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Relay Synthesis of AzadirachtinContributor(s):Philip Kocienski
University of Cambridge, UK
Synthesis of Azadirachtin: A Long but Successful Journey
Angew. Chem. Int. Ed. 2007, 46: 7629-7632
23 January 2008 (online)
Claisen rearrangement - radical cyclization - peroxidation - allenes
Azadirachtin is an insect antifeedant. A major challenge in this 22-year odyssey was the construction of 16 contiguous stereogenic centers seven of which are quaternary. The excerpt depicted here focuses on the Claisen rearrangment used to construct the congested C8-C14 bond.
Compounds A and G were obtained by total synthesis and by degradation of azadirachtin. The degradation route enabled exploration of the difficult closing stages of the synthesis. Note the harsh conditions required to effect the difficult epoxidation F→G.