Reaction of ninhydrin with β‐dicarbonyl compounds

The reactions of ninhydrin (1) with β‐ketoesters and β‐diketones were studied. Two kinds of products were observed, which were either simple adducts of the dicarbonyl compounds to the center carbonyl group of ninhydrin, or tricyclic indeno[1,2‐b]furans. Treatment of methyl 3‐aminocrotonate (2) with ninhydrin led in analogous fashion to an indeno[1,2‐b]pyrrole (3) which was reduced to a complex tetracyclic dihydropyridine 12. A mechanism for the production of 12 is presented. Structures for the novel heterocycles are supported by high resolution nmr studies and X‐ray crystallography.