Showing papers in "Journal of The Chemical Society-perkin Transactions 1 in 1999"
TL;DR: In this article, the radiative rate constants for depopulation of the excited states of closely-related series of anionic, neutral and cationic europium, terbium and ytterbium complexes have been measured in H2O and D2O.
Abstract: The radiative rate constants for depopulation of the excited states of closely-related series of anionic, neutral and cationic europium, terbium and ytterbium complexes have been measured in H2O and D2O. With the aid of selective ligand deuteriation, the relative contributions of OH, NH (both amide and amine) and CH oscillators have been measured and critically assessed. Quenching of the Eu 5D0 excited state by amine NH oscillators is more than twice as efficient as OH quenching. The importance of the distance between the excited Ln ion and the XH oscillator is described with recourse to published crystallographic information. The general equation, q = A′(ΔkH2O–kD2O)corr is presented and revised values of A′ for Eu (1.2 ms), Tb (5 ms) and Yb (1 µs) given, which allow for the quenching contribution of closely diffusing OH oscillators. The relevance of such studies to the hydration state of certain gadolinium complexes is described and clear evidence provided for a break in hydration at gadolinium.
1,233 citations
224 citations
TL;DR: In this article, the authors present a review of the literature in the area of the Perkin Transformer Transformer (PTT) and its application in the field of bioinformatics.
Abstract: Covering: July 1998 to the end of July 1999. Previous review: J. Chem. Soc., Perkin Trans. 1, 1999, 641.
191 citations
TL;DR: In this paper, the antioxidant properties of quercetin and its derivatives were investigated in both protic and non-proto-probic solvents, and the effect of these observations on the antioxidant performance was discussed.
Abstract: Quercetin (3,3′,4′,5,7-pentahydroxyflavone) and quercetin derivatives (3-methylquercetin, rutin) are strong flavonoid antioxidants abundant in plants and in human diet. Their oxidation by DPPH, CAN or dioxygen (autoxidation) is studied in protic and non protic solvents. From kinetic investigations by UV–visible spectroscopy, oxidation rate constants are estimated. Fast disproportionation of flavonoid radicals is shown to give quinones which can be identified by their adducts with methanol (quercetin quinone) or sodium benzenesulfinate (rutin quinone). In strongly alkaline non aqueous conditions, the quercetin quinone can also be evidenced by strong charge transfer absorption bands in the range 700–800 nm.The consequences of these observations for the antioxidant properties of quercetin and quercetin derivatives are discussed.
144 citations
141 citations
TL;DR: In this article, four parent carbolines and their corresponding 5- or 9-methylsulfonyl derivatives were synthesized by the palladium-catalyzed intramolecular arylation reaction of ortho-bromo-substituted anilinopyridines.
Abstract: Four parent carbolines and the corresponding 5- or 9-methylsulfonyl derivatives were synthesized by the palladium-catalyzed intramolecular arylation reaction of ortho-bromo-substituted anilinopyridines which were prepared by the palladium-catalyzed amination reaction of iodobenzenes with aminopyridines. Carbazole and its 9-methylsulfonyl derivative were also synthesized by the same method.
139 citations
TL;DR: A product study of the sonolysis of the volatile substrate t-butanol in aqueous solution indicates that substrate decomposition is practically completely determined, even at concentrations as low as millimolar, by oxidative pyrolysis going on in the gas phase within the collapsing cavitational bubble as mentioned in this paper.
Abstract: A product study of the sonolysis of the volatile substrate t-butanol in aqueous solution indicates that substrate decomposition is practically completely determined, even at concentrations as low as millimolar, by oxidative pyrolysis going on in the gas phase within the collapsing cavitational bubble. OH-Radical-induced reactions in solution are insignificant since the volatility of this substrate, its gas-phase concentration within the bubble enhanced by a certain degree of hydrophobicity, causes OH radicals generated thermolytically from water vapour to be intercepted before they can reach the aqueous phase. The nature of the products, as well as the t-butanol-concentration dependence of the product yields, can be qualitatively explained on the basis of the t-butanol-pyrolysis mechanism. Kinetic considerations involving the relative yields of the pyrolysis products ethane, ethylene and acetylene lead to an estimate of a value of 3600 K for the average pyrolysis temperature at a t-butanol bulk concentration of 10–3 molar.
