Product diversity in cyclisations of maleamic acids: The imide-isoimide dichotomy

Cyclisation of maleamic acids with acetic anhydride in dimethylacetamide (DMA) at ca. 75 degrees C, with or without added cobalt naphthenate, gives predominantly maleimides when the parent maleic anhydride is unsubstituted. However, when the maleic anhydride has either one or two methyl substituents, the products are greater than or equal to 95% maleisoimides (5-imino-2,5-dihydrofuran-2-ones). In contrast all maleamic acids investigated, regardless of the extent of substitution, give exclusively maleimides when heated in acetic acid under reflux. Isoimide formation in the Ac2O-DMA-cobalt naphthenate procedure appears to arise from kinetic control, since the isoimide preference was reduced at higher reaction temperature. The preferred Z-stereochemistry of the isoimide 3 is confirmed by X-ray crystallography.