The present invention provides a compound represented by formula (I). (Informula (I), (2) is, for example,(3); R1 is a hydrogen atom; R2a and R2b have been bonded to each other to forma ring B, which is (5); R3a is a hydrogenatom; R3b is a hydrogen atom; R4a is carboxy, etc.; and R4b is, for example,an alkyl optionally substituted by any ofsubstituent group .alpha..)