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Acids
Kinds of Acids Terms modified by Acids Selected AbstractsRELATION BETWEEN THE FREE AMINO ACIDS, ANSERINE AND THE TOTAL VOLATILE BASIC NITROGEN PRODUCED IN MUSCLE OF HAKE (MERLUCCIUS MERLUCCIUS, L.) DURING ICED STORAGEJOURNAL OF FOOD BIOCHEMISTRY, Issue 1 2002CLAUDIA RUIZ-CAPILLAS This work studied the development of free amino acids (FAAs) and dipeptide anserine as quality indices for gutted hake stored in ice for 25 days. The correlation of these compounds was determined with total volatile basic nitrogen (TVBN) which has been used as a quality index, for fish stored in ice. The most abundant free amino acids in hake muscle were found to be threonine, glycine, alanine, glutamic acid, ,-alanine methylhistidine. lysine and the dipeptide, anserine. The only hydrophobic free ammo adds which exhibit significant differences (P<0.05) throughout storage was tryptophan. moreover, this amino acid exhibited a very high correlation (r=0.951) with TVBN. A significant decrease in anserine (P<0.05) correlated with the increases in 1-methylhistidine and ,-alanine throughout storage. These changes also exhibited a very high correlation with TVBN. Therefore, 1-methylhistidine, ,-alanine anserine and tryptophan could be used as quality parameters for hake stored in ice. [source] Electrocatalytic Oxidation of Sulfur Containing Amino Acids at Renewable Ni-Powder Doped Carbon Ceramic Electrode: Application to Amperometric Detection L -Cystine, L -Cysteine and L -MethionineELECTROANALYSIS, Issue 21 2006Abdollah Salimi Abstract A sol-gel technique was used here to prepare a renewable carbon ceramic electrode modified with nickel powder. Cyclic voltammograms of the resulting modified electrode show stable and a well defined redox couple due to Ni(II)/Ni(III) system with surface confined characteristics. The modified electrode shows excellent catalytic activity toward L -cystine, L -cysteine and L -methionine oxidation at reduced overpotential in alkaline solutions. In addition the antifouling properties at the modified electrode toward the above analytes and their oxidation products increases the reproducibility of results. L -cystine, L -cysteine and L -methionine were determined chronoamperometricaly at the surface of this modified electrode at pH range 9,13. Under the optimized conditions the calibration curves are linear in the concentration range 1,450,,M, 2,90,,M and 0.2,75,,M for L -cystine, L -methionine and L -cysteine determination, respectively. The detection limit and sensitivity were 0.64,,M, 3.8,nA/ ,M for L -cystine, 2,,M, 5.6,nA/ ,M for L -methionine and 0.2,,M and 8.1,nA/,M for L -cysteine. The advantageous of this modified electrode is high response, good stability and reproducibility, excellent catalytic activity for oxidation inert molecules at reduced overpotential and possibility of regeneration of the electrode surface by potential cycling for 5,minutes. Furthermore, the modified electrode has been prepared without using specific reagents. This sensor can be used as an amperometric detector for disulfides detection in chromatographic or flow systems. [source] The Use of N -Type Ligands in the Enantioselective Liquid,Liquid Extraction of Underivatized Amino AcidsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 27 2010Bastiaan J. V. Verkuijl Abstract The first palladium based extraction system using chiral N -based ligands in the enantioselective liquid,liquid extraction (ELLE) of underivatized amino acids, is presented. The system shows the highest selectivity for the ELLE of methionine with metal complexes as hosts reported to date. Furthermore, the host can be prepared in situ from commercially available compounds. The dependency of the system on parameters such as pH, organic solvent, and temperature has been established. The intrinsic selectivity was deduced by determination of the association constants of the palladium complex with the tryptophan enantiomers. [source] Chemo-, Regio- and Stereospecific Synthesis of Unnatural, Fluorescent Amino Acids by Condensation of L -Lysine and 1-Vinylpyrrole-2-carbaldehydesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 24 2010Andrey V. Ivanov Abstract A new family of unnatural, optically active amino acids containing the pyrrole moiety have been synthesized by condensation of 1-vinylpyrrole-2-carbaldehydes with L -lysine under mild conditions (EtOH, room temp., 2.5,3 h, 0.5 wt.