137 citations
TL;DR: In this paper, the photophysical properties of the tumour photosensitisers m-THPP, mTHPC, and mTHPBC in methanol have been measured: absorption and fluorescence spectra, ES, Stokes shift, Φf, τ, τT, dioxygen quenching rates, and ΦΔ for air-saturated and oxygen saturated solutions.
Abstract: For the tumour photosensitisers m-THPP, m-THPC, and m-THPBC in methanol the following photophysical properties have been measured: absorption and fluorescence spectra, ES, Stokes shift, Φf, ΦT, τT, dioxygen quenching rates, and ΦΔ for air-saturated and oxygen-saturated solutions. The properties of the first excited triplet state are quite uniform across this series. The quantum yields of singlet oxygen formation are high, being 0.43–0.46 in air-saturated methanol, and 0.59–0.62 in oxygen-saturated methanol. For a constant substitution pattern and a standard given light dose, tumour photonecrosis in vivo appears to parallel the product of drug dose and the molar extinction at the irradiation wavelength in the red region of the visible spectrum.
133 citations
TL;DR: In this article, the performance of a variety of theoretical methods in computing stabilization energies of the substituted methyl and vinyl radicals is examined, and the results indicate that generally reliable radical stabilization energies can be obtained at modest cost using RMP2/6-311+G(2df,p)//RMP2 /6-31G(d) energies.
Abstract: The performance of a variety of theoretical methods in computing stabilization energies of the substituted methyl and vinyl radicals ˙CH2F, ˙CH2CN, ˙CH2CHCH2, ˙CH2CHO, CH2C˙F and CH2C˙CN is examined. The influence of electron correlation (UHF, UMP2, PMP2, RMP2, UB3-LYP, UQCISD, UQCISD(T), UCCSD(T), URCCSD(T) and RRCCSD(T)) and basis set size (from 6-31G(d) to 6-311++G(3df,3pd)) on stabilization energies is evaluated, as well as the performance of compound methods such as G2, G3, CBS-Q and CBS-APNO and their variants. The results indicate that generally reliable radical stabilization energies can be obtained at modest cost using RMP2/6-311+G(2df,p)//RMP2/6-31G(d) energies. A slightly less accurate but more economical procedure is RMP2/6-311+G(d)//B3-LYP/6-31G(d). UMP2 and PMP2 are unsuitable for obtaining radical stabilization energies for spin-contaminated radicals, while UB3-LYP appears generally to overestimate stabilization energies.
126 citations
TL;DR: In this paper, the Gibbs energies, enthalpies and entropies of complexation have been determined both in acidic medium (pH 1) and in slightly basic medium(pH 8).
Abstract: The complexes formed between three p-sulfonatocalix[n]arenes (n = 4, 6 and 8) and the amino acids lysine and arginine in water have been studied by microcalorimetry, at 298.15 K. For each system, both the apparent association constant and enthalpy of reaction have been extracted from the calorimetric data. The Gibbs energies, enthalpies and entropies of complexation have been determined both in acidic medium (pH 1) and in slightly basic medium (pH 8). The thermodynamic parameters for the complexation of arginine markedly differ from those for the complexation of lysine. The three hosts show very different thermodynamic behaviours. Our results are consistent with the formation of 1∶1 complexes with the calix[4]arenesulfonate and the calix[6]arenesulfonate and with the formation of 1∶1 and 1∶2 complexes with the calix[8]arenesulfonate. Whereas the calix[4]arenesulfonate forms relatively strong complexes, the calix[6]arenesulfonate and the calix[8]arenesulfonate form only weak complexes. In all cases, the complexation is driven by a favourable enthalpy change. The enthalpies and entropies of complexing of arginine by the calix[6]arenesulfonate are remarkably negative. The enthalpies and entropies of complexation of the two amino acids by the cyclic tetramer and by the cyclic hexamer become more negative when the pH is changed from 8 to 1; the same effect is observed upon binding of the cyclic octamer with the first guest whereas the opposite effect is observed upon addition of the second guest.
