Photochemistry of Cyclic Ketones in Solution

ion from the solvent by the excited cyclocamphanone to give (XXV), cleavage of the cyclopropane ring to give (XXXVI), and hydrogen atom transfer and ketonization. This postulated route for the isomerization of cyclocamphanone to (XXXIV) is based on the proposal by Srinivasan for the route of isomerization of camphor to the ketone (XIV)'°. The photolysis of (XXXIV) to give (XVI) finds close analogy in work on the photolysis of dehydronorcamphor (XXXVII a) which has shown that cleavage occurs to give cyclopentadiene and ketene'5. A similar reaction has also been observed in the case of dehydrocamphor with a 5-aryl substituent (XXXVII b) which gives a 3-aryl-1 ,5,5-trimethylcyclopentadiene (XXXVIII b)'6. Formation of cyclic acetals by irradiation of other cyclic ketones Since our original preliminary report on the novel photochemical ring expansion of cyclic ketones to cyclic acetals'7 many other examples have come to light. Hostettler18 found that irradiation of the bicyclic ketone (XXXIX) gave two major products. One of these is the cyclic acetal (XL), which is the analogue of the acetals obtained from cyclocamphanone and nortricyclanone. The other is the ester (XLI), which is considered to be formed from the corresponding ketene, itself formed by photocleavage of the four-membered ring in a well-known fashion that was first observed in the vapour phase photolysis of cyclobutanone itself19. Hostettler20 has also observed cyclic acetal formation in the photochemical reactions of a series of 3-substituted-2,2,4,4-tetramethylcyclobutanones (XLII). Irradiation in alcohols gave a mixture of the corresponding ring-expanded cyclic acetal