The Circular Dichroism of 5,6‐Dimethylidene‐2‐bicyclo[2.2.n]alky Esters. Chiral Exciton Coupling between Benzoate and Exocyclic s‐cis ‐Butadiene Chromophores

The optically pure aryl‐substituted 5,6‐dimethylidene‐2‐bicyclo[2.2.1]heptyl benzoates 12–21 were prepared; their UV absorption and CD spectra are reported. The (−)‐(1 S ,2 S )‐esters 17–21 with carbonyl groups in endo ‐position exhibit typical excitonsplit Cotton effects whereas the corresponding (−)‐(1 S ,2 R )‐esters 12 ‐ 16 with carbonyl groups in exo ‐position do not present such effects. The chiral exciton coupling between the exocyclic diene and a remote p ‐substituted benzoate chromophore can be used for unambiguous assignment of the absolute configuration of 5,6‐dimethylidene‐2‐ endo ‐bicyclo[2.2.1]heptyl derivatives. The method is applied to establish the absolute configuration of 5,6‐dimethylidene‐2‐ exo and ‐2‐ endo ‐bicyclo[2.2.2.]octyl p ‐bromobenzoates (−)‐ 24 and (−)‐ 25 .