1-Azabicyclo[3,2,0]hept-3-ene-2,7-diones. A new heterocyclic system

4-Methoxy-1-azabicyclo[3,2,0]hept-3-ene-2,7-dione (III) has been prepared both by a Beckmann rearrangement of 4-methoxy-o-benzoquinone 1-(O-phenyl-sulphonyl)oxime (I) followed by ring contraction and by cyclization of 5-cyano-3-methoxypenta-cis-2,trans-4-dienoic acid (V).