Synthesis and chemistry of endoperoxides derived from 3,4-dihydroazulen-1(2H)-one: An entry to cyclopentane-anellated tropone derivatives

Reduction of trienone 1 and subsequent treatment with acid in MeOH furnishe d 1-methoxy-1,2,3,4-tetrahydroazulene (13). Photo-oxygenation of 13 provide d the two bicyclic endoperoxides 14 and 15. Pyrolysis of 14 and 15 gave the corresponding bis-epoxides 17 and 18, which have been synthesized also upo n treatment with a catalytic amount of CoTPP (TPP = tetraphenylporphyrin). That an unusual endoperoxide-endoperoxide rearrangement has not been observ ed strongly supports the assumption that the carbonyl group in 2-4 is respo nsible for this unprecedented endoperoxide-endoperoxide rearrangement. Trea tment of the endoperoxides 14 and 15 with a catalytic amount of Et3N at 0 d egrees provided the azulenones 22 and 23 in high yield. Attempted cleavage of the O-O peroxide linkage in 14 and 15 with thiourea resulted, contrary t o our expectation, in the formation of 22 and 23. That thiourea acts as a b ase instead of a reducing reagent has been observed for the first time in p eroxide chemistry.