Phenylalanine Methyl Ester Research Articles

2,5-Dimethylphenacyl carbamate: a photoremovable protecting group for amines and amino acids

2,5-Dimethylphenacyl (DMP) carbamates (1a-c) released the corresponding free amines or amino acids in high chemical yields, albeit with quantum yields Phi of only 0.04-0.09, upon irradiation in either aprotic or protic solvents. The photoreaction proceeded principally from the triplet excited state via the E-photoenol. The lifetimes of the triplet enol and the E- and Z-enols in the ground state were determined by laser flash photolysis. The primary photoinitiated transformation liberated a carbamic acid derivative, which subsequently decarboxylated to the amino group-containing compound. Exhaustive irradiation of a DMP-protected aniline (1a) in acetonitrile did not provide aniline in quantitative chemical yields, because it was involved in reductive cleavage of the starting material as an electron donor, thereby decreasing the overall deprotection yield (86%). Phenylalanine methyl ester, liberated from 1c, was, however, obtained in excellent chemical yield (97%). It was also found that the carbamates, while thermally stable, released amines with higher quantum yields in acidic methanol solutions. The DMP chromophore is proposed as an excellent photoremovable protecting group for amino acids and, under specific conditions, for amines in organic synthesis and biochemistry.

Enantioselective hydrogenation of N-acetyl dehydrophenylalanine methyl ester using cinchonine-modified Pd/Al 2O 3 catalysts

The enantioselective hydrogenation of ( E)- N-acetyl dehydrophenylalanine methyl ester (NADPME) to N-acetyl phenylalanine methyl ester was investigated using cinchonine-modified Pd/Al 2O 3 catalysts. The catalysts were evaluated for this reaction using methanol as solvent with various cinchonine:NADPME molar ratios. Enantioselectivity was sensitive to this ratio as well as to the solvent, and in general, ee increased with the polarity of the solvent. The highest ee of 33% was observed with DMF/water as solvent, but attempts to improve upon this have been unsuccessful. We used combined structural modification of the reactant and alkaloid together with computer simulations to gain insight into why the low enantioselectivity persists. Using this approach, we propose a model in which NADPME interacts as a monomer with cinchonine via hydrogen bonding between the protonated quinuclidine- N of cinchonine and the hydrogen bond acceptor functional groups of NADPME, which induces enantioselection. DFT level calculations of these interactions show that N H O C hydrogen bonding gives the most stable complexes and that the lowest pro- S and pro- R dimer forms are practically isoenergetic, although higher energy dimer pairs show discrimination, thereby helping to explain the disappointing ee obtained to date.

Synthesis, Resolution, and Enantiomeric Purity Assay of 2- n-Butylbutanedioic Acid 4- t-Butyl Esters

Racemic 2- n -butylbutanedioic acid 4- t -butyl esters were synthesized from methyl hexanoate and t -butyl α-iodoacetate via alkylation and subsequently selective hydrolyzation. The ( R )-and ( S )-2- n -butylbutanedioic acid 4- t -butyl esters were obtained by the resolution of the above-mentioned racemic compounds with ( S )-(-) or ( R )-(+)-α-methylbenzylamine, respectively. The e.e. values of the two optical active products were determined to be above 99% by HPLC after the formation of two pairs of diastereoisomers with ( R )-(+)-α-methylbenzylamine and ( S )-phenylalanine methyl ester.

Spirobicyclic diamines. Part 2: Synthesis of homochiral diastereoisomeric proline derived [4.4]-spirolactams

l-Proline derived diastereoisomeric [4.4]-spirolactams have been prepared by a reductive-amination reaction of ( R)- or ( S)-alanine methyl ester, followed by thermal cyclisation of the resulting amine onto the proline ester group in refluxing toluene. Under similar conditions ( R)- or ( S)-phenylalanine methyl ester gave no cyclisation products, while R- or S-α-methylbenzylamine required treatment with NaNH 2 in refluxing toluene to induce cyclisation giving diastereoisomeric [4.4]-spirolactams.

