Optical resolution of 7-oxabicyclo[2.2.1]hept-21-ene derivatives. Diastereoselectivity in the formation of cyanohydrine-brucine complexes

A new, practical method for the optical resolution of bicyclic ketones if illusttrated by the preparation of (+)-(1R,4R)-7-oxabicyclo[2.2.1]bept-5-en-2-one ((+)- 1 ) and (+)-(1R, 2S,4R)-2-cyano-7-oxabicyclo[2.2.1]hept-5-en-2-yul acetate ((+)- 4 ). It involves the diastereoselective formation of a brucine complex with the corresponding cyanhydrine mixture.     

High yielding synthesis of 3a-hydroxypyrrolo[2,3-b]indoline dipeptide methyl esters: synthons for expedient introduction of the hydroxypyrroloindoline moiety into larger peptide-based natural products and for the creation of tryptathionine bridges

This work describes a rapid and high yielding oxidation of 14 tryptophanylated amino acid methyl esters to the corresponding 3a-hydroxypyrrolo[2,3-b]indoline (Hpi) amino acids with generally facile separation of syn-cis and anti-cis diastereomers. Structural X-ray diffraction data are presented for both diastereomers of Tr-Hpi-Gly-OMe, which allow for a putative assignment of the other 13 pairs of diastereomers reported herein, based on correlations with 1H NMR chemical shifts. Selective and high yielding deprotection at either the N or C terminus is described, allowing the Hpi motif to be introduced efficiently into potential targets with minimal protecting group manipulation. Two tripeptides containing Hpi and cysteine were prepared and treated with acid in the Savige-Fontana reaction to produce a cyclic tryptathionine linkage, characteristic of both amatoxins and phallotoxins.     

Theoretical studies of molecular conformation. II: Application of the SIBFA procedure to molecules containing carbonyl and carboxylate oxygens and amide nitrogens

The SIBFA procedure (Sum of Interactions Between Fragments computedAb initio, Ref [1]) is extended to the study of the conformational behavior of representative molecules containing amide nitrogens and carbonyl and carboxylate oxygens. The molecules studied are C- and N-ethylammonium formamide, C- and N-ethanol-formamide, ethylammonium formate and ethanolformate. The cases investigated include interactions of the types which occur between functional groups in proteins or ionophores. The accuracy of the procedure, assessed by comparing the results to those of correspondingab initio SCF computations, is very satisfactory. An application of the procedure to study the conformation of the glycyl and alanyl dipeptides as a function of the backbone torsional angles and is presented.     

Equilibrium and Kinetics of NO and CO Chemisorptions on Nonstoichiometric Nickel-Copper Manganites

The equilibrium and kinetics of adsorption of NO and CO on nonstoichiometric nickel-copper manganites have been investigated through volumetric measurements. The adsorption isotherms were satisfactorily fitted to the Freundlich equation. The equilibrium coverages at 298 K were found to depend closely on the chemical composition of the oxide; thus, a decrease in the coverage beyond a maximum copper extent was observed. The adsorption isotherms of NO at various temperatures in the range from 298 to 473 K showed that the equilibrium coverage decreases with increasing temperature. This behavior enabled us to follow the logarithmic decrease of the heat of adsorption of NO on such surfaces. The adsorptions of NO and CO on surfaces preadsorbed with CO and NO, respectively, were also studied. These experiments showed the ability of NO to displace CO preadsorbed molecules whereas the contrary did not hold, suggesting the existence of common adsorption sites as well as some specific CO adsorption sites. Finally, some kinetic data are reported showing that the experimental adsorption results fit the Elovich equation (with t(0) approximately 0), although two distinct rate processes could be identified. Copyright 2000 Academic Press.     

