Diastereodifferentiating the [2+2] photocycloaddition of ethylene to arylmenthyl cyclohexenonecarboxylates: stacking-driven enhancement of the product diastereoselectivity that is correlated with the reactant ellipticity.

Upon diastereodifferentiating the [2+2] photocycloaddition of ethylene to a series of p-substituted (-)-8-phenylmenthyl cyclohexenonecarboxylates, the diastereoselectivity was critically controlled by the nature of the substituent introduced to the chiral auxiliary, and the p-nitro-substituted substrate afforded the cycloadducts in 90% diastereomeric excess (de) and with 97% isolated yield. Detailed experimental and theoretical conformation analyses revealed that the stacking interaction of the aromatic auxiliary with the cyclohexenone moiety plays the decisive role in determining the substrate conformation and is, therefore, responsible for the dramatic enhancement of the de. Of particular interest, the product de was directly related to the ellipticity of the substrate, enabling us to "predict" the de prior to photoirradiation.

Asymmetric [2+2] photocycloaddition of cycloalkenone-cyclodextrin complexes to ethylene.

The enantioselective [2+2] photocycloaddition of cycloalkenone-cyclodextrin complexes to ethylene was examined. Photoreaction of alpha-, beta-, and gamma-cyclodextrin complexes of various cyclohexenonecarboxylates and cyclopentenonecarboxylates in the presence of ethylene in the solid state or in aqueous suspension gave corresponding photocycloadducts with chiral induction.

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.