-% of CF3CO2H) in up to 90,% yields. Unlike non-vinylated analogues, 1-vinylpyrrole-2-carbaldehydes react chemo-, regio- and stereospecifically with an ,-amino group only to afford products of exclusively (E) configuration. The amino acids synthesized containing aromatic or condensed aromatic substitutents in the pyrrole ring fluoresce in the UV/Vis region (,max = 350,382 nm, Stokes shifts 6150,7800 cm,1). [source] Synthesis of New ,-(Polyfluoroalkyl)-,-hydroxy-,-amino AcidsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 29 2009Nataliya A. Tolmacheva Abstract New ,-(polyfluoroalkyl)-,-hydroxy-,-amino acids were synthesized from the corresponding starting 3-(benzoylamino)-6-(polyfluoroalkyl)-2H -pyran-2-ones. The key step of the synthesis was the hydrogenation of the pyrone ring. Stereoselectivity and yields depended dramatically on the reaction conditions and the nature of the polyfluoroalkyl group. Various conditions were used for the preparation of both mixtures of diastereomers and pure diastereomers of the target amino acids. The obtained ,-(polyfluoroalkyl)-,-hydroxy-,-amino acids are of interest as analogues of 2-amino-5-hydroxyvaleric acid and glutamic acid.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Click Chemistry Inspired Synthesis of Novel Ferrocenyl-Substituted Amino Acids or PeptidesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 13 2009V. Sai Sudhir Abstract This work reports on the synthesis of a wide range of ferrocenyl-substituted amino acids and peptides in excellent yield. Conjugation is established via copper-catalyzed 1,3-dipolar cycloaddition. Two complementary strategies were employed for conjugation, one involving cycloaddition of amino acid derived azides with ethynyl ferrocene 1 and the other involves cycloaddition between amino acid derived alkynes with ferrocene-derived azides 2 and 3. Labeling of amino acids at multiple sites with ferrocene is discussed. A new route to 1,1,-unsymmetrically substituted ferrocene conjugates is reported. A novel ferrocenophane 19 is accessed via bimolecular condensation of amino acid derived bis-alkyne 9b with the azide 2. The electrochemical behavior of some selected ferrocene conjugates has been studied by cyclic voltammetry.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Resolution of Racemic N -Benzyl ,-Amino Acids by Liquid-Liquid Extraction: A Practical Method Using a Lipophilic Chiral Cobalt(III) Salen Complex and Mechanistic StudiesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 7 2008Pawel Dzygiel Abstract The efficient resolution of racemic N -benzyl ,-amino acids (N -Bn-AA) has been achieved by a liquid-liquid extraction process using the lipophilic chiral salen,cobalt(III) complex [CoIII(3)(OAc)]. As a result of the resolution by extraction, one enantiomer (S) of the N -benzyl ,-amino acid predominated in the aqueous phase, while the other enantiomer (R) was driven into the organic phase by complexation to cobalt. The complexed amino acid (R) was then quantitatively released by a reductive (CoIII,,,CoII) counter-extraction with aqueous sodium dithionite or L -ascorbic acid in methanol. Thereductive cleavage allowed to recover the [CoII(3)] complex in good yield, which could be easily re-oxidized to[CoIII(3)(OAc)] with air/AcOH and reused with essentially no loss of reactivity and selectivity. Investigation on the nitrogen substitution indicates that the presence of a single benzyl group on the amino acid nitrogen is important to obtain high enantioselectivity in the extraction process. The kinetic vs. thermodynamic nature of the resolution process was also investigated with an enantiomeric exchange experiment, which shows that the liquid-liquid extraction with [CoIII(3)(OAc)] is an equilibrium process operating under thermodynamic control. In the absence of a suitable crystal structure of the [CoIII(3)(N -Bn-AA)] complexes, computational and spectroscopic studies were used to investigate how the N -benzyl ,-amino acids are accommodated in the "binding pocket" of the chiral cobalt complex. