cis-Disubstituted Cyclopropanesvia Asymmetric Catalytic Cyclopropenation: Synthesis of Cyclopropyl-dehydroamino Acids and of Dictyopterene C′

Abstract

The cyclopropenation of diethoxypropyne (1) with methyl diazoacetate in the presence of [Rh2{(2S)-mepy}4] (mepy=methyl 5-oxopyrrolidine-2-carboxylate) proceeds with >95% ee. The resulting cyclopropenecarboxylate 2 underwent stereoselective hydrogenation to the cis-cyclopropane 3. Hydrolysis of the acetal function of 3 liberated the formyl cyclopropenecarboxylate 4, which was transformed by Wittig reaction with the phosphonate 5 to afford dehydroamino acid 6 as a mixture of (Z)- and (E)-isomers in various proportions. The (Z)-isomer 6a was hydrolyzed, and the structure and the absolute configuration of the (Z)-dicarboxylic acid 7a were established by X-ray crystallography. The cis-divinylcyclopropane 11 (ee>95%), in turn, was synthesized from 4 via Wittig reaction to afford 8, which was transformed to the aldehyde 10 and subjected to a second Wittig reaction. Thermolysis of 11 afforded (+)-dictyopterene C′ (12) in quantitative yield.