3.5- Disubstituted pyrrole-2-carboxylate esters

DBU is capable of inducing the elimination of /3-acetoxy nitroalkanes and deprotonation of benzyl isocyanoacetate. A tandem Michael-Henry reaction of the resulting species ensues, leading to 3,4-disubstituted pyrrole-2-carboxylic esters. 

Disubstituted pyrroles lacking substituents at the 3- and S-positions are often required in porphyrin synthesis, and these can now be prepared from pyrrole itself. Thus, trichloroacetylation or trifluoroacetylation of pyrrole affords the 2-trihaloacetylpyrrole 8, which can be converted into the corresponding esters 9, carboxylic acids, or amides. The amides have been acetylated, or formylated, at the 4-position specifically, and the resulting acyl pyrroles (10) have been reduced to the alkyl pyrroles (11). This general procedure has now been improved by direct Friedel-Crafts formylation of the trichloroacetylpyrrole (8a) with dichloromethyl methyl ether (aluminium chloride) to the diacylpyrroles... 

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