In certain cases, these bicyclic systems have been specifically designed to mimic the geometries of arenes; examples include bridgehead-disubstituted bicyclo[1.1.1]pentanes (BCPs, Fig. ) and bicyclo[3.1.1]heptanes (BCHeps), which act as bioisosteres for para-and meta-substituted benzenes respectively due to the equivalent exit vectors of their bridgehead substituents to their aromatic counterparts; examples include the BCP analogue of the g-secretase inhibitor avagacestat and the BCHep analogue of the anticancer agent sonidegib 4 . These motifs are most commonly accessed by ring-opening reactions of [n. 1.1]propellanes (1) , hydrocarbons that feature a strained C-C bond to which are fused two cyclopropanes and an (n+2)-membered ring; these are most commonly prepared from tetrahalogenated precursors 2 on treatment with organolithium reagents 4, . Under radical or anionic conditions, propellane ring-opening via cleavage of the strained central C-C bond enables the installation of a diversity of substituents at the bridgehead positions .