Angewandte Chemie International Edition

Biologically active small molecules derived from natural sources, such as Panax ginseng plants and Streptomyces bacteria are shown in the title picture. In their Review on page 6032 ff., S. J. Danishefsky and R. M. Wilson demonstrate how chemical synthesis can be used to derive drug candidates from natural products.

Ambiphilic phosphine–boronates readily split singlet dioxygen under mild conditions. As described by D. Bourissou and co-workers in their Communication on page 6186 ff., the initially formed phosphine peroxides spontaneously rearrange into stable crystalline peroxoboronates. Artwork: Christian Pradel.

“My favorite subjects at school were mathematics, chemistry, and French. The three qualities that make a good scientist are curiosity, enthusiasm, and persistence …” This and more about Carsten Schmuck can be found on page 6024.

John Wiley & Sons, Hoboken 2009. 536 pp., hardcover € 139.00.—ISBN 978-0470519981

Controlling carbonyl groups: Identification of the correct ligand/counterion combination was essential to allow the efficient and selective rhodium-catalyzed conversion of a series of ketobenzaldehydes into the corresponding phthalides using the title reaction (see scheme; cod=cycloocta-l,5-diene).

Let's twist to the music: In an unprecedented reconfiguration of thermally stable atropisomers, mechanical stress induced by ultrasound waves on a polymer-bound substrate (see structure) did effectively cause racemization facilitating enantiomer interconversion.

Successful journey: This retrospective Review describes investigations into the total synthesis and evaluation of biologically active small molecules (such as iso-fludelone; see structure), as well as the development of a program directed toward the chemical synthesis of therapeutically relevant larger molecules, including the glycoprotein erythropoietin.

Getting CO₂under control: This Review highlights the challenges for carbon capture and storage technologies which have been proposed to reduce CO₂ emissions from large point sources. The most recent developments in new materials and emerging concepts for CO₂ separations by absorption, adsorption, and membranes, amongst other approaches, are discussed, with particular attention on progress in the burgeoning field of metal–organic frameworks (see example).

Looking good: High-resolution TEM can be used to determine the surface structure of a metal oxide catalyst in real space. The picture shows HR-TEM images of one catalyst particle along 〈001〉 direction far from Scherzer focus. The perimeter of the particle is highlighted by the fractured structural units.

Now available! Farnesylated and carboxymethylated Rheb (see picture) and K-Ras4B GTPases were synthesized in useful amounts by a combination of expressed protein ligation and solid-phase lipopeptide synthesis. The functionality of the proteins was proven by biochemical, biophysical, and cell-based investigations.

Zinc in: Polyoxometalate 1 reacts with Zn²⁺ ions in acetone to form a novel sandwich-type POM 2 in almost quantitative yield. The H₂O₂-based oxidation of secondary alcohols with 2 efficiently proceeds, with an activity and chemoselectivity that is very different from those of tungsten-based catalysts including 1 (see scheme; green Zn).

Starting from the same bisacetylene, different reaction conditions (palladium or copper catalysis) selectively yielded cyclic or acyclic oligomers with n=2–6 (see picture for n=3) linked by freely rotating corner units. STM images of self-assembled monolayers revealed the difference in the adsorption behavior of the acyclic and cyclic oligomers.

Wet chemistry: Organo-SOMO activation is an intricate process. The catalyst is deactivated in the absence of H₂O and its concentration is maintained with 2 equivalents of H₂O. The kinetic role of ceric ammonium nitrate (CAN) is masked by phase transfer and its limited solubility is enhanced by added H₂O. Mechanistic studies show that careful addition of H₂O to dried reagents greatly enhances reaction. TMS=trimethylsilyl.

In the loop: Engineering of the surface loop in haloalkane dehalogenases affects their enantiodiscrimination behavior. The temperature dependence of the enantioselectivity (lnE versus 1/T) of β-bromoalkanes by haloalkane dehalogenases is reversed (red data points) by deletion of the surface loop; the selectivity switches back when an additional single-point mutation is made. This behavior is not observed for α-bromoesters.

