Effect of asymmetry on the electronic delocalization in diiron and iron-cobalt mixed valence metallocenic compounds

In this work, we report the synthesis and a study on the degree of electronic delocalization in the asymmetric mixed valence complexes [CpFe(C(8)H(6))Fe(C(8)H(7))](+), 3a(+), and [CpCo(C(8)H(6))Fe(C(8)H(7))](+), 3b(+), (Cp = C(5)Me(5), C(8)H(6) = pentalenyde, C(8)H(7) = hydropentalenyde, and = ((3,5(CF(3))(2)C(6)H(3))(4)B(-))). Electrochemical methods, (57)Fe M?ssbauer spectroscopy, electronic spectroscopy, and electron paramagnetic resonance were used for this purpose. Although the anti conformation of the complexes precludes any metal-metal interaction, all the techniques employed show that 3a(+) is a electronic delocalized system, while 3b(+) behaves as two individual metallic centers with localized electron density.     

Elaboration and characterization of hybrid organic-inorganic gels obtained by reaction of 1,4-butanediol on tetramethoxysilane

New hybrid organic-inorganic gels have been obtained by reaction of 1,4-butanediol, on tetramethoxysilane Si(OMe)₄ dissolved in CCl₄. This reaction does not require water and leads to the formation of polymeric transparent materials.Infrared, ²⁹Si and ¹³C NMR spectroscopy shows that interchange reactions between OMe groups of alkoxide and -O-(CH₂)₄-O of 1,4-butanediol occurred, leading to the monolithic transparent gels in which both organic (Si-O-(CH₂)₄-O-Si) and inorganic (Si-O-Si) bridges are formed.     

Enzymatic desymmetrization of a meso polyol corresponding to the C(19)-C(27) segment of rifamycin S

The stereoselective acylation of meso polyol 2 by vinyl acetate (solvent and acyl donor) in the presence of porcine pancreas lipase gave the corresponding monoester 5 in good yield (76%) and high enantiomeric purity (ee > 98%). The enzymatic reaction was also highly regioselective for a primary alcohol end group, and the two unprotected secondary alcohols were not involved. Compound 5 corresponds to the C(19)-C(27) fragment of rifamycin S.     

Methylbis (dithiocarbamato)stibane Synthese und Kristallstruktur

Methylbis(dithicarbamato) stibanes. Synthesis and Crystal Structure . The title compounds are obtained by reaction of methyldibromostibane with the sodium dithiocarbamates in methanol at low temperature (?78°C). The crystal structure of CH₃Sb(S₂CNEt₂)₂ as elucidated by X-ray diffraction shows no special intermolecular interactions. It can be compared to that of the corresponding bismutane CH₃Bi(S₂CNEt₂)₂ and the arsane C₆H₅As(S₂CNEt₂)₂.     

Tetramethyldisilan-1,2-diyl-verbrückte Dicyclopentadienyl-und Diindenylmetalldichloride der 4. Gruppe – Kristallstruktur von

Tetramethyldisilane-1,2-diyl bridged Dicyclopentadienyl and Diindenyl Metal Dichlorides of the Group 4 Metals – Crystal Structure of Dicyclopentadienyl and diindenyl metal dichlorides of the type Cp′? SiMe₂SiMe₂? Cp′MCl₂ (Cp′ = C₅H₄, M = Ti ( 1 ), Zr ( 2 ), Hf ( 3 ); Cp′ = C₉H₇, M = Zr ( 4 ), Hf ( 5 )) were synthesized and characterized by means of their ¹H, ¹³C, ²⁹Si-n.m.r., MS, and IR spectra. The crystal structure of 2 was determined.     

Synthesis and structure of 5-methyl-1-(2-R-phenyl)-dihydro-2,4(1H,3H)-pyrimidinediones and their 2-thiono analogs

The reaction of o-substituted aromatic amines with methacrylic acid gave N-aryl--methyl--alanines, which were converted to dihydro-2,4-pyrimidinedione and dihydro-4-pyrimidinone-2-thione derivatives. The alkylation, acylation, and oximation of the dihydro-2,4-pyrimidinediones were accomplished. Conformational analysis of the compounds obtained was carried out by dynamic NMR methods.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1240–1247, September, 1991.     

Tautomerism and stereochemistry of dihydroxyperylenequinones: Force field investigations

Summary Stereochemistry and tautomerism of cercosporin and several of its partial structure models were investigated using an MM2 derived force field method. Besides the propeller type conformer, which was found before by X-ray crystallography, the complicated energy hypersurface was shown to contain a novel double-butterfly conformer of similar stability. The interconversion barrier between these conformers and their enantiomers was found to be unusually high due to buttressing effects of neighbor substituents. Judged from the calculations, the 4,9-tautomer of cercosporin could also be present in favoring instances besides the 3,10-tautomer, whereas the 3,9-tautomer is strongly destabilized.

