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Tadamichi Nagashima

Publications and source records attributed to Tadamichi Nagashima.

8 recordsLinked to original sources
PubMed2005

Increasing fluorous partition coefficients by solvent tuning.

Low partition coefficients of fluorous components have been a persistent problem in liquid-liquid separations using perfluoroalkanes as the fluorous phase. Solvent tuning of both the nonfluorous and the fluorous phase dramatically enhances the partitioning of light or polar fluorous molecules into the fluorous liquid phase, while minimally effecting partition coefficients of nonfluorous molecules. These findings suggest an expanded scope for liquid-based separations in fluorous biphasic catalysis, fluorous-tagged reagents, fluorous-supported oligomer synthesis, and related areas. [reaction: see text]

Alkanes↗
PubMed2004

Radical and palladium-catalyzed cyclizations to cyclobutenes: an entry to the BCD ring system of penitrem D.

A novel approach toward the synthesis of the BCD ring system of penitrem D is described. The strategy capitalizes on the fast cyclization rates of aryl radicals into cyclobutenes and allows access to a variety of fused tricyclic structures. Radical/polar crossover reactions of precursors 24-29 promoted by samarium diiodide in the presence of HMPA and acetone allow access to the fully functionalized BCD ring system of penitrem D. The stereochemical implications of these processes are evaluated, and a Pd-mediated cyclization approach toward the penitrems is also introduced.

Catalysis↗
PubMed2004

A highly efficient microwave-assisted suzuki coupling reaction of aryl perfluorooctylsulfonates with boronic acids.

[reaction: see text] A new strategy to improve the efficiency of Suzuki coupling reactions is introduced by combining fast microwave reaction with easy fluorous separation. Aryl perfluorooctylsulfonates derived from the corresponding phenols are coupled with aryl boronic acids to form biaryls under general microwave conditions. Both intermediates and products are purified by solid-phase extraction over FluoroFlash silica gel. Application of this tagging strategy to multistep synthesis of biaryl-substituted hydantoin is also described.

Alkanesulfonic Acids↗
PubMed2004

Simple strategy for the immobilization of dirhodium tetraprolinate catalysts using a pyridine-linked solid support.

Dirhodium tetracarboxylates are readily immobilized on agitation in the presence of highly cross-linked polystyrene resins with a pyridine attachment. A systematic study demonstrates that the polymer backbone, the linker, the terminal pyridine group, and the catalyst structure all contribute to the efficiency of dirhodium catalyst immobilization. The immobilization is considered to be due to the combination of ligand coordination and encapsulation. The dirhodium tetraprolinate catalysts, Rh2(S-DOSP)4 (1a), Rh2(S-TBSP)4 (1b), and Rh2(S-biTISP)2 (2), are all efficiently immobilized. The resulting heterogeneous complexes are very effective catalysts for asymmetric cyclopropanation between methyl phenyldiazoacetate and styrene, and under optimized conditions they can be recycled five times with virtually no loss in enantioselectivity. The three-phase test studies indicated that a very slow reaction occurs when both the catalyst and the diazo compound were immobilized, but the slow rate precluded the likelihood that the cyclopropanation was predominately occurring by a release-and-capture mechanism.

Journal Article↗
PubMed2003

Samarium(II) iodide mediated radical/polar crossover reactions of cyclobutenes. An efficient approach to the BCD ring system of the penitrems.

[reaction: see text] Radical/polar crossover reactions of derivatives of 1-(2-cyclobutenyl)-2-(2-iodoaryl)ethanones with acetone promoted by samarium diiodide and HMPA provide 1-(1-hydroxy-1-methylethyl)-2,2a,4,8b-tetrahydro-1H-cyclobuta[a]naphthalen-3-one derivatives in about 50% isolated yield. This reaction shows promise for construction of the BCD ring fragment of the penitrems.

Alkaloids↗
PubMedDate not supplied

Solution-phase parallel synthesis of an N-alkylated dihydropteridinone library from fluorous amino acids.

Parallel synthesis of an N-alkylated dihydropteridinone library has been accomplished in five steps starting from two displacement reactions of 4,6-dichloro-5-nitropyrimidine, first with fluorous amino acids, then with secondary amines. The hydrogenation of the nitro group followed by microwave-assisted cyclization gave the dihydropteridinones. Further diversification was achieved by the reaction of dihydropteridinones with benzyl halides to afford mono-N-alkylated products. All the reaction intermediates and final products were purified by SPE or precipitation without the need to perform chromatography.

Amino Acids↗
PubMedDate not supplied

Plate-to-plate fluorous solid-phase extraction for solution-phase parallel synthesis.

A commercially available Argonaut VacMaster-96 plate-to-plate solid-phase extraction (SPE) station equipped with 24 FluoroFlash cartridges is employed for parallel purification of fluorous reaction mixtures. Each cartridge charged with 3 g of fluorous silica gel has the capability to produce up to 100 mg of purified small molecules. The 24-well receiving plate has a standard footprint that can be directly concentrated in a Genevac vacuum centrifuge. Important issues such as sample loading, product cross-contamination, cartridge reuse, and reproducibility are investigated. The SPE system has been demonstrated in the purification of three small libraries that were produced involving amine scavenging reactions with fluorous isatoic anhydride, amide coupling reactions with 2-chloro-4,6-bis[(perfluorohexyl)propyloxy]-1,3,5-triazine (fluorous CDMT), and amide coupling reactions with a newly developed fluorous Mukaiyama condensation reagent.

Chromatography↗