In the given transformation, the intermediate [A] and the major product B are

Concept:

​Carbene:

• Carbenes are neutral substances with six valence shell electrons on the carbon atom.
• Carbenes are divalent compounds with two valence electrons distributed between two nonbonding orbitals and the carbon atom is subsequently bonded to two same or different atoms.

Cycloaddition reaction:

• The cycloaddition reaction indicates the addition of the two unsaturated compounds to form a cyclic compound with the formation of a sigma (\(\sigma \)) bond at the end of the \(\pi\) component.
• An example of [2+2] cycloaddition reaction is shown below:

Explanation:

• The reaction pathway is shown below:

• From the above reaction, we can see that the first step involves the formation of a carbene intermediate upon heating Ethyl 2-diazoacetate (N₂CHCO₂Et) with Cu salt.
• The generated carbene undergoes nucleophilic attack by the lone pair of electrons of the oxygen atom of benzaldehyde (PhCHO) and forms a dipolar ion A.
• The dipolar ion undergoes [4+2]\(\pi \) cycloaddition reaction to give the final product B.

Conclusion:

Hence, in the given transformation, the intermediate [A] and the major product B are