Syntheses of 2-hydroxypyrano[3,2-c]quinolin-5-ones from 4-hydroxyquinolin-2-ones by tandem Knoevenagel condensation with aldehyde and Michael addition of enamine with the quinone methide—thermo- and photochemical approaches

Abstract

2,3,4,6-Tetrahydro-2-hydroxypyrano[3,2-c]quinolin-5-one derivatives 3 are conveniently synthesized from 4-hydroxyquinolin-2(1H)-ones 4 by tandem Knoevenagel condensation of 4 with aliphatic aldehyde–Michael-type 1,4-addition of the enamine (derived from the aldehyde and diethylamine in situ) with the quinone methide (quinomethane) 9. This reaction sequence can be achieved in one pot by either direct reaction of 4 with an aldehyde in the presence of diethylamine as a base in refluxing benzene. Alternatively, in the case of 3a–c and 3g from 4 and acetaldehyde, these syntheses were carried out by way of a photochemical variant by photolysis of a benzene solution of 4 as an electron acceptor and triethylamine as an electron donor, where acetaldehyde and diethylamine are generated in situ from triethylamine in redox processes initiated by single-electron transfer (SET) between photoexcited 4 and triethylamine.