High diastereoselectivity in lewis acid mediated aldol condensations using thioester silyl ketene acetals.

BF₃ OEt₂ mediated aldol condensations of thioester silyl ketene acetals are stereoconvergent, and exhibit high internal (anti) and relative (Cram) diastereoselectivity.     

Structural and spectroscopic investigations of blue, vanadium-doped ZrSiO4 pigments prepared by a sol-gel route

A sol-gel reaction starting from silicon and zirconium alkoxides, in water-ethanol mixtures, was employed to obtain vanadium-doped zirconium silicate powders (zircon). The reactions were performed by modulating both (a) the amount of the vanadium salt in the starting mixture and also (b) the amount of mineralizer (NaF). The products of the sol-gel reaction were calcined at 600, 800, 1000, and 1200 degrees C. The samples were characterized by X-ray powder diffraction (XRPD), electron paramagnetic resonance spectroscopy (EPR), scanning electron microscopy (SEM), X-ray absorption near-edge spectroscopy (XANES), and diffuse UV-vis-near-IR reflectance spectroscopy. Results from the structural, morphological, and optical characterization are examined and cross-compared to produce a consistent picture of the key factors leading to the formation, growth, and optical properties of the reaction products.     

Ethyl-(r)-(4-methylphenylsulfinyl)-n-methoxy acetimidate a useful chiral acetate equivalent in aldol type condensations

Ethyl N-methoxyacetimidate was metallated by lithium amides and reacted with (-)-(S)-menthyl p-toluenesulfinate to afford, in excellent yield, (R) - (4-methylphenylsulfinyl)-ethyl-N-methoxy acetimidate (2). Compound (2) was tested in aldol-type condensations with various aldehydes, and the adducts, after desulfurization, were converted into optically active β-hydroxyesters with good ?80%) e.e. The stereochemical outcome of the reaction of benzaldehyde was found to be dramatically affected by changing either the counterion of the enolate from lithium to zirconium or by going from kinetic to thermodynamic control.     

Competitive Na(+) and Rb(+) binding in the minor groove of DNA

Sequence-dependent coordination of alkali ions to the nucleotide bases in the minor groove of AT-tract B-DNA has recently been inferred from X-ray crystallography, solution NMR and computer simulations. Here, we present new (23)Na and (87)Rb magnetic relaxation dispersion (MRD) data that demonstrate competitive and long-lived binding of Na(+) and Rb(+) ions in the minor groove of the B-DNA duplex [d(CGCGAATTCGCG)](2). The Na(+)/Rb(+) selectivity of the minor groove is found to be weak, consistent with local structural flexibility. The ion occupancies derived from the MRD data are substantially higher than previously reported, suggesting that groove-bound ions significantly influence the energetics and structural polymorphism of DNA in vivo. For example, in the presence of 0.20 M Na(+) and 0.56 M Rb(+) at 4 degrees C, the ApT site in the minor groove is occupied by a Rb(+) ion, a Na(+) ion, or a water molecule 40, 10, and 50% of the time, respectively. In the absence of Rb(+), the Na(+) occupancy increases to 50%. At 4 degrees C, the mean residence time of groove-bound ions is 0.2 +/- 0.1 micros for Rb(+) and 10 ns to 100 micros for Na(+). A shorter correlation time of 2 ns is attributed to counterions bridging cross-strand phosphate groups.     

Microbial growth and air pollution in carbonate rock weathering. Preliminary results of a in situ experimental study

Preliminary results on limestone weathering caused by air pollution and microbial colonization are presented in this study. Outdoor exposure experimental assays were performed on Scaglia limestone samples. Samples were exposed in two areas in Perugia (Italy) that differ for degree of urban air pollution. At different times of exposure, ranging from 1 to 12 months, microbial contamination and textural modifications of sampled surfaces were evaluated by microbiological procedures, X-ray diffraction, and scanning electron microscopy. After one year of exposure a significant fungal colonization and the presence of weathering products (i.e. gypsum) were detected on sampled surfaces.     

