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Asymmetric Synthesis of Neolignans (−)-epi-Conocarpan and (+)-Conocarpan via Rh(II)-Catalyzed C−H Insertion Process and Revision of the Absolute Configuration of (−)-epi-Conocarpan

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journal contribution
posted on 05.06.2009, 00:00 by Yoshihiro Natori, Hideyuki Tsutsui, Naoki Sato, Seiichi Nakamura, Hisanori Nambu, Motoo Shiro, Shunichi Hashimoto
Catalytic asymmetric synthesis of neolignan natural products (−)-epi-conocarpan and (+)-conocarpan has been achieved by exploiting an enantio- and diastereoselective intramolecular C−H insertion reaction to construct a cis-2-aryl-2,3-dihydrobenzofuran ring system as a key step. The C−H insertion reaction of 5-bromoaryldiazoacetate catalyzed by Rh2(S-PTTEA)4, a new dirhodium(II) carboxylate complex that incorporates N-phthaloyl-(S)-triethylalaninate as chiral bridging ligands, provided 2-aryl-5-bromo-3-methoxycarbonyl-2,3-dihydrobenzofuran with exceptionally high diastereoselectivity (cis/trans = 97:3) and high enantioselectivity for the cis isomer (84% ee).