Leading Question

The product of this reaction is unstable and has to be quickly moved into the next reaction. Why?

Molecule 19 is unstable for a few reasons. First, it contains a reactive aldehyde group, whose electrophilic carbon center is prone to reactions with nucleophiles. Secondly, it has an acidic hydrogen atom in the carboxylic acid functional group with a pKa around 4.8, and this can be relatively easily deprotonated by some base. Finally, the small 4-membered ring in the center of the molecule is subjected to great ring strain, contributing to the instability of this molecule.