The following transformation

is an example of

Concept:

• Cycloaddition reactions are combination of two or more unsaturated conjugated systems.
• The combination results in formation of another cyclic system with 2 lesser \(\pi\) bond but 2 more \(\sigma\) bond.
• Formation of new \(\sigma\) bonds is the driving force of the reaction.
• The number of participating \(\pi\) electrons is used to define the type of cycloaddition occurring between two conjugate systems. for example, cycloaddition between a diene and simplest alkene (dienophile) is called as [4\(\pi\) + 2\(\pi\)] cycloaddition reaction.

Explanation:

• Benzyne is very reactive species and  shares its

 \(\pi\) electron of non-linear triple bond.

• The given conjugated unsaturated system undergoes cycloaddition with benzyne as follows:

8\(\pi\) electrons of bicyclic conjugate system are involved in cyclization with 2\(\pi\) electrons of benzyne system.

Conclusion:

Hence, given transformation is an example of [8π + 2π] cycloaddition reaction.