Atmospheric solids analysis probe mass spectrometry: An easy bolt-on for the synthetic undergraduate teaching laboratory.

RATIONALE: High costs and student numbers can often hinder implementation of mass spectrometry (MS) in the undergraduate teaching laboratory, often with technicians running samples on students' behalf, and the implementation of MS only in discrete or isolated experiments. This study explores the use of atmospheric solids analysis probe MS (ASAP-MS) as a relatively low-cost, benchtop instrument, and its potential for application as a 'bolt-on' to existing undergraduate organic chemistry experiments. METHODS: Thirteen products synthesised in undergraduate laboratory experiments were analysed by ASAP-MS, along with their starting materials. Analysis was carried out with a Waters RADIAN ASAP mass spectrometer, at four different cone voltages simultaneously to provide fragmentation information. RESULTS: Out of the 13 undergraduate experiments, ASAP-MS was shown to be complementary in 11 of these, either through simple analysis of the precursor ion or by a more complex analysis of the fragments. CONCLUSIONS: ASAP-MS provided spectra that both complement and enhance intended learning outcomes in existing organic chemistry experiments, showing its versatility as a bolt-on technique. Moving forward, ASAP-MS will be integrated into the University of Surrey's undergraduate teaching laboratory.

The Essence in Selectivity of Copper-Mediated Intermolecular Nucleophilic Substitution of a meta C-H Bond in 2-Methyl-N-methoxyaniline: A Theoretical Study.

The detailed mechanism for NHC-Cu(I)-catalyzed intermolecular nucleophilic substitution of the C-H bonds at aniline (2-methyl-N-methoxyaniline) was studied via DFT methods to reveal the essence of the selectivity. Calculations revealed that the meta C-H functionalization proceeds via two nucleophilic attacks on the aromatic ring rather than a one-step meta C-H substitution to give the experimentally observed major product. The reaction is initiated by activation of the substrate via oxidative addition with an NHC-Cu(I) catalyst, through which an umpolung occurs at the ring. From the activated intermediate, methoxyl group transfer to benzyl forms a resting state, while a nucleophile can attack the ortho position of benzyl to form a more stable intermediate. The nucleophile group can then transfer to the meta position by a 1,2-Wagner-Meerwein rearrangement to form the final product through a proton shuttle. In contrast, other transfer processes affording ortho- or para-substituted products encounter higher activation barriers. This work investigates the relationship of product selectivity with the umpolung of the aromatic ring, as well as the priority of a nucleophilic attack at the ortho position of the aromatic, 1,2-Wagner-Meerwein rearrangement from the ortho-substituted intermediate, and proton shuttle from the meta-substituted intermediate.

Metal-ligand Lability and Ligand Mobility Enables Framework Transformation via Ligand Release in a Family of Crystalline 2D Coordination Polymers.

A family of seven silver(I)-perfluorocarboxylate-quinoxaline coordination polymers, [Ag4 (O2 CRF )4 (quin)4 ] 1-5 (RF =(CF2 )n-1 CF3 )4 , n=1 to 5); [Ag4 (O2 C(CF2 )2 CO2 )2 (quin)4 ] 6; [Ag4 (O2 CC6 F5 )4 (quin)4 ] 7 (quin=quinoxaline), denoted by composition as 4 : 4 : 4 phases, was synthesised from reaction of the corresponding silver(I) perfluorocarboxylate with excess quinoxaline. Compounds 1-7 adopt a common 2D layered structure in which 1D silver-perfluorcarboxylate chains are crosslinked by ditopic quinoxaline ligands. Solid-state reaction upon heating, involving loss of one equivalent of quinoxaline, yielding new crystalline 4 : 4 : 3 phases [Ag4 (O2 C(CF2 )n-1 CF3 )4 (quin)3 ]n (8-10, n=1 to 3), was followed in situ by PXRD and TGA studies. Crystal structures were confirmed by direct syntheses and structure determination. The solid-state reaction converting 4 : 4 : 4 to 4 : 4 : 3 phase materials involves cleavage and formation of Ag-N and Ag-O bonds to enable the structural rearrangement. One of the 4 : 4 : 3 phase coordination polymers (10) shows the remarkably high dielectric constant in the low electric field frequency range.

