Formation of a Dimeric Camphor Derivative with an Unusually Stable 3-exo Substituent

The reductive dimerization of E-3-benzylidenecamphor with sodium proceeds with homoacyloin regiochemistry to give a complex mixture of products from which two major products have been isolated by chromatography. The major product is formed by dimerization of the E radical anion through the re face of one radical anion to the si face of the other. This compound lacks a true inversion center of symmetry, but the atoms of the 1,2-diphenylethane moiety, as well as the camphor oxygen atoms and C(1)–C(4) and C(10) of the camphor groups, are related by a local inversion center of symmetry. One of the camphor moieties carries an exo substituent, but the compound does not readily equilibrate to the epimeric compound where both camphor moieties carry an endo substituent.