Angewandte Chemie International Edition

A porous copper coordination framework grew epitaxially as a single crystal on the surface of a single crystal of a porous zinc coordination framework, as described by S. Kitagawa and co-workers in their Communication on page 1766 ff. The picture shows the structural relationship between the copper and zinc frameworks, which has been unveiled by synchrotron surface X-ray diffraction measurements; in-plane rotational epitaxial growth compensates for the different lattice constants of the two crystals.

Carbohydrates are slippery customers in water. Camouflaged by solvent-mimicking hydroxy groups, they make challenging targets, even for natural receptors. In their Communication on page 1775 ff., A. P. Davis and co-workers describe a synthetic receptor that is remarkably effective for the important β-N-acetylglucosaminyl (β-GlcNAC) unit. The affinities of the receptor are good and its selectivities are excellent, even by the standards of natural carbohydrate-binding proteins.

“I chose chemistry as a career because… I like the creative challenge. My favorite subjects at school were mathematics and music…!” These facts and more information about Alan E. Rowan can be found on page 1717

Improving on Mother Nature? The carbohydrate recognition demonstrated by supramolecular systems in water can now compete with that of natural systems, both in terms of affinity and selectivity. A synthetic carbohydrate receptor displays similar affinity for N-acetyl-D-glucosamine derivatives as the lectin wheat germ agglutinin and even greater selectivity (see picture: gray C, white H, blue N, red O).

Undue influence: N-heterocyclic carbenes (NHCs) were found to reduce the strength of the BH bonds of borane by a surprisingly large amount upon the formation of NHC–BH₃ complexes. This property was exploited in the development of a suite of NHC–borane complexes for the reduction of xanthates in radical-mediated Barton–McCombie-type deoxygenation reactions (see scheme). AIBN=azobisisobutyronitrile, Bn=benzyl.

The light-induced formation of a covalent bond between an alkene group and a tetrazole moiety has been used for the selective labeling of proteins in vitro and in live cells. This bioorthogonal ligation initiated by brief irradiation with UV light leads to a fluorescent pyrazoline adduct (see scheme).

Surfaces with purposes: The electroinitiated patterning of self-assembled monolayers enables the fabrication of a variety of complex nanostructures (see picture). The possibilities offered by the introduction of chemical selectivity through the local generation of chemically active groups and subsequent derivatization are reviewed, with a focus on progress in this area of research over the last four years.

Small molecules against Alzheimer's: The pathological aggregation of the tau protein is a major hallmark of neurodegenerative diseases such as Alzheimer's disease. The inhibition or reversal of tau aggregation is a potential therapeutic strategy that is currently undergoing clinical trials. The image shows pathological fibers assembled from tau protein, which are the main components of the neurofibrillary tangles of Alzheimer's disease.

Getting organized: Assemblies of ferromagnetic FePt nanoparticles were generated with large perpendicular magnetic anisotropy by a magnetic-field-assisted layer-by-layer method, and subsequently layer-by-layer films consisting of L1₀-FePt nanoparticles and organic polymers were prepared. These films are phototunable when photochromic molecules are used as polymer layers.

It's not easy being green: Real-time visualization of labeled ribosomes and de novo synthesized green fluorescent protein molecules using single-molecule-sensitive fluorescence microscopy demonstrates that the mutant GFPem is produced with a characteristic time of five minutes. Fluorescence of the fastest GFP molecules appears within one minute (see picture).

Complex patterns: The arene manganese tricarbonyl complexes [Mn(η⁵-2,5-didodecoxy-1,4-semiquinone)(CO)₃] and [Mn(η⁶-1,4-dioctyloxybenzene)(CO)₃] BF₄ form patterned monolayers on the surface of highly ordered pyrolytic graphite (HOPG), as a result of hydrogen-bonding, hydrophobic, and electrostatic interactions, leading to an ordered 2D array of manganese atoms or ions.

MOF on MOF: Core–shell porous coordination polymer (PCP) crystals are fabricated at the single-crystal level by epitaxial growth in solution. Synchrotron X-ray diffraction measurements unveiled the structural relationship between the shell crystal and the core crystal, where in-plane rotational epitaxial growth compensates the difference in lattice constant.

