Electron impact mass spectrometry of some macrocyclic polyether lactones

Electron impact mass spectrometry has been used together with linked scans, exact mass measurements and mass-analysed ion kinetic energy spectrometry to study nine inacrocyclic polyether lactones in detail. The presence of abundant hydrogenated oxirane ions has been observed for all the compounds examined, and confirmed by kinetic energy release measurements.     

Spectrophotometric, EPR and kinetic characterisation of the >N-O* radical from 1-hydroxybenzotriazole, a key reactive species in mediated enzymatic oxidations

Characterisation of the aminoxyl (>N-O*) radical BTNO, generated from 1-hydroxybenzotriazole (HBT) by the one-electron oxidant CAN (a Ce(IV) salt), confirms BTNO as the reactive intermediate in oxidations run with the laccase/HBT system.     

Synthesis, biological activity and modelling studies of two novel anti HIV PR inhibitors with a thiophene containing hydroxyethylamino core

An efficient method has been developed for the synthesis of a versatile intermediate bearing azido, hydroxyl and ester functions, a useful precursor for peptidomimetic compounds. The two main features for this synthesis were the use of the Sharpless asymmetric dihydroxylation on thiophene acrylate and the subsequent regioselective ring opening by sodium azide of the cyclic sulfite. Highly chemoselective reduction of the azido alcohol led to a key compound which was utilized for the synthesis of two analogues of commercial anti HIV PR such as nelfinavir and saquinavir. The biological activity and molecular modelling study on these two new potential drugs have been evaluated.     

Molecular switches of fluorescence operating through metal centred redox couples

Three-component molecular systems (redox active subunit)-spacer-(light-emitting fragment) can operate as fluorescence switches, following the alternate addition of an oxidizing agent and a reducing agent (or the adjustment of the potential of the working electrode in an electrolysis experiment). The redox active subunit typically consists of a metal centred redox couple (M⁽ⁿ⁺¹⁾⁺/Mⁿ⁺), encircled by a macrocyclic receptor, and switching efficiency requires that one of the two oxidation states quenches the proximate fluorophore and the other does not. Four ON/OFF systems, based on either the Cu^II/Cu^I or Ni^III/Ni^II couple, will be discussed. The nature of the quenching process responsible for the OFF state, either electron transfer or energy transfer, is related to the length and to the flexibility-rigidity of the spacer.     

Adsorption of 3-pyrroline on Si(100) from first principles

The chemisorption of 3-pyrroline (C(4)H(7)N) on Si(100) is studied from first principles. Three different structures can be realized for which, depending on the temperature, the chemisorption process is facile (for two of them it is essentially barrierless); among these configurations the most favored one, from a thermodynamical point of view, is a dissociated structure obtained through an exothermic reaction characterized by the formation of a N-Si bond and a H-Si bond in which the H atom is detached from the molecule. Several other chemisorption structures are possible which, however, require overcoming a significant energy barrier and often breaking multiple bonds. A number of reaction paths going from one stable structure to another have been investigated. We have also generated, for the two basic adsorption structures, theoretical scanning tunneling microscopy images which could facilitate the interpretation of experimental measurements, and we propose a possible reaction mechanism for nitrogen incorporation.     

An iterative extended Hückel calculation for some amino acids containing sulphur and selenium

Semiempirical iterative extended Hückel calculations are performed for the dipolar ions of cystine, cysteine, methionine and their selenium analogues. An attempt has been made to interprete the wave function of cysteine in terms of localized molecular orbitals.

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Zusammenfassung Semiempirische iterative Rechnungen wurden nach der erweiterten Hückeltheorie für die dipolaren Ionen Cystin, Cystein, Methionin und ihre Selen-Homologen durchgeführt. Für den Fall des Cysteins wurde die Wellenfunktion auf lokalisierte Orbitale transformiert.

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Résumé On a analisé les ions dipolaires de la cystine, cystéine, methionine et leurs analogues séléniates avec la méthode semiempirique de Hückel étendue itérative. On a cherché à intérpreter la fonction d'onde pour la cystéine dans le schème des orbitales moleculaires localisées.

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Work performed with the financial support of the Consiglio Nazionale delle Ricerche.     

Smiles rearrangement for the synthesis of 5-amino-substituted [1]benzothieno[2,3-b]pyridine

The Smiles rearrangement was successfully applied to 4-hydroxybenzo[b]thiophene furnishing a facile entry to the 4-amino derivative. The rearrangement was extended to 5-methoxy-4-methoxycarbonyl[1]benzothieno[2,3-b]pyridine obtained via aza-Wittig/electrocyclization reaction of novel N-(4-methoxybenzothiophen-2-yl)iminomethyldiphenylphosphorane with methyl trans-4-oxo-2-pentenoate. The preparation of a novel 5-amino-4-methoxycarbonyl[1]benzothieno[2,3-b]pyridine, which is of interest as a potential secondary peptide structure mimic, was successfully achieved.     

Electrochemical behaviour of some polyoxo clusters

We describe the redox behaviour in non-aqueous solvents of some cyclopentadienyl(oxo)titanium derivatives. The derivative [Ti₄{η⁵-C₅H₄(SiMe₃)}₄(μ-O)₆] shows an electrochemically and chemically reversible le reduction process, followed by a multi-electron, chemically complicated reduction at a fairly cathodic potential. On the basis of the overall electrochemical features and the comparison with the redox behaviour of the quasi-planar compound [[Ti{η⁵-C₅H₄(SiMe₃)}Cl(μ-O)]₄] we propose an EECCEE mechanism for the first derivative, where the second electron-transfer induces a cascade of chemical reactions giving rise to irreversible cluster breakdown. The electrochemically induced fragmentation can be viewed as a retrosynthetic pathway. The heterometallic derivative [{Ti(η⁵-C₅H₄Me)₂(μ₂-MoO₄)₂}₂] shows two consecutive reduction processes; the first is chemically reversible, and the second quasi-reversible. The molybdate bridges apparently increase the stability of the electrogenerated anions. However none of these poly-oxo clusters can be considered as good models of electron ‘sinks’.     

Anisotropic contributions in the ligand polarization model for the f-f transition probabilities of Eu(III) complexes

The intensities of the ΔM = ± 1 component of the ⁷f_o→ ⁵D₂ transition in D₃ complexes containing the [ EuO₉] cluster, and of other d-d or f-f transitions in D₃ complexes with the yz- and xz-components of the electric quadrupole as the leading moment, are found to be dependent upon the polarizability anisotropy of the ligands. The expectation is supported by determinations of the single-crystal absorption intensities of Na₃ [Eu(diglycollate)₃]·2NaClO₄·6H₂O and [Eu(H₂O)₉] (C₂H₅SO₄)₃.     

Effects of variable selection on CoMFA coefficient contour maps in a set of triazines inhibiting DHFR

Summary An example of a CoMFA study is described with the aim to discuss one of the major problems of this 3D QSAR method: lack of variable selection. It is shown that the use of nonrelevant energy parameters might produce CoMFA contour maps which poorly reflect the actual nature of the binding site and are in part statistical artefacts. The data set employed in our analysis comparises triazine inhibitors of dihydrofolate reductase (DHFR), isolated from chicken liver, which have already been the object of a QSAR study by other authors. Since three-dimensional structures of triazine-DHFR complexes are known, it was possible not only to reduce ambiguities in the superimposition of the ligands, but also to compare the resulting CoMFA contour maps with the enzyme active site.Supplementary material available: The Cartesian coordinates and the atomic charges of the PM3-optimized structures used in the CoMFA study are available as MOL2 files upon request.To whose memory this paper is dedicated.