygenase mimic?

Lipoxygenases are mononuclear non-heme metalloenzymes that regio- and stereospecifically convert 1,4-pentadiene subunit-containing fatty acids into alkyl peroxides. The rate-determining step is generally accepted to be hydrogen atom abstraction from the pentadiene subunit of the substrate by an active metal(III)-hydroxide species to give a metal(II)-water species and an organic radical. All known plant and animal lipoxygenases contain iron as the active metal; recently, however, manganese was found to be the active metal in a fungal lipoxygenase. Reported here are the synthesis and characterization of a mononuclear Mn(III) complex, [Mn(III)(PY5)(OH)](CF(3)SO(3))(2) (PY5 = 2,6-bis(bis(2-pyridyl)methoxymethane)pyridine), that reacts with hydrocarbon substrates in a manner most consistent with hydrogen atom abstraction and provides chemical precedence for the proposed reaction mechanism. The neutral penta-pyridyl ligation of PY5 endows a strong Lewis acidic character to the metal center allowing the Mn(III) compound to perform this oxidation chemistry. Thermodynamic analysis of [Mn(III)(PY5)(OH)](2+) and the reduced product, [Mn(II)(PY5)(H(2)O)](2+), estimates the strength of the O-H bond in the metal-bound water in the Mn(II) complex to be 82 (+/-2) kcal mol(-)(1), slightly less than that of the O-H bond in the related reduced iron complex, [Fe(II)(PY5)(MeOH)](2+). [Mn(III)(PY5)(OH)](2+) reacts with hydrocarbon substrates at rates comparable to those of the analogous [Fe(III)(PY5)(OMe)](2+) at 323 K. The crystal structure of [Mn(III)(PY5)(OH)](2+) displays Jahn-Teller distortions that are absent in [Mn(II)(PY5)(H(2)O)](2+), notably a compression along the Mn(III)-OH axis. Consequently, a large internal structural reorganization is anticipated for hydrogen atom transfer, which may be correlated to the lessened dependence of the rate of substrate oxidation on the substrate bond dissociation energy as compared to other metal complexes. The results presented here suggest that manganese is a viable metal for lipoxygenase activity and that, with similar coordination spheres, iron and manganese can oxidize substrates through a similar mechanism.     

Copper complexes of pyridyl–tetrazole ligands with pendant amide and hydrazide arms: synthesis,characterization, DNA-binding and antioxidant properties

Pyridyl–tetrazole ligands 2-(5-(pyridin-2-yl)-1H-tetrazol-1-yl)acetamide (L1), 2-(5-(pyridin-2-yl)-2H-tetrazol-2-yl)acetamide (L2), 2-(5-(pyridin-2-yl)-1H-tetrazol-1-yl)acetohydrazide (L3) and 2-(5-(pyridin-2-yl)-2H-tetrazol-2-yl)acetohydrazide (L4) have been prepared and coordinated with CuCl₂·2H₂O to furnish the corresponding complexes [Cu(L1) ₂ ]–[Cu(L4) ₂ ]. EPR spectra of the complexes are characteristic of square planar geometries, with nuclear hyperfine spin 3/2. DNA-binding studies using UV–Vis absorption spectroscopy, viscosity and thermal denature studies revealed that all of these complexes are avid binders of calf thymus DNA. The antioxidant properties of the free ligands and the Cu(II) complexes were investigated using the p-nitrosodimethyl aniline hydroxyl radical scavenging method, and [Cu(L4) ₂ ] was found to show the highest activity.     

Supramolecular engineering of a 2D Kagomé lattice: synthesis, structures, and magnetic properties

Two 2D Mn (II) complexes, [Mn3(TzDC)2(phen)3] x 2 H2O (1; H3TzDC = 1,2,3-triazole-4,5-dicarboxylic acid, phen = 1,10-phenanthroline) and [Mn3(TzDC)2(bipy)3] x 4 H2O (2; bipy = 2,2'-bipyridine), were synthesized by hydrothermal reactions and characterized magnetically, and complex 1 was the first example of the chiral complex with a Kagomé lattice connectivity obtained through spontaneous resolution.     

