Umpolung Activation of Bicyclobutanes via N‐Heterocyclic Carbene Catalysis

Abstract Bicyclo[1.1.0]butane (BCB) has predominantly been explored as an electrophile or radical acceptor owing to its inherent polarity. However, its reactivity as a nucleophile remains largely unexplored. Herein, we report an umpolung strategy that reverses the innate polarity of bicyclobutane aldehydes via N‐heterocyclic carbene (NHC) catalysis. This transformation enables the construction of novel rigidified lactones and lactams via coupling with diverse electrophilic partners, including aldehydes, ketones, and imines. The synthetic versatility of this method highlights its potential for applications in medicinal chemistry by providing new building blocks with defined exit vectors.