Acid-catalyzed transannular cyclization of 3aH-cyclopentene[8]annulene-1,4-(5H,9aH)-diones and some proposed mechanisms

Bicyclic 3aH-cyclopentene8]annulene-1,4-(5H,9aH)-diones underwent three types of acid-induced transannular reactions, Michael cyclization, 3 + 2] cycloaddition, and Friedel-Crafts ipso-alkylation, depending on the cyclopentenone ring substituent (Me or Ph) and the position of 8]annulenone substituent as well as the nature of acids (BF3, MeSO3H, CF3SO3H). The Me-substituent permitted the Michael reaction for all acids used to give tricyclic diones by the activation of cyclopentenone carbonyl group. However, the Ph-substituent inhibited the Michael reaction for BF3 and MeSO3H but allowed the 3 + 2] cycloaddition and Friedel-Crafts reaction for CF3SO3H depending on the position of annulenone substituent. These CF3SO3H reactions exhibited the following novel rearrangements, affording 2-naphthalenone and 7-acenaphthylene derivatives, respectively. The factors that control the reaction mode of these transannular cyclizations were discussed in view of the constraint twist-boat conformation of 8]annulenone ring as well as the ring substituent effects on the intramolecular cyclization. In addition, these 8]annulenone rings were found to easily undergo the intramolecular 2 + 2] photocyclization to provide the tetracyclic cage compounds which exhibited the facile cycloreversion under the influence of acid.