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Simple Derivatives of Natural Amino Acids as Chiral Ligands in the Catalytic Asymmetric Addition of Phenylacetylene to AldehydesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 5 2005Zhi-jian Han Abstract Optically active propargylic alcohols are important chiral building blocks in asymmetric synthesis, and asymmetric addition of terminal alkynes to aldehydes is one of the most important and interesting procedures by which to prepare these chiral building blocks. In this work we have identified some simple derivatives of (S)-proline and other natural amino acids as chiral ligands that can be combined with Ti(OiPr)4 and then used to catalyze the asymmetric addition of zinc acetylide, produced in situ by the reaction of phenylacetylene with diethylzinc, to aldehydes. The ee value was as high as 90,%. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Asymmetric Synthesis of Isoquinoline Derivatives from Amino AcidsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 4 2005Oxana Sieck Abstract Reaction of isoquinolines 1 with N -arylsulfonylamino acid fluorides 2 provides a highly stereoselective access to new dihydroimidazo[2,1 -a]isoquinolin-3-ones 5 via intermediate N -acylisoquinolinium salts 3. Addition reactions to the en-amine double bond, such as hydrogenation or epoxidation with dimethyldioxirane, leads to tetrahydroimidazo[2,1 -a]isoquinoline-3-ones 6, 7 and oxiranes 8, respectively. Opening of the oxirane ring of the 8 with nucleophiles allows the synthesis of hydroxytetrahydroimidazo[2,1 -a]isoquinolin-3-ones 10 or 12 or of the polycyclic 1,4-dioxane 13 in high stereoselectivity. The regioselectivity of the oxiran ring opening depends on the kind of nucleophile and the conditions. Reaction of dihydroisoquinoline with O -TBDMS-mandelic acid chloride 15 leads to a tetrahydrooxazolo[2,3 -a]isoquinoline 17 with opposite facial selectivity as compared with dihydroimidazo[2,1 -a]isoquinolin-3-ones 5. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Preparation of Sugar Amino Acids by Claisen-Johnson Rearrangement: Synthesis and Incorporation into Enkephalin AnaloguesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2004Ana Montero Abstract We have developed a convenient route for the synthesis of an unsaturated branched sugar bearing a carboxylic acid and an amino group (masked as an azide group) by employing a totally stereoselective Claisen,Johnson rearrangement as the key step. Several Met- and Leu-enkephalin analogues with different substitution patterns at the N - and C -termini were prepared by incorporating this sugar amino acid (SAA) as a substitute for the central Gly,Gly fragment of the parent pentapeptides. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Design of Cyclopentaisoxazoline Amino Acids as Conformationally Constrained Agonists at Glutamate ReceptorsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2003Paola Conti Abstract We have prepared four isomeric 3-hydroxycyclopentaisoxazoline amino acids 12,15, which represent analogues of glutamic acid having restricted conformations, through a strategy based on the 1,3-dipolar cycloaddition of bromonitrile oxide to a suitably protected 1-aminocyclopent-2-enecarboxylic acid. These target compounds proved to be inactive when assayed at ionotropic and metabotropic glutamate receptors, except for 12 which is an agonist primarily at mGluR5 (EC50 = 79 ,M), but is less active at mGluR2 and only marginally active at mGluR1. The biological data are accounted for through comparison of the conformational profiles of the test compounds with that of reference agonists, i.e., N -methyl- D -aspartate (NMDA, 2), and 1-aminocyclopentane-1,3-dicarboxylic acids [trans -ACPD, 10; cis -ACPD, 11]. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Synthesis of Novel Nucleo-,-Amino Acids and Nucleobase-Functionalized ,-PeptidesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 18 2003Arndt M. Brückner Abstract Four novel ,-amino acids bearing the canonical nucleobases guanine, cytosine, adenine, and thymine in the side chain, are synthesized starting from Boc- L -aspartic acid 4-benzyl ester. The syntheses are accomplished in six steps by the nucleophilic substitution of (S)-,-(tert -butoxycarbonylamino)-,-bromopentanoic acid benzyl ester with the corresponding nucleobase derivative as the key step. The guaninyl and cytosinyl ,-amino acids were built into ,-peptides that were studied by temperature-dependent CD and UV spectroscopy. