[4+2]‐Cycloadditionen von α,β‐ungesättigten Hydrazonen. Teil 2. Pyridin‐2,3‐dicarboximide aus 1,4‐Dihydropyridin‐Derivaten

[4+2] Cycloaddition of α,β‐Unsaturated Hydrazones to Pyridine‐2,3‐dicarboximides via 1,4‐Dihydropyridine Derivatives The [4 + 2] cycloaddition of α,β‐unsaturated hydrazones of type 1 (1‐aza‐1,2‐butadienes) with maleimides ( e.g. 2b ) affords the tetrahydropyridines of type 8. Elimination of dimethylamine to 9 is easily achieved by treatment of 8 with silica gel at elevated temperature. Oxidation of the dihydropyridines 9 leads to highly substituted pyridine‐2,3‐dicarboxylic‐acid dervatives 4 .