Photochemical reactions. Part II. Cycloaddition reactions with photoenols from 2-methylbenzaldehyde and related systems

On photolysis, 2-methylbenzaldehyde can form the corresponding (E)-photoenol which can be trapped with dienophiles in a highly stereoselective manner. Maleic anhydride gives a single adduct (11), the reactions of which have been studied. From trapping experiments no evidence for the formation of (Z)-photoenol intermediates could be found. Ketones which cannot form (E)-photoenols for steric reasons do not form (Z)-photoenols. Both 2-benzylbenzaldehyde and 2-methylbenzoyl cyanide can undergo photoenolisation upon irradiation.