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 anthracene-carboxylates. Of these chiral sensitizers, (-)-menthyl benzenehexacarboxylate afforded the highest enantiomeric excesses (ee's) up to 10% and 18% in pentane at 25 and -40 degrees 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: [alpha](25)(D) 1380 degrees (CH2Cl2), Delta epsilon 12.8 M(-1) cm(-1) (lambda(ext) 228 mm, cyclohexane). The chiroptical properties observed not only correct the previous data but also present the conclusive evidence for the recent theoretical predictions.