3-Benzoyl-3,4-dihydro-2H,5H-1-benzopyrano[4,3-b]pyran-5-ones by condensation of 4-hydroxycoumarins with enone mannich bases

Starting from 4-hydroxycoumarins 4 and 1-aryl-2-(dimethylaminomethyl)prop-2-en-1-ones 2 the title compounds 6 have been synthesized. The reaction is initiated by addition of the C-3 carbon atom of 4-hydroxycoumarin to the enone double bond of the Mannich base, subsequent deamination affords the intermediate 3-(2-benzoylallyl)-4-hydroxycoumarin 8, followed by ring closure. Depending on the reaction time, varying proportions of the ketones 6 and 8 are formed and support the mechanism defined. The spectroscopic data of all products unambiguously distinguish between the coumarin and the chromone structures in favor of the former.