NOVEL HETEROCUMULENE (RN=C=C=C=X) AND KETENE REARRANGEMENTS

The novel iminopropadienones RN=C=C=C=O (8) and the corresponding thio ne 21 have been synthesized by FVP of isoxazole or Meldrum's acid deri vatives. Bisimines 16 and the unusual, linear ketenimine 18 were gener ated in related reactions. In the Meldrum's acid series, a competing f ragmentation leads to imidoylketenes (R'N=CR-CR ''=C=O) (28). alpha-Ox oketenes (O=CR-CR'=C=O) (22), imidoylketenes (28), and vinylketenes (3 9c) undergo reversible interconversion with alpha-oxoketenes (23), oxo ketenimines (29), and acylallenes (40c), respectively, via a thermal 1 ,3-shift of the group R. This is favored by electron-donating substitu ents R by means of interaction with the ketene LUMO.