125 citations
TL;DR: In this article, the solid-state supramolecular complexation of fullerenes C60 and C70 as well as some derivatives of C60 was attained by the use of a mechanochemical high-speed vibration milling technique.
Abstract: The solid-state supramolecular complexation of fullerenes C60 and C70 as well as some derivatives of C60 was attained with γ-cyclodextrin by the use of a mechanochemical high-speed vibration milling technique. Similarly C60 and the fullerene dimer C120 were complexed with sulfocalix[8]arene. The complexes produced were found to be soluble in water, and their solubility was examined by UV-vis spectroscopy.
TL;DR: In this paper, the synthesis of three new derivatives of tert-butylcalix[4]arene bearing four carbamoylmethylphosphine oxide (CMPO) groups as ligating functions on the narrow rim is described.
Abstract: The synthesis of three new derivatives of tert-butylcalix[4]arene bearing four carbamoylmethylphosphine oxide (CMPO) groups as ligating functions on the narrow rim is described. These are easily prepared in 65–80% yield via acylation of the corresponding fixed cone aminoalkoxy (C2 to C4) derivatives with p-nitrophenyl (diphenylphosphoryl)acetate. Extraction studies of selected lanthanides (La3+, Eu3+, Yb3+) and thorium (Th4+) from 1 M HNO3 to CH2Cl2 show a strong increase of the percentage extracted (%E) with increasing length of the spacer. Extraction of Am3+, La3+, Nd3+, Sm3+ and Eu3+ from (0.1–4) M HNO3 into o-nitrophenyl hexyl ether shows, in contrast to the wide rim CMPO derivative, no decrease of the distribution coefficient for higher concentrations of nitric acid.
TL;DR: In this paper, the structure of the bisboronic acid-glucopyranose bisboronate complex was determined based on 1H and 13C NMR data as well as information obtained from 1JCC coupling constants.
Abstract: The structures of the complexes between a fluorescent bisboronic acid 7 and glucose have been determined. Shinkai et al.1 previously studied the complex between 7 and glucose and they deduced a 1,2∶4,6-α-D-glucopyranose bisboronate structure. We have shown that this structure is only valid as an initial complex formed under completely nonaqueous conditions. In the presence of water the pyranose complex rearranges rapidly into an α-D-glucofuranose-1,2∶3,5,6-bisboronate in which all five free hydroxy groups of glucose are covalently bound by the sensor molecule. A favourable B–N interaction around the 1,2-binding site and the effect of an o-ammoniomethyl group on the pKa value of the second boronic acid group allow for the observed binding at neutral pH. The structure evaluations are based on 1H and 13C NMR data as well as information obtained from 1JCC coupling constants. The fluorescence spectra of both complexes were measured and discussed. MALDI TOF-MS experiments showed competitive formation of 1∶2 (boronic acid∶glucose) complexes under conditions of physiological glucose levels.
TL;DR: The cyclization reaction of various 2-ethynylanilines, which were easily synthesized from 2-haloanilines by the palladium-catalyzed reaction with terminal alkynes, with tetrabutylammonium fluoride (TBAF) to yield 2-substituted indoles proceeded at refluxing or room temperature in THF in excellent yields without affecting the bromo, chloro, cyano, ethoxycarbonyl, and ethynyl groups as discussed by the authors.
Abstract: The cyclization reaction of various 2-ethynylanilines, which were easily synthesized from 2-haloanilines by the palladium-catalyzed reaction with terminal alkynes, with tetrabutylammonium fluoride (TBAF) to yield 2-substituted indoles proceeded at refluxing or room temperature in THF in excellent yields without affecting the bromo, chloro, cyano, ethoxycarbonyl, and ethynyl groups.