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Enantioselective Recognition of Aliphatic Amino Acids by ,-Cyclodextrin Derivatives Bearing Aromatic Organoselenium Moieties on the Primary or Secondary SideEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2003Yu Liu Abstract Spectrophotometric titrations have been performed in order to determine the stability constants of inclusion complexation of some aliphatic amino acids with four structurally related organoselenium-modified ,-cyclodextrins: mono(6-phenylseleno-6-deoxy)-,-cyclodextrin (1a), mono[6-(p -methoxyphenylseleno)-6-deoxy]-,-cyclodextrin (1b), mono(2-phenylseleno-2-deoxy)-,-cyclodextrin (2a), and mono[2-(p -methoxyphenylseleno)-2-deoxy]-,-cyclodextrin (2b). Conformation analysis by circular dichroism and 2D NMR spectroscopic studies revealed that the aryl-substituted ,-cyclodextrins gave self-inclusion intramolecular complexes in aqueous solution, while the extent of penetration depended both on the positions and on the structures of substituents. Quantitative investigation on the binding ability of the hosts with amino acids showed that they were able to recognize the size and the shape of guests, affording supramolecular complexes with quite small stability constants ranging from 24 to 355 M,1. The molecular recognition abilities are discussed from the viewpoints of induced-fitting mechanisms, geometric complementary, and cooperative binding processes. Furthermore, these ,-cyclodextrin derivatives displayed considerable enantioselectivity towards L/D -amino acid isomers, giving the highest L -enantioselectivity (up to 8.4) for inclusion complexation between leucine and 2a. The binding modes of L/D -leucine with 1b were elucidated from NOESY studies and the chiral recognition phenomena were interpreted accordingly. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Synthesis of Novel Amino Acids and Dehydroamino Acids Containing the Benzo[b]thiophene MoietyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2003Ana S. Abreu Abstract Several novel amino acids and dehydroamino acids containing the benzo[b]thiophene moiety were prepared by Michael addition or sequential Michael addition and palladium-catalyzed C,C or C,N cross couplings. The substrates for Michael addition were the methyl esters of N,N -bis(tert -butyloxycarbonyl)dehydroalanine [Boc2,,Ala,OMe] and N -(4-toluenesulfonyl)- N -(tert -butyloxycarbonyl)dehydroalanine [Tos,,Ala(N -Boc),OMe], and the nucleophiles were aromatic thiols and 3-iodobenzylamine. The addition of mercaptobenzo[b]thiophenes directly to Tos,,Ala(N -Boc),OMe gave stereoselectively, in good yields, the E -isomer of the corresponding dehydrocysteine. When thiophenols and 3-iodobenzylamine were used as nucleophiles the presence of an additional function (halogen or amine) allowed a subsequent palladium-catalyzed cross-coupling reaction with functionalized benzo[b]thiophenes (boronic acids, a halogen or an amine). Using this strategy, several racemic amino acid and dehydroamino acid derivatives, which are linked to the benzo[b]thiophene moiety by an aromatic spacer, were obtained in good yields. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] O -Glycosyl Amino Acids by 2-Nitrogalactal Concatenation , Synthesis of a Mucin-Type O -GlycanEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 6 2003Gottfried A. Winterfeld Abstract Base-promoted Michael-type addition of N -Boc- and N -Fmoc-protected serine and threonine esters to 2-nitrogalactal derivatives 2 and 26 led highly selectively to ,-glycosides 4a,d and 27a,c, respectively. Ensuing transformation of threonine derivative 4d and serine derivatives 4a,b resulted in compounds useful as lysine and dipeptide mimetics. 