Shinobu Takizawa
Osaka University
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Featured researches published by Shinobu Takizawa.
Tetrahedron | 2001
Hirofumi Tohma; Hironori Morioka; Shinobu Takizawa; Mitsuhiro Arisawa; Yasuyuki Kita
Abstract Oxidative biaryl coupling reaction of phenol ether derivatives with the hypervalent iodine reagent, phenyliodine(III) bis(trifluoroacetate) (PIFA), in the presence of BF3·Et2O gave a variety of substituted biphenyl and binaphthyl compounds in high yields. Replacement of PIFA with a more practical polymer-supported hypervalent iodine reagent has also been achieved.
Angewandte Chemie | 2010
Shinobu Takizawa; Naohito Inoue; Shuichi Hirata; Hiroaki Sasai
The organocatalytic asymmetric domino process is a veryattractive method because of its ability to construct complexchiral molecules from readily available substrates under mildreaction conditions in two or more steps in a single operation.These reactions can save the time and the chemicals usuallyrequired for isolation or purification of the synthetic inter-mediates.Inaddition,noconcernformetalcontamination of the products is necessarybecause organocatalysts do not containtoxic or expensive metals. Examples oforganocatalytic asymmetric domino reac-tions mediated by chiral secondaryamines
Organic Letters | 2011
Kazuhiro Takenaka; Mitsutoshi Akita; Yugo Tanigaki; Shinobu Takizawa; Hiroaki Sasai
An enantioselective intramolecular oxidative cyclization of 4-alkenoic acids was developed. The reaction proceeded via a π-allyl Pd intermediate generated by an allylic C-H activation to give γ-lactone derivatives with moderate to good enantioselectivity. Spiro bis(isoxazoline) ligand, SPRIX, was indispensable for this asymmetric transformation.
Advanced Synthesis & Catalysis | 2002
Hirofumi Tohma; Tomohiro Maegawa; Shinobu Takizawa; Yasuyuki Kita
The facile and efficient oxidation of various alcohols such as benzylic alcohols, primary alcohols, secondary alcohols, and diols in water using the hypervalent iodine(III) reagent, iodosobenzene (PhI=O), with KBr is described. Electrospray ionization (ESI) mass spectrometric studies on the behavior of PhI=O-KBr in aqueous solution suggested that these reactions are induced by the formation of highly reactive iodine species [PhI(Br)nO –]. Further development to recyclable polymer-supported iodine(III) reagent extends the utility of this reaction to afford an environmentally benign method.
Journal of Organic Chemistry | 2009
Tetsuya Tsujihara; Toshio Shinohara; Kazuhiro Takenaka; Shinobu Takizawa; Kiyotaka Onitsuka; Minoru Hatanaka; Hiroaki Sasai
An enantioselective synthesis of tetrahydropyrrolo[1,2-c]pyrimidine-1,3-diones via a palladium-catalyzed intramolecular oxidative aminocarbonylation is described. The carbon-carbon double bond of alkenylurea substrates has been shown to react intramolecularly with a nitrogen nucleophile in the presence of a palladium catalyst under a carbon monoxide atmosphere. The use of a chiral spiro bis(isoxazoline) ligand (SPRIX) is essential to obtain the desired products in optically active forms. In comparison with the coordination ability of other known ligands, this peculiar character of SPRIX originates from two structural characteristics: low sigma-donor ability of the isoxazoline coordination site and rigidity of the spiro skeleton.
Journal of the American Chemical Society | 2009
Joie Garfunkle; F. Scott Kimball; John D. Trzupek; Shinobu Takizawa; Hiroyuki Shimamura; Masaki Tomishima; Dale L. Boger
The first total synthesis of chloropeptin II (1, complestatin) is disclosed. Key elements of the approach include the use of an intramolecular Larock indole synthesis for the initial macrocyclization, adopting conditions that permit utilization of a 2-bromoaniline, incorporating a terminal alkyne substituent (-SiEt(3)) that sterically dictates the indole cyclization regioselectivity, and benefiting from an aniline protecting group (-Ac) that enhances the atropdiastereoselectivity and diminishes the strained indole reactivity toward subsequent electrophilic reagents. Not only did this key reaction provide the fully functionalized right-hand ring system of 1 in superb conversion (89%) and good atropdiastereoselectivity (4:1 R:S), but it also represents the first reported example of what will prove to be a useful Larock macrocyclization strategy. Subsequent introduction of the left-hand ring system enlisting an aromatic nucleophilic substitution reaction for macrocyclization with biaryl ether formation completed the assemblage of the core bicyclic structure of 1. Intrinsic in the design of the approach and by virtue of the single-step acid-catalyzed conversion of chloropeptin II (1) to chloropeptin I (2), the route also provides a total synthesis of 2.
Tetrahedron Letters | 1998
Hirofumi Tohma; Shinobu Takizawa; Hiroaki Watanabe; Yasuyuki Kita
Abstract Catalytic activation of iodosobenzene (PhIO) by cetyltrimethylammonium bromide (CTAB) allows sulfide oxidation reaction to occur in high yields under mild conditions in a variety of solvents. This PhIO activation may be due to the formation of micellar or reversed micellar systems.
Angewandte Chemie | 2015
Shinobu Takizawa; Kenta Kishi; Yasushi Yoshida; Steffen Mader; Fernando Arteaga Arteaga; Shoukou Lee; Manabu Hoshino; Magnus Rueping; Makoto Fujita; Hiroaki Sasai
An enantio-, diastereo-, regio-, and chemoselective phosphine-catalyzed β,γ-umpolung domino reaction of allenic esters with dienones has been developed for the first time. The designed sequence, involving oxy-Michael and Rauhut-Currier reactions, produced highly functionalized tetrahydrobenzofuranones, bearing a chiral tetrasubstituted stereogenic center, in up to 96 % ee.
Organic Letters | 2013
Shinobu Takizawa; Fernando Arteaga Arteaga; Yasushi Yoshida; Michitaka Suzuki; Hiroaki Sasai
An enantioselective organocatalyzed aza-MBH-type reaction of ketimines and allenoates has been developed. The present formal [2 + 2] cycloaddition produces highly functionalized azetidines with a chiral tetrasubstituted carbon stereogenic center in good to excellent yields and high enantioselectivities.
Chemical Communications | 2008
Shinobu Takizawa; Tomomi Katayama; Chiaki Kameyama; Kiyotaka Onitsuka; Takeyuki Suzuki; Takeshi Yanagida; Tomoji Kawai; Hiroaki Sasai
Preparation and structural analysis of chiral dinuclear vanadium(v) catalysts with high catalytic activity for the oxidative coupling of 2-naphthols are described.