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Douglas C. Neckers - One of the best experts on this subject based on the ideXlab platform.
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Analysis of the time-resolved FTIR spectra produced by the photolysis of alkyl Phenylglyoxylates.
Photochemical & Photobiological Sciences, 2007Co-Authors: Alexei G. Merzlikine, Douglas C. Neckers, Sergey V. Voskresensky, Eugene O. Danilov, Andrei V. FedorovAbstract:The photochemistry of alkyl Phenylglyoxylates (APG) was further investigated using time-resolved infrared spectroscopy. The primary focus was on the analysis of weak transient bands around 1828 and 1730 cm−1 in the time-resolved FTIR spectra of glyoxylates. The observed transients were assigned to benzoyl and alkyl mandelate ester radicals, respectively. The formation of benzoyl radical was fast and attributed to the Norrish Type I process. In addition, the intensities of the strong FTIR bands around 1680 and 2100 cm−1 were used to analyze the yields of the triplet state and ketene, respectively. These new and previous data on APG photochemistry are discussed in relation to the acrylate polymerization photoinitiation by alkyl Phenylglyoxylates.
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Application of Time-Resolved Step-Scan Fourier Transform Infrared Spectroscopy to Photochemical Mechanistic Investigations of Alkyl Phenylglyoxylates†,‡
The Journal of Physical Chemistry A, 2003Co-Authors: A. V. Fedorov, Evgeny O. Danilov, A. G. Merzlikine, And M. A. J. Rodgers, Douglas C. NeckersAbstract:The photochemical reactivity of alkyl Phenylglyoxylates has been investigated using time-resolved step-scan Fourier transform infrared spectroscopy. The time-resolved FTIR spectra of hexane solutions of methyl (MPG), ethyl (EPG), and isopropyl (iPPG) Phenylglyoxylates and benzene solutions of MPG display two major transient absorption peaks in the carbonyl fundamental region around 1661 and 2105 cm-1. On the basis of the spectroscopic and kinetic evidence, the 1661 cm-1 transitions are assigned to the carbonyl fundamental of the triplet state of the corresponding alkyl Phenylglyoxylate. The 2105 cm-1 band is attributed to the carbonyl stretch of α-hydroxyphenyl ketene, which is the postulated intermediate of the intramolecular hydrogen abstraction in alkyl Phenylglyoxylates. These assignments are supported by the results of DFT calculations. From kinetic time profiles for the two observed transient infrared bands, the rate constants for the intra- and intermolecular γ-hydrogen abstraction were obtained an...
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Observations of different triplet conformations in time-resolved infrared spectra of alkyl Phenylglyoxylates.
Journal of the American Chemical Society, 2002Co-Authors: Alexei G. Merzlikine, Sergey V. Voskresensky, Eugene O. Danilov, Michael A. J. Rodgers, Douglas C. NeckersAbstract:Two different conformations of the triplet state of alkyl Phenylglyoxylates were observed by means of time-resolved step-scan FT-IR spectroscopy. The amplitude of the peak corresponding to the sterically hindered conformation decreases as the size of the alkyl group increases. Both conformations exhibit similar reactivity in intermolecular hydrogen abstraction, but only one of them undergoes Norrish Type II photoelimination.
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Mechanism of alkyl Phenylglyoxylates photoreaction in the presence of oxygen
Journal of Photochemistry and Photobiology A: Chemistry, 1998Co-Authors: Douglas C. NeckersAbstract:Abstract The rate of starting material disappearance for Phenylglyoxylate esters possessing a reactive γ-hydrogen is not significantly influenced by the presence of oxygen. However, the products produced and their yields are quite different. A mechanism involving trapping of the 1,4-biradical intermediate by ground state oxygen following the triplet state γ-hydrogen abstraction is proposed. Rate constants for the various processes are deduced from literature values and from laser flash photolysis of the starting Phenylglyoxylates.
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Photochemically Active Polymers Containing Pendant Ethyl Phenylglyoxylate
Macromolecules, 1998Co-Authors: Douglas C. NeckersAbstract:Photoreactions of acrylic polymers containing pendant ethyl Phenylglyoxylate groups at different distances from the polymeric backbone have been studied. Hydrogen abstraction between pendant groups by an excited triplet Phenylglyoxylate carbonyl group from adjacent pendant groups is the primary process. The expected γ-hydrogen abstraction within one pendant group (the Norrish type II reaction) is not observed. The radicals formed react with each other to form cross-linked structures in the polymer. This effects solubility in various solvents which changes significantly after irradiation. The potential of the polymer as an imageable material has been evaluated.
