75  US 9,989,847 B2  76  The acid labile group  Rb  in formula ( e2) is not particularly  limited as long as it is deprotected under the action of acid  to generate a hydroxyl group. Suitable acid labile groups  include acetal structure groups, ketal structure groups, and 65  alkoxycarbonyl groups, examples of which are shown  below.  Of the acid labile groups represented by  Rb  in formula  (e2), alkoxymethyl groups having the general formula (e3)  are most preferred.  Herein, R 1$_{a }$ and  Rb  are as defined above. 