 tert-Butyl ester

A tert-Butyl ester contains the group $\mathrm{R}-\mathrm{C}(=\mathrm{O})-\mathrm{OC}(\mathrm{C}{\mathrm{H}}_3{)}_3$. Its bulky alcohol-derived group differs from the smaller group in a methyl ester.

A tert-Butyl ester can protect a carboxylic acid during a synthesis and then be cleaved under suitable acidic conditions. This is an example of selecting a protecting group by the conditions required for both its survival and removal.