Abstract 1-Ethyl-6-fluoro-7-hydroxylamino-1, 4-dihydro-4-oxoquinoline-3-carboxylic acid 7 can be either oxidized to the corresponding nitroso intermediate 8 or converted to the methylene nitrone 9. Both intermediates form cycloaddition products with selected dienes and olefins respectively. Also, the preparation of 1-ethyl-6-fluoro-7-(hexahydro-2-methyl-5H- pyrrolo [3, 4-d] isoxazol-5-yl)-3-quinolinecarboxylic acid 5 via a nitrone cycloaddition is ...