Hydro­gen‐bonded assemblages in 2,6‐bis­(hydroxy­methyl)‐4‐R‐phenol,with R = methyl,methoxy, phenoxy and 1‐(4‐methoxy­phenyl)‐1‐methyl­ethyl

Four derivatives of 2,6‐bis­(hydroxy­methyl)­phenol, with various para substituents, have been investigated; these are 2,6‐bis­(hydroxy­methyl)‐4‐methyl­phenol, C₉H₁₂O₃, (I), 2,6‐bis­(hydroxy­methyl)‐4‐methoxy­phenol, C₉H₁₂O₄, (II), 2,6‐bis­(hydroxy­methyl)‐4‐phenoxy­phenol, C₁₄H₁₄O₄, (III), and 2,6‐bis­(hydroxy­methyl)‐4‐[1‐(4‐methoxy­phenyl)‐1‐methyl­ethyl]­phenol, C₁₈H₂₂O₄, (IV). All four structures display hydrogen‐bonding networks resulting in sheets, with possible weak inter‐sheet π–π interactions in one case. In all the structures but one, the mol­ecules form centrosymmetric dimeric subunits held together by two hydrogen bonds between the hydroxy­methyl groups and, in two cases, by probable π–π interactions.     

Synthesis and structure of Pt(II) phosphonato-phosphine complexes and of a P,O-stabilized metal-metal-bonded Pt2Ag2 complex

As part of our interest in the design and reactivity of P,O ligands, and because the insertion chemistry of small molecules into a metal alkyl bond is very dependent on the ancillary ligands, the behavior of Pt-methyl complexes containing the beta-phosphonato-phosphine ligand rac-Ph2PCH(Ph)P(O)(OEt)2 (abbreviated PPO in the following) toward CO insertion has been explored. New, mononuclear Pt(II) complexes containing one or two PPO ligands, [PtClMe(kappa2-PPO)] (1), [Pt{C(O)Me}Cl(kappa2-PPO)] (2), [PtMe(CO)(kappa2-PPO)]OTf (3 x OTf), [PtMe(OTf)(kappa2-PPO)] (4), trans-[PtClMe(kappa1-PPO)2] (5), [PtMe(kappa2-PPO)(kappa1-PPO)]BF4 (6 x BF4), [PtMe(kappa2-PPO)(kappa1-PPO)]OTf (6 x OTf), and [Pt{C(O)Me}(kappa2-PPO)(kappa1-PPO)]BF4 (7 x BF4) have been prepared and characterized. Hemilability of the ligands is observed in the cations 6 and 7 in which the terminally bound and chelating PPO ligands exchange their role on the NMR time-scale. The acetyl complexes 2 and 7 are stable in solution, but the former deinserts CO upon chloride abstraction. We also demonstrate the ability of PPO to behave as an assembling ligand and to stabilize a heterometallic Pt-Ag metal complex, [PtMe(kappa2-PPO){mu-(eta1-P;eta1-O)PPO)}Ag(OTf)(Pt-Ag)]OTf (8 x OTf), which was obtained by reaction of 5 with AgOTf to generate more reactive, cationic complexes. Whereas the first equivalent of AgOTf abstracted the chloride ligand, the second equivalent added to the cationic complex with formation of a Pt-Ag bond (2.819(1) A). The complexes 1, 2, 4, 5 x CH2Cl2, and (8 x OTf)2 have been structurally characterized by single-crystal X-ray diffraction. The latter has a dimeric nature in the solid state, with two silver-bound triflates acting as bridging ligands between two Pt-Ag moieties. In addition to the Ag-Pt bond, the Ag+ cation is stabilized by a dative O -->Ag interaction involving one of the PPO ligands.     

Eta6-mesityl,eta1-imidazolinylidene-carbene-ruthenium(II) complexes: catalytic activity of their allenylidene derivatives in alkene metathesis and cycloisomerisation reactions