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Tautomerie und Stereochemie von Dihydroxyperylenchinonen: Untersuchungen mit Hilfe der Kraftfeld-Methodik

Zusammenfassung Die Stereochemie von Cercosporin und einigen seiner Partialstrukturmodelle wurde Hilfe einer von MM2 abgeleiteten Kraftfeldmethodik untersucht. In der komplizierten Energiehyperfläche wurde neben dem aus der Röntgenstrukturanalyse bekannten Propeller-Konformeren auch ein neues Doppelschmetterling-Konformer ähnlicher Stabilität aufgefunden. Die Interkonversionsbarrieren zwischen diesen Konformeren und ihren Enantiomeren sind außergewöhnlich hoch, was auf einen Buttressing-Effekt der benachbarten Substituenten zurückgeführt wurde. Aus den Rechnungen folgte auch, daß neben dem 3,10-Tautomeren in günstigen Fällen auch das 4,9-Tautomere des Cercosporins vorliegen kann, wogegen das 3,9-Tautomere stark destabilisiert ist.

     

Metal island coated polymer sensor for direct determination of the volume effect of chaotropic agents

A new optical sensor is presented, based on the analyte reaction resulting in swelling and shrinking of a thin polymer layer. Changing the concentration of ions in a new bisazide photo-cross-linked poly(vinylpyrrolidone) polymer results in a concentration-dependent volume change of the hydrated gel. The volume response of the sensor induced by different ions is fully reversible over more than 250 cycles. The response of the device depends on the type, the charge and the concentration of the ions. The sensor material is part of an optical thin film system which transforms the variations in volume of the polymer into spectral information. The steady state of the sensor response is obtained within 60 s. The response time is mainly limited by the pump rate, the back pressure and the total volume of the system but not by the swelling of the sensor polymer. A comparative study of ion effects has demonstrated a fundamental correlation of the polymer swelling properties with the Hofmeister series of chaotropic agents. Thus it is concluded that the photopolymer, which is solubilized in aqueous solutions by the interaction of its amide structure with the solvent, behaves like the backbone amide structure of proteins.     

pH-induced destabilization of lipid bilayers by a peptide from the VP3 protein of the capsid of hepatitis A virus.

The membrane destabilizing and fusogenic properties of the synthetic peptide VP3(110-121), corresponding to an immunogenic sequence of the hepatitis A virus (HAV) VP3 capsid protein, were studied. By tryptophan fluorescence and acryalmide quenching it was demonstrated that the peptide binds liposomes of POPC-SM-DPPE (47 + 39 + 14) and POPC-SM-DPPE-DOTAP (40 + 33 + 12 + 15) and penetrates the membrane, at both neutral and acidic pH (POPC = 1-palmitoyl-2-oleoylglycero-sn-3-phosphocholine; SM = sphingomyelin; DPPE = 1,2-dipalmitoylphosphatidylethanolamine; DOTAP = 1,2-dioleoyl-3-trimethylammoniumpropane). VP3(110-121) did not have membrane-destabilizing properties at neutral pH. Acid-induced destabilization of the vesicles was demonstrated by fluorescence techniques and dynamic light scattering. VP3(110-121) induced aggregation of POPC-SM-DPPE-DOTAP (40 + 33 + 12 + 15) vesicles, lipid mixing and leakage of vesicle contents, all consistent with fusion of vesicles. In POPC-SM-DPPE (47 + 39 + 14) vesicles, at acidic pH, VP3(110-121) induced membrane destabilization with leakage of contents but without aggregation of vesicles or lipid mixing. The peptide only showed fusogenic properties when bound to the vesicles at neutral pH before acidification to pH below 6.0, and no effect was seen if the peptide was added to vesicles already set at acidic pH. These results may have physiological significance in the mechanism of infection of host hepatic cells by HAV.     

An unexpected Pummerer rearrangement in the synthetic route to ethyl (2′-hydroxy-4′,5′-methylenedioxyphenyl)acetate: An alternative approach to 2,3-dimethylthio benzofurans

The synthesis of ethyl (2′-hydroxy-4′,5′-methylendioxophenyl)acetate, a fragment of the antihyperglycemic natural coumarin subcoriacin, is reported. We found an expeditious route to the title compound in five steps. Final metal catalyzed acid ethanolysis of the vinylic 1,1-methylthio methylsulfoxide derivative afforded the required aryl acetic ester, but in the absence of metal catalyst, an unexpected Pummerer rearrangement produced the 2,3-dimethylthiofuran derivative as the major product. This last result provides an alternative entry to 2,3-dimethlythiobenzofurans.