Synthesis of symmetric and asymmetric diamides of citric acid and amino acids

Summary A convenient method for the synthesis of symmetric and asymmetric diamides of amino acids including DOPA and citric acid from 2-tert-butyl-1,3-di(N-hydroxysuccinimidyl)citrate and 1-tert-butyl-2,3-di(N-hydroxysuccinimidyl)citrate is described.Abbreviations AcOtBu tert-butyl acetate - i-Bu iso-butyl - tBu tert-butyl - Bzl benzyl - p-OH-Bzl p-hydroxybenzyl - m,p-(OH)₂-Bzl m,p-dihydroxybenzyl - DCCI dicyclohexylcarbodiimide - Et ethyl - Me methyl - Su succinimidyl - SuOH N-hydroxysuccinimide - Ph phenyl     

Alternative syntheses and reactivity of platinum(II) terpyridyl acetonitrile complexes

New direct syntheses of [Pt(trpy)(NCCH₃)](CF₃SO₃)₂2 (where trpy = 2,2′:6′,2′′-terpyridine) and [Pt(tBu₃-trpy)(NCCH₃)](CF₃SO₃)₂3 (where tBu₃-trpy = 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine) via the displacement of acetonitrile from [Pt(NCCH₃)₄](CF₃SO₃)₂ have been developed. The synthetic utility of 2 was investigated in reactions with triphenylphosphine (PPh₃), 2,6-dimethylphenyl isocyanide (CN-Xyl), 2,5-dimethyl-2,5-diisocyanohexane (TM4), and tert-butyl isocyanide (CN-tBu). Whereas the expected substitution products were observed for reactions with PPh₃, CN-Xyl, and CN-tBu, dealkylation of TM4 occurred to afford [Pt(trpy)(CN)](CF₃SO₃) 6. The structures of [Pt(trpy)L]²⁺ dications show little intermolecular interactions in the solid state, with the exception of the tBu₃-trpy complex 3 which exists as head-to-tail dimers with a Pt-Pt distance of 3.29 Å. The cyano product 6 was found to stack in infinite chains of cations with a Pt-Pt distance of 3.45 Å.     

Investigating the free radical trapping ability of NXY-059, S-PBN and PBN

The spin trapping ability of the nitrones 2,4-disulphophenyl-N-tert-butyl nitrone (NXY-059), 2-sulphophenyl-N-tert-butyl nitrone (S-PBN) and α-phenyl-N-tert-butyl nitrone (PBN) for both hydroxyl and methanol radicals was investigated using electron paramagnetic resonance (EPR) spectroscopy. The radicals of interest were generated in situ in the spectrometer under constant flow conditions in the presence of each nitrone. The spin adducts formed were detected by EPR spectroscopy. This approach allowed for quantitative comparison of the EPR spectra of the spin adducts of each nitrone. The results obtained showed that NXY-059 trapped a greater number of hydroxyl and methanol radicals than the other two nitrones, under the conditions studied.     

Facile synthesis and reactivity study of mixed phosphane-isocyanide Pd(II) and Pd(0) complexes

The reaction between an equimolecular mixture of isocyanide CNR (CNR = di-methylphenyl isocyanide (DIC), tert-butyl isocyanide (TIC), triphenyl phosphane (PPh₃) and a dechlorinated solution of the palladium allyl dimers [Pd(η³-allyl)Cl]₂ (allyl = 2-Meallyl, 1,1-Me₂allyl) in stoichiometric ratio yields the mixed derivative [Pd(η³-allyl)(CNR)(PPh₃)] only. Apparently, the mixed derivative represents the most stable species among all the possible ones that might be formed under those experimental conditions. Theoretical calculations are in agreement with the experimental observation and the energy stabilization of the mixed species with respect to the homoleptic derivatives is traced back to an overall push-pull effect exerted by the isocyanide and the phosphane acting synergically. Similar behavior is observed in the case of the synthesis of the palladacyclopentadienyl complexes [Pd(C₄(COOMe)₄)(CNR)(PPh₃)] and of the palladium(0) olefin complexes whose synthesis invariably yields the mixed [Pd(η²-olefin)(CNR)(PPh₃)] derivatives. The paper includes studies on the reactivity toward allylamination in the case of the palladium(II) allyl complexes. A diffractometric investigation on the solid state structures of four different palladium isocyanide-phosphane complexes is also included.     

