Intramolecular Asymmetric Cyclopropanation Using Air Stable Alkylboronic Esters

The preparation of polysubstituted bicyclo[3.1.0]hexanes starting from air stable substituted pent-4-en-1-ylboronic esters has been investigated. The method involves a Matteson homologation with LiCHCl2 leading to intermediates 1-chlorohex-5-en-1-yl boronic esters. The intramolecular cyclopropanation step was performed in a one-pot process. With pinacol boronic esters, the cyclopropanation step was either performed thermally at 140 °C or at 70 °C after in situ transesterification to a catechol boronic ester. This last approach is suitable for the preparation of enantioenriched bicyclo[3.1.0]hexanes using either chiral auxiliary control or by taking advantage of the sparteine controlled enantioselective boroalkylation of alcohols.