Showing papers by "Hisashi Yamamoto published in 2005"

"Designer acids": combined acid catalysis for asymmetric synthesis.

Abstract: Lewis and Bronsted acids can be utilized as more-effective tools for chemical reactions by sophisticated engineering ("designer acids"). The ultimate goal of such "designer acids" is to form a combination of acids with higher reactivity, selectivity, and versatility than the individual acid catalysts. One possible way to take advantage of such abilities may be to apply a "combined acids system" to the catalyst design. The concept of combined acids, which can be classified into Bronsted acid assisted Lewis acid (BLA), Lewis acid assisted Lewis acid (LLA), Lewis acid assisted Bronsted acid (LBA), and Bronsted acid assisted Bronsted acid (BBA), can be a particularly useful tool for the design of asymmetric catalysis, because combining such acids will bring out their inherent reactivity by associative interaction, and also provide more-organized structures that allow an effective asymmetric environment.

Brønsted acid catalysis of achiral enamine for regio- and enantioselective nitroso aldol synthesis

Abstract: Two types of chiral Bronsted acid catalysts have been shown to catalyze regio- and enantioselective nitroso aldol synthesis between nitrosobenzene and achiral enamine. The combination of Bronsted acidity and amine moiety of enamine realizes complete regioselectivity with high enantioselectivity. After a survey of Bronsted acid catalysts, 1-naphthyl glycolic acid is found to be optimal in the O-nitroso aldol pathway, while 1-naphthyl TADDOL is the best catalyst for the N-nitroso aldol pathway. This is based on our finding on the control of regioselectivity by changing the amine moiety of enamine and the choice of Bronsted acidity.

Axially chiral biaryl diols catalyze highly enantioselective hetero-Diels-Alder reactions through hydrogen bonding.

Abstract: Axially chiral 1,1'-biaryl-2,2'-dimethanol (3, BAMOL) family of diols are highly effective catalysts for enantioselective hetero-Diels-Alder reactions between aminosiloxydiene 1 and a wide variety of unactivated aldehydes. The reactions proceed in useful yields and excellent enantioselectivities. The diols function in the same capacity as Lewis acids, by activating the aldehyde carbonyl group through hydrogen bonding.

Kombinierte Säurekatalyse in der asymmetrischen Synthese durch “Designer‐Säuren”

Abstract: Lewis- und Bronsted-Sauren konnen durch verfeinerte Techniken in effektivere Reagentien uberfuhrt werden. Ziel der Entwicklung solcher “Designer-Sauren” sind hohe Reaktivitaten und Selektivitaten sowie eine vielseitige Anwendbarkeit bei Reaktionen, die das Potenzial von Saurekatalysatoren nicht vollstandig ausschopfen. In der Katalysatorplanung wird insbesondere das Konzept der kombinierten Sauren zum Design enantioselektiver Katalysen genutzt. Diese sind unterteilt in Bronsted-Saure-aktivierte Lewis-Sauren (BLAs), Lewis-Saure-aktivierte Lewis-Sauren (LLAs), Lewis-Saure-aktivierte Bronsted-Sauren (LBAs) und Bronsted-Saure-aktivierte Bronsted-Sauren (BBAs). Bei der kombinierten Anwendung von Sauren bleibt zum einen die inharente Reaktivitat der assoziierten Komponenten erhalten, zum anderen stehen hoher organisierte Strukturen zur Verfugung, die eine wirksame asymmetrische Umgebung schaffen.

