Abstract
The presented examples demonstrate that 1,3-oxazol-5(4H)-ones are attractive intermediates not only for intermolecular but also for intramolecular reactions with nucleophiles. With a series of examples, the potential of these reactions was exemplified. The reactions of 1,3-oxazol-5(4H)-ones, as starting materials or as intermediates, bearing the nucleophilic group in ω-postion of the substituent at C(2), belong to real ring enlargement reactions as all atoms of the oxazolone ring are included in the new ring. They proceed under acid catalysis, e.g. under the conditions of the ‘direct amide cyclization’. Together with the ‘azirine/oxazolone method’ for the introduction of α,α-disubstituted α-amino acids into peptides, these reactions are ideally suited for the synthesis of cyclodepsipeptides and cyclopeptides, especially of sterically congested ones, which contain α-aminoisobutyric acid (Aib) or other α,α-disubstituted glycines. On the other hand, 4,4-disubstituted 1,3-oxazol-5(4H)-ones with the substituent containing the nucleophilic group located at C(4), undergo acid-catalyzed ring transformations leading to lactones or lactams with an acylamino group in α-position.
Heimgartner, H