Recent advances in the cyclization reactions of pyridinium 1,n-zwitterions (n = 4 and 5): scope and mechanism

Nitrogen- and sulfur-containing heterocyclