Electrochemical approach to the repair of oxetanes mimicking DNA (6-4) photoproducts

Electrochemical study of oxetanes mimicking DNA (6-4) photoproducts gives new insight into the repair mechanism by (6-4) photolyase. Both electrochemical oxidation and electrochemical reduction at carbon electrodes lead to the cleavage of the oxetanes in a retro-Paterno-Büchi sequence. Within the family of compounds investigated and the range of driving forces offered, transient formation of unstable radical ions is observed, for both oxidative and reductive cleavage. Taking advantage of the electrochemical signature of these mimics, enzymatic assay with Escherichia coli CPD photolyase coupled to electrochemical monitoring of the reaction brings evidence that enzymatic repair of (6-4) DNA photoproducts does involve a catalytic dissociative electron-transfer mechanism at the level of an oxetane intermediate.     

Contribution of the ligand to the electroreduction of CO2 catalyzed by a cobalt(II) macrocyclic complex

The electrochemical reduction of carbon dioxide using hexa-aza-macrocycles derived from the condensation of 1,10-phenanthroline and its Co(II) complex as an electrocatalyst dissolved in dimethylformamide has been studied by cyclic voltammetry and UV-visible spectroscopy. The ligand does not show catalytic activity and only generates hydrogen when it is reduced under carbon dioxide. The cobalt complex shows electrocatalytic activity toward the reduction of carbon dioxide, generating carbon monoxide and formic acid. Cyclic voltammetry and UV-visible spectroscopy show that the active site for the reduction is the metal center in oxidation state (I), although the reduced cobalt center alone is not enough to promote reduction of the carbon dioxide. Electrolysis at controlled potential shows that only at potentials corresponding to reduction of the ligand (second reduction) does carbon dioxide reduction occur. Cobalt(I) probably reacts with CO₂ forming a non-isolated intermediate which, when reduced, gives CO and formic acid. The second reduction that takes place on the ligand regenerates the catalyst and gives products, thus becoming the rate-determining step of the reaction.     

钴(II)离子阴极还原的研究

研究Co~(2+)离子的阴极还原,主要是由于钴的湿法冶金和电镀生产的需要,以及理论研究上的兴趣.研究表明,钴离子的阴极还原过程是复杂的,很大程度上取决于电解液的酸度.Simonova 和Rotinyan 根据pH=2.5-4的实验数据,提出了二电子还原和分步还原平行进行的钴还原机理,用以解释85—150mV 的塔费尔斜率,这实际上反映了酸度对钴还原的影响.Epelboin 和Wiart 根据阴极极化特征和阻抗分析,认为Co~(2+)离子还原时可能     

The synthesis of 2-(alkylthio)- and 2-(arylthio)-3-cyanothiophenes. The nucleophilic displacement of the alkylsulfinyl group by thiols.

The nucleophilic substitution of activated 2-(alkylsulfinyl)thiophenes with alkyl- or arylmercaptans gives the corresponding 2-(alkylthio)- or 2-(arylthio)- substituted thiophenes. When thiolate anion is used instead of thiol, sulfoxide reduction is the main reaction.     

Enantioselective total synthesis of the potent antitumor agent (-)-mucocin using a temporary silicon-tethered ring-closing metathesis cross-coupling reaction

The enantioselective total synthesis of the annonaceous acetogenin (-)-mucocin (1) was accomplished using a triply convergent 12-step sequence (longest linear sequence) in 13.6% overall yield. This represents the first application of the temporary silicon-tethered (TST) ring-closing metathesis (RCM) cross-coupling reaction and the enantioselective alkyne/aldehyde addition to the synthesis of a complex annonaceous acetogenin. Moreover, all three fragments required for the coupling reactions are conveniently prepared in 5-6 steps from two readily available enantiomerically enriched epoxides. Finally, this synthesis stimulated the development of a new approach for the construction of 3-hydroxy-2,6-disubstituted tetrahydropyrans, using the bismuth tribromide-mediated reductive etherification reaction, which represents a motif that is prevalent in a wide range of pharmacologically significant natural products.     

Synthesis of (E)-2-chlorovinyltellurium trichloride and (E,E)-bis(2-chlorovinyl) ditelluride

The reaction of tellurium tetrachloride with acetylene in CCl₄ at atmospheric pressure and ambient temperature affords earlier unknown (E)-2-chlorovinyltellurium trichloride in 30% yield, whose reduction with sodium bisulfite gives (E,E)-bis(2-chlorovinyl) ditelluride in 64% yield.     

