Pyrroles mesoionic oxazolones

Mesoionic oxazolones (munchnones) 297 can be generated by cyclodehydration of N-substituted a-amino acids 295 or by alkylation of oxazolones 296 (Scheme 7.98). These compounds are reactive and versatile 1,3-dipoles that undergo cycloaddition reactions with dipolarophiles to generate a variety of heterocyclic systems. In particular, this is an extremely versatile methodology to prepare pyrroles that result from elimination of carbon dioxide from the initial cycloadduct. Numerous examples have appeared in the literature in recent years and several have been selected for discussion. The reader should consult Part A, Chapter 4 for an extensive discussion and additional examples. 

Huisgen and co-workers486,490 have described a useful synthesis of N-substituted pyrroles (41) from mesoionic oxazolones (39) via the intermediates 40, which were not isolated. A variety of acetylenic esters (phenylpropiolic, propiolic, tetrolic, and DMAD) were used. The kinetics of these reactions have been studied.491 The addition of carbon... 

Mesoionic oxazolones 154, prepared from 153, allowed the synthesis of 3-CF3-pyrroles. Thus, their reactions with fluorinated derivatives of ethyl propiolate 155 led to the alkyl pyrrole-3-carboxylates 156 [65]. Similarly, the reaction of compound 157 with hexafluorobut-2-yne gave the pyrrole 158 [66]. 

As it was mentioned previously, mesoionic oxazolones were used as dipolaro-philes in the synthesis of 2-CF3-pyrroles. A tandem addition of 1,2-binucleophiles to oxazolones also led to pyrroles, but bearing the CFs-group in 3-position. For instance, the reaction of the oxazolones 426 with aminomalonate 427 gave the pyrrolidine derivative 428, which formed 429 by reaction with acetic anhydride. Treatment of 429 with lithium hydroxide resulted in a decarboxylation giving the pyrrole 430 [140]. 

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