Quinoline ring isatins

Indoles yield 3-chloroquinolines by addition of dichlorocarbene and HCl elimination (see p 101). A number of ring enlargement reactions of isatins into quinolines are known (as example, see the Pfitzinger synthesis, p 328, and [107/108]). 

In the alkaline medium, isatin is cleaved at the amide bond and converted to the salt 82 of isatinic acid, which undergoes aldol condensation with a-methylene ketones and cyclization with H2O elimination affording quinoline-4-carboxyhc acids (83). Notably, N-acetylisatins (84), after base-induced ring-opening (— -85), undergo an intramolecular aldol condensation giving rise to 2-quinolone-4-carboxylic acids (86). 

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