Asymmetric Synthesis of alpha-Amino Phosphonic Acids by Diastereoselective Addition of Trimethyl Phosphite onto Chiral Oxazolidines(1)

A simple and general asymmetric synthesis of alpha-amino phosphonic acids is described. The method involves the highly selective addition of trialkyl phosphite onto various chiral oxazolidines. Oxazaphosphorinanes thus obtained with an excellent diastereoselectivity furnish the corresponding (S)-alpha-substituted amino phosphonic acids in good overall yields and high ee (77-->97%) after simple deprotection.     

Metal-directed synthesis and photophysical studies of trinuclear v-shaped and pentanuclear x-shaped ruthenium and osmium metallorods and metallostars based upon 4'-(3,5-dihydroxyphenyl)-2,2':6',2''-terpyridine divergent units

A new series of V-shaped trinuclear metallorods and X-shaped pentanuclear metallostars has been prepared by the reaction of metal complexes bearing pendant phenolic functionalities with complexes containing electrophilic ligands. Specifically, {M(tpy)2} motifs (M=Ru or Os; tpy=2,2':6',2'-terpyridine) bearing one or two pendant 3,5-dihydroxyphenyl substituents at the 4-position of the central ring of the tpy have been reacted with the complexes [Ru(tpy)(Xtpy)]2+ (X=Cl or Br) to form new ether-linked species. The energy transfer from ruthenium to osmium in these complexes has been investigated in detail and the efficiency of transfer shown to be highly temperature dependent; the energy transfer is highly efficient at low temperature, whereas at room temperature nonradiative and nontransfer deactivation of the excited {Ru(tpy)2}* domains is most significant.     

Conformational analysis of self-organized monolayers with scanning tunneling microscopy at near-atomic resolution

We describe the synthesis and a novel approach to the conformational analysis of 2,2'-bipyridines (bpy) bearing aromatic rich Frechet-type dendritic wedges of the first and second generation as substituents. The evaporation of solutions of these new ligands on graphite surfaces under ambient conditions results in the formation of self-organized monolayers. Scanning tunneling microscopy (STM) investigations of the monolayers under ambient conditions (air, 298 K) gave images at submolecular and near-atomic resolution. The analysis of the STM images includes the following processes: (i) identification and reproduction of potential homoconformational domains, (ii) exclusion of improper data using quality criteria for drift and feedback artifacts, (iii) compilation of running averages and checking for averaging artifacts, (iv) analysis of three-dimensional and contour plots, (v) calculation of the HOMO properties of the free molecules, and (vi) final conformational assignment based on all accessible information. Following this procedure, two different conformations could be assigned to domains observed in the monolayers of the first-generation (G1) and second-generation (G2) dendritic compounds. Homoconformational domains are observed side-by-side. The different conformations arise from syn or anti arrangements at the ether substituents. An additional conformational effect is found upon treating the G1 domains with HCl gas, when a partial rearrangement of the bpy from trans to cis occurs, concomitant with protonation.     

Improvements to the distance geometry algorithm for conformational sampling of cyclic structures

Modifications to the distance geometry algorithm as embodied in the program DGEOM have been made to improve sampling capabilities. Specifically, torsion angle sampling replaces distance sampling for 1,4 atomic relationships and correlated distance sampling is disabled. The effects of these modifications are illustrated by comparing the different sets of conformations produced for butane. In addition, these changes are shown to increase the conformational sampling of two medium-sized rings, cycloheptadecane and caprylolactam. The current results for these molecules are compared to those of other conformational searching methods.     

