Reactions of alkyl diazoacetates with pyridinium ylides

Pyridinium (methoxycarbonyl)methylide generated from (methoxycarbonyl)methylpyridinium halides under the action of K2CO3 reacts with alkyl diazoacetates in CH2Cl2 at 20 degrees C, resulting in the successive addition of three CHCOOMe fragments from the ylide to form 3,6-bis(alkoxycarbonyl)-4,5-diazaoctadienoic acid diesters. Beating of the latter in the presence of pyridine leads to their isomerization to give tetraalkyl tetrahydropyridazine-tetracarboxylates in high yields. Under more drastic conditions (refluxing xylene in the presence of pyridine), the acyclic tetraesters undergo another transformation to form pyrroletetracarboxylic acid esters in yields of up to 60%.