Arene Selectivity by a Flexible Coordination Polymer Host.

The coordination polymers [Ag4 (O2 CCF3 )4 (phen)3 ]⋅ phen⋅arene (1⋅phen⋅arene) (phen=phenazine; arene=toluene, p-xylene or benzene) have been synthesised from the solution phase in a series of arene solvents and crystallographically characterised. By contrast, analogous syntheses from o-xylene and m-xylene as the solvent yield the solvent-free coordination polymer [Ag4 (O2 CCF3 )4 (phen)2 ] (2). Toluene, p-xylene and benzene have been successfully used in mixed-arene syntheses to template the formation of coordination polymers 1⋅phen⋅arene, which incorporate o- or m-xylene. The selectivity of 1⋅phen⋅arene for the arene guests was determined, through pairwise competition experiments, to be p-xylene>toluene≈benzene>o-xylene>m-xylene. The largest selectivity coefficient was determined as 14.2 for p-xylene:m-xylene and the smallest was 1.0 for toluene:benzene.

Synthesis and receptor binding in trans-CD ring-fused A-CD estrogens: comparison with the cis-fused isomers.

Ligands which selectively activate only one of the estrogen receptors, ERα or ERß, are current pharmaceutical targets. Previously, we have reported on substituted cis A-CD ligands in which the B-ring of the steroidal structure has been removed and cis refers the stereochemistry of the CD ring junction as compared to trans in estradiol. These compounds often showed good potency and selectivity for ERß. Here we report the synthesis and binding affinities for a similar series of trans A-CD ligands, and compare them to the cis-series. Counterintuitively, trans A-CD ligands, which are structurally more closely related to the natural ligand estradiol, show weaker binding and less ß-selectivity than their cis-counterparts.

Experimental versus predicted affinities for ligand binding to estrogen receptor: iterative selection and rescoring of docked poses systematically improves the correlation.

The computational determination of binding modes for a ligand into a protein receptor is much more successful than the prediction of relative binding affinities (RBAs) for a set of ligands. Here we consider the binding of a set of 26 synthetic A-CD ligands into the estrogen receptor ERα. We show that the MOE default scoring function (London dG) used to rank the docked poses leads to a negligible correlation with experimental RBAs. However, switching to an energy-based scoring function, using a multiple linear regression to fit experimental RBAs, selecting top-ranked poses and then iteratively repeating this process leads to exponential convergence in 4-7 iterations and a very strong correlation. The method is robust, as shown by various validation tests. This approach may be of general use in improving the quality of predicted binding affinities.

Scalable Electrochemical Aerobic Oxygenation of Indoles to Isatins without Electron Transfer Mediators by Merging with an Oxygen Reduction Reaction.

An approach to electrochemical oxygenation of indoles leading to isatins was developed by merging with a complementary cathode oxygen reduction reaction. The features of this green protocol include the use of molecular oxygen as the sole oxidant, it being free of an electron transfer mediator, and gram-scale preparation. Mechanistic studies suggested a radical process, and the two oxygen atoms in the isatins were both most likely from molecular oxygen. A detailed mechanism of the reaction utilizing density functional theory calculations was elucidated.

Hypervalent iodine promoted the synthesis of cycloheptatrienes and cyclopropanes.

A new strategy is reported for intramolecular Buchner-type reactions using PIDA as a promotor. Traditionally, the Buchner reaction is achieved via Rh-carbenoids derived from RhII catalysts with diazo compounds. Herein, the first metal-free Buchner-type reaction to construct highly strained cycloheptatriene- and cyclopropane-fused lactams is presented. The advantage of these transformations is in their mild reaction conditions, simple operation, broad functional group compatibility and rapid synthetic protocol. In addition, scaled-up experiments and a series of follow-up synthetic procedures were performed to clarify the flexibility and practicability of this method. DFT calculations were carried out to clarify the mechanism.

