Irradiation of 2,4-cyclohexadienones (L) usually produces ketenes. Most commonly, these ketenes either thermally recyclize to dienone or, in the presence of nucleophiles, react to form open-chain unsaturated acids or their derivatives.lm4 In special cases, however, the ketene may thermally cyclize to a bicyclo[3.l.O]hexenone (:).3*5*6 Cyclization to z is favored by polar solvents3 and even occurs in methanol, though in the presence of stronger nucleophiles, such as amines, open-chain products are formed.
The structural features which determine whether a particular ketene will cyclize to dienone or bicyclic ketone, or will react with a nucleophile, are still not well delineated, so that predictions are difficult, especially for highly substituted ketenes. Those ketenes which do give bicyclic products are usually highly substituted.5-7
In the present paper, we describe the synthesis and irradiation of dienone t, and compare the results with those previously obtained5a for the closely related la.
allylic methyls. Careful examination of the uv and nmr spectra of all four isomers of 2 allowed each structure to be uniquely assigned.
All new compounds gave elemental analyses in accord with the assigned structures.