Total Synthesis of Cryptocaryol A by Enantioselective Iridium‐Catalyzed Alcohol C−H Allylation

The polyketide natural product cryptocaryol A is prepared in 8 steps via iridium catalyzed enantioselective diol double C?H allylation, which directly generates an acetate‐based triketide stereodiad. In 4 previously reported total syntheses, 17–28 steps were required.     

Low‐Temperature Cationic Rearrangement in a Bulk Metal Oxide

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic‐scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn₂FeMoO₆ at unparalleled low temperatures of 150–300 ^oC. The irreversible ionic motion at ambient pressure, as evidenced by real‐time powder synchrotron X‐ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni₃TeO₆–type to an ordered‐ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.     

Peripheral Templation Generates an MII6L4 Guest‐Binding Capsule

Pseudo‐octahedral M^II₆L₄ capsules result from the subcomponent self‐assembly of 2‐formylphenanthroline, threefold‐symmetric triamines, and octahedral metal ions. Whereas neutral tetrahedral guests and most of the anions investigated were observed to bind within the central cavity, tetraphenylborate anions bound on the outside, with one phenyl ring pointing into the cavity. This binding configuration is promoted by the complementary arrangement of the phenyl rings of the intercalated guest between the phenanthroline units of the host. The peripherally bound, rapidly exchanging tetraphenylborate anions were found to template an otherwise inaccessible capsular structure in a manner usually associated with slow‐exchanging, centrally bound agents. Once formed, this cage was able to bind guests in its central cavity.     

Dehydrocoupling and Silazane Cleavage Routes to Organic–Inorganic Hybrid Polymers with NBN Units in the Main Chain

Despite the great potential of both π‐conjugated organoboron polymers and BN‐doped polycyclic aromatic hydrocarbons in organic optoelectronics, our knowledge of conjugated polymers with B?N bonds in their main chain is currently scarce. Herein, the first examples of a new class of organic–inorganic hybrid polymers are presented, which consist of alternating NBN and para‐phenylene units. Polycondensation with B?N bond formation provides facile access to soluble materials under mild conditions. The photophysical data for the polymer and molecular model systems of different chain lengths reveal a low extent of π‐conjugation across the NBN units, which is supported by DFT calculations. The applicability of the new polymers as macromolecular polyligands is demonstrated by a cross‐linking reaction with Zr^IV.     

Late-stage C−H Functionalization of Tryptophan-Containing Peptides with Thianthrenium Salts: Conjugation and Ligation

Bioorthogonal late-stage diversification of structurally complex peptides bears enormous potential for drug discovery and molecular imaging, among other applications. Herein, we report on a palladium-catalyzed C−H arylation of tryptophan-containing peptides with readily accessible and modular arylthianthrenium salts. Under exceedingly mild reaction conditions, the late-stage diversification of structurally complex peptides was accomplished. The tunability and ease of preparation of arylthianthrenium salts allowed the expedient stitching of tryptophan-containing peptides with drug, natural product, and peptidic scaffolds by forging sterically congested biaryl linkages. The robustness of the palladium catalysis regime was reflected by the full tolerance of a plethora of sensitive and coordinating functional groups. Hence, our manifold enabled efficient access to highly decorated, labelled, conjugated, and ligated linear and cyclic peptides.     

On the Brink of Decomposition: Controlled Oxidation of a Substituted Arsanylborane as a Way to Labile Group 13-15-16 Compounds

The reactivity of the organic-substituted arsanylborane tBuAsHBH₂NMe₃ ( 1 ) towards different elemental chalcogenes as well as organic oxidants such as O-NMe_3, Me₃Si−O−O-SiMe₃, MesCNO and cyclohexenesulfide is reported. By the reaction of 1 with grey selenium, the selenium oxidation product tBuAs(Se)HBH₂NMe₃ ( 2 ) was obtained. For the oxidation with sulfur, the two products tBuAs(S)HBH₂NMe₃ ( 3 a ) and tBuAs(S)SHBH₂NMe₃ ( 3 b ) could be isolated as oils. The structural characterization of As(tBuAs(S)SHBH₂NMe₃)₃ ( 4 ) as well as corresponding DFT computations allow insights into the decomposition behavior of 3 a and 3 b in solution. For the reaction of MesCNO with 1 , the formation of an unusual As−H activation product Mes-C(NOH)-AstBu-BH₂NMe₃ ( 5 ) is observed. In the reaction with Me₃N−O, the first isolatable oxo-arsanylboranes tBuAs(O)HBH₂NMe₃ ( 6 a ) and tBuAs(O)OHBH₂NMe₃ ( 6 b ) are obtained, with 6 b also being accessible via the controlled reaction of 1 with air.     

Anti-inflammatory and antioxidant activities and constituents of Platostoma africanum P. Beauv

Hexane and dichloromethane extracts of Platostoma africanum P. Beauv., a plant popularly employed in Nigeria in the treatment of rheumatic symptoms, were evaluated for anti-inflammatory and antioxidant activities. Acute inflammatory effects were studied in the egg-albumin-induced rat paw edema. Both extracts produced significant (p < 0.05) and dose-dependent inhibition of the egg-albumin-induced pedal edema with the 400 mg kg(-1) dose being markedly better than piroxicam. The hexane and dichloromethane extracts also showed significant antioxidant activity with the dichloromethane extract exhibiting an IC(50) comparable to that of BHT, a synthetic antioxidant. Phytochemical investigation of the extracts afforded eight acidic pentacyclic triterpenes, namely, ursolic acid, oleanolic acid, epimaslinic acid, maslinic acid, corosolic acid, hyptadienic, euscaphic acid and tomentic acid, and a mixture of beta-sitosterol and stigmasterol. These constituents are reported for the first time in the genus.     

Analytical speciation of chromium in in-vitro cultures of chromate-resistant filamentous fungi

In this work, different analytical speciation schemes have been used to study the reduction of Cr(VI) by a chromate-resistant strain of filamentous fungi Ed8 (Aspergillus sp), indigenous to contaminated industrial wastes. As demonstrated previously, this strain has the capability to reduce chromate present in the growth medium without its accumulation in the biomass, yet the reduced chromium end-products have not been characterized. Liquid growth medium, initially containing 50 mg L(-1) Cr(VI), was analyzed for Cr(III)/Cr(VI) and for total Cr at different time intervals (0-24 h) after inoculation with fungi. Three hyphenated procedures, based on the Cr(III)-EDTA formation and species separation by anion-exchange or ion-pairing reversed-phase chromatography with ICP-MS or DAD detection were used. The results obtained for Cr(VI) in each case were consistent, demonstrating efficient reduction of chromate during 24 h of Ed8 growth. However, pre-column complexation with EDTA did not ensure complete recovery of the reduced forms of chromium in the above procedures. An alternative speciation scheme, based on extraction of Cr(VI)-benzyltributylammonium bromide (BTAB) ion pairs into chloroform and subsequent determination of residual chromium by ICP-MS has provided evidence on the effective conversion of chromate into reduced chromium species in the growth medium. The results indicate the feasibility of using Ed8 strain for chromate bioremediation purposes. Analytically it can be concluded that speciation of chromium in biological systems should not be limited to its two most common oxidation states, because the actual reduced chromium species are not converted quantitatively to Cr(III)-EDTA.