Enhancing protease enantioselectivity by ionic liquids based on chiral- or ω-amino acids

Ionic liquids (ILs) carrying anions of chiral- or ω-amino acids were prepared. The enzymatic hydrolysis of phenylalanine methyl ester was studied in aqueous solutions of these ILs. These ILs were found capable of stabilizing the protease activity and enantioselectivity at low concentrations. Interestingly, higher ees and yields of l-phenylalanine were generally observed in ILs based on d-amino acids rather than in those derived from l-isomers. The reason could be that d-amino acids are more kosmotropic than l-isomers. Meanwhile, the IL–D 2O solution was able to further enhance the enzymatic resolution, when comparing with that in an IL–H 2O system.

Enantioselective hydrogenation of N-acetyl dehydrophenylalanine methyl ester using cinchonine-modified Pd/TiO 2 catalysts

The enantioselective hydrogenation of N-acetyl dehydrophenylalanine methyl ester (NADPME) to N-acetyl phenylalanine methyl ester is investigated using cinchonine-modified Pd/TiO 2 catalysts. The catalysts were prepared using deposition–reduction and wet impregnation methods and were evaluated for reaction using methanol as solvent with various cinchonine/NADPME molar ratios. Enantioselectivity was sensitive to this ratio and comparison with Pd/γ-Al 2O 3 showed that the Pd/TiO 2 gave marginally higher enantioselectivity when tested under comparable conditions. The effect of Pd loading and dispersion was investigated and the maximum enantiomeric excess observed was dependent on the dispersion; the role of the Pd particle size is discussed in the context of cinchonine adsorption. The reactions were carried out in solvents (methanol and dimethyl formamide) and mixed solvents (methanol–water and dimethyl formamide–water); the enantiomeric excess was significantly enhanced using dimethyl formamide. The addition of water does not have a significant effect for the Pd/TiO 2 catalysts. In this paper we report the highest enantiomeric excess for the hydrogenation of a prochiral ester using an immobilised catalyst.

An improved synthesis of deuterated Schöllkopf’s bis-lactim ether and its use for the asymmetric synthesis of ( R)-[α- 2H]-phenylalanine methyl esters

Treatment of ( S)-3-isopropyl-2,5-dimethoxy-3,6-dihydropyrazine with trifluoroacetic acid in MeOD results in regioselective deuteration at its C 6-position affording its corresponding ( S)-[6- 2H 2]-isotopomer in excellent yield with no loss of stereochemical integrity at its C 3-stereocentre. The lithium aza-enolate of this deuterated chiral template has been alkylated with a range of substituted benzyl bromides to afford (3 S,6 R)-[6- 2H]-3-isopropyl-6-benzyl-bis-lactim ethers that were hydrolysed to afford their corresponding ( R)-[α- 2H]-phenylalanine methyl esters as hydrochloride salts in good yield.

Hofmeister series of ionic liquids: kosmotropic effect of ionic liquids on the enzymatic hydrolysis of enantiomeric phenylalanine methyl ester

The kinetic hydrolysis of enantiomeric phenylalanine methyl ester catalyzed by Bacillus licheniformis protease was performed in aqueous solutions of several hydrophilic ionic liquids (ILs). The protease enantioselectivity was found related to the kosmotropicity of individual cations and anions of ILs. The ion effectiveness in enhancing the enzyme enantioselectivity follows the Hofmeister series: kosmotropic anions and chaotropic cations stabilize the enzyme. In this application, the Hofmeister series of ILs was established in an order of decreasing effectiveness for anions: PO 4 3 - > citrate 3−, CH 3COO −, EtSO 4 - , CF 3COO − > Br − > OTs −, BF 4 - and for cations: [EMIM] + > [BMIM] + > [HMIM] +. The overall IL kosmotropicity was quantified by the δ value (difference in the Jones–Dole viscosity B-coefficients of anion and cation). In general, a high enzyme enantioselectivity was observed in a solution of IL with a high δ value.

