The srtructure of A in the following reaction is

Concept:

• The [2+2] cycloaddition reaction involves the addition reaction of two diene molecules to give a cyclic compound. The [2+2] cycloaddition reaction between two alkenes is a photochemically allowed process.
• An example is shown below:

• The [2+2] cycloaddition reaction between an alkyne and an alkene is photochemically allowed but thermally forbidden because of symmetry mismatch between the overlapping orbitals.
• The intramolecular aldol reaction is the condensation reaction of two aldehyde groups or ketone groups in the same molecule in the presence of a base.
• An example is shown below:

Explanation:

• The reaction pathway is shown below:

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• From the above reaction, we see that the first step involves the [2+2] photochemical cycloaddition reaction between an alkyne and an alkene to give a cyclic compound followed by an ozonolysis reaction.
• In the last step of the reaction, it undergoes an intramolecular aldol reaction to give the desired product.

Conclusion:

Hence, the structure of A in the following reaction is

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