Photochemistry of Tricyclo[3.3.1.0^2,7]nonan‐6‐ones. Preliminary communication

Abstract

Summary. Sensitized irradiation (A = 254 nm) of the title compounds 2a and 2b in benzene affords the tetracyclic oxetanes 3a and 3b. The irradiation of 2c under similar conditions yields equal amounts of 3c and the cyclopentadiene derivative 5. The formation of the photoproducts is discussed.The photochemical behaviour of bicyclic ketones has attracted the interest of several research groups [14]. We have recently reported [7] the intramolecular photocyclization of the 6-allyl-2-cyclohexenones 1 a and 1 b (Scheme 1). We now discuss results on the photochemistry of the tricyclo[3.3.1.02~7]nonan-6-ones 2. to the formation of 3a and 3b. These were shown to be 2-oxatetracyclo[4.2.2.0~~9.0~~~] decanes on the basis of the spectroscopic data summarized in the Table. Their formation from 2 can readily be explained by preferential u-cleavage on the side of the carbonyl group remote from the four-membered ring [5] and abstraction of the more easily accessible hydrogen atom on C(4) to yield the intermediate aldehyde 4 which then photocyclizes to 3 (Scheme 2). Such oxetane formations have also been observed in direct irradiations of bicyclic ketones [l] [3].1) Minor amounts of aldehydic products were also detected and isolated by GLC. It remains unknown for the moment if thesc correspond to thermal cleavage products of 3 or to the unreacted precursors 4.