Recovery of pure products from ionic liquids using supercritical carbon dioxide as a co-solvent in extractions or as an anti-solvent in precipitations

In this paper two advanced methods for separation and purification of products from ionic liquids by using supercritical carbon dioxide as a co-solvent in extractions or as an anti-solvent in precipitations are demonstrated. As an example, the recovery of the product N-acetyl-(S)-phenylalanine methyl ester (APAM) from the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) was studied experimentally. APAM is the product of the asymmetric hydrogenation of methyl-(Z)-α-acetamidocinnamate (MAAC). For extraction of the product, the solubility of APAM in CO2 should be sufficiently high. This solubility is 1.78 g kg−1 at 12.0 MPa and 323 K, whereas [bmim][BF4] has a negligible solubility in scCO2. The extracted product was found to contain no detectible amount of ionic liquid. The solubility of the reactant MAAC in scCO2 is five times lower than the solubility of APAM, which means that a selectivity towards extraction of APAM exists. The product APAM was also precipitated out of the ionic liquid phase using scCO2 as an anti-solvent, enabled by the lower solubility of APAM in ionic liquid/scCO2 mixtures compared to the solubility in the pure ionic liquid at atmospheric conditions (650 g l−1). For example, the solubility of APAM in ionic liquid + scCO2 (1∶1.34 g g−1) at 313 K and 18.0 MPa is 162 g l−1. After precipitation the formed crystals can be washed using CO2 to obtain pure product.

Organopalladium Complexes 5-Amino-3H-1,3,4-thiadiazoline-2-thione as Metalloreceptor

Macrocycles containing two 5-amino-3H-1,3,4-thiadiazoline-2-thione 5 linking the 2- and 5-positions of the heterocycle unit and one 1,3-benzenedimethanethiol were prepared via the regiospecific S-alkylation of 5-amino-3H-1,3,4-thiadiazoline-2-thione. The 1,3-benzenedimethanethiol sites chelated to palladium metal ion to afford an organopalladium metalloreceptor. The structures of the macrocycles and metalloreceptors were established using 1 H and 13 C NMR, IR, MS spectrometry, and elemental analysis. The molecular recognition of the metalloreceptors (5a and 5b) was examined for some DNA/RNA nucleobases and some amino acid methyl ester by 'H NMR spectrometry. In case of 5a, the complexation ability increased in the order pyrazine / uracil /acetanilide < adenine < cytosine < phenylalanine methyl ester / tyrosine methyl ester. With 5b, the complexation ability also increased in the same order as 5a pyrazine / uracil <acetanilide <adenine < cytosine, however, the formation constants are larger than in case of 5a.

Unexpected Inversion in Enantioselectivity in the Hydrogenation N-acetyl Dehydrophenylalanine Methyl Ester using Cinchona-Modified Pd/Al2O3 catalyst

The enantioselective hydrogenation of N-acetyl dehydrophenylalanine methyl ester (NADPME) to N-acetyl phenylalanine methyl ester is investigated using cinchona-modified Pd/Al2O3 catalysts. The catalyst was prepared using deposition-reduction and was evaluated for the reaction using methanol as solvent with various cinchonine alkaloid/NADPME molar ratios. Enantioselectivity was sensitive to this ratio. For cinchonine at low cinchonine:NADPME molar ratios the S-N-acetyl phenylalanine methyl ester was formed with low enantioselection, and as the cinchonine:NADPME ratio was increased the reaction became less enantioselective. In the extreme the solubility of cinchonine limited the extent of the experimental conditions that could be explored. As expected cinchonidine modified Pd/Al2O3 initially gave R-N-acetyl phenylalanine methyl ester, again with low enantioselection. However, as the cinchonidine:NADPME molar ratio was increased the reaction initially became racemic and then was selective to the formation of S-N-acetyl phenylalanine methyl ester. This unexpected inversion in the sense of enantioselection was observed in a range of solvents.

