Boron-Substituted Difluorocyclopropanes:  New Building Blocks of <i>gem</i>-Difluorocyclopropanes

Cycloaddition of difluorocarbene to alkenyl boronates <b>3</b> gave boron-substituted <i>gem</i>-difluorocyclopropanes <b>2</b> in stereospecific fashion. Upon treatment with lithium carbenoids, cyclopropyl boronates <b>2</b> underwent one-carbon homologation to afford a variety of <i>gem</i>-difluorocyclopropanes in good yields.