Recent Advances in Base-Mediated [5 + 1] Annulation Reactions

The [5 + 1] annulation is a highly efficient strategy for constructing six-membered carbocycles, heterocycles, and spiro/fused ring skeletons, playing a crucial role in the synthesis of natural products, pharmaceuticals, and functional molecules. Traditional transition-metal-catalyzed methods often suffer from limitations such as metal contamination, high costs, and tedious workup procedures. In contrast, base-mediated/catalyzed systems have emerged as a prominent research hotspot owing to their metal-free nature, mild conditions, excellent functional group tolerance, and green economy. This review systematically summarizes the recent advances in base-mediated [5 + 1] cycloadditions over the past two decades. Based on the catalytic systems, the reactions are categorized into organic bases (tertiary amines, N-heterocyclic carbenes, organophosphines) and inorganic bases (potassium tert-butoxide, carbonates, and potassium methoxide). The cyclization modes involving various five-atom synthons with one-carbon/one-nitrogen synthons are discussed in detail. This review aims to provide a comprehensive theoretical reference for the future design of novel base-mediated cycloadditions and the construction of complex cyclic pharmaceutical skeletons.