Asymmetric conjugate reductions with samarium diiodide: asymmetric synthesis of (2S,3R)- and (2S,3S)-[2-2H,3-2H]-leucine-(S)-phenylalanine dipeptides and (2S,3R)-[2-2H,3-2H]-phenylalanine methyl ester

The highly diastereoselective samarium diiodide and D(2)O-promoted conjugate reduction of homochiral (E)- and (Z)-benzylidene and isobutylidene diketopiperazines (E)-5,7 and (Z)-6,8 has been demonstrated. This methodology allows the asymmetric synthesis of methyl (2S,3R)-dideuteriophenylalanine 27 in > or = 95% de and >98% ee, and (2S,3R)- or (2S,3S)-dideuterioleucine-(S)-phenylalanine dipeptides 37 and 38 in moderate de, 66% and 74% respectively. A mechanism is proposed to account for this process.

Mild, rapid and selective alcoholysis of terpyridine-appended amide substrates by Cu 2+-catalysis: protonation state and reactivity of the complex

An amide bond of a terpyridine-appended substrate, 6-(L-phenylalanylamino)-2,2′:6′,2″-terpyridine ( 2a), was cleaved to yield phenylalanine methyl ester quantitatively in the presence of catalytic amounts of Cu 2+ not only in methanol but also in aqueous methanol at 30 °C. The reaction proceeds via formation of an N 3O (three terpyridine nitrogens and one carbonyl oxygen) type 1:1 metal complex 2a-Cu 2+. From spectral titration, the structure of the 2a-Cu 2+ complex was confirmed to have three different protonation states, i.e., A (non-deprotonated amide with α-ammonium), D1 (deprotonated-amide with α-ammonium) and D2 (deprotonated-amide with α-amino) states. Among them, the complex in the D2 state was exclusively responsible for the observed mild, rapid and selective alcoholysis, showing the first-order rate constant of 6 × 10 −3 s −1 or half-decay time of 2 min in methanol at 30 °C. Acidity of the amide proton was found to be higher than that of the ammonium proton in the complex, allowing formation of the highly reactive amide-deprotonated D2 state in methanol even without addition of external bases. Factors contributing to the high reactivity of the complex were discussed.

Mechanism-based inactivation of human leukocyte elastase via an enzyme-induced sulfonamide fragmentation process

We describe herein the design and in vitro biochemical evaluation of a novel class of mechanism-based inhibitors of human leukocyte elastase (HLE) that inactivate the enzyme via an unprecedented enzyme-induced sulfonamide fragmentation cascade. The inhibitors incorporate in their structure an appropriately functionalized saccharin scaffold. Furthermore, the inactivation of the enzyme by these inhibitors was found to be time-dependent and to involve the active site. Biochemical, HPLC, and mass spectrometric studies show that the interaction of these inhibitors with HLE results in the formation of a stable acyl complex and is accompanied by the release of (L) phenylalanine methyl ester. The data are consistent with initial formation of a Michaelis–Menten complex and subsequent formation of a tetrahedral intermediate with the active site serine (Ser 195). Collapse of the tetrahedral intermediate with tandem fragmentation results in the formation of a highly reactive conjugated sulfonyl imine which can either react with water to form a stable acyl enzyme and/or undergo a Michael addition reaction with an active site nucleophilic residue (His 57). It is also demonstrated herein that this class of compounds can be used in the design of inhibitors of serine proteases having either a neutral or basic primary substrate specificity. Thus, the results suggest that these inhibitors constitute a potential general class of mechanism-based inhibitors of (chymo)trypsin-like serine proteases.

One‐Carbon Chain Extension of Esters to α‐Chloroketones: A Safer Route Without Diazomethane.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Metal Phthalocyanine and Crown Ether–Based Membranes for the Potentiometric Determination of Phenylalanine Methyl Ester Using an Ion-Selective Electrode

Tetracrown-substituted metal phthalocyanines and metal phthalocyanine–crown ether mixtures are used as ionophores for plasticized PVC membranes of ion-selective electrodes for the ditopic binding and potentiometric determination of phenylalanine methyl ester. The complexation of the reagent with phenylalanine methyl ester is confirmed by extraction data and absorption spectra. Ion-selective electrodes are proposed for the monitoring of phenylalanine methyl ester in the biocatalytic synthesis of aspartame.

