case of cyano-substituted mesitaldehydes has permitted directobservation, for the first time, of the E-enols en route to benzocyclobutenols. The AM1 calculations suggest that the cyano-substitution causes intrinsic stabilization of the E-enols relative to the corresponding cyclobutenols, while the bromo groups do the opposite. The lack of observation of the red color in bromo- and formyl-substituted aldehydes
less bulky alkyl side chain and fragmentation leading to a carbonyl—alkyl bond fission with a more bulky one. A correlation is found between the photoreactions and the massspectrometric decomposition of the ketones. These photoreactions have been interpreted in terms of stereoelectronic requirements for the hydrogen abstraction from the n,π* triplet state. Unusual photoreduction of 2,4,6-trimethylbenzophenone