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

Dipartimento di Chimica Organica E. Paterno, Università degli Studi di Palermo, Viale delle Scienze, Parco d'Orleans II, Edificio 17, I-90128 Palermo, Italy.
The Journal of Organic Chemistry (Impact Factor: 4.64). 10/2007; 72(20):7656-66. DOI: 10.1021/jo701306t
Source: PubMed

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.