Catalytic asymmetric aza Diels-Alder reactions of hydrazones using a chiral zirconium catalyst

Catalytic asymmetric aza Diels-Alder reactions of acylhydrazones with Danishefsky’s dienes have been developed. A chiral zirconium complex derived from zirconium propoxide and 3,3′,6,6′-I₄BINOL was found to be effective in this reaction, and the desired optically active 2,3-dihydro-4-pyridone derivatives were obtained with high enantioselectivities. Asymmetric formal synthesis of a natural product, coniine, was conducted using this catalytic asymmetric reaction as a key step.     

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.     

Synthesis and Diels-Alder reactions of 9-(4-benzyloxazolin-2-yl) anthracene

The synthesis of 9-(4-benzyloxazolin-2-yl)anthracene is described employing a new approach for the cyclisation of β-hydroxy amides to oxazolines. Thermal Diels-Alder reactions with N-methyl maleimide were found to be considerably slower than those previously observed. Essentially no diastereoselectivity was observed in these reactions as the benzyl stereodirecting group is remote from the reactive site. Minor rate enhancements were noticeable in the presence of some added Lewis acids, but with no diastereoselection.     

Cinchona-derived ammonium salts-catalyzed aza Diels-Alder reaction of Danishefsky’s diene with imines

Described is the efficiency of cinchona-derived quaternary ammonium salts as Lewis acid organocatalysts in aza Diels-Alder reaction of Danishefsky’s diene 1 with imines 2 and 16, providing 1,2-dialkyl-2,3-dihydro-4-pyridinones 3 and cyclic dihydropyridones 17, respectively. Among the nine of cinchonidine-derived quaternary ammonium catalysts prepared, N-2′,3′,4′-trifluorobenzyl-O-benzylcinchonidinum bromide (6) exhibited the highest chemical yield (up to 99%). Systematic study on structure-catalytic efficiency relationship revealed that 2′,3′,4′-trifluorobenzyl, quinuclidine, and quinoline moieties are essential.     

Diastereo- and regioselective Diels-Alder reactions of 2-phosphaindolizines

1,3-Bis(alkoxycarbonyl)-2-phosphaindolizines undergo Diels-Alder reactions at the CP- functionality with 2,3-dimethylbutadiene and with isoprene in the presence of sulfur with complete diastereoselectivity. The reaction with isoprene occurs with 100% regioselectivity as well. 3-Ethoxycarbonyl-1-methyl-2-phosphaindolizine, however, fails to undergo Diels-Alder reaction under these conditions. Difference in the dienophilic reactivities of mono- and bis(alkoxycarbonyl) substituted 2-phosphaindolizines and the observed regioselectivity in the Diels-Alder reaction has been rationalized on the basis of DFT calculations. The relative stabilities of the transition structures have been explained on the basis of the NBO analysis.     

对映选择催化 Diels-Alder 反应

本文综述了对映选择催化 Diels-Alder 反应的进展。     

Diels-Alder reactivity of 4-aryl-1-phthalimido-2-siloxy-1,3-butadienes

The reaction of (1Z,3E) and (1E,3E) 4-aryl-1-phthalimido-2-trialkylsiloxy-1,3-butadienes with maleimides and quinones has been studied. The observed exo-stereospecifity can be attributed to the simultaneous presence of the phthalimido and aryl groups, which produce strong hindrance during the endo approach.     

Studies into the Diels-Alder reactions of 5-trimethylsilylthebaine

The introduction of a 5-trimethylsilyl group on the least hindered face of the diene thebaine was anticipated to favor attack by dienophiles from the alternate face, but only gave rise to a rearrangement product when treated with 3-butene-2-one at 110 °C. Reaction with the more reactive benzoquinone at lower temperature gave rise to a very slow reaction from the same face as the silyl group, indicating that a trimethylsilyl group does not sufficiently hinder this face to achieve reaction at the other face.     

Polymer-supported electron-rich diene for hetero Diels-Alder reactions

A Brassard diene analogue was grafted on a modified Merrifield resin and used in a hetero Diels-Alder reaction with various aldehydes or ketones. The reaction gave access to 6-substituted 5,6-dihydropyrones (5a-u). This new immobilized electron-rich diene is more stable than the corresponding native diene, and is a versatile supported reagent.     

Rhodium catalyzed tandem Diels-Alder/hydrolysis reactions of 2-boron-substituted 1,3-dienes

2-Boron substituted 1,3-dienes have been prepared and used in preliminary experiments to demonstrate that it is possible to effect rhodium catalyzed Diels-Alder/hydrolysis tandem reactions.     

