Reactions Alder-ene

Various oligomers formed by Diels-Alder/ene reactions are observed.333 334 For S-MA11 polymerization Sato et ci//31 used spin trapping to identify the initialing species. On the other hand, in the case of S-AN copolymerization, Ihe... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

Treating diene-yne derivatives 50 with ferrate 40 does not lead to the expected ene-allenes, instead the [4 + 2]-cycloaddition products 51 are obtained in moderate yields (eq. 1 in Scheme 11). As metal-catalyzed Diels-Alder-reactions of unactivated aUcynes and dienophiles are assumed to proceed via metaUacyclic intermediates, this supports the mechanism for the Alder-ene-reaction discussed before. 

Alder-Ene Reactions. An ene-iminium one-pot cyclization proceeds smoothly in a mixture of water-THF (Eq. 12.66). 138 The reactivity of the ene-iminium substrates is highly dependent on the substitution pattern of the ethylenic double bond. This methodology can be used to form homochiral pipecolic acid derivatives. 

C-C Bond Formation (Part 1) by Addition Reactions Alder-ene Reaction... 

Transition Metal-Catalyzed Intermolecular Alder-Ene Reactions 565... 

Transition Metal-Catalyzed Intramolecular Alder-Ene Reactions 568... 

Applications of the Alder-Ene Reaction to the Synthesis of Biologically Relevant... 

The ene reaction, an electronic relative of the Diels-Alder cycloaddition, is a six-electron process involving the reaction of the n and allylic reactions these reactions are reviewed in detail (for reviews of the Alder-ene reaction and related chemistry, see Refs 1,1a, and lb). Recent developments in the area of hetero-ene chemistry (X1 = H, X2 = N,0) are also surveyed. 

This excellent regiocontrol was exploited by subjecting terminal alkenes and hydroxyalkynoates to ruthenium catalysis conditions to afford butenolides and pentenolides (Equation (23)).36 The Alder-ene reaction occurs preferentially to form the G-G bond at the alpha-carbon of the alkynoate. The unusually high regioselectivity is attributed to a synergistic effect derived from an enhanced coordination of the hydroxyl group to the ruthenium. 

A mechanistic pathway is proposed based upon the observed regioselectivities and other results that were obtained during the exploration of the scope and limitations of the Alder-ene reaction.38 Initially, coordination of the alkene and alkyne to the ruthenium catalyst takes place (Scheme 5). Next, oxidative addition affords the metallocycles 42 and 43. It is postulated that /3-hydride elimination is slow and that the oxidative addition step is reversible. Thus, the product ratio is determined by the rate at which 42 and 43 undergo /3-hydride elimination. 

The intermolecular ruthenium enyne Alder-ene reaction has been extended to the stereoselective preparation of enamines (Equation (26)).39 The yields obtained for this reaction were high with allylacetamides, -benzamides,... 

Enantio- and diastereoselective syntheses of a variety of heterocycles were accomplished by combining the ruthenium-catalyzed Alder-ene reaction with a palladium-catalyzed asymmetric allylic alkylation (AAA) (Scheme 7). For the AAA, y>-nitrophenol was found to function as a suitable leaving group and yet was stable to the Alder-ene conditions. Extensive solvent studies were performed to determine the best conditions for the one-pot procedure. 

The Alder-ene reaction has traditionally been performed under thermal conditions—generally at temperatures in excess of 200 °C. Transition metal catalysis not only maintains the attractive atom-economical feature of the Alder-ene reaction, but also allows for regiocontrol and, in many cases, stereoselectivity. A multitude of transition metal complexes has shown the ability to catalyze the intramolecular Alder-ene reaction. Each possesses a unique reactivity that is reflected in the diversity of carbocyclic and heterocyclic products accessible via the transition metal-catalyzed intramolecular Alder-ene reaction. Presumably for these reasons, investigation of the thermal Alder-ene reaction seems to have stopped almost completely. For example, more than 40 papers pertaining to the transition metal-catalyzed intramolecular Alder-ene reaction have been published over the last decade. In the process of writing this review, we encountered only three recent examples of the thermal intramolecular Alder-ene reaction, two of which were applications to the synthesis of biologically relevant compounds (see Section 10.12.6). 

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