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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-1H-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)