Recent applications of anionic oxy-cope rearrangements
About: This article is published in Tetrahedron.The article was published on 1997-10-13. It has received 108 citations till now.
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TL;DR: It is highlighted how "growing" rings from existing cyclic systems via ring expansion can expedite the efficient, practical and scalable synthesis of macrocycles and medium-sized rings.
Abstract: Functionalised macrocycles and medium-sized rings have applications in a number of scientific fields, ranging from medicinal chemistry and supramolecular chemistry, to catalysis and nanotechnology. However, their value in these areas can be undermined by a simple, but important limitation: large ring systems are very often difficult to make. Traditional end-to-end cyclisation reactions of long linear precursors are typically unpredictable and impractical processes, mainly due to unfavourable enthalpic and entropic factors. Most published methods to make large rings focus on minimising the damage inflicted by performing the difficult cyclisation step; in contrast, ring-expansion reactions enable it to be avoided altogether. In this Review article, it is highlighted how “growing” rings from existing cyclic systems via ring expansion can expedite the efficient, practical and scalable synthesis of macrocycles and medium-sized rings.
163 citations
TL;DR: Iridium(I)-catalyzed olefin isomerization in bis(allyl) ethers is integrated into a generally applicable strategy for affecting highly stereoselective Claisen rearrangements.
Abstract: Iridium(I)-catalyzed olefin isomerization in bis(allyl) ethers is integrated into a generally applicable strategy for affecting highly stereoselective Claisen rearrangements. Catalyzed alkene isomerization affords allyl vinyl ethers from easily prepared di(allyl) ethers; direct thermolysis of these reaction mixtures leads to highly diastereoselective [3,3] sigmatropic rearrangements affording syn-2,3-dialkyl-4-pentenal derivatives. An easily executed strategy for realizing asymmetric variants of the isomerization-Claisen rearrangement (ICR) reactions is also described.
90 citations
TL;DR: The origin of stereoselectivity of NHC-catalyzed annulation reactions of ynals and stable enols was studied with Density Functional Theory and the data suggest that the C-C bond formation is the stereo-determining step.
Abstract: The origin of stereoselectivity of NHC-catalyzed annulation reactions of ynals and stable enols was studied with Density Functional Theory. The data suggest that the C–C bond formation is the stereo-determining step. Only the deprotonated pathway (containing an oxy-anion and overall neutral species) was found to give rise to discrimination of the competing stereoisomers. This is due predominantly to electrostatic repulsion of the β-stabilizing enolate functionality with the π-cloud of the aryl group in the NHC-catalyst.
80 citations
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TL;DR: In this paper, a theory of charge transfer stabilization of the transition state for nucleophilic addition to a carbonyl group by electron donors is proposed, using the Bell-Evans-Polanyi principle.
Abstract: A theory of charge-transfer stabilization of the transition state for nucleophilic addition to a carbonyl group by electron donors is proposed. It is postulated, using the Bell-Evans-Polanyi principle, that the feature of this transition state critical for stereoselectivity of the reaction is the existence of a low-lying vacant orbital u** associated with the u bond being formed in the reaction and that electron delocalization into that orbital will stabilize the transition state and may thereby enhance the reaction rate: the kinetic anomeric effect and the kinetic a effect are considered as the examples of such a stabilizing interaction. Stereochemistry of nucleophilic addition to cyclohexanone is determined by two factors according to this model: steric hindrance which favors the equatorial approach and electron donation from the cyclohexanone ucc and ucH bonds into the u** orbital, which favors the axial approach since the carbon-hydrogen bonds are better electron donors. Consequently, nucleophile structure, metal cations complexing the carbonyl oxygen, solvent, and counterions or other solutes may influence stereoselectivity of the reaction by changing the e(.**). Furthermore, it is shown that this model offers a simple and consistent way to rationalize kinetic and stereochemical effects of the so-called "remote polar substituents" which cannot be explained in terms of steric or electrostatic interactions but appear to be controlled by the overlap and energy gap between the remote electron-donor orbitals and the u** orbital.
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TL;DR: In this paper, the authors concluded that the four-centered, chair-like transition state is favored in the Cope rearrangement over the six-centered boat-like structure.
258 citations
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