title: 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.
abstract: The experimental investigation of the base-catalyzed rearrangements of 3-acylamino-1,2,4-oxadiazoles evidenced 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 that is base-activated, occurs at a higher temperature than the BKR, and irreversibly leads to the corresponding 2-acylamino-1,3,4-oxadiazoles. An extensive DFT study is reported to elucidate the proposed reaction mechanism and to compare the three possible inherent routes: (i) the reversible three-atom side-chain ring-degenerate BKR, (ii) the ring contraction-ring expansion route (RCRE), and (iii) the one-atom side-chain rearrangement. The results of the computational investigation point out that the latter route is kinetically preferred over the RCRE and can be considered as the ground-state analogue of a previously proposed C(3)-N(2) migration-nucleophilic attack-cyclization (MNAC) photochemically activated pathway. 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 single intramolecular nucleophilic attack-cyclization step leading to the final 2-acylamino-1,3,4-oxadiazole.
authors.affiliation: Dipartimento di Chimica Organica E. Paterno, Università degli Studi di Palermo, Viale delle Scienze, Parco d'Orleans II, Edificio 17, I-90128 Palermo, Italy. pace@unipa.it
authors.forename: Andrea
authors.initials: A
authors.lastname: Pace
authors.forename: Ivana
authors.initials: I
authors.lastname: Pibiri
authors.forename: Antonio Palumbo
authors.initials: AP
authors.lastname: Piccionello
authors.forename: Silvestre
authors.initials: S
authors.lastname: Buscemi
authors.forename: Nicolo
authors.initials: N
authors.lastname: Vivona
authors.forename: Giampaolo
authors.initials: G
authors.lastname: Barone