Oxidation of l-methionine with aqueous chromic acid: a kinetic study

The kinetics of oxidation of l-methionine by chromic acid in acidic medium (pH=0.83–2.2) has been studied spectrophotometrically. The effect of l-methionine and chromium(VI) concentrations on the rate of the reaction was determined. The reaction rate decreases with increasing the pH of the medium. The kinetics of the formation of a chromium(III) complex conform to the rate law:with k1=7.5×10–2s–1 and Kes1=43.85 at constant [H+]=1.9×10–2moldm–3 and [l-methionine]T= 2.0×10–2moldm–3. The same values were found with [l-methionine]T variation at constant [H+]. The reaction proceeds through formation of chromium(VI)-l-methionine ester in a rapid pre-equilibrium step, followed by a slower redox reaction of the ester. The present study provides kinetic evidence for formation of a complex ion (ester). One mole of Cr2O72– oxidizes four moles of l-methionine, which acts as a monodentate ligand and binds to chromium(VI) through the sulfur atom. The coordinated sulfur atom of l-methionine–chromium(VI) ester is responsible for the oxidative degradation (breaking of the C1-C2 bond) of l-methionine. Coordinated oxygen of the carboxylate group inhibits the cleavage of the C1-C2 bond.     

Caffeic acid phenethyl ester (CAPE): synthesis and X-ray crystallographic analysis

The structure of caffeic acid phenethyl ester [2-propenoic acid, 3-(3,4-dihydroxyphenyl)-, 2-phenethyl ester] (I), C17H16O4 x 1/2C6H6, synthesized by base-catalyzed alkylation of caffeic acid salt with beta-bromoethylbenzene in HMPA (hexamethylphosphoramide) and recrystallized from benzene, was confirmed by single crystal X-ray diffraction. The crystals are triclinic, space group P1, Z=2, unit cell dimension a=5.8129 (9) A, b=11.122 (2) A, c=13.226 (2) A, alpha=97.080 (3) degrees, beta=101.467 (3) degrees, gamma=95.405 (3) degrees , V=825.4 (2) A3, Dcalc=1.301 g/cm3, F(000)=342. The packing of the molecule is stabilized by intermolecular O1H...O4 (2.69 A) and O1...HO2 (2.82 A) hydrogen bonds.     

系列同三核铬、锰、铁羧酸配合物的FAB-MS研究

进行了系列同三核羧酸配合物〔Ｍ３Ｏ（Ｏ２ＣＲ）６Ｐｙ３〕Ｘ（Ｍ＝Ｃｒ,Ｍｎ,Ｆｅ;Ｒ＝ＣＨ３,Ｃ２Ｈ５,Ｃ６Ｈ５;Ｘ＝Ｃｌ－,ＣｌＯ４－;Ｐｙ为吡啶）的快原子轰击质谱（ＦＡＢ－ＭＳ）研究。获得了包括配位吡啶在内的完整阳离子峰。在研究其断裂规律时,主要观察到４个系列碎片离子：Ⅰ．〔Ｍ３Ｏ（Ｏ２ＣＲ）ｎ〕＋,ｎ＝６～２;Ⅱ．〔Ｍ３Ｏ（Ｏ２ＣＲ）ｎＯ〕＋,ｎ＝５～１;Ⅲ．〔Ｍ２Ｏ（Ｏ２ＣＲ）ｎ〕＋,ｎ＝３～１;Ⅳ．〔Ｍ２（Ｏ２ＣＲ）ｎ〕＋,ｎ＝４～２。通过对该系列配合物质谱断裂过程的比较和分析,获得了配合物稳定性随金属离子及配体的变化如下：金属离子,Ｃｒ＞Ｍｎ＞Ｆｅ;桥配基,－ＣＨ３ＣＯ２＞－Ｃ２Ｈ５ＣＯ２＞－Ｃ６Ｈ５ＣＯ２;端配基,Ｐｙ＞Ｈ２Ｏ。本研究及先前的工作〔１,８〕还为某些三核铬,铁羧酸配合物在以乙炔加水或加氢为探针反应中存在活性物种：〔Ｃｒ３Ｏ（Ｏ２ＣＲ）３～４〕,〔Ｆｅ３Ｏ（Ｏ２ＣＲ）３〕和〔Ｆｅ３Ｏ－（Ｏ２ＣＲ）Ｏ〕～〔Ｆｅ３Ｏ４〕提供了佐证     

