Diverged Plant Terpene Synthases Reroute the Carbocation Cyclization Path towards the Formation of Unprecedented 6/11/5 and 6/6/7/5 Sesterterpene Scaffolds

Sesterterpenoids are a relatively rare class of plant terpenes. Sesterterpene synthase (STS)‐mediated cyclization of the linear C₂₅ isoprenoid precursor geranylfarnesyl diphosphate (GFPP) defines sesterterpene scaffolds. So far only a very limited number of STSs have been characterized. The discovery of three new plant STSs is reported that produce a suite of sesterterpenes with unprecedented 6/11/5 and 6/6/7/5 fused ring systems when transiently co‐expressed with a GFPP synthase in Nicotiana benthamiana. Structural elucidation, feeding experiments, and quantum chemical calculations suggest that these STSs catalyze an unusual cyclization path involving reprotonation, intramolecular 1,6 proton transfer, and concerted but asynchronous bicyclization events. The cyclization is diverted from those catalyzed by the characterized plant STSs by forming unified 15/5 bicyclic sesterterpene intermediates. Mutagenesis further revealed a conserved amino acid residue implicated in reprotonation.     

Catalytic Gas‐Phase Production of Lactide from Renewable Alkyl Lactates

A new route to lactide, which is a key building block of the bioplastic polylactic acid, is proposed involving a continuous catalytic gas‐phase transesterification of renewable alkyl lactates in a scalable fixed‐bed setup. Supported TiO₂/SiO₂ catalysts are highly selective to lactide, with only minimal lactide racemization. The solvent‐free process allows for easy product separation and recycling of unconverted alkyl lactates and recyclable lactyl intermediates. The catalytic activity of TiO₂/SiO₂ catalysts was strongly correlated to their optical properties by DR UV/Vis spectroscopy. Catalysts with high band‐gap energy of the supported TiO₂ phase, indicative of a high surface spreading of isolated Ti centers, show the highest turnover frequency per Ti site.     

Chiral Bifunctional Phosphine‐Carboxylate Ligands for Palladium(0)‐Catalyzed Enantioselective C−H Arylation

Previous enantioselective Pd⁰‐catalyzed C?H activation reactions proceeding via the concerted metalation‐deprotonation mechanism employed either a chiral ancillary ligand, a chiral base, or a bimolecular mixture thereof. This study describes the development of new chiral bifunctional ligands based on a binaphthyl scaffold which incorporates both a phosphine and a carboxylic acid moiety. The optimal ligand provided high yields and enantioselectivities for a desymmetrizing C(sp²)?H arylation leading to 5,6‐dihydrophenanthridines, whereas the corresponding monofunctional ligands showed low enantioselectivities. The bifunctional system proved applicable to a range of substituted dihydrophenanthridines, and allowed the parallel kinetic resolution of racemic substrates.     

Catalytic Desymmetrizing Dehydrogenation of 4‐Substituted Cyclohexanones through Enamine Oxidation

A desymmetrizing dehydrogenation process catalyzed by a chiral primary amine is described herein. The reaction proceeds through the oxidation of a ketone enamine by IBX and enables the highly enantioselective desymmetrization of 4‐substituted cyclohexanones with the generation of chiral 4‐substituted cyclohexenones containing a remote γ‐stereocenter.     

Copper‐Catalyzed Enantioselective Arylative Desymmetrization of Prochiral Cyclopentenes with Diaryliodonium Salts

A copper‐catalyzed enantioselective arylative desymmetrization of prochiral cyclopentenes with diaryliodonium salts was developed. In the presence of a catalytic amount of a chiral copper–bisoxazoline complex, which was generated in situ, the reaction of 4‐substituted or 4,4‐disubstituted cyclopent‐1‐enes with diaryliodonium hexafluoroarsenates afforded the chiral arylated products in good yields with excellent enantioselectivity. A cyclohexyl‐containing Box ligand was essential for the high enantioselectivity. Transformation of the enantiomerically enriched adducts into other chiral building blocks is also documented.     

Thermally Rearranged Polymer Membranes Containing Tröger's Base Units Have Exceptional Performance for Air Separations

The influence of segmental chain motion on the gas separation performance of thermally rearranged (TR) polymer membranes is established for TR polybenzoxazoles featuring Tröger's base (TB) monomer subunits as exceptionally rigid sites of contortion along the polymer backbone. These polymers are accessed from solution‐processable ortho‐acetate functionalized polyimides, which are readily synthesized as high‐molecular‐weight polymers for membrane casting. We find that thermal rearrangement leads to a small increase in d‐spacing between polymer chains and a dramatic pore‐network reconfiguration that increases both membrane permeability and O₂/N₂ selectivity, putting its performance above the 2015 upper bound.     

Reversible Clustering of Gold Nanoparticles under Confinement

A limiting factor of solvent‐induced nanoparticle self‐assembly is the need for constant sample dilution in assembly/disassembly cycles. Changes in the nanoparticle concentration alter the kinetics of the subsequent assembly process, limiting optical signal recovery. Herein, we show that upon confining hydrophobic nanoparticles in permeable silica nanocapsules, the number of nanoparticles participating in cyclic aggregation remains constant despite bulk changes in solution, leading to highly reproducible plasmon band shifts at different solvent compositions.     

Helicenophyrins: Expanded Carbaporphyrins Incorporating Aza[5]helicene and Heptacyclic S‐Shaped Aza[5]helicene Motifs

Incorporation of phenanthrene into a hexaphyrin(1.1.1.1.1.0) frame resulted in intramolecular ring fusion, thus giving rise to chiral helicenophyrins. These molecules contain helicene and porphyrin features by incorporating either an aza[5]helicene or heptacyclic S‐shaped aza[5]helicene.     

Synthesis of (−)‐Hebelophyllene E: An Entry to Geminal Dimethyl‐Cyclobutanes by [2+2] Cycloaddition of Alkenes and Allenoates

The first synthesis of hebelophyllene E is presented, along with assignment of its previously unknown relative configuration through synthesis of epi‐ent‐hebelophyllene E. Development of a catalytic enantioselective [2+2] cycloaddition of alkenes and allenoates provides access to the required chiral geminal dimethylcyclobutanes. Key to its success is the identification of a novel oxazaborolidine catalyst which promotes the cycloaddition in high enantioselectivities with good functional‐group tolerance (9 examples, up to 97:3 e.r.). Thus, a late‐stage cycloaddition using a fully functionalized alkene, followed by a diastereoselective reduction allows a concise entry to this class of natural products.     

Copper‐Catalyzed Heteroarylboration of 1,3‐Dienes with 3‐Bromopyridines: A cine Substitution

A method for the heteroarylboration of 1,3‐dienes is presented. The process involves an unusual cine substitution of 3‐bromopyridine derivatives to deliver highly functionalized heterocyclic products. Mechanistic studies are included that clarify the details of this unusual process.