| http://www.w3.org/ns/prov#value | - In the general formula (1) described above, preferable examples of R4 include a hydrogen atom, an alkyl group having 1 to 4 carbon atoms or an aryl group having 6 to 10 carbon atoms and more preferably a hydrogen atom, a methyl group or a phenyl group, of which an particularly preferable example is a methyl group.
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