Direct, stereospecific generation of (Z)-disubstituted allylic alcohols

A one-pot method to prepare highly functionalized (Z)-disubstituted allylic alcohols is introduced. Hydroboration of a variety of 1-bromo-1-acetylenes with dicyclohexyl borane, reaction with t-BuLi, and transmetalation to zinc generates a (Z)-disubstituted vinylzinc reagent. In situ reaction of this reagent with aldehydes generates (Z)-disubstituted allylic alcohols in high yields (81-97%). Addition to chiral protected alpha- or beta-oxygenated aldehydes proceeds with diastereoselectivities between 6:1 and 18:1. The anti-Felkin product is obtained in most cases.     

Biosynthesis of pipecolic acid by RapL, a lysine cyclodeaminase encoded in the rapamycin gene cluster

Rapamycin, FK506, and FK520 are immunosuppressant macrolactone natural products comprised of predominantly polyketide-based core structures. A single nonproteinogenic pipecolic acid residue is installed into the scaffold by a nonribosomal peptide synthetase that also performs the subsequent macrocyclization step at the carbonyl group of this amino acid. It has been assumed that pipecolic acid is generated from lysine by the cyclodeaminases RapL/FkbL. Herein we report the heterologous overexpression and purification of RapL and validate its ability to convert L-lysine to L-pipecolic acid by a cyclodeamination reaction that involves redox catalysis. RapL also accepts L-ornithine as a substrate, albeit with a significantly reduced catalytic efficiency. Turnover is presumed to encompass a reversible oxidation at the alpha-amine, internal cyclization, and subsequent re-reduction of the cyclic delta1-piperideine-2-carboxylate intermediate. As isolated, RapL has about 0.17 equiv of tightly bound NAD+, suggesting that the enzyme is incompletely loaded when overproduced in E. coli. In the presence of exogenous NAD+, the initial rate is elevated 8-fold with a Km of 2.3 microM for the cofactor, consistent with some release and rebinding of NAD+ during catalytic cycles. Through the use of isotopically labeled substrates, we have confirmed mechanistic details of the cyclodeaminase reaction, including loss of the alpha-amine and retention of the hydrogen atom at the alpha-carbon. In addition to the characterization of a critical enzyme in the biosynthesis of a medically important class of natural products, this work represents the first in vitro characterization of a lysine cyclodeaminase, a member of a unique group of enzymes which utilize the nicotinamide cofactor in a catalytic manner.     

The electrodeposition of highly reflective lead dioxide coatings

In the presence of a suitable surfactant, such as hexadecyltrimethylammonium chloride or bromide, highly reflective and hard lead dioxide coatings with a black appearance can be electrodeposited from methanesulfonic acid media at room temperature (295 K). The reflective PbO₂ coatings are compact, adherent to the (vitreous carbon or carbon-polymer) substrate and can be formed at current densities of 10 to 100 mA cm^?2 at a thickness up to several hundred microns. The coatings were characterised by measurement of surface optical reflectance, surface roughness, surface microstructure, phase composition and crystallite size. The reflective PbO₂ films were found to mainly consist of the alpha (orthorhombic) phase with feather-like and orientated microstructures. The crystallite size and surface roughness were in the order of tens of nanometres and their optical reflectance was several orders of magnitude higher than matte coatings produced in the absence of additives.     

Microemulsions.Background,New Concepts,Applications, Perspectives. Edited by Cosima Stubenrauch.

John Wiley & Sons, Hoboken 2008. 400 pp., hardcover € 129.00.—ISBN 978‐1405167826

     

Minimum broadcast tree decompositions

Recursive constructions for decomposing the complete directed graph D_n into minimum broadcast trees of order n are given, thereby showing the existence of such decompositions for all n. Such decompositions can be used for a routing system in a network where every participant has the ability to broadcast a message to the group; as each arc is used in only one tree, a participant’s further actions upon receipt of a message depend only on its sender, and so all routing information can be stored locally rather than in the message itself.     

Measurement of vibrational energy flow in a plate with high energy flow boundary crossing using electronic speckle pattern interferometry

The measurement of vibrational energy flow is an important tool in understanding the vibrational behaviour of structures. In the past, because of transducer constraints, the measurement of vibrational energy flow was mostly restricted to single point measurements. However, recent developments in advanced laser measurement techniques, such as electronic speckle pattern interferometry (ESPI), have gained interest in applying two-dimensional, multi-point measurement techniques to the estimation of vibrational energy flow. This paper addresses the measurement of vibrational energy flow in a plate by using an ESPI based vibrational energy flow measurement technique. A radially symmetric bending wave plate vibration model is introduced and theoretical expressions for energy-based quantities are derived. To assess the accuracy of the measurement method, these theoretical quantities are compared to synthetic results derived from the ESPI energy flow measurement technique. The ESPI measurement technique is also applied to an experimental ‘infinite’ plate. Thus, a specially designed experimental apparatus was constructed so as to minimise undesired wave reflections in the plate and, thus, achieve a high energy flow boundary crossing at the edges of the plate. To reduce the effect of optical noise contamination on the ESPI measured out-of-plane plate displacement data, optimal filters were applied prior to the vibrational energy flow computation. To appraise the accuracy of the experimental method, measured vibrational power on the plate is compared with measured vibrational input power. A difference of less than 1 dB between both quantities indicates that vibrational energy flow within a rectangular plate that contains radially symmetric wave propagation can be measured to a good degree of accuracy if appropriate filtering is applied.     

Sulfenate Anion Catalyzed Diastereoselective Synthesis of Aziridines

Aziridines are highly valued synthetic targets in organic and medicinal chemistry. The organocatalytic synthesis of such structures with broad substrate scope and good diastereoselectivity, however, is rare. Herein, we report a broadly applicable and diastereoselective synthetic method for the synthesis of trans-aziridines from imines and benzylic or alkyl halides utilizing sulfenate anions (PhSO^–) as the catalyst. Substrates bearing heterocyclic aromatic groups, alkyl, and electron-rich and electron-poor aryl groups were shown to be compatible with this method (33 examples), giving good yields and high diastereoselectivities (trans : cis >20 : 1). Further functionalization of aziridines containing cyclopropyl or cyclobutyl groups was achieved through ring-opening reactions, with a cyclobutyl-substituted norephedrine derivative obtained through a four-step synthesis. We offer a mechanistic proposal involving reversible addition of the deprotonated benzyl sulfoxide to the imine to explain the high trans-diastereoselectivity.     

Palladium-Catalyzed Benzylic C(sp3)−H Carbonylative Arylation with Aryl Bromides

A novel, selective and high-yielding palladium-catalyzed carbonylative arylation of a variety of weakly acidic (pK_a 25–35 in DMSO) benzylic and heterobenzylic C(sp³)−H bonds with aryl bromides has been achieved. This system is applicable to a range of pro-nucleophiles for access to sterically and electronically diverse α-aryl or α,α-diaryl ketones, which are ubiquitous substructures in biologically active compounds. The Josiphos SL-J001-1-based palladium catalyst was identified as the most efficient and selective, enabling carbonylative arylation with aryl bromides under 1 atm CO to provide the ketone products without the formation of direct coupling byproducts. Additionally, (Josiphos)Pd(CO)₂ was identified as the catalyst resting state. A kinetic study suggests that the oxidative addition of aryl bromides is the turnover-limiting step. Key catalytic intermediates were also isolated.     

Catalytic Chemo- and Regioselective Radical Carbocyanation of 2-Azadienes for the Synthesis of α-Amino Nitriles

α-Amino nitriles are versatile structural motifs in a variety of biologically active compounds and pharmaceuticals and they serve as valuable building blocks in synthesis. The preparation of α- and β-functionalized α-amino nitriles from readily available scaffolds, however, remains challenging. Herein is reported a novel dual catalytic photoredox/copper-catalyzed chemo- and regioselective radical carbocyanation of 2-azadienes to access functionalized α-amino nitriles by using redox-active esters (RAEs) and trimethylsilyl cyanide. This cascade process employs a broad scope of RAEs and provides the corresponding α-amino nitrile building blocks in 50–95 % yields (51 examples, regioselectivity >95 : 5). The products were transformed into prized α-amino nitriles and α-amino acids. Mechanistic studies suggest a radical cascade coupling process.     

ChemInform Abstract: Polymorph Engineering of TiO2: Demonstrating How Absolute Reference Potentials Are Determined by Local Coordination.

A multiscale quantum‐chemical approach is used for examining the electronic structure of eight known TiO₂ polymorphs and aligning their ionization potential and electron affinity relative to an absolute energy reference.