One-pot enantioselective formation of eight-membered rings from alkenyl Fischer carbene complexes and ketone enolates

Eight-membered carbocycles with up to five new stereogenic centers are enantioselectively obtained following a one-pot procedure that involves the coupling of three components: an alkenyl Fischer carbene complex, a ketone enolate, and allyl lithium.     

An NMR study of merocyanine-type dyes derived from barbituric acid

The 13C NMR of two solvatochromic dyes derived from a barbituric acid acceptor and dimethylaminophenyl donor fragments, compound 1 and the related merocyanine 2, were recorded in various solvents. The observed chemical-shift variations were used to interpret their structural differences and solvatochromic behavior in solution.     

Formation of Tertiary 3-Nitro Allylic Alcohol by Condensation of 2-Chloroiso- butyrophenone with Nitromethanide Anion: Estimation of Gibbs Free Enthalpies of Reaction in Solution for the Reaction Mechanism [1]

Summary. The reaction of 2-chloroisobutyrophenones and nitromethanide anion gives stereoselectively (E)-3-nitro allylic alcohols. The Gibbs free enthalpies of reaction in DMSO for carbanion addition, epoxide formation, and rearrangement to 3-nitro allylic alcohol, as elementary steps for the reaction, were estimated from corresponding neutral gas reactions and using a thermodynamical approach to the transfer of gaseous compounds to DMSO. A criterion for assigning the sign of affinity of liquid compounds to DMSO was developed on the basis of the Gibbs enthalpies of liquefaction. The information obtained on reaction rate and thermodynamic viability of the steps indicates that carbanion addition is the rate-determining step.In memory of Prof. Dr. M. Ballester, deceased on April 6, 2005     

Thermal requirements to obtain calcined and frits of ulexite

Ulexite has a substantial amount of hydration water, which allows using calcination methods to increase the B₂O₃ content from 42.97 to 66.69%, due to the loss of 35.54% of its weight, corresponding to its hydration water.In this study, determinations of changes in the heat content at each temperature were made using an isoperibol calorimeter. The mean specific heat was calculated from these values. The reaction enthalpy was also determined by DTA methods.The sample used in the experiments contains 39.04% B₂O₃. The temperature was between 80 and 1000 °C. Curves of weight loss and enthalpy changes are presented and analysed.The results have industrial application in order to determine the thermal requirements for calcining ulexite at different temperatures, as well as its complete dehydration.     

Effect of the parent ligand on the photophysical properties of closely-coupled, binuclear ruthenium(II) tris(2,2'-bipyridine) complexes

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) tris(2,2'-bipyridine) complexes via an alkynylene group differ significantly from those of the relevant mononuclear complex. In particular, the energy of the first triplet excited state is lowered relative to the parent complex, because of the presence of the alkynylene substituent, while the triplet lifetime is prolonged, in part, because of extended electron delocalization. We now report that the triplet lifetime is also affected by the nature of the spectator 2,2'-bipyridyl ligands. Thus, replacing the parent 2,2'-bipyridine ligands with the corresponding 4,4'-dinitro-substituted ligands serves to decrease the luminescence yield and lifetime. With the corresponding carboxylate ester, the luminescence yield and lifetime are increased. Perdeuteration of the parent 2,2'-bipyridine ligands also leads to a modest increase in the luminescence yield. Such observations are indicative of electronic coupling between the various metal-to-ligand, charge-transfer excited triplet states. Temperature dependence studies confirm that these excited states are closely spaced and thermally accessible at ambient temperature. For some of the binuclear complexes, the quantum yield for formation of the lowest-energy triplet state is significantly less than unity.     

Stereocontrolled synthesis of iminocyclitols with an ether bridge

Nor-tropane related bicyclic (6+5) iminocyclitols with an ether bridge and different substituents (hydroxymethyl, aminomethyl, and aminoethyl) on C-1 are prepared starting from a β-d-psicofuranosyl cyanide. The method involves an internal nucleophilic attack of a stabilized amide ion on a 5-mesyloxy derivative. The intermediate N-acetyl O-protected iminocyclitols present atropoisomerism due to restricted rotation of the N-CO amido bond. Conformational aspects of the prepared compounds are discussed.     

Cyclodextrin effects on physical–chemical properties of novocaine

The UV-vis absorption and the fluorescence emission spectra of novocaine were analysed in aqueous cyclodextrin (CD) solutions. Either the absorbance read at λ_max 290 nm or the fluorescence emission intensity at λ_ems 346 nm increase in the presence of both α- and β-CD due to the formation of 1:1 inclusion complexes. The lower polarity of the CD-cavity sensed by the included drug enhances the emitted fluorescence; in fact, the same effect was observed in aqueous mixtures of acetonitrile, dioxane, or dimethylsulfoxide. The inclusion complex formation between the monocation of novocaine and CDs diminishes the electrical conductance of aqueous solutions of novocaine hydrochloride (NoHCl). Both the nitrosation reaction in aqueous acid medium and the ester hydrolysis in alkaline medium are retarded in the presence of CDs. The strongest effect was observed with β-CD as a consequence of the higher stability inclusion complex.     

Role of the tautomerism of 2-azaadenine and 2-azahypoxanthine in substrate recognition by xanthine oxidase

The tautomerism of 2-azaadenine and 2-hypoxanthine has been examined in the gas phase and in aqueous solution. The tautomerism in the gas phase has been studied by means of semiempirical and ab initio quantum-mechanical computations, as well as density-functional calculations. The influence of the aqueous solvent on the relative stability between tautomers has been estimated from self-consistent reaction field calculations performed with different high-level continuum models. The results provide a detailed picture of the tautomeric preference for these purine bases. The importance of tautomerism in the substrate recognition by xanthine oxidase is discussed. Finally, the rate of oxidation of 2-azaadenine and 2- hypoxanthine by xanthine oxidase is discussed in terms of the recognition model at the enzyme active site.