Starting from bulk chemicals, novel isoxazoledicarboxylic acid esters were prepared and converted to the corresponding dicarboxylic acid monoamides in a few reaction steps and in high overall yields. Key intermediates for the cycloaddition step were chlorooximinoacetates or amides as nitrile oxide equivalents, prepared in one-pot reactions from diketene or acetoacetic acid esters. Isoxazoledicarboxylic acid monoamides combined good herbicidal activity with chemical flexibility. They were characterized as inhibitors of photosynthesis. Particularly, they affected the photosynthetic electron transport in photosystem I1 and the rate of carbon dioxide assimilation. Applied postemergence, the new compounds were found to control a broad spectrum of key weeds in corn. Excellent activity was found on Abutilon theophrasti (L.) Medik., Amaranthus retrojlexus L., Chenopodium album L., Ipomoea spp., Polygonum persicaria L., Solanum nigrum L. and Xanthium strumarium L. with rates between 0.2 and 0.5 kg ha-'. As a side-effect, the compounds also showed activity against grass weeds. The compounds are excellent tank-mix partners, e.g. for sulfonylureas, to complete the weed spectrum (Chenopodium album L., Solanum nigrum L.) and/or to reduce the risk of developing herbicide-resistant weeds.