Compounds of formula (I) and their pharmaceutically acceptable salts are described. Processes for their preparation, pharmaceutical compositions containing them, their use as medicaments and their use in the treatment of bacterial infections are also described.本發明係描述式(I)化合物及其藥學上可接受之鹽。亦描述其製法,含有彼等之醫藥組合物,其作為藥劑之用途,及其在細菌感染治療上之用途。