Base-catalyzed hydrolysis of 6-aminopenicillanic acid. The kinetic and thermodynamic products

In the presence of sodium hydroxide or a β-lactamase, 6-APA has been shown to hydrolyze rapidly at room temperature to penicic acid 3 , the kinetic product of the reaction. In a subsequent equilibration 3 isomerizes at C-5, by way of intermediate imine 4 , affording 5-epi-penicic acid 6 as the major hydrolysis product (～ 95% at equilibrium). The pH and temperature parameters of equilibration are discussed and HPLC, optical rotation, proton nmr and ¹³C nmr data are presented.     

Micropatterning of semicrystalline poly(vinylidene fluoride) (PVDF) solutions

Micropatterning of a semicrystalline poly(vinylidene fluoride) (PVDF) solution was performed by a temperature controlled capillary micromolding where the rate of solvent evaporation was controlled by substrate temperature. In order to choose proper solvents for micropatterning, we have investigated the solubility of PVDF in various organic solvents and crystal structures of the PVDF bulk films cast from the solvents. The films prepared from the polar solvents such as dimethylformamide (DMF), dimethyl sulfoxide (DMSO) dominantly showed γ type crystals regardless of preparation temperature, while the films from tetrahydrofuran (THF) exhibit α type crystals and the ones from acetone and methyl ethyl ketone (MEK) show the characteristics of both α- and γ-PVDF. The quality of micropatterns and shapes of the PVDF crystals in the patterns significantly depend on solvent evaporation rates. Micropatterns of PVDF formed in DMF at 120 °C showed the best uniformity in shape. Crystals of the PVDF nucleated at the center regions of microchannels tended to be elongated with the b-axis of γ-PVDF crystals along the channels as the concentration of the solution decreased. In contrast, crystals nucleated at the corner regions of the channels had their b-axis oriented perpendicular to the channels. In line patterns with the width of 2 μm, the corner nucleated crystals were dominant and a resulting bamboo-like crystalline microstructure was observed in which the b-axis of γ-PVDF crystals, fast growth direction, is oriented normal to the microchannels. The crystal structures of the bulk films and the micropatterns were characterized by X-ray diffractometer, Fourier transform infrared spectroscope in Attenuated Total Reflection mode, Polarized Optical and Scanning Electron Microscope.     

Total synthesis and absolute stereochemistry of (9R,10S)-epoxyheptadecan-4,6-diyn-3-one, a diacylglycerol acyltransferase inhibitor from Panax ginseng

Asymmetric synthesis of four possible stereoisomers of (9,10)-epoxyheptadecan-4,6-diyn-3-one was accomplished, and the absolute configuration of the naturally occurring (9R,10S)-epoxyheptadecan-4,6-diyn-3-one (1) was elucidated.     

Photoreaction of 2-halo-N-pyridinylbenzamide: intramolecular cyclization mechanism of phenyl radical assisted with n-complexation of chlorine radical

The photochemical behavior of 2-halo-N-pyridinylbenzamide (1-4 in Chart 1) was studied. The photoreaction of 2-chloro-N-pyridinylbenzamides 1a, 2a, 3a, and 4 afforded photocyclized products, benzo[c]naphthyridinones (6-9 and 16), in high yield, whereas the bromo analogues 1b, 2b, and 3b produced extensively photoreduced products, N-pyridinylbenzamides (1c, 10, and 11), with minor photocyclized product. Since the photocyclization reaction of 2-chloro-N-pyridinylbenzamide is retarded by the presence of oxygen and sensitized by the presence of a triplet sensitizer, acetone or acetophenone, a triplet state of the chloro analogue is involved in the reaction. Since several radical intermediates, particularly n-complexes of chlorine radical, are identified in the laser flash photolysis of 2-chloro-N-pyridinylbenzamide, an intramolecular cyclization mechanism of phenyl radical assisted with n-complexation of chlorine radical for the cyclization reaction is proposed: the triplet state (78 kcal/mol) of the chloro analogue (1a), which is populated by the excitation of 1a undergoes a homolytic cleavage of the C-Cl bond to give phenyl and chlorine radicals; while chlorine radical holds the neighbor pyridinyl ring with its n-complexation, the intramolecular arylation of the phenyl radical with the pyridinyl ring proceeds to produce a conjugated 2,3-dihydropyridinyl radical and then the conjugated radical aromatizes to afford a cyclized product, benzo[c]naphthyridinone by ejecting a hydrogen. The photoreduction product can be formed by hydrogen atom abstraction of the phenyl sigma radical from the environment.     

Convergent synthesis of PAMAM dendrimers using click chemistry of azide-functionalized PAMAM dendrons

Azide-functionalized PAMAM dendrons containing an azidopropylamine focal point were synthesized by the divergent method and applied for the construction of symmetric PAMAM-like dendrimers containing 1,2,3-triazole rings as connectors via stitching with two different multi-terminal alkynes. The stitching method was based on the click chemistry protocol, i.e., the copper-catalyzed cycloaddition reaction between an alkyne and an azide.     