Stability of carbon-centered radicals: effect of functional groups on the energetics of addition of molecular oxygen.

In this paper we examine a series of hydrocarbons with structural features which cause a weakening of the C-H bond. We use theoretical calculations to explore whether the carbon-centered radicals R(*) which are created after breaking the bond can be stabilized enough so that they resist the addition of molecular oxygen, i.e. where the reaction R(*) + O(2) --> ROO(*) becomes energetically unfavorable. Calculations using a B3LYP-based method provide accurate bond dissociation enthalpies (BDEs) for R-H and R-OO(*) bonds, as well as Gibbs free energy changes for the addition reaction. The data show strong correlations between R-OO(*) and R-H BDEs for a wide variety of structures. They also show an equally strong correlation between the R-OO(*) BDE and the unpaired spin density at the site of addition. Using these data we examine the major functional group categories proposed in several experimental studies, and assess their relative importance. Finally, we combine effects to try to optimize resistance to the addition of molecular oxygen, an important factor in designing carbon-based antioxidants.

Rational design, synthesis, and optical properties of film-forming, near-infrared absorbing, and fluorescent chromophores with multidonors and large heterocyclic acceptors.

A new series of film-forming, low-bandgap chromophores (1 a,b and 2 a,b) were rationally designed with aid of a computational study, and then synthesized and characterized. To realize absorption and emission above the 1000 nm wavelength, the molecular design focuses on lowering the LUMO level by fusing common heterocyclic units into a large conjugated core that acts an electron acceptor and increasing the charge transfer by attaching the multiple electron-donating groups at the appropriate positions of the acceptor core. The chromophores have bandgap levels of 1.27-0.71 eV, and accordingly absorb at 746-1003 nm and emit at 1035-1290 nm in solution. By design, the relatively high molecular weight (up to 2400 g mol(-1)) and non-coplanar structure allow these near-infrared (NIR) chromophores to be readily spin-coated as uniform thin films and doped with other organic semiconductors for potential device applications. Doping with [6,6]-phenyl-C(61) butyric acid methyl ester leads to a red shift in the absorption only for 1 a and 2 a. An interesting NIR electrochromism was found for 2 a, with absorption being turned on at 1034 nm when electrochemically switched (at 1000 mV) from its neutral state to a radical cation state. Furthermore, a large Stokes shift (256-318 nm) is also unique for this multidonor-acceptor type of chromophore, indicating a significant structural difference between the ground state and the excited state. Photoluminescence of the film of 2 a was further probed at variable temperatures and the results strongly suggest that the restriction of bond rotations certainly helps to diminish non-radiative decay and thus enhance the luminescence of these large chromophores.

Deconstructing estradiol: removal of B-ring generates compounds which are potent and subtype-selective estrogen receptor agonists.

Estradiol and related estrogens have been widely used as supplements to relieve menopausal symptoms, but they lead to an increased risk of breast and endometrial cancer. Here we report the synthesis of a new family of compounds where we have removed the B-ring from the steroid ABCD structure, and functionalized the A-ring. These A-CD compounds show a preferential affinity for the estrogen receptor subtype ERbeta. Some show binding affinities which are greater than estradiol. The presence of electron-withdrawing substituents on the A-ring should reduce the tendency of these compounds to form carcinogenic metabolites, so they might lead to a safer approach to hormone replacement therapy.

Oxidative modification of citrate synthase by peroxyl radicals and protection with novel antioxidants.

