A compound of formula I ** Formula ** in which R1a represents H or carboxy and R1b represents H, or R1a and R1b together represent the group * -C (O) -NH-S- or the group * -C (OH )>； = NS-, in which "*" represents the junction point of R1a and " represents the junction point of R1b； R2 represents H, (C1-C3) alkyl, hydroxy (C1-C3) alkyl, benzyl or cycloalkyl (C3-C5)； R3 represents H or halogen； U represents N or CR4； wherein R4 is H or (C1-C3) alkoxy； A represents CH, B represents NH, m represents 1 or 2 and n represents 1 or 2； or A represents N, B is absent, m represents 2 and n represents 2； Y represents CH or N； and Q represents O or S； or a salt of said compound.Un compuesto de fórmula I**Fórmula** en la que R1a representa H o carboxi y R1b representa H, o R1a y R1b representan junto el grupo *-C(O)-NH-S- o el grupo *-C(OH)>；=N-S- , en el que "*" representa el punto de unión de R1a y " " representa el punto de unión de R1b； R2 representa H, alquilo (C1-C3), hidroxi-alquilo (C1-C3), bencilo o cicloalquilo (C3-C5)； R3 representa H o halógeno； U representa N o CR4； en el que R4 es H o alcoxi (C1-C3)； A representa CH, B representa NH, m representa 1 o 2 y n representa 1 o 2； o A representa N, B está ausente, m representa 2 y n representa 2； Y representa CH o N； y Q representa O o S； o una sal de dicho compuesto.
