Syntheses of 2-arylimidazole derivatives through annelations employing benzylamines

Annelations of benzylamines with hydroximino derivatives of pentane-2,4-dione or of alkyl acetoacetates, acetoacetamides, and acetoacetanilides, afford imidazole derivatives and provide easy incorporation of the benzylic carbon and nitrogen in the heterocycle. The limits of the reaction have been explored.     

Oxidation by chlorine dioxide of methionine and cysteine derivatives to sulfoxides

Methionine and cysteine derivatives were oxidized asymmetrically by chlorine dioxide to sulfinyl derivatives. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 608–610, November–December, 2008.     

Chemical modification of dispersed silica by pyrimidine derivatives

Results are given of the chemical modification of silicas containing and groups on their surface by pyridimidines in the gaseous phase. IR spectroscopy and field desorption mass spectrometric methods were used to obtain information on the structure of the surface compounds prepared. Possible reaction schemes are suggested. The high hydrolytic stability of some of the immobilized hydroxypyrimidines has been established.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 4, pp. 495–499, July–August, 1986.     

Accessing the disallowed conformations of peptides employing amide-to-imidate modification

Selective modification of the C-terminal amide in peptides to dihydrooxazine (a novel stable imidate isostere) by intramolecular nucleophilic cyclo-O-alkylation of the corresponding N-(3-bromopropyl)amides results in constraining of the C-terminal residue in natively disallowed conformations both in crystals and in solution.     

The reduction of oxidized methionine residues in peptide thioesters with NH4I-Me2S

Oxidized methionine residues in peptide thioesters can be reduced rapidly with NH4I to the corresponding sulfide by using Me2S as coreductant. Comparative reduction studies employing a 28-amino acid peptide thioester with an N-terminal methionine oxide as model system revealed the importance of the Me2S addition to avoid hydrolysis of the reactive thioester functionality. In addition, an NH4I-Me2S containing cleavage cocktail has been used for the global deprotection of various thioesters which revealed no hydrolysis or oxidative side products. These results demonstrate the general applicability of sulfoxides as protecting groups in advanced peptide synthesis techniques by facilitating the preparation and handling of methionine containing peptide thioesters for native chemical ligation (NCL).     

Enhancement in the rate of conversion of peptide Cys-Pro esters to peptide thioesters by structural modification

We previously reported that the peptide containing a Cys-Pro ester (CPE) moiety is spontaneously transformed into a peptide thioester via an N to S acyl shift followed by diketopiperazine formation. In an attempt to identify more reactive structures for the formation of a peptide thioester, we modified the CPE structure, in which the Pro residue in the CPE moiety was replaced with N-substituted glycine derivatives. These peptides were transformed into a peptide thioester more rapidly. Alternatively, the addition of an amino acid residue at the C-terminus of the CPE moiety also accelerated thioester formation.     

Racemization of an asparagine residue during peptide deamidation

The Asn residue in the pentapeptide Asn-Gln-Asn-Glu-Gly undergoes racemization at the Calpha center in the course of deamidation of this residue through a succinimide intermediate. The succinimide intermediate is known to racemize at the corresponding center, leading to racemized products of deamidation. Return of this intermediate to reactant Asn is very unlikely in dilute solution where attack of product ammonia on the succinimide is precluded, and Asn racemization has not been previously observed. We give evidence that the observed racemization occurs at the tetrahedral-intermediate stage preceding the succinimide intermediate. The observation is significant for protein stability in vivo and in vitro and has importance in medicine, food chemistry, and archaeological/palaeological dating.     

Chemoselective and enantioselective oxidation of indoles employing aspartyl peptide catalysts

Catalytic enantioselective indole oxidation is a process of particular relevance to the chemistry of complex alkaloids, as it has been implicated in their biosynthesis. In the context of synthetic methodology, catalytic enantioselective indole oxidation allows a rapid and biomimetic entry into several classes of alkaloid natural products. Despite this potentially high utility in the total synthesis, reports of catalytic enantioselective indole oxidation remain sparse. Here we report a highly chemoselective catalytic system for the indole oxidation that delivers 3-hydroxy-indolenines with good chemical yields and moderate to high levels of enantio- and diastereoselectivity (up to 95:5 er and up to 92:8 dr). These results represent, to our knowledge, the most selective values yet reported in the literature for catalytic asymmetric indole oxidation. Furthermore, the utility of enantioenriched hydroxy-indolenines in stereospecific rearrangements is demonstrated.     

