Showing papers in "Helvetica Chimica Acta in 2001"

Reliable chemical synthesis of oligoribonucleotides (RNA) with 2"-O-[(triisopropylsilyl)oxy]methyl(2"-O-tom)-protected phosphoramidites

Abstract: A method for the introduction of the 2'-O-[(triisopropylsilyl)oxy]methyl (=tom) group into N-acetylated, 5'-O-dimethoxytritylated ribonucleosides is presented. The corresponding 2'-O-tom-protected phosphoramidite building blocks were obtained in pure form and were successfully employed for the routine synthesis of oligoribonucleotides on DNA synthesizers. Under DNA coupling conditions (2.5 min coupling time for a 1.5-mmol synthesis scale) and with 5-(benzylthio)-1H-tetrazole (BTT) as activator. 2'-O-tom-protected phosphoramidites exhibited av. coupling yields >99.4%. The combination of N-acetyl and 2'-O-tom protecting groups allowed a reliable and complete two-step deprotection, first with MeNH2 in EtOH/H2O and then with Bu4NF in THF, without concomitant destruction of the product RNA sequences. [on SciFinder (R)]

Electrophilic Terminal‐Phosphinidene Complexes: Versatile Phosphorus Analogues of Singlet Carbenes

Abstract: Electrophilic and nucleophilic terminal-phosphinidene complexes are compared in terms of electronic structures and reactivities. Various precursors of the unstable electrophilic species [R−P−M] (M=Cr, Mo, W(CO)5 and Fe(CO)4) are discussed. The addition reactions of the electrophilic phosphinidene complexes with Lewis bases, insertion reactions into O−H, N−H, and activated C−H bonds, and cycloaddition reactions with double and triple bonds are described, as well as some rearrangements and autocondensations. Various applications to the synthesis of new organophosphorus molecules are discussed and techniques available for demetallation are given.

Selenocysteine-mediated native chemical ligation

Abstract: C-Terminal peptide thioesters are shown to react efficiently with peptide fragments containing an N-terminal selenocysteine to give selenoproteins. In analogy to the native chemical ligation of thioesters and peptides containing N-terminal cysteines, the selenol presumably attacks the thioester nucleophilically to give a selenoester intermediate that subsequently rearranges to give a native chemical bond. The utility of this procedure was demonstrated by the synthesis of a selenium-containing derivative of bovine pancreatic trypsin inhibitor (BPTI) in which Cys38 is replaced by selenocysteine. The artificial selenoprotein folds into a conformation similar to that of wild-type BPTI and inhibits trypsin and chymotrypsin with unaltered affinity.

Photolabile Protecting Groups for Nucleosides: Mechanistic Studies of the 2-(2-Nitrophenyl)ethyl Group

Abstract: The photochemistry of several 2-(2-nitrophenyl)ethyl-caged compounds including caged thymidine nucleosides was studied by nanosecond laser flash photolysis and stationary illumination experiments with quantitative HPLC analysis for quantum yields and product distribution. Effects of solvent basicity and acidity were investigated by varying the H2O content and HCl concentration, respectively, in MeCN/H2O mixtures. For all compounds 1 ‐ 7 investigated, intramolecular H abstraction by the nitro group from the exocyclic a-position with respect to the aryl moiety was found to be the primary process. The protolytic dissociation equilibrium of the resulting aci-nitro compound was kinetically characterized in the 0.1 ‐ 10 ms time region. In general, two reaction channels compete for the aci-nitro compound and its anion: b-elimination of the caged compound occurs from the anion, while from the undissociated aci-nitro compound, a nitrosobenzene derivative is formed with no release of the caged compound. The yield ratio of these two reaction channels can be controlled through shifts in the protolytic dissociation equilibrium of the aci-nitro compound. In solutions with either low basicity (H2O-free MeCN) or high acidity (higher concentration of HCl in H2O/MeCN), two as yet unidentified products are formed, each one specifically for one of the mentioned conditions.