TL;DR: In this paper, the problem of the intramolecular proton transfer isomerism in arginine, leading to conventional neutral and zwitterionic forms of this compound, is addressed by high level theoretical models.
Abstract: The problem of the intramolecular proton transfer isomerism in arginine, leading to conventional neutral and zwitterionic forms of this compound, is addressed by high level theoretical models. It is shown that arginine has two neutral and two zwitterionic isomers implying that there exist two additional unconventional isomers, which have not been identified so far. It appears also that the most stable neutral isomer is energetically more favourable than both zwitterions, which implies that the former should be preferred in the gas phase. Examination of atomic charges obtained by the electron density partitioning techniques reveals that the charge distributions of neutral and zwitterionic isomers are not as widely different as expected. This finding is counterintuitive, since it contradicts the classical notion of chemical bonding and a customary picture of zwitterions involving two local complementary fragments possessing unit charges of opposite sign. The true distribution of the electron density is more uniform and quite similar to that of the neutral form. The proton affinity of arginine is estimated to be 249 kcal mol–1. Hence, it follows that arginine is a very basic compound although it belongs to a family of 20 fundamental α-amino acids. A very high proton affinity is interpreted in terms of the resonance effect spurred by protonation in the guanidine moiety and by a strong hydrogen bonding taking place in the protonated form.
TL;DR: In this article, an efficient combinatorial route to substituted 3-phenylbenzofurans was achieved by the bromination of acetophenones to α-bromoacetophenones by polymer-supported pyridinium bromide perbromide (PSPBP).
Abstract: An efficient combinatorial route to substituted 3-phenylbenzofurans, is achieved by the bromination of acetophenones to α-bromoacetophenones by polymer-supported pyridinium bromide perbromide (PSPBP). The subsequent clean substitution of the obtained bromides by phenols using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD-P) and cyclodehydration of the resulting α-phenoxyacetophenones using Amberlyst 15, affords pure products without the need for any chromatographic purification step.
TL;DR: A novel optical sensing methodology for nitrate ions has been developed using an ensemble of a synthetic receptor and colorimetric dyes as indicators and the association constants between receptor 1 and various anions were determined by analyzing the intensity changes.
Abstract: A novel optical sensing methodology for nitrate ions has been developed using an ensemble of a synthetic receptor and colorimetric dyes as indicators. Neutral receptor 1 forms one to one complexes with anionic dyes such as Resorufin or Methyl Red in organic solvents. The equilibrium between receptor and dye is perturbed by added nitrate ion, resulting in a receptor–nitrate complex. The displacement of the indicator by nitrate causes large absorbance changes, because of alternation of the microenvironment of the chromophore. The association constants between receptor 1 and various anions were determined by analyzing the intensity changes.
TL;DR: In this paper, a new catalytic enantioselective aziridination reaction of α-imino esters with diazo compounds catalyzed by chiral Lewis acid complexes is presented.
Abstract: A new catalytic enantioselective aziridination reaction of α-imino esters with diazo compounds catalyzed by chiral Lewis acid complexes is presented. A series of N-substituted α-imino esters has been tested as substrates for the aziridination reaction using trimethylsilyldiazomethane as the carbenoid fragment donor catalyzed by various chiral complexes. Both chiral BINAP and bisoxazoline† ligands, in combination with copper(I) salts in particular can catalyze the aziridination reaction leading to the cis-aziridine with up to 72% ee applying BINAP–copper(I) complexes as the catalyst, while the bisoxazoline–copper(I) catalysts give an aziridination reaction with lower diastereoselectivity, however, the trans-aziridine was formed in 69% ee. The influence of diazo compounds, Lewis acids, chiral ligands, solvents and counterion on the reaction course has been investigated and a mechanism for the reaction is discussed in which the α-imino ester coordinates to the chiral catalyst. For trimethylsilyldiazomethane, nucleophilic attack on the coordinated α-imino ester probably takes place in the case of copper(I) as the Lewis acid, while for ethyl diazoacetate the reaction course is dependent on the Lewis acid; for silver(I) a nucleophilic attack is probably also operating, while a metal–carbene intermediate is involved when copper(I) is used.
