Benzoquinones and related compounds. Part 5. Nuclear magnetic resonance study of the conformation in solution of some Diels–Alder adducts between 1,4-benzoquinones and cyclopentadiene

Abstract

The Diels–Alder reaction of cyclopentadiene with the 5,6-double bond of benzyl-, α-hydroxybenzyl-, and α-acetoxybenzyl-1,4-benzoquinone, and of 1-hydroxy-1-phenyl-, 1-phenyl-, and 1-hydroxy-indan-4,7-quinone gives in each case a pair of diastereoisomeric endo adducts. For those which carry a phenyl group, the chemical shift of one of the norbornene olefinic protons in one member of each pair is significantly different from that of the corresponding proton in the other member. The conformational significance of this effect is discussed. Proton longitudinal relaxation and nuclear Overhauser relaxation techniques are used to show that the cyclopentadiene endo monoadducts of 1,4-benzoquinone and α-hydroxybenzyl-1,4-benzoquinone prefer to adopt an ‘open’ rather than a ‘closed’ conformation.