The Briggs‐Rauscher Reaction as a Test to Measure the Activity of Antioxidants

Abstract: A new method for monitoring the relative activity of antioxidants is presented, and its advantages and limits are discussed. The method is based on the previously reported inhibitory effects of free-radical scavengers on the oscillations of the Briggs-Rauscher reaction. The effect consists of an immediate cessation of oscillations, an inhibition time that linearly depends on the concentration of the antioxidant added, and subsequent regeneration of oscillations. Here the effects of ten antioxidants (pyrocatechol (=benzene-1,2-diol), ferulic acid (=3-(4-hydroxy-3-methoxyphenyl)prop-2-enoic acid), caffeic acid (=3-(3,4-dihydroxyphenyl)prop-2-enoic acid), 2,6-, 3,4-, 2,4-, 3,5-, and 2,5-dihydroxybenzoic acids, homovanillic acid (=4-hydroxy-3-methoxybenzeneacetic acid), and resorcinol (=benzene-1,3-diol)) were studied in detail. Relative antioxidant activities of these substances with respect to resorcinol were determined in different ways on the basis of inhibition times. The limits of the calculated values of relative activity based on the Briggs-Rauscher reaction are the same as those obtained with other analytical procedures and are discussed here. The new method is inexpensive: reagents and apparatus are commonly used in all chemical laboratories. The thermochemical behavior of the Briggs-Rauscher reaction and the dependence of inhibition time on the temperature were also carefully investigated and taken into account. A semiquantitative mechanistic interpretation of the inhibitory effects based on a suitable kinetic model is given.

Facile, Asymmetric Addition of Acetylene to Aldehydes: In Situ Generation of Reactive Zinc Acetylide

Abstract: This study documents a process in which acetylene is employed directly in nucleophilic additions to aldehydes to give adducts in high levels of enantiomeric induction, up to 98% ee. To the best of our knowledge, this represents the first time in which acetylene itself has been utilized in such ligand-controlled enantioselective additions.

Preparation of TADOOH, a Hydroperoxide from TADDOL, and Use in Highly Enantioface‐ and Enantiomer‐Differentiating Oxidations

Abstract: Replacement of one OH group in TADDOL (=α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5-dimethanol) by an OOH group gives a stable, crystalline chiral hydroperoxy alcohol TADOOH (={(4R,5R)-5-[(hydroperoxydiphenyl)methyl]-2,2-dimethyl-1,3-dioxolan-4-yl}diphenylmethanol) 3, the crystal structure of which resembles those of numerous other TADDOL derivatives (Fig. 2). The new hydroperoxide was tested as chiral oxidant in three types of reactions: the epoxidation of enones with base catalysis (Scheme 2), the sulfoxidation of methyl phenyl sulfide (Scheme 3), and the Baeyer-Villiger oxidation of bicyclic and tricyclic cyclobutanones, rac-10a – d with kinetic resolution (Scheme 4, Fig. 3, and Table). Products of up to 99% enantiomer puritiy were isolated (the highest values yet observed for oxidations with a chiral hydroperoxide!). Mechanistic models are proposed for the stereochemical courses of the three types of reactions (Schemes 5 and 6, and Fig. 4). Results of AM1 calculations of the relative transition-state energies for the anionic rearrangements of the exo Criegee adducts of TADOOH to the enantiomeric bicyclo[3.2.0]heptan-6-ones are in qualitative agreement with the observed relative rates (Table and Fig. 5).

Damage to Isolated DNA Mediated by Singlet Oxygen

Abstract: In the present work, we study the reaction of singlet oxygen (1O2) with isolated DNA. Emphasis is placed on the identification and quantitative measurement of the DNA modifications that are produced by the reaction of 1O2 with DNA. For this purpose, calf-thymus DNA was incubated with the endoperoxide of N,N′-di(2,3-dihydroxypropyl)-1,4-naphthalenedipropanamide, a chemical generator of 1O2. Thereafter, DNA was digested, and the resulting oxidized nucleosides were measured by means of a recently optimized high-performance-liquid-chromatography tandem-mass-spectrometry assay. It was found that, among the different DNA lesions observed, 7,8-dihydro-8-oxo-2′-deoxyguanosine is the major 1O2-mediated DNA-damage product. Interestingly, cyclobutane pyrimidine dimers, oxidized pyrimidine bases, 7,8-dihydro-8-oxo-2′-deoxyadenosine, and 2,6-diamino-5-formamido-4-hydroxypyrimidine are not formed, at least not in detectable amounts, following treatment of DNA with the 1O2 generator. The reported results strongly suggest that the decomposition of the endoperoxide provides a pure source of 1O2, and that reaction of 1O2 with isolated DNA induces the specific formation of 7,8-dihydro-8-oxo-2′-deoxyguanosine.

