Chemistry - A European Journal

A chiral tripodal receptor has been developed by S. Roelens, J. Jiménez-Barbero et al. in their Communication on page 414 ff., to enantioselectively recognize β-mannose and β-mannosides. The chiral receptor is the most effective synthetic receptor for mannosides reported to date and highlights the key role of receptor chirality in the recognition of carbohydrates. The structure of the receptor has been analyzed by using a combination of NMR spectroscopy and molecular modeling.

A novel polyketide Kendomycin with a unique quinone methide ansa structure and an impressive biological activity profile is obtained through two concise and efficient syntheses of the compound, one of which features a novel photo-Fries rearrangement to close a highly strained all carbon macrocycle. For details see the Full Paper by J. Mulzer et al. on page 507 ff.

Go catalytic! Simple transmetalation from lithium to zinc attenuates the basicity and nucleophilicity of silicon and tin main group organometallics, thereby rendering the stoichiometric use of copper superfluous. All common carbonsilicon and selected carbontin bond formations are now catalytic in copper (see scheme; r.s.=regioselectivity).

Chiral is better! Superior recognition properties are shown by a pyrrolic tripodal receptor featuring a chiral diamine in its architecture. Highly enantioselective recognition of the β-mannosyl residue is achieved by one of the two enantiomers of the receptor, with remarkable affinity in a polar medium and distinct α/β selectivity.

Which way to go? The cyclometallated compounds [(η⁵-C₅Me₅)Ir(X){PMe(2,6-CH₂(Me)C₆H₃)(2,6-Me₂C₆H₃)}] (X=Cl, H, Me) have been synthesized in high yields and their potential for CH activation demonstrated. Deuterium incorporation into the metallated ligand backbone has been achieved for X=Cl and X=H, with dissimilar results and hence through different reaction pathways.

Silver-assisted detection: A novel circular dichroism (CD) method was described for detecting L-cysteine in biological fluids on the basis of chiral nanoparticle self-assembly from Ag^I and L-cysteine (see graphic). The method allows highly selective and sensitive detection of L-cysteine in biological fluids at the μM level without the need for any special synthesis or sample pretreatment other than an appropriate dilution.

All at once: Microwave irradiation of a metal-free mixture of propargyl enol ethers and primary amines generates substituted alkyl 1,2-dihydropyridine-3-carboxylates in excellent yields through a domino process (see scheme). The obtained 1,2-dihydropyridines feature four possible diversity points and a chemical handle for complexity-diversity generation (carboxylic ester at the C₃ position).

Diarsenic as a Lewis acid: An N-heterocyclic carbene stabilized diarsenic molecule [L:AsAs:L] (L:=:C{N(2,6-iPr₂C₆H₃)CH}₂) (see scheme) has been prepared by potassium/graphite reduction of [L:AsCl₃]. The nature of the bonding in this molecule was probed by DFT computations.

P₄activation but without reduction is the most striking result of the reaction of Ni^I precursors (LNi) with white phosphorus. This reaction leads to the complexes 1 a and 1 b, which feature the scarce η³-coordination of the P₄ ligand between two metal centers. Remarkably, 1 a undergoes facile dissociation in solution to give the paramagnetic Ni^I complex 2. L=β-diketiminate; 1 a: R=iPr₂C₆H₃, 1 b: R=2,6-Et₂C₆H₃.

Like water from a duck's back: A highly enantioselective organocatalytic cascade aza-Michael/aldol reaction of indole-2-carbaldehydes with α,β-unsaturated aldehydes has been developed, providing an easy access to pyrrolo[1,2-a]indole-2-carbaldehydes with moderate to excellent enantioselectivities in one single reaction (see scheme).

Trinity: The combination of rhodium/phosphine/amine⋅HBr enables the highly selective catalytic cross-cyclodimerization of diarylacetylenes with aliphatic alkynes to afford the corresponding multiply substituted naphthalenes in good-to-high yields. The catalyst system also allows the use of some alkenes as the coupling partners, which provides a facile route to highly functionalized dihydronaphthalenes (see scheme).

Acidity effect: Correlations of pK_a with catalytic activity and stereoselectivity were determined and linear free energy relationships (LFERs) were observed for both pK_a−log (k) and pK_a−log (R/S) correlations in meta- and/or para-substituted aromatic thioureas (see figure). These results provided a basis for new catalyst development and several improved catalysts were identified in our initial attempts.

Rapid access to heterobicycles: A novel cationic gold(I)-catalyzed tandem heterobicyclization and 1,2-alkyl migration was developed, which provides a rapid, efficient, and stereoselective access to highly substituted furo[3,4-c]azepines from simple, readily available 2-(1-alkynyl)-2-alken-1-ones and α,β-unsaturated imines under mild conditions (see scheme). In contrast, when heteroaryl imines were used, only direct formal [4+3] cycloadducts were formed.

