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    General Chemistry

    Crystal‐to‐Gel Transformation Stimulated by a Solid‐State E→Z Photoisomerization

    Extreme Makeover: The molecule (E)‐(5‐(3‐anthracen‐9‐yl‐allylidene)‐2,2‐dimethyl‐[1,3] dioxane‐4,6‐dione) ((E)‐AYAD) undergoes EZ photoisomerization in the crystal state, which drives a dramatic physical transformation characterized by a large volume expansion, loss of crystallinity, and high porosity. This photoinduced crystal‐to‐gel transition can be used to entrap superparamagnetic nanoparticles from the surrounding liquid.

    Abstract

    The molecule (E)‐(5‐(3‐anthracen‐9‐yl‐allylidene)‐2,2‐dimethyl‐[1,3] dioxane‐4,6‐dione) (EAYAD) undergoes EZ photoisomerization. In the solid state, this photoisomerization process can initiate a physical transformation of the crystal that is accompanied by a large volume expansion (ca. 10 times), loss of crystallinity, and growth of large pores. This physical change requires approximately 10 % conversion of the E isomer to the Z isomer and results in a gel‐like solid with decreased stiffness that still retains its mechanical integrity. The induced porosity allows the expanding gel to engulf superparamagnetic nanoparticles from the surrounding liquid. The trapped superparamagnetic nanoparticles impart a magnetic susceptibility to the gel, allowing it to be moved by a magnetic field. The photoinduced phase transition, starting with a compact crystalline solid instead of a dilute solution, provides a new route for in situ production of functional porous materials.

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    Source:: Angewandte Chemie Int. edition

          

    Identification of the C‐Glycoside Synthases during Biosynthesis of the Pyrazole‐C‐Nucleosides Formycin and Pyrazofurin

    C‐Nucleosides are characterized by a C‐C rather than a C‐N linkage between the heterocyclic base and the ribofuranose ring. While the biosynthesis of pseudouridine‐C‐nucleosides has been studied, less is known about the pyrazole‐C‐nucleosides such as the formycins and pyrazofurin. Herein, genome screening of Streptomyces candidus NRRL 3601 led to the discovery of the pyrazofurin biosynthetic gene cluster pyf. In vitro characterization of gene product PyfQ demonstrated that it is able to catalyze formation of the C‐glycoside carboxylhydroxylpyrazole ribonucleotide (CHPR) from 4‐hydroxyl‐1H‐pyrazole‐3,5‐dicarboxylic acid and phosphoribosyl pyrophosphate (PRPP). Similarly, ForT, the PyfQ homologue in the formycin pathway, can catalyze the coupling of 4‐amino‐1H‐pyrazole‐3,5‐dicarboxylic acid and PRPP to form carboxylaminopyrazole ribonucleotide. Finally, PyfP and PyfT are shown to catalyze amidation of CHPR to pyrazofurin 5′‐phosphate thereby establishing the latter stages of both pyrazofurin and formycin biosynthesis.

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    Source:: Angewandte Chemie Int. edition

          

    Organic free radicals as circularly polarized luminescence emitters

    Chiroptical properties of two chiral atropisomers of propeller‐like trityl‐based radical derivatives have been analyzed. A new absolute configuration (AC) assignment has been made, according to the combination of experimental and theoretical data. In this sense, their ACs have been determined through the comparison of the Cotton effects recorded by electronic circular dichroism (ECD) with the theoretical ECD of the open shell structures obtained by TD‐DFT calculations. Finally, their circularly polarized luminescence (CPL) responses have been addressed. Remarkably, this is the first description of organic free radicals as intrinsic CPL emitters. Opposite signed CPL has been detected for each pair of conform‐ers, with acceptable luminescent dissymmetry factors (|glum| ≈ 0.5‐0.8 x 10‐3) considering their pure organic nature. In fact, highly efficient chiral emissions have been demonstrated, according to the comparison of |glum| with their respective absorption anisotropy factors (|gabs|). This study lays the foundations for the optimizations of new magnetically active organic chiral emitters

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    Source:: Angewandte Chemie Int. edition

          

    Accessing Equatorially Di‐ and Trifluoromethylated cis‐Cycloalkanes and Saturated Heterocycles by Preferential Hydrogen‐Addition to the Substitution Sites for Dearomatization

    We report here a straightforward process in which cyclic (alkyl)(amino)carbene‐Rh‐catalyzed preferential addition of hydrogen to the substitution sites of difluoromethylated and trifluoromethylated arenes and heteroarenes occurs, leading to dearomative reduction. This strategy enables the diastereoselective synthesis of cis‐difluoromethylated and trifluoromethylated cycloalkanes and saturated heterocycles, and even allows forming all‐ cis multi‐trifluoromethylated cyclic products with defined equatorial orientation of di‐ and trifluoromethyl groups. Deuterium‐labeling studies indicate that hydrogen preferentially attacks the substitution sites of planar arenes, resulting in dearomatization with possible heterogeneous Rh as reactive species, followed by reversible or irreversible H‐addition to the nonsubstitution sites.