TL;DR: In this paper, the energies and geometries of a series of 2-substituted-1-fluoroethanes were computed at the MP2/6-311++G**(6D)//MP2/ 6-31+G* level of theory for both the maxima and minima of the rotation about the C-C bond.
Abstract: The energies and geometries of a series of 2-substituted-1-fluoroethanes were computed at the MP2/6-311++G**(6D)//MP2/6-31+G* level of theory for both the maxima and minima of the rotation about the C–C bond. The results did not support the predictions of a hyperconjugative model, that both 1,2-difluoroethane and 1-chloro-2-fluoroethane would strongly prefer a gauche conformation, and that 1-fluoro-2-silylethane would strongly prefer an anti conformation. The existence of competing electrostatic interactions between the fluorine and the substituents at C-2 was indicated by the detailed geometries of the gauche conformers and by the calculated sensitivity of the gauche–anti energy differences to the presence of a polar solvent. However, Fourier analyses of the torsional potential energies were wholly consistent with hyperconjugative electron donation into the C–F σ* orbital contributing to the conformational preferences of these 1-fluoroethanes. Fourier analyses also showed that hyperconjugation contributes to the small variations in C–C and C–F bond lengths and in fluorine atomic charges that were computed. The torsional potential energies, variations in geometry and atomic charge, and sensitivity to solvent were all in accord with the expected ranking of hyperconjugative electron donating ability of bonds to carbon, C–Si > C–H > C–C > C–Cl > C–F.
TL;DR: In this article, the synthesis of the alkaloids (±)-oxomaritidine 1 and (−)-epimaritidine 2 in high yield was described, which employs a sequence of five-and six-step reactions respectively, using solely polymer supported reagents in an orchestrated successive manner.
Abstract: The concise synthesis of the alkaloids (±)-oxomaritidine 1 and (±)-epimaritidine 2 in high yield are described, which employs a sequence of five- and six-step reactions respectively, using solely polymer supported reagents in an orchestrated successive manner.
TL;DR: A clean oxidation reaction of a variety of substrates using polymer-supported (diacetoxyiodo)benzene (PSDIB) which proceeds in high to excellent yield with maximum purity is described including isolation and regeneration of the polymer reagent.
Abstract: A clean oxidation reaction of a variety of substrates using polymer-supported (diacetoxyiodo)benzene (PSDIB) which proceeds in high to excellent yield with maximum purity is described including isolation and regeneration of the polymer reagent.
TL;DR: In this paper, the vertical ionization potentials of free-base and zinc porphyrins and free base and zinc phthalocyanine, including all those for which experimental ultraviolet photoelectron spectral (UPS) data are presently known, are computed using six semi-empirical molecular orbital methods.
Abstract: The vertical ionization potentials (IP's) of a variety of free-base and zinc porphyrins and free-base and zinc phthalocyanine, including all those for which experimental ultraviolet photoelectron spectral (UPS) data are presently known, are computed using six semiempirical molecular orbital methods. Koopman's Theorem (KT), second order outer valence Green's function methods with a large number of active orbitals (OVGF), and explicit computation of the relative energies of neutral species and vertically ionized radical cations (ΔSCF IP) are used in combination with both PM3 and AM1 parameterizations, and the results are compared to experimental data. On average, both the OVGF and ΔSCF IP approximations reproduce the first vertical IP's, as determined by UPS, far more accurately than KT at minimal extra computational costs. Over the full set of available experimental data, the average error for the lowest IP with both OVGF and ΔSCF IP is only ca. 40% of that of KT (AM1 data, AM1 being generally more accurate than PM3). Inclusion of higher order terms in the OVGF treatment (third order truncation or full expression of the self-energy part) does not affect the computed IP's significantly, but inclusion of a large number of active orbitals in the OVGF technique is shown to be essential for this class of molecules. In agreement with the experimental data, zinc porphyrins and zinc phthalocyanines are computed to be better electron donors than their free-base analogues. Conformational differences of the peripheral substituents have no significant effects on the valence IP's.
