This invention provides a compound represented by formula (1) or a pharmaceutically acceptable salt thereof. Specifically, the present invention provides a compound represented by formula (1) or a pharmaceutically acceptable salt thereof [Therein, A is O, S, or N—R
6
; ring G is a 5-membered or 6-membered aromatic ring, etc., including 1-3 heteroatoms selected from O, S and N as constituent atoms; R
1
and R
2
are each independently a hydrogen atom, a halogen atom, or an optionally-substituted C
1-6
alkyl carbonyl group, etc.; R
3
, R
4
and R
5
are each independently a hydrogen atom, a halogen atom, or an optionally-substituted C
1-6
alkyl carbonyl group, etc.; and R
6
is a hydrogen atom or an optionally-substituted C
1-6
alkyl group, etc.].