開催日: 2017/09/20 - 2017/09/22
Avenaol (1), which was isolated from the allelopathic plant black oat (Avena strigosa Schreb.) by Yoneyama and coworkers, is the first reported natural C20 germination stimulant structurally related to strigolactones (SLs). The structure of avenaol is characterized by a bicyclo[4.1.0]heptanone skeleton with an all-cis substituted cyclopropane on which the three main substituents are positioned in the same direction. Avenaol shows potent germination-stimulating activity for Phelipanche ramose seeds, but low activities for Striga hermonthica and Orobanche minor. To date, no total synthesis of avenaol (1) has been reported. Because of its unique structure, important biological activity, and limited availability from the natural source, we started a synthetic project of avenaol. In this presentation, we would like to present the first total synthesis of avenaol (1), which features the following reactions: (i) Rh-catalyzed intramolecular cyclopropanation of an allene (6a → 11a), (ii) Ir-catalyzed stereoselective double-bond isomerization (5d → 4), (iii) differentiation of two hydroxymethyl groups by intramolecular SN1 reaction (3 → 16), and (iv) regioselective C-H oxidation of tetrahydropyran (17 → 19). This synthetic route through an alkylidenecyclopropane effectively avoids undesired side reactions including ring-opening of a cyclopropane. The proposed structure of avenaol, especially the all-cis-substituted cyclopropane was confirmed to be correct by our total synthesis.