Sniffing out proteins: Fluorescent DNA G-quadruplexes have been used for building versatile signaling receptors for proteins in a single solution. Introducing a protein sample to the ensemble results in a unique emission signature for unambiguous identification (see scheme, R=fluorophore). The self-assembled, pattern-based protein detection systems are easily fabricated, have the potential for high-throughput operations, and have the ability to handle small protein samples.

Changing employment: Receptor 1 binds β-N-acetylglucosaminyl (β-GlcNAc) up to 100 times more strongly than it does glucose. This synthetic lectin shows affinities similar to wheat germ agglutinin (WGA), a natural lectin used to bind GlcNAc. Remarkably, 1 is more selective than WGA. It favors especially the glycoside unit in glycopeptide 2, a model of the serine-O-GlcNAc posttranslational protein modification.

Picky ferryl: The complex [Fe(Tp)(BF)] (Tp=hydrotris(3,5-diphenylpyrazolyl)borate; BF=benzoylformate) reacts with O₂ to generate an oxidant (see picture; O red, pink; Fe yellow; N blue; C gray; H white) that oxidizes added hydrocarbons shape-selectively. Discrimination derives from a cleft formed by two phenyl groups of the Tp ligand, favoring oblate spheroidal substrates.

How PHDs achieve specificity: trans-4-prolyl hydroxylation of the transcription factor HIF occurs with stereochemical retention. Substrate-analogue studies show how the von Hippel Lindau tumor suppressor protein (pVHL) and the oxygen-sensing hydroxylases (PHDs) achieve specificity for hydroxyprolyl/prolyl residues for the C⁴-exo/endo prolyl conformations, respectively.

Through different windows: One major obstacle in energy research is the complexity and variety of compounds present in crude oil. A study of different ionization methods for mass spectrometry shows that the mass spectrum very strongly depends on which method is used.

Side-chain poly[2]catenanes at the click of a switch! A bistable side-chain poly[2]catenane has been synthesized and found to form hierarchical self-assembled hollow superstructures of nanoscale dimensions in solution. Molecular electromechanical switching (see picture) of the material is demonstrated, and the ground-state equilibrium thermodynamics and switching kinetics are examined as the initial steps towards processible molecular-based electronic devices and nanoelectromechanical systems.

Responsive supramolecular gels were constructed from crown ether terminated four-arm star poly(ε-caprolactone) (PCL–DB24C8) and dibenzylammonium-terminated two-arm PCL–DBAS (see scheme), exploiting the formation of pseudorotaxane linkages between crown ether and ammonium moieties. The resultant supramolecular gels exhibit thermo- and pH-induced reversible gel–sol transition.

Give me an “O”! Mononuclear nonheme iron(IV) oxo complexes have been generated using water as an oxygen source and cerium(IV) as an oxidant. The high-yield oxygenation of organic substrates in this system (see picture, Fe green, O red, N blue, C gray) is catalyzed by iron(II) complexes. The source of oxygen in the iron(IV) oxo complexes and the oxygenated products has been assigned unambiguously by isotopic labeling experiments.

Set free by light: Near-IR (NIR) laser-initiated remote release of fluorescent dye from complexes of liposome–gold-nanoparticle aggregates is demonstrated (see fluorescence images). Complexes of the desired size are shown to be a viable approach to the construction of vesicle-based drug-delivery systems with light-triggered remote release characteristics. This opens up a new method to manipulate liposome-based drug-delivery systems in a biocompatible way by using the near-IR spectral range.

Cobalt cyclopentadienyl complexes incorporating a fumarate and a CO ligand (see picture) efficiently catalyze inter- and intramolecular [2+2+2] cycloadditions of alkynes, nitriles, and/or alkenes to give benzenes, pyridines, or 1,3-cyclohexadienes. Unlike catalysts such as [CpCo(CO)₂] or [CpCo(C₂H₄)₂] (Cp=C₅H₅), they are air-stable, easy to handle, compatible with microwave conditions, and do not necessarily require irradiation to be active.

