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,^[1] 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.