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A MECHANISTIC PROPOSAL FOR THE FORMATION OF UNEXPECTED PYRROLES IN REACTIONS OF CARBOETHOXYCARBENE WITH ENAMINONES

A reaction mechanism is proposed for the unexpected formation of 3-carboethoxy-2-methyl pyrroles in reactions of acyclic enaminones with carbethoxycarbene, formed in situ from cupric acetilacetonate catalyzed decomposition of ethyl diazoacetate. Carbenes, very useful and highly reactive intermediates in organic synthesis (1, 2), are generated when diazo compounds decompose in the presence of copper and rhodium salts (3). Enaminones (4, 5, 6) are also easily accessible, multifunctional, and versatile reagents in organic synthesis as they contain several centers subject to either nucleophilic or electrophilic attack. We have studied the structure (7) and reactivity (8) of enaminones, and the synthetic utility of their reactions with α-ketocarbenes, generated in situ from cupric acetylacetonate catalyzed decomposition of a-diazoketones (9), and developed new methods for the synthesis of pyrroles (9) and triazoles (10, 11). When reacting the carboethoxycarbene 1 (generated in situ from the cupric acetilacetonate catalyzed decomposition of ethyl diazoacetate) with acyclic enaminones 2, we observed the unexpected formation of pyrroles 3 (12) (Scheme 1). Introduction