Flash Vacuum Pyrolysis of o-Xylylene Dimers

Flash vacuum pyrolysis of [4+4] dimers of o-xylylenes gives anthracenes as the major products, and this remarkable transformation is highly regiospecific. Thus, the methyl-substituted derivative 1 gives 2 while 3 gives 4. 

Elucidate the overall synthesis and give a mechanism for the final step. 

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