Reaction Paradigms that Leverage Cycloaddition and Ring Strain to Construction Bicyclic Aryl Bioisosteres from Bicyclo[1.1.0]butanes.

Within a medicinal chemist’s toolbox, one of the most effective strategies to improve the overall properties of a biologically active compound is bioisosteric replacement. Ever since the first example of replacing benzene with a bicyclo[1.1.1]pentane (BCP) group was published in the late 1990s, the medicinal chemistry community has continually been expanding the scope of such phenyl bioisosteric replacements. Recent interest from academia has focused on novel synthetic strategies to access C(sp3)‐rich bicyclic hydrocarbons with expanded ring sizes. Herein, we summarize some of these transformations and reveal that most rely on strain releasing cycloadditions with bicyclo[1.1.0]butane (BCB) and bicyclo[2.1.0]pentane (housane). We have organized this review based on the mechanism of such strain release strategies, namely, carbene cycloadditions, energy transfer photocatalyzed cycloadditions, electron transfer catalyzed cycloadditions, and polar cycloadditions.