Reactions of cyclo-octatetraene and its derivatives—IX: Cyclopentadienone adducts of tricyclok[4.2.2.02,5]deca-3,7-dienes and tricyclo[4.2.1.02,5]nona-3,7-dienes1

Abstract Tetracyclol[8.2.2.0 2,9 .0 3,8 ]tetradeca-4,6,11-trienes 5 are produced from tricyclo[4.2.2.0 2,5 ]deca-3,7-dienes 1 by reaction with cyclopentadienones 2 (or equiv) followed by thermal decarbonylation. With tetraphenylcyclopentadienone, however, the product 5e rearranges under the conditions required for its formation to give the dihydrosemibullvalene 11 . The rearrangement evidently proceeds via the cyclo-octa-1,3,5-triene 14 , the ring-opening 5e→14 being facilitated by a phenyl-conjugation effect, uncovered in another series of experiments involving the reaction of cyclopentadienones with the tricyclo[4.2.1.0 2,5 ]nona-3,7-diene 16 . Thus the tetraphenylcyclohexa-1,3-diene 18c undergoes ring-opening in refluxing xylene to afford the eyclo-octa-1,3,5-triene 21a , whereas the dimethyldiphenyl-analogue 18b resists valence isomerisation under these conditions.