Addition of dichlorocarbene to tricyclo[4.3.0.0(3,7)]nona-4,8-diene (b
rexadiene) under conditions of phase transfer catalysis occurs from th
e exo side. Cyclopropyl-allyl rearrangement of intermediate chlorocycl
opropanes yields tricyclo[5.4.0.0(3,8)]undecadienes.