三重桥氧三核铁脂肪酸配合物FAB和EI质谱研究

本文报道三重桥氧三核铁脂肪酸配合物[Fe_3(μ_3～O)(μ-O_2CR)_6(H_2O)]_3~+(R=CH_3,C_2H_5,C_3H_7,C_4H_9,C_(15)H_(31)和C_(17)H_(35))的快原子轰击(FAB)和电子轰击(EI)质谱。在FAB—MS谱中观察到四种系列离子:Ⅰ.[Fe_3O(O_2CR)_n]~+n=2-6;Ⅱ.[Fe_3O(O_2CR)_nO]~+n=1-4;Ⅲ.[Fe_2O(O_2CR)_n]~+n=1-3;Ⅳ.[Fe_2(O_2CR)_n]~+n=2-4;以及与底物所形成的加合离子[Fe_3O(O_2CR)_n·NBA-1]~+n=3-5(NBA是间-硝基苄醇)。在EI—MS谱中,除上述Ⅰ—Ⅳ离子(其中n的数目略有差异)外,还观察到含三个配位水的完整阳离子[Fe_3(O_2CR)_6(H_2O)_3]~+及其与配体羧酸的加合离子[Fe_3O(0_2CR)_n(H_20)_3·RCOOH+H]~+n=4-6,(Fe_3O(O_2CR)_nO(H_2O)3·RCOOH+H]~+n=2—4和Fe(O_2CR)_n]~+n=1—3。分析了它们的断裂规律,探讨了从中所获得的有益启示。     

B-失碳-3,5-环谷甾烷-6-羟基-6-甲酸及B-失碳-3,5-环谷甾烷-6-酮的合成

油菜甾醇内酯(brassinolide,1)是从油菜花粉中分离到的一种新型高效植物生长激素.自1979年确定其结构以来,一些实验室先后成功地进行了合成.尽管这些合成路线各有特点,但在B环内酯的建立上,大都采用了Baeyer-Villiger方法氧化6-酮化合物的路线;该方法在生成预期的B-高6-酮-7-氧化合物的同时,得到了约15％的7-酮-6-氧化合物异构体.     

Pd(II)-catalyzed vinylic polymerization of norbornene and copolymerization with norbornene and copolymerization with norbornene carboxylic acid esters

The vinylic polymerization of norbornene and its copolymerization with norbornene carboxylic acid methyl esters were investigated. Norbornene was polymerized by us using di-μ-chloro-bis-(6-methoxybicyclo[2.2.1]hept-2-ene-endo-5σ,2π)-palladium(II) as catalyst. The polymerization time can be decreased by a factor of 100000 by activation of the catalyst with methylaluminoxane (MAO). With this palladium catalyst activated by MAO, 140 t of norbornene can be polymerized per mol palladium per h. This catalyst system was much more active than [Pd(CH₃CN)₄](BF₄)₂ ( I ). The polymerization of norbornene by (6-methoxybicyclo[2.2.1]hept-2-ene-endo-5σ,2π)-palladium(II) tetrafluoroborate was also possible but it was not as fast as the polymerization by Pd catalysts activated with MAO. We were also able to obtain copolymers of norbornene and 5-norbornene-2-carboxylic acid methyl ester (exo/endo = 1/4 or 2/3) containing between 15 and 20 mol-% ester units. The copolymerization of norbornene and 2-methyl-5-norbornene-2-carboxylic acid methyl ester (exo/endo = 7/3) was faster than the copolymerization mentioned before. In contrast the homopolymerization of 2-methyl-5-norbornene-2-carboxylic acid methyl ester was 10 times slower than that of 5-norbornene-2-carboxylic acid methyl ester (exo/endo = 1/4).     

Reactive sulfur species: kinetics and mechanisms of the oxidation of cysteine by hypohalous acid to give cysteine sulfenic acid

Cysteine sulfenic acid has been generated in alkaline aqueous solution by oxidation of cysteine with hypohalous acid (HOX, X = Cl or Br). The kinetics and mechanisms of the oxidation reaction and the subsequent reactions of cysteine sulfenic acid have been studied by stopped-flow spectrophotometry between pH 10 and 14. Two reaction pathways were observed: (1) below pH 12, the condensation of two sulfenic acids to give cysteine thiosulfinate ester followed by the nucleophilic attack of cysteinate on cysteine thiosulfinate ester and (2) above pH 10, a pH-dependent fast equilibrium protonation of cysteine sulfenate that is followed by rate-limiting comproportionation of cysteine sulfenic acid with cysteinate to give cystine. The observation of the first reaction suggests that the condensation of cysteine sulfenic acid to give cysteine thiosulfinate ester can be competitive with the reaction of cysteine sulfenic acid with cysteine.     