Photochemical Reaction Mechanisms of 2-Nitrobenzyl Compounds in Solution, I. 2-Nitrotoluene: Thermodynamic and Kinetic Parameters of the aci-Nitro Tautomer

Abstract: The largely reversible, light-induced tautomerization of 2-nitrotoluene (1) to the quinonoid aci-nitro tautomer aci-1 was studied by flash photolysis as a benchmark for comparison with the widely used nitrobenzyl phototriggers (`caged compounds'). The pH-rate profile for the decay of aci-1 in aqueous solution exhibits downward curvature at pH 3 – 4, which is attributed to pre-equilibrium ionization of the nitronic acid aci-1 to its anion 1− (pKa=3.57). Two regions of upward curvature, at pH ca. 6 and 6. A hird, irreversible Nef-type isomerization of aci-1 prevails in highly acidic solutions (pH<0). The equilibrium constant for the thermal tautomerization of 1 to aci-1 is estimated as pKT=17.0±0.2 based on kinetic data.

Conformational Switching of Resorcin[4]arene Cavitands by Protonation, Preliminary Communication

Abstract: The synthesis of the quinoxaline-bridged resorcin[4]arene cavitand 1 was accomplished from 2-[3,5-di(tert-butyl)phenyl]acetaldehyde via formation of the intermediate octol 2. Such cavitands are known to occur in an open `kite' conformation at low temperature ( 318 K). We discovered that protonation of cavitand 1 at room temperature by common acids, such as CF3COOH, also causes reversible switching from `vase' to `kite', and that this conformational change can be conveniently monitored by both 1H-NMR and UV/VIS spectroscopy.

The Insulated Solar Fenton Hybrid Process: Fundamental Investigations

Abstract: With the so-called advanced oxidation processes (AOPs) gaining more and more importance, the Fenton process has proven to be a particularly effective method for the treatment of wastewater. 4-Nitrophenol (4-NP) was selected as the model pollutant to study the influence of temperature and solar illumination on its degradation rate in batch Fenton and photo-Fenton experiments. Based upon these results, the insulated solar Fenton hybrid (ISFH) process was developed, combining solar-chemical and solar-thermal processes. With a flat-plate reactor, it was found to be of particular importance to determine the optimal reactor depth for this continuous process, since excessive reactor depth increased the heat capacity, resulting in an increase of the heating period. Constructive limits were also clearly established.

Synthesis of Citronellal by RhI-Catalysed Asymmetric Isomerization ofN,N-Diethyl-Substituted Geranyl- and Nerylamines or Geraniol and Nerol in the Presence of Chiral Diphosphino Ligands, under Homogeneous and Supported Conditions

Abstract: For the asymmetric isomerization of geranyl- or neryldiethylamine ((E)- or (Z)-1, resp.) and allyl alcohols geraniol or nerol ((E)- or (Z)-2, resp.) to citronellal (4) in the presence of a [RhI(ligand)cycloocta-1,5-diene)]+ catalyst, the atropic ligands 5 – 11 are compared under homogeneous and polymer-supported conditions with the non-C2-symmetrical diphosphino ferrocene ligands 12 – 16. The tBu-josiphos ligand 13 or daniphos ligand 19, available in both antipodal series, already catalyse the reaction of (E)-1 at 20° (97% e.e.) and favourably compare with the binap ligand 5 (see Table 1). Silica-gel- or polymer-supported diphosphino ligands usually afford similar selectivity as compared to the corresponding ligands applied under homogeneous conditions, but are generally less reactive. In this context, a polymer-supported ligand of interest is the polymer-anchored binap (R)-6, in terms of reactivity, selectivity, and recoverability, with a turnover of more than 14400.

Optically Active Cyclophane Receptors for Mono- and Disaccharides: The Role of Bidentate Ionic Hydrogen Bonding in Carbohydrate Recognition