Dialumenes – aryl vs. silyl stabilisation for small molecule activation and catalysis

Main group multiple bonds have proven their ability to act as transition metal mimics in the last few decades. However, catalytic application of these species is still in its infancy. Herein we report the second neutral NHC-stabilised dialumene species by use of a supporting aryl ligand (3). Different to the trans-planar silyl-substituted dialumene (3Si), compound 3 features a trans-bent and twisted geometry. The differences between the two dialumenes are explored computationally (using B3LYP-D3/6-311G(d)) as well as experimentally. A high influence of the ligand''s steric demand on the structural motif is revealed, giving rise to enhanced reactivity of 3 enabled by a higher flexibility in addition to different polarisation of the aluminium centres. As such, facile activation of dihydrogen is now achievable. The influence of ligand choice is further implicated in two different catalytic reactions; not only is the aryl-stabilised dialumene more catalytically active but the resulting product distributions also differ, thus indicating the likelihood of alternate mechanisms simply through a change of supporting ligand.

Ligand controlled reactivity: a trans-bent and twisted geometry enables dihydrogen activation and enhanced catalytic activity for NHC-stabilised dialumenes.     

Fragmentation sous Impact Electronique du Phényl-2 Oxétanne

The mass spectral fragmentation processes of 2-phenyloxetane have been studied by means of isotopic labelling. The origin of the benzoyl ion, which is the base peak, is discused     

Structural characterization of lipopeptide biomarkers isolated from Bacillus globigii

Spectra obtained using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of Bacillus globigii (Bacillus subtilis niger) spores, vegetative cells and the culture supernatant show a cluster of biomarkers centered at a molecular mass of 1478 Da. Three biomarkers were isolated from the cell-free culture supernatant by solid-phase extraction and reversed-phase high-performance liquid chromatography, and characterized using various kinds of mass spectrometry. A Fourier transform mass spectrometer with a MALDI source was used to determine the monoisotopic protonated masses at 1463.8, 1477.8, and 1505.8 Da in order of elution. The mass differences of 14 and 28 Da suggest that they are homologous molecules. Alkaline hydrolysis of each species showed that it contained a lactone linkage. Strong acid hydrolysis released a fatty acid from an amide bond, consistent with a lipopeptide. A quadrupole time-of-flight instrument with a nanospray source was used to sequence the hydrolyzed forms of the three biomarkers. The cyclic lipopeptides were found to have amino acid sequences identical with those in fengycins and plipastatins, antimicrobial compounds with phospholipase inhibitor activity, previously identified in related species of Bacillus subtilis and Bacillus cereus.     

Asymmetric enamide hydrogenation in the synthesis of N-acetylcolchinol: a key intermediate for ZD6126

A synthesis of N-acetylcolchinol, a key intermediate in the synthesis of ZD6126, was developed. The enantiodifferentiating step required the catalytic asymmetric hydrogenation of an enamide. After screening a range of metal and ligand combinations it was found that (S,S)-ⁱPr-FerroTANE Ru(methallyl)₂ and [(S,S)-^tBuFerroTANE Rh(COD)]BF₄ gave both high enantioselectivity (>90% ee) and high catalyst utility (molar S/C = 1000).     

Selectivity of long chain stationary phases in reversed phase liquid chromatography

A series of commercial monomeric and polymeric C(18), C(27), and C(30) stationary phases were compared with immobilized poly(ethylene-co-acrylic acid) stationary phases synthesized in-house. The columns were characterized on the basis of methylene selectivity, silanol activity, metal activity, pore size, shape selectivity, and the ability to separate tocopherol isomers and carotenoid isomers. Monomeric and polymeric C(30) phases were shown to yield excellent separations of the tocopherol isomers while the polymeric C(30) and polyethylene phases were more appropriate to the separation of carotenoids.     

Practical synthesis of 1-deoxy-d-xylulose and 1-deoxy-d-xylulose 5-phosphate allowing deuterium labelling

An optimised gram scale synthesis allows the production of 1-deoxy-d-xylulose and 1-deoxy-d-xylulose 5-phosphate with possible deuterium labelling at C-5. Such substrates are required for investigations on the mevalonate-independent 2-C-methyl-d-erythritol 4-phosphate pathway for isoprenoid biosynthesis in bacteria and chloroplasts of phototrophic eukaryotes and for the biosynthesis of vitamins B₁ (thiamine diphosphate) and B₆ (pyridoxol phosphate) in bacteria.