Unprecedented Reaction of Ninhydrin with Ethyl Cyanoacetate and Diethyl Malonate on Ultrasonic Irradiation.

Abstract Ultrasonic-assisted, catalyst-free reactions of ninhydrin with ethyl cyanoacetate and diethyl malonate led to the unprecedented formation of an indenopyran and a spiroindenofuran viz. diethyl 4-cyano-2-hydroxy-5-oxo-4,5-dihydroindeno[1,2-b]pyran-3,4-dicarboxylate 1 and diethyl 3a′,8b′-dihydroxy-1,3,4′-trioxo-1,3,3a′,4′-tetrahydrospiro[indene-2,2′-indeno[1,2-b]furan]-3′,3′(8b′H)-dicarboxylate 3, respectively. In addition to these unprecedented results, reactions of ninhydrin with dimedone, ethyl acetoacetate and ethyl nitroacetate yielded 4b,9b-dihydroxy-7,7-dimethyl-7,8-dihydro-4bH-indeno[1,2-b]benzofuran-9,10(6H,9bH)-dione 5, ethyl 3a,8b-dihydroxy-2-methyl-4-oxo-4,8b-dihydro-3aH-indeno[1,2-b]furan-3-carboxylate 6 and 2-hydroxy-2-(nitromethyl)-1H-indene-1,3(2H)-dione 7, respectively. The structures of 1, 3, 5, 6 and 7 were determined by X-ray crystallography and attempts have been made to propose the mechanism of their formation.