Para-Benzoquinone, Diels-Alder reaction

In total syntheses where a homogeneously catalyzed transfer hydrogenation is applied, almost exclusively aluminum(III) isopropoxide is utilized as the catalyst. At an early stage in the total synthesis of (-)-reserpine (65) by Woodward [106], an intermediate with two ketone groups and two C-C double bonds is formed (66) by a Diels-Alder reaction of para-benzoquinone (67) and vinylacry-late (68). The two ketone groups were reduced with aluminum(III) isopropoxide... 

Olah et al.902 demonstrated in 1979 that Nafion-H is able to catalyze Diels-Alder reaction of anthracene with a number of dienophile (maleic anhydride [Eq. (5.329)], para-benzoquinone, dimethyl maleate, dimethyl fumarate) in chloroform or benzene... 

A different synthetic approach to benzoannelated [2.2]paracyclophanes was realized by the Diels-Alder reaction of bis (methylene) [2.2] paracyclophane (13) and tetrakismethylene[2.2]paracyclophane (15) with para-benzoquinone [20]. The bis(exomethylene)paracyclophane derivatives were obtained from 6 and 7 via a copper-mediated coupling with methyl magnesium bromide, bromination and debromination. The dienes 13 and 15 react readily to the bis- and tris-phanes 16 and 17 when heated with para-benzoquinone in dichlorobenzene. However, both compounds are extremely poorly soluble in organic solvents and... 

Scheme 5. Synthesis of benzoanellated [2.2]paracyclophanes via Diels-Alder reactions with para-benzoquinone [20]...

The Diels-Alder Reaction essentially consists in the direct combination of a compound possessing a conjugated diene system with a reagent that contains either a double-bond or a triple-bond, usually activated by conjugation with additional multiply-bonded systems, such as cyano, carbonyl, nitro, phenyl functions. It ultimately adds on to the 1, 4-positions of a conjugated diene system e.g., buta-1, 3-diene) with the formation of a 6-membered ring. Importantly, the ethylenic (double-bond) or acetylenic (tripple-bond) compormd is normally termed as the dienophile, the second reactant as the diene and the final desired product as the adduct. A few typical examples of such reagents are, namely maleic anhydride, para-benzoquinone, acetaldehyde and acetylene dicarboxylic esters. 

Cycloaddition (Diels-Alder) reactions have been reported for [6]radia-lene (5) and its hexaalkyl derivatives 113 and 115, but not for the permethylated radialene 72, which was inert even to the reactive dienophiles TCNE and Af-phenyltriazolinedione [67]. The sterically least hindered radialene 5 reacted with acetylenic and olefinic dienophiles in a 1 3 ratio to give triphenylene derivatives such as 139 in low yield (Scheme 4.30) [5, 95]. On the other hand, radi-alenes 113 and 115 gave linear,/)-quinodimethane-type 1 2-adducts, when they were exposed to an excess of various common dienophiles inter alia maleic anhydride, tetracyanoethylene, />-benzoquinone, acrolein, ethyl acrylate, acetylenedi-carboxylic acid) [89, 96, 97]. The 1 1 adduct 140, which was isolated so far only from the reaction with an equimolar amount of TCNE (92% yield) [97], presumably prefers the second cycloaddition step in the linear (para) position (141) over that in the angular (meta) position (142) for steric reasons. 

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