Recent Advances in Transition‑Metal‑Catalyzed Formal [5+1] Annulations Toward Six‑Membered Rings

ABSTRACT

Transition metal‐catalyzed [5+1] annulation reactions represent one of the efficient strategies for the construction of six‐membered (hetero) cycles. In recent years, diverse catalytic systems based on rhodium, copper, palladium, and ruthenium have been established successively, providing versatile access to a broad spectrum of substituted six‐membered (hetero) cycles. In this review, according to the classification by the types of transition metal catalysts employed, we systematically summarize representative studies in this field from 2010 to the present, with an emphasis on the reaction characteristics, substrate scope, and mechanistic investigations of each catalytic system. Finally, this review summarizes the current challenges and future directions in this field, with the aim of providing a reference for the construction of six‐membered (hetero) cycles via [5+1] annulation reactions.