The 1:1 and 2:1 adducts of cyclopentadiene with p-benzoquinone

Abstract

The exo-cis isomer (8) of the well-known endo-cis 1:1 adduct (4) of cyclopentadiene and p-benzoquinone has been obtained by heating the endo-cis-anti-cis-endo 2:1 adduct (13) with p-benzoquinone and also by heating 4 alone. The exo-cis isomer of the 1:1 adduct (11) of cyclopentadiene and p-toluquinone has also been obtained for the first time. Heating of the 2:1 adduct 13 gives a mixture containing the endo-anti-exo isomer 15, the exo-anti-exo isomer 16, and 13. The structures of these two new isomers are assigned on the basis of (i) spectroscopic comparison with 13 and the known endo-syn-endo and endo-syn-exo isomers, 12 and 14; (ii) spectroscopic comparison of the corresponding tetrahydro compounds; and (iii) the conversion of tetrahydro 15 by oxidation and acetylation to the sny-hydroquinone diacetate 28, which is stereoisomeric with the anti-hydroquinone diacetate 25, prepared by similar treatment of tetrahydro 13, 14, and 16. It is proposed that interconversion of the 1:1 adducts occurs via dissociation–recombination, interconversion of the tetrahydro 2:1 adducts occurs via enolization, while interconversion of the 2:1 adducts themselves can occur by both routes. Keywords: Diels–Alder, benzoquinone, cyclopentadiene, stereoisomerism, cycloaddition.