A synthesis of the tetracyclic carboskeleton of isaindigotidione

An efficient synthesis of the unique indolizino[7,6-c]quinoline carboskeleton of isaindigotidione has been achieved. This strategy employed l-proline and isatin as the main building blocks in the construction of the framework. Four transformations occurred in a one-pot operation to furnish the tetracyclic nucleus.     

Nowhere zero 4‐flow in regular matroids

Jensen and Toft 8 conjectured that every 2‐edge‐connected graph without a K₅‐minor has a nowhere zero 4‐flow. Walton and Welsh 19 proved that if a coloopless regular matroid M does not have a minor in {M(K_3,3), M*(K₅)}, then M admits a nowhere zero 4‐flow. In this note, we prove that if a coloopless regular matroid M does not have a minor in {M(K₅), M*(K₅)}, then M admits a nowhere zero 4‐flow. Our result implies the Jensen and Toft conjecture. © 2005 Wiley Periodicals, Inc. J Graph Theory     

Phase behaviour of colloid-polymer mixtures

Summary We review the phase behaviour of mixtures of colloids and non-adsorbing polymers. The exclusion of polymer molecules from overlapping ?depletion zones? between two neighbouring colloidal particles results in an unbalanced osmotic pressure pushing the particles together. This depletion potential is separately tunable in range and depth. Theory predicts that the resulting phase behaviour is sensitive to ξ=r _g/R, the ratio of the radius of gyration of a polymer molecule, to the radius of the colloid. At large ξ, a stable colloidalliquid phase becomes possible. This has been confirmed by recent experiments. The formation of non-equilibrium ?transient gel? states when the size ratio is small (≈0.08) is also introduced briefly. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Conformationally controlled (entropy effects), stereoselective vibrational quenching of singlet oxygen in the oxidative cleavage of oxazolidinone-functionalized enecarbamates through solvent and temperature variations

On photooxygenation (methylene blue as sensitizer) of E/Z enecarbamates, equipped with the oxazolidinone chiral auxiliary, the oxidative cleavage of the alkenyl functionality releases the enantiomerically enriched methyldesoxybenzoin (MDB) product. The extent (% ee) as well as the sense (R vs S) of the stereoselectivity in the MDB formation depends on the choice of the alkene configuration; the efficacy of stereocontrol may be tuned by appropriate solvent and temperature conditions. Highlighted is the finding that the formation of the preferred MDB enantiomer (R or S) depends for the E isomer on the chosen solvent and temperature, but not for the corresponding Z isomer. The activation parameters for the various solvents disclose that differential entropy effects (ΔΔS^‡) dominate the conformationally more flexible E diastereomers. As mechanistic rationale for this unprecedented conformationally imposed stereochemical behavior, we propose the competitive action of stereoselective vibrational quenching of the attacking singlet oxygen by the enecarbamate versus sterically controlled stereoselective oxidative cleavage of its double bond.     

Tetrapyrrole derivatives substituted with ferrocenylethynyl moieties. synthesis and electrochemical studies

Up to eight redox-active ferrocenyl units have been incorporated, through the unsaturated ethynyl linkers, on the periphery of a series of cyclic tetrapyrrole derivatives including zinc(II) phthalocyanine and 2,3-naphthalocyanine, and nickel(II) meso-diphenylporphyrin. The synthesis of the former two macrocycles 4 and 7 involves the Sonogashira coupling reaction of ferrocenylethyne with 4,5-dichlorophthalonitrile (1) or 6,7-dibromonaphthalonitrile (5), respectively, followed by a base-promoted cyclization. The meso-bis(ferrocenylethynyl)porphyrin 11 has been prepared from the dibromo analogue 10 also by a palladium-catalyzed coupling reaction. These novel macrocyclic compounds have been spectroscopically and electrochemically characterized. As revealed by cyclic voltammetry, the ferrocenyl moieties appear to be electrochemically independent in these complexes and there is no significant electronic coupling among the iron(II) centers.     

A New Method for Coarse-Grained Elastic Normal-Mode Analysis

In this paper, we report a new method for coarse-grained elastic normal-mode analysis. The purpose is to overcome a long-standing problem in the conventional analysis called the tip effect that makes the motional patterns (eigenvectors) of some low-frequency modes irrational. The new method retains the merits of a conventional method such as not requiring lengthy initial energy minimization, which always distorts structures, and also delivers substantially more accurate low-frequency modes with no tip effect for proteins of any size. This improvement of modes is crucial for certain types of applications such as structural refinement or normal-mode-based sampling.