In mammals, aging is linked to a decline in the activity of citrate synthase (CS; E.C. 2.3.3.1), the first enzyme of the citric acid cycle. We used 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH), a water-soluble generator of peroxyl and alkoxyl radicals, to investigate the susceptibility of CS to oxidative damage. Treatment of isolated mitochondria with AAPH for 8-24 h led to CS inactivation; however, the activity of aconitase, a mitochondrial enzyme routinely used as an oxidative stress marker, was unaffected. In addition to enzyme inactivation, AAPH treatment of purified CS resulted in dityrosine formation, increased protein surface hydrophobicity, and loss of tryptophan fluorescence. Propyl gallate, 1,8-naphthalenediol, 2,3-naphthalenediol, ascorbic acid, glutathione, and oxaloacetate protected CS from AAPH-mediated inactivation, with IC(50) values of 9, 14, 34, 37, 150, and 160 muM, respectively. Surprisingly, the antioxidant epigallocatechin gallate offered no protection against AAPH, but instead caused CS inactivation. Our results suggest that the current practice of using the enzymatic activity of CS as an index of mitochondrial abundance and the use of aconitase activity as an oxidative stress marker may be inappropriate, especially in oxidative stress-related studies, during which alkyl peroxyl and alkoxyl radicals can be generated.

Understanding the toxicity of phenols: using quantitative structure-activity relationship and enthalpy changes to discriminate between possible mechanisms.

Experimental studies of the "extended toxicity" of substituted phenols are mainly of two types: the toxicity due to phenoxyl radical formation and the toxicity caused by metabolites, for example, the formation of quinones. Quantitative structure-activity relationship (QSAR) studies of phenol toxicity have dealt with the formation of phenoxyl radicals using bond dissociation enthalpy (BDE) of parent phenols, have obtained good correlations with experimental data, and have concluded that phenoxyl radicals are the toxic agent. However, the actual toxic mechanism has remained poorly defined. In this study, we follow the metabolic pathways of monosubstituted phenols to their quinone end products and calculate enthalpy changes for all relevant reactions. These enthalpy changes are first used as descriptors for a QSAR analysis. Many of these new descriptors, including some relevant to quinone formation, are highly correlated with the BDE values of the parent phenols. Therefore, a QSAR analysis by itself is inconclusive as to the mechanism of toxicity. To better define the problem, we have returned to a detailed analysis of net enthalpy changes. We show that the formation of phenoxyl radical is the rate-determining step: This step is slow for electron-withdrawing group substituted phenols (EWG-phenols), whereas it is fast for electron-donating group substituted phenols (EDG-phenols). The study of net enthalpy changes of reactions reveals that once the phenoxyl radical is present, the corresponding quinone is rapidly formed, so that quinone formation may be ultimately responsible for toxicity of EDG-phenols. We then demonstrate how the suggested mechanism (quinone formation) is successful in predicting the toxicity of some complex phenols, which are predicted poorly using the phenoxyl radical argument. We also discuss the toxicities of some estrogens in light of the quinone mechanism.

Computational modeling of substituent effects on phenol toxicity.

Standard computational models of cytotoxicity of substituted phenols relate the toxicity to a set of quatitative structure-activity relationship (QSAR) descriptors such as log P, p K a, OH bond dissociation enthalpy (BDE), etc. Implicit in this approach is the idea that the phenoxyl radical is disruptive to the cell and factors increasing its production rate will enhance the toxicity. To improve the QSAR correlations, substituents are usually divided into electron-donating groups (EDG) and electron-withdrawing groups (EWG), which are treated separately and thought to follow different mechanisms of toxicity. In this paper, we focus on one important aspect of toxicity, the rate constant for production of phenoxyl radical. Activation energies are obtained for the reaction of X-phenol with peroxyl radical by using the Evans-Polanyi principle, giving rate constants as a function of DeltaBDE values for both EDG and EWG sets. We show that (i) a plot of log k for phenoxyl formation vs DeltaBDE shows a double set of straight lines with different slopes, justifying the usual EDG and EWG separation but without requiring any change in mechanism; (ii) the same method can be effectively used for different target radicals (e.g., tert-butoxyl) or different sets of parent compounds (e.g., substituted catechols), thus giving a useful general approach to analysis of toxicity data; (iii) regions of constant toxicity in all cases are predicted; and (iv) we argue that competing parallel mechanisms of toxicity are likely to be dominant for EWG-substituted phenols.

