Zn(II)-catalyzed thiolysis of oxiranes in water under neutral conditions

Thiolysis of a variety of 1,2-epoxides in water at 30 degrees C and pH 7.0 is strongly accelerated by ZnCl(2) (10 mol %) except when amino- and carboxythiophenol are used. The aqueous medium and the catalyst were recovered and reused in various runs without affecting the efficiency of the process.     

On lattice-like properties of orthoquaternary categories

Summary We point a class of categories for which necessary and sufficient conditions in order they have an orthodox quaternary symmetrization are expressible by means of lattice-like properties of subobjects, these one given by axiomsD1)-5). This implies that canonical isomorphisms between subquotients are composable and that Noether isomorphism theorems hold.

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Riassunto Si individua una classe di categoric per le quali l’esistenza e l’ortodossia della simmetrizazione quaternaria è exprimibile mediante proprietà di tipo reticolare di sottooggetti, enunciate nella forma di assiomiD1)-5). Ciò garantisce la componibilità degli isomorfismi canonici tra subquozienti e la validità dei teoremi di isomorfismo di Noether.

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Written while the authors were members of C.N.R. sec. G.N.S.A.G.A.     

Synthesis and acylating properties of 4,5-dihydro-6H-1,3,4- oxadiazin-6-ones

Cyclization of α-acylhydrazinoacids with dicyclohexylcarbodiimide or with acetic anhydride allows an easy entry to the little known class of 4, 5-dihydro-6H-1, 3, 4-oxadiazin-6-ones. Such compounds maintain the optical activity of the starting hydrazino acids and may be exploited as useful acylating agents.     

Acid soap and phase behavior of stearic acid and triethanolamine stearate

Crystals of partially neutralized stearic acid with triethanolamine (TEA) were prepared by mixing these two materials above 80 degrees C and then cooling. The crystalline composition and the structure and melting behavior of the resultant products were characterized with small-angle and wide-angle X-ray diffraction, thermal analysis, microscopy, and infrared spectroscopy. It was discovered that an acid-soap complex of 2:1 fixed stoichiometric ratio exists between stearic acid and TEA stearate. A binary phase diagram of stearic acid and TEA soap is built based on the experimental results; this is the first published record of a binary phase diagram for amine-based soap. Its behavior is significantly different from that of binary systems of fatty acid and alkali soap.     

Electronic and steric substituent influences on the conformational equilibria of cyclohexyl esters: the anomeric effect is not anomalous!

The cyclohexyl esters of a series of carboxylic acids, RCO₂H, spanning a range of electronegativities and quotients of steric hindrance for the R substituent (R=Me, Et, iPr, tBu, CF₃, CH₂Cl, CHCl₂, CCl₃, CH₂Br, CHBr₂, and CBr₃) were prepared. Their conformational equilibria in CD₂Cl₂ were examined by low‐temperature ¹H NMR spectroscopy to study the axial or equatorial orientation of the ester functionality with respect to the adopted chair conformation of the cyclohexane ring. The ab initio and DFT geometry‐optimized structures and relative free energies of the axial and equatorial conformers were also calculated at the HF/6‐311G**, MP2/6‐311G**, and B3LYP/6‐31G** levels of theory, both in the gas phase and in solution. In the latter case, a self‐consistent isodensity polarized continuum model was employed. Only by including electron correlation in the modeling calculations for the solvated molecules was it possible to obtain a reasonable correlation between ΔG°_calcd and ΔG°_exp. Both the structures and the free energy differences of the axial and equatorial conformers were evaluated with respect to the factors normally influencing conformational preference, namely, 1,3‐diaxial steric interactions in the axial conformer and hyperconjugation. It was assessed that hyperconjugative interactions, σ_{C? C}/σ_{C? H} and σ*_{C? O}, together with a steric effect—the destabilization of the equatorial conformer with increasing bulk of the R group—were the determinant factors for the position of the conformational equilibria. Thus, because hyperconjugation is held responsible as the mitigating factor for the anomeric effect in 2‐substituted, six‐membered saturated heterocyclic rings, and since it is also similarly responsible, at least partly, in these monosubstituted cyclohexanes for a preferential shift towards the axial conformer, the question is therefore raised: can the anomeric effect really be construed as anomalous?     

