Heterocyclic photorearrangements. Photoinduced rearrangements of 1,2,4‐oxadiazoles substituted by an XYZ side chain sequence

Abstract

Photoinduced rearrangements of 1,2,4‐oxadiazoles substituted by an XYZ side chain sequence at position 3 of the ring have been recognized. Examples taken out from previous results have been emphasized and some other patterns dealing with a 3‐phenoxy and 3‐enaminoketone sequence have been investigated. An intermediate species derived from photolysis of the ring O‐N bond and characterized by a continuous 6π electron system involving the side chain sequence, was suggested to give ring closure to the rearrangement product.

Solvent dependent photochemical reactivity of 3-allyloxy-1,2,4-oxadiazoles

“…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 solvent was then evaporated, the residue treated with water and neutralized with HCl 1M and extracted with EtOAc. The organic layer was dried over Na 2 SO 4 …”

Photoinduced Molecular Rearrangements. The Photochemistry of Some 1,2,4-Oxadiazoles in the Presence of Nitrogen Nucleophiles. Formation of 1,2,4-Triazoles, Indazoles, and Benzimidazoles

“…The photochemistry of 3,5-disubstituted 1,2,4-oxadiazoles in methanol is characterized by photolysis of the ring O−N bond into open-chain species which develop into final products depending on the nature and position of substituents. − A quite generalizable behavior was recognized in the formation of compounds arising from a reaction of the electrophilic nitrogen of the photolytic species with the nucleophilic solvent. 1b, In some cases, nitrene-type intermediates can even isomerize into a carbodiimide which the solvent will then capture. , Moreover, in the irradiation of 5-aryl substituted oxadiazoles, the formation of quinazolinones has been reported and explained by a heterocyclization reaction involving the electrophilic nitrogen of the photolytic species and the C(5)-aryl moiety. 1b,, Furthermore, photoinduced rearrangements of oxadiazoles containing certain three-(four-)atom side-chains linked at C(3) have been also recognized. − …”