Synthesis of new camptothecin analogues with the E-lactone ring replaced by α,β-cyclohexenone

The total synthesis of racemic camptothecin analogues 12a and 12b, in which the E-lactone ring has been replaced by an α,β-cyclohexenone ring and the ethyl and hydroxy substituents have been retained, was achieved by first preparing the ABCD fragments 31a and 31b, which were then converted into the tetracyclic triol 36a and 36b by osmium-mediated dihydroxylation. Compounds 36a and 36b were oxidized in one-pot reactions, followed by intramolecular aldol condensation to furnish the desired pentacyclic 12a and 12b, which retained topoisomerase I inhibitory activity and exhibited cytotoxicity to tumor cell growth in culture.