Abstract: A new family of optically active cyclophane receptors for the complexation of mono- and disaccharides in competitive protic solvent mixtures is described. Macrocycles (−)-(R,R,R,R)-1 – 4 feature preorganized binding cavities formed by four 1,1′-binaphthalene-2,2′-diyl phosphate moieties bridged in the 3,3′-positions by acetylenic or phenylacetylenic spacers. The four phosphodiester groups converge towards the binding cavity and provide efficient bidentate ionic H-bond acceptor sites (Fig. 2). Benzyloxy groups in the 7,7′-positions of the 1,1′-binaphthalene moieties ensure solubility of the nanometer-sized receptors and prevent undesirable aggregation. The construction of the macrocyclic framework of the four cyclophanes takes advantage of Pd0-catalyzed aryl—acetylene cross-coupling by the Sonogashira protocol, and oxidative acetylenic homo-coupling methodology (Schemes 2 and 8 – 10). Several cleft-type receptors featuring one 1,1′-binaphthalene-2,2′-diyl phosphate moiety were also prepared (Schemes 1, 6, and 7). An undesired side reaction encountered during the synthesis of the target compounds was the formation of naptho[b]furan rings from 3-ethynylnaphthalene-2-ol derivatives, proceeding via 5-endo-dig cyclization (Schemes 3 – 5). Computer-assisted molecular modeling indicated that the macrocycles prefer nonplanar puckered, cyclobutane-type conformations (Figs. 7 and 8). According to these calculations, receptor (−)-(R,R,R,R)-1 has, on average, a square binding site, which is complementary in size to one monosaccharide. The three other cyclophanes (−)-(R,R,R,R)-2 – 4 feature, on average, wider rectangular cavities, providing a good fit to one disaccharide, while being too large for the complexation of one monosaccharide. This substrate selectivity was fully confirmed in 1H-NMR binding titrations. The chiroptical properties of the cyclophanes and their nonmacrocyclic precursors were investigated by circular dichroism (CD) spectroscopy. The CD spectra of the acyclic precursors showed a large dependence from the number of 1,1′-binaphthalene moieties (Fig. 9), and those of the cyclophanes were remarkably influenced by the nature of the functional groups lining the macrocyclic cavity (Fig. 11). Profound differences were also observed between the CD spectra of linear and macrocyclic tetrakis(1,1′-binaphthalene) scaffolds, which feature very different molecular shapes (Fig. 10). In 1H-NMR binding titrations with mono- and disaccharides (Fig. 13), concentration ranges were chosen to favor 1 : 1 host−guest binding. This stoichiometry was experimentally established by the curve-fitting analysis of the titration data and by Job plots. The titration data demonstrate conclusively that the strength of carbohydrate recognition is enhanced with an increasing number of bidentate ionic host−guest H-bonds (Table 1) in the complex formed. As a result of the formation of these highly stable H-bonds, carbohydrate complexation in competitive protic solvent mixtures becomes more favorable. Thus, cleft-type receptors (−)-(R)-7 and (−)-(R)-38 with one phosphodiester moiety form weak 1 : 1 complexes only in CD3CN. In contrast, macrocycle (−)-(R,R,R,R)-1 with four phosphodiester groups undergoes stable inclusion complexation with monosaccharides in CD3CN containing 2% CD3OD. With their larger number of H-bonding sites, disaccharide substrates bind even more strongly to the four phosphodiester groups lining the cavity of (−)-(R,R,R,R)-2 and complexation becomes efficient in CD3CN containing 12% CD3OD. Finally, the introduction of two additional methyl ester residues further enhances the receptor capacity of (−)-(R,R,R,R)-3, and efficient disaccharide complexation occurs already in CD3CN containing 20% CD3OD.

Photo-Assisted ImmobilizedFenton Degradation up to pH 8 of Azo Dye Orange II Mediated by Fe3+/Nafion/Glass Fibers

Abstract: Fe3+ Ions have been immobilized into very thin Nafion films cast onto a glass-fiber mat immersed in an alcoholic solution of Nafion oligomers. This immobilized Fenton catalyst was used to abate/mineralize the azo dye Orange II, taken as a model organic compound. The abatement of Orange II on the Fe3+/Nafion/glass fibers was observed to proceed within the same time period as when Nafion alone was used to immobilize the Fe3+ ions during the photo-Fenton reaction. The amount of Nafion in the Nafion Fe3+/Nafion/glass fibers was ca. 15 times less per unit surface area compared to Fe3+-exchanged on conventional Nafion membranes used to immobilize Fe3+ ions. Orange II solutions under visible-light irradiation in the presence of H2O2 were mineralized up to pH 8 with a kinetics comparable to that found during the degradation runs at pH 3. Repetitive mineralization cycles mediated by the Fe3+/Nafion/glass fibers under visible light did not show any decrease in the activity of the immobilized catalysts. A reaction mechanism consistent with the experimental data is suggested. The morphology of the Fe3+/Nafion/glass fibers was characterized by scanning electron microscopy (SEM) showing thin Nafion films cast deposited on the glass fibers. Transmission-electron-microscopy (TEM) micrographs reveal Fe3+-oxy-hydroxide particles of 3 – 6 nm before and after repetitive Orange II photodegradation. X-Ray photoelectron spectroscopy (XPS) provided the evidence for the existence of Fe clusters on the topmost layer of the catalyst mainly as FeIII. The improvements brought by the glass fibers are a) the use of low quantities of expensive Nafion supported on glass mats to achieve dye degradation rates comparable to Nafion alone and b) Fenton-mediated degradation of azo dyes at pH 8 without the costly initial acidification usually needed for this type of treatment.

