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DETAILED DESCRIPTION OF THE INVENTION As previously indicated, R.
sup.
1 of general formula (I) represents a monocyclic or bicyclic aryl group.
The aryl group may be substituted with one or more substituents which are not electron attractive, such as an alkyl group (which may be straight chained or branched chained) having 1 to 20 carbon atoms, an aralkyl group in which the alkyl moiety has 1 to 3 carbon atoms and may be straight chain or branched chain, an alkoxy group having 1 to 20 carbon atoms and which may be straight chain or branched chain, an amino group mono- or di-substituted with an alkyl group having 1 to 20 carbon atoms and which may be straight chain or branched chain, an aliphatic acylamino group having 2 to 21 carbon atoms which may be straight chain or branched chain, and an aromatic acylamino group which contains a monocyclic aryl moiety.
R.
sup.
2 of general formula (I) can be a hydrogen atom, a phenyl group, or an alkyl group having 1 to 3 carbon atoms, which may be straight chained or branched chained.
The alkyl group is unsubstituted.
The phenyl group may be substituted and it is preferred for the substituent to be an electron attractive substituent such as, for example, a halogen atom (a chlorine atom, a bromine atom, etc.
), a cyano group, a trifluoromethyl group, a carboxyl group, a sulfo group, etc.
Specific examples of substituents represented by R.
sup.
1 in general formula (I) are a phenyl group, an .
alpha.
-naphthyl group, a .
beta.
-naphthyl group, a p-tolyl group, a m-tolyl group, an o-tolyl group, a p-methoxyphenyl group, a m-methoxyphenyl group, a p-dimethylaminophenyl group, a p-diethylaminophenyl group, a p-(acetylamino)phenyl group, a p-(capryloylamino)phenyl group, p-(benzylamino)phenyl group, a p-benzylphenyl group, etc.
Also, specific examples of substituents represented by R.
sup.
2 of general formula (I) other than a hydrogen atom are a methyl group, a ethyl group, an n-propyl group, a isopropyl group, a phenyl group, a 4-chlorophenyl group, a 4-bromophenyl group, a 3-chlorophenyl group, a 4-cyanophenyl group, a 4-carboxyphenyl group, a 4-sulfophenyl group, a 3,5-dichlorophenyl group, a 2,5-dichlorophenyl group, etc