Hydrogen bonding vs. halogen bonding: the solvent decides.

Control of intermolecular interactions is integral to harnessing self-assembly in nature. Here we demonstrate that control of the competition between hydrogen bonds and halogen bonds, the two most highly studied directional intermolecular interactions, can be exerted by choice of solvent (polarity) to direct the self-assembly of co-crystals. Competitive co-crystal formation has been investigated for three pairs of hydrogen bond and halogen bond donors, which can compete for a common acceptor group. These competitions have been examined in seven different solvents. Product formation has been determined and phase purity has been examined by analysis of powder X-ray diffraction patterns. Formation of hydrogen-bonded co-crystals is favoured from less polar solvents and halogen-bonded co-crystals from more polar solvents. The solvent polarity at which the crystal formation switches from hydrogen-bond to halogen-bond dominance depends on the relative strengths of the interactions, but is not a function of the solution-phase interactions alone. The results clearly establish that an appreciation of solvent effects is critical to obtain control of the intermolecular interactions.

Highly selective CO2vs. N2 adsorption in the cavity of a molecular coordination cage.

Two M8L12 cubic coordination cages, as desolvated crystalline powders, preferentially adsorb CO2 over N2 with ideal selectivity CO2/N2 constants of 49 and 30 at 298 K. A binding site for CO2 is suggested by crystallographic location of CS2 within the cage cavity at an electropositive hydrogen-bond donor site, potentially explaining the high CO2/N2 selectivity compared to other materials with this level of porosity.

Cytotoxicity and cytoprotective activity in naphthalenediols depends on their tendency to form naphthoquinones.

We consider the cytotoxicity and the protection against oxidative stress for members of the naphthalenediol family and the known antioxidant epigallocatechin gallate (EGCG). Compounds include the 1,2-naphthalenediol (1,2-ND), 1,4-ND, 2,3-ND, 1,8-ND, and 1,4-dipropyl-2,3-naphthalenediol (DPND). The cell line is an adherent clone of rat pheochromocytoma (PC12-AC). Oxidative stress was induced by the peroxyl radical generator AAPH. The relative order of cytotoxicity was 1,4-ND > 1,2-ND > DPND > 2,3-ND > 1,8-ND > EGCG, with EC(50)'s of 15, 40, 160, >250, >250, >>250 muM, respectively. Despite their high toxicity, both 1,4-ND and 1,2-ND showed narrow zones of protective behavior whereas DPND, 2,3-ND and 1,8-ND and especially EGCG showed an extended protective range. The total protection obtained for the combination of cells/oxidative stressor/protective compounds (PC12-AC/AAPH/naphthalenediols) was defined by an integrated measure, the cytoprotective area (CPA). We relate the observed cytotoxicity and CPA to the different electronic structures of the naphthalenediols, characterized by the first and second bond dissociation enthalpies and the pK(a)'s for parent (diol) and semiquinone. Since the 2,3- and 1,8-naphthalenediols do not form quinones, their cytotoxicity is much lower than for the compounds which do. Thus selected members of the naphthalenediol family show promise as antioxidants.

Mechanism of cytotoxicity of catechols and a naphthalenediol in PC12-AC cells: the connection between extracellular autoxidation and molecular electronic structure.

ortho-Hydroxyphenols (catechols) form a common structural unit in naturally occurring antioxidants such as polyphenols. They also show pro-oxidant characteristics which depend on their particular structure. Here we examined the acetylated versions of three catechols and a naphthalenediol for cytotoxicity to adrenal PC12-AC cells. We found that the three catechols H1 (a p-methoxycatechol), H2 (a catechol analog of alpha-tocopherol), and H4 (a dioxymethylene-substituted catechol) strongly upregulate glutathione (GSH) in 24 h, whereas 1,4-dipropyl-2,3-naphthalenediol (DPND) does not. Upregulation of GSH is primarily caused by oxidative stress in the form of hydrogen peroxide generation, and both GSH upregulation and the rate of H(2)O(2) generation correlate well with the cytotoxicity. The major source of H(2)O(2) is autoxidation in the extracellular space, which results from transport of the (deacetylated) hydroquinone form outside the cell, rather than internal redox cycling. DPND is much less cytotoxic than any of the catechols. We show that this is because it cannot form a naphthoquinone due to the energy penalty associated with the loss of aromaticity in the benzene ring adjacent to the diol functional group. The relevance of these results to the design of antioxidants is discussed.

