Synthesis of indolizinoquinolinones through three- and four-component domino Knoevenagel/hetero-Diels-Alder reactions: novel access to (+)-camptothecin

The fused heterocyclic indolizinoquinolinone system is a key structural feature of several highly bioactive alkaloids, including camptothecin. Camptothecin has been efficiently obtained by a three- or four-component domino Knoevenagel / hetero-Diels-Alder reaction of aldehyde, Meldrum's acid, and enol ether in the presence or absence of alcohol, followed by reductive cleavage of the amine protecting group. The obtained products were further transformed along several different routes leading to camptothecin.