PubMed Health⌕ Search

Biomedical subjects

Yoshito Tobe

Publications and source records attributed to Yoshito Tobe.

At least 19 recordsLinked to original sources
PubMed2006

Two-dimensional porous molecular networks of dehydrobenzo[12]annulene derivatives via alkyl chain interdigitation.

The self-assembly of a series of hexadehydrotribenzo[12]annulene (DBA) derivatives has been scrutinized by scanning tunneling microscopy (STM) at the liquid-solid interface. First, the influence of core symmetry on the network structure was investigated by comparing the two-dimensional (2D) ordering of rhombic bisDBA 1a and triangular DBA 2a (Figure 1). BisDBA 1a forms a Kagomé network upon physisorption from 1,2,4-trichlorobenzene (TCB) onto highly oriented pyrolytic graphite (HOPG). Under similar experimental conditions, DBA 2a shows the formation of a honeycomb network. The core symmetry and location of alkyl substituents determine the network structure. The most remarkable feature of the DBA networks is the interdigitation of the nonpolar alkyl chains: they connect the pi-conjugated cores and direct their orientation. As a result, 2D open networks with voids are formed. Second, the effect of alkyl chain length on the structure of DBA patterns was investigated. Upon increasing the length of the alkyl chains (DBAs 3c-e) a transition from honeycomb networks to linear networks was observed in TCB, an observation attributed to stronger molecule-substrate interactions. Third, the effect of solvent on the structure of the nonpolar DBA networks was investigated in four different solvents: TCB as a polar aromatic solvent, 1-phenyloctane as a solvent having both aromatic and aliphatic moieties, n-tetradecane as an aliphatic solvent, and octanoic acid as a polar alkylated solvent. The solvent dramatically changes the structure of the DBA networks. The solvent effects are discussed in terms of factors that influence the mobility of molecules at the liquid-solid interface such as solvation.

Alkylation↗
PubMed2006

Preparation and evaluation of a chiral stationary phase covalently bound with a chiral pseudo-18-crown-6 ether having a phenolic hydroxy group for enantiomer separation of amino compounds.

In order to develop a chiral stationary phase (CSP), which has even higher separation ability than the corresponding commercially available crown ether based CSP (OA-8000 having a pseudo-18-crown-6 ether with an OMe group as a selector), chemically bonded type CSP having a phenolic OH group on a crown ring was developed. Normal mobile phases with or without acid additive can be used with this OH type CSP in contrast to the conventional OMe type CSP which has a neutral chiral selector. Enantiomers of 25 out of 27 amino compounds, including 20 amino acids, 5 amino alcohols, and 2 lipophilic amines, were efficiently separated on a column with this CSP. Nine amino compounds out of 27 were separated with better separation factors than the corresponding OMe type CSP. It is noteworthy that the chromatography on this CSP exhibited excellent enantiomer-separations for amines and amino alcohols when triethyl amine was used as an additive in the mobile phase. Comparison of enantiomer separation ability on this OH type of CSP and on the OMe type of CSP and correlation between the enantioselectivity in chiral chromatography and that of the corresponding model compounds in solution imply that the chiral separation arose from chiral recognition in host guest interactions.

Amines↗
PubMed2006

Molecular geometry directed Kagomé and honeycomb networks: toward two-dimensional crystal engineering.

We present here the formation of a molecular Kagomé network within a two-dimensional (2D) crystal on a surface. This system provides a clear example of how, by design, molecular geometry can be expressed at the level of the 2D crystal lattice, leading to the formation of open networks. Key elements to control molecular network formation are core symmetry, location and orientation of interacting and connecting substituents, as well as symmetry matching between the networks and the surface.

Journal Article↗
PubMed2006

Asymmetric [2+2] photocycloaddition of cycloalkenone-cyclodextrin complexes to ethylene.

The enantioselective [2+2] photocycloaddition of cycloalkenone-cyclodextrin complexes to ethylene was examined. Photoreaction of alpha-, beta-, and gamma-cyclodextrin complexes of various cyclohexenonecarboxylates and cyclopentenonecarboxylates in the presence of ethylene in the solid state or in aqueous suspension gave corresponding photocycloadducts with chiral induction.

Journal Article↗
PubMed2005

Gas-phase generation of highly reactive hexatriyne-bridged [6n]paracyclophynes.

[6n]Paracyclophenediynes 2a-d (n = 3-6) having [4.3.2]propellatriene units were prepared as precursors of the corresponding [6n]paracyclophynes. Laser irradiation of 2a-d produced the negative ions of [6n]paracyclophynes 1a-d (n = 3-6) by extrusion of the indan fragments, which were detected by time-of-flight mass spectrometry.

Journal Article↗
PubMed2005

Preparation and evaluation of a chiral stationary phase covalently bound with chiral pseudo-1 8-crown-6 ether having 1-phenyl-1,2-cyclohexanediol as a chiral unit.