6- O -Desilylation of 27a,c, then 6- O -sialylation, and transformation of the nitro group of the galactose moiety into a 2-acetamido functionality, afforded N -Boc-protected serine and threonine tert -butyl esters 31a,c carrying the O -protected STN -antigen at the hydroxy group. The threonine derivative 31c was then transformed into the N -Fmoc-protected amino acid building block 33, which was employed for the synthesis of mucin repeating unit partial structure Ac-GS(STN)-TAPPAHG-NH2 (1). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] ,N-Stereogenic Quaternary Ammonium Salts' from L -Amino Acids: Synthesis, Separation, and Absolute ConfigurationHELVETICA CHIMICA ACTA, Issue 4 2009Hua-Fang Wu Abstract Diastereoisomeric linear and cyclic ,N-chiral quaternary ammonium salts' (QASs) were efficiently synthesized from corresponding L -amino acids in high yields. The diastereoisomers of each pair of ,N-chiral QASs' were successfully separated. The absolute configurations of these QASs were determined predominately by X-ray single-crystal analysis. The 1H-NMR data of ,N-chiral QASs' provided characteristic information on the configuration of the N-chiral center. ,N-Chiral QASs' exemplified by [N(R)]- 2a and [N(S)]- 2a are stable in protic and aprotic solvents within a broad pH and temperature range. [source] Modified Cyclodextrins as Enantioselective Hosts for Amino AcidsHELVETICA CHIMICA ACTA, Issue 4 2008Pavan Kumar, Vydyula Abstract Two modified , -cyclodextrins, H-2 and H-3, having a flexible appended moiety were studied for the chiral discrimination of the enantiomers of various amino acids by means of fluorescence as signaling option. These hosts quenched the fluorescence intensities of amino acids upon binding. The d- enantiomers were better recognized by these hosts. The association constants (Ks) and enantioselectivity factors (,) of the host,guest complexes were calculated. [source] Stereoselective Aldol Reactions Catalyzed by Acyclic Amino Acids in Aqueous MicellesHELVETICA CHIMICA ACTA, Issue 1 2007Dong-Sheng Deng Abstract The catalytic properties of all proteinogenic, acyclic amino acids for direct aldol reaction in H2O, assisted by various surfactants, were investigated. The basic and neutral amino acids were shown to be efficient catalysts, giving rise to good-to-excellent yields of adducts (up to 95%), with moderate-to-good diastereoselectivities (up to 86%), L -arginine being the most-effective catalyst. The syn/anti diastereoisomer ratio could be readily tuned by proper choice of the amino acid used. Also, the range of substrates that underwent the reaction was extended to less-reactive aldehydes carrying electron-donating Br substituents. [source] Effective Methods for the Synthesis of N -Methyl , -Amino Acids from All Twenty Common , -Amino Acids Using 1,3-Oxazolidin-5-ones and 1,3-Oxazinan-6-onesHELVETICA CHIMICA ACTA, Issue 11 2006Andrew Abstract N -Methyl , -amino acids are generally required for application in the synthesis of potentially bioactive modified peptides and other oligomers. Previous work highlighted the reductive cleavage of 1,3-oxazolidin-5-ones to synthesise N -methyl , -amino acids. Starting from , -amino acids, two approaches were used to prepare the corresponding N -methyl , -amino acids. First, , -amino acids were converted to N -methyl , -amino acids by the so-called ,1,3-oxazolidin-5-one strategy', and these were then homologated by the Arndt,Eistert procedure to afford N -protected N -methyl , -amino acids derived from the 20 common , -amino acids. These compounds were prepared in yields of 23,57% (relative to N -methyl , -amino acid). In a second approach, twelve N -protected , -amino acids could be directly homologated by the Arndt,Eistert procedure, and the resulting , -amino acids were converted to the 1,3-oxazinan-6-ones in 30,45% yield. Finally, reductive cleavage afforded the desired N -methyl , -amino acids in 41,63% yield. One sterically congested , -amino acid, 3-methyl-3-aminobutanoic acid, did give a high yield (95%) of the 1,3-oxazinan-6-one (65), and subsequent reductive cleavage gave the corresponding AIBN-derived N -methyl , -amino acid 61 in 71% yield (Scheme,2). Thus, our protocols allow the ready preparation of all N -methyl , -amino acids derived from the 20 proteinogenic , -amino acids. [source] The ,Azirine/Oxazolone Method' on Solid Phase: Introduction of Various ,,, -Disubstituted , -Amino AcidsHELVETICA CHIMICA ACTA, Issue 