Best of both worlds: Reduction of an organometallic Co precursor on preformed CdSe nanorods yields two distinct semiconducting–magnetic heterostructures (see picture). The selective growth of Co on the tips of CdSe first gives nanosphere–nanorod dimers, which evolve into nanorod–nanorod structures. In the hybrid objects the magnetic properties of Co remain intact, while the luminescence properties of CdSe are affected but not completely quenched.

Fluorinated building blocks by CF bond cleavage: Catalytic CF activation reactions that give novel dioxaborolanes have been developed (see scheme). The reactions proceed at room temperature, and catalytic intermediates are presumably rhodium hydride and boryl species.

Special delivery: Liposomal drug-delivery systems in which prodrugs are activated specifically by disease-associated enzymes have great potential for the treatment of severe diseases, such as cancer. A new type of phospholipid-based prodrug has the ability to form stable small unilamellar vesicles (see picture). Activation of the prodrug vesicles by the enzyme sPLA₂ initiates a cyclization reaction, which leads to the release of the drug.

The quaternary king: Azithromycin (1), which has improved pharmacological profiles compared with erythromycins, was the target of an enantioselective synthesis. All the stereogenic quaternary carbon centers were elaborated by a desymmetrization of 2-substituted glycerols using a chiral imine/CuCl₂ catalyst.

Pd and CO—ureally got me! The title reaction proceeds efficiently at 18 °C under CO (1 atm) with 5 % [Pd(OTs)₂(MeCN)₂] as precatalyst. Depending on the solvents used, either anthranilates or cyclic imides can be obtained in high yields (see picture, BQ=benzoquinone, Ts=4-toluenesulfonyl).

The silene (Me₃Si)₂SiAd is polymerized to produce a polycarbosilane with an unusual SiSiC repeating backbone, rather than the SiC or SiSiCC units expected for olefinic radical polymerization. The polymer structure and the polymerization mechanism (see scheme) were studied by GPC, EPR, and NMR spectroscopy and by trapping experiments.

The moderate nucleophilicity of the axially chiral amino sulfonamide (S)-1 suppresses the problematic side reactions, including aldol reactions, in the asymmetric Mannich reaction of N-Boc-protected imines with aldehydes. The corresponding adducts are obtained in good yield and excellent stereoselectivity (see scheme; Boc=tert-butoxycarbonyl, Tf=trifluoromethanesulfonyl).

Light, inexpensive, effective: Nanostructured Co₃O₄ clusters (see picture) in mesoporous silica are the first example of a nanometer-sized multielectron catalyst made of a first-row transition-metal oxide that evolves oxygen from water efficiently. The nanorod bundle structure of the catalyst results in a very large surface area, an important factor contributing to the high turnover frequency.

(−)-Dysibetaine has been synthesized in 11 steps from readily available L-malic acid (see scheme). The key step is a unique Ugi 4-center-3-component cyclization reaction, where an ester group acts as the carboxylic acid component. The use of 1,1,1,3,3,3-hexamethyldisilazane as an ammonia equivalent and a specially designed isocyanide leads to an expeditious synthesis.

They all fall down: The value of domino processes can be greatly enhanced when the possibility exists for one to selectively diverge from a common intermediate. In preliminary studies the dual reactivity of aryl–palladium intermediates is exploited. A diverse array of fluorene and phenanthrene derivatives were synthesized in a rapid and efficient manner (see scheme).

Phosphorus meets carbohydrates: Dimethyl phosphite reacts with ceric(IV) ammonium nitrate (CAN) to give phosphonyl radicals that add to glycals 1. The derivatives 2 were isolated in high yields and during a subsequent Horner–Emmons reaction underwent an interesting elimination to give 3,6-dihydro-2H-pyrans 3. The short sequence with simple precursors is applicable to the transformation of hexoses, pentoses, and disaccharides. Bn=benzyl.

An adaptable approach: The first highly convergent stereoselective synthesis of feglymycin (see structure) and its enantiomer is based on the coupling of repeating peptide fragments. The use of weakly basic conditions throughout the synthesis suppressed the epimerization of sensitive aryl glycine units. Feglymycin has strong anti-HIV activity as well as potent (previously identified as weak) antibacterial activity against Staphylococcus aureus.