Ombretta Porta - One of the best experts on this subject based on the ideXlab platform.
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Acylation of a transient Ti(IV)-enolate by acyl halides and anhydrides. Facile synthesis of α-hydroxy-β-ketoesters
Tetrahedron, 1996Co-Authors: Angelo Clerici, Laura Clerici, Ombretta PortaAbstract:Abstract Regioselective C-acylation at the carbonyl carbon of methyl Phenylglyoxylate 1 occurs by reaction with a variety of acyl halides 3 and anhydrides 4 in the presence of TiCl 3 /py system in THF at room temperature. α-Hydroxy-β-ketoesters 5 are the only reaction products and pyridine is essential to obtain useful yields (50–90%). The mechanism of acylation involves the intermediacy of a nucleophilic a Ti(IV)-ene diolate B .
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A tricomponent reaction promoted by a titanium trichloride/pyridine system diastereoselective synthesis of β-amino-α-hydroxyesters
Tetrahedron Letters, 1995Co-Authors: Angelo Clerici, Laura Clerici, Ombretta PortaAbstract:Abstract Methyl Phenylglyoxylate, aniline and aromatic aldehydes, on treatment with TiCl 3 /pyridine in anhydrous THF at room temperature, undergo rapid condensation to produce syn -β-amino-α-hydroxyesters. The mechanism of the reaction is rationalized.
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Efficient diastereoselective synthesis of α,β-Dihydroxyesters from methyl Phenylglyoxylate and aldehydes mediated by Titanium Trichloride/Pyridine System
Tetrahedron, 1995Co-Authors: Angelo Clerici, Laura Clerici, Luciana Malpezzi, Ombretta PortaAbstract:Abstract The reductive coupling of methyl Phenylglyoxylate 2 with aliphatic and aromatic aldehydes 3 , promoted by TiCl 3 /Py system in anhydrous THF. affords α,β-dihydroxyesters 4 in good yields (60–94%) and high syn -diastereoselectivity (up to 85%) The type of ligand at the metal ion necessary to achieve a high level of diastereocontrol and the possible mechanism involved are discussed
Angelo Clerici - One of the best experts on this subject based on the ideXlab platform.
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Acylation of a transient Ti(IV)-enolate by acyl halides and anhydrides. Facile synthesis of α-hydroxy-β-ketoesters
Tetrahedron, 1996Co-Authors: Angelo Clerici, Laura Clerici, Ombretta PortaAbstract:Abstract Regioselective C-acylation at the carbonyl carbon of methyl Phenylglyoxylate 1 occurs by reaction with a variety of acyl halides 3 and anhydrides 4 in the presence of TiCl 3 /py system in THF at room temperature. α-Hydroxy-β-ketoesters 5 are the only reaction products and pyridine is essential to obtain useful yields (50–90%). The mechanism of acylation involves the intermediacy of a nucleophilic a Ti(IV)-ene diolate B .
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A tricomponent reaction promoted by a titanium trichloride/pyridine system diastereoselective synthesis of β-amino-α-hydroxyesters
Tetrahedron Letters, 1995Co-Authors: Angelo Clerici, Laura Clerici, Ombretta PortaAbstract:Abstract Methyl Phenylglyoxylate, aniline and aromatic aldehydes, on treatment with TiCl 3 /pyridine in anhydrous THF at room temperature, undergo rapid condensation to produce syn -β-amino-α-hydroxyesters. The mechanism of the reaction is rationalized.
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Efficient diastereoselective synthesis of α,β-Dihydroxyesters from methyl Phenylglyoxylate and aldehydes mediated by Titanium Trichloride/Pyridine System
Tetrahedron, 1995Co-Authors: Angelo Clerici, Laura Clerici, Luciana Malpezzi, Ombretta PortaAbstract:Abstract The reductive coupling of methyl Phenylglyoxylate 2 with aliphatic and aromatic aldehydes 3 , promoted by TiCl 3 /Py system in anhydrous THF. affords α,β-dihydroxyesters 4 in good yields (60–94%) and high syn -diastereoselectivity (up to 85%) The type of ligand at the metal ion necessary to achieve a high level of diastereocontrol and the possible mechanism involved are discussed
Georg Fuchs - One of the best experts on this subject based on the ideXlab platform.
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Phenylacetyl-CoA:acceptor oxidoreductase, a membrane-bound molybdenum-iron-sulfur enzyme involved in anaerobic metabolism of phenylalanine in the denitrifying bacterium Thauera aromatica.
