Synthesis of 3‐Substituted Dihydro‐1‐phenylamino‐1 H ‐pyrrolo[1,2‐ a ]imidazole‐2,5(3 H ,6 H )‐diones from α‐Amino Acid Phenylhydrazides and Levulinic Acid

α‐Amino acid phenylhydrazides 1 readily react with levulinic acid to produce the imidazolidin‐4‐one intermediates 4 , which undergo a second ring closure to afford the dihydro‐1 H ‐pyrrolo[1,2‐ a ]imidazole‐2,5‐dione derivatives 5 . It has been established that the solvent polarity has a great influence on the rate of the second condensation reaction, but not on the first. A mechanism, supported by experimental evidence, has been proposed to explain how the imidazolidin‐4‐one intermediates 4 , obtained as expected as a mixture of two diastereoisomers, give a single isomer for the bicyclic derivatives 5 ; the absolute stereochemistry of these compounds has been determined by X‐ray crystallographic analysis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)