79  US 9,221,742 B2  80  Herein, R $^{1 }$and  Rb  are as defined above.  The acid labile group  Rb  in formula  (1  b) is not particularly  limited as long as it is deprotected under the action of acid to  generate a hydroxyl group. Suitable acid labile groups  RI  RI  RI  I  I  I  --+CH2-Ct- OAO  --+CH2-Ct- OAO  --+ CH2 -C-,---- ~  (!  ~  ORb  ORb  ORb  ORb  ORb  ORb  RI  RI  I  I  --+CH2 -Ct- ~x  OAO  ~  ORb  ORb  ORb  ORb  RI  RI  I  I  --+CH2 -C-,---- OAO  --+ CH2-C  t- OAO  ~  ORb  ORb  ORb  -continued 