A chiral stationary phase (CSP) has been prepared by chemically bonding a chiral pseudo-18-crown-6 type host having a 1-phenyl-1,2-cyclohexanediol unit to 3-aminopropyl silica gel. The chiral column was prepared by the slurry-packing method in a stainless steel HPLC column. Normal mobile phases can be used with this CSP in contrast to conventional dynamic coating type CSPs. Enantiomers of 20 out of 30 amino compounds, including 20 amino acids, 2 amino acid methyl esters, 6 amino alcohols, and 2 lipophilic amines, were efficiently separated on columns with this CSP. It is noteworthy that 15 amino compounds out of 30 were separated with better separation factors and shorter retention times compared to the corresponding CSP having pseudo-18-crown-6 with 1-phenyl-1,2-ethanediol as a chiral unit. In view of the correlation between the enantiomer selectivities observed in chromatography and those obtained in gas phase FABMS-EL methods and solution phase titrations, chiral recognition in the host-guest interaction likely contributes to enantiomer separation.

Amino Acids↗
PubMed2005

Generation and characterization of highly strained dibenzotetrakisdehydro[12]- and dibenzopentakisdehydro[14]annulenes.

To generate dibenzotetrakisdehydro[12]- and dibenzopentakisdehydro[14]annulenes ([12]- and [14]DBAs) having a highly deformed triyne moiety, [4.3.2]propellatriene-anneleted dehydro[12]- and dehydro[14]annulenes were prepared as their precursors. UV irradiation of the precursors resulted in the photochemical [2 + 2] cycloreversion to generate the strained [12]- and [14]DBAs, respectively. The [12]DBA was not detected by 1H NMR spectroscopy, but it was intercepted as Diels-Alder adducts in solution, suggesting its intermediacy. Its spectroscopic characterization was successfully carried out by UV-vis spectroscopy in a 2-methyltetrahydrofuran (MTHF) glass matrix at 77 K and by FT-IR spectroscopy in an argon matrix at 20 K. On the other hand, the [14]DBA was stable enough for observation by 1H and 13C NMR spectra in solution, though it was not isolated because of the low efficiency of the cycloreversion. The [14]DBA was also characterized by interception as Diels-Alder adducts in solution and by UV-vis spectroscopy in a MTHF glass matrix at 77 K. The kinetic stabilities of the DBAs are compared with the related dehydrobenzoannulenes with respect to the topology of the pi-systems. In addition, the tropicity of the [14]DBA is discussed based on its experimental and theoretical 1H NMR chemical shifts.

Cold Temperature↗
PubMed2005

Size-selective formation of C78 fullerene from a three-dimensional polyyne precursor.

Multicyclic cagelike cyclophanes 2 a and 2 b containing cyclobutene rings have been prepared as precursors of three-dimensional polyynes C78H18 (1a) and C78H12Cl6 (1b), respectively. Laser irradiation of 2a and 2b induced expulsion of the aromatic fragment, indane, to give the three-dimensional polyyne anions C78H18- and C78H12Cl6-, respectively. Whereas the former anion lost only four hydrogen atoms to form C78H14-, complete loss of all hydrogen and chlorine atoms was observed from the latter anion, to yield a C78- ion that has a fullerene structure which was proven by its characteristic fragmentation pattern.

Journal Article↗
PubMed2005

Chiral stationary phase covalently bound with a chiral pseudo-18-crown-6 ether for enantiomer separation of amino compounds using a normal mobile phase.

In order to apply the excellent chiral recognition ability of chiral pseudo-18-crown-6 ethers that we developed to chiral separation, we prepared a chiral stationary phase (CSP) by immobilizing a chiral pseudo-18-crown-6-type host on 3-aminopropyl silica gel. A chiral column was prepared by the slurry-packing method in a stainless steel HPLC column. A liquid chromatography system using this CSP combined with the detection by mass spectrometry was used for enantiomer separation of amino compounds. A normal mobile phase can be used on this CSP as opposed to conventional dynamic coating-type CSPs. Enantiomers of 18 common natural amino acids were efficiently separated. The chiral separation observed for amino acid methyl esters, amino alcohols, and lipophilic amines was fair using this HPLC system. In view of the correlation between the enantiomer selectivity observed in chromatography and the complexion in solution, the chiral recognition in host-guest interactions might contribute to this enantiomer separation.

Amino Acids↗
PubMed2004

Chiral recognition in NMR spectroscopy using crown ethers and their ytterbium(III) complexes.

