The intermediate A and the major product B formed in the following reaction are

Concept:-

The goes via cis hydrogenation followed by the electrocyclic reaction.

Lindlar catalyst:
5%Pd-CaCO3 + Pb(OCOCH3)2 + 
Function:

Cis hydrogenation of alkyne to form an alkene

Example:

Electrocyclic reaction:-

It involves ring opening and ring-closing by the formation or breaking of new \(\sigma \) bond from \(\pi\) bond through the concerted mechanism.

Points to remember:

• Be it thermal/photochemical both are allowed reactions.
• Under the thermal condition

[ 4n= conrotation, 4n+2= disrotation ]

• Under the photochemical condition

[4n = disrotation, 4n + 2 = conrotation]

Example:
 
Explanation:-

Here is the mechanism of the reaction;

• The first step is cis-hydrogenation of alkyne followed by electrocyclic ring-closing.
  

Conclusion:-

Therefore the intermediate A and the major product B formed in the following reaction are