Microwave-assisted reactions of nitroheterocycles with dienes. Diels-Alder and tandem hetero Diels-Alder/[3,3] sigmatropic shift

Diels-Alder cycloaddition of 3-nitro-1-(p-toluenesulfonyl)indole 1 with dienes 2-6 under microwave irradiation in solvent-free conditions gave carbazole derivatives after elimination of the nitro group and in situ aromatization. While 3-nitro-1-(p-toluenesulfonyl)pyrrole 11 afforded indole derivatives, 4-nitro-1-(p-toluenesulfonyl)pyrazole 13 with cyclohexadiene 2 did not afford the expected cycloadduct but instead gave 1-cyclohexen-2-ylpyrazole 16. This process occurred by hydrolysis of the 1-(p-toluenesulfonyl) group, protonation of the diene and nucleophilic addition of the pyrazolate ion, as elucidated by computational studies. Reaction of nitroindole 1 with cyclohexadiene 2 afforded exclusively the endo stereoisomer (10endo) in a tandem hetero Diels-Alder/[3,3] sigmatropic shift, as determined by computational calculations.     

Regioselective Diels-Alder reactions of 3-indolylquinones

6-(3-Indolyl)quinolinequinone derivatives gave regioselective Diels-Alder reactions with a variety of dienophiles, yielding polycyclic carbazole derivatives. One-pot reactions, proceeding through a cascade of reactions including regioselective Michael and Diels-Alder steps, gave heptacyclic derivatives starting from indoles and 2,5,8(1H)-quinolinetriones. Double Diels-Alder reactions of 6-(3-indolyl)quinolinequinones and dihalobenzoquinones gave eleven-cycle products in one step.     

Remarkable effects of titanium tetrachloride in diastereoselective aza Diels-Alder cycloaddition: synthesis of (S)-piperazic acid

Titanium tetrachloride-mediated aza Diels-Alder cycloaddition using a chiral diene and an aza dienophile proceeds in highly diastereoselective manner to form a dehydropiperazic acid derivative in high yield, and diastereoselectivity of the reaction depends on the quantity of titanium tetrachloride.     

Behaviour of selenophenes substituted with electron-withdrawing groups in polar Diels-Alder reactions

The normal electron-demand Diels-Alder reactions between substituted selenophenes, nitro being one of these groups, and dienes of diverse reactivity give benzoselenophenes derivatives.     

Intramolecular hetero Diels-Alder reactions of vinyl allenes and aldehydes

Compounds containing a vinyl allene moiety linked to an aldehyde by a tether have been synthesised and shown to undergo the intramolecular hetero Diels-Alder reaction. Only one compound with stereochemistry consistent with an exo approach of the dienophile to the dienic portion of the vinyl allene was obtained in each case. Depending on the length of the tether and on the substitution pattern on the allene the reactions were spontaneous at room temperature or required heating or the presence of a Lewis acid to proceed.     

Sequential ene, Diels-Alder reactions of AF4-yne with 1,3,5-cycloheptatriene

(4,5-Dehydro)-1,1,2,2,9,9,10,10-octafluoro [2.2]paracyclophane (AF4-yne) undergoes an ene reaction with 1,3,5-cycloheptatriene, the adduct of which subsequently undergoes a further Diels-Alder reaction with a second equivalent of AF4-yne to give two stereoisomeric 2:1 adducts. A very small amount of the classic 1:1 Diels-Alder adduct also can be isolated from the reaction. Structure assignments of all products were determined by NMR through a series of H1-H1, H1-C13 one bond, and H1-C13 two and three bond correlation experiments as well as H1-H1 NOE experiments.     

Diels-Alder reactions in pyridinium based ionic liquids

Diels-Alder reactions have been investigated in pyridinium based ionic liquids. These solvents are found to be more effective compared to the organic solvent studied, in enhancing the reaction rate and product yields. Recycled ionic liquids also gave excellent results.     

Perfluoroacetylenephosphonates in Diels-Alder reactions: Synthesis of perfluoroalkylated heterocyclic and carbocyclic phosphonates

Diethyl 3,3,3-trifluoroprop-1-ynylphosphonate and diethyl 3,3,4,4,4-pentafluorobut-1-ynylphosphonate are obtained by the dehydration of the corresponding enols using P₂O₅-Et₃N system as a dehydrating agent, affording acetylenes in 50-60% yield. By the reaction of these perfluoroacetylenephosphonates with acyclic and cyclic 1,3-dienes or diene-like heteroaromatic and aromatic compounds corresponding Diels-Alder cyclo- and bicycloadducts were prepared in good yields (65-90%). The reactivity of the dienes and acetylenes which depends on their structure, as well as the regioselectivity of the reaction are established.     

Easy access to N-aryl, N-heteroarylbenzoxazolinones and 4-aza analogues via Diels-Alder cycloaddition reactions

An improved method for the synthesis of N-aryl, N-heteroarylbenzoxazolinones and their 4-aza analogues is described. The process involves the Diels-Alder cycloaddition of benzoxazinic or pyridoxazinic dienic systems with dienophiles as a key step.