Palladium(II) amido complexes with an unsymmetrical PNP′ pincer-type coordination and a new (E,E)-tetradentate diphosphinoazine coordination mode

Cationic palladium(II) complexes [PdCl{PR₂CH₂C(Bu^t)NNC(Bu^t)CH₂PR₂}]Cl, where R = isopropyl, cyclohexyl or tert-butyl, were synthesized by the reactions of the corresponding diphosphinoazines with bis(acetonitrile)palladium(II) dichloride. When bis(benzonitrile)palladium(II) dichloride was used instead, in the molar ratio of 2:1 to the diphosphinoazine, a new complex was isolated with the isopropyl ligand showing a previously unknown (E,E) tetradentate coordination mode. Crystal and molecular structure was determined by X-ray diffraction. The solid complex was a racemate of two axially chiral enantiomers and the chirality was preserved in solution. Reactions of the cationic complexes with triethylamine gave complexes [PdCl{PR₂CHC(Bu^t)NNC(Bu^t)CH₂PR₂}], containing deprotonated diphosphinoazines in ene-hydrazone unsymmetrical pincer-like configuration. The complexes represent several of the still rare examples of Pd(II) amido bis(phosphine) complexes with a chlorine atom covalently bonded trans to the amide nitrogen.     

n-Butyl isocyanide oxidation at the [NiFe4S4OH x ] cluster of CO dehydrogenase

Carbon monoxide dehydrogenases (CODHs) catalyze the reversible oxidation of carbon monoxide by reaction with water to yield carbon dioxide, two protons, and two electrons. Two principal types of CODHs can be distinguished. Ni,Fe-containing CODHs contain a [NiFe₄S₄OH _x ] cluster within their active site, to which the direct binding of the substrates water and carbon dioxide has been revealed by protein X-ray crystallography. n-Butyl isocyanide is a slow-turnover substrate of CODHs, whose oxidation at the active site shows several parallels to the oxidation of carbon monoxide. Here, we report the crystal structure of CODH-II from Carboxydothermus hydrogenoformans resulting from the enzymatic oxidation of n-butyl isocyanide to n-butyl isocyanate at its active site cluster. The high resolution of the structure (d _min = 1.28 ?) revealed n-butyl isocyanate bound to the active site cluster and identified a novel type of Ni–C bond in CODHs. The structure suggests the occurrence of tetrahedral in addition to square-planar nickel complexes in product-bound states of this enzyme. Furthermore, we discovered a molecule of n-butyl isocyanide in a hydrophobic channel leading to the active site, revealing a unique architecture for the substrate channel of CODH-II compared with the bifunctional CODHs.     

Methyl <Emphasis Type="Italic">tert</Emphasis>-butyl Ether and <Emphasis Type="Italic">tert</Emphasis>-butyl Alcohol Degradation by <Emphasis Type="Italic">Fusarium solani</Emphasis>

Fusarium solani degraded methyl tert-butyl ether (MTBE) and other oxygenated compounds from gasoline including tert-butyl alcohol (TBA). The maximum degradation rate of MTBE was 16 mg protein h and 46 mg/g protein h for TBA. The culture transformed 77% of the total carbon to ¹⁴CO₂. The estimated yield for MTBE was 0.18 g dry wt/g MTBE.     

Kinetics of oxidation of ferrocene by tris-1,10-phenanthrolinecobalt(III) in t-butyl alcoholwater and acetonewater mixtures

Rate constants and activation parameters (ΔH^≠ and ΔS^≠)are reported for the oxidation of ferrocene by the tris-1,10-phenanthrolinecobalt(III) cation in t-butyl alcoholwater and in acetonewater solvent mixtures. Solvent effects on reactivity trends for these systems, for this same reaction in methanolwater mixtures, and for cobalt(II)-catalysed racemisation of Co(phen)_3³⁺ in t-butyl alcoholwater solvent mixtures are analysed into initial state and transition state contributions. The dependences of solubilities on solvent composition for ferrocene and for [Co(phen)₃](ClO₄)₃ in methanol, t-butyl alcohol, and acetonewater mixtures are also reported; these results are needed in order to establish solvent effects on the initial states of the reactions studied.     

