Synthesis of polyacenequinones via crossed aldol condensation in pressurized hot water in the absence of added catalysts

Phthalaldehyde and 1,4-cyclohexanedione react in the 2:1 molar ratio via a crossed aldol condensation in sub- and supercritical water to produce 6,13-pentacenequinone. The reaction conditions were optimized at 77% yield at 250 °C for 60 min with the starting molar ratio of 2:1:800 for phthalaldehyde, 1,4-cyclohexanedione and water, respectively. Whereas the initial rate of the reaction increased at higher pH, as suggested by earlier studies of the crossed aldol condensation of the two carbonyl compounds using a base catalyst in an organic solvent, the present results of successful reactions without the addition of any acid or base show the role of the increased solubility of the starting materials and the higher ion product in pressurized hot water. 6,13-Pentacenequinone was obtained as fine needles from the cooled reaction mixture just by filtration. Under similar conditions, 7,16-heptacenequinone and 5,12-naphthacenequinone were obtained in 46 and 70% isolated yields, respectively.