Experimental and DFT studies on competitive heterocyclic rearrangements. part 2: A one-atom side-chain versus the classic three-atom side-chain (Boulton-Katritzky) ring rearrangement of 3-acylamino-1,2,4-oxadiazoles

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Abstract

The experimental investigation of the base-catalyzed rearrangements of 3-acylamino-1,2,4-oxadiazolesevidenced a new reaction pathway which competes with the well-known ring-degenerate Boulton-Katritzky rearrangement (BKR). The new reaction consists of a one-atom side-chain rearrangement thatis base-activated, occurs at a higher temperature than the BKR, and irreversibly leads to the corresponding2-acylamino-1,3,4-oxadiazoles. An extensive DFT study is reported to elucidate the proposed reactionmechanism and to compare the three possible inherent routes: (i) the reversible three-atom side-chainring-degenerate BKR, (ii) the ring contraction-ring expansion route (RCRE), and (iii) the one-atomside-chain rearrangement. The results of the computational investigation point out that the latter route iskinetically preferred over the RCRE and can be considered as the ground-state analogue of a previouslyproposed C(3)-N(2) migration-nucleophilic attack-cyclization (MNAC) photochemically activatedpathway. The MNAC consists of the formation of a diazirine intermediate, involving the exocyclic nitrogen,that eventually evolves into a carbodiimide intermediate (migration); the latter undergoes a singleintramolecular nucleophilic attack-cyclization step leading to the final 2-acylamino-1,3,4-oxadiazole.
Lingua originaleEnglish
pagine (da-a)7656-7666
Numero di pagine11
RivistaJournal of Organic Chemistry
Volume72
Stato di pubblicazionePublished - 2007

All Science Journal Classification (ASJC) codes

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