A novel route to 1-substituted 3-(dialkylamino)-9-oxo-9H-indeno[2,1-c]-pyridine-4-carbonitriles

Heptalenecarbaldehydes 1/1' as well as aromatic aldehydes react with 3-(dicyanomethylidene)-indan-1-one in boiling EtOH and in the presence of secondary amines to yield 3-(dialkylamino)-1,2-dihydro-9-oxo-9H-indeno[2,1-c]pyridine-4-carbonitriles (Schemes 2 and 4, and Fig. 1). The 1,2-dihydro forms can be dehydrogenated easily with KMnO(4) in acetone at 0 degrees (Scheme 3) or chloranil (= 2,3,5,6-tetrachlorocyclohexa-2,5-diene-1,4-dione) in a 'one-pot' reaction in dioxane at ambient temperature (Table 1). The structures of the indeno[2,1-c]pyridine-4-carbonitriles 5' and 6a have been verified by Xray crystal-structure analyses (Fig. 2 and 4). The inherent merocyanine system of the dihydro forms results in a broad absorption band in the range of 515-530 nm in their UV/VIS spectra (Table 2 and Fig. 3). The dehydrogenated compounds 5, 5', and 7a-7f exhibit their longest-wavelength absorption maximum at ca. 380 nm (Table 2). In contrast to 5 and 5', 7a-7f in solution exhibit a blue-green fluorescence with emission bands at around 460 and 480 nm (Table 4 and Fig. 5).