The reaction of electron-rich carbene-precursor olefins containing two imidazolinylidene moieties [(2,4,6-Me(3)C(6)H(2)CH(2))NCH(2)CH(2)N(R)Cdbond;](2) (2a: R=CH(2)CH(2)OMe, 2 b R=CH(2)Mes), bearing at least one 2,4,6-trimethylbenzyl (R=CH(2)Mes) group on the nitrogen atom, with [RuCl(2)(arene)](2) (arene=p-cymene, hexamethylbenzene) selectively leads to two types of complexes. The cleavage of the chloride bridges occurs first to yield the expected (carbene) (arene)ruthenium(II) complex 3. Then a further arene displacement reaction takes place to give the chelated eta(6)-mesityl,eta(1)-carbene-ruthenium complexes 4 and 5. An analogous eta(6)-arene,eta(1)-carbene complex with a benzimidazole frame 6 was isolated from an in situ reaction between [RuCl(2)(p-cymene)](2), the corresponding benzimidazolium salt and cesium carbonate. On heating, the RuCl(2)(imidazolinylidene) (p-cymene) complex 8, with p-methoxybenzyl pendent groups attached to the N atoms, leads to intramolecular p-cymene displacement and to the chelated eta(6)-arene,eta(1)-carbene complex 9. On reaction with AgOTf and the propargylic alcohol HCtbond;CCPh(2)OH, compounds 4-6 were transformed into the corresponding ruthenium allenylidene intermediates (4-->10, 5-->11, 6-->12). The in situ generated intermediates 10-12 were found to be active and selective catalysts for ring-closing metathesis (RCM) or cycloisomerisation reactions depending on the nature of the 1,6-dienes. Two complexes [RuCl(2)[eta(1)-CN(CH(2)C(6)H(2)Me(3)-2,4,6)CH(2)CH(2)N- (CH(2)CH(2)OMe)](C(6)Me(6))] 3 with a monodentate carbene ligand and [RuCl(2)[eta(1)-CN[CH(2)(eta(6)-C(6)H(2)Me(3)-2,4,6)]CH(2)CH(2)N-(CH(2)C(6)H(2)Me(3)-2,4,6)]] 5 with a chelating carbene-arene ligand were characterised by X-ray crystallography.     

Characterization of dextransucrases fromLeuconostoc mesenteroides NRRL B-1299

Leuconostoc mesenteroides NRRL B-1299 dextransucrase was fractionated into soluble (SGT) and insoluble (IGT) enzyme preparations differing by their dextran content. In spite of this, they displayed the same Km for sucrose (10 g/L) and the same activation energy (35 kJ/mol). But the presence of cells and insoluble dextran led to the IGT behaving like an immobilized enzyme: stabilization against thermal denaturation and diffusional limitations at low substrate concentrations were observed. On the other hand, the behavior of SGT was influenced by the presence, in the preparation, of soluble dextran that reduced enzyme inhibition by excess substrate. SGT and IGT present very different pH profiles. In the presence of 4 g/L of soluble dextran, IGT was activated and displayed the same susceptibility to pH as SGT. The activation of IGT was highly dependent on the nature of the acceptor added but also on the pH of the reaction medium. IGT and SGT synthesize both soluble and insoluble polymer containing α(l → 2), α(l → 3), and α(l → 6) linkages. A larger amount of insoluble dextran is elaborated by SGT. The polymer structures, examined by¹³C NMR spectrometry, revealed that they differ mainly by their α(l → 3) linkage content (from 0 to 11%). This linkage seems to be partly responsible for the dextran insolubility and can be completely eliminated by carrying out the synthesis of soluble polymer at pH 7.4 with SGT.     

Méthodes chimiques d'analyse des polyacroléines linéaires

Linear polyacroleins prepared by anionic polymerization give the structural repeat units of the types \documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{--}[{\rm CH}\left( {{\rm CHO}} \right)\hbox{--} {\rm CH}_{\rm 2} {\rm \rlap{--} ], \rlap{--} [CH}_{\rm 2} \hbox{--} {\rm CH}\left( {{\rm CHO}} \right)\rlap{--} ], $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{--} [{\rm CH}\left( {{\rm CH}\hbox {\rm CH}_2 } \right)\hbox{\rm O\rlap{--} ]} $\end{document} without any cyclization. Analysis of these polymers by several methods reveal the nature and amount of each structural species, and an estimation of their distribution along the polymeric chain.     

Dirac's aether in relativistic quantum mechanics

The introduction by Dirac of a new aether model based on a stochastic covariant distribution of subquantum motions (corresponding to a vacuum state alive with fluctuations and randomness) is discussed with respect to the present experimental and theoretical discussion of nonlocality in EPR situations. It is shown (1) that one can deduce the de Broglie waves as real collective Markov processes on the top of Dirac's aether; (2) that the quantum potential associated with this aether's modification, by the presence of EPR photon pairs, yields a relativistic causal action at a distance which interprets the superluminal correlations recently established by Aspect et al.; (3) that the existence of the Einstein-de Broglie photon model (deduced from Dirac's aether) implies experimental predictions which conflict with the Copenhagen interpretation in certain specific testable interference experiments.