Towards new camptothecins. Part 3: Synthesis of 5-methoxycarbonyl camptothecin

The synthesis of two camptothecin analogs substituted by a carbonyl function on position 5 of cycle C was realized. New conditions were studied to obtain the E-lactone ring of these heterocycles. These compounds were obtained from the reaction of Bredereck's reagent with indolizines derived from pyroglutamic acid. This yielded dimethylaminovinyl groups whose oxidation by NaIO₄ yielded ketones. The indolizinones obtained were reacted in Friedlander condition, to give the scaffold of the desired camptothecins.