ACYLATION OF PYRROLIDINE-2,4-DIONES - A SYNTHESIS OF 3-ACYLTETRAMIC ACIDS - X-RAY MOLECULAR-STRUCTURE OF 3-[1-(DIFLUOROBORYLOXY)ETHYLIDENE]-5-ISOPROPYL-1-METHYL-PYRROLIDINE-2,4-DIONE

Pyrrolidine-2,4-diones, prepared from the corresponding α-amino acid esters by condensation with ethoxycarbonylacetic acid, Dieckmann cyclisation, and hydrolysis-decarboxylation, are acylated at C-3 by the acid chlorides of saturated, unsaturated, and arenecarboxylic acids in the presence of Lewis acids. The most efficient protocol involves boron trifluoride-diethyl ether as Lewis acid, isolation of the neutral boron difluoride complexes of the 3-acyltetramic acids, and their subsequent methanolysis. The quaternary ammonium enolates of 5-substituted pyrrolidine-2,4-diones undergo isomerisation to the exocyclic AM-isomers during O-acylation with acid chlorides.