The Diels-Alder adduct formed from p-benzoquinone and 2 mol of 1,3-cyclohexadiene has been shown by 13C NMR spectra of the adduct and its derivatives and by direct single-crystal X-ray crystallographic analysis to have the endo-anti-endo configuration E. Heating with alkali isomerizes the adduct to the exo-syn-endo isomer C.The adducts of 1,3-dienes with p-benzoquinone occupy a special place in the history of the Diels-Alder reaction, as it was during the reinvestigation of an earlier incorrect structure proposal1 for the cyclopentadiene-benzoquinone adduct that Diels and Alder realized the true nature of the reaction and first demonstrated its versatility.2 The stereospecificity, which has proven to be such a valuable characteristic of the reaction, is exemplified in the adduct (I) formed3 from p-benzoquinone and 2 mol of 1,3-cyclohexadiene: even when the possibilities are restricted to those structures containing only cis ring junctions there are still six possible stereoisomers (A-F) of I; however, only a single isomer is formed in the Diels-Alder reaction. Alder and Stein4 showed that a second isomer (II) is produced
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