This paper is dedicated to Professor Ross Stewart on the occasion of his 65th birthday PETER YATES and KRZYSZTOF SWITLAK. Can. J. Chem. 68, 1894Chem. 68, (1990.The exo-cis isomer (8) of the well-known endo-cis 1: 1 adduct (4) of cyclopentadiene andp-benzoquinone has been obtained by heating the endo-cis-anti-cis-endo 2: 1 adduct (13) withp-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-e.ro isomer 15, the exo-anti-e.ro 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 syrz-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:l adducts occurs via dissociation-recombination, interconversion of the tetrahydro 2: 1 adducts occurs via enolization, while interconversion of the 2:l adducts themselves can occur by both routes.
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