European Journal of Biochemistry, 1999Co-Authors: Sung-keun Rhee, Georg FuchsAbstract:Phenylacetic acids are common intermediates in the microbial metabolism of various aromatic substrates including phenylalanine. In the denitrifying bacterium Thauera aromatica phenylacetate is oxidized, under anoxic conditions, to the common intermediate benzoyl-CoA via the intermediates phenylacetyl-CoA and Phenylglyoxylate (benzoylformate). The enzyme that catalyzes the four-electron oxidation of phenylacetyl-CoA has been purified from this bacterium and studied. The enzyme preparation catalyzes the reaction phenylacetyl-CoA + 2 quinone + 2 H2O --> Phenylglyoxylate + 2 quinone H2 + CoASH. Phenylacetyl-CoA:acceptor oxidoreductase is a membrane-bound molybdenum-iron-sulfur protein. The purest preparations contained three subunits of 93, 27, and 26 kDa. Ubiquinone is most likely to act as the electron acceptor, and the oxygen atom introduced into the product is derived from water. The protein preparations contained 0.66 mol Mo, 30 mol Fe, and 25 mol acid-labile sulfur per mol of native enzyme, assuming a native molecular mass of 280 kDa. Phenylglyoxylyl-CoA, but not mandelyl-CoA, was observed as a free intermediate. All enzyme preparations also catalyzed the subsequent hydrolytic release of coenzyme A from phenylglyoxylyl-CoA but not from phenylacetyl-CoA. The enzyme is reversibly inactivated by a low concentration of cyanide, but is remarkably stable with respect to oxygen. This new member of the molybdoproteins represents the first example of an enzyme which catalyzes the alpha-oxidation of a CoA-activated carboxylic acid without utilizing molecular oxygen.
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Phenylglyoxylate : NAD+ oxidoreductase (CoA benzoylating), a new enzyme of anaerobic phenylalanine metabolism in the denitrifying bacterium Azoarcus evansii
European Journal of Biochemistry, 1998Co-Authors: Wolfram Hirsch, Hermann Schägger, Georg FuchsAbstract:Phenylglyoxylate (benzoylformate) is an intermediate in the anoxic metabolism of phenylalanine and phenylacetate. It is formed by alpha-oxidation of phenylacetyl-CoA. Phenylglyoxylate is oxidatively decarboxylated by Phenylglyoxylate-oxidoreductase to benzoyl-CoA, a central intermediate of anaerobic aromatic metabolism. The Phenylglyoxylate oxidizing enzyme activity in the denitrifying bacterium Azoarcus evansii was induced during anaerobic growth with phenylalanine, phenylacetate and Phenylglyoxylate, but not with benzoate. The new enzyme Phenylglyoxylate:acceptor oxidoreductase was purified and studied. The oxygen-sensitive enzyme reduced both NAD+ and viologen dyes. It was composed of five subunits of approximately 50, 48, 43, 24, and 11.5 kDa; the native mass as determined by gel filtration was 370 kDa, suggesting an alpha2 beta2 gamma2 delta2 epsilon2 composition. Phenylglyoxylate:acceptor oxidoreductase exhibited an ultraviolet/visible spectrum characteristic for an iron-sulfur protein and contained 35 +/- 4 mol Fe, 36 +/- 4 mol acid-labile sulfur, and 1.1 +/- 0.2 mol FAD/mol. The enzyme was specific for Phenylglyoxylate (Km 45 microM) and coenzyme A (Km 55 microM); 2-oxoisovalerate was oxidized with 15% of the rate. The turnover number with benzyl viologen at 37 degrees C was 46 s(-1) at the optimal pH of 8. The enzyme catalyzed a NAD(P)H:viologen dye transhydrogenation reaction, NAD(H) being the preferred coenzyme. It also catalyzed an isotope exchange between CO2 and the carboxyl group of the substrate. The data are consistent with the following hypothesis. The enzyme complex consists of a core enzyme of four subunits with the composition alpha2 beta2 gamma2 delta2, as reported for archaeal 2-oxoacid:ferredoxin oxidoreductases; this complex is able to reduce viologen dyes. The holoenzyme contains in addition an epsilon2 unit that catalyzes the transfer of electrons from a small ferredoxin-like subunit of the core complex to NAD+; this unit also catalyzes the transhydrogenase reaction, carries FAD and resembles ferredoxin:NAD(P)+-oxidoreductase.
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Anaerobic metabolism of L-phenylalanine via benzoyl-CoA in the denitrifying bacterium Thauera aromatica.
