Novel Enhancement of Diastereoselectivity of [2 + 2] Photocycloaddition of Chiral Cyclohexenones to Ethylene by Adding Naphthalenes

The additive effect on the diastereoselective [2 + 2] photocycloaddition of chiral cyclohexenones 1
to ethylene is examined. A novel and fairly efficient method of increasing the diastereoselectivity
in the reaction of 1a was elucidated. The de value increased from 56% to 83% by the addition of
1-phenylnaphthalene. The major product 2a was isolated by the recrystallization of the diastereomeric mixture (major/minor = 11/1), of which X-ray analysis confirmed the absolute configuration
of the bicyclic system of 2a. Hydrolysis for removing the chiral auxiliary and subsequent
esterification afforded the optically pure bicyclo[4.2.0]octanone derivative 5. From the fluorescence
spectral analyses and other experimental results, the additive effect is attributed to the complex
formation of chiral cyclohexenone 1a and added naphthalenes.