Regioselectivity on the palladium-catalyzed intramolecular cyclization of indole derivatives

Indole 2-carboxamide derivatives 4 underwent palladium-catalyzed intramolecular cyclization reactions to afford beta-carbolinones or pyrazino[1,2-a]indoles according to different reaction pathways. The complete regioselectivity of the reactions was obtained in different reaction conditions.     

Development and conformational analysis of a pseudoproline-containing turn mimic

The liquid-phase synthesis and the conformational analysis of a small library of fully protected tetramers containing L-pyroglutamic acid (L-pGlu), (4S,5R)-4-methyl-5-carboxybenzyloxazolidin-2-one (L-Oxd), or (4R,5S)-4-methyl-5-carboxybenzyloxazolidin-2-one (D-Oxd) as residue i + 1 are reported to test the tendency of these oligomers to assume a -hairpin conformation. The most promising molecule is Boc-L-Val-D-Oxd-Gly-L-Ala-OBn, which assumes a preferential -turn conformation in CDCl3, as shown by IR and 1H NMR analysis. These findings have been confirmed by DFT calculations, which provide an interpretation for the available experimental data and agree with the reported observations.     

Dipolar waves as NMR maps of protein structure

The anisotropy of nuclear spin interactions results in a unique mapping of structure to the resonance frequencies and split tings observed in NMR spectra, however, the determination of molecular structure from experimentally measured spectral parameters is complicated by angular ambiguities resulting from the symmetry properties of dipole-dipole and chemical shift interactions. This issue can be addressed through the periodicity inherent in secondary structure elements, which can be used as an index of topology. Distinctive wheel-like patterns are observed in two-dimensional 1H-15N heteronuclear dipolar/15N chemical shift PISEMA (polarization inversion spin-exchange at the magic angle) spectra of helical membrane proteins in highly aligned lipid bilayer samples. One-dimensional dipolar waves are an extension of two-dimensional PISA (polarity index slant angle) wheels to map protein structure in NMR spectra of both highly and weakly aligned samples. Dipolar waves describe the periodic wavelike variations of the magnitudes of the static heteronuclear dipolar couplings as a function of residue number in the absence of chemical shift effects. Weakly aligned samples of proteins display these same effects, primarily as residual dipolar couplings (RDCs), in solution NMR spectra. The corresponding properties of the RDCs in solution NMR spectra of weakly aligned helices represent a convergence of solid-state and solution NMR approaches to structure determination.     

The diastereoisomers methyl 5‐(S)‐[2‐(R)/(S)‐methoxycarbonyl)‐2,3,4,5‐tetrahydropyrrol‐1‐ylcarbonyl]‐1‐(4‐methylphenyl)‐4,5‐dihydropyrazole‐3‐carboxylate

The title diastereoisomers, methyl 5‐(S)‐[2‐(S)‐methoxy­carbonyl)‐2,3,4,5‐tetra­hydro­pyrrol‐1‐yl­carbonyl]‐1‐(4‐methyl­phenyl)‐4,5‐di­hydro­pyrazole‐3‐carboxyl­ate and methyl 5‐(S)‐[2‐(R)‐methoxycarbonyl)‐2,3,4,5‐tetrahydropyrrol‐1‐ylcarbonyl]‐1‐(4‐methyl­phenyl)‐4,5‐di­hydro­pyrazole‐3‐carboxylate, both C₁₉H₂₃N₃O₅, have been studied in two crystalline forms. The first form, methyl 5‐(S)‐[2‐(S)‐methoxy­carbonyl)‐2,3,4,5‐tetrahydropyrrol‐1‐ylcarbonyl]‐1‐(4‐methylphenyl)‐4,5‐di­hydro­pyrazole‐3‐carboxyl­ate–methyl 5‐(S)‐[2‐(R)‐methoxy­carbonyl)‐2,3,4,5‐tetra­hydro­pyrrol‐1‐yl­carbonyl]‐1‐(4‐methylphenyl)‐4,5‐dihydropyrazole‐3‐carboxylate (1/1), 2(S),5(S)‐C₁₉H₂₃N₃O₅·2(R),5(S)‐C₁₉H₂₃N₃O₅, contains both S,S and S,R isomers, while the second, methyl 5‐(S)‐[2‐(S)‐methoxycarbonyl)‐2,3,4,5‐tetrahydro­pyrrol‐1‐ylcarbonyl]‐1‐(4‐methyl­phenyl)‐4,5‐di­hydro­pyrazole‐3‐carboxyl­ate, 2(S),5(S)‐C₁₉H₂₃N₃O₅, is the pure S,S isomer. The S,S isomers in the two structures show very similar geometries, the maximum difference being about 15° on one torsion angle. The differences between the S,S and S,R isomers, apart from those due to the inversion of one chiral centre, are more remarkable, and are partially due to a possible rotational disorder of the 2‐­(methoxycarbonyl)tetrahydropyrrole group.     

GaN nanowires as a reusable photoredox catalyst for radical coupling of carbonyl under blacklight irradiation

Employing photo-energy to drive the desired chemical transformation has been a long pursued subject. The development of homogeneous photoredox catalysts in radical coupling reactions has been truly phenomenal, however, with apparent disadvantages such as the difficulty in separating the catalyst and the frequent requirement of scarce noble metals. We therefore envisioned the use of a hyper-stable III–V photosensitizing semiconductor with a tunable Fermi level and energy band as a readily isolable and recyclable heterogeneous photoredox catalyst for radical coupling reactions. Using the carbonyl coupling reaction as a proof-of-concept, herein, we report a photo-pinacol coupling reaction catalyzed by GaN nanowires under ambient light at room temperature with methanol as a solvent and sacrificial reagent. By simply tuning the dopant, the GaN nanowire shows significantly enhanced electronic properties. The catalyst showed excellent stability, reusability and functional tolerance. All reactions could be accomplished with a single piece of nanowire on Si-wafer.

A highly efficient re-usable semiconductor as a radical coupling catalyst in MeOH.     

Analytical electron microscopy studies of lithium aluminum hydrides with Ti- and V-based additives

The microstructure of LiAlD(4) with TiCl(3).1/3(AlCl(3)) and VCl(3) additives has been studied during different steps of the decomposition process using electron energy loss spectroscopy and energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope. Energy filtered transmission electron microscopy was used to show elemental distributions in the samples. The spatial distribution of the additives and the main elements within the alanate particles was examined with a resolution of a few nanometers. The analysis of the electron energy loss spectra reveals the chemical state of Al, O, and the additives. Ti and V do not appear to mix chemically with Al to a significant degree. V was found in high concentration in just a few particles, while Ti is more uniformly distributed. All the samples showed evidence of oxidation despite procedures being adopted to avoid exposing the material to air. The additives are oxidized in all the samples, and Al(2)O(3) forms a thin layer at the surface of the particles. This paper gives a comparison between samples at different stages of the decomposition process using different additives.     

Cosmological black holes are coloured too

A new solution of the Einstein-Yang-Mills system with axial symmetry and cosmological constant is given here. This new metric, which in the absence of matter becomes the de Sitter universe, is the natural generalization of both a recent result of Perry for stellar (asymptotically flat) black holes and a well-knwn family of solutions of the Einstein-Maxwell system with nonvanishing cosmological constant given some time ago by Carter. Calculations have been carried out making extensive use of the coordinates found by Plebanski.