Synthesis and evaluation of new fluorescent derivatization reagents for resolution of chiral amines by RP-HPLC

New fluorescent chiral derivatization reagents (i.e., DBD- trans-4-hydroxy- l-proline, DBD- cis-4-hydroxy- l-proline, DBD- cis-4-hydroxy- d-proline, and DBD- trans-3-hydroxy- l-proline) were synthesized from the reaction of 4-( N, N-dimethylaminosulfonyl)-7-fluoro-2,1,3-benzoxadiazole with corresponding hydroxy-prolines. These reagents reacted with chiral amine to produce a couple of diastereomers. The labeling efficiently proceeded in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and pyridine as the activation reagents. The reaction conditions are mild and no racemization occurred during both the reagent synthesis and the diastereomer formation (<0.4%). The resulting diastereomers fluoresce at around 560 nm (excitation at around 450 nm). Good linearity of the calibration curves was obtained in the range of 1–75 pmol and the detection limits on chromatogram were less than 1 pmol. The separability of the diastereomers was evaluated in terms of separation factor ( α) and resolution value (Rs). DBD- trans-4-hydroxy- l-proline was efficient for the resolution of dl-phenylalanine methylester; while DBD- cis-4-hydroxy- d(or l)-proline was excellent for the separation of 1-(1-naphtyl)ethylamines, as comparing with trans-4-hydroxy isomer. The reagents of cis-isomer seemed to be predominant for the resolution of hydrophobic enantiomers. On the other hand, trans-isomers were suitable for the separation of the racemic amines containing ester in the structure. With respect to the position of OH group, the effect seems to be less, judging from the results of DBD- trans-4-hydroxy- l-proline and DBD- trans-3-hydroxy- l-proline toward phenylalanine methylester. The results suggest that the separation is dependent upon both structures of the amines and the reagent used. Thus, the stereostructure, hydrophobicity and hydrogen bonding of the diastereomer, etc. seem to be affecting the separation.

One-carbon chain extension of esters to alpha-chloroketones: a safer route without diazomethane.

The reaction of a variety of methyl esters with dimethylsulfoxonium methylide at 0-25 degrees C affords the chain-extended beta-keto dimethylsulfoxonium ylides. Subsequent treatment with hydrogen chloride in THF proceeds with loss of DMSO to afford the corresponding alpha-chloroketones. This sequence has been utilized to convert the methyl esters of CBZ-protected alanine and valine to the anti N-protected alpha-amino epoxides, which are important pharmaceutical intermediates. When the same protocol is applied to BOC-protected phenylalanine methyl ester, epimerization occurs so that the use of a more reactive aryl ester is required. This chemistry provides a practical route to alpha-chloroketones that avoids the use of toxic and explosive diazomethane.

Monitoring the Intracellular Metabolism of Nucleoside Phosphoramidate Pronucleotides by 31P NMR †

The intracellular metabolism of 3′‐azido‐3′‐deoxythymidine (AZT)‐(L)‐tryptophan methyl ester phosphoramidate (L‐ATO) and AZT‐(L)‐phenylalanine methyl ester phosphoramidate (L‐APO) by the human T‐lymphoblastoid cell line CCRF‐CEM (CEM‐1.3) and peripheral blood mononuclear cell line (PBMC) was investigated with high field 31P NMR spectroscopy. The AZT amino acid phosphoramidates were shown to accumulate intracellularly and to be readily converted into AZT‐MP by both tissues types. Thus, the efficient delivery of nucleoside monophosphates to cells can be facilitated by nucleoside phosphoramidate pronucleotides. †In honor and celebration of the 70th birthday of Professor Leroy B. Townsend.

Synthesis and Resolution of Dinucleotide(TpAZT) Phosphoramidates.

AbstractFor Abstract see ChemInform Abstract in Full Text.