Catching the end groups: A simple procedure was used for the covalent layer-by-layer assembly of homobifunctional H₂N-poly(dimethoxysilane)-NH₂ on SiO₂ surfaces that leads to robust layer-by-layer films of optical quality (see picture; photo on left) despite the use of non-purified commercial starting materials. The films show a solvent memory for swelling and de-swelling when immersed in the corresponding solvent for each polymer.

Playing nano-tectonics: The interaction of atomic hydrogen with a single layer of hexagonal boron nitride on rhodium leads to the removal of the h-BN surface corrugation (see picture; blue region: corrugated, orange region: flat). This change of surface texture arises from the intercalation of hydrogen atoms between the h-BN skin and the metal, and can be restored by annealing to about 600 K to expel the hydrogen atoms.

Not just a pretty face: Non-invasive X-ray fluorescence spectroscopy was used to reveal the sfumato paint layer stacking method that was used by Leonardo da Vinci to paint the faces in seven of his paintings. A strong diversity in his technique could clearly be seen with this method.

Heal thyself! Self-healing superhydrophobic coatings are fabricated by preserving healing agents of reacted fluoroalkylsilane in layered polymeric coatings that are porous and rigidly flexible. When the top layer of fluoroalkyl chains decomposes or the coatings are scratched, the healing agents migrate to the surface to restore the superhydrophobicity of the coatings (see picture).

A breath of fresh air: The rate of aerial oxidation of dihydroethidium and hydrocyanine radical oxidant probes can be selectively reduced by deuteration (see picture). The reaction rate between the deuterated compounds and the superoxide radical was reduced by a much smaller factor because of mechanistic differences between the two reactions. The deuterated probes are more effective than their hydrogen analogues in vitro, in cell culture, and in vivo.

Two is better that one: A new organocatalytic route for the asymmetric preparation of the embedded piperazic acids and a Stille coupling of an ortho-chloropyrroloindole served as key steps in the total synthesis of the dimeric cyclopeptide chloptosin (see structure).

Securely nested: The first examples of σ-organoniobium compounds with [Nb^IIIR₄]⁻ and Nb^IVR₄ stoichiometries are reported. The Nb centers within the [Nb(C₆Cl₅)₄]^q− units (q=0, 1) are located in triakis tetrahedral environments formed by the combination of inner NbC₄ (see picture; gray) and outer NbCl₄ (green) tetrahedra.

Short and sweet: Chiral epoxides, prepared using (salen)cobalt-catalyzed ring-opening reactions, and a chromium catalyst controlled hetero-Diels–Alder reaction were used to set most of the stereocenters in the total synthesis of the microtubule-stabilizing agent peloruside A. The overall highly convergent route required only 20 steps in the longest linear sequence. MOM=methoxymethyl, TBS=tert-butyldimethylsilyl.

The other side: A convergent total synthesis of peloruside A (1) is described. The key strategic features are a diastereoselective lactonization to generate a C5–C9 valerolactone from the C₂-symmetric ketone 3, and a relay ring-closing metathesis reaction to produce a dehydrovalerolactone 2. A new isomer of 1, the valerolactone isopeloruside A (iso-1), was identified. MOM=methoxymethyl.

Putting the pedal to the metal: A facile strategy for the synthesis of metal nanocrystals is demonstrated that employs carbon monoxide as a reducing agent. Highly monodisperse platinum nanocubes, spherical palladium nanocrystals, and ultrathin gold nanowires can be produced within 15 minutes.

Stand and deliver: The first highly regio- and enantioselective bromoamination of chalcones has been developed which proceeds via an unusual bromonium-based mechanism to deliver the title compounds. Excellent results were obtained using 0.05 mol % of the C₂-symmetric N,N′-dioxide/scandium(III) complex under mild conditions (see scheme).

Has a nice ring to it: A concise and modular total synthesis of the naturally occurring antibiotic virginiamycin M₂ is described. A Barbier-type cyclization was used to close the 23-membered macrocycle and deliver virginiamycin M₂ in 19 steps from a chiral organosilane.

A balancing act: Complementary catalytic systems are described, in which the reactivity/selectivity balance in Pd^II-catalyzed ortho-CH olefination can be modulated to enable sequential CH functionalization for the rapid preparation of 1,2,3-trisubstituted arenes 1. Additionally, a rare example of iterative CH activation, in which a newly installed functional group directs subsequent CH activation has been demonstrated (2).

