PRODUCT DIVERSITY IN CYCLIZATIONS OF MALEAMIC ACIDS - THE IMIDE-ISOIMIDE DICHOTOMY

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