紫草萘醌类化合物对乳酸脱氢酶和乙醇脱氢酶的共价修 饰作用

为研究紫草萘醌类化合物的细胞毒性作用机制,从新疆软紫草根中分离出四种紫草萘醌类化合物β,β-二甲基丙烯酰阿卡宁(1),乙酰阿卡宁(2),β-乙酰氧基异戊酰阿卡宁(3)和阿卡宁(4)。研究了四种天然紫草萘醌类化合物对乳酸脱氢酶和乙醇脱氢酶的共价修饰作用。酶活力测定结果表明,这四种紫草萘醌类化合物对两种脱氢酶都具有不同程度的抑制作用;酶分子中游离氨基和巯基修饰率的测定结果表明,紫草萘醌类化合物对两种酶的抑制作用主要是通过与酶分子中的巯基共价结合产生的。     

Photosensitized oxidation of methionine derivatives. Laser flash photolysis studies

The early events in the triplet 4-carboxybenzophenone (CB)-induced oxidation of N-acetyl-methionine methyl ester (N-Ac-Met-OCH₃) are investigated in aqueous solution. Upon electron transfer from the methionine residue of N-Ac-Met-OCH₃ to ³CB*, the resulting sulfur radical cation undergoes further reactions: (1) back electron transfer, (2) escape of the radical ions from the solvent cage, or (3) proton transfer and escape of the radicals. The yields and paths of these reactions are shown to depend strongly on the pH of the solution, and, similar to the previously reported results for dipeptides (Met-Gly and Gly-Met), on the structural nature of the methionine substituents. In the experiments performed in this work, low quencher concentrations were used to avoid formation of intermolecular transients (e.g., dimeric sulfur-centered radical cation (S∴S)⁺). Under these experimental conditions, the one-electron oxidized sulfur does not seem to become stabilized in an (S∴N)⁺ three-electron bonded intramolecular complex. The proposed mechanism is further supported by the stable products analysis. A detailed mechanism involving characterization of the transients is discussed and compared to that of methionine and methionine-containing dipeptides (Met-Gly and Gly-Met). Moreover, a newly installed transient absorption laser system is described in details.     

Synthesis of formyl and carboxy derivatives of heterocycles by modification of 2-(2-aminovinyl)- benzofuran-5,6-dicarbonitriles

Procedures have been developed for the synthesis of 2-formylbenzofuran-5,6-dicarbonitriles, 5,6-dicyanobenzofuran-2-carboxylic acids, and formyldibenzo[b,d]furan-2,3-dicarbonitriles by modification of 2-(2-aminovinyl)benzofuran-5,6-dicarbonitriles with sodium periodate or Vilsmeier reagent.     

Improved deprotection in solid phase peptide synthesis: quantitative reduction of methionine sulfoxide to methionine during HF cleavage

The reduction of methionine sulfoxide residues in peptides was found to be quantitative by treatment with a low concentration of HF in dimethylsulfide (HF:dimethylsulfide, 1:3, v/v). The reduction was dependent on the acidity function of HF and was optimal with an HF concentration between 25% and 40%. The low HF concentration cleavage reagent also deprotected most of the benzyl alcohol-derived protecting groups.     

Surface modification of synthetic and natural fibres by fixation of cyclodextrin derivatives

It is demonstrated that cyclodextrin derivatives can be fixed permanently onto polymer surfaces by functional groups using conventional technologies of textile processing. As a result of the modification process some properties of the fibres are directly influenced. But also special effects may be obtained by complexing specific chemical substances, because the fixed cyclodextrin cavities do not loose their complexing power. Conceivable fields of application are medical or technical textiles and textiles for clothing.