Stereoselective aldol condensations via alkenyloxy dialkoxyboranes: synthetic applications using thioesters

A detailed Investigation of the enolization of phenyl thiopropionate with ethylenechloroboronate (ECB) and diisopropylethylamine (DPEA) and the subsequent aldol condensations of these enolates was conducted. Alkenyloxy dialkoxyboranes derived from thioesters were found to be stereoconvergent: both Z and E enolates give syn aldol condensation products. The thioester additions to chiral aldehydes were studied. Internal selectivity (syn) was usually very high, while the relative stereoselectivity ranged from poor to good, depending on the specific aldehyde used. The aldol products were transformed to known compounds for correlation.     

Ab initio HF versus semi-empirical results of chemisorption calculations of hydrogen on graphite

The regular chemisorption of hydrogen on graphite in four different configurations has been studied with a Hartree-Fock ab initio technique. No stable phase was found; previous CNDO calculations indicated stable adsorption to occur in three cases out of four. However, the positions of relative minima and the corresponding electronic structure are reasonably described by the semi-empirical scheme.     

Rh-catalyzed asymmetric hydrogenation of prochiral olefins with a dynamic library of chiral TROPOS phosphorus ligands

A library of 19 chiral tropos phosphorus ligands, based on a flexible (tropos) biphenol unit and a chiral P-bound alcohol (11 phosphites) or secondary amine (8 phosphoramidites), was synthesized. These ligands were screened, individually and as a combination of two, in the rhodium-catalyzed asymmetric hydrogenation of dehydro-alpha-amino acids, dehydro-beta-amino acids, enamides and dimethyl itaconate. ee values up to 98% were obtained for the dehydro-alpha-amino acids, by using the best combination of ligands, a phosphite [4-P(O)2O] and a phosphoramidite [13-P(O)2N]. Kinetic studies of the reactions with the single ligands and with the combination of phosphite [4-P(O)2O] and phosphoramidite [13-P(O)2N] have shown that the phosphite, despite being less enantioselective, promotes the hydrogenation of methyl 2-acetamidoacrylate and methyl 2-acetamidocinnamate faster than the mixture of the same phosphite with the phosphoramidite, while the phosphoramidite alone is much less active. In this way, the reaction was optimized by lowering the phosphite/phosphoramidite ratio (the best ratio is 0.25 equiv phosphite/1.75 equiv phosphoramidite) with a resulting improvement of the product enantiomeric excess. A simple mathematical model for a better understanding of the variation of the enantiomeric excess with the phosphite/phosphoramidite ratio is also presented.     

EPR study of iron-doped MFI zeolite and silicalite catalysts: effect of treatments after synthesis

The systematic study of iron-doped MFI zeolite and MFI silicalite by X- and Q-band EPR spectroscopy is reported in this paper. Both samples, which contain Fe(III) ions in the MFI lattice, underwent three sequential treatments—calcination, steaming, and washing with sodium dithionite solution—which are needed to make the catalyst active by extracting iron ions from the lattice. EPR spectra could be simulated by assuming a broad distribution of spin-spin (zero-field) coupling parameters. The simulation parameters depend on sample history and reproduce the selective intensity changes that are observed in X-band spectra after each treatment. In particular, the distribution width decreases by calcining and steaming samples and then it increases by treating with aqueous dithionite solution. This is in agreement with the view that iron ions are extracted from the framework by calcining and by steaming and may form relatively large iron oxide particles which become smaller after the chemical treatment. We have also studied the temperature dependence of the EPR spectra. The most interesting results are from the steamed samples. Their behavior has been interpreted as due to the presence of superparamagnetic single domain particles of iron oxide. This agrees with the view that iron oxide particles form during steaming.     

Determination of structure and conformation in solution of syringotoxin,a lipodepsipeptide fromPseudomonas syringae pv.syringae by 2D NMR and molecular dynamics

Strain B427 ofPseudomonas syringae pv.syringae, originally isolated from lemon, produces several bioactive lipodepsipeptides. The structure of Syringotoxin (ST) and Syringopeptins (SPs) has been investigated in these last years. This paper reports the 2D NMR data collected in the study of ST covalent structure. The study was performed in different solvents in order both to prevent aggregation and to completely characterize the side chains features. These include the presence of common and less common aminoacids, which compose the macrocyclic ring and fatty acid side-chain moieties of ST. The nature and position in the molecule of the residues involved in the lactonic ring closure of ST have been identified with certainty. The interpretation of NOE data obtained in acetonitrile/water solution was performed by molecular dynamics calculations in vacuo. This procedure has allowed determination of the nature and number of intramolecular hydrogen bonds and the predominant conformation of ST.