Two-Directional Synthesis of Polycyclopropanes. An Approach to the Quinquecyclopropane Fragment of U-106305

The stereoselective preparation of three stereoisomeric tercyclopropanes and a quinquecyclopropane was investigated. Two of the tercyclopropanes were C 2 -symmetric and were prepared efficiently through the two-directional application of Charette's reagent-stereocontrolled cyclopropanation methodology. The nonsymmetric tercyclopropane was prepared by an iterative one-directional application of the same reagent-mediated cyclopropanation method. It was shown that the reagent-controlled transformations are far more effective for the stereoselective preparation of the tercyclopropanes than are the reactions which rely upon the influence of the substrate stereocenters. A C 2 -symmetric quinquecyclopropane, which possesses the repeating trans-syn stereochemistry, was prepared by iterative application of the two-directional strategy.