Photoinduced charge separation of phenothiazine–platinum–naphthalene diimide triads linked by twisted phenylene bridges

Two triads (i.e., 3PTZ–Pt–MNDI and 10PTZ–Pt–MNDI) consisting of 3-phenothiazine (3PTZ) or 10-phenothiazine (10PTZ), bipyridine–diacetylide platinum complex (Pt), and naphthalene diimide (MNDI) chromophores linked by highly twisted biphenylene spacers have been prepared. The formation and decay of the charge-separated (CS) states in toluene were studied by use of picosecond and nanosecond laser photolysis via selective excitation of the Pt moiety. The time required for formation of the CS state, PTZ⁺–Pt–MNDI^?, from PTZ–³Pt*–MNDI was determined to be τ _CS = 280 ps for 3PTZ⁺–Pt–MNDI^? and τ _CS = 230 ps for 10PTZ⁺–Pt–MNDI^?. The lifetimes of the CS states were determined to be τ _CR1 = 75 ns (95 %) and τ _CR2 = 285 ns (5 %) for 3PTZ⁺–Pt–MNDI^? and τ _CR = 830 ns for 10PTZ⁺–Pt–MNDI^?. Formation and decay of the CS states are discussed in terms the Marcus theory and the spin-correlated radical pair mechanism.     

Highly enantioselective michael addition of silyl nitronates to alpha,beta-unsaturated aldehydes catalyzed by designer chiral ammonium bifluorides: efficient access to optically active gamma-nitro aldehydes and their enol silyl ethers

Highly enantioselective Michael addition of silyl nitronates to alpha,beta-unsaturated aldehydes has been accomplished by the utilization of designer N-spiro C2-symmetric chiral quaternary ammonium bifluoride 1 as an efficient catalyst, providing direct access to both optically active gamma-nitro aldehydes, a very useful precursor to various complex organic molecules including aminocarbonyls, and their enol silyl ethers, a Mukaiyama donor of potential synthetic utility for further selective transformations. For instance, the reaction of trimethylsilyl nitronate 2 (R1 = Me) with trans-cinnamaldehyde (R2 = Ph, R3 = H) in toluene in the presence of (R,R)-1 (2 mol %) proceeded smoothly at -78 degrees C to give the desired enol silyl ether 3 (R1 = Me, R2 = Ph, R3 = H) in 90% isolated yield (anti/syn = 83:17) with 97% ee (anti isomer), and simple treatment of 3 thus obtained with 1 N HCl in THF at 0 degrees C afforded the corresponding gamma-nitro aldehyde 4 quantitatively without loss of diastereo- and enantioselectivity.     

Geometry optimization with QM/MM,ONIOM, and other combined methods. I. Microiterations and constraints

Hybrid energy methods such as QM/MM and ONIOM, that combine different levels of theory into one calculation, have been very successful in describing large systems. Geometry optimization methods can take advantage of the partitioning of these calculations into a region treated at a quantum mechanical (QM) level of theory and the larger, remaining region treated by an inexpensive method such as molecular mechanics (MM). A series of microiterations can be employed to fully optimize the MM region for each optimization step in the QM region. Cartesian coordinates are used for the MM region and are chosen so that the internal coordinates of the QM region remain constant during the microiterations. The coordinates of the MM region are augmented to permit rigid body translation and rotation of the QM region. This is essential if any atoms in the MM region are constrained, but it also improves the efficiency of unconstrained optimizations. Because of the microiterations, special care is needed for the optimization step in the QM region so that the system remains in the same local valley during the course of the optimization. The optimization methodology with microiterations, constraints, and step-size control are illustrated by calculations on bacteriorhodopsin and other systems.     

Asymmetric synthesis of cyclic β-amino acids and cyclic amines via sequential diastereoselective conjugate addition and ring closing metathesis

Diastereoselective conjugate addition of lithium (S)-N-allyl-N-α-methylbenzylamide to a range of α,β-unsaturated esters followed by ring closing metathesis is used to afford efficiently a range of substituted cyclic β-amino esters in high d.e. Alternatively, conjugate addition to α,β-unsaturated Weinreb amides, functional group conversion and ring closing metathesis affords cyclic amines in high d.e. The further application of this methodology to the synthesis of a range of carbocyclic β-amino esters via conjugate addition, enolate alkylation and ring closing metathesis is also described. Application of this methodology affords, after deprotection, (S)-homoproline, (S)-homopipecolic acid, (S)-coniine and (1S,2S)-trans-pentacin.     

Evaluation of reactivity for nitroxide radical trapping by correlation analysis using steric substituent parameter (ΩS)

The reactivity of nitroxide radical trapping (NRT), where stable nitroxide radicals react with transient carbon-centered radicals to form diamagnetic molecules, was evaluated. This was done with the use of the steric substituent parameter of both radicals by correlation analysis on the basis of the reactivity data determined by Ingold's group. In the case where the transient carbon-centered radicals were not resonance stabilized, the rate constant could be well correlated by using only Ω_S parameters of R₂NO⋅ and U⋅. If the transient radicals were stabilized by resonance, the parameter to estimate the amount of the resonance stabilization of U⋅ was necessary in the regression equation to evaluate the reactivity correctly. When the spin density, calculated by PM3 UHF molecular orbital calculations, was used as the resonance stabilization parameter, the rate constant could be well evaluated by a dual-parameter regression equation. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 215–221, 1998.     

