Stereoselective Synthesis of Polysubstituted Spiropentanes.

A new approach to polysubstituted spiropentanes is developed through a regio- and diastereoselective carbometalation of sp2-disubstituted cyclopropenes. The control of selectivity originates from a combined syn-facial diastereoselective carbometalation with a regio-directed addition. The regio-controlling element subsequently serves as a leaving group in an intramolecular nucleophilic substitution. This method allows the preparation of various polysubstituted spiropentanes with up to five contiguous stereocenters.