An approach toward the alkaloid (±)-mersicarpine using a rhodium(II) carbenoid cyclization-cycloaddition cascade of an α-diazo dihydroindolinone

A tandem carbonyl ylide/1,3-dipolar cycloaddition cascade of α-diazo indole-2,3-dione with several different dipolarophiles was investigated. The intermolecular Rh(II)-catalyzed reaction occurs efficiently and affords dipolar cycloadducts in high yields. The analogous intramolecular reaction also takes place and gives an azapolycyclic product derived from trapping of the carbonyl ylide dipole with a tethered alkene. The power of the intramolecular cascade sequence is that it rapidly assembles polycyclic ring systems containing both multiple stereocenters and adjacent quaternary carbon centers in a single step in high yield. This cascade reaction was successfully utilized in a model study directed toward the total synthesis of mersicarpine.     

Differential determination of antimony(III) and antimony(V) by indirect spectrophotometry with chromium(VI) and diphenylcarbazide,after reduction of antimony(V)

Antimony(III) is determined indirectly through its reaction with excess of chromium(VI), the excess being quantified with diphenylcarbazide and measurement at 540 nm. Antimony(V) is reduced to antimony(III) with sodium sulfite in hydrochloric acid solution; excess of sulfite is eliminated by boiling. The subsequent determination of antimony(III) gives the concentration of total antimony, and antimony(V) is found from the difference between the results before and after reduction. Antimony in its different oxidation states can be determined in the range 0.04–0.7 mg l^?1 within an error of about 10%.     

Syntheses of new poly (arylene vinylene) analogues: Poly (4,7-benzofuran vinylene) and poly (4,7-benzothiophene vinylene)

The reaction of bis(4,7-tetrahydrothiopheniomethyl) benzofuran dibromide with aqueous tetramethylammonium hydroxide leads to a water-soluble polyelectrolyte which can be film cast and thermolytically eliminated to give poly(4,7-benzofuran vinylene) (PBFV). Subjection of bis(4,7-tetrahydrothiopheniomethyl) benzothiophene dibromide to the same reaction sequence gives poly(4,7-benzothiophene vinylene) (PBTV). UV-VIS studies show that PBFV has a band gap of 2.76 eV, while PBTV has a band gap of 2.92 eV. These polymers are members of a new class of conjugated poly (arylene vinylene)s, in which heterocyclic pseudoaromatic rings are fused onto a poly(1,4-phenylene vinylene) backbone. © 1994 John Wiley & Sons, Inc.     

Ruthenium(III)-catalyzed oxidation of thallium(i) by 12-tungstocobaltate(III) in hydrochloric acid medium

The reaction between thallium(I) and [Co^IIIW₁₂O₄₀]^5- in the presence of ruthenium(III) as catalyst proceeds viainitial outer-sphere oxidation of the catalyst to ruthenium(VI). The ruthenium(IV) thus generated will oxidize thallium(I) to an unstable thallium(II) which by reacting with oxidant gives the final product, thallium(III). The formation of ruthenium(II) by direct two-electron reduction of the catalyst by thallium(I) is thermodynamically less favorable. The reaction rate is unaffected by the [ H⁺ ], whereas it is catalyzed by chloride ion . The formation of reactive chlorocomplex,TlCl, in a prior equilibrium is the reason for the chloride ion catalysis. Increasing the relative permittivity of the medium increases the rate of the reaction, which is attributed to the formation of an outer-sphere complex between the catalyst and oxidant. This revised version was published online in June 2006 with corrections to the Cover Date.     

Two successive one-pot reactions leading to the expeditious synthesis of (−)-centrolobine

A very short and efficient synthesis of (−)-centrolobine has been achieved by using two successive one-pot reactions. The first sequence involves a cross-metathesis reaction/hydrogenation/lactonization and the second sequence is a Grignard addition/reduction.     

Enantioselective synthesis of (-)-jiadifenin, a potent neurotrophic modulator

The first enantioselective synthesis of (-)-jiadifenin (1), a potent neurite outgrowth promoter isolated from the Illicium species, is described. The synthetic strategy builds upon bicyclic motif 6, which represents the AB ring of the natural product and proceeds in 19 steps and 1.1% overall yield. Key to our approach is a Mn(III)-mediated oxidation reaction of A ring that, following a regio- and diastereoselective α-hydroxylation and methylation sequence, produces the desired functionalities of (-)-jiadifenin. The effect of synthetic 1 in NGF-mediated neurite outgrowth was also measured in PC-12 cells.     

