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Very High Diastereoselectivities (very + high_diastereoselectivity)
Selected AbstractsUnusual Magnesium Chloride Catalyzed Non-Evans anti -Aldol Reactions of an Enolizable L -Threose DerivativeEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 34 2007James McNulty Abstract The magnesium chloride catalyzed anti -aldol reaction of phenyl acetate derived oxazolidinones proceeds readily with enolizable L -threose derivative 8 to provide anti -aldol adducts in high yields and with very high diastereoselectivities. The reaction is also efficient with aromatic aldehydes and provides slightly lower diastereoselectivities. This extension allows access to stereochemically defined fragments applicable to the synthesis of alkaloid and phenylpropanoid derivatives.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Highly Enantio- and Diastereoselective One-Pot Reactions in Aqueous Media: Combined Asymmetric Rh-Catalyzed Conjugate Addition/Metal-Mediated AllylationEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 17 2006Sara Källström Abstract 1,3-Disubstituted, enantiopure cyclohexanols have been prepared in very high diastereoselectivities and good yields by a concise one-pot method combining the enantioselective rhodium-catalyzed conjugate addition of arylboronic acids with indium-mediated allylation into a highly efficient one-pot reaction in aqueous media. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] The Use of Samarium Enolates, A Novel Alternative in the Addition Reactions to Imines.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 20094-Diamino Esters, Amides, Enantiopure , Synthesis of 3-Amino Esters Abstract An efficient reaction of tosylimines with a range of samarium enolates (derived from esters, and amides) is reported. The reaction with the ,-dibenzylamino- N - tert- butanesulfinimine derived from chiral phenylalaninal afforded the corresponding enantiopure 3,4-diamino ester with very high diastereoselectivity. [source] Vinyl Sulfoxides as Stereochemical Controllers in Intermolecular Pauson,Khand Reactions: Applications to the Enantioselective Synthesis of Natural CyclopentanoidsCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2004Marta Rodríguez Rivero Abstract The use of sulfoxides as chiral auxiliaries in asymmetric intermolecular Pauson,Khand reactions is described. After screening a wide variety of substituents on the sulfur atom in ,,,-unsaturated sulfoxides, the readily available o -(N,N -dimethylamino)phenyl vinyl sulfoxide (1,i) has proved to be highly reactive with substituted terminal alkynes under N -oxide-promoted conditions (CH3CN, 0,°C). In addition, these Pauson,Khand reactions occurred with complete regioselectivity and very high diastereoselectivity (de=86,>96,%, (S,RS) diastereomer). Experimental studies suggest that the high reactivity exhibited by the vinyl sulfoxide 1,i relies on the ability of the amine group to act as a soft ligand on the alkyne dicobalt complex prior to the generation of the cobaltacycle intermediate. On the other hand, both theoretical and experimental studies show that the high stereoselectivity of the process is due to the easy thermodynamic epimerization at the C5 center in the resulting 5-sulfinyl-2-cyclopentenone adducts. When it is taken into account that the known asymmetric intermolecular Pauson,Khand reactions are limited to the use of highly reactive bicyclic alkenes, mainly norbornene and norbornadiene, this novel procedure constitutes the first asymmetric version with unstrained acyclic alkenes. As a demonstration of the synthetic interest of this sulfoxide-based methodology in the enantioselective preparation of stereochemically complex cyclopentanoids, we have developed very short and efficient syntheses of the antibiotic (,)-pentenomycin I and the (,)-aminocyclopentitol moiety of a hopane triterpenoid. Se ha estudiado la utilización de sulfóxidos como auxiliares quirales en reacciones de Pauson,Khand intermoleculares. Tras considerar una amplia variedad de vinil sulfóxidos diferentemente sustituidos en el átomo de azufre, se ha encontrado que el o -(N,N -dimetilamino)fenil sulfóxido (1,i) presenta una elevada reactividad frente a alquinos terminales en reacciones de Pauson,Khand. Además, estas reacciones transcurren con regioselectividades completas y diastereoselectividades muy elevadas (ed=86,>96,%). Estudios teóricos y experimentales sugieren que la gran reactividad mostrada por el vinil sulfóxido 1,i se debe a la capacidad del grupo amino para coordinarse al complejo de dicobalto del alquino, favoreciendo así la posterior formación del cobaltaciclo intermedio. Por otro lado, estudios tanto teóricos como experimentales han demostrado que la elevada diastereoselectividad del proceso es consecuencia de la fácil epimerización termodinámica en la posición C-5 de las 5-sulfinil-2-ciclopentenonas finales. Teniendo en cuenta que hasta el momento la reacción de Pauson,Khand intermolecular estaba limitada al empleo de alquenos bicíclicos muy reactivos, principalmente norborneno y norbornadieno, este nuevo procedimiento constituye la primera versión asimétrica con alquenos acíclicos no tensionados. Como demostración de la utilidad sintética de esta nueva metodología en la preparación enantioselectiva de sistemas ciclopentánicos complejos, se han desarrollado síntesis muy eficaces del antibiótico (,)-pentenomicina I y de la unidad de aminociclopentitol de un triperpenoide. [source] | ||||||||||