Stereoselective Synthesis of Highly Functionalized Bicyclo[2.1.0]pentanes by Sequential [2 + 1] and [2 + 2] Cycloadditions
Brockton Keen, Christina Cong, Alberto Castanedo, Geraint H. M. Davies, Robert R. Knowles, Huw M. L. Davies
Abstract
This study describes a method for the stereoselective synthesis of highly functionalized bicyclo[2.1.0]pentanes (housanes). The approach utilizes a two-step sequence, a silver- or gold-catalyzed cyclopropenation of alkynes followed by an intermolecular [2 + 2] photocycloaddition reaction with electron-deficient alkenes. The cyclopropenation is an established reaction of aryldiazoacetates. A regioselective [2 + 2] cycloaddition of the cyclopropane was developed using blue LED irradiation, a commercially available photocatalyst as a triplet-sensitizer, and low reaction temperature (-40 °C). The [2 + 2] cycloaddition is highly diastereoselective, and when enantioenriched cyclopropenes are used, it proceeds with enantioretention.