Pyrroles and related compounds. Part XXVI. Pyrrole β-keto-esters

Abstract

Various attempts to synthesise β-keto-esters of pyrroles for use as model compounds, or as intermediates in chlorophyll synthesis, are described. Carboxylation of acetylpyrroles, using dialkyl carbonates or chloroformates, gave a variety of products, including a β-keto-ester and (unexpectedly)N-alkyl- as well as N-alkoxycarbonyl-pyrroles. Attempts to utilise pyrrylacetylenes were unsuccessful, as were direct acylation reactions with derivatives of malonic ester. The most successful approach involved the coupling of a pyrryl acid chloride with the sodium salts of monoalkyl monobenzyl (or mono-t-butyl) esters of malonic acid; the β-keto-diester thus obtained could then be converted (by hydrogenolysis or treatment with cold trifluoroacetic acid) into the desired β-keto-ester.