<Emphasis Type="Italic">n</Emphasis>-Alkane assimilation and <Emphasis Type="Italic">tert</Emphasis>-butyl alcohol (TBA) oxidation capacity in <Emphasis Type="Italic">Mycobacterium austroafricanum</Emphasis> strains

Mycobacterium austroafricanum IFP 2012, which grows on methyl tert-butyl ether (MTBE) and on tert-butyl alcohol (TBA), the main intermediate of MTBE degradation, also grows on a broad range of n-alkanes (C₂ to C₁₆). A single alkB gene copy, encoding a non-heme alkane monooxygenase, was partially amplified from the genome of this bacterium. Its expression was induced after growth on n-propane, n-hexane, n-hexadecane and on TBA but not after growth on LB. The capacity of other fast-growing mycobacteria to grow on n-alkanes (C₁ to C₁₆) and to degrade TBA after growth on n-alkanes was compared to that of M. austroafricanum IFP 2012. We studied M. austroafricanum IFP 2012 and IFP 2015 able to grow on MTBE, M. austroafricanum IFP 2173 able to grow on isooctane, Mycobacterium sp. IFP 2009 able to grow on ethyl tert-butyl ether (ETBE), M. vaccae JOB5 (M. austroaafricanum ATCC 29678) able to degrade MTBE and TBA and M. smegmatis mc2 155 with no known degradation capacity towards fuel oxygenates. The M. austroafricanum strains grew on a broad range of n-alkanes and three were able to degrade TBA after growth on propane, hexane and hexadecane. An alkB gene was partially amplified from the genome of all mycobacteria and a sequence comparison demonstrated a close relationship among the M. austroafricanum strains. This is the first report suggesting the involvement of an alkane hydroxylase in TBA oxidation, a key step during MTBE metabolism.     

Hydroalumination of bis(alkynyl)germanes and -silanes: Generation of a chelating Lewis-acid and observation of a dismutation reaction

Treatment of diphenyl-di(phenylethynyl)germane with two equivalents of di(tert-butyl)aluminum hydride afforded the corresponding dialkenyl derivative, Ph₂Ge[C(AltBu₂)C(H)-Ph]₂ (1) by dual hydroalumination. The aluminum atoms of 1 are attached to the carbon atoms in α-position to germanium. They are coordinatively unsaturated and are able to act as chelating Lewis-acids and to coordinate donors such as chloride or bromide anions in a chelating manner (2, 3). The analogous reaction of the corresponding silicon-centered dialkyne with two equivalents of dimethylaluminum hydride gave a mixture of unknown compounds. Interestingly, equimolar quantities of the hydride and the dialkyne resulted in dismutation and the formation of the unprecedented compound MeAl[C(CH-Ph)-SiPh₂-CC-Ph]₂ (4). Compound 4 has two alkenyl groups bonded to the central aluminum atom and a terminal alkynyl group attached to each silicon atom. An attempt to reduce the remaining triple bonds by reaction with di(tert-butyl)aluminum hydride resulted in cleavage and isolation of the monoalkenyl compound tBu₂Al-C[C(H)-Ph]-SiPh₂-CC-Ph (5). The molecular structure of 5 showed a close interaction between the α-carbon atom of the triple bond and the coordinatively unsaturated aluminum atom.     

Hemoglobin-sulfhydryls from tortoise (Geochelone carbonaria) can reduce oxidative damage induced by organic hydroperoxide in erythrocyte membrane

Sulfhydryl groups are important to avoid oxidative damage to the cell. In RBC, tert-butyl hydroperoxide (tert-BOOH) and hydrogen peroxide (H₂O₂) are capable of oxidizing heme and promoting lipid peroxidation. H₂O₂ caused greater oxidation of heme than tert-BOOH, although the oxidation of sulfhydryl groups was similar. Geochelone carbonaria Hb, a rich sulfhydryl protein, inhibited the TBA-reactive substances formation of human erythrocytes exposed to tert-BOOH by about 30%; this decrease was smaller with Geochelone denticulata Hb. Sulfhydryl reagents diminished the number of reactive sulfhydryl groups in the G. carbonaria Hb resulting in a decrease of its antioxidant power, suggesting the involvement of sulfhydryls of Hb in the protection against lipid peroxidation.     