Structure and properties of lithium thio-boro-germanate glasses

Structural studies of the ternary xLi₂S + (1 − x)[0.5B₂S₃ + 0.5GeS₂] glasses using IR, Raman, and ¹¹B NMR show that the Li₂S is not shared proportionately between the GeS₂ and B₂S₃ sub-networks of the glass. The IR spectra indicate that the B₂S₃ glass network is under-doped in comparison to the corresponding composition in the xLi₂S + (1 − x)B₂S₃ binary system. Additionally, the Raman spectra show that the GeS₂ glass network is over-modified. Surprisingly, however, the ¹¹Boron static NMR gives evidence that ∼80% of the boron atoms are in tetrahedral coordinated. A super macro tetrahedron, B₁₀S₁₈⁻⁶ is proposed as one of the structures in these glasses in which can account for the apparent low fraction of Li₂S present in the B₂S₃ sub-network while at the same time enabling the high fraction of tetrahedral borons in the glass.     

Inorganic-organic hybrids formed by P,P'-diphenylmethylenediphosphinate, pcp2-, with the Cu2+ ion. X-ray crystal structures of [Cu(pcp)(H2O)2] x H2O and [Cu(pcp)(bipy)(H2O)

Weakly coordinated [Cu(pcp)(H2O)n] complexes are formed in aqueous solution, at room temperature, by interaction of P,P'-diphenylmethylene diphosphinic acid (H2pcp) with copper(II) ions. However, heating of the solutions gives rise to the formation of two extended metal-oxygen networks of formulas [Cu(pcp)(H2O)2] x H2O, 1, and [Cu(pcp)(H2O)2], 2. In the presence of 2,2'-bipyridyl (bipy) the diamine derivative [Cu(pcp)(bipy)(H2O)], 4, has been isolated. Complex 1 easily loses water to form a monohydrated derivative [Cu(pcp)H2O], 3, whereas 2 is completely dehydrated after prolonged heating at 150 degrees C, under vacuum. The compounds 1 and 2 have substantially different solid-state structures as shown by X-ray powder diffraction spectra, IR spectra, and thermogravimetric analyses. Consistently, the two complexes cannot be directly interconverted and present different dehydration pathways. Rehydration of these materials in both cases allows quantitative formation of 1. X-ray analysis established that the structure of 1 consists of a corrugated two-dimensional layered polymeric array, where infinite zigzag chains of Cu centers and bridging phenylphosphinate ligands are linked together through strong hydrogen-bonding interactions; the structure of 4 consists of monodimensional polymers, where the hydrogen-bonding interactions play an essential bridging role in the extended architecture. In both structures the metal center displays a five-coordinate environment with approximate square pyramidal geometry, with the pcp ligand acting as bidentate and monodentate in 1 and solely as bidentate in 4. In 1 the coordination sphere is completed through water molecules; in 4, through water and diamine ligands. The thermogravimetric analyses of the complexes are compared with those of the related hybrids [M(pcp)(H2O)3] x H2O, where M = Mn, Co, or Ni, confirming that noncoordinated water molecules also play a basic role in determining the molecular packing.     

Polyfluorinated amino acids for sensitive 19F NMR-based screening and kinetic measurements

Two novel series of polyfluorinated amino acids (PFAs) were designed and synthesized according to a very short and scalable synthetic sequence. The advantages and limitations of these moieties for screening purposes are presented and discussed. The potential applications of these PFAs were tested with their incorporation into small arginine-containing peptides that represent suitable substrates for the enzyme trypsin. The enzymatic reactions were monitored by 19F NMR spectroscopy, using the 3-FABS (three fluorine atoms for biochemical screening) technique. The high sensitivity achieved with these PFAs permits a reduction in substrate concentration required for 3-FABS. This is relevant in the utilization of 3-FABS in fragment-based screening for identification of small scaffolds that bind weakly to the receptor of interest. The large dispersion of 19F isotropic chemical shifts allows the simultaneous measurement of the efficiency of the different substrates, thus identifying the best substrate for screening purposes. Furthermore, the knowledge of KM and Kcat for the different substrates allows the identification of the structural motifs responsible for the binding affinity to the receptor and those affecting the chemical steps in enzymatic catalysis. This enables the construction of suitable pharmacophores that can be used for designing nonpeptidic inhibitors with high affinity for the enzyme or molecules that mimic the transition state. The novel PFAs can also find useful application in the FAXS (fluorine chemical shift anisotropy and exchange for screening) experiment, a 19F-based competition binding assay for the detection of molecules that inhibit the interaction between two proteins.