Elimination-addition mechanism for nucleophilic substitution reaction of cyclohexenyl iodonium salts and regioselectivity of nucleophilic addition to the cyclohexyne intermediate

The reaction of 4-substituted cyclohex-1-enyl(phenyl)iodonium tetrafluoroborate with tetrabutylammonium acetate gives both the ipso and cine acetate-substitution products in aprotic solvents. The isomeric 5-substituted iodonium salt also gives the same mixture of the isomeric acetate products. The reaction is best explained by an elimination-addition mechanism with 4-substituted cyclohexyne as a common intermediate. The cyclohexyne formation was confirmed by deuterium labeling and trapping to lead to [4 + 2] cycloadducts and a platinum-cyclohexyne complex. Cyclohexyne can also be generated in the presence of some other mild bases such as fluoride ion, alkoxides, and amines, though amines are less effective bases for the elimination. Kinetic deuterium isotope effects show that the anionic bases induce the E2 elimination (k(H)/k(D) > 2), while the amines allow formation of a cyclohexenyl cation in chloroform to lead to E1 as well as S(N)1 reactions (k(H)/k(D) approximately 1). Bases are much less effective in methanol, and methoxide was the only base to efficiently afford the cyclohexyne intermediate. Nucleophiles react with the cyclohexyne to give regioisomeric products in the ratio dependent on the ring substituent. The observed regioselectivity of nucleophilic addition to substituted cyclohexynes is rationalized from calculated LUMO populations, which are governed by the bond angles at the acetylenic carbons: The less deformed carbon has a higher LUMO population and is preferentially attacked by the nucleophile.     

Amphiphilic guest sorption of K2[Cr3O(OOCC2H5)6(H2O)3]2[alpha-SiW12O40] ionic crystal

An ionic crystal K2[Cr3O(OOCC2H5)6(H2O)3]2[alpha-SiW12O40] x 3H2O (1a) is synthesized by the complexation of a Keggin-type polyoxometalate of [alpha-SiW12O40]4- with K+ and a macrocation of [Cr3O(OOCC2H5)6(H2O)3]+. Compound 1a possesses both hydrophilic and hydrophobic channels in the crystal lattice. The 3 mol mol(-1) of the water of crystallization in 1a resides in the hydrophilic channel. The water of crystallization is removed by the evacuation at 303 K to form the guest-free phase 1b with small changes in the lattice lengths (+/-0.2 A). The water sorption profile is reproduced by the single rate constant. Therefore, the water sorbed probably resides in the hydrophilic channel. Compound 1b sorbs various kinds of polar organic molecules, and the amounts of < or = C3 alcohols are comparable to or larger than that of water, while chlorocarbons with no hydrogen-bonding ability and nonpolar molecules are excluded. Thus, 1b showed the amphiphilic sorption property. The states of the polar organic molecules sorbed in 1b have been quantitatively investigated using ethanol as a probe molecule. The IR, NMR, and single-crystal X-ray diffraction studies combined with the sorption kinetics reveal that ethanol molecules are mainly sorbed into the hydrophilic channel at P/P0 < or = 0.5, while the sorption into the hydrophobic channel is dominant at P/P0 > or = 0.6. Thus, it is demonstrated that ethanol molecules enter both hydrophilic and hydrophobic channels of 1b.     

Synthesis of the C42-C52 part of ciguatoxin CTX3C

The C42-C52 part of ciguatoxin CTX3C (1) was synthesized from tri-O-acetyl d-glucal. The synthetic segment had a tetrahydropyran ring corresponding to the ‘C49-reduced’ L-ring of 1, designed to avoid side reactions due to acid-labile C49 acetal carbon during acidic reductive conditions planned in further synthesis toward 1. The vicinal dimethyl part at C47-C48 was constructed by a stepwise conjugate addition/methylation procedure. The C50-C52 unit was installed by Grignard addition of the C₃ unit followed by spirocyclization and reductive cleavage of the spirocyclic acetal. Stereoselective assembly of the C42-C44 part was achieved by Brown’s asymmetric crotylboration.     

