As ???acyl group??? represented by R1a, for example, there are an acyl group represented by the formula: ???(C???O)???R5a, ???(C???O)???OR5a, ???(C???O)???NR5aR6a, ???(C???S)???NHR5a or ???SO2???R7a (wherein R5a represents a hydrogen atom, a hydrocarbon group optionally having substituents or a heterocyclic group optionally having substituents, R6a represents a hydrogen atom or a C1-6 alkyl, R7a r