A Novel Skeletal Rearrangement of 2-Azabicyclo[2.2.1]hept-5-ene-3-carboxylic Acid Derivatives into 2-Oxabicyclo[3.3.0]-oct-7-en-3-ones under Acidic Conditions

The reaction of ethyl 2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate with aroyl chloride and the subsequent hydrolysis of the ester group provided 2-aroyl-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylic acid derivatives, which underwent a stereospecific rearrangement giving 4-aroylamino-2-oxabicyclo[3.3.0]oct-7-en-3-ones by treatment with trifluoroacetic acid. On the other hand, the reaction of 2-benzoyl-2-azabicyclo[2.2.1]hept-5-ene with trifluoroacetic acid in heated benzene afforded 1-[(benzoylamino)methyl]-1,3-cyclopentadiene which afforded a Diels–Alder cycloadduct with N-phenylmaleimide. A plausible mechanism for the rearrangement is presented.