Synthese optisch aktiver 2‐Siloxycyclopropancarbonsäureester durch asymmetrische Katalyse, I. Katalysatoreinfluß auf die Cyclopropanierung von (Z)‐1‐Phenyl‐1‐(trimethylsiloxy)prop‐1‐en

Abstract

Synthesis of Optically Active Siloxycyclopropanes by Asymmetric Catalysis, I. – Influence of the Catalyst on the Cyclopropanation of (Z)‐1‐Phenyl‐1‐(trimethylsiloxy)prop‐1‐eneThe cyclopropanation of silyl enol ether 1a with methyl diazoacetate (2a) and diazoesters 2b–d in the presence of optically active copper salicylimine complexes 4‐Cu was systematically studied. Up to 88% enantiomeric excess in products 3 were obtained by employing the appropriate reaction conditions and the optimal catalyst ligands. Thus, for the first time respectable optical yields were achieved in asymmetric cyclopropanations of silyl enol ethers with easily available copper‐Schiff base catalysts. The ring opening of the separated diastereomers of 3a employing nBu4NF provided methyl γ‐oxocarboxylate 6a in good optical purity. This demonstrates that this process occurs without racemization and also that 3a is formed with the same absolute configuration at C‐1.