Triazolyl C-nucleosides via the intermediacy of β-1′-ethynyl-2′-deoxyribose derived from a Nicholas reaction: Synthesis,photophysical properties and interaction with BSA

We report the design and synthesis of triazolyl donor/acceptor unnatural C-nucleosides via alkyne (sugar)—azide (aromatic) 1, 3-dipolar cyclo-addition reaction as a key step and studies on their photophysical properties. We have chosen β-1′-ethynyl-2′-deoxyribose as a precursor to synthesize triazolyl-C-nucleosides. Overcoming the difficulties, we obtain β-1′-ethynyl-2′-deoxyribose as a major product following a Co₂(CO)₈ catalyzed intramolecular Nicholas reaction. The 1,3-diaxial interaction is the driving force for the α to β-anomeric conversion while performing cobalt complexation followed by oxidation to afford β-1′- ethynyl-2′-deoxyribose as the major product. A Cu(I)-catalyzed click reaction between different aromatic donor/acceptor azides and β-1′- ethynyl-2′-deoxyribose generates the desired unnatural triazolyl donor-acceptor aromatic C-nucleosides (^cTB_Do/Ac) within 30 min. Single crystal X-ray structure shows the puckered conformation of sugar as C3′-exo. Studies on the photophysical properties suggests good fluorophoric as well as solvatochromic characteristics of these nucleosides. Two of the synthesised nucleosides, ^cTAnthB_Do and ^cTPyB_Do, are found to interact with BSA as the only tested protein with quenching of fluorescence signal. The designed bases, thus, might find applications in stabilizing a DNA and in the biophysical study thereof, if a pair of such donor acceptor C-nucleosides could be incorporated into a DNA sequence.     

Diastereoselective synthesis of β- and γ2-muurolene: a carbocationic pathway from mono- to sesquiterpenes

The Lewis acid catalysed addition of the piperityl chlorides6 to isoprene yields adduct7, which undergoes a cis-stereoselective cyclisation to give the diastereoisomeric muurolene monohydrochlorides8. Treatment with potassium tert-butoxide affords β- and γ₂- muurolene 2.     

Straightforward and highly efficient catalyst-free regioselective reaction of thiol to β-nitrostyrene: a concise synthesis of vinyl sulfide and nitro sulfide

Under catalyst-free reaction conditions, solvent-mediated addition of thiol 2 to β-nitrostyrene 1 proceeded with regioselective control to afford either adduct 3 or vinyl sulfide 4 in good to excellent yield. Thermodynamic and autocatalytic reaction mechanisms were proposed to rationalize the products thus formed.     

In situ synthesis of calcium pimelate as a highly dispersed β-nucleating agent for improving the crystallization behavior and mechanical properties of isotactic polypropylene

The reaction precursors pimelic acid (Pi) and calcium stearate (CaSt) were added in situ during extrusion of isotactic polypropylene (iPP) to generate self-dispersed calcium pimelate (CaPi[IS]). The results reveal that the nucleating agents (NAs) synthesized in situ obviously affected the nucleation effect in iPP. The crystallization, mechanical properties and melting behavior of nucleated iPP were investigated. The crystallization temperature (T_c) of iPP modified with CaPi(IS) increased by 4–5°C when compared to that of pure iPP. Especially, when the addition amount of CaPi(IS) in iPP was 0.30 wt%, the relative β-crystal concentration of iPP/CaPi(IS) reached the highest level of 96.47%, 22.71% higher than that of iPP/CaPi. However, Pi and CaSt has hardly impact on the nucleation effect in iPP. The mechanical properties of iPP show that CaPi and CaPi(IS) have excellent toughening effect on iPP while Pi and CaSt greatly improved the stiffness of iPP. Furthermore, the dispersion of these NAs in iPP was investigated by scanning electronic microscope (SEM). It can be clearly seen that the CaPi(IS) particles are uniformly distributed in the iPP after magnification.     

Palladium-catalyzed cycloisomerizations of diarylacetylenes: synthesis, structures, and physical properties of highly substituted naphthalenes and 8,8 a-dihydrocyclopenta[a]indenes

Depending on the electronic properties of their substituents, the major products generated by palladium‐catalyzed cycloisomerizations of diarylalkynes are either highly substituted 8,8a‐dihydrocyclopenta[a]indenes 3 or naphthalenes 4 . The structures of these compounds were verified by X‐ray crystallographic analysis. Many functional groups tolerated the reaction conditions evaluated in this study. The isotope‐labeled experiments indicated that added water has a critical role in forming both classes of compounds. The photophysical and electrochemical properties of cycloadducts 3 and their analogues were systematically studied and compared with computational predictions based on density functional theory. Dihydrocyclopenta[a]indenes 3 in either solid or liquid form display strong luminescence, whereas cyclopenta[a]indene 11 j is practically nonfluorescent. The functional groups directly attached to the backbone of compound 3 significantly influenced physical properties. The steric effect arising from the aryl substituents caused different luminescence phenomena, including aggregation‐induced and ‐enhanced emission.