The synthesis and chemistry of azolenines. Part 18. Preparation of 3-ethoxycarbonyl-3 [formula omitted]-pyrroles [formula omitted] the paal-knorr reaction, and sigmatropic rearrangements involving competitive ester migrations to c-2, c-4 and n.

Abstract

3 H -Pyrrole-3-carboxylic esters ( 4 ) have been prepared, in some cases together with isomers ( 5 ) and ( 6 ) having exocyclic double bonds, by cyclization of suitably substituted 2-ethoxycarbonyl-1,4-diketones ( 1 ) with liquid ammonia, followed by dehydration of the isolable 2-hydroxy-3,4-dihydro-2 H -pyrrole intermediates ( 2 ) and ( 3 ) with aluminia in boiling solvents. Prolonged heating in toluene or p-xylene coverts the 3H pyrroles ( 4 ) quantitatively into isomeric 4-esters ( 11 ) and N -esters ( 13 ) of 1 H -pyrroles via competitive [1,5]sigmatropic rearrangements. Isolable intermediate 2 H -pyrrole-2-carboxylic esters ( l2 ) are converted similarly into the same products, under the same conditions. Detection of 3 H -pyrroles ( 4 ) as intermediates in the latter reaction demonstrates for the first time the reversibility of the thermal 2 H -pyrrole to 3 H -pyrrole interconversion.