Solvent-vapour-assisted pathways and the role of pre-organization in solid-state transformations of coordination polymers.

A family of one-dimensional coordination polymers, [Ag4(O2C(CF2)2CF3)4(phenazine)2(arene) n ]·m(arene), 1 (arene = toluene or xylene), have been synthesized and crystallographically characterized. Arene guest loss invokes structural transformations to yield a pair of polymorphic coordination polymers [Ag4(O2C(CF2)2CF3)4(phenazine)2], 2a and/or 2b , with one- and two-dimensional architectures, respectively. The role of pre-organization of the polymer chains of 1 in the selectivity for formation of either polymorph is explored, and the templating effect of toluene and p-xylene over o-xylene or m-xylene in the formation of arene-containing architecture 1 is also demonstrated. The formation of arene-free phase 2b , not accessible in a phase-pure form through other means, is shown to be the sole product of loss of toluene from 1-tol·tol [Ag4(O2C(CF2)2CF3)4(phenazine)2(toluene)]·2(toluene), a phase containing toluene coordinated to Ag(I) in an unusual µ:η(1),η(1) manner. Solvent-vapour-assisted conversion between the polymorphic coordination polymers and solvent-vapour influence on the conversion of coordination polymers 1 to 2a and 2b is also explored. The transformations have been examined and confirmed by X-ray diffraction, NMR spectroscopy and thermal analyses, including in situ diffraction studies of some transformations.

Novel ligands balance estrogen receptor ß and α agonism for safe and effective suppression of the vasomotor response in the ovariectomized female rat model of menopause.

Vasomotor thermo-dysregulation (hot flashes) are an often debilitating symptom of menopause. Effective treatment is achieved primarily through activation of the estrogen receptor (ER)α with estrogens but is also associated with increased risk for breast and uterine cancer. In this study, we have tested novel compounds lacking the B ring of 17-hydroxy-ß-estradiol (E2) (A-CD compounds) with differing ratios of ERα:ERß binding affinities for the ability to reduce diurnal/nocturnal tail-skin temperatures (TSTs) in the ovariectomized female rat menopausal hot flash model. Normal mammary tissue expresses the predominantly antiproliferative ERß. Therefore, we hypothesized that a preferential ERß agonist with fractional ERα activity would safely reduce TSTs. The A-CD compound, L17, is a preferential ERß agonist that has a ratio of ERß:ERα binding affinity relative to E2 of 9.3 (where ERß:ERα for E2, 1.0). In the ovariectomized rat, daily administration of low doses (1 mg/kg) of the A-CD compound TD81 (ERα:ERß relative affinity, 15.2) was ineffective in temperature regulation, whereas L17 showed a trend toward TST reduction. Both E2 and the A-CD compound, TD3 (ERß:ERα relative affinity, 5.0), also reduced TSTs but had marked proliferative effects on mammary and uterine tissues. At 2 mg/kg, L17 strongly reduced TSTs even more effectively than E2 but, importantly, had only minimal effect on uterine weight and mammary tissues. Both E2- and L17-treated rats showed similar weight reduction over the treatment period. E2 is rapidly metabolized to highly reactive quinones, and we show that L17 has 2-fold greater metabolic stability than E2. Finally, L17 and E2 similarly mediated induction of c-fos expression in neurons within the rat thermoregulatory hypothalamic median preoptic nucleus. Thus, the A-CD compound, L17, may represent a safe and effective approach to the treatment of menopausal hot flashes.