Ab initio molecular orbitals study of the conformational preference in alpha-cyano-alpha-fluorophenylacetic acid ester

The origin of conformational preference in alpha-cyano-alpha-fluorophenylacetic acid (CFPA) methyl ester that is a model system of alpha-cyano-alpha-fluoro-p-tolylacetic acid (CFTA) esters was theoretically investigated by means of DFT and MP2 calculations. Two stable conformations having the C-F bond syn and anti to the C=O bond, respectively, were obtained for CFPA methyl ester. A small energy difference (0.9 kcal mol-1 at the MP2(fc)/6-31++G(d,p)) was found between the two conformations. From the molecular orbital analysis based on the Natural Bond Orbital analysis and supported by calculations using the Orbital Deletion Procedure technique, we found that sigma-(sigma*+pi*)(C=O) and sigma-sigma*(Ph) and pi(Ph)-sigma* hyperconjugations are the main factors responsible for the conformational preference. The role of the fluorine atom on the stereogenic center was also clarified.     

An unexpected epoxidation of benzil derivatives in their reaction with a germene

The germene Mes(2)Ge=CR(2) (Mes = 2,4,6-trimethylphenyl, CR(2) = fluorenylidene) reacts with various benzil derivatives to lead to germanium-containing bicyclic epoxides by an unexpected new type of epoxidation reaction.     

General base and general acid catalyzed intramolecular aminolysis of esters. Cyclization of esters of 2-aminomethylbenzoic acid to phthalimidine

Plots of log k(0) vs pH for the cyclization of trifluoroethyl and phenyl 2-aminomethylbenzoate to phthalimidine at 30 degrees C in H(2)O are linear with slopes of 1.0 at pH >3. The values of the second-order rate constants k(OH) for apparent OH(-) catalysis in the cyclization reactions are 1.7 x 10(5) and 5.7 x 10(7) M(-)(1) s(-)(1), respectively. These rate constants are 10(5)- and 10(7)-fold greater than for alkaline hydrolysis of trifluoroethyl and phenyl benzoate. The k(OH) for cyclization of the methyl ester is 7.2 x 10(3) M(-)(1) s(-)(1). Bimolecular general base catalysis occurs in the intramolecular nucleophilic reactions of the neutral species. The value of the Bronsted coefficient beta for the trifluoroethyl ester is 0.7. The rate-limiting step in the general base catalyzed reaction involves proton transfer in concert with leaving group departure. The mechanism involving rate-determining proton transfer exemplified by the methyl ester in this series (beta = 1.0) can then be considered a limiting case of the concerted mechanism. General acid catalysis of the neutral species reaction or a kinetic equivalent also occurs when the leaving group is good (pK(a) 相似文献   

Synthesis and structural characterization of M(PMe3)3(O2CR)2(OH2)H2 (M = Mo, W): aqua-hydride complexes of molybdenum and tungsten

Mo(PMe(3))(6) and W(PMe(3))(4)(eta(2)-CH(2)PMe(2))H undergo oxidative addition of the O-H bond of RCO(2)H to yield sequentially M(PMe(3))(4)(eta(2)-O(2)CR)H and M(PMe(3))(3)(eta(2)-O(2)CR)(eta(1)-O(2)CR)H(2) (M = Mo and R = Ph, Bu(t); M = W and R = Bu(t)). One of the oxygen donors of the bidentate carboxylate ligand may be displaced by H(2)O to give rare examples of aqua-dihydride complexes, M(PMe(3))(3)(eta(1)-O(2)CR)(2)(OH(2))H(2), in which the coordinated water molecule is hydrogen-bonded to both carboxylate ligands.     

Reaction of Carbodiimides with trifluoromethanesulfonic acid

Carbodiimides RN=C=NR (R = i-Pr, c-C₆H₁₁, Ph) react with trifluoromethanesulfonic acid with successive formation of O-triflyl isoureas RNHC(OTf)=NR, the isomeric N-triflyl ureas RN(Tf)C(O)NHR, and symmetrically substituted ureas RNHC(O)NHR. Carbodiimide Me₃SiN=C=NSiMe₃ in the reaction with TfOH undergoes desilylation to afford ester CF₃SO₂OSiMe₃ and unsubstituted carbodiimide (cyanamide), which gives the products of di- and trimerization and urea.     

