4-cinnamylideneamino-3-methyl-5-styrylisoxazoles (1) with dimethylacetylenedicarboxylate has given an isoxazolyldihydropyridine (2) as major and an isoxazolylaminofumarate (3) as minor products respectively. 3-Cinnamylideneamino-5-methylisoxazoles (4) behave similarly. Isoxazolylaminofumarates (3 and 6) have been identified by unambiguous synthesis.