201  US 9,291,896 B2  202  -continued  In formula ( d-1 ), each R’ independently represents a hydro› gen atom, a linear or branched alkyl group, a cycloalkyl  group, an aryl group, an aralkyl group or an alkoxyalkyl  group. R’ may combine with each other to form a ring.  R’ is preferably a linear or branched alkyl group, a  cycloalkyl group or an aryl group, more preferably a linear or  branched alkyl group or a cycloalkyl group.  Specific structures of the protective group are illustrated  below.  (d-1) 