Interaction of tert -butyl Hydroperoxide with Hypochlorous Acid. A Spin Trapping and Chemiluminescence Study

The formation of radical species during the reaction of tert-butyl hydroperoxide and hypochlorous acid has been investigated by spin trapping and chemiluminescence. A superposition of two signals appeared incubating tert-butyl hydroperoxide with hypochlorous acid in the presence of the spin trap &#102 -(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN). The first signal (a_N = 1.537mT, a^&#103_H = 0.148mT) was an oxidation product of POBN caused by the action of hypochlorous acid. The second spin adduct (a_N = 1.484mT, a^&#103_H = 0.233mT) was derived from a radical species that was formed in the result of reaction of tert-butyl hydroperoxide with hypochlorous acid. Similarly, a superposition of two signals was also obtained using the spin trap N-tert-butyl- &#102 -phenylnitrone (PBN). tert-Butyl hydroperoxide was also treated with Fe²⁺ or Ce⁴⁺ in the presence of POBN. Using Fe²⁺ a spin adduct with a _N= 1.633mT and a^&#103_H = 0.276mT was observed. The major spin adduct formed with Ce⁴⁺ was characterised by α_N = 1.480mT and a^&#103_H = 0.233mT. The reaction of tert-butyl hydroperoxide with hypochlorous acid was accompanied by a light emission, that time profile and intensity were identical to those emission using Ce⁴⁺. The addition of Fe²⁺ to tert-butyl hydroperoxide yielded a much smaller chemiluminescence. Thus, tert-butyl hydroperoxide yielded in its reaction with hypochlorous acid or Ce⁴⁺ the same spin adduct and the same luminescence profile. Because Ce⁴⁺ is known to oxidise organic hydroperoxides to peroxyl radical species, it can be concluded that a similar reaction takes place in the case of hypochlorous acid.     

Synthesis and use of <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-di-Boc-glutamate γ-semialdehydes and related aldehydes

Summary.  This review article focuses on the synthesis and reactions of N,N-di-Boc glutamate and aspartate semialdehydes as well as related aldehydes. These building blocks are prepared according to various strategies from glutamic and aspartic acids and find interesting synthetic applications. In the first part, the methods for the synthesis of N,N-di-Boc-amino aldehydes are summarized. The applications of these chiral synthons for the synthesis of unnatural amino acids and other bioactive compounds are discussed in the second section. Received April 24, 2002 Accepted August 13, 2002 Published online January 30, 2003 Authors' address: Prof. Violetta Constantinou-Kokotou, Chemical Laboratories, Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece, E-mail: vikon@aua.gr Abbreviations: AcNH-TEMPO, 4-acetamido-2,2,6,6-tetramethyl-1-piperidinyloxy free radical; AIBN, 2,2′-azobis(2-methylpropionitrile); Aliquat, methyltrioctylammonium chloride; Bn, benzyl; Boc, tert-butoxycarbonyl; Bu^t, tert-butyl; m-CPBA, 3-chloroperoxybenzoic acid; DAST, diethylaminosulfur trifluoride; DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; (R,R)-(+)-DET, (R,R)-(+)-diethyltartrate; DIBALH, diisobutyl aluminium hydride; DMAP, 4-dimethylaminopyridine; DMF, dimethylformamide; Et₃N, triethylamine; KHMDS, potassium bis(trimethylsilyl)amide; (S)-LLB, lanthanium-lithium-bis-metallic binaphthol catalyst; MsCl, methanesulfonyl chloride; NEM, N-ethylmorpholine; NMO, 4-methylmorpholine N-oxide; PPA, propylphosphonic acid anhydride; TBHP, tert-butyl hydroperoxide; TFA, trifluoroacetic acid; THF, tetrahydrofuran; TMSI, 1-(trimethylsilyl)imidazole; Trt, trityl.     