Archives of Microbiology, 1997Co-Authors: Sabine Schneider, Magdy El-said Mohamed, Georg FuchsAbstract:The anaerobic metabolism of phenylalanine was studied in the denitrifying bacterium Thauera aromatica, a member of the β-subclass of the Proteobacteria. Phenylalanine was completely oxidized and served as the sole source of cell carbon. Evidence is presented that degradation proceeds via benzoyl-CoA as the central aromatic intermediate; the aromatic ring-reducing enzyme benzoyl-CoA reductase was present in cells grown on phenylalanine. Intermediates in phenylalanine oxidation to benzoyl-CoA were phenylpyruvate, phenylacetaldehyde, phenylacetate, phenylacetyl-CoA, and Phenylglyoxylate. The required enzymes were detected in extracts of cells grown with phenylalanine and nitrate. Oxidation of phenylalanine to benzoyl-CoA was catalyzed by phenylalanine transaminase, phenylpyruvate decarboxylase, phenylacetaldehyde dehydrogenase (NAD+), phenylacetate-CoA ligase (AMP-forming), enzyme(s) oxidizing phenylacetyl-CoA to Phenylglyoxylate with nitrate, and Phenylglyoxylate:acceptor oxidoreductase. The capacity for phenylalanine oxidation to phenylacetate was induced during growth with phenylalanine. Evidence is provided that α-oxidation of phenylacetyl-CoA is catalyzed by a membrane-bound enzyme. This is the first report on the complete anaerobic degradation of an aromatic amino acid and the regulation of this process.
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Anaerobic oxidation of phenylacetate and 4-hydroxyphenylacetate to benzoyl-coenzyme A and CO2 in denitrifying Pseudomonas sp.
Archives of Microbiology, 1993Co-Authors: Magdy El-said Mohamed, Birgit Seyfried, Andreas Tschech, Georg FuchsAbstract:Anaerobic degradation of (4-hydroxy)phenylacetate in denitrifying Pseudomonas sp. was investigated. Evidence is presented for α-oxidation of the coenzyme A (CoA)-activated carboxymethyl side chain, a reaction which has not been described. The C6−C2 compounds are degraded to benzoyl-CoA and furtheron to CO2 via the following intermediates: Phenylacetyl-CoA, Phenylglyoxylate, benzoyl-CoA plus CO2; 4-hydroxyphenylacetyl-CoA, 4-hydroxyPhenylglyoxylate, 4-hydroxybenzoyl-CoA plus CO2, benzoyl-CoA. Trace amounts of mandelate possibly derived from mandelyl-CoA were detected during phenylacetate degradation in vitro. The reactions are catalyzed by (i) phenylacetate-CoA ligase which converts phenylacetate to phenylacetyl-CoA and by a second enzyme for 4-hydroxyphenylacetate; (ii) a (4-hydroxy)-phenylacetyl-CoA dehydrogenase system which oxidizes phenylacetyl-CoA to (4-hydroxy)Phenylglyoxylate plus CoA; and (iii) (4-hydroxy)Phenylglyoxylate: acceptor oxidoreductase (CoA acylating) which catalyzes the oxidative decarboxylation of (4-hydroxy)Phenylglyoxylate to (4-hydroxy)benzoyl-CoA and CO2. (iv) The degradation of 4-hydroxyphenylacetate in addition requires the reductive dehydroxylation of 4-hydroxybenzoyl-CoA to benzoyl-CoA, catalyzed by 4-hydroxybenzoyl-CoA reductase (dehydroxylating). The whole cell regulation of these enzyme activities supports the proposed pathway. An ionic mechanism for anaerobic α-oxidation of the CoA-activated carboxymethyl side chain is proposed. Phenylacetic acids are plant constituents and in addition are formed from a large variety of natural aromatic compounds by microorganisms; their degradation therefore plays a significant role in nature, as illustrated in the preceding paper (Mohamed and Fuchs 1993). We have investigated and purified an enzyme which catalyzes the first step in the anaerobic degradation of phenylacetate in a denitrifying Pseudomonas sp. Phenylacetate is converted to phenylacetyl-CoA by phenylacetate-CoA ligase (AMP forming). The postulated function of this enzyme is corroborated by the strict regulation of its expression. 4-Hydroxyphenylacetate appears to be similarly activated by an independent enzyme prior to further degradation.
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Anaerobic oxidation of phenylacetate and 4-hydroxyphenylacetate to benzoyl-coenzyme A and CO_2 in denitrifying Pseudomonas sp.
