1-Bromo-2-ethoxycyclopropyllithium: A synthetic equivalent of 2-lithio- or 3-lithiopropenal. Application to the synthesis of juvenile hormone (JH-II), .BETA.-sinensal, and jasmonoids.

概要

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The ethyl vinyl ether-dibromocarbene adduct was lithiated with butyllithium at −95 °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. The transformation has been utilized in the synthesis of a homoterpenoid (JH-II) or a terpenoid (β-sinensal) structure by SN2′ 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.
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