A tandem oxidative ring expansion of a six- to seven-membered ring was developed to construct a new class of negatively curved polycyclic arenes embedded with oxepine and thiepine, namely, dibenzo[b,f]phenanthro[9,10-d]oxepine (DBPO) and dibenzo[b,f]phenanthro[9,10-d]thiepine (DBPT), respectively. Mechanistic studies disclosed that an unexpected [4 + 2] cycloadduct formed between the alkene moiety of o-biphenyl-linked methylenexanthenes and o-chloranil serves as a radical cation or a dication equivalent that facilitates the FeCl3-mediated tandem ring expansion process.