A triangle-square equilibrium of metallosupramolecular assemblies based on pd(II) and pt(II) corners and diazadibenzoperylene bridging ligands

Tetraaryloxy-substituted diazadibenzoperylene bridging ligands 1a,b were employed in transition metal-directed self-assembly with Pd(II) and Pt(II) phosphane triflates 2a,b which resulted in complex dynamic equilibria between molecular triangles 3a-d and molecular squares 4a-d in solution. Characterization of the equilibria and assignment of the metallacycles was accomplished by (1)H and (31)P[(1)H] NMR spectroscopy in combination with electrospray ionization Fourier transform ion cyclotron mass spectrometry (ESI-FTICR-MS). It was found that the equilibria depend on several factors, such as the metal ion (Pd(2+) or Pt(2+)), the solvent, and the steric demand of the phenoxy substituents of the diazadibenzoperylene ligands 1a,b. Introduction of bulky tert-butyl groups in 1b shifts the equilibrium significantly in the direction of the molecular squares. Molecular dynamics simulations of the triangle and square structures revealed critical steric effects and restricted conformational flexibilities of the phosphane and diazadibenzoperylene ligands that help explain the distinct dynamic behavior observed in variable-temperature NMR studies. Concentration-dependent UV/vis and fluorescence spectroscopy revealed the limited stability of the assemblies and confirmed the reversible nature of the dynamic equilibria.     

Hydrogen peroxide oxidation of di(hydroxo)platinum(II) species in carboxylic acids

A facile procedure for synthesizing the mono(hydroxo)tris(carboxylato)platinum(IV) species has been achieved. The reaction of [Pt^II(OH)₂(dmpda)] (dmpda=2,2-dimethyl-1,3-propanediamine) with a 30% aqueous solution of H₂O₂ in the presence of a carboxylic acid produces a stable [Pt^IV(OOCR)₃(OH)(dmpda)] (R=Me, Et) complex in high yield. The crystal structures of [Pt^IV(OOCMe)₃(OH)(dmpda)] . H₂O (triclinic P1 bar, a=8.761(2) Å, b=9.245(3) Å, c=10.659(2) Å, =106.25(2)°, =93.90(2)°, =98.92(2)°, V=813.1(3) ų, Z=2, R= 0.0474) and [Pt^IV(OOCEt)₃(OH)(dmpda)] (monoclinic P2₁/c, a=12.777(4) Å, b=10.514(2) Å, c=14.971(3) Å, =107.40(2)°, V=1919.2(8) ų, Z=4, R=0.0611) show that the hydroxyl group has been selectively positioned at an axial site. The intramolecular hydrogen bond between the OH and C=O moiety exists (O(H)...=C, 2.83 Å for [Pt^IV(OOCMe)₃(OH)(dmpda)] · H₂O; 2.72 Å for [Pt^IV(OOCEt)₃(OH)(dmpda)]. Formation of the axial-mono(hydroxo)tris(carboxylato)platinum(IV) species may be ascribed to a combination of `reactive-equatorial effects' with `cis-addition' in the carboxylic acid.     

Photoluminescence of AlxGa1−xAs/GaAs quantum well heterostructures grown by molecular beam epitaxy

Low-temperature photoluminescence measurements on nominally undoped Al_xGa_1–xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the Al_xGa_1–xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in Al_xGa_1–xAs/GaAs QWHs starts at a well width ofL _z30 nm when x0.25. The minor average well thickness fluctuation ofL _z=4×10^–2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asL _z=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al_0.43Ga_0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.     

Intraframework potential energy function of zeolites

Potential energy functions suitable for the (Cs_1–xNa_xT₂O₄)-A type zeolite family were obtained from the known crystal structure of Cs₇Na₅-A zeolite. Using these potential functions, several other crystal structures in these family were obtained by molecular mechanical calculations. The activation energy - to -cage transmission of the H₂ molecule and the void volume of Cs₃Na₉-A zeolite were calculated.     

Photoluminescence of Al x Ga1−x As/GaAs quantum well heterostructures grown by molecular beam epitaxy

Nominally undoped Al_xGa_1–xAs grown by molecular beam epitaxy from As₄ species at elevated substrate temperatures of 670°C exhibits well-resolved excitonic fine structure in the low-temperature photoluminescence spectra, if the effective As-to-(Al+Ga) flux ratio on the growth surface is kept within a rather narrow range of clearly As-stabilized conditions. In contrast to previous results on Al_xGa_1–xAs of composition 0.15not to shift in energy by changing the excitation intensity. This implies a simple freeelectron carbon-acceptor recombination mechanism for the line without any participation of a donor. In Al_xGa_1–xAs of composition close to the direct-to-indirect cross-over point, two distinct LO-phonons separated by 34 and 48 meV from the (D ⁰,C ⁰) peak position at x=0.43 were observed which were before only detectable by Raman scattering experiments. The intensity of the carbon-impurity related luminescence lines in bulk-type Al_xGa_1–xAs and GaAs layers was found to be strongly reduced, as compared to the excitonic recombination lines, if the respective active layer was covered by a very thin confinement layer of either GaAs on top of Al_xGa_1–xAs or vice versa grown in the same growth cycle.