The proton-initiated cyclization of N-alkylamides of styrylacetic acids. The synthesis of 5-arylpirrolidine-2-ones

Abstract

Aim. To study the effect of the structural parameters of styrylacetic acid amides on the course of the reaction of the electrophilic intramolecular cyclization under the action of polyphosphoric acid and search the rational approaches to obtain N-unsubstituted 5-arylpyrrolidine-2-ones. Results and discussion. The literature sources related to the main methods of synthesis, as well as the biological activity of 5-arylpyrrolidine-2-ones, have been analyzed and systematized. The regiochemistry of the cyclization of N-unsubstituted and N-alkyl amides of styrylacetic acids has been studied using polyphosphoric acid (PPA). Experimental part. It has been found that N-unsubstituted styrylacetic acid amides when heating at 100 °C in PPA are cyclized to 5-arylpyrrolidine-2-ones with the yields of 44-58 %. For N-tert-butylamides with donor substituents in the styrenic moiety of the molecule the cyclization under similar conditions is accompanied with elimination of the N-alkyl fragment resulting in N-unsubstituted 5-arylpyrrolidine-2-ones with the yields of 50-95 %. Lactamization of N-benzylamide and N-isopropylamides under the action of PPA proceeds with formation of 1-alkyl-5-arylpyrrolidine-2-ones. Conclusions. It has been found that the proton-initiated cyclization of N-unsubstituted and N-tert-butylamides of styrylacetic acid with donor substituents in the styrene fragment in polyphosphoric acid when heating at 100 °C is a preparative convenient method for the synthesis of 1-unsubstituted 5-arylpyrrolidine-2-ones. A similar reaction of N-benzyl (isopropyl) amides leads to the preferential formation of 1-alkyl-5-arylpyrrolidine-2-ones.