Intramolecular Hydroxycyclopropanation of ω-Vinyl Carboxylic Esters

摘要

Reactions of .Grignard reagents in the presence of transition metals have found numerous synthetic applications in organic chemistry. Recently, use of organotitanium and zirconium compounds has resulted in the design and development of new stereoselective carbon--carbon bond-forming reactions. As part of our research program in cyclopropane-mediated natural product synthesis, we became interested in the Kulinkovich hydroxycyclopropanation (1 -- 2), which involves treatment of a carboxylic ester withan excess (3 equiv) of Grignard reagent at -78 to 0 °C in the presence of Ti(Oi-Pr)_4 (1 equiv), affording cis-1,2-dialkylcyclopropan-l-ols in good yields (Scheme I). Herein we report an intramolecular version of the Kulinkovich hydroxycyclopropanation, which entails treatment of ω-vinyl carboxylates with n-BuMgCl in the presence of Ti(Oi-Pr)_4 or ClTi(Oi-Pr)_3.