Cobalt-Mediated [2+2+2] Cycloaddition of Alkynes to the Enamine Double Bond: A Formal Total Synthesis of γ-Lycorane
作者:Douglas B. Grotjahn、K. Peter C. Vollhardt
DOI:10.1055/s-1993-25910
日期:——
In the presence of CpCo(CO)2, bis(trimethylsilyl)acetylene reacts with the alkyne and alkene functions of 1-(6-ethynyl-1,3-benzodioxol-5-ylcarbonyl) azacyclopent-2-ene (5c) to create CpCo-cyclohexadiene complexes embodying the polycyclic framework of γ-lycorane. A novel regioselective fluoride-mediated removal of the 2-trimethylsilyl group from these 1,2-bis (trimethylsilyl)-substituted complexes under mild conditions allowed their successful conversion into 1,12b-didehydro-7-oxo-γ-lycorane (10 steps from 6-bromo-1,3-benzodioxole-5-carboxylic acid in 9.4% overall yield). Unusual problems associated with the application of palladium catalysts in the construction of 2-functionalized alkynylarenes related to 5c were circumvented. In addition, CpCo(CO)2 catalyzes the isomerization of the double bond of 1-acylazacyclopent-3-enes to the 2-position, but apparently is ineffective in the isomerization of N-(cyclopropyl-methylidene)amines to azacyclopent-2-enes.
在 CpCo(CO)2 的存在下,双(三甲基硅基)乙炔与 1-(6-乙炔基-1,3-苯并二恶茂-5-羰基)氮杂环戊-2-烯 (5c) 的炔和烯官能团发生反应,生成体现 δ³-lycorane 多环框架的 CpCo-Cyclohexadiene 复合物。在温和的条件下,通过新型的区域选择性氟化物介导,从这些 1,2-双(三甲基硅基)取代的络合物中去除 2-三甲基硅基,成功地将这些络合物转化为 1,12b-二脱氢-7-氧代δ-lycorane(从 6-溴-1,3-苯并二恶茂-5-羧酸经过 10 个步骤制得,总产率为 9.4%)。 在构建与 5c 有关的 2-官能化炔烃时,与钯催化剂应用相关的不寻常问题得到了解决。此外,CpCo(CO)2 还能催化 1-酰氮杂环戊-3-烯的双键向 2-位异构化,但对 N-(环丙基亚甲基)胺向氮杂环戊-2-烯的异构化显然无效。