1 2006Simon Stamm Abstract Peptides containing various ,,, -disubstituted , -amino acids, such as , -aminoisobutyric acid (Aib), 1-aminocyclopentane-1-carboxylic acid, , -methylphenylalanine, and 3-amino-3,4,5,6-tetrahydro-2H -pyran-3-carboxylic acid have been synthesized from the N- to the C-terminus by the ,azirine/oxazolone method' under solid-phase conditions. In this convenient method for the synthesis of sterically demanding peptides on solid-phase, 2H -azirin-3-amines are used to introduce the ,,, -disubstituted , -amino acids without the need for additional reagents. Furthermore, the synthesis of poly(Aib) sequences has been explored. [source] Transformation of Amino Acids into Nonracemic 1-(Heteroaryl)ethanamines by the Enamino Ketone MethodologyHELVETICA CHIMICA ACTA, Issue 1 2006Samo Pirc Abstract N -Protected L -phenylalanines 1a,b were transformed, via the corresponding Weinreb amides 2 and ethynyl ketones 3, into chiral enamino ketones 4 (Scheme,1). Similarly, L -threonine 6 was transformed in four steps into the enamino ketone 10. Cyclocondensations of 4 and 10 with pyrazolamines 11, benzenecarboximidamide (12), and hydrazine derivatives 18 afforded N -protected 1-heteroaryl-2-phenylethanamines 15a,e, 16, 17, and 21a,k and 1-heteroaryl-1-aminopropan-2-ols 23a,b in good yields (Schemes,2 and 3). Finally, deprotection by catalytic hydrogenation furnished free amines 22a,g and 24a,b (Scheme,3). [source] The Nonchiral Bislactim Diethoxy Ether as a Highly Stereo-Inducing Synthon for Sterically Hindered, , -Branched , -Amino Acids: A Practical, Large-Scale Route to an Intermediate of the Novel Renin Inhibitor AliskirenHELVETICA CHIMICA ACTA, Issue 8 2003Richard Göschke The diastereoselective synthesis of the sterically hindered, , -branched , -amino acid derivative (2S,4S)- 24a and its N -[(tert -butoxy)carbonyl](Boc)-protected alcohol (2S,4S)- 19, both key intermediates of a novel class of nonpeptide renin inhibitors such as aliskiren (1), is described. Initially, the analogous methyl ester (2S,4S)- 17 was obtained by alkylation of the chiral Schöllkopf dihydropyrazine (R)- 12a with the dialkoxy-substituted alkyl bromide (R)- 11a, which proceeded with explicitly high diastereofacial selectivity (ds ,98%) to give (2S,5R,2,S)- 13a (Scheme,4), followed by mild acid hydrolysis and N -Boc protection (Scheme,5). Conversely, the complete lack of stereocontrol and poor yields for the reaction of (R)- 11a with the enantiomeric (S)- 12b suggested, in addition to the anticipated shielding effect by the iPr group at C(2) of the auxiliary, steric repulsion between the MeOC(6) and the bulky residues of (R)- 11a in the proposed transition state, which would strongly disfavor both the Si and Re attack of the electrophile (see Fig.). Based on this rationale, alkylation of the readily accessible achiral diethoxy-dihydropyrazine 21 with (R)- 11a was found to provide a 95,:,5 mixture of diastereoisomers (2S,2,S)- 22a and (2R,2,S)- 23a in high yield (Scheme,6), which afforded in two steps and after recrystallization enantiomerically pure (2S,4S)- 24a. Similarly, the stereochemical course for the alkylation reactions of the related alkyl bromides (S)- 28a and (R)- 28b with both (R)- 12a and (S)- 12b as well as with the achiral 21 was investigated (Schemes,7,9). The precursor bromides (R)- 11a, (S)- 11b, (R)- 28a, and (S)- 28b were efficiently synthesized via the diastereoselective alkylation of the Evans 3-isovaleroyloxazolidin-2-ones (R)- 7a and (S)- 7b either with bromide 6 or with benzyl chloromethyl ether, and subsequent standard transformations (Schemes,3 and 7). A practical and economical protocol of the preparation of (2S,4S)- 24a on a multi-100-g scale is given. This is the first report of the application of an achiral dihydropyrazine, i.e., in form of 21, as a highly stereo-inducing synthon providing rapid access to a N -protected , -branched , -amino acid with (2S) absolute configuration. [source] Allylsilane-Modified Amino Acids from the Claisen RearrangementHELVETICA CHIMICA ACTA, Issue 12 2002Mustafa Mohamed The Claisen rearrangement of the N -protected, silylated allyl glycinates 11 and 12 led to the formation of allyl/silyl-functionalized amino acids 13 and 14 in yields up to 