The invention relates to a process for the preparation of pyridine-2,3-dicarboxylic acid esters using novel maleic acid esters as starting materials. In the process, in a first process step, 2,3-dihalomaleic acid esters are reacted with ammonia, or ammonium salts, with the exclusion of water at a temperature of at least 50.degree.0 C. to form novel 2-amino-3-halomaleic acid esters, and these are subsequently reacted further in a second process step with .alpha.,.beta.-unsaturated aldehydes, or ketones, in the presence of an acidic catalyst and acid-binding agents at a temperature of at least 50.degree. C. with heat being supplied until the pyridine-2,3-dicarboxylic acid esters have been formed. Pyridine-2,3-dicarboxylic acid esters of this type can be used for example, as intermediates for synthesis of herbicides based on imidazoline. The present invention also relates to novel 2-amino-3-halomaleic acid esters, which are useful for producing the final product pyyridine-2,3-dicarboxylic acid esters.
本发明涉及一种使用新型
马来酸酯作为起始材料制备
吡啶-2,3-二
羧酸酯的方法。在该方法中,首先在第一步反应中,将2,3-二卤代
马来酸酯与
氨或
铵盐在排除
水的条件下在至少50°C的温度下反应,形成新型的2-
氨基-3-卤代
马来酸酯,然后在第二步反应中,在存在酸性催化剂和酸中和剂的条件下,将这些新型的2-
氨基-3-卤代
马来酸酯与α,β-不饱和醛或酮在至少50°C的温度下反应,并提供热量,直到形成
吡啶-2,3-二
羧酸酯。这种类型的
吡啶-2,3-二
羧酸酯可以用作例如基于
咪唑啉的
除草剂合成的中间体。本发明还涉及新型的2-
氨基-3-卤代
马来酸酯,这些酯对于制备最终产物
吡啶-2,3-二
羧酸酯是有用的。