Development of the First P‐Stereogenic PCP Pincer Ligands, Their Metallation by Palladium and Platinum, and Preliminary Catalysis

Abstract: )BijvoetCenterforBiomolecularResearch,CrystalandStructuralChemistry,UniversiteitUtrecht,Padualaan8,3584CH,Utrecht,TheNetherlandsInmemoriamProfessorLuigiM.Venanziforhispioneeringworkinorganometallicchemistry,andinPCP-pincerchemistryinparticularThepotentiallytridentateP-stereogenic[P*CP*]ligands1,3-{bis[(tert-butyl)(phenyl)phosphino]methyl}-benzene and 1,3-{bis[(tert-butyl)(phenyl) phosphino]methyl}-2-bromobenzene have been synthesized as theprotectedphosphine-boraneadducts.Deprotectionwithasecondaryamineaffordsthefreephosphineligandwhich can be metallated by Pd and Pt with standard metal synthons. Two of the resultant [P*CP*] metalcomplexes have been characterized by X-ray crystallography. The complexes exhibit a C

Quercetin (=2-(3,4-Dihydroxyphenyl)-3,5,7-trihydroxy-4H-1-benzopyran-4-one) Glycosides and Sulfates: Chemical Synthesis, Complexation, and Antioxidant Properties

Abstract: Glycosides, acylated glycosides, and sulfates of quercetin (=3,3′,4′,5,7-pentahydroxyflavone; 1), which is, together with its derivatives, among the most common polyphenols found in plants and in the human diet, were prepared and quantitatively investigated for their ability to bind metal ions (AlIII, FeII, FeIII), enhance and vary natural colours (anthocyanin copigmentation), and trap potentially damaging radicals (antioxidant activity).

Photoswitchable Tetraethynylethene‐Dihydroazulene Chromophores

Abstract: The synthesis, characterization, and photophysical as well as electrochemical properties of the photochromic hybrid systems 11 – 16 and 18, which contain photoswitchable tetraethynylethene (TEE; 3,4-diethynylhex-3-ene-1,5-diyne) and dihydroazulene (DHA) moieties, are presented. The molecular photoswitches were synthesized by a Sonogashira cross-coupling reaction between an appropriate TEE precursor (6 – 10 and 17) and an iodinated DHA 1 or its vinylheptafulvene (VHF) isomer (4) (Schemes 5 – 7). X-Ray crystal structures of five DHA derivatives (1, trans-11a, cis-11a, 12, and 13) are discussed (Figs. 2 – 5). In all compounds, the cyclohexatriene moiety of the DHA chromophore adopts a clear boat conformation (Table 1). Presumably due to crystal-packing effects, the arylated TEE moieties in the hybrid systems show substantial distortions from planarity, with the dihedral angles between the planes of the central TEE core and the adjacent aryl substituents amounting to 44°. The switching properties were investigated by electronic absorption spectroscopy. Upon light absorption, DHAs 1, 12 – 16, and 18 underwent retro-electrocyclization in solution to give the corresponding VHFs (Figs. 6, 11, and 12). The reaction is thermally reversible, with half-lives τ1/2 between 3.9 and 5.8 h at 25° in CH2Cl2 (Figs. 7 and 13 and Table 3). A comparatively slower (E)(Z) isomerization process about the central C=C bond of the TEE moiety was also observed. The N,N-dimethylanilino-(DMA) substituted TEE-DHA hybrid systems trans-11a and cis-11a did not react to the corresponding VHFs upon irradiation (Scheme 9). Instead, only the reversible (E)(Z) photoisomerization of the TEE core occurred (Fig. 16 and Table 4). This process was further investigated for photofatigue by electronic-emission spectroscopy (Fig. 17). After protonation of the DMA group, the usual DHAVHF photoreaction took place. Compound 11 represents a three-way chromophoric molecular switch with three addressable sub-units (TEE core, DHA/VHF moiety, and proton sensitive DMA group) that can undergo individual, reversible switching cycles (Scheme 9). A process modeling the function of an `AND' logic gate (Fig. 19) and three write/erase processes could be performed with this system. Cyclic and linear sweep-voltammetry studies in CH2Cl2 (+Bu4NPF6) revealed the occurrence of characteristic first-reduction steps in the TEE-DHA hybrid systems between −1.6 and −1.8 V vs. Fc/Fc+ (ferrocene/ferricinium couple) (Table 5). Oxidations occur at ca. +1.10 V. After photoisomerization to the VHF derivatives, reduction steps at more positive and oxidation steps at more negative potentials were recorded. No DHAVHF isomerization took place upon electrochemical oxidation or reduction (Fig. 20).

