A compound of the formula (I):
1
wherein X
1
and X
2
are the same or different and each is a bond or a spacer having 1 to 20 atom(s) in the main chain;
one of R
1
and R
2
is a cycle group having substituent(s) selected from 1) an optionally substituted carboxy-C
1-6
alkoxy group and 2) an optionally substituted carboxy-C
1-6
aliphatic hydrocarbon group, wherein the cycle group optionally has additional substituent(s), and the other is an optionally substituted cycle group or a hydrogen atom; and
R
3
, R
4
and R
5
are the same or different and each is a hydrogen atom or a substituent, or R
4
may link together with R
3
or R
5
to form an optionally substituted ring;
provided that when R
3
is a hydrogen atom, R
4
is a hydrogen atom and R
5
is methyl, X
2
—R
2
is not 4-cyclohexylphenyl; when R
3
is 4-methoxyphenyl, R
4
is a hydrogen atom and R
5
is methyl, X
2
—R
2
is not 4-methoxyphenyl; and when R
1
or R
2
is a hydrogen atom, the adjacent X
1
or X
2
is not a C
1-7
alkylene;
or a salt thereof exhibits a protein tyrosine phosphatase inhibitory action and is useful as a prophylactic or therapeutic agent for diabetes or the like.