We report a new method to synthesize rigid ladder polymers using efficient palladium catalyzed annulation reactions with low catalyst loading (1 mol %). Rigid ladder polymers with benzocyclobutene backbone linkages can be synthesized from copolymerization of readily accessible aryl dibromides and norbornadiene or polymerization of AB type monomers bearing norbornene and aryl bromide or triflate moieties. High molecular weight (10-40 kDa) rigid ladder polymers can be obtained with complete monomer conversions. Diverse monomers also gave different, fixed ladder polymer conformations. The ladder polymers exhibited excellent thermal stability, high carbonization yield, and large intrinsic porosity.
Rhodium carboxylate complexes (1 mol %) catalyze the migration of electron withdrawing groups to selectively produce 3-substituted indoles from β-substituted styryl azides. The relative order of migratorial aptitude for this transformation is ester ≪ amide < H < sulfonyl < benzoyl ≪ nitro.
A synthetic strategy featuring efficient ruthenium-catalyzed asymmetric hydrogenation of racemic α-aryloxy cyclic ketone via dynamic kinetic resolution and palladium-catalyzed intramolecular reductive Heck cyclization has been developed for the asymmetric total synthesis of (-)-galanthamine (20.1%, 12 steps) and (-)-lycoramine (40.2%, 10 steps).
Experiments show that particles smaller than the throat size of converging-diverging microchannels can sometimes be trapped near the throat. This critical phenomenon is associated with the negative dc dielectrophoresis arising from nonuniform electric fields in the microchannels. A finite-element model, accounting for the particle-fluid-electric field interactions, is employed to investigate the conditions for this dielectrophoretic (DEP) choking in a converging-diverging microchannel for the first time. It is shown quantitatively that the DEP choking occurs for high nonuniformity of electric fields, high ratio of particle size to throat size, and high ratio of particle's zeta potential to that of microchannel.
A short total synthesis of bioactive alkaloid calothrixin B have been accomplished with atom and step economy from commercially available starting materials. The key cyclization involved a Baylis−Hillman/6π electrocyclization/dehydroaromatiz‐ation sequence to construct the pentacyclic skeleton. Reduction of the resulting lactam followed by oxidation led to the characteristic quino[4,3‐b]carbazole framework. This intermediate underwent a direct oxidation with high regioselectivity to yield the final product.magnified image
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