Natural and Synthetic Xanthones as Monoamine Oxidase Inhibitors: Biological Assay and 3D QSAR.

Abstract: Fifty-nine xanthones (= 9H-xanthen-9-ones) of natural or synthetic origin were investigated for their inhibitory activity toward monoamine-oxidase A (MAO-A) and MAO-B. The compounds demonstrated reversible. time-independent activities, with selectivity toward MAO-A. The most active inhibitor had an IC50 of 40 nM. Electron absorption spectroscopy revealed the formation of a 1:1 charge-transfer complex between lumiflavine and xanthones. 3D-QSAR Studies according to the CoMFA/GOLPE procedure provided information on the relationship between steric and electrostatic fields and MAO-A inhibition. The ALMOND procedure yielded additional topographical information on structural factors favoring activity

Self-Assembly of Tricuprous Double Helicates: Thermodynamics, Kinetics, and Mechanism

Abstract: We report in this paper the coordination and kinetic properties of two oligobipyridine strands, which contain three 2,2′-bipyridine subunits separated by oxydimethylene bridges, the 4,4′-bis(CONET2)-substituted L and the 4,4′-bis(CO2Et)-substituted L′. Spectrophotometric measurements allowed the characterization of thermodynamic complexes and kinetic intermediates* which are involved in the self-assembly process of L2Cu3 and L2Cu3 helicates. The reaction presents positive cooperativity for the binding of two 2,2′-bipyridine strands to the cuprous cations. While reactive kinetic intermediates* present distorted coordination geometries around Cu1, the final rearrangement of the tricuprous bistranded helicates allows more closely tetrahedral coordination of each cation and reduces the interactions. Differences in the bulkiness and electronic properties of the L and L′ substituents do not affect significantly the stability of the corresponding helicates, but greatly influence binding rates in the self-assembly process.

Photochemistry of 1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-7- (piperazin-1-yl)quinoline-3-carboxylic acid (=ciprofloxacin) in aqueous solutions

Abstract: The 1-cyclopropyl-6-fluoro-1,4-dihydro-4-oxo-7-(piperazin-1-yl)quinoline-3-carboxylic acid (=ciprofloxacin; 1) undergoes low-efficiency (Φ=0.07) substitution of the 6-fluoro by an OH group on irradiation in H2O via the ππ* triplet (detected by flash photolysis, λmax 610 nm, τ 1.5 μs). Decarboxylation is a minor process (≤5%). The addition of sodium sulfite or phosphate changes the course of the reaction under neutral conditions. Reductive defluorination is the main process in the first case, while defluorination is accompanied by degradation of the piperazine moiety in the presence of phosphate. In both cases, the initial step is electron-transfer quenching of the triplet (kq=2.3⋅108M−1 s−1 and 2.2⋅107M−1 s−1, respectively). Oxoquinoline derivative 1 is much more photostable under acidic conditions, and in this case the F-atom is conserved, and the piperazine group is stepwise degraded (Φ=0.001).

Photoinduced Electron Transfer in Tetrathiafulvalene−Porphyrin−Fullerene Molecular Triads

Abstract: The two molecular triads 1a and 1b consisting of a porphyrin (P) covalently linked to a fullerene (C60) electron acceptor and tetrathiafulvalene (TTF) electron-donor moiety were synthesized, and their photochemical properties were determined by transient absorption and emission techniques. Excitation of the free-base-porphyrin moiety of the TTF−P2 H−C60 triad 1a in tetrahydro-2-methylfuran solution yields the porphyrin first excited singlet state TTF−1P2 H−C60, which undergoes photoinduced electron transfer with a time constant of 25 ps to give TTF−P2 H.+−C60.−. This intermediate charge-separated state has a lifetime of 230 ps, decaying mainly by a charge-shift reaction to yield a final state, TTF.+−P2 H−C60.−. The final state has a lifetime of 660 ns, is formed with an overall yield of 92%, and preserves ca. 1.0 eV of the 1.9 eV inherent in the porphyrin excited state. Similar behavior is observed for the zinc analog 1b. The TTF-PZn.+−C60.− state is formed by ultrafast electron transfer from the porphyrinatozinc excited singlet state with a time constant of 1.5 ps. The final TTF.+−PZn−C60.− state is generated with a yield of 16%, and also has a lifetime of 660 ns. Although charge recombination to yield a triplet has been observed in related donor-acceptor systems, the TTF.+−P−C60.− states recombine to the ground state, because the molecule lacks low-energy triplet states. This structural feature leads to a longer lifetime for the final charge-separated state, during which the stored energy could be harvested for solar-energy conversion or molecular optoelectronic applications.

