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Ken Tanaka

Publications and source records attributed to Ken Tanaka.

At least 19 recordsLinked to original sources
PubMed2006

Enantioselective Synthesis of Tetra-ortho-Substituted Axially Chiral Biaryls through Rhodium-Catalyzed Double [2 + 2 + 2] Cycloaddition.

[reaction: see text] We have established an enantioselective synthesis of both C2 symmetric and unsymmetric tetra-ortho-substituted axially chiral biaryls through rhodium-catalyzed double [2 + 2 + 2] cycloaddition (up to >99% ee). This method serves as an attractive new route to enantioenriched tetra-ortho-substituted axially chiral biaryls in view of the one-step access to substrate diynes and tetraynes starting from readily available alkynes.

Journal Article↗
PubMed2006

Enantioselective synthesis of axially chiral anilides through rhodium-catalyzed [2+2+2] cycloaddition of 1,6-diynes with trimethylsilylynamides.

We have developed a rhodium-catalyzed enantioselective intermolecular [2+2+2] cycloaddition of 1,6-diynes with trimethylsilylynamides for the synthesis of axially chiral anilides. The axial chirality is constructed at the formation of benzene rings with high enantioselectivity (up to 98% ee). It should be noted that the present reaction employs the readily prepared trimethylsilylynamides starting from commercially available bis(trimethysilyl)acetylene and the trimethylsilyl group of the product anilides is expected to be utilized for further functionalization.

Journal Article↗
PubMed2006

Comparative study of chemical constituents of rhubarb from different origins.

A comparative study of the pharmacologically active constituents of 24 rhubarb samples, which were identified genetically as Rheum tanguticum, 3 intraspecies groups of R. palmatum and R. officinale, was conducted using reversed-phase high performance liquid chromatography (HPLC) methods. Thirty compounds belonging to anthraquinones, anthraquinone glucosides, dianthrones, phenylbutanones, stilbenes, flavan-3-ols, procyanidins, galloylglucoses, acylglucoses, gallic acid, and polymeric procyanidins were analyzed quantitatively. The drug samples derived from the same botanical source showed similar chromatographic profiles, and the comparable specific shape that appeared in the 10-directed radar graphs constructed on the basis of the results of quantitative analysis indicated the relationship between chemical constituent patterns and genetic varieties of rhubarb samples.

Anthracenes↗
PubMed2006

Development of a high performance liquid chromatographic method for systematic quantitative analysis of chemical constituents in rhubarb.

HPLC methods for the systematic determination of 30 compounds in Rhei Rhizoma (rhubarb) were developed. Using a combination of mobile phase gradient conditions and UV detection at 280 nm, all 30 compounds were separated satisfactorily with low detection limits (0.05-2 microg/ml). The developed methods provided a reliable calibration curve for each compound. By adopting these methods, the determination of 30 compounds in three kinds of rhubarb samples, derived from Rheum tanguticum, R. palmatum and R. officinale, was achieved. The constituent pattern of each rhubarb was clearly characterized through the quantitative composition of 30 major constituents of rhubarb.

Calibration↗
PubMed2005

Rhodium-catalyzed chemo-, regio-, and enantioselective [2 + 2 + 2] cycloaddition of alkynes with isocyanates.

[reaction: see text] We have developed a cationic rhodium(I)/modified-BINAP complex-catalyzed chemoselective [2 + 2 + 2] cycloaddition of alkynes with isocyanates leading to a wide range of 2-pyridones. This method was successfully applied to the chemo-, regio-, and enantioselective synthesis of axially chiral 2-pyridones from unsymmetrical alpha,omega-diynes, bearing an ortho-substituted phenyl at one terminal position, and alkyl isocyanates.

Journal Article↗
PubMed2005

Rhodium-catalyzed reductive coupling of disulfides and diselenides with alkyl halides, using hydrogen as a reducing agent.

[reaction: see text] We have established that RhCl(PPh3)3 catalyzes a reductive coupling of disulfides and diselenides with alkyl halides in the presence of triethylamine using hydrogen as a reducing agent. This reaction serves as a convenient new method to produce unsymmetrical sulfides and selenides from disulfides and diselenides instead of unstable and odoriferous thiols and selenols.

Journal Article↗
PubMed2005

Cationic rhodium(I)/BINAP complex-catalyzed isomerization of secondary propargylic alcohols to alpha,beta-enones.

We have developed a cationic rhodium(I)/BINAP complex-catalyzed isomerization of secondary propargylic alcohols to alpha,beta-enones. The asymmetric variant of this reaction, a kinetic resolution of secondary propargylic alcohols, was also developed with good selectivity. The mechanistic study revealed that the isomerization proceeds through intramolecular 1,3- and 1,2-hydrogen migration pathways. [reaction: see text]

Journal Article↗
PubMed2005

Enantioselective synthesis of axially chiral biaryls through rhodium-catalyzed complete intermolecular cross-cyclotrimerization of internal alkynes.

[reaction: see text] We have developed a cationic rhodium(I)/H8-BINAP complex-catalyzed complete intermolecular cross-cyclotrimerization of internal alkynes with dialkyl acetylenedicarboxylates. This reaction was successfully applied to enantioselective synthesis of axially chiral biaryls utilizing internal alkynes bearing ortho-substituted phenyl and acetoxymethyl in each terminal position. The axial chirality is constructed at the formation of benzene rings with high enantioselectivity (up to 96% ee).

Journal Article↗
PubMed2005

Synthesis and activity of oleanolic acid derivatives, a novel class of inhibitors of osteoclast formation.

Two series of oleanolic acid derivatives were synthesized and their inhibitory activity on the formation of osteoclast-like multinucleated cells (OCLs) induced by 1alpha,25-dihydroxy vitamin D3 was evaluated in a co-culture assay system. The structure-activity relationships, together with electronic structure based on the frontier molecular orbitals, for example, HOMO and LUMO, related to different amino acid substituents were studied. Derivatives with proline or phenylalanine showed a tendency to enhance the inhibitory activity.

Animals↗
PubMed2005

Chemo- and regioselective intermolecular cyclotrimerization of terminal alkynes catalyzed by cationic rhodium(I)/modified BINAP complexes: application to one-step synthesis of paracyclophanes.

A highly regioselective intermolecular cyclotrimerization of terminal alkynes has been developed based on the use of the cationic rhodium(I)/DTBM-Segphos complex. This method can be applied to a variety of terminal alkynes to provide 1,2,4-trisubstituted benzenes in high yield and with high regioselectivity. A chemo- and regioselective intermolecular crossed-cyclotrimerization of dialkyl acetylenedicarboxylates with a variety of terminal alkynes has also been developed based on the use of the cationic rhodium(I)/H8-BINAP complex, furnishing 3,6-disubstituted phthalates in high yields. It constitutes a highly efficient new method for intermolecular crossed-cyclotrimerization of two different monoynes in terms of catalytic activity, chemo- and regioselectivity, scope of substrates, and ease of operation. The versatility of this new crossed-alkyne cyclotrimerization procedure is demonstrated through its application to one-step synthesis of a [6]metacyclophane and [7]-[12]paracyclophanes from the corresponding terminal alpha,omega-diynes. Mechanistic studies have revealed that the chemo- and regioselectivity of this crossed-alkyne cyclotrimerization are determined by the preferential formation of a specific rhodium metallacycle derived from a terminal alkyne and a dialkyl acetylenedicarboxylate.

Journal Article↗
PubMed2005

Cassane- and norcassane-type diterpenes from Caesalpinia crista of Indonesia and their antimalarial activity against the growth of Plasmodium falciparum.

The CH2Cl2 extract of the seed kernels of Caesalpinia crista, which exhibited promising antimalarial activity against Plasmodium berghei-infected mice in vivo, was examined and resulted in the isolation of seven new furanocassane-type diterpenes [caesalpinins C-G (1-5) and norcaesalpinins D and E (6, 7)] together with norcaesalpinins A-C (8-10) and 11 known compounds (norcaesalpinins A-C, 2-acetoxy-3-deacetoxycaesaldekarin e, caesalmin B, caesaldekarin e, caesalpin F, 14(17)-dehydrocaesalpin F, 2-acetoxycaesaldekarin e, 7-acetoxybonducellpin C, and caesalmin G). Their structures were determined on the basis of spectroscopic analysis. The isolated diterpenes showed significant dose-dependent inhibitory effects on Plasmodium falciparum FCR-3/A2 growth in vitro. Their IC50 values ranged from 90 nM to 6.5 microM, and norcaesalpinin E (7) showed the most potent inhibitory activity (IC50, 90 nM).

Animals↗
PubMed2005

Nitric oxide (NO) production inhibitory constituents of Tabebuia avellanedae from Brazil.

From the water extract of Brazilian Tabebuia avellanedae, two new iridoids (1, 2) and a new phenylethanoid glycoside (3) have been isolated together with twelve known compounds (4-15). Their structures were determined based on the spectroscopic data. The isolated compounds inhibited nitric oxide (NO) production in lipopolysaccharide (LPS)-activated macrophage-like J774.1 cells. Compounds 1, 3, 10, 11, and 12 showed inhibitory activities more potent (IC50, 13.8-26.1 microg/ml) than a positive control N(G)-monomethyl-L-arginine (L-NMMA; IC50, 27.4 microg/ml).

Brazil↗