Synthesis of Curved Polycyclic Arenes Embedded with Oxepine and Thiepine Using Ring Expansion Strategy.

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 FeCl ₃ -mediated tandem ring expansion process.