On the isomerization of ent-kaurenic acid

Kaurenic acid (1a) is a tetracyclic diterpene that has an exocyclic double bond at delta16. Isokaurenic acid (2a) has an endocyclic delta15double bond. This compound has been isolated from Espeletia tenore (Espeletinae), a resinous plant from the Venezuelan Andes, but its occurrence is rare. In order to obtain a larger amount of 2a, the isomerization of la, which is easily obtained from other Espeletinae, was tried. Kaurenic acid methyl ester (1b) was treated with dil. HCl in CH3Cl/EtOH, after 6 h under reflux a yield of 41.5% isokaurenic acid methyl ester (2b) was obtained but 35.7% 16alpha-ethoxy-kauran-19-oic acid methyl ester (3b) had formed as a byproduct. Treating 1b with CF3COOH in refluxing CH2Cl2 permitted to obtain a yield of 66.6% of 2b in 4 h and only traces of 16alpha-hydroxy-kauran-19-oic acid methyl ester (3a) as a byproduct. Both isomers were separated on a silica gel column impregnated with 20% AgNO3. Treating 2b with KOH in refluxing DMSO yielded pure isokaurenic acid, no back isomerization was observed.     

Synthesis of boronic acid analogues of alpha-amino acids by introducing side chains as electrophiles

A synthetic route has been developed which has allowed us to prepare novel alpha-aminoboronic acids as inhibitors of serine proteases. These compounds were prepared to study the roles of proteases in biological systems. This methodology affords alpha-aminoboronic acids with the general formula R'-NHCH(R)BO(2)-pinanediol, where R = -CH(2)CHF(2), -CH(2)CO(2)tBu, and -(CH(2))(2)CO(2)Me and R' = either H or C(O)R". The latter two compounds are the boronic acid analogues of the natural amino acids aspartic acid and glutamic acid with the side chain carboxylate protected as a tert-butyl or a methyl ester, respectively. Following acylation of the amino group, the side chain tert-butyl ester of boroaspartic acid was removed by treatment with TFA. Boroglutamic acid was obtained as the free boronic acid by hydrolysis with HCl. Prior syntheses of alpha-aminoboronic acids involve the initial addition of an organometallic reagent to a trialkyl borate ester. These conditions do not allow the preparation of compounds with functionalities that are not stable to the strongly basic reaction conditions. The methodology described here allows the preparation of alpha-aminoboronic acids by introducing side chains as electrophiles. This is particularly advantageous for side chains which are prone to elimination or unwanted enolate formation. Specifically, BrCH(2)CHF(2), BrCH(2)COO(t)Bu, and CH(2)=CHCOOMe were allowed to react with the stabilized anion of (phenylthio)methane boronate, PhSCH(2)BO(2)C(6)H(12), to give the substituted boronate. The substituted (phenylthio)methane boronate was converted to the corresponding sulfonium ion by treatment with methyl iodide and subsequently displaced with iodide. The alpha-iodo derivative was converted to the amine by conventional methods.     

Kinetics and mechanism of oxidation with peroxides of ferrocenylacetic acid and methyl ferrocenylacetate

The reaction of ferrocenylacetic acid and its methyl ester with peroxides ROOR (R = H, t-Bu) in organic solvents was studied. It is shown that direct involvement of the substituent in this process leads to abnormally high reactivity of the acid toward H₂O₂ and t-BuOOH as compared with both the ester and ferrocene in a model system of Cp₂Fe + PhCOOH. An unusual shift of the absorption maximum of the ferrocenyl cation formed in the reaction course to longer wavelengths reaching up to several tens of nanometers was discovered.     

Synthesis and structure of new polynuclear cobalt(ii) complexes with 3,5-di-tert-butylbenzoic acid anions

Trinuclear complex Co₃(O₂CR)₆(EtOH)₂ (1) crystals were obtained in good yield (75%) by the exchange reaction between cobalt(II) chloride and potassium 3,5-di-tert-butylbenzoate (KO₂CR) in EtOH. Complex 1 undergoes hydrolysis during recrystallization from non-anhydrous benzene to form the hexanuclear hydroxocarboxylate complex Co₆(OH)₂(O₂CR)₁₀(HO₂CR)₄ (2).     

