Mechanisms of Cascade Reactions in the Syntheses of Camptothecin-Family Alkaloids: Intramolecular [4(+)+2] Reactions of N-Arylimidates and Alkynes

The key steps of cascade reactions employed In the syntheses of camptothecin-family alkaloids by Fortunak and Yao are intramolecular aza-Diels-Alder (IADA) reactions between in situ generated N-arylimidates and alkynes. The efficiencies of the IADA reactions are different but not well-understood. DFT calculations shown here provide insights Into these two IADA reactions and well-rationalize why hexaphenyloxodiphosphonium triflate (Hendrickson reagent) as an amide-activating reagent Is superior to trimethyloxonium fluoroborate.Chemistry, OrganicSCI(E)PubMed14ARTICLE225302-53051