What is formed when carbon monoxide reacts with $Cl_2$ in the presence of sunlight?

$A \xrightarrow{} (CH_3)_2C=CH-CO-CH_3$. $A$ is:

$p$-cresol reacts with chloroform in an alkaline medium to give compound $A$,which adds hydrogen cyanide to form compound $B$. The latter,on acidic hydrolysis,gives a chiral carboxylic acid. The structure of the carboxylic acid is:

Reagents to carry out the above conversion,$P$,$Q$,$R$ respectively are:

Explain the cross aldol condensation reaction.

The major product of the following reaction sequence is