Diastereoselective [2+2] photocycloaddition of chiral cyclohexenonecarboxylates to ethylene

The diastereoselective [2+2] photocycloaddition of cyclohexenonecarboxylates containing various chiral auxiliaries to ethylene is described. The effect of the auxiliary, reaction temperature, and solvent on diastereoselectivity was examined. The (−)‐8‐( p ‐methoxyphenyl)menthyl group was found to be the most effective chiral auxiliary. The photoreaction of (−)‐8‐( p ‐methoxyphenyl)menthyl cyclohexenonecarboxylate in methylcyclohexane at −78°C gave the corresponding bicyclo[4.2.0]octanone derivative in 81% diastereomeric excess (d.e.). The extent of diastereoselectivity was found to be closely related to the most stable π‐stack conformation of the starting cyclohexenones. Chirality 15:504–509, 2003.