Alkenylidenecyclopropanes (ACPs) as Three-Carbon Synthons in Transition-Metal-Catalyzed Cycloadditions

Alkenylidenecyclopropanes (ACPs) have emerged as versatile and highly reactive building blocks in transition-metal-catalyzed transformations. Their strained cyclopropane framework, combined with an exocyclic alkene, enables diverse bond-activation pathways and promotes efficient cycloaddition reactions. In recent years, ACPs have been widely developed as three-carbon synthons in a variety of higher-order cycloadditions. This review provides a systematic overview of transition-metal-catalyzed ACP transformations, focusing on their applications in [3+2], [3+2+2], [3+2+1], [4+3], and [4+3+2] cycloaddition reactions with reaction partners such as alkenes, alkynes, carbonyl compounds, imines, dienes, and carbon monoxide. Particular attention is given to mechanistic aspects, including cyclopropane ring-opening processes and the formation of key metal–carbene and π-allyl intermediates that govern reactivity and selectivity. Factors influencing regioselectivity, stereoselectivity, and catalyst design are also discussed. The synthetic potential of ACP chemistry is illustrated through representative applications in the total synthesis of complex natural products, such as pyrovellerolactone and (+)-zizaene. Overall, this review highlights recent advances in ACP-based cycloaddition strategies and emphasizes their growing significance in modern synthetic chemistry.