Synthesis of (all-rac)-α-Tocopherol in Supercritical Carbon Dioxide: Tuning of the Product Selectivity in Batch and Continuous-Flow Reactors

Abstract: α-Tocopherol was synthesized using a condensation reaction of 2,3,6-trimethylhydroquinone with isophytol in supercritical CO 2 using batch and continuous-flow reactors. In the batch reaction catalyzed by a fluorinated molecular catalyst bearing strong Bronsted acidity, C 6 F 5 CHTf 2 (Ti=SO 2 CF 3 ), an increase in the CO 2 pressure causes a marked increase in the product selectivity for α-tocopherol, albeit with a slight decrease in the product yield. The solubility measurements by extraction experiments and the supercritical fluid NMR (scNMR) indicate that the homogeneous and non-polar reaction phase in scCO 2 is crucial to obtain α-tocopherol with high selectivity. A continuous flow scCO 2 process for the condensation reaction can be performed with a strong acid resin, Amberlyst 15, as a solid acid catalyst to give the desired product with high selectivity.

Rich Chemistry of Nitroso Compounds

Abstract: Nitrosobenzene or nitrosopyridine are found to be attractive electrophiles in catalytic enantioselective carbon–nitrogen and/or carbon–oxygen bond forming reactions. In the presence of designer Lewis or Bronsted acid catalysts, catalytic enantioselective O- and N-nitroso aldol reaction or nitroso Diels–Alder reaction proceed smoothly. The scope and limitation of new catalytic processes are described.

Axially Chiral Biaryl Diols Catalyze Highly Enantioselective Hetero-Diels—Alder Reactions Through Hydrogen Bonding.

Abstract: Axially chiral 1,1'-biaryl-2,2'-dimethanol (3, BAMOL) family of diols are highly effective catalysts for enantioselective hetero-Diels-Alder reactions between aminosiloxydiene 1 and a wide variety of unactivated aldehydes. The reactions proceed in useful yields and excellent enantioselectivities. The diols function in the same capacity as Lewis acids, by activating the aldehyde carbonyl group through hydrogen bonding.

“Designer Acids”: Combined Acid Catalysis for Asymmetric Synthesis

Abstract: Lewis and Bronsted acids can be utilized as more-effective tools for chemical reactions by sophisticated engineering ("designer acids"). The ultimate goal of such "designer acids" is to form a combination of acids with higher reactivity, selectivity, and versatility than the individual acid catalysts. One possible way to take advantage of such abilities may be to apply a "combined acids system" to the catalyst design. The concept of combined acids, which can be classified into Bronsted acid assisted Lewis acid (BLA), Lewis acid assisted Lewis acid (LLA), Lewis acid assisted Bronsted acid (LBA), and Bronsted acid assisted Bronsted acid (BBA), can be a particularly useful tool for the design of asymmetric catalysis, because combining such acids will bring out their inherent reactivity by associative interaction, and also provide more-organized structures that allow an effective asymmetric environment.

Procede de preparation de composes d'$g(a)-aminooxycetone/$g(a)-aminooxyaldehyde et d'$g(a)-hydroxycetone/$g(a)-hydroxyaldehyde et procede de preparation de produits de reaction a partir de substrats de cetone non satures $g(a),$g(b) cyclique et de substrats nitreux

Abstract: L'invention concerne un procede de preparation de composes α-aminooxycetone et d'α-hydroxycetone. La voie de synthese applique generalement la mise en reaction d'un substrat aldehyde ou cetone et d'un substrat nitreux en presence d'un catalyseur represente par la formule (IV). Dans cette formule, X1-X3 representent de facon independante de l'azote, du carbone, de l'oxygene ou du soufre et Z represente un anneau a 4 a 10 elements avec ou sans substituant et eventuellement un etage supplementaire destine a convertir le compose d'α-aminooxycetone forme en compose α-hydroxycetone. Cette invention permet d'obtenir des composes α-aminooxycetone et α-hydroxycetone a enantioselectivite et purete elevees. Cette invention concerne egalement une reaction de Michael avec de l'Aldol O-nitreux asymetrique catalytique. Les substrats de cette reaction consistent generalement en un substrat de cetone α,β non sature cyclique ou en un substrat nitreux. Cette methodologie implique generalement la mise en reaction du substrat de cetone α,β non sature cyclique et du substrat nitreux en presence d'un catalyseur a base de proline, afin d'obtenir un produit heterocyclique.