This rapidly led to the development of several other 4-quinolone-based antibiotics such as oxolinic acid, cinoxacin and flumequine (Fig. 1), used clinically to treat Gram-negative bacterial infections, and later on to second-generation drugs such as norfloxacin and ciprofloxacin (Fig. 1), also effective against some Gram-positive bacteria. All of the quinolone antibiotics are characterized by the presence of a carboxylic acid function at C-3