Highly regioselective Wittig reactions of cyclic ketones with a stabilized phosphorus ylide under controlled microwave heating

Significant base and temperature effects on the Wittig reactions of cyclohexanones with (carbethoxymethylene)triphenylphosphorane under microwave irradiation were observed. For the Wittig reactions carried out in a domestic microwave oven under solvent-free conditions, the initially formed exo-olefin products isomerized into the thermodynamically more stable endo-olefins due to uncontrolled high reaction temperature. In sharp contrast, under controlled microwave heating, both exo- and endo-olefins have been selectively synthesized by accurately regulating the reaction temperature with or without a base.     

An azomethine ylide approach to complex alkaloid-like heterocycles

The nitrone above is readily available via the intramolecular aza Diels-Alder reaction of an amino acid derived triene in acetic acid. Subsequent treatment of the nitrone in refluxing toluene with substituted actetylenes produced the pictured pyrrole. At lower temperatures a 2,3-dihydroisoxazole, which is the product of a 3+2 dipolar cycloaddition, is produced. Upon heating in refluxing toluene the 2,3-dihydroisoxazole is converted to the pyrrole.     

Kinetically controlled self-assembly of pseudorotaxanes on crystallization

[reaction: see text] Mixing of equimolar amounts of cyclobis(paraquat-p-phenylene) (CBPQT(4+)) with a bis-4-methylphenyl ether (MPE twice) of a 1,5-dioxynaphthalene (DNP) derivative in MeCN/CH(2)Cl(2) (3:1) results in the formation of a [2]pseudorotaxane which, on crystallization, yields a [4]pseudorotaxane in the solid state that is stabilized by multiple [C-H...F] interactions: a mixture of the same components in a 1:3 ratio affords a crystalline [2]pseudorotaxane after vapor diffusion of methyl-tert-butyl ether into a solution of these components in MeCN/CH(2)Cl(2) (3:1).     

Microwave-assisted regioselective olefinations of cyclic mono- and di-ketones with a stabilized phosphorus ylide

A number of cyclic mono- and di-ketones underwent regioselective olefination with (carbethoxyethylidene)triphenylphospharane under controlled microwave heating. The Wittig reaction of 4-substituted cyclohexanones or 1,2- and 1,4-cyclohexanediones with the ylide at 190 °C for 20 min in MeCN afforded the exocyclic olefins in >94:6 isomer ratios. On the other hand, the same reactions carried out at 230 °C for 20 min in the presence of 20 mol % DBU furnished the endocyclic olefins in >83:17 isomer ratios. The base-mediated isomerization of the exocyclic olefins into the endocyclic isomers was primarily driven by thermodynamic stability of the products and the effect of ring structures on deconjugation was examined.     

Kinetically controlled Ag+-coordinated chiral supramolecular polymerization accompanying a helical inversion

We report kinetically controlled chiral supramolecular polymerization based on ligand–metal complex with a 3 : 2 (L : Ag⁺) stoichiometry accompanying a helical inversion in water. A new family of bipyridine-based ligands (d-L1, l-L1, d-L2, and d-L3) possessing hydrazine and d- or l-alanine moieties at the alkyl chain groups has been designed and synthesized. Interestingly, upon addition of AgNO₃ (0.5–1.3 equiv.) to the d-L1 solution, it generated the aggregate I composed of the d-L1AgNO₃ complex (d-L1 : Ag⁺ = 1 : 1) as the kinetic product with a spherical structure. Then, aggregate I (nanoparticle) was transformed into the aggregate II (supramolecular polymer) based on the (d-L1)₃Ag₂(NO₃)₂ complex as the thermodynamic product with a fiber structure, which led to the helical inversion from the left-handed (M-type) to the right-handed (P-type) helicity accompanying CD amplification. In contrast, the spherical aggregate I (nanoparticle) composed of the d-L1AgNO₃ complex with the left-handed (M-type) helicity formed in the presence of 2.0 equiv. of AgNO₃ and was not additionally changed, which indicated that it was the thermodynamic product. The chiral supramolecular polymer based on (d-L1)₃Ag₂(NO₃)₂ was produced via a nucleation–elongation mechanism with a cooperative pathway. In thermodynamic study, the standard ΔG° and ΔH_e values for the aggregates I and II were calculated using the van''t Hoff plot. The enhanced ΔG° value of the aggregate II compared to that of the formation of aggregate I confirms that aggregate II was thermodynamically more stable. In the kinetic study, the influence of concentration of AgNO₃ confirmed the initial formation of the aggregate I (nanoparticle), which then evolved to the aggregate II (supramolecular polymer). Thus, the concentration of the (d-L1)₃Ag₂(NO₃)₂ complex in the initial state plays a critical role in generating aggregate II (supramolecular polymer). In particular, NO₃⁻ acts as a critical linker and accelerator in the transformation from the aggregate I to the aggregate II. This is the first example of a system for a kinetically controlled chiral supramolecular polymer that is formed via multiple steps with coordination structural change.

The nanoparticles were transformed into the supramolecular polymer as the thermodynamic product, involving a helical inversion from left-handed to right-handed helicity.     

Diastereoselective tandem Michael-intramolecular Wittig reactions of a cyclic phosphonium ylide with 8-phenylmenthyl enoates.

