A Deoxygenative [4+1] Annulation Involving N-Acyldiazenes for an Efficient Synthesis of 2,2,5-Trisubstituted 1,3,4-Oxadiazole Derivatives

An unprecedented and highly efficient tris(dimethylamino)phosphine [P(NMe2)(3)]-mediated deoxygenative [4 + 1] annulation of N-acyldiazenes with alpha-dicarbonyl compounds such as isatins, -keto esters, and alpha-diketones is reported. The annulation reactions proceed smoothly under mild conditions to deliver a broad range of 2,2,5-trisubstituted 1,3,4-oxadiazole derivatives in moderate to excellent yields from readily available starting materials. It represents the first realization of the [4+1] annulation mode involving N-acyldiazenes to construct five-membered heterocycles.