Polymer depletion-induced instability of silica suspensions in the presence of flexible and semiflexible polymers

Polymer depletion-induced instability of silica suspensions in dilute toluene solutions of flexible polystyrene (PS) or semiflexible poly(hexyl isocyanate) (PHIC) was investigated by direct observation and oscillatory moduli measurements as functions of silica volume fraction and polymer molecular weight. Addition of the respective polymers to the gelled silica suspensions induces a gel settling. Below the silica volume fraction phi=4 vol%, PS chains compress the volume of the sediment silica phase, whereas PHIC chains expand it and play a role in swelling agents. Thus, PHIC chains lead to the formation of aggregates in the silica suspensions that are larger and less compact than those formed by the PS chains. On the other hand, above phi=4 vol% where the silica suspension occurs gelation, the effect of nonadsorbing polymers on changes in the volume of the silica phase is opposite. Moreover, polymer depletion interaction results in a mechanically stronger gelled phase, leading to the storage modulus G' values larger than those without polymer, but the effects of polymer concentration and polymer molecular weight on the G' values are not clear.     

Solution structures of 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid saturated with CO2: Experimental evidence of specific anion-CO2 interaction

X-ray diffraction measurements for 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid ([BMIM][PF6])-CO2 systems were carried out at high pressures with a newly developed polymer cell. The intermolecular distribution functions (g(inter)(r)) were obtained at 25 degrees C for neat [BMIM][PF6] and its solutions saturated with CO2 at 4 and 15 MPa, where the mole fractions (x) of CO2 correspond to 0.5 and 0.7, respectively. In g(inter)(r) for x = 0.5, two peaks appeared at around 2.8 and 3.2 A. These two peaks in g(inter)(r) appreciably increased for x = 0.7; moreover, there was another peak observed at approximately 3.8 A. Only assuming the correlations between CO2 and [PF6]-, it is reasonably determined that the nearest-neighbor P([PF6]-). . .C(CO2) distances are 3.57 and 3.59 A with the coordination numbers being 1.8 and 4.0 for x = 0.5 and 0.7, respectively. It is concluded that CO2 molecules are preferentially solvated to the [PF6]- anion.     

Nitrogen-14 NMR Study on Solvent Exchange of the Octahedral Cobalt(II) Ion in Neat 1,3-Propanediamine and n-Propylamine at Various Temperatures and Pressures. Tetrahedral-Octahedral Equilibrium of the Solvated Cobalt(II) Ion in n-Propylamine As Studied by EXAFS and Electronic Absorption Spectroscopy

Solvated cobalt(II) ions in neat 1,3-propanediamine (tn) and n-propylamine (pa) have been characterized by electronic absorption spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. The equilibrium between tetrahedral and octahedral geometry for cobalt(II) ion has been observed in a neat pa solution, but not in neat diamine solutions such as tn and ethylenediamine (en). The thermodynamic parameters and equilibrium constant at 298 K for the geometrical equilibrium in pa were determined to be DeltaH degrees = -36.1 +/- 2.3 kJ mol(-1), DeltaS degrees = -163 +/- 8 J mol(-1) K(-1), and K(298) = 6.0 x 10(-3) M(-2), where K = [Co(pa)(6)(2+)]/{[Co(pa)(4)(2+)][pa](2)}. The equilibrium is caused by the large entropy gain in formation of the tetrahedral cobalt(II) species. The solvent exchange of cobalt(II) ion with octahedral geometry in tn and pa solutions has been studied by the (14)N NMR line-broadening method. The activation parameters and rate constants at 298 K for the solvent exchange reactions are as follows: DeltaH() = 49.3 +/- 0.9 kJ mol(-1), DeltaS() = 25 +/- 3 J mol(-1) K(-1), DeltaV() = 6.6 +/- 0.3 cm(3) mol(-1) at 302.1 K, and k(298) = 2.9 x 10(5) s(-1) for the tn exchange, and DeltaH() = 36.2 +/- 1.2 kJ mol(-1), DeltaS() = 35 +/- 6 J mol(-1) K(-1), and k(298) = 2.0 x 10(8) s(-1) for the pa exchange. By comparison of the activation parameters with those for the en exchange of cobalt(II) ion, it has been confirmed that the kinetic chelate strain effect is attributed to the large activation enthalpy for the bidentate chelate opening and that the enthalpic effect is smaller in the case of the six-membered tn chelate compared with the five-membered en chelate.     

