Stereochemical Analysis of an Aromatic Triplet Di-?-methane Rearrangement

Catalytic, Enantioselective Cyclopropanation of Allylic Alcohols. Substrate Generality

Catalytic, enantioselective cyclopropanation of a broad range of allylic alcohols and one homoallylic alcohol was carried out. The cyclopropanation reagent employed was bis(iodomethyl)zinc generated by the method of Furukawa, and the chiral promoter used (10 mol %) was the N,N-bis(methanesulfonyl) derivative of (R,R)-1,2-diaminocyclohexane. Three experimental features were found to be critical for the rapid and selective cyclopropanation: (1) use of the ethylzinc alkoxide of the allylic alcohol as the substrate by prior deprotonation of the allylic alcohols by diethylzinc, (2) the formation of the zinc complex of the promoter by prior deprotonation of the bis-sulfonamide with diethylzinc, and (3) the use of added zinc iodide generated in situ from diethylzinc and iodine. The stereoselectivity of cyclopropanation was found to be independent of olefin geometry and worked well for substrates bearing both aliphatic and aromatic substituents at either or both 3-positions of the allylic alcohol. However, a methyl substituent on the 2-position of the allyl alcohol was not well tolerated and led to slow reactions and poor enantioselectivities. A rationale for the observed selectivities is proposed.

Studies on the Diastereoselective Preparation of Bis-cyclopropanes

The identification of two natural products, FR-900848 and U-106305, has stimulated interest concerning the relationship between configurational isomerism, conformational isomerism, and biological activity of polycyclopropanes. Efforts to investigate the relationship between configurational and conformational isomerism through molecular modeling suggest that significantly different three-dimensional structures will result from unique primary structures. Any effort to address these issues demands that stereoselective methods for the preparation of polycyclopropanes be developed. We have investigated the application of zinc-carbenoid cyclopropanation in the presence of chiral dioxaboralanes to the preparation of eight stereochemically unique bicyclopropanes. The trans-vinylcyclopropane starting materials demonstrated very little substrate-induced stereoselectivity, while the cis-vinylcyclopropane demonstrates modest to excellent stereocontrol. A model for the substrate-based stereocontrol is proposed. We also used the spectroscopic data gathered in this investigation to probe the substrate-mediated stereocontrol in the rhodium(II)-catalyzed cyclopropanation of vinylcyclopropanes with ethyl diazoacetate.