Serendipitous Preparation of a Pyrrole Precursor to Porphyrins

Ethyl 5-methylpyrrole-2-carboxylate has been used as a precursor to porphyrins, and is accessible by a number of multistage syntheses. A new, one-pot preparation has been discovered recently by accident, which gives this pyrrole in 35-40% yield from cheap and readily available starting materials. 

American workers needed to prepare the bis-amino acid 1 and adopted a literature procedure in which two equivalents of diethyl acetamidomalonate were to be alkylated with one equivalent of l,4-dichloro-2-butyne using two equivalents of sodium ethoxide in hot ethanol. Hydrolysis and decarboxylation of the dialkylated malonate would then give 1. This alkylation reaction was carried out, but ten equivalents of sodium ethoxide were used rather than two. This resulted in formation of ethyl 5-methylpyrrole-2-carboxylate in ca. 40% yield. Further study showed that the reaction to produce the pyrrole required equimolar amounts of the acetamidomalonate and the dichlorobutyne, excess of sodium ethoxide, and heating. No pyrrole was formed at room temperature. 

Devise a mechanism to account for the formation of ethyl 5-methylpyrrole-2-carboxylate. 

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