Photochemistry of 3-Oxacycloalkenes

Direct and sensitized photolyses of 3-oxacycloalkenes 1a-c has been studied in n-pentane and in alcohols. In contrast to the photochemistry of cycloalkenes, the direct photolyses of 1a and 1b in pentane gave the products 2, 3, and 4, 5, respectively, which are derived from an allylic O-C bond cleavage. In addition, in the case of lb, a small amount of the carbene-derived product 6 was also obtained. The seven-membered oxacycloalkene 1c gave no volatile product upon direct irradiation in pentane. Upon direct or sensitized photolyses in methanol, oxacycloalkenes lb or lc gave the adducts 7b,c as major products. Irrespective of the method of excitation, the yield of the adduct in the photolyses in a series of alcohols decreased with decreasing pXa value of the alcohol which can be explained in terms of the formation and subsequent trapping of the intermediate trans isomer 14 by the alcohol. These results can be rationalized in terms of the reactivities of the, * and, * states and the increased stability of the Rydberg state. © 1981, American Chemical Society. All rights reserved.