Application of a new tandem isomerization-aldolization reaction of allylic alcohols to the synthesis of three diastereoisomers of (2R)-1,2-O-isopropylidene-4-methylpentane-1,2,3,5-tetraol

The tandem isomerization-aldolization reaction of (2R)-1,2-O-isopropylidene-4-penten-1,2,3-triol 3 and formaldehyde gives a mixture of two aldol products 2a and 2b. The stereoselective reduction of each compound by l-Selectride affords two diastereoisomers of (2R)-1,2-O-Isopropylidene-4-methylpentane-1,2,3,5-tetraol while a third diastereoisomer is obtained by stereoselective reduction with Me(4)NHB(OAc)(3).     

Preparation and properties of the aqua ions [W(4)S(4)(H(2)O)(12)](n+) (n = 5, 6) and crystal structure of (Me(2)NH(2))(6)[W(4)S(4)(NCS)(12)].0.5H(2)O

The [3 + 1] reaction of [W(3)S(4)(H(2)O)(9)](4+) with [W(CO)(6)] in 2 M HCl under hydrothermal conditions (130 degrees C) gives the [W(4)S(4)(H(2)O)(12)](6+) cuboidal cluster, reduction potential 35 mV vs NHE (6+/5+ couple). The reduced form is obtained by controlled potential electrolysis. X-ray crystal structure was determined for (Me(2)NH(2))(6)[W(4)S(4)(NCS)(12)].0.5H(2)O. The W-W and W-S bond lengths are 2.840 and 2.379 A, respectively.     

Synthesis of 1-substituted (S)-4-acetoxy-1-penten-3-ones using (S)-lactic acid as a chiral source and their synthetic reactions

Ylide reaction of (S)-3-acetoxy-1-triphenylphosphoranylidene-2-butanone, derived from (S)-lactic acid, with aldehydes gives 1-substituted (S)-4-acetoxy-1-penten-3-one derivatives, which are shown to be transformed into useful chiral compounds by stereoselective reduction or the Johnson-Claisen rearrangement.     

Stabilization of high-valent metals by corroles: oxo[tris(pentafluorophenyl)corrolato]chromium(V)

The aerobic reaction of Cr(CO)6 with tris(pentafluorophenyl)corrole (H3(TpFPC)) in toluene gives the dark red oxochromium(V) compound (TpFPC)Cr(O), which has been characterized by X-ray crystallography, electrochemistry, and EPR spectroscopy. Short Cr-N (1.927-1.943 A) bonds as well as relatively large 14N and small 53Cr coupling constants suggest that sigma (N-->Cr) donation is responsible for the unusual stability of chromium(V) in this complex. The CrV/IV reduction potential (0.11 V vs Ag/AgCl) is 0.65 V below that of oxo(tetramesitylporphinato)chromium(V).     

A practical synthesis of (1S,4S)-2,5-diazabicyclo[2.2.1]heptane

The rigid piperazine homologue 2,5-diazabicyclo[2.2.1]heptane (DBH) finds extensive application in medicinal chemistry and pharmaceutical research, but access to this scaffold is non-trivial. This letter details a concise synthetic sequence that gives ready access to DBH on gram scale. The route features a Staudinger reduction of an azide to facilitate a transannular cyclisation     

高效亲和色谱研究含有特定结构域肽与DNA的结合

高效亲和色谱研究含有特定结构域肽与ＤＮＡ的结合张若蘅,杨栩,李崇熙,徐筱杰（北京大学化学系北京１００８７１）１前言在脱氧核糖核酸（ＤＮＡ）结合蛋白质中,除了螺旋－转折。螺旋、锌指和亮氨酸拉链三种主要结构域之外￣［１］,近年来,一类含有β－转折结构的Ｓ...     

New approach to Ru(II) pincer ligand chemistry. Bis(tert-butylaminomethyl)pyridine coordinated to ruthenium(II)

Reaction of 2,6-bis-(tBuNHCH2)2NC5H3 ("N2py") with RuCl2(PPh3)3 gives two isomers of Ru(N2py)Cl2(PPh3), 5, while reaction with RuCl2(DMSO)4 (DMSO = Me2SO) gives isomerically pure Ru(N2py)Cl2(DMSO), whose structure is reported. The PPh3 of 5 can be replaced by CO, P(OPh)3, or pyridine. The chlorides in Ru(N2py)Cl2(CO) can both be replaced by F3CSO3-. Isomer structure preferences are discussed, and the reaction of Ru(N2py)Cl2(pyridine) with O2 gives apparent oxidation of N2py to give the diimine.