TL;DR: In this paper, the binding of quercetin to Bovine serum albumin (BSA) was quantitatively investigated by fluorescence spectroscopy, and it was shown that a significant fraction of queretin adopts a pyrylium-like structure in the complex.
Abstract: Quercetin (3,3′,4′,5,7-pentahydroxyflavone) and quercetin derivatives (3-methylquercetin, isoquercitrin, rutin) are strong polyphenolic antioxidants abundant in plants and in the human diet. Recent investigations have shown that significant concentrations of albumin-bound quercetin conjugates are present in the plasma of humans fed a quercetin-rich diet.In this work, binding of quercetin and quercetin glycosides to bovine serum albumin (BSA) is quantitatively investigated by fluorescence spectroscopy. The strong fluorescence enhancement of quercetin upon binding points to the fact that a significant fraction of quercetin adopts a pyrylium-like structure in the complex. On the other hand, the observation of a very efficient quenching of tryptophan fluorescence by quercetin is consistent with a binding occurring in the IIA domain.Flavonoid-derived quinones may be formed upon quenching of reactive oxygen species by flavonoids (antioxidant activity). In this work, the quinones are conveniently formed upon periodate oxidation of the selected flavonoids in methanol and in aqueous buffers with and without BSA. A kinetic investigation by UV–visible spectroscopy shows that albumin-bound flavonoids are oxidized as quickly as free flavonoids. Interestingly, the quercetin quinone, which is merely detectable in the absence of BSA because of fast solvent addition, is efficiently stabilized in the complex by charge transfer interactions (pH 9). No evidence for quercetin–BSA conjugates could be found, thus showing that water addition (and subsequent degradation) remains the sole significant pathway of quinone transformation in the complex.
TL;DR: Evidence for the use of these receptors as ATP luminescent chemosensors is advanced and unambiguous criteria for defining thermodynamic selectivity based on theUse of effective stability constants are presented.
Abstract: The interaction of the open-chain polyamine N-(3-aminopropyl)-N′-[3-(anthrylmethyl)aminopropyl]ethane-1,2-diamine (L) with the relevant anionic forms of adenosine 5′-triphosphate (ATP), adenosine 5′-diphosphate (ADP) and adenosine 5′-monophosphate (AMP) is described. Unambiguous criteria for defining thermodynamic selectivity based on the use of effective stability constants are presented. The interaction of L and several other topologically similar polyammonium receptors with ATP has been shown to occur through electrostatic and π-stacking intermolecular forces. The π-stacking binding mode is modulated by the protonation degree of ATP as indicated by fluorescence emission titrations. Evidence for the use of these receptors as ATP luminescent chemosensors is advanced.
TL;DR: The steady state concentration of semiquinones determined by EPR in the mixtures of eleven alkyl-, methoxy- and chloro-substituted 1,4-benzoquinones as well as 1, 4-naphthoquinone (Q) with corresponding hydroquinones (QH2) in aqueous buffer, pH 7.40, was used to calculate a constant for equilibrium as discussed by the authors.
Abstract: The steady-state concentration of semiquinones (Q˙–) determined by EPR in the mixtures of eleven alkyl-, methoxy- and chloro-substituted 1,4-benzoquinones as well as 1,4-naphthoquinone (Q) with corresponding hydroquinones (QH2) in aqueous buffer, pH 7.40, was used to calculate a constant for equilibrium (1) Q + QH2⇌Q˙– + Q˙– + 2H+ (k1; 2k–1; K1 = k1/2k–1). The rate constants for comproportionation between Q and QH2, k1, were calculated from the combination of K1 determined in this work and 2k–1 reported previously. The Nernst equation was applied to calculate the change in one-electron reduction potential ΔE1 = E(Q/Q˙–) – E(Q˙–/QH2) in equilibrium (1). The E(Q˙–/QH2) values were calculated from ΔE1 and the values of E(Q/Q˙–) known from the literature. The correlations between E(Q˙–/QH2) and E(Q/Q˙–) as well as between ΔE1 (k1) and E(Q/Q˙–) are discussed. The values of ΔE1 and k1 are suggested to be the key factors governing the autoxidation of QH2.
