Optically Active (E,Z)-1,3-Cyclooctadiene: First Enantioselective Synthesis through Asymmetric Photosensitization and Chiroptical Property

Enantiodifferentiating photoisomerizations of ( Z,Z )-1,3-cyclooctadiene ( 1ZZ ) to the chiral E,Z -isomer 1EZ were performed at varying temperatures in the presence of some benzenepoly-, naphthalene(di)-, and anthracenecarboxylates. Of these chiral sensitizers, (−)-menthyl benzenehexacarboxylate afforded the highest enantiomeric excesses (ee's) up to 10% and 18% in pentane at 25 and −40 °C, respectively. In contrast, the use of polar solvents greatly diminished the product's ee, suggesting intervention of a radical ionic rather than exciplex intermediate in these solvents. Optically pure 1EZ is shown to possess anomalously high specific rotation and circular dichroism as a simple diene chromophore: [α] 25 D 1380° (CH 2 Cl 2 ), Δε 12.8 M - 1 cm - 1 (λ ext 228 nm, cyclohexane). The chiroptical properties observed not only correct the previous data but also present the conclusive evidence for the recent theoretical predictions.