Synthesis and antibacterial activities of new quinolone derivatives utilizing 1-azabicyclo[1.1.0]butane

The ring-opening reactions of 1-azabicyclo[1.1.0]butane 3 with thiols 6a–f gave 3-sulfenylazetidine derivatives 7a–f in 50-92% yields. Treatment of 3 with aromatic amines 11a–e and dibenzylamine 11f in the presence of Mg(ClO4)2 afforded the corresponding 3-aminoazetidine derivatives 12a–f in 24–53% yields. These azetidine derivatives were introduced into the C7 position of a quinolone nucleus 8 to afford the corresponding fluoroquinolones 9a–f and 13a–f in 21–83% yields. Some of them exhibited superior antibacterial activity against quinolone-susceptible MRSA in comparison with clinically used fluoroquinolones, such as levofloxacin, ciprofloxacin, and gatifloxacin.