Dansylated Polyamines as Fluorescent Sensors for Metal Ions: Photophysical Properties and Stability of Copper(II) Complexes in Solution

Abstract: Protonation and the CuII complexation constants of the dansylated polyamines N-dansylethylenediamine (1), N-dansyldiethylenetriamine (2), N-dansyltriethylenetetramine (3), N′-[2-(dansylamino)ethyl]diethylenetriamine (4), and tris(2-dansylaminoethyl)amine (5) were determined by glass-electrode potentiometry in MeOH/H2O 9 : 1 (v/v) solution. For ligands 3 and 4, the determinations were also performed in aqueous solution. The complexes formed by these ligands in neutral form correspond to those observed for the analogous unsubstituted monoprotonated amines, whereas, when the ligands are deprotonated at the sulfonamide moiety, the species parallel those of the corresponding amines. The molecular structures of the complexes were deduced from the VIS absorption spectra. The crystal structure of the [CuL2H−2] complex 6 of ligand 1 (L) was determined by X-ray diffraction. The study of the photophysical properties of the ligands 3 – 5 showed that they are good fluorescent sensors for copper(II), which induced fluorescence quenching. Time-resolved fluorescence measurements allowed us to clarify the sensing mechanism. The pH dependence of the quenching effect demonstrated that it occurs for all Cu2+ complexes, even for species in which the sulfonamide moiety is not deprotonated. Sensing of Cu2+ was compared with that of other metal ions (Co2+, Ni2+, Zn2+, Cd2+, Hg2+), and selectivity was studied as a function of pH. Ligands 3 and 4 were found to be selective chemosensors for Cu2+ in weakly acidic solution (pH ca. 4 – 5).

Carbenoid Pathways in Copper‐Catalyzed Intramolecular Cyclopropanations of Phenyliodonium Ylides

Abstract: The enantioselectivity of the copper-catalyzed intramolecular cyclopropanation of allyl diazomalonates and the corresponding phenyliodonium ylides was investigated with a series of chiral, non-racemic ligands. The reaction of 6b in the presence of the bis[dihydrooxazole] ligand Xa in refluxing 1,2-dichloroethane proceeded to 8b with an enantiomer excess (ee) of up to 72% under optimized conditions. In contrast, 8b resulting from reaction of ylide 7b with the same ligand, but in CH2Cl2 at 0°, had an ee of only 30%. With other ligands, diazomalonate 6b reacted with a lower enantioselectivity than ylide 7b, however. The intramolecular cyclopropanation of the acetoacetate-derived phenyliodonium ylide 15b afforded 16b with 68% ee with ligand Xa, but the corresponding diazo compound was unreactive when exposed to chiral copper catalysts. The observation of asymmetric induction in the Cu-catalyzed reactions of the ylides 7 and 15 is consistent with a carbenoid mechanism; however, the discrepancy of the enantioselectivities observed between diazomalonate 6b and ylide 7b suggests a competing unselective pathway for cyclopropanation outside of the coordination sphere of copper.

Synthesis of cyclo-β-tripeptides and their biological in vitro evaluation as antiproliferatives against the growth of human cancer cell lines

Abstract: A number of cyclo-β-tripeptides and their linear precursors were subjected to primary biol. evaluation for cancer-cell growth inhibition (one-dose, three-cell essay), and the five most active ones were then tested in the anti-tumor screen of the National Cancer Institute (Bethesda, USA) with 60 human cancer cell lines. Growth inhibition values GI50 in the one-digit micromolar, and in one case in the nanomolar range were obtained. The effects show selectivities for certain types of cancer cells and for certain cell lines within these types; the screen includes leukemia, non-small-cell lung, colon, and central-nervous-system (CNS) cancer, melanoma, ovarian, renal, prostate, and breast cancer cell lines. The synthesis and full characterization of two new cyclo-β-peptides,(β3-HSer(OBn))3 (I; R = β-CH2OCH2Ph) and (β3-HMet)3 (I; R = β-CH2CH2SMe) are described. Other cyclo-β-peptides included in this investigation are (β-Asp(Bn))3 (I; R = α-C(O)OCH2Ph), (β-HGlu(Bn))3 (I; R = β-(CH2)2C(O)OCH2Ph), and (β-HAla)3 (R = β-CH3), compounds which had been previously prepared by us. Strongest activities were measured with the cyclo-β-peptides bearing benzyl-ester or benzyl-ether groups in the side chains. The cytotoxic activity of the compds. included in this investigation is much lower (LC50>100 μM) than their antiproliferative activity (GI50).