The diastereoselective tandem Michael-intramolecular Wittig reactions of a five-membered cyclic phosphonium ylide 2 using 8-phenylmenthyl enoates were examined. The reaction of the phosphonium ylide with 8-phenylmenthyl cinnamate followed by the hydrolysis of the resulting enol ether 4a afforded (3R,4S)-4-(diphenylphosphinyl)-3-phenylcycloheptanone (3R,4S)-5a as the major isomer. The diastereoselectivity of the initial tandem reactions was estimated to be 94:6 from the 31P NMR of a mixture of the diastereomeric ketal derivatives 6a and 6'a which were obtained by the reaction of 5a with (2R,3R)-2,3-butanediol, and the absolute configuration of the major isomer was determined by the single-crystal X-ray analysis. Similar reactions using some 8-phenylmenthyl alkenoates were attempted. As a result, it was clarified that the corresponding trans-ketones 5b-d were obtained and that the diastereomer ratios of their ketal derivatives were 60:40-73:27.     

Kinetically controlled synthesis of wurtzite ZnS nanorods through mild thermolysis of a covalent organic-inorganic network

The high-temperature (over 1020 degrees C) polymorph of ZnS, wurtzite ZnS, has been successfully prepared through a low-temperature (180 degrees C) hydrothermal synthesis route in the presence of ethylenediamine (en). The effects of en concentrations, reactant concentrations, reaction temperatures, and reaction times on crystal structures and shapes of ZnS have been investigated. We have demonstrated that the wurtzite ZnS showing rodlike morphology can be kinetically stabilized in the presence of en, especially at a high reactant concentration under appropriate hydrothermal conditions. Besides phase evolution of ZnS from hexagonal to cubic, morphological transformation from nanorods to nanograins has also been observed in the present investigation. Nanograins of phase-pure cubic ZnS, the thermodynamically stable polymorph, are easily prepared, and no hexagonal ZnS nanorods are detected in "pure" water, i.e., in the absence of en molecules. The above investigations indicate that the controlled fabrication of wurtzite ZnS nanorods is due to a mediated generation of the lamellar phase, ZnS.0.5en, a covalent organic-inorganic network based on ZnS slabs, and to its subsequent thermolysis in aqueous solution. The controlled growth of wurtzite ZnS nanorods and sphalerite ZnS nanograins provides us an opportunity to structurally modulate physical properties. These wurtzite ZnS nanorods display narrower and stronger blue emission than sphalerite ZnS nanograins.     

Kinetically controlled ring-closing metathesis: synthesis of a potential scaffold for 12-membered salicylic macrolides

For the synthesis of a 12-membered salicylic macrolide scaffold, ring-closing metathesis (RCM) of a omega-diene compound was planned. The stereochemical outcome of the RCM reaction changed depending on the type of Ru catalyst that was used; a "first-generation" Grubbs catalyst produced exclusively the E isomer and "second-generation" catalysts provided a mixture of the E and Z isomers under kinetic control (not thermodynamic control). Considerations for the E/Z selectivity are described.     

Peak distortion in chromatography. Part 2: Kinetically controlled factors

The review of the characterization, diagnosis, and treatment of peak distortion begun in Part 1 is concluded. Part 1 dealt with peak distortion caused by nonlinear (concentration-dependent) chromatographic behavior in the column. The present part covers peak distortion due to either the apparatus, incomplete resolution, slow kinetic processes, reaction on the column, or column voids. All these sources are controlled by the kinetics of transport or reaction processes.     

Kinetically controlled growth of gold nanotriangles in a vesicular template phase by adding a strongly alternating polyampholyte

This paper is focused on the temperature-dependent synthesis of gold nanotriangles in a vesicular template phase, containing phosphatidylcholine and AOT, by adding the strongly alternating polyampholyte PalPhBisCarb.

UV-vis absorption spectra in combination with TEM micrographs show that flat gold nanoplatelets are formed predominantly in the presence of the polyampholyte at 45°C. The formation of triangular and hexagonal nanoplatelets can be directly influenced by the kinetic approach, i.e., by varying the polyampholyte dosage rate at 45°C. Corresponding zeta potential measurements indicate that a temperature-dependent adsorption of the polyampholyte on the {111} faces will induce the symmetry breaking effect, which is responsible for the kinetically controlled hindered vertical and preferred lateral growth of the nanoplatelets.     

NMR structure of a cyclic polyamide-DNA complex

The solution structure of a cyclic polyamide ligand complexed to a DNA oligomer, derived from NMR restrained molecular mechanics, is presented. The polyamide, cyclo-gamma-ImPyPy-gamma-PyPyPy-, binds to target DNA with a nanomolar dissociation constant as characterized by quantitative footprinting previously reported. 2D (1)H NMR data were used to generate distance restraints defining the structure of this cyclic polyamide with the DNA duplex d(5'-GCCTGTTAGCG-3'):d(5'-CGCTAACAGGC-3'). Data interpretation used complete relaxation matrix analysis of the NOESY cross-peak intensities with the program MARDIGRAS. The NMR-based distance restraints (276 total) were applied in restrained molecular dynamics calculations using a solvent model, yielding structures with an rmsd for the ligand and binding site of approximately 1 A. The resulting structures indicate some distortion of the DNA in the binding site. The constraints from cyclization lead to altered stacking of the rings in the halves of the cyclic ligand relative to unlinked complexes. Despite this, the interactions with DNA are very similar to what has been found in unlinked complexes. Measurements of ligand amide and DNA imino proton exchange rates indicate very slow dissociation of the ligand and show that the DNA can undergo opening fluctuations while the ligand is bound although the presence of the ligand decreases their frequency relative to the free DNA.