Two pathways in the cycloaddition of 4-nitrobenzaldehyde to acyl(imidoyl)ketene

Aroyl(benzoxazinyl)ketenes I generated by thermal decarbonylation of 3-aroylpyrrolo[2,1-c][1,4]benzoxazine-1,2,4-triones II undergo [4 + 2]-cyclodimerization in the absence of other reaction partners. In this transformation, one molecule I acts as dienophile via the ketene C=C bond, and the other acts as diene through the conjugated C=C–C=N bond sequence in the imidoylketene fragment [1]. Attempts to trap aroyl(benzoxazinyl)ketenes I with 4-bromobenzaldehyde or Schiff base lead to the formation of the corresponding [4 + 2]-cycloadducts with participation of the C=C–C=N fragment in imidoylketene I and C=O bond of 4-bromobenzaldehyde [2] or C=N bond of Schiff base [3]. On the other hand, [4 + 2]-cycloaddition of N,N′-dicyclohexylcarbodiimide to ketenes I involves the C=C–C=O bond sequence of the aroylketene fragment in I and the C=N bond of N,N′-dicyclohexylcarbodiimide [4]. According to the TLC data, all the above reactions are regioselective.