Fluoropropenoates as dienophiles in the Diels-Alder reaction

Several fluorinated alkenes were prepared from known ethyl (R)-2-fluoro-4,5-dihydroxyisopropylidine-2-pentenoate (1). The fluoroalkenoates were tested as dienophiles with several dienes and showed cycloaddition only with the very reactive diene, 1,3-diphenylisobenzofuran (5). Fluorobutenolide 2 reacted with 5 to produce the endo-syn adduct 9 in a highly stereoselective manner.     

Diels-Alder reaction of furan with some dienophiles

The reaction of furan and maleic acid was carried out in several solvents. The endo-adduct was isolated and the structure established by its spectral properties and conversion into known compounds. The adducts of furan with fumaric acid, diethyl fumarate and diethyl maleate are reported.     

2-Nitrofurans as dienophiles in Diels-Alder reactions

α-Nitrofuran derivatives are studied in Diels-Alder reactions under thermal conditions. In contrast to α-acylfurans, they proved to be efficient dienophiles.     

Diels-Alder reaction of 4-halogenated masked o-benzoquinones with electron-rich dienophiles

Inverse-electron-demand Diels-Alder reaction of masked o-benzoquinones (MOBs) ensuing from the corresponding 4-halo-2-methoxyphenols with styrene, dihydrofuran and ethyl vinyl ether, butyl vinyl ether, phenyl vinyl sulfide and vinyl acetate to afford the highly functionalized halogen substituted bicylclo[2.2.2]octenones are described.     

A novel application of the Diels-Alder reaction: nitronaphthalenes as normal electron demand dienophiles

Thermal reactions between nitronaphthalenes and butadienes were studied. It was demonstrated that these reactions are capable of undergoing the normal electron demand Diels-Alder reaction, with a variety of dienes affording the phenanthrene derivatives. The influence of the extension and type of substitution was also discussed. When the electron-withdrawing activation of the naphthalenic nucleus or the donor properties of the dienes were not enough, N-naphthylpyrroles were detected as main product, suggesting that a competitive reaction would probably take place. The results clearly confirmed the dienophilic nature of nitronaphthalenic double bonds and provided an alternative procedure for phenanthrene derivatives and N-naphthylpyrroles' synthesis. The relative reactivity of the reactants and the viability of the reactions were discussed from a theoretical point of view.     

Copper(II)-bis(oxazoline) catalyzed asymmetric Diels-Alder reaction with alpha'-arylsulfonyl enones as dienophiles

Alpha'-arylsulfonyl enones are efficient bidentate dienophiles for the Cu(II)-bis(oxazoline) catalyzed enantioselective Diels-Alder reaction with a number of dienes, affording the corresponding products with good to high enantiomeric excesses. The resulting products can be alkylated and the sulfone removed, so alpha'-arylsulfonyl enones can be regarded as surrogates of simple monodentate enones, which are poor dienophiles with this catalytic system.     

Alkenylimmonium salts as dienophiles in Diels-Alder cycloadditions with high reactivity and stereoselectivity

Alkenylimmonium salts, such as the substituted vinylpyridinium salts, 6, 7, 12, and 16, are easily prepared and are very reactive dienophiles in Diels-Alder cycloadditions.     

A novel Ireland-Claisen rearrangement/Diels-Alder tandem reaction of propargylic acrylates with acyclic dienophiles

A novel DABCO-catalyzed Ireland-Claisen rearrangement/Diels-Alder tandem reaction of propargylic acrylates 3 with dienophiles 4 was developed in the presence of an excess TMSCl and DBU with 1% hydroquinone as polymerization inhibitor in acetonitrile at refluxing temperature under air.This protocol gave cyclicα,β-unsaturated carboxylic acids 5 with complete regioselectivity.     

Electron transfer activation of the Diels-Alder reaction: Quantitative relationship to charge transfer excited states

The Diels-Alder cycloaddition of anthracene to tetracyanoethylene (TCNE) is quantitatively compared to alkylmetal insertion under the same reaction conditions. In both systems, the observation of transient charge transfer (CT) absorption bands is related to the presence of 1:1 electron donor-acceptor complexes of anthracene (Ar) and alkylmetal (RM) donors with the TCNE acceptor. The activation free energies ΔG3 for anthracene cycloaddition and alkylmetal insertion are found to be equal to the energies of ion-pair formation, i.e. [Ar⁺TCNE^?] and RM⁺TCNE^?], which are evaluated from the CT transition energies hν_CT. Indeed, the differences in the rates of alkylmetal insertion and anthracene cycloaddition by a factor of more than 10⁹, are shown quantitatively to arise from the differences in ion-pair solvation ΔG^s. The same differences in ΔG^s also apply quantitatively to the free ions, [Ar⁺] and [RM⁺], independently derived from the electrochemical and iron(III) oxidations of alkylmetals and aromatic compounds, respectively, by outer- sphere electron transfer. The charge transfer formulation of the activation process but provides a unifying basis for comparing such diverse processes as Diels-Alder cycloadditions and organometal cleavages, when a common electron-deficient acceptor is employed. The relationship to the concerted mechanisms of the Diels-Alder reaction is discussed.     