Syntheses and Properties of (Nitronyl nitroxide)-substituted Tri-phenylamine ortho-Bridged by Two Oxygen and Sulfur Atoms

A nitronyl nitroxide unit ( NN ) was linked with a triphenylamine-based condensed polycyclic skeleton DOTT to form a radical substituted donor NN - DOTT . X-ray crystal structure analysis demonstrated a flat bowl shape of the DOTT unit. EPR spectra showed the localization of electron spin on the NN unit. Chemical oxidation of the DOTT unit produced radical-substituted radical cation salts NN - DOTT ⁺ ⋅ SbF₆⁻ and NN - DOTT ⁺ ⋅ FeBr₄⁻ that are stable under ambient conditions. The magnetic behavior of NN - DOTT ⁺ ⋅ SbF₆⁻ is characterized by the strong intramolecular ferromagnetic interaction between NN and DOTT ⁺. The X-ray structural analysis of NN - DOTT ⁺ ⋅ FeBr₄⁻ shows planar structure of DOTT and 1D mixed-stack column of NN-DOTT ⁺ and FeBr₄⁻. Magnetic measurements established that NN - DOTT ⁺ ⋅ FeBr₄⁻ undergoes magnetic phase transition into a weak ferromagnet at 7 K.     

Surface plasmon resonance-based immunoassay for 17β-estradiol and its application to the measurement of estrogen receptor-binding activity

A rapid and simple surface plasmon resonance (SPR)-based immunoassay for detection of 17-estradiol was developed. The assay was designed as an inhibitive format, in which 17-estradiol–BSA conjugates are immobilized on an SPR sensor chip and the binding of antibody to the chip is measured. The binding was inhibited by 17-estradiol in the concentration range 0.468 to 21.4 nmol L^–1 with an IC₅₀ value of 2.29±0.10 nmol L^–1. Although not as sensitive as traditional radioimmunoassay (RIA) and enzyme-linked immunoassay (ELISA), this method requires no separation and washing after addition of the antibody, steps which are relatively time-consuming. Estrogen receptor (ER)-binding was then investigated using this SPR immunoassay for the determination of the amount of unbound 17-estradiol after competition with test compounds for the ER-binding. Inhibition of the binding of 17-estradiol to ER by diethylstilbestrol (DES) was successfully measured by injecting the reaction mixture into the SPR sensor after addition of the antibody. This binding assay requires no separation of unbound 17-estradiol from the mixture and no radioisotope- or fluorescence-labeling of 17-estradiol. These results show the potential usefulness of the SPR sensor both detecting 17-estradiol and evaluating the ER-binding activity of xenoestrogens such as DES in a single assay system.     

Synthetic studies of tedanolide, a marine macrolide displaying potent antitumor activity. Stereoselective synthesis of the C13-C23 segment

[structure: see text] A highly stereoselective synthesis of the C13-C23 segment of tedanolide (1), an 18-membered macrolide isolated from the Caribbean sponge Tedania ignis, displaying significant cytotoxicity against KB and PS tumor cell lines, is described which involves two stereoselective epoxidations of regioisomeric trisubstituted double bonds and a stereospecific S(N)2' methylation reaction of a trans-gamma,delta-epoxy-cis-alpha,beta-unsaturated ester as the key steps.     

Template effect in the competition between Haeckelite and graphene growth on Ni(111): quantum chemical molecular dynamics simulations

Quantum chemical molecular dynamics (QM/MD) simulations of ensembles of C(2) molecules on the Ni(111) terrace show that, in the absence of a hexagonal template or step edge, Haeckelite is preferentially nucleated over graphene as a metastable intermediate. The nucleation process is dominated by the swift transition of long carbon chains toward a fully connected sp(2) carbon network. Starting from a pentagon as nucleus, pentagons and heptagons condense during ring collapse reactions, which results in zero overall curvature. To the contrary, in the presence of a coronene-like C(24) template, hexagonal ring formation is clearly promoted, in agreement with recent suggestions from experiments. In the absence of step edges or molecular templates, graphene nucleation follows Ostwald's "rule of stages" cascade of metastable states, from linear carbon chains, via Haeckelite islands that finally anneal to graphene.     

Kinetically controlled one-pot formation of DEFGH-rings of type B physalins through domino-type transformations

The characteristic DEFGH-ring system of type B physalins has been synthesized by means of a one-pot procedure incorporating domino-type ring transformations. Unexpectedly, we found that introduction of an α-hydroxyester functionality at C17 in ring E allowed the key 7-endo oxy-Michael reaction to proceed. Originally this was thought to be an unfavored process. This afforded the desired caged ring system to be formed in a kinetically controlled manner. Consecutive treatment with AcOH at 100 °C furnished the DEFGH-ring system in one pot.