Improved cyclic reduction for solving queueing problems

The cyclic reduction technique (Buzbee et al., 1970), rephrased in functional form (Bini and Meini, 1996), provides a numerically stable, quadratically convergent method for solving the matrix equation X = ∑^{+ ∞} _i=0 X_i A_i, where the A_i's are nonnegative k × k matrices such that ∑^{+ ∞} _i=0 A_i is column stochastic. In this paper we propose a further improvement of the above method, based on a point-wise evaluation/interpolation at a suitable set of Fourier points, of the functional relations defining each step of cyclic reduction (Bini and Meini,1996). This new technique allows us to devise an algorithm based on FFT having a lower computational cost and a higher numerical stability. Numerical results and comparisons are provided. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electron impact mass spectrometry of some 2-(p-bromophenyl)-4,4-disubstituted-5(4H)-oxazolones

The mass spectrometric behaviour of four 2-(p-bromophenyl)-4,4-disubstituted-5(4H)-oxazolones has been investigated using linked scans, mass analyzed ion kinetic energy spectrometry, accurate mass measurements and collisional spectroscopy. The study shows the formation of similar ions in all cases, with the presence of skeletal rearrangements, leading to highly stable product ions.     

Highly enantioselective catalytic asymmetric ring opening reaction employing the Daniphos ligand

A set of the recently published planar-chiral Daniphos diphosphine ligands, based on an arene chromium tricarbonyl scaffold, has been applied to the rhodium-catalyzed asymmetric ring opening (ARO) reaction of 1,4-dihydro-1,4-epoxynaphthalene with methanol as the nucleophile. Enantioselectivities of up to 97.5% ee at satisfactory conversions have been obtained. The most successful ligand showed to be a PPh₂/P(t-Bu)₂-substituted derivative. An X-ray structure of this ligand is presented and discussed.     

(S)-(+)-N-acetylphenylglycineboronic acid: a chiral derivatizing agent for Ee determination of 1,2-diols

A new chiral derivatizing agent for ee determination of 1,2-diols via (1)H NMR is described. (S)-(+)-N-acetylphenylglycineboronic acid (1) is synthesized in enantiomerically pure form; its reaction with chiral diols quantitatively yields cyclic boronic esters 5a-g. The latter show a remarkably high diastereodifferentiation of proton NMR signals useful for de determination. [reaction: see text]     

A convenient synthesis by microwave irradiation of an active metabolite (EXP-3174) of losartan

A novel and convenient microwave-assisted synthesis of an active metabolite (EXP-3174) of losartan is described. Room temperature and microwave irradiation of the reactions are compared. Synthesis by microwave irradiation gave the desired compound in higher yields and in shorter reaction times than those obtained at room temperature.     

Ion trap mass spectrometry in the structural analysis of haemoglobin peptides modified by epichlorohydrin and diepoxybutane

Ion trap mass spectrometry has been shown to be particularly suitable for the structural analysis of high molecular weight peptides directly fragmented in the mass analyser without needing further sub-digestion reactions. Here we report the advantages of using multi-stage ion trap mass spectrometry in the structural characterisation of haemoglobin alkylated with epichlorohydrin and diepoxybutane. Alkylated globins were digested with trypsin and the peptide mixtures were analysed by MS(3). This technique allows the sequential fragmentation of peptides under analysis, giving rise to MS(3) product ion spectra with additional information with respect to MS(2) mass spectra. The results obtained complete the previously reported structural characterisation of alkylated haemoglobin, demonstrating the potential of ion trap mass spectrometry.     

Enzyme-cleavable linkers for peptide and glycopeptide synthesis

Hydroxymethylphenoxy linkers that are commonly used in solid phase peptide synthesis are surprisingly susceptible to efficient cleavage by the protease chymotrypsin with a broad range of amino acid residues being tolerated at the scissile bond; this enzyme-cleavable linker system has been applied to peptide and glycopeptide synthesis.     

Efficient microwave-assisted synthesis of highly functionalized pyrimidine derivatives

A generally applicable one-pot procedure for the rapid, easy, and diverse asymmetric functionalization of pyrimidines was developed that requires minimum efforts for the purification of the final products. 4-Amino-6-aryl-substituted pyrimidines are prepared in good yields by combined microwave-assisted amination and Suzuki-Miyaura cross-coupling reactions. The acid-mediated amination reaction affords the products as easily separable salts in 30-40 min reaction time.