Reactivity and stereoselectivity of the Diels-Alder reaction using cyclic dienophiles and siloxyaminobutadienes

Several bicyclic compounds were synthesized by the Diels-Alder reaction using aminodiene and a cyclic dienophile. The stereochemistries of the obtained adducts were determined by X-ray crystallography or NMR analysis. The stereoselectivity of this Diels-Alder reaction was based on the interaction of molecular orbitals between the diene and dienophile. The reactivities of these Diels-Alder reactions were estimated, and the generality of this reaction is discussed.     

Diels-Alder reaction of sugar-derived cyclic dienophiles with cyclopentadiene. Factors affecting the reactivity and stereoselectivity

[formula: see text] A sugar-derived cyclic dienophile showed a high exo and face selectivity in the Diels-Alder reaction with cyclopentadiene. Structural modifications on the dienophile structure were made, and the outcomes of the cycloaddition reactions were investigated. These results and the spectroscopic data recorded within a series of analogues were used to analyze the factors affecting the reactivity and exo selectivity.     

Diels-Alder reactions of nickel(II) N-confused porphyrins as dienophiles

Diels-Alder reactions of nickel(II) N-confused tetraarylporphyrins as dienophiles with o-benzoquinodimethane yield nickel(II) N-confused isoquinoporphyrins.     

Diels-Alder approach to tetra-ortho-substituted biaryls employing propargylic tertiary alcohols as dienophiles

The efficient synthesis of a series of tetra-ortho-substituted biaryls is described utilizing a Diels-Alder reaction between propargylic tertiary alcohols and cyclic oxygenated dienes. The successful resolution of one of the biaryls is achieved through derivatization with menthyl chloroformate followed by crystallization. The menthyl carbamate is cleaved under basic conditions to reveal enantiomerically pure biaryl compounds.     

Cyclic vinyl p-tolyl sulfilimines as chiral dienophiles: Diels-Alder reactions with furan and acyclic dienes

The dienophilic behavior of the sulfilimine 2, synthesized from (Z)-3-p-tolylsulfinylacrylonitrile 1, in its Diels-Alder reactions with furan and acyclic dienes has been investigated. A complete pi-facial selectivity for 2, opposite to that observed from its precursor 1, is the main feature of all these cycloadditions. Moreover, the high exo selectivity observed in reactions of 2 with furan (not observed for 1) contrasts with the almost complete endo selectivity with other cyclic and acyclic dienes. Additionally, the opposite regioselectivities obtained for 2 with Dane's diene and 1-substituted butadienes (not observed for 1) are also noteworthy. This behavior allows dienophiles 1 and 2 to be considered as complementary precursors from a synthetic point of view.     

Nitrosonium (NO) initiated and cation radical-mediated imino Diels-Alder reaction

An efficient aza-Diels-Alder reaction of N-arylimines with N-vinylpyrrolidinone was achieved using nitrosonium tetrafluoroborate as a cation radical initiator, giving cis-4-(2-oxopyrrolidin-1-yl)tetrahydroquinolines in various yields.     

Lewis acid-promoted hetero Diels-Alder cycloaddition of alpha-acetoxynitroso dienophiles

[reaction: see text] alpha-Acetoxynitroso compound 3 has been prepared as a new stable, isolable, and reactive dienophile in nitroso Diels-Alder reactions. The yield of the [4 + 2] cycloaddition of alpha-acetoxynitroso dienophile with 1,3-dienes could be enhanced in the presence of 20 mol % Lewis acid. An unexpected retro hetero-Michael reaction from 26 was observed, leading to the cleavage of the N-O bond of the cycloadduct. This tandem nitroso Diels-Alder/retro hetero-Michael sequence has been used with cyclic and acyclic 1,3-dienes.     

Captodative α,β-unsaturated oxazolines as dienophiles in the asymmetric Diels-Alder reaction

Captodative oxazoline derivative 5a gave rise to four-cycloadducts 6, 7, 8 and 9 in the presence of trifluoroacetic anhydride and cyclopentadiene. Hydrolysis of 6 led to α-phenylthio carboxylic acid 14.