TL;DR: In this article, the thermolysis of 2-methoxyphenol has been studied between 680 and 790 K, applying cumene as a radical scavenger, and two pathways were observed: a homolytic route involving the cleavage of the methoxyl O-C bond, leading to methane and 1,2-dihydroxybenzene, obeying kuni (s−1)Õ(−1015.2−0.2 exp(−239−±8 (kJ mol−1)/RT), and an
Abstract: The thermolysis of 2-methoxyphenol has been studied between 680 and 790 K, applying cumene as a radical scavenger. Two pathways were observed: a homolytic route involving the cleavage of the methoxyl O–C bond, leading to methane and 1,2-dihydroxybenzene, obeying kuni (s–1) = 1015.2 ± 0.2 exp(–239 ± 8 (kJ mol–1)/RT), and an induced route starting with abstraction of the phenolic hydrogen by cumyl radicals. After intramolecular hydrogen transfer a cascade of reactions yields phenol, 2-hydroxybenzaldehyde, and 2-hydroxybenzyl alcohol. The latter compound decomposes instantaneously into o-quinone methide (o-QM) which is reduced to o-cresol.
TL;DR: In this paper, the relative reactivities of enzyme-like catalysts for regioselective functionalization of unprotected sugars were investigated in chloroform and showed the decisive role of intramolecular hydrogen bonding networks among carbohydrate OH groups.
Abstract: In our efforts toward the development of enzyme-like catalysts for regioselective functionalization of unprotected sugars, DMAP-catalyzed acetylation of unprotected carbohydrates in chloroform was investigated. Product distributions of monoacetylated sugars were determined under kinetic control. To accurately evaluate the relative reactivity of each OH group, reaction conditions were used under which only monoacetylated sugars were obtained and almost no diacetylated sugars were formed as by-products. Systematic acetylation experiments of glucose, mannose, and galactose revealed the decisive role of intramolecular hydrogen-bonding networks among carbohydrate OH groups. The relative reactivities in the DMAP-catalyzed acetylation were successfully correlated with the calculated proton affinity of each OH group in carbohydrates.
TL;DR: A ten-step synthesis of (±)-epibatidine 1 is described, using an organised array of polymer supported reagents and sequestering agents in a successive manner to afford the product in >90% purity.
Abstract: A ten-step synthesis of (±)-epibatidine 1 is described, using an organised array of polymer supported reagents and sequestering agents in a successive manner. No chromatographic purification steps are required to afford the product in >90% purity.
TL;DR: In this article, the quadratic hyperpolarisability measured by hyper-Rayleigh scattering was used to estimate excited state couplings between mono-, bis-, and tris(tricyanovinyl)-substituted triphenylamines (1, 2, 2-OMe and 3).
Abstract: The electronic coupling between the phenyl substituents of mono-, bis-, and tris(tricyanovinyl)-substituted triphenylamines (1, 2, 2-OMe and 3) have been investigated by various methods: the splitting of excited states was taken as a measure of the interaction. These splittings can be seen in the optical absorption spectra; exciton coupling theory was also used to estimate electronic coupling of the excited states; redox potentials gave coupling energies of charged ground state species as well as Hush analysis of intervalence charge-transfer bands (IV-CT), which were observed by UV-Vis-NIR spectroelectrochemistry, and finally, the quadratic hyperpolarisability measured by hyper-Rayleigh scattering was used to estimate excited state couplings. The results of these methods were compared to semiempirical AM1 calculations and suggest the subchromophore interactions to be weak compared to the band width of the UV-spectra. Nevertheless, the quadratic hyperpolarisability is enhanced in the two-dimensional tris(tricyanovinylphenyl)amine compared to its one-dimensional analogue.