Excited-State Properties of Camphorquinone Based Monomeric and Polymeric Photoinitiators

Abstract: The excited-state properties of a new polymeric photoinitiator-bearing camphorquinone or/and amine moieties were studied and compared to the behaviour of the precursor molecules. The triplet state of the polymeric system was extremely short-lived, due to the close vicinity of the amino group. In addition, the singlet state also reacted with the amino group. A study on camphorquinone and methyldiethanolamine (=2,2′-(methylimino)bis[ethanol]) revealed that both these pathways led to the formation of a ketyl radical and an amine-derived radical. Therefore, high efficiency of the radical generation was expected. However, the radical photopolymerization of a polyfunctional mixture of acrylic monomers with various combinations of monomeric and polymeric photoinitiators gave evidence that the polymeric structure of the photoinitiating system may differently affect the overall cure rate of the formulation.

Regioselective 1,3‐Dipolar Cycloadditions of (1Z)‐1‐(Arylmethylidene)‐5,5‐dimethyl‐3‐oxopyrazolidin‐1‐ium‐2‐ide Azomethine Imines to Acetylenic Dipolarophiles

Abstract: The 5,5-dimethylpyrazolidin-3-one (4), prepared from ethyl 3-methylbut-2-enoate (3) and hydrazine hydrate, was treated with various substituted benzaldehydes 5a – i to give the corresponding (1Z)-1-(arylmethylidene)-5,5-dimethyl-3-oxopyrazolidin-1-ium-2-ide azomethine imines 6a – i. The 1,3-dipolar cycloaddition reactions of azomethine imines 6a – h with dimethyl acetylenedicarboxylate (=dimethyl but-2-ynedioate; 7) afforded the corresponding dimethyl pyrazolo[1,2-a]pyrazoledicarboxylates 8a – h, while by cycloaddition of 6 with methyl propiolate (=methyl prop-2-ynoate; 9), regioisomeric methyl pyrazolo[1,2-a]pyrazolemonocarboxylates 10 and 11 were obtained. The regioselectivity of cycloadditions of azomethine imines 6a – i with methyl propiolate (9) was influenced by the substituents on the aryl residue. Thus, azomethine imines 6a – e derived from benzaldehydes 5a – e with a single substituent or without a substituent at the ortho-positions in the aryl residue, led to mixtures of regioisomers 10a – e and 11a – e. Azomethine imines 6f – i derived from 2,6-disubstituted benzaldehydes 5f – i gave single regioisomers 10f – i.

Single and Double Ionization of Corannulene and Coronene

Abstract: Electron-transfer processes that involve single and doubly charged cations of corannulene (C20H10) and coronene (C24H12) are examined by three different mass-spectrometric techniques. Photoionization studies give first-ionization energies of IE(C20H10)=7.83±0.02 eV and IE(C24H12)=7.21 ±0.02 eV. Photoionizations of the neutrals to the doubly charged cations occur at thresholds of 20.1±0.2 eV and 18.5±0.2 eV for corannulene and coronene, respectively. Energy-resolved charge-stripping mass spectrometry yields kinetic energy deficits of Qmin(C20H=13.8±0.3 eV and Qmin(C24H=12.8±0.3 eV for the transitions from the mono- to the corresponding dications in keV collisions. Reactivity studies of the C20H and C24H dications in a selected-ion flow-tube mass spectrometer are used to determine the onsets for the occurrence of single-electron transfer from several neutral reagents to the dications, affording two different monocationic products. With decreasing IEs of the neutral reagents, electron transfer to doubly charged corannulene is first observed with hexafluorobenzene (IE=9.91 eV), while neutrals with lower IEs are required in the case of the coronene dication, e.g., NO2 (IE=9.75 eV). Density-functional theory is used to support the interpretation of the experimental data. The best estimates of the ionization energies evaluated are IE(C20H10)=7.83±0.02 eV and IE(C24H12)=7.21 ±0.02 eV for the neutral molecules, and IE(C20H)=12.3±0.2 eV and IE(C24H)=11.3±0.2 eV for the monocations.