Copper(II)-Catalyzed Room Temperature Aerobic Oxidation of Hydroxamic Acids and Hydrazides to Acyl-Nitroso and Azo Intermediates, and Their Diels–Alder Trapping

CuCl2, in the presence of a 2-ethyl-2-oxazoline ligand, is an effective catalyst for the room temperature, aerobic oxidation of hydroxamic acids and hydrazides, to acyl-nitroso and azo dienophiles respectively, which are efficiently trapped in situ via both inter- and intramolecular hetero-Diels–Alder reactions with dienes. Both inter- and intramolecular variants of the Diels–Alder reaction are suitable under the reaction conditions using a variety of solvents. Under the same conditions, an acyl hydrazide was also oxidized to give an acyl-azo dienophile which was trapped intramolecularly by a diene.