Chemistry of the benzotropone endoperoxides and their conversion into tropolone derivatives: Unusual endoperoxide rearrangements

The chemistry of two bicyclic endoperoxides, obtained by photooxygenation of 2,3-benzotropone (= 5H-benzocyclohepten-5-one 5) and of its ethyl carboxylate derivative 15, was investigated with the aim of synthesizing the respective benzotropolone derivatives. The reaction of the endoperoxide 10 derived from 5 with thiourea gave the desired benzotropolone, i.e., 6-hvdroxy-5H-benzocyclohepten-5-one (11), in high yield (Scheme 1). On the other hand, the endoperoxide 16 derived from the ethyl carboxylate derivative 15 underwent an unprecedented transformation yielding mainly the ring-contracted lactones 28 and 29 besides the expected substituted benzotropolone derivative 27 (Scheme 5). However, the thermolysis reaction of the same endoperoxide 16 resulted in the formation of four rearranged compounds with different skeletons (Scheme 3). The formation mechanism of all products is discussed (Schemes 4 and 6).