In the reaction :
${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{OC}}_2{\mathrm{H}}_5+\mathrm{CO} \underset{150°\mathrm{C}}{\overset{{\mathrm{BF}}_3,500\mathrm{atm}}{\to }}\mathrm{X}$.
What is the IUPAC name of X?

The reaction is insertion, where CO gets inserted into the C-O Bond

${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{OC}}_2{\mathrm{H}}_5+\mathrm{CO}\underset{{150}^0\mathrm{C}}{\overset{{\mathrm{BF}}_3,{500}^0\mathrm{C}}{\to }}{\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{COOC}}_2{\mathrm{H}}_5$