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Synthesis 1993; 1993(6): 579-605
DOI: 10.1055/s-1993-25910
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Cobalt-Mediated [2+2+2] Cycloaddition of Alkynes to the Enamine Double Bond: A Formal Total Synthesis of γ-Lycorane

Douglas B. Grotjahn* , K. Peter C. Vollhardt
  • *Department of Chemistry, University of California at Berkeley, and the Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720, USA
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Publication Date:
17 September 2002 (online)

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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.

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