Thermal analysis of the double-melting behavior of poly(L-lactic acid)

The melting and crystallization behavior of poly(L -lactic acid) (PLLA; weight-average molecular weight = 3 × 10⁵) was studied with differential scanning calorimetry (DSC). DSC curves for PLLA samples were obtained at various cooling rates (CRs) from the melt (210 °C). The peak crystallization temperature and the exothermic heat of crystallization determined from the DSC curve decreased almost linearly with increasing log(CR). DSC melting curves for the melt-crystallized samples were obtained at various heating rates (HRs). The double-melting behavior was confirmed by the double endothermic peaks, a high-temperature peak (H) and a low-temperature peak (L), that appeared in the DSC curves at slow HRs for the samples prepared with a slow CR. Peak L increased with increasing HR, whereas peak H decreased. The peak melting temperatures of L and H [T_m(L) and T_m(H)] decreased linearly with log(HR). The appearance region of the double-melting peaks (L and H) was illustrated in a CR–HR map. Peak L decreased with increasing CR, whereas peak H increased. T_m(L) and T_m(H) decreased almost linearly with log(CR). The characteristics of the crystallization and double-melting behavior were explained by the slow rates of crystallization and recrystallization, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 25–32, 2004     

Synthesis and characterization of cationic heteronuclear complexes of platinum(II) and silver( I) bridged by alkynyl ligands

The dialkynyl complexes cis-[Pt(C CR)₂L₂] [R = Ph, L₂ = 2PPh₃, 2PEt₃, dppe (dppe = 1,2-bis(diphenylphosphino)ethane]; R ---^tBu, L₂ = 2PPh₃, dppe) react with silver perchlorate in a molar ratio 1:0.5 to give platinum-silver perchlorate salts of the type [Pt₂ Ag(C CR)₄L₄](ClO₄) in excellent yield. The X-ray crystal structure of [Pt₂Ag(C = CPh)₄(PPh₃)₄](ClO₄) 1 shows that the cation is formed by two nearly orthogonal cis-[Pt(C CPh)₂(PPh₃)₂] units connected through a silver cation which is unsymmetrically π-bonded to all four acetylene fragments. Similar reactions of cis-[Pt(C CR)₂L₂] with one equivalent of AgClO₄ afford cationic complexes of general formula [PtAg(C CR)₂L₂](ClO₄), which are believed to be salts, [Pt₂Ag₂(C CR)₄L₄](ClO₄)₂.     

Use of enantio-, chemo- and regioselectivity of acylase I. Resolution of polycarboxylic acid esters

Acylase I was used to catalyze the enantioselective butanolysis of trimethyl 2-[(carboxymethyl)oxy]succinate (E=30) and N-carboxymethylaspartate (E=9) exclusively at the most sterically hindered of the three ester groups (the position α to the asymmetric centre). Gram-scale resolution allowed the preparation of the less reactive trimethyl (S)-2-[(carboxymethyl)oxy]succinate (96% e.e.), that of the (R)-butyldimethyl regioisomer (78% e.e.) at 55% conversion and finally the preparation of the corresponding trisodium carboxylate by saponification. Acylase I was shown to transform (±)-methyl N-acetylmethionine and (±)-valine to the corresponding (S)-amino acids through ester hydrolysis-N-acetyl transfer sequence with absolute chemo- and enantioselectivity. Butanolysis of methyl N-acetylmethionine stopped in the formation of the butyl ester (E=12), the valine derivative being totally unreactive.     

Synthesis and single cell pharmacology of potential heterocyclic bioisosteres of the excitatory amino acid antagonist glutamic acid diethyl ester

A series of heterocyclic analogues of glutamic acid diethyl ester (GDEE), an antagonist at central excitatory amino acid receptors, have been synthesized and tested biologically. (RS)-Ethyl alpha-amino-alpha-(3-ethoxyisoxazol-5-yl)acetate (7), (RS)-ethyl 2-amino-3-(3-ethoxy-5-methylisoxazol-4-yl)propionate (16) and closely related analogues were synthesized. Compound 7, a diethyl derivative of the naturally occurring excitatory amino acid ibotenic acid (IBO), was synthesized from 3-hydroxy-5-methylisoxazole (1) via 3-ethoxyisoxazol-5-ylacetic acid (5) and its ethyl ester. Nitrosation of this ester followed by catalytic reduction gave 7. The ethyl ester of IBO, 9, was synthesized in a similar manner from 3-benzyloxyisoxazol-5-ylacetic acid (8). Ethyl derivatives of the synthetic excitatory amino acid 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) were synthesized from 3-hydroxy-4,5-dimethylisoxazole (10) through a diethyl acetylaminomalonate derivative, which upon deprotection gave the 3-ethoxy derivative of AMPA (15). Esterification of 15 gave the diethyl derivative 16 and the ethyl ester of AMPA (18) as well as N-ethylated derivatives of AMPA, 21 and 22 were synthesized. The final products were tested microelectrophoretically. The derivatives 7, 9, 15, 16 and 18 were weak and non-selective excitatory amino acid antagonists, whereas 21 and 22 were found to be inactive.