Acid-mediated cyclotransformations of 4a-substituted 4H-4a,5-dihydroindeno[1,2-b]pyridines as a new route to 9a-substituted 1H-fluorenes

4a-Substituted dihydroindenopyridines undergo cleavage of the C(9b)=N bond in water-containing acidic medium, resulting in the formation of diastereomeric ethyl α-acetyl-β-(2,3-dihydro-1,3-dioxo-1H-inden-2-yl)-β-phenylpropionates and 9a-substituted 1,9a-dihydrofluore-none derivatives. The cyclization of α-acetyl-β-(2,3-dihydro-1,3-dioxo-1H-inden-2-yl)-β-phenyl-propionates with benzylammonium acetate affords 3-benzylamino-1,9a-dihydrofluorenones. The methanobenzo[a]azulene compound is also found among cyclization products.