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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. 

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-continued 

