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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.”
Notes
aSample preparation involved mixing a 0.1 mg/mL sample in acetonitrile‐water with an equal volume of a saturated solution of 2,5‐dihydroxybenzoic acid (used as matrix) in acetonitrile: 0.1% TFA (water) (70:30) solution. An aliquot (1 μL) of the resulting mixture was deposited onto a gold‐plated MALDI sample plate for the actual analysis.
bA portion of the sample was dissolved in a m‐nitrobenzyl alcohol matrix and positioned at the end of the FAB probe. The probe was then introduced directly into the ion source of the mass spectrometer at ambient temperature and high vacuum. Ions were generated by exposure of the sample to a beam of neutral xenon atoms from the FAB gun at a voltage of 8 keV and an ion current of 1 mA. The mass spectrometer was operated at a mass resolution of approximately 4000 over a range of 50–1800 m/z, with a second analysis being performed over a mass range of 1400–3100 m/z.