α-Selective Ring-Opening of Bicyclo[1.1.0]butanes to Access Spirocyclobutyl Lactams

A variety of spirocyclobutyl lactams were conveniently prepared by the Brønsted acid-catalyzed α-selective ring-opening of bicyclo[1.1.0]butane (BCB) within 15 min under mild conditions without any photocatalysts, oxidants, metal catalysts, or radical initiators. This reaction involved the rapid ring-opening of BCB to generate carbocation, which further efficiently promoted intramolecular Friedel-Crafts alkylation or intramolecular spirocyclization to access diverse spirocyclobutyl lactams.