SYNTHESIS OF OPTICALLY-ACTIVE SILOXYCYCLOPROPANES BY ASYMMETRIC CATALYSIS .2. INFLUENCE OF THE SILYL ENOL ETHER STRUCTURE ON THE ENANTIOSELECTIVE CYCLOPROPANATION WITH METHYL DIAZOACETATE

On the basis of reactions of (Z)-1 -phenyl-1-(trimethylsiloxy)-1-propene (1a) with methyl diazoacetate (2) in the presence of different chiral (Schiff base) copper catalysts 4-Cu we extended our study to the silyl enol ethers 1b-1j. The highest enantiomeric excesses were obtained in most cases with catalyst 4b-Cu. Whereas cyclopropanations of 1-aryl- and 1-alkenyl-substituted silyl enol ethers 1b-1h gave moderate to good enantioselectivities (up to 80%ee), alkyl-substituted olefins 1i and 1j provided less satisfactory results. By deprotonation and alkylation of 3c, 3h, and 3i it could be established that for these cyclopropanes the predominating absolute configuration of cis- and trans-cyclopropanes is equal at C-1, but consequently opposite at C-2. The influence of the silyl enol ether structure on the enantioselectivities achieved is briefly discussed.