Archives of Microbiology, 1993Co-Authors: Magdy El-said Mohamed, Birgit Seyfried, Andreas Tschech, Georg FuchsAbstract:Anaerobic degradation of (4-hydroxy)phenylacetate in denitrifying Pseudomonas sp. was investigated. Evidence is presented for α-oxidation of the coenzyme A (CoA)-activated carboxymethyl side chain, a reaction which has not been described. The C_6−C_2 compounds are degraded to benzoyl-CoA and furtheron to CO_2 via the following intermediates: Phenylacetyl-CoA, Phenylglyoxylate, benzoyl-CoA plus CO_2; 4-hydroxyphenylacetyl-CoA, 4-hydroxyPhenylglyoxylate, 4-hydroxybenzoyl-CoA plus CO_2, benzoyl-CoA. Trace amounts of mandelate possibly derived from mandelyl-CoA were detected during phenylacetate degradation in vitro. The reactions are catalyzed by (i) phenylacetate-CoA ligase which converts phenylacetate to phenylacetyl-CoA and by a second enzyme for 4-hydroxyphenylacetate; (ii) a (4-hydroxy)-phenylacetyl-CoA dehydrogenase system which oxidizes phenylacetyl-CoA to (4-hydroxy)Phenylglyoxylate plus CoA; and (iii) (4-hydroxy)Phenylglyoxylate: acceptor oxidoreductase (CoA acylating) which catalyzes the oxidative decarboxylation of (4-hydroxy)Phenylglyoxylate to (4-hydroxy)benzoyl-CoA and CO_2. (iv) The degradation of 4-hydroxyphenylacetate in addition requires the reductive dehydroxylation of 4-hydroxybenzoyl-CoA to benzoyl-CoA, catalyzed by 4-hydroxybenzoyl-CoA reductase (dehydroxylating). The whole cell regulation of these enzyme activities supports the proposed pathway. An ionic mechanism for anaerobic α-oxidation of the CoA-activated carboxymethyl side chain is proposed. Phenylacetic acids are plant constituents and in addition are formed from a large variety of natural aromatic compounds by microorganisms; their degradation therefore plays a significant role in nature, as illustrated in the preceding paper (Mohamed and Fuchs 1993). We have investigated and purified an enzyme which catalyzes the first step in the anaerobic degradation of phenylacetate in a denitrifying Pseudomonas sp. Phenylacetate is converted to phenylacetyl-CoA by phenylacetate-CoA ligase (AMP forming). The postulated function of this enzyme is corroborated by the strict regulation of its expression. 4-Hydroxyphenylacetate appears to be similarly activated by an independent enzyme prior to further degradation. We have suggested before that phenylacetyl-CoA is anaerobically converted by α-oxidation of the side chain to Phenylglyoxylate^1, which is oxidatively decarboxylated to benzoyl-CoA plus CO_2 (Seyfried et al. 1991; Dangel et al. 1991). 4-Hydroxyphenylacetate was proposed to be similarly oxidized to 4-hydroxybenzoyl-CoA plus CO_2, followed by reductive dehydroxylation to benzoyl-CoA. The evidence was not presented in full, and the crucial α-oxidation was not demonstrated in vitro. We present here ample evidence for this pathway. A hypothetical mechanism is proposed by which the oxidation of the α-methylene group to an α-carbonyl group may occur.
Samir Bondock - One of the best experts on this subject based on the ideXlab platform.
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A Concise Diastereoselective Photochemical Synthesis of 3‐Hydroxyfuran‐2(3H)‐ones.
ChemInform, 2012Co-Authors: Samir BondockAbstract:Oxetanes (III) are obtained with high to moderate diastereoselectivity by photocycloaddition of alkyl Phenylglyoxylates (II) to allylic alcohols (I).
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A Concise Diastereoselective Photochemical Synthesis of 3‐Hydroxyfuran‐2(3H)‐ones
Journal of Heterocyclic Chemistry, 2012Co-Authors: Samir BondockAbstract:The photocycloaddition of alkyl Phenylglyoxylates to allylic alcohols leads to oxetanes 3a–h with high to moderate (2R∗,4R∗)-diastereoselectivity that can be easily ring-opened to give 3-hydroxyfuran-2(3H)-ones 4a–b.
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stereoselective generation of vicinal stereogenic quaternary centers by photocycloaddition of 5 methoxy oxazoles to α keto esters synthesis of erythroβ hydroxy dimethyl aspartates
Organic and Biomolecular Chemistry, 2004Co-Authors: Axel G Griesbeck, Samir BondockAbstract:The photocycloaddition of methyl pyruvate and methyl Phenylglyoxylate, respectively, to 5-methoxy oxazoles bearing additional substituents at C-2 and C-4 leads to bicyclic oxetanes 2 and 3 with high to moderate (exo) diastereoselectivity that can be easily ring-opened to give bis-quaternary aspartic acid diester derivatives 4 and 5.