Camptothecine-type alkaloids

Nowadays, many elegant syntheses of steroids, alkaloids and affiliated products are prepared from several five- or six-membered rings. Usually, the preferred steps of such syntheses correspond to the polycyclizations of educts, that contain many multiple bonds, and one or two other functional groups of further educts can often also participate [17]. However, in recent years a new type of the radical chemistry of the isocyanides has been introduced [47].This is illustrated by the one-pot synthesis of (20S)-camptothecin 25 from phenyl isocyanide 13a and 24. Such preparations of cyclopenta-fused quinolines are seldom found in tandem-domino chemistry [16], and this procedure is also closely related to the MCRs of the isocyanides. The previous multistep synthesis of 25 was accomplished by Danishefsky and Volkmann [47b], and uses a large number of linear synthetic steps with only small overall transformations towards the target. 

Cyclizations of substructure type C (Figure 6.3) are also used in total synthesis of natural products bearing five-membered heterocycles, as shown by the example of (S)-camptothecin (83), a natural anticancer agent (Scheme 6.23) [71]. This pentacyclic alkaloid was synthesized in 10 steps, the last one being a 5-exo-intramolecular Mizoroki-Heck reaction. 

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