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

AbstractThe 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 (−)‐(1S,2S)‐esters 17–21 with carbonyl groups in endo‐position exhibit typical excitonsplit Cotton effects whereas the corresponding (−)‐(1S,2R)‐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.