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Complete Regioselectivity (complete + regioselectivity)
Selected AbstractsHighly Efficient Asymmetric Three-Component Vinylogous Mannich Reaction Catalyzed by a Chiral Scandium(III)- N,N,-Dioxide ComplexADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2010Qi Zhang Abstract The asymmetric three-component vinylogous Mannich reaction of an acyclic silyl dienol ester, an aldehyde and 2-aminophenol was accomplished using a chiral N,N,-dioxide-scandium(III) complex as the catalyst. A variety of aldehydes were found to be suitable substrates for the reaction and the desired ,-amino-,,,-unsaturated esters were obtained in 90,99% yields with good to excellent enantioselectivities (up to >99% ee) and complete regioselectivities. Moreover, the simple experimental procedures were air-tolerant and convenient. The present catalytic process provides the potential for large-scale syntheses of the chiral ,-amino-,,,-unsaturated esters. [source] Highly Regio- and Stereoselective Diels,Alder Cycloaddition of Difluoro(methylene)cyclopropanesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 6 2008Xiao-Chun Hang Abstract The Diels,Alder reactions of difluoro(methylene)cyclopropanes (F2MCPs) with cyclic dienes are described. These cycloaddition reactions exhibited complete regioselectivity and high endo -stereoselectivity. The obtained cycloadducts underwent a retro-Diels,Alder reaction to give the original dienophiles and dienes when heated, reflecting the reversible Diels,Alder reactivity of F2MCPs.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Iridium-Catalyzed Selective Synthesis of 4-Substituted Benzofurans and Indoles via Directed CyclodehydrationADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 17 2009Kyoji Tsuchikama Abstract A directed cyclization-dehydration cascade of ,-aryloxy ketones and ,-arylamino ketones was efficiently catalyzed by a cationic iridium-BINAP complex, which afforded various types of 4-substituted benzofurans and indoles in high yields with complete regioselectivity. The newly developed protocol also enabled the enantioselective preparation of chiral 4-acetyloxindole using a chiral iridium catalyst. [source] Use of some aryl and heteroaryl nitrilimines and nitrones in the synthesis of spiroheterocyclesJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 5 2004Eva Jedlovská 1,3-Dipolar cycloadditions of C-(5-nitro-2-furyl)- N -methyl nitrilimine (2a), C -(5-nitro-2-furyl)- N -phenyl nitrilimine (2b), C -4-nitrophenyl- N -methyl nitrilimine (2c) and C,N -diphenyl nitrilimine (2d) with 1-R-substituted 3,3-methylene-5,5-dimethylpyrrolidin-2-ones (1a-d) where R is H, acetyl, 1,1-dimethylethoxycarbonyl and 1-methylethenyl proceed with complete regioselectivity in good yields to afford 1,3,7-trisubstituted-6-oxo-8,8-dimethyl-1,2,7-triazaspiro[4,4]non-2-enes (5a-g) exclusively. Cycloaddition of C -(5-nitro-2-furyl)- N -phenylnitrone (3b) to the exocyclic double bond of the dipolarophile 1a proceeds to 2-phenyl-3-(5-nitro-2-furyl)-6-oxo-8,8-dimethyl-1-oxa-2,7-diazaspiro[4,4]nonane (7) with complete regio- and stereoselectivity. [source] Origin of Diastereoselectivity in the Organolanthanide-Mediated Intramolecular Hydroamination/Cyclisation of Aminodienes: A Computational Exploration of Constrained Geometry CGC,Ln Catalysts,CHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2007Sven Tobisch Dr. Abstract The regulation of ring-substituent diastereoselectivity in the intramolecular hydroamination/cyclisation (IHC) of ,-substituted aminodienes by constrained geometry CGC,lanthanide catalysts (CGC=[Me2Si(,5 -Me4C5)(tBuN)]2,) has been elucidated by means of a reliable DFT method. The first survey of relevant elementary steps for the 1-methyl-(4E,6)-heptadienylamine substrate (1) and the [{Me2Si(,5 -Me4C5)(tBuN)}Sm{N(TMS)2}] starting material (2) identified the following general mechanistic aspects of Ln-catalysed aminodiene IHC. The substrate-adduct 3 -S of the active CGC,Ln,amidodiene compound represents the catalyst's resting state, but the substrate-free form 3, with a chelating amidodiene functionality is the direct precursor for cyclisation. This step proceeds with almost complete regioselectivity through exocyclic ring closure by means of a frontal trajectory, giving rise to the CGC,Ln,azacycle intermediate 4. Subsequent protonolysis of 4 is turnover limiting, whilst the ring-substituent diastereoselectivity is dictated by exocyclic ring closure. Unfavourable close interatomic contacts between the substrate's ,-substituent and the catalyst backbone have been shown to largely govern the trans/cis selectivity. Substituents of sufficient bulk in the ,-position of the substrate have been identified as being vital for stereochemical induction. The present study has indicated that the diastereoselectivity of ring closure can be considerably modulated. The variation of the lanthanide's ionic radius and introduction of extra steric pressure at the substrate's ,-position and/or the CGC N centre have been identified as effective handles for tuning the selectivity. The quantification of these factors reported herein represents the first step toward the rational design of improved CGC,Ln catalyst architectures and will thus aid this process. [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] | ||||||||||