In the air tonight: A novel approach to symmetric and unsymmetric aromatic azo compounds from simple anilines catalyzed by inexpensive CuBr has been disclosed. Air (or dioxygen) was used as an oxidant under mild reaction conditions, with H₂O as the byproduct, to make this transformation environmentally benign and very easy to handle.

A door to new opportunities: The stepwise hydrolysis of a diphosphorus trication is an efficient method for the preparation of an unusual ligand-stabilized dication that contains a novel cationic [P₄O₄]²⁺ framework (see Scheme; gray C, blue N, red O, orange P). This approach demonstrates the potential of the diphosphorus trication as a source for phosphorus building blocks to be used in the construction of novel cationic ring and cluster systems.

Vitamin C for longer lifetimes: N-oxide radicals that are widely used for dynamic nuclear polarization can be reduced by scavengers such as sodium ascorbate (vitamin C) during the dissolution process, thus diminishing losses of polarization during the transfer and extending transverse and longitudinal relaxation times in NMR spectroscopy (see picture).

Singlet dioxygen is readily split by phosphine–boronates under mild conditions. The initially formed phosphine peroxides spontaneously rearrange by B→O migration. The resulting peroxoboronates have been structurally characterized, and their ability to undergo oxygen transfer reactions substantiated.

Mind the band gap: π-conjugated pentacene di-, tri-, and tetramers can be synthesized by using a versatile building block in Hay homocoupling as well as Cadiot–Chodkiewicz cross-coupling reactions. This modular approach allows the evaluation of solubility, stability, and the HOMO–LUMO gap as a function of compound length. Long-range three-dimensional π overlap occurs in the iBu₃Si-substituted pentacene dimer (see figure; n=2).

Matched or mismatched, that is not the question! The anti,anti configuration of the C7–C16 fragment of (+)-neopeltolide is stereoselectively installed in an iterative sequence of catalyst-controlled Si group and Me group transfers, even with mismatched selectivity in the former (Si=Me₂PhSi, see scheme; TBS=tert-butyldimethylsilyl).

Three strikes and you're out! A concise, asymmetric synthesis of pycnanthuquinone C underscores the biosynthetic relevance of Diels–Alder reactions of vinyl quinones. The relative and absolute configuration of the natural product has been elucidated.

Cool chirality: L-Menthol (see ball-and-stick model), a most important fragrance, is characterized in terms of its conformation, selective aggregation, and sublimation. The data indicate a preference for homoconfigured aggregates.

Homoconjugated push–pull chromophores are obtained by [2+2] cycloaddition of 2,3-dichloro-5,6-dicyano-p-benzoquinone to anilino or ferrocene donor-substituted alkynes, and in one case a spiro compound (see picture: examples with corresponding electron adsorption spectra; C gray, Cl green, N blue, O red). Significant third-order optical nonlinearities could be measured for the first time for homoconjugated push–pull systems.

The stoichiometric reduction of the MIL-47(V) framework (see picture; green VO₆ octahedra) was performed using cobaltocene as an organometallic reducing agent. The formation of a mixed-valence compound with a V³⁺/V⁴⁺ ratio of 1:1 was confirmed by magnetic susceptibility measurements. Incorporation of cobaltocene into the MIL-47(V) framework can be reversed upon treatment with water.

Be prepared: The structure of an synthetic neomycin riboswitch RNA (N1) is investigated by NMR spectroscopy. A largely disordered free structural ensemble also contains a compact conformation that resembles the ligand-bound state thus suggesting a binding mechanism by conformational capture (see scheme, red=ligand).

Standing on their own two feet: Underpotential deposition of Cu on Au(111) yields a surface onto which 1,3-benzenedicarboxylic acid (IPA) and 1,3,5-benzenetricarboxylic acid (TMA) adsorb in a bipodal configuration. Both molecules form highly crystalline isostructural monolayers, thus demonstrating the potential of the IPA moiety as tecton for self-assembled monolayers. A thin film of a Cu–TMA coordination polymer was grown on a patterned TMA monolayer.