Characterization of a putative thioredoxin peroxidase prx1 of <Emphasis Type="Italic">Candida albicans</Emphasis>

In this study, we characterized a putative peroxidase Prx1 of Candida albicans by: 1) demonstrating the thioredoxin-linked peroxidase activity with purified proteins, 2) examining the sensitivity to several oxidants and the accumulation of intracellular reactive oxygen species with a null mutant (prx1Δ), a mutant (C69S) with a point mutation at Cys69, and a revertant, and 3) subcelluar localization. Enzymatic assays showed that Prx1 is a thioredoxin-linked peroxidase which reduces both hydrogen peroxide (H₂O₂) and tert-butyl hydroperoxide (t-BOOH). Compared with two other strong H₂O₂ scavenger mutants for TSA1 and CAT1, prx1Δ and C69S were less sensitive to H₂O₂, menadione and diamide at all concentrations tested, but were more sensitive to low concentration of t-BOOH. Intracellular reactive oxygen species accumulated in prx1Δ and C69S cells treated with t-BOOH but not H₂O₂. These results suggest that peroxidase activity of Prx1 is specified to t-BOOH in cells. In both biochemical and physiological cases, the evolutionarily conserved Cys69 was found to be essential for the function. Immunocytochemical staining revealed Prx1 is localized in the cytosol of yeast cells, but is translocated to the nucleus during the hyphal transition, though the significances of this observation are unclear. Our data suggest that PRX1 has a thioredoxin peroxidase activity reducing both t-BOOH and H₂O₂, but its cellular function is specified to t-BOOH.     

Isolation and characterization of a new Mycobacterium austroafricanum strain, IFP 2015, growing on MTBE

A new Mycobacterium austroafricanum strain, IFP 2015, growing on methyl tert-butyl ether (MTBE) as a sole carbon source was isolated from an MTBE-degrading microcosm inoculated with drain water of an MTBE-supplemented gasoline storage tank. M. austroafricanum IFP 2015 was able to grow on tert-butyl formate, tert-butyl alcohol (TBA) and α-hydroxyisobutyrate. 2-Methyl-1,2-propanediol was identified as the TBA oxidation product in M. austroafricanum IFP 2015 and in the previously isolated M. austroafricanum IFP 2012. M. austroafricanum IFP 2015 also degraded ethyl tert-butyl ether more rapidly than M. austroafricanum IFP 2012. Specific primers designed to monitor the presence of M. austroafricanum strains could be used as molecular tools to detect similar strains in MTBE-contaminated environment.     

New method for determination of fecal sterols in urine using non-chlorinated solvents

A new method has been developed to determine a number of sterols in urine using non-chlorinated solvents, namely methyl tert.-butyl ether and methanol (the MTB method). The method involves liquid–liquid extraction, saponification, reextraction, silylation and final identification and quantification by GC–FID. The sterols determined were coprostanol, epicoprostanol, cholesterol and dihydrocholesterol. 5α-Cholestane was used as internal standard. The limit of detection for the sterols in this experiment was 2 μg l⁻¹ urine. Recovery of coprostanol over the range 5–100 μg l⁻¹ urine by this method was between 80 and 92%.     

Estimation of the fraction of biologically active methyl <Emphasis Type="Italic">tert</Emphasis>-butyl ether degraders in a heterogeneous biomass sample

The fraction of biologically active methyl tert-butyl ether degraders in reactors is just as important for prediction of removal rates as knowledge of the kinetic parameters. The fraction of biologically active methyl tert-butyl ether degraders in a heterogeneous biomass sample, taken from a packed bed reactor, was determined using a batch kinetic based approach. The procedure involved modeling of methyl tert-butyl ether removal rates from batch experiments followed by parameter estimations. It was estimated to be 5–14% (w/w) of the measured volatile suspended solids concentration in the reactor.     

Syntheses of Group 14 bis(tert-butylamido)cyclodiphosph(III)azane dichlorides, and the solid-state structures of the silicon and tin derivatives

