The Chemistry of 2‐Alkenyl‐5(4H)‐ oxazolones. IX. Acid‐Catalyzed Oligomerization

Abstract Results of the acid catalyzed oligomerization of 2‐alkenyl‐5(4H)‐oxazolones are reported. Employing LC–MS and preparative LC methods, the oligomeric mixtures were characterized by NMR analyses and were discovered to consist of exclusively cyclic trimers to decamers, with tetramers and pentamers predominating. A nucleophilic oligomerization mechanism involving Michael addition and C‐alkylation of a ketene‐aminal to protonated monomer was proposed that resulted in irreversible cyclization at the trimer propagation stage. Subsequent oligomerization proceeded via enolization of α‐hydrogens on 2‐substituted 5(4H)‐oxazolone products and continued Michael addition to protonated monomer. In the sense that when both enolizable hydrogens and protonated monomer are present, the oligomerization can be regarded as being “living.”