As part of our continuing study of the use of N-aryl nitrones in ortho-selective aromatic substitution reactions l-3 , we have investigated the reaction of a number of such nitrones with dichloroketene. These reactions lead to adducts which can be hydrolysed by boiling water or aqueous acid to isatin (l.) or a derivative of isatin.
This new isatin synthesis seems of significance because of its simplicity, few steps, and good yields. Isatins are valuable intermediates in the preparation of pharmaceuticals and dyes, adding to the usefulness of the reaction. While this manuscript was in preparation, an account of another new ieatin synthesis (based on sulfur ylid chemistry) was published4; this drew attention to the shortcomings of existing routes to the isatin system.
Our method, as mentioned above, involves the reaction of A-aryl nitrones with dichloroketene.
Although this reaction has not been investigated previously, other workers have investigated the reactions of nitrones with dialkyl and diary1 ketenes. Staudinger and Miescher were the first to report on the reactions of diphenylketene with various nitrones5, noting the formation of a 1:l adduct with diphenylnftrone and a 1:2 adduct (nitrone:ketene) with triphenylnitrone. Later workers, notably Rassall and Lippman6, and Taylor and co-workers 7 , reinvestigated the work of Staudinger and Wiescher. Taylor and his associates also worked with variously substituted N-a@