Toward an improved ring-forming method: I. 1,1-Dihalo-1-ethenyl vinylcyclopropanes--useful building blocks for the stereoselective synthesis of highly functionalized cyclopentanes. II. Further attempts toward the synthesis of the Brefeldin class of natura

摘要

Methods for improving stereoselectivity in a novel radical-mediated cyclopentannulation reaction were examined. Complexation protocols using $beta$-cyclodextrin did not lead to enhancements in the stereochemical outcome of ring-closure. Covalently modified three atom components did provide stereoselectivity enhancements. Thus, upon reaction with activated olefins, 1,1 dihalo-2-ethenyl vinylcyclopropanes gave rise to moderate yields of functionalized cyclopentanes with concomittantly high levels of stereoselectivity favoring cis stereochemistry relative to the C-(1)-C-(5) appendages. This general methodology improvement was applied in an approach to the preparation of the brefeldin class of natural products. In a related study, the feasibility of controlling remote relative stereoselectivity in the 6-hexenyl radical cyclization of 7-cyclopropyl-6-heptene-2-thiol derivatives was examined.