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  • Title: Experimental and DFT studies on competitive heterocyclic rearrangements. 3. A cascade isoxazole-1,2,4-oxadiazole-oxazole rearrangement.
    Author: Pace A, Pierro P, Buscemi S, Vivona N, Barone G.
    Journal: J Org Chem; 2009 Jan 02; 74(1):351-8. PubMed ID: 19046122.
    Abstract:
    The thermal rearrangements of 3-acylamino-5-methylisoxazoles 1 have been investigated under basic and neutral conditions and interpreted with the support of computational data. The density functional theory (DFT) study on the competitive routes available for the base-catalyzed thermal rearrangement of isoxazoles 1 showed that the Boulton-Katritzky (BK) rearrangement, producing the less stable 3-acetonyl-1,2,4-oxadiazoles 5, is a much more favored process than either the migration-nucleophilic attack-cyclization (MNAC) or the ring contraction-ring expansion (RCRE). In turn, an increase in reaction temperature will promote the MNAC of oxadiazoles 5, producing the more stable 2-acylaminooxazoles 8. The thermal rearrangement of 3-acylaminoisoxazoles 1 into oxazoles 8 can therefore be interpreted in terms of a cascade BK-MNAC rearrangement involving 3-acetonyl-1,2,4-oxadiazoles 5 as ancillary intermediates.
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