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Herein, R 1$_{a }$ and Rb are as defined above. 

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. 

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Of the acid labile groups represented by Rb in formula (e2), alkoxymethyl groups having the general formula (e3) are most preferred. 

