Vinylogous vs arylogous isocephems

The synthesis of 2-isocephems bearing multiple bonds between the thiazoline ring and the 3-substituent is reported. When a leaving group was present, The E-vinylogous derivatives were shown to be more active than the parent compounds, essentially against staphylococci. Insertion of an aryl group in this position, however, sharply reduces the activity against Gram-negative bacteria.