- Alkyne metathesis: Development of a novel molybdenum-based catalyst system and its application to the total synthesis of epothilone A and C
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Sterically hindered molybdenum(III) amido complexes of the general type [Mo{(tBu)(Ar)N}3] (1), upon treatment with CH2Cl2 or other halogen donors, have been converted into highly effective catalysts for all kinds of alkyne metathesis reactions. Although the actual nature of the propagating species formed in situ is still elusive, halogen transfer to the Mo center of 1 plays a decisive role in the activation of such precatalysts. It was possible to isolate and characterize by X-ray crystallography some of the resulting molybdenum halide derivatives such as 15, 16 and 20 which themselves were shown to be catalytically active. Numerous applications illustrate the performance of the catalytic system 1/CH2Cl2 which operates under mild conditions and tolerates an array of polar functional groups. The wide scope allows the method to be implemented into the total synthesis of sensitive and polyfunctional natural products. Most notable among them is a concise entry into the potent anticancer agents epothilone A (86) and C (88). The macrolide core of these targets is forged by ring closing alkyne metathesis (RCAM) of diyne 113, followed by Lindlar hydrogenation of cycloalkyne 114 thus formed. Since this strategy opens a stereoselective entry into (Z)-alkene 115, the approach is inherently more efficient than previous syntheses based on conventional RCM. Wiley-VCH Verlag GmbH, 2001.
- Fuerstner, Alois,Mathes, Christian,Lehmann, Christian W.
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p. 5299 - 5317
(2007/10/03)
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- Scope and Limitations of the Pd/BINAP-Catalyzed Amination of Aryl Bromides
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Mixtures of Pd2(dba)3 or Pd(OAc)2 and BINAP catalyze the cross-coupling of amines with a variety of aryl bromides. Primary amines are arylated in high yield, and certain classes of secondary amines are also effectively transformed. The process tolerates the presence of several functional groups including methyl and ethyl esters, enolizable ketones, and nitro groups provided that cesium carbonate is employed as the base. Most reactions proceed to completion with 0.5-1.0 mol % of the palladium catalyst; in some cases, catalyst levels as low as 0.05 mol % Pd may be employed. Reactions are considerably faster if Pd(OAc)2 is employed as the precatalyst, and the order in which reagents are added to the reaction has a substantial effect on reaction rate. It is likely that the catalytic process proceeds via bis(phosphine)palladium complexes as intermediates. These complexes are less prone to undergo undesirable side reactions which lead to diminished yields or catalyst deactivation than complexes of the corresponding monodentate triarylphosphines.
- Wolfe, John P.,Buchwald, Stephen L.
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p. 1144 - 1157
(2007/10/03)
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- SYNTHESE DE N,N-DIALKYL-3,5-DIALKYL-ANILINES A PARTIR DES SELS DE TRIALKYL-2,4,6-PYRYLIUM
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We have optimized through an experimental design the synthesis of N,N-diethyl-3,5-dimethylaniline from 2,4,6-trimethylpyrylium terafluoroborate (I) and diethylamine in a new reaction medium (acetonitrile/triethylamine).Optimal conditions have been applied with high yields to the synthesis of various 3,5-dimethylanilines derived from I and dimethylamine, morpholine, N-methylpiperazine, piperazine, dihexylamine, piperidine, and pyrrolidine respectively. 2-Alkyl-4,6-dimethylpyrylium tetrafluoroborates react regioselectively with diethylamine to afford N,N-diethyl-3-alkyl-5-methyl anilines under these conditions.
- Vernaudon, Pascal,Rajoharison, Harivelo G.,Roussel, Chritian
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p. 205 - 211
(2007/10/02)
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