Chiral crown ethers 1 and 5 are useful enantiomeric discriminating agents in 1H NMR spectroscopy for neutral and protonated primary amines, amino acids, and amino alcohols. The presence of the carboxylic acid groups in 1 and 5 provide sites at which ytterbium(III) can bind. Adding ytterbium(III) nitrate to crown-substrate mixtures in methanol-d4 causes shifts in the spectra of substrates and often enhances the chiral discrimination in the 1H NMR spectrum. The enhancement in enantiomeric discrimination that occurs in the presence of ytterbium(III) allows lower concentrations of the crown ether to be used in chiral recognition studies. Several amide derivatives of 1 were prepared and evaluated as chiral NMR discriminating agents, although except for 1e, these were less effective than 1.

Journal Article↗
PubMed2004

Depression of the apparent chiral recognition ability obtained in the host-guest complexation systems by electrospray and nano-electrospray ionization mass spectrometry.

Chiral recognition in the host-guest complexation systems of chiral crown ether hosts and amino ester guests was thoroughly examined using the electrospray ionization (ESI) mass spectrometry/enantiomer labeled (EL)-guest method. In this method, the mass spectra of a mixture of three components in a solution, a chiral host (H), an equal amount of an (S)-enantiomer guest labeled with deuterium atoms (G(S-dn)(+)) and an unlabeled (R)-enantiomer guest (G(R)+), were measured and the relative peak intensity value [I(H + G(R))(+) / I(H + G(S-dn))(+) = IRIS] of the host-guest complex ions, observed with an excess guest concentration, was taken to provide the chiral recognition ability of the host. In our earlier report (1996), we demonstrated that the apparent chiral recognition abilities using a mass spectrometer with a homemade ESI interface were depressed by about one tenth compared with the corresponding abilities obtained by fast-atom bombardment (FAB) MS. In the present study, the enantioselective complexation behaviors of various combinations of chiral crown hosts with chiral guests were further investigated in detail mainly using a modern commercial ESI/ion trap (IT) mass spectrometer. Consequently, it was found that the apparent IRIS values from the ESI-MS/EL-guest method changed significantly, depending upon the instrument used, and in particular, upon the ESI interfaces. Moreover, under the specific measuring conditions in ESI-IT-MS, the degrees of depression of the apparent chiral recognition abilities are roughly grouped into three classes, depending upon the number (or probably the type) of the hydrophobic substituents of the hosts. Representing the degrees by the slopes when plotting the apparent IRIS values in ESI-MS versus those in FAB-MS, the slopes for the three classes are (1) 1.0, (2) 0.7 and (3) 0.3; the higher the hydrophobicity of the hosts (and then, the host-guest complex ions), the lower the slope (the apparent enantioselectivity). Strengthening the degree of depression may be caused by an increase in the local concentration of the host close to the surface of the droplets produced during the electrospary ionization process. The chiral recognition ability (K(R )/ K(S)) in an equilibrated solution agrees quite well with the IRIS value in FAB-MS rather than that in ESI-MS.

Journal Article↗
PubMed2003

Expanded radialenes with bicyclo[4.3.1]decatriene units: new precursors to cyclo[n]carbons.

A new method for the formation of conjugated polyynes has been developed based on both the rearrangement of vinylidenes to alkynes and the [2+1] cheletropic fragmentation of dialkynylmethylenebicyclo[4.3.1]deca-1,3,5-triene derivatives. A model study of the photolysis of simple dialkynylmethylenebicyclo[4.3.1]deca-1,3,5-trienes resulted in cheletropic fragmentation followed by 1,2-migration to give the corresponding linear polyynes, although undesired isomerization to methylenebicyclo[5.3.0]triene derivatives took place concurrently. Expanded [3]-, [4]-, [5]-, and [6]radialene derivatives with exocyclic bicyclo[4.3.1]decatriene units were prepared by oxidative coupling of the monomeric units as precursors to the corresponding cyclo[n]carbons, monocyclic forms of carbon clusters. The spectroscopic properties of the expanded radialenes were investigated in connection with cross conjugation of the core pi system and with its perturbation by the extraannular bicyclic pi system. In negative-mode laser-desorption time-of-flight (LD-TOF) mass spectra, the expanded radialenes exhibited peaks due to the corresponding cyclo[n]carbon anions (n = 18, 24, 30, and 36) formed by the stepwise loss of the aromatic indane fragments.

Journal Article↗
PubMed2003

Generation and characterization of highly strained dibenzotetrakisdehydro[12]annulene.

We report here the generation of tetrakisdehydro[12]annulene possessing a highly deformed triyne component from the [4.3.2]propellatriene-annelated precursor by its photolysis extruding indan and the characterization of the highly reactive annulene by chemical and spectroscopic methods. In addition to the chemical evidence for the formation of the title compound in solution such as interception as a Diels-Alder adduct, we succeeded in its characterization by UV-vis and FTIR spectra in an argon matrix at 20 K. The experimental IR spectrum agreed well with the theoretical one calculated by the DFT method.

Journal Article↗