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    Source:: Angewandte Chemie Int. edition

          

    Volatile Organic Compound (VOC)‐Based Probe for Induced Volatolomics of Cancers

    The development of efficient protocols for cancer diagnosis remains highly challenging. An emerging approach relies on the detection in exhaled breath of volatile organic compounds (VOC) produced by tumours. In this context, we propose a novel strategy in which a VOC‐based probe is converted selectively in malignant tissues by a tumour‐associated enzyme for releasing the corresponding VOC. The latter is then detected in the breath as a tumour marker for cancer diagnosis. This approach allows the detection of several different tumours in mice, the monitoring of tumour growth and tumour response to chemotherapy. Thus, the concept of ‘induced volatolomics’ provides a new way to explore biological processes through the use of VOC‐based probes that could be adapted to many biomedical applications.

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    Source:: Angewandte Chemie Int. edition

          

    Unique Bora[1]ferrocenophanes with Sterically Protected Boron: A Potential Gateway to Helical Polyferrocenes

    Silicon‐bridged [1]ferrocenophanes are a versatile class of monomers to obtain well‐defined metallopolymers, however, their boron‐bridged analogs are far less utilized despite being significantly higher strained. We speculate that the reactivity of known bora[1]ferrocenophanes towards ring‐opening polymerization is hampered by pi‐donating R2N groups at the bridging boron atom and, therefore, prepared the first bora[1]ferrocenophanes lacking such electronic stabilization. The new isolated ferrocenophane with a 2,4,6‐triisopropylphenyl group attached to the bridging boron atom exhibits the most tilted Cp rings among all isolated strained sandwich compounds [α (DFT) = 33.3°] with a measured record value of the bathochromic shift (λmax = 516 nm). Attempts to purify the mesityl analog by vacuum sublimation transformed this monomer to a purple‐colored polymer that resulted in Cotton effects in Circular Dichroism spectroscopy. DFT calculations revealed a left‐handed helical structure for this polymer. This is the first evidence for a poly(ferrocene) with a chiral secondary structure.

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    Source:: Angewandte Chemie Int. edition

          

    Molecular Pivot‐Hinge Installation to Evolve Topology in Rare‐Earth Metal–Organic Frameworks

    Linker desymmetrization has been witnessed as a powerful design strategy for the discovery of highly connected metal‐organic frameworks (MOFs) with unprecedented topologies. Herein, we introduce molecular pivot‐hinge installation as a linker desymmetrization strategy to evolve topology of highly connected rare‐earth (RE) MOFs, where a pivot group is placed in the center of a linker similar to a hinge. By tuning the composition of pivot groups and steric hindrances of the substituents on various linker rotamers, MOFs with various topologies can be obtained. The combination of L‐SO2 with C2v symmetry and 12‐connected RE9 clusters leads to the formation of a fascinating (4,12)‐c dfs new topology. Interestingly, when replacing L‐SO2 with a tetrahedra linker L‐O, the stacking behaviors of RE‐organic layers switch from an eclipsed mode to a staggered stacking mode, leading to the discovery of an intriguing hjz topology. Additionally, the combination of the RE cluster and a linker [(L‐(CH3)6 )] with more bulky groups gives rise to a flu topology with a new 8‐c inorganic cluster. The diversity of these RE‐MOFs was further enhanced through post‐synthetic installation of linkers with various functional groups. Functionalization of each linker with acidic and basic units in the mesoporous RE‐based PCN‐905‐SO2 allows for efficient cascade catalytic transformation within the functionalized channels.

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    Source:: Angewandte Chemie Int. edition

          

    A molecular string phone at work

    Researchers from the Department of Atomically Resolved Dynamics of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg, the University of Potsdam (both in Germany) and the University of Toronto (Canada) have pieced together a detailed time-lapse movie revealing all the major steps during the catalytic cycle of an enzyme. Surprisingly, the communication between the protein units is accomplished via a water network akin to a string telephone. This communication is aligned with a ‘breathing’ motion, that is the expansion and contraction of the protein. This time-lapse sequence of structures reveals dynamic motions as a fundamental element in the molecular foundations of biology. …read more

    Source:: PhysOrg Chemistry

          

    Establishment of Guidelines for the Control of Glycosylation Reactions and Intermediates by Quantitative Assessment of Reactivity

    Stereocontrolled chemical glycosylation remains a major challenge despite vast efforts reported in the literature over many decades and so far still mainly relies on trial and error. Herein, we describe that the relative reactivity value (RRV) of thioglycosides is an indicator for revealing stereoselectivities according to four types of acceptors. Mechanistic studies show that the reaction is dominated by two distinct intermediates: glycosyl triflates and glycosyl halides from N ‐halosuccinimide (NXS)/TfOH. The formation of glycosyl halide is highly correlated with the production of α‐glycoside. These findings enable us to foresee glycosylation reactions by using RRVs as an α/β‐selectivity indicator and develop guidelines and rules for stereocontrolled glycosylation.

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    Source:: Angewandte Chemie Int. edition

          

    A Photoaffinity Displacement Assay and Probes to Study the Cyclin‐Dependent Kinase Family

    The CDK family plays a crucial role in the control of the cell cycle. Dysregulation and mutation of the CDKs has been implicated in cancer and the CDKs have been investigated extensively as potential therapeutic targets. Selective inhibition of specific isoforms of the CDKs is crucial to achieve therapeutic effect while minimising toxicity. We present a group of photoaffinity probes designed to bind to the family of CDKs. The site of crosslinking of the optimised probe, as well as its ability to enrich members of the CDK family from cell lysates, was investigated. In a proof of concept study, we subsequently developed a photoaffinity probe‐based competition assay to profile CDK inhibitors. We anticipate that this approach will be widely applicable to the study of small molecule binding to protein families of interest.

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    Source:: Angewandte Chemie Int. edition

          

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