Issue in Honor of Prof Nicolò Vivona ARKIVOC 2009 (viii) 156-167 Solvent dependent photochemical reactivity of 3-allyloxy-1,2,4- oxadiazoles Antonio Palumbo Piccionello, Andrea Pace, Ivana Pibiri, and Silvestre Buscemi* Dipartimento di Chimica Organica “E. Paternò”, Università degli Studi di Palermo, Viale delle Scienze-Parco d’Orleans II, I-90128, Palermo, Italy. E-mail: sbuscemi@unipa.it Dedicated to Prof. Nicolò Vivona on his 70 th birthday Abstract The photochemistry of some 1,2,4-oxadiazoles containing a double bond at the side chain has been investigated. The irradiation yielded different 2-N-benzoylamino-2-oxazoline derivatives depending on the employed reaction solvent (DCM or THF). Photorearrangements occur through an intramolecular aziridination reaction yielding a bicyclic intermediate, which was subjected to in situ aziridine ring opening. Photoinduced addition of HCl, involving the chlorinated solvent as a reagent, gave the corresponding chlorinated 2-oxazoline. In THF, reductive aziridine ring opening was observed. Keywords: Oxadiazole, photochemical rearrangement, 2-oxazoline, intramolecular aziridination Introduction The 1,2,4-oxadiazole 1 is an interesting heterocycle as it presents many useful applications ranging from pharmaceutical 1 to materials science (ionic liquids, liquid crystals, OLED). 2 In the last years the photochemical behaviour of the 1,2,4-oxadiazole system has been the object of several studies that showed the use of this heterocycle as synthon in the construction of different heterocyclic systems such as 1,3,4-oxadiazoles, 3 benzimidazoles, 4,5 benzoxazoles, 4 indazoles, 5 quinolines, 6 quinazolinones, 7 and triazoles. 5,8 Generally, the photochemical reactivity of the 1,2,4-oxadiazole ring involves the cleavage of the O-N bond. The photolytic intermediate 2 (Scheme 1), with zwitterionic, radicalic or nitrene- like character, will follow different reaction patterns depending on the nature of the substituents on the ring, the kind of solvent and the presence of other reactive species in solution. In many cases the N(2) of the oxadiazolic system acts as electrophilic center such as in the reactions with ISSN 1551-7012 Page 156  ARKAT USA, Inc.