1-Bromo-2-ethoxycyclolithium: A Synthetic Equivalent of 2-Lithio or 3-Lithiopropenal. Application to the Synthesis of Juvenile Hormone (JH-II), β-Sinensal, and Jasmonoids.

Article abstract of DOI:10.1246/bcsj.57.1935

The ethyl vinyl ether-dibromocarbene adduct was lithiated with butyllithium at -95 deg C in tetrahydrofuran.The resulting lithium carbenoid 3 was allowed to react with various electrophiles to give 1-substituted trans-1-bromo-2-ethoxycyclopropanes (1) in good yields.The trans relationship of Br and OEt groups was found particularly pertinent to the ethanolysis of 1 producing 2-substituted propenal diethyl acetal derivatives.The reaction has been applied to 1-methoxycyclohexene-dibromocarbene adducts, giving rise hereby 2-substituted 2-cyclohepten-1-one dimethyl acetals under ring enlargement.T he transformation has been utilized in the synthesis of a homoterpenoid (JH-II) or a terpenoid (β-sinensal) structure by S_N2' substitution of allylic acetates with lithium dimethylcuprate(I) or iron pentacarbonyl respectively.The reaction products of 3 with aldehydes are oxidized with dimethyl sulfoxide to give cyclopropyl ketones whose ethanolysis in the presence of boron trifluoride ether complex gives β-bromo γ-keto aldehyde acetals.Debromination followed by acidic hydrolysis produces γ-keto aldehydes serving as precursors of dihydrojasmone and cis-jasmone.