To test the synthetic utility of bis(tert-butylamido)cyclodiphosph(III)azanes as ligands we extended the coordination chemistry of these diamides from Group 4 to Group 14. The syntheses of compounds of the formula cis-[^tBuNP(μ-^tBuN)₂PN^tBu]ECl₂, E = Si (1), Ge (2), Sn (3) and the solid-state structures of 1 and 3 are reported. Silicon tetrachloride reacted with dilithiobis(tert-butylamido)cyclodiphosph(III)azane to cleanly produce cis-[^tBuNP(μ-^tBuN)₂PN^tBu]SiCl₂, but for the germanium and tin analogues the interaction of GeCl₄ or SnCl₄ with the diazastannylene cis-[^tBuNP(μ-^tBuN)₂PN^tBu]Sn proved to be a better method. Single-crystal X-ray studies on both 1 and 3 revealed that they had C_s-symmetric structures, the central element being coordinated by two amide nitrogens and two chlorides, in addition to being weakly coordinated by one of the cyclodiphosph(III)azane ring nitrogens. Using structural comparisons between crystallographically-independent 1a and 1b, between 1 and 3, and between 3 and its isomorphous zirconium analogue, the nature of this donor bond is discussed.     

3-(1-Piperidinyl)alanine formation during the preparation ofC-terminal cysteine peptides with the Fmoc/t-Bu strategy

Summary Several side reactions have been detected for cysteine-containing peptides. During the synthesis ofC-terminal cysteine peptides, a base-catalyzed elimination of the sulfhydryl-protected side-chain to afford the dehydroalanine derivative followed by a nucleophilic addition to the alkene was observed. MALDI-TOF analysis was a useful analytical technique to determine this phenomenon.Abbreviations Acm acetamidomethyl - Boc tert-butyloxycarbonyl - t-Bu tert-butyl - DBU 1,8-diazabicyclo[5.4.0]undec-7-ene - DIEA N,N-diisopropylethylamine - DMF N,N-dimethylformamide - Fmoc 9-fluorenylmethyloxycarbonyl - HATU N-[(dimethylamino)-1H-1,2,3-triazolo[4,5-b]pyridin-1-yl-methylene]-N-methylmethanaminium hexafluorophosphateN-oxide - HBTU N-[(1H-benzotriazol-1-yl)(dimethylamino)methylene]-N-methylmethanaminium hexafluorophosphateN-oxide - HOAt 1-hydroxy-7-azabenzotriazole - HOBt 1-hydroxybenzotriazole - HPLC high-performance liquid chromatography - MALDI-TOF matrix-assisted laser desorption/ionization time-of-flight mass spectrometry - PAC 4-hydroxymethylphenoxyacetic acid handle - PAL 5-(4-(9-fluorenylmethyloxycarbonyl)aminomethyl-3,5-dimethoxy-phenoxy)valeric acid handle - PEG-PS polyethylene glycol-polystyrene graft supports - PS polystyrene - Reagent R TFA/thioanisole/1,2-ethanedithiol/anisole (90:5:3:2) - S-t-Bu S-tert-butyl mercapto - TFA trifluoroacetic acid - Trt triphenylmethyl. Amino acid symbols denote thel-configuration, unless indicated otherwise     

Comparison of the radical trapping ability of PBN,S-PBN and NXY-059

The nitrones α-phenyl-N-tert-butyl nitrone (PBN), sodium 2-sulfophenyl-N-tert-butyl nitrone (S-PBN) and disodium 2,4-disulfophenyl-N-tert-butyl nitrone (NXY-059) are neuroprotective in a variety of rodent models. The objective of the current studies was to compare the ability of PBN, S-PBN, and NXY-059 to form radical adducts and to prevent salicylate oxidation in an aqueous system. For the electron spin resonance (ESR) studies, hydroxyl radicals were generated with ultraviolet (UV) light and hydrogen peroxide. Secondary radicals were then produced by the addition of methanol, ethanol, isopropanol, dimethylsulfoxide, tetrahydrofuran or 1,4-dioxane. In addition, competition spin trapping studies were performed using PBN-α-¹³C and either S-PBN or NXY-059. In the salicylate studies, PBN, S-PBN and NXY-059 were compared to a variety of other antioxidants and reference compounds (cysteine, glutathione, ascorbate, uric acid, Tempo, Trolox, and Tirilizad) for their ability to prevent 2,3- and 2,5-dihydroxybenzoic acid formation induced by hydroxyl radical generating systems. All 3 nitrones trapped carbon- and oxygen-centered radicals to produce ESR-detectable radical adducts. Each nitrone also prevented salicylate oxidation, with PBN being the most effective. The ability of these 3 nitrones to prevent salicylate oxidation resembled that of most of the other compounds tested.