Flow-through micro sensor using immobilized peroxidase with chemiluminometric FIA system for determining hydrogen peroxide.

A micromachined flow cell (overall size; 25 x 25 x 1 mm3) was designed for the fast determination of hydrogen peroxide, based on a luminol-H2O2 chemiluminescence reaction catalyzed by immobilized peroxidase (POD). The flow cell consisted of a sandwich of anisotropically etched silicon and glass chips and contained a spiral channel (20 turns, 50 cm long, 150 microm wide, 20 microm depth, channel volume 1.4 microl) and two holes (1 mm diameter). POD was covalently immobilized with 3-(trimethoxysilyl)propyldietylenetriamine and glutaraldehyde on the inner surface of the channel. The chip was placed in front of a window of a photomultiplier tube and used as a flow cell in a single-line flow-injection analysis system using a luminol solution as a carrier solution. The sample volume for one measurement was 0.2 microl. The maximal sampling rate was 315 h(-1) at a carrier solution flow rate of 10 microl min(-1). A calibration graph for H2O2 was linear for 5 nM - 5 microM; the detection limit (signal-to-noise = 3) was 1 nM (7 fg in 0.2 microl injection). The H2O2 concentration in rainwater was determined using this sensor system.     

Tomato new sapogenols, isoesculeogenin A and esculeogenin B

Two novel sapogenols, isoesculeogenin A (1) and esculeogenin B (2) of steroidal alkaloid glycosides, lycoperoside F and esculeoside B, respectively, isolated from the ripe tomato have been characterized as (5alpha,22R,23R,25S)-3beta,23,27-trihydroxyspirosolane and (5alpha,22S,23R,25S)-22,26-epimino-16beta,23-epoxy-3beta,23,27-trihydroxycholestane, respectively.     

Isothiouronium-modified gold nanoparticles capable of colorimetric sensing of oxoanions in aqueous MeOH solution

Isothiouronium-modified gold nanoparticles 1 with an average diameter of 5.8 nm were synthesized from bis(5-(N,N′-dimethyl-S-isothiouronio)pentyl) disulfide 2. The surface plasmon band at 524 nm in 10% (v/v) H₂O-MeOH showed a selective red shift upon adding anions, making this potentially useful as a new anion sensor material.     

The Effects of Serum on Cellular Uptake and Phototoxicity of Mono-L-Aspartyl Chlorin e6 (NPe6) In Vitro

Abstract— Previous reports showed that the photosensitizer mono- l -aspartyl chlorin e6 (NPe6) binds to serum proteins. However, the influence of this binding on the cellular uptake and photodynamic therapy (PDT) phototoxicity of NPe6 is still undefined. In this paper, we studied how serum in medium affected the P388 cellular uptake and PDT phototoxicity of NPe6 in vitro. This was assessed by (1) detection of the red shift (654 nm Q band peak of absorption) induced by protein binding NPe6; (2) detection of intracellular concentration of NPe6 by HPLC and (3) measurements of the cell survival ratio after PDT by MTT assay. The 654 nm Q band peak of NPe6 shifted to 665 nm after binding of NPe6 and serum proteins. The protein-bound NPe6 cannot be uptaken by cells, thus there was no PDT phototoxicity. Nevertheless, phototoxicity recovered when the concentration of NPe6 excessed the serum protein binding ability or there was free serum protein in the medium. These data suggested that the cellular uptake of NPe6 is inhibited by serum components in the medium, and that only free NPe6 is accumulated by P388 cells even during relatively long incubations. The cytotoxicity of PDT mainly depends on the free NPe6 level in the medium.