80%. The diastereoisomer ratio varied from 2,:,1 to 29,:,1 for 11mb, and from 2,:,1 to 46,:,1 (syn/anti) for 12mb, depending on reaction conditions, as shown by X-ray crystallographic analysis of 14mb. The relationship between the size of the alkyl groups on the chlorosilane reagent (Me2R,SiCl, R,=Cl, Me, t -Bu, Ph) used as an enolate trap and the observed stereoselectivity was investigated in the case of the Ireland,Claisen variant. Me3SiCl gave the best results. However, the size of the alkyl groups on the silylated ester (Me2R,Si, R=Me, t -Bu, Ph, i-Pr) did not exert a significant effect on the diastereoselectivity or yield of the rearrangement. [source] Polystyrene-Supported Amino Acids as Efficient Catalyst for Chemical Fixation of Carbon DioxideADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2010Chaorong Qi Abstract Four new polystyrene-supported amino acids have been synthesized and applied to the chemical fixation of carbon dioxide for the first time. Two series of experiments with polystyrene-supported threonine (PS-Thr) and polystyrene-supported tyrosine (PS-Tyr) as catalyst, respectively, were conducted to study the effect of the reaction conditions on the carboxylation of propylene oxide/carbon dioxide. There was no considerable decrease in the yield of propylene carbonate after the polystyrene-supported amino acids were used five times, indicating that these catalysts are very stable. It was demonstrated that these catalysts were very efficient in the carboxylation of various epoxides and aziridines with carbon dioxide under mild conditions without any solvents. The mechanism for this carboxylation is also discussed. [source] Efficient Tandem Biocatalytic Process for the Kinetic Resolution of Aromatic ,-Amino AcidsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2010Bian Wu Abstract We describe a simple and efficient enzymatic tandem reaction for the preparation of enantiomerically pure , -phenylalanine and its analogues from the corresponding racemates. In this process, phenylalanine aminomutase (PAM) catalyzes the stereoselective isomerization of (R)- , -phenylalanines to (S)- , -phenylalanines, which are in situ transformed to cinnamic acids by phenylalanine ammonia lyase (PAL). Preparative scale conversions are done with a mutated PAM with enhanced catalytic activity. [source] Design and Application of 2,2-Dibromodimedone as Organic Brominating Reagent for Asymmetric Bromination of 1,3-Dicarbonyl Compounds and Ketones Catalysed by Chiral Amino AcidsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 10 2009Papori Goswami Abstract A green and ecofriendly enantioselective ,-bromination of carbonyl and 1,3-dicarbonyl compounds is reported involving the synthesis of a novel organic brominating source. The organic brominating reagent can be recovered after each cycle, rebrominated and reused. The reaction is catalysed by chiral amino acids and completed within a short reaction time with good enantioselectivity and exclusive formation of only ,-monobrominated carbonyl compounds. [source] Primary Amine-Thioureas based on tert -Butyl Esters of Natural Amino Acids as Organocatalysts for the Michael ReactionADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2009Christoforos Abstract A new class of primary amine-thioureas based on tert -butyl esters of (S)-,-amino acids and (1S,2S)-diphenylethylenediamine was synthesized and their activity as catalysts in Michael additions was evaluated. Derivatives based on di- tert -butyl aspartate and tert -butyl O - tert -butyl threoninate provided the product of the reaction between trans- ,-nitrostyrene and acetone in quantitative yield and high enantioselectivity (87,91% ee). All the thioureas based on tert -butyl esters of amino acids catalyzed the reaction of nitroolefins with acetophenone with high enantioselectivity (92,98% ee). Thus, low-cost, commercially available tert -butyl esters of natural amino acids are very important chiral building blocks for the construction of novel chiral thioureas able to catalyze asymmetric Michael additions with high enantioselectivity. [source] Synthesis of Ruthenium Hydride Complexes Containing beta-Aminophosphine Ligands Derived from Amino Acids and their use in the H2 -Hydrogenation of Ketones and IminesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2005Kamaluddin Abdur-Rashid Abstract The new complexes RuHCl(PPh2CH2CHRNH2)2 and RuHCl(PPh2CH2CHRNH2)(R- binap), R=H (Pgly), R=Me [(R)-Pala] were prepared by the substitution of the PPh3 ligands in RuHCl(PPh3)3 or RuHCl(PPh3)[(R)-binap] with beta-aminophosphines derived from amino acids. The complex trans -RuHCl(Pgly)[(R)-binap] has been characterized by X-ray crystallography. The complex trans -RuHCl[(S)-Ppro]2 where (S)-Ppro is derived from proline was also prepared and characterized by X-ray crystallography. These were used as catalyst precursors in the presence of a base (KOPr- i or KOBu- t) for the hydrogenation of various ketones and imines to the respective alcohols and amines with H2 gas (1,11 atm) at room temperature. Acetophenone was hydrogenated to (S)-1-phenylethanol in low ee (up to 40%) when catalyzed by the enantiomerically pure complexes. These complexes are especially active in the hydrogenation of sterically congested and electronically deactivated ketones and imines and are selective for the hydrogenation of CO bonds over CC bonds. [source] Identification of Candidate Amino Acids Involved in the Formation of Blue Pigments in Crushed Garlic Cloves (Allium sativum L.)JOURNAL OF FOOD SCIENCE, Issue 1 2009Jungeun Cho ABSTRACT:, The color-forming ability of amino acids with thiosulfinate in crushed garlic was investigated. We developed reaction systems for generating pure blue pigments using extracted thiosulfinate from crushed garlic and onion and all 22 amino acids. Each amino acid was reacted with thiosulfinate solution and was then incubated at 60 °C for 3 h to generate pigments. Unknown blue pigments, responsible for discoloration in crushed garlic cloves (Allium sativum L.), were separated and tentatively characterized using high-performance liquid chromatography (HPLC) and a diode array detector ranging between 200 and 700 nm. Blue pigment solutions exhibited 2 maximal absorbance peaks at 440 nm and 580 nm, corresponding to yellow and blue, respectively, with different retention times. Our findings indicated that green discoloration is created by the combination of yellow and blue pigments. Eight naturally occurring blue pigments were separated from discolored garlic extracts using HPLC at 580 nm. This suggests that garlic discoloration is not caused by only 1 blue pigment, as reported earlier, but by as many as 8 pigments. Overall, free amino acids that formed blue pigment when reacted with thiosulfinate were glycine, arginine, lysine, serine, alanine, aspartic acid, asparagine, glutamic acid, and tyrosine. Arginine, asparagine, and glutamine had spectra that were more similar to naturally greened garlic extract. [source] Free Amino Acids in Botanicals and Botanical PreparationsJOURNAL OF FOOD SCIENCE, Issue 5 2008B. Carratù ABSTRACT:, Numerous studies were carried out about aminoacidic composition of vegetable proteins, but information about the free amino acid pool and the role of these substances is very incomplete. The aim of this paper was to contribute to the scarce knowledge concerning the composition of free amino acids in botanicals and botanical preparations widely used as food, in dietary supplements, and in pharmaceutical products. This work studied the composition of free amino acids, identified the major components of 19 species of plants, and evaluated the influence of different types of extraction on the amino acid profile. Amino acids were determined using an automatic precolumn derivatization with fluorenylmethyl-chloroformate and reversed-phase liquid chromatography with fluorescence and ultraviolet detection. The amounts of total free amino acids varied widely between plants, from approximately 12 g in 100 g of Echinacea pallida extract to less than 60 mg in the same amount of Coleus forskohlii, Garcinia cambogia, and Glycine max. In 13 plants arginine, asparagine, glutamine, proline, and ,-aminobutyric acid were the free amino acids found in preponderant quantities. The levels of free amino acids above the quantification limit in 36 assayed samples of botanicals, extracts, and supplements are shown. [source] | ||||||||||||||||||||||||||||||||||||||||||||||