Inverse gear! The hydrogen atom abstraction from [(L)ClRh^IIIH/D]⁺ by methyl radicals (see scheme) involves a substantial inverse isotope effect. This result suggests that the detailed mechanism of this process involves the formation of a transient complex with a RhC σ bond followed by reductive elimination.

Get cross! A cruciform 6,6′-dipentacenyl, Dip-TIPS, was prepared as a unique semiconductor with two-dimensional isotropic face-to-face π-stacking in the crystal structure. High FET mobilities were obtained from the vapor-deposited thin films. The use of cruciform molecules in the design of organic semiconductors is a new concept that is expected to diminish the effect of molecular orientation on device performance.

Ground-state character of a series of quinoidal oligothiophenes Q2–Q6 (n=2–6 in the depicted formula) was investigated experimentally and by DFT calculations. For Q4–Q6, the latter indicate biradical character that increases with increasing oligomer length. Furthermore, a low-lying triplet state very close in energy to the singlet ground state is predicted. Raman and IR spectra support the quinoidal structures computed for Q2 and Q3 and aromatic biradical structures predicted for Q5 and Q6.

A new hydroxide-induced decarboxylative Grob-type fragmentation for the stereoselective synthesis of methyl-branched trisubstituted olefins, an important motif in many polyketides with interesting biological activity, such as peloruside A, discodermolide, and epothilone D, is presented. This strategy centers on mesyloxy lactones (see scheme) with three stereogenic centers, one quaternary, which upon treatment with hydroxide fragment extrusion of the leaving group and carbon dioxide to the olefin.

Underestimated: The thus far underestimated photo-Fries reaction serves as an efficient tool for the total synthesis of (−)-kendomycin. A powerful ring-closing metathesis is the basis of the second total synthesis. The installation of the lactol unit was performed by a chemoselective oxidation–hydrolysis sequence, thus avoiding additional protecting groups.

Solvent-controlled right or left turn in hemicryptophane–oxidovanadium(V) complexes! The helical chirality of the propeller-like vanatrane moiety gives rise to different diastereomers (see figure). The interconversion between the Λ and Δ forms of the propeller is strongly solvent-dependent and the energy barrier is the highest ever reported for atrane derivatives, a consequence of the cage structure.

Chiral molecular conductors: Electrocrystallization experiments with the chiral (R)-, (S)-, and (rac)-ethylenedithio-tetrathiafulvalene-methyl-oxazoline (EDT-TTF-OX) donors provide two series of crystalline mixed-valence salts with the PF₆⁻ and [Au(CN)₂]⁻ anions (see figure). The conductivity measurements show metallic behavior in the high-temperature regime with a much lower conductivity when the oxazoline ring is structurally disordered.

Get in shape! Ag§AgCl samples with hierarchical microstructures including rods, irregular balls, and hollow spheres (see picture) have been fabricated by an ion-exchange process and light-induced chemical reduction reaction. The photocatalytic properties of the samples were evaluated by measuring the decomposition of methyl orange under visible light.

Significant substitution: Two coupled photochromic systems that involve hydroxy-4-pyridinechromene and pyridinechalcone are obtained when the phenyl group in 2-hydroxychalcones is substituted by a 4-pyridine unit (see scheme).

Orange agent: The optical absorbance and fluorescence characteristics of acridine orange single-wall carbon nanotube (AO–SWCNT) conjugates display interesting pH-dependent properties. The AO–SWCNTs can enter HeLa cells and are located inside lysosomes by means of clathrin-mediated endocytosis (see picture).

Modifiable inside and out: A single-step preparation of chitosan core–shell microspheres (see picture) by a sonochemical method is described. The surface of the microspheres can easily be modified by different electrophiles, while the inside can be loaded with different agents.

Keen sensors! We present a highly sensitive fluorescent chemosensor based on 4-amino-1,8-naphthalimide, which shows an off–on fluorescence change concomitantly with the appearance of a new absorption band at λ=380 nm in response to zinc ions.

Super acid for super people: A superacid-like activation of alkanes occurs over zeolites. A longstanding controversy about the initial step of hydrocarbon activation on industrial catalysts (a or b, see figure), such as acidic zeolites, has been solved by experiments combining gas chromatography, mass spectroscopy and isotopic labelling of starting materials.

What an effect: A new subclass of β-aminoxy peptides, that is, peptides of acyclic β^2, 3-aminoxy acids (see picture), has been synthesized and the effect of the backbone stereochemistry on their conformations provides a useful insight into the design and construction of new foldamers with predicable structures.

The journey is the reward: DFT calculations and experimental observations were combined to elucidate the mechanistic pathway of rhodium-catalyzed aziridination and ring opening on a glycal scaffold (see scheme). The installation of a sulfamate moiety at different positions of the glycal scaffold could present a new concept for synthesizing α- and β-aminoglycosides.

Cat got your tungsten? In the present work the transition between complex formation and redox chemistry of tungsten(VI) fluorides with phosphanes is studied (see figure). The complexes studied are characterised by using H, F, and P NMR spectroscopy; EPR and IR spectroscopy; and single-crystal X-ray analysis.

Nitrogen-rich antibiotic: The synthetic venture of glycocinnasperimicin D, an amino sugar antibiotic decorated with a variety of nitrogen-containing functional groups, has been completed by a three-component coupling strategy (see scheme). The convergent tactic is realized by the Heck–Mizoroki reaction and construction of the urea glycoside by the reaction of glycosyl isocyanate with an amino sugar.

Ultrafast studies: Two polypyridyl complexes—[M(bpy)₂L]²⁺, in which M=Ru^II or Os^II, bpy=2,2′-bipyridyl, and L=4-(2,2′-bipyridinyl-4-yl)benzene-1,2-diol—have been synthesized and their interfacial electron-transfer process on a TiO₂ nanoparticle surface has been studied with femtosecond transient-absorption spectroscopy (see image).

Substrate substitutions: We describe the synthesis and application of a library of phosphite–oxazole/thiazole ligands in the Pd-catalyzed allylic substitution reactions of several substrate types (see scheme). High regio- and enantioselectivities and good activities have been achieved.

The synthesis of a tetracyclic ring system under Au catalysis: Macrocyclic propargyl esters (1) undergo a tandem 3,3-rearrangement/transannular [4+3] cycloaddition reaction in the presence of a Au^III catalyst to give a multiple fused-ring structure (2). The tetracyclic carbon core structure (3) of the potent antiangiogenesis natural product cortistatin A, which features a conjugated diene functionality, was easily obtained from 2.

π–π versus π–π: Stereoselectivity of an oxygen-transfer reaction catalysed by Ti^IV trialkanolamines (see scheme) is determined by the subtle interplay of edge-to-face and face-to-face aromatic interactions.

Golden honeycombs: Ligand-stabilized gold nanoparticles are used as building blocks to fabricate macroporous films with ordered honeycomb structures at an air/water interface. The films' fabrication is based on solvent evaporation and templated by water droplets; morphological changes in the films are dependent on the concentration of gold nanoparticles. Both two- and three-dimensional pore arrays are observed (see picture).

Unusual insertion: A series of ortho-substituted aryl–palladium(II) complexes have been reacted with unsaturated N-donor ligands giving insertion and/or addition products (see scheme).

Sulfonimidamides in three dimensions: The absolute configurations of enantiopure N-protected sulfonimidamides, which have been accessed through straightforward syntheses, were assigned by X-ray structure determination and comparison of calculated and measured CD spectraj (see scheme).

A smooth transition: X-ray absorption near-edge structure (XANES) spectroscopy has been found to be the ideal experimental technique to shed light on the structure of the hydration complexes of the Ln^III ions in aqueous solution. The analysis of the XANES spectra has provided definitive experimental proof of the retention of the tricapped trigonal prism geometry (see figure) of the Ln^III hydrated ions along the series.

Keeping our eye on the ball (milling)! The reaction pathways for hydrogen desorption/absorption of the LiNH₂/MgH₂ (1:1) system were found to depend strongly on the ball-milling duration (BM; see figure). This is due to the presence of two competing reactions in different stages including the ball milling and the heating process.

Catalytic zincate: A novel rearrangement of 2-(1-hydroxyalkyl)-1-alkylcyclopropanol, which proceeds in the presence of a catalytic amount of organozinc ate complex to give vic-diols (see scheme), has been found. The rearrangement can be applied to various types of 2-(1-hydroxyalkyl)-1-alkylcyclopropanol, which can be easily prepared from the corresponding α,β-epoxyketones and bis(iodozincio)methane.

Grown in a cavity: The magnetic properties of 3 nm iron oxide nanoparticles grown in the cavity of the DNA-binding protein from starved cells of the bacterium Listeria innocua, and in its triple-mutant, lacking the catalytic ferroxidase centre, have been investigated by TEM and static and dynamic magnetic and electron magnetic resonance measurements (see figure).

Byproduct catalysis: Four-component reactions of aldehydes with hexamethyldisilazane (HMDS), chloroformates, and nucleophiles have been developed by using a catalytic amount of water to produce various protected primary amines, β-amino esters, and β-amino ketones (see scheme; CbzCl: benzyl chloroformate). No catalyst is needed for the reactions with aldehydes bearing hydroxy groups.