Copper activation of boronic acids: factors affecting reactivity

The generation of nucleophiles from the combination of aryl boronic acids and catalytic amounts of copper salt allows a reactivity distinct from other organometallic species, such as organolithiums or Grignard reagents. Here we examine how the electronic and steric properties of the boronic acid affect the formation of active nucleophiles and their subsequent reactivity with iminium‐type compounds, showing that electron‐rich substrates display reduced reactivity. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis of 2-C-methylerythritols and 2-C-methylthreitols via enantiodivergent Sharpless dihydroxylation of trisubstituted olefins

The mevalonate-independent pathway (MIP) is an interesting avenue for antimicrobial lead discovery. Here, we present a unified enantioselective synthesis of all four stereoisomers of 2-C-methyltetrol. These are useful building blocks of many bioactive natural products, including 2-C-methylerythritol phosphate (MEP) of the MIP biosynthetic pathway.     

Characterization of the Charge Transfer Complex from Furan and Maleic Anhydride and the Alternating Copolymer

A previously unreported copolymer between furan and maleic anhydride was readily obtained by free radical initiation of benzene solutions of the comonomers. The product copolymers exhibit remarkably uniform composition regardless of monomer feed ratios. A donor-acceptor complex was suspected to account for the 1:1 alternating sequence of the monomers in the product. The complex was easily detected and partially characterized, although its exact role in the mechanism is uncertain.     

Studies in Cyclocopolymerization. X. Cyclocopolymerization of Tetrahydronaphthoquinone and Dimethyl Tetrahydronaphthoquinone with Divinyl Ether

Abstract

Tetrahydronaphthoquinone (THNQ) and dimethyl tetrahydronaphthoquinone (DMTHNQ) were found by UV spectroscopy to form donor-acceptor complexes with divinyl ether (DVE), the latter being the electron donor. Since the participation of such complexed species has been considered in the cyclocopolymerization of a 1,4-diene with a monoolefin such as DVE-maleic anhydride (MA) and DVE-fumaronitrile (FN) systems, radical copolymerization of THNQ and DMTHNQ with DVE was studied. It was found that these copolymers have constant 1:1 composition regardless of the feed composition. The terpolymerization of DVE-THNQ-DMTHNQ confirmed the 1:1 donor-acceptor composition in the polymer. The integration of the NMR spectrum was used in determining the copolymer composition. The spectroscopic data suggest a cyclized repeating unit in which the copolymer main chain consists of only DVE units. There is a marked difference between these copolymers and the typical cyclo-copolymers, such as DVE-MA and DVE-FN, in which the copolymer main chains consist of DVE and the comonomer alternately, with the overall composition being 1:2. These results are interpreted in terms of the steric effect by the bulky acceptor monomers and the electronic interaction between the comonomers. The competition between an acceptor monomer and the charge-transfer (CT) complex toward the cyclized DVE radical in the propagation step appears to favor the CT complex.     

The Fundamental Basis for Cyclopolymerization. VII. A Cyclopolymerization Study of Certain Allylcycloalkenes

The cyclopolymerization characteristics of 3-allylcyclopentene, 3 allylcyclopentene, 3-allylcyclohexene, and 4-allylohexene were studied and the extent of their cyclopolymerization by cationic initiation were compared with previously reported solvolysis results involving the same intermediate carbonium ion. The results of this study were also compared with previously reported results on 4-vinylcyclohexene and 1, 4-dimethylenecyclohexane. The allylcycloalkenes were also polymerized by metal alkyl coordination catalysts and the extent of cyclization in each case was determined. The polymers derived from these monomers invariably contained polymeric fractions (27 to 99%) which were soluble in non-polar organic solvents. Calculations based on NMR spectral data indicated the cationic polymers were 68 to 95% cyclized. The extent of cyclization was found to parallel the interolefinic separation in the monomer. Coordination polymerization yielded somewhat lower ratios of cyclizations (54 to 80%) in all cases except for 4-allylcyclohexene. 4-Allylcyclohexene gave only 5 to 54% cyclization, 4-Vinylcyclohexene, 3-allylcyclopentene, and 3-allylcyclohexene produced soluble copolymers with maleic anhydride. Elemental analysis indicated the composition of these copolymers to be 2:1 molar in maleic anhydride and diene units. A cyclic copolymerization mechanism suggested the formation of a multicyclic repeating unit. The other monomers gave crosslinked polymers since maleic anhydride could not be incorporated into a six-membered ring.     

Palladium on Carbon-Catalyzed Cross-Coupling of Aryl Halides with Potassium p-Tolyltrifluoroborate in Air

Palladium supported on carbon (Pd/C) has been shown to be an effective catalyst for the cross-coupling of potassium p-tolyltrifluoroborate with a variety of aryl bromides and iodides. Yields ranging from moderate to good were obtained using Pd/C in ethanol/water mixtures with potassium carbonate as base at 50 °C under an air atmosphere.     

Diverted total synthesis of falcitidin acyl tetrapeptides as new antimalarial leads

We report not only the convergent total synthesis of falcitidin, a natural inhibitor of falcipain-2 from myxobacterium Chitinophaga, but also its diversification into a new antimalarial class of N-acyl tetrapeptides (Acyl-His-Ile-Val-Pro-NH₂). Despite the lack of whole-cell activity of falcitidin itself, our study led to the identification of a trifluoromethyl (CF₃) analogue displaying sub-micromolar IC₅₀ activity against Plasmodium falciparum 3D7 in a standard blood-cell assay, but only when N-tritylated on its histidine (imidazole) residue.     

Novel microwave plasma‐assisted CVD reactor for diamond delta doping

We report on building a novel chemical vapor deposition (CVD) reactor for diamond delta‐doping. The main features of our reactor are: a) the use of rapid gas switching system, (b) the reactor design providing the laminar gas flow. These features provide the creation of ultra‐sharp interfaces between doped and undoped material and minimize the prolonged ”tails” formation in the doping profile. It is proved by optical emission spectroscopy that gas switching time is not more than 10 seconds. Using the novel reactor we have grown the nanometer‐thin layers of boron doped diamond. The FWHM of boron concentration profile is about 2 nm which is proved by SIMS. It is shown that the both single delta‐layer and multiple delta‐layers could be grown using the novel CVD reactor. In principle, the reactor could be used for diamond delta doping with other dopants, like nitrogen, phosphorus etc. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

High sensitivity and high selectivity terahertz biomedical imaging

We demonstrate two distinct emerging terahertz (THz) biomedical imaging techniques.One is based on the use of a new single frequency THz quantum cascade laser and the other is based on broadband THz time domain spectrocopy.The first method is employed to derive a metastasis lung tissue imaging at 3.7 THz with clear contrast between cancerous and healthy areas.The second approach is used to study an osseous tissue under several imaging modalities and achieve full THz spectroscopic imaging based on the freque...