Saturated polycyclic carbocycles have gained growing attention in both medicinal and organic chemistry. Molecules incorporating these motifs exhibit enhanced pharmacokinetic and physiochemical properties compared to more common Csp 2 -rich bioactive synthetic compounds, and have become privileged candidates for drug-discovery. The increased conformational rigidity of these polycyclic frameworks is especially important as it can lead to improved affinity to their biological targets, as demonstrated also in many bioactive natural products. Accordingly, the efficient construction of bicycloalkanes as core elements of more complex systems has become an important goal for synthetic chemists, although it demands addressing the challenges coming from their inherent complexity. In the past few years, the use of carbonyl-substituted bicyclo [1.1.0]butanes (BCBs) in strain-releasing annulations has emerged as a modular approach for the generation of bicyclic carbocycles and their heterocyclic analogs. The synthesis of bicyclo [2.1.1]hexanes (BCHex's) through the formal (2π+2σ) cycloaddition of BCBs has been extensively studied to access new bioisosteres of the benzene ring. Following on the seminal reports by the groups of Glorius and Brown, several methods have appeared that rely on radical pathways, either under light induced energy transfer (Scheme 1.A.1: Glorius, Brown, Bach ) or electron-transfer conditions (Scheme 1.A.2: Li, Procter, Wang ). Lewis acid catalysis has also proven effective to promote annulations following a polar mechanism (Scheme 1.A.3: Leitch, Studer, Glorius, Deng ).