Syntheses, structures and tautomers of 2,5-disubstituted pyrroles

The preparation of diethyl 2,2'-(2,5-pyrrolidinediylidene)bisacetate (2) from diethyl 3,6-dioxooctanedioate (1) is detailed, as is the subsequent conversion of 2 into its tautomer, diethyl pyrrole-2,5-diacetate (3), by acid catalysis. Alternatively, base hydrolysis of 2 gives pyrrole-2,5-diacetic acid (4) in good yield as long as the reaction solution is subsequently acidified to pH = 1 before extraction. In D2O solution, in the absence of added base, the pyrrole ring C-H protons are rapidly exchanged for deuterons. The X-ray structures of 2 and 4 are reported and confirm the earlier prediction, made primarily on the basis of the H-1 NMR data, that 2 is a pyrrolidine tautomer while 4 is a pyrrole tautomer. Hydrogen bonding, intramolecular in the case of 2 and intermolecular in the case of 4, is a feature of both structurally characterised compounds.