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Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
Published on: June 21, 2017
Total Synthesis of Euphane Triterpenoids Using Metal-Catalyzed Hydrogen Atom Transfer
Chengying Zhou1, Weitian Qin1, Canhui Tu1
1Key Laboratory of Green Chemistry and Technology of the Ministry of Education, College of Chemistry Sichuan University, 29 Wangjiang Rd., Chengdu, Sichuan, 610064, China.
Abstract:
Euphane triterpenoids are widely distributed in nature and show various intriguing bioactivities, but relatively few synthetic routes to them have been described. Here, we report asymmetric convergent total syntheses of euphanes involving two triterpenoids and two nortriterpenoids: euphol, 25,26,27-trisnor-3β-hydroxy-euphan-24-al, euphorbiumrin D, and 3-oxo-tirucall-7-ene-3,20-dione. The syntheses employ an enantioselective Antilla allylboration and intramolecular radical cyclization to construct ring A, a palladium-catalyzed Liebeskind stannane-thioester coupling to connect ring A with the bicyclic CD system, and a novel radical cascade with metal-catalyzed hydrogen atom transfer (MHAT) to complete the polycyclic architecture. The late-stage syntheses of both triterpenoids feature a diimide reduction and a MHAT/1,5-hydrogen transfer cascade to diastereoselectively forge the C20 and C17 stereogenic centers.
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