Biomedical subjects
Publications and source records attributed to Shinobu Takizawa.
[reaction: see text] Novel chiral imidazolium salts have been synthesized as examples of chiral ionic liquids with a spiro skeleton. Effects of N-substituents and counteranions on the melting point of spiro imidazolium salts and their chiral discrimination abilities are described.
The efficient and novel bifunctional organocatalyst for the enantioselective aza-Morita-Baylis-Hillman (aza-MBH) reaction has been established with (S)-3-(N-isopropyl-N-3-pyridinylaminomethyl)BINOL for the first time. The reaction proved to be deeply influenced by the position of the Lewis base attached to BINOL. The acid-base-mediated functionalities for the activation of the substrate and the fixing of conformation of the organocatalyst are harmoniously performed to promote the reaction with high enantiocontrol.
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[reaction: see text] Disulfides bearing (R)-1,1'-bi-2-naphthol ((R)-BINOL) moieties at each terminal position have been successfully introduced on the surface of Au cluster. Ti-BINOLate complex generated from the obtained monolayer-protected Au cluster (MPC) promoted catalytic asymmetric alkylation of benzaldehyde with Et(2)Zn to afford the adduct in up to 98% yield with 86% ee. After completion of the reaction, the BINOL-functionalized MPC was easily recovered.
[reaction: see text] Polymer-supported bisBINOL ligands were successfully utilized for the immobilization of multicomponent asymmetric catalysts. The polymer-supported Al-Li-bis(binaphthoxide) (ALB) catalyst was more effective than the dendrimer-supported ALB in the Michael reaction of 2-cyclohexen-1-one with dibenzyl malonate affording the adduct in 91% yield with 96% ee. The polymer was also effective for the immobilization of a mu-oxodititanium complex that promoted carbonyl-ene reaction of ethyl glyoxalate with alpha-methyl styrene to provide the adduct with up to 98% ee.
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Although several types of chiral hypervalent iodine reagents have been used for asymmetric induction, all of them have needed more than a stoichiometric amount of chiral reagents and have shown low enantioselectivities. The described new catalytic asymmetric oxidation using a hypervalent iodine(V) reagent, iodoxybenzene (PhIO(2)), in a cationic reversed micellar system provides the first example of a catalytic asymmetric oxidation of sulfides to sulfoxides in high chemical yield with moderate to good enantioselectivity without the use of any transition-metal catalysts. The solubilization and activation of PhIO(2) by adding catalytic amounts of both cetyltrimethylammonium bromide (CTAB) and a chiral tartaric acid derivative were found to be indispensable for the enhancement of chemical and optical yields.