A-Level Carboxylic acids, acyl chlorides and esters: worked solution

Question

During the manufacture of zanamivir, step a (H+ with MeOH) turns a carboxylic acid group into the corresponding methyl ester (-CO2H becomes -CO2Me), and step b (AcCl) puts acetyl groups onto the hydroxyl functions; only afterwards are the chlorination, elimination and remaining transformations performed. Explain why steps a and b have to be done before those later stages.

Steps a and b are protecting-group steps. The -CO2H (carboxylic acid) and -OH (hydroxyl) groups are reactive and would interfere with, or be attacked during, the later chlorination and elimination steps, giving unwanted side reactions and low yield. Converting -CO2H to the methyl ester (step a) and acetylating the -OH groups (step b) masks/protects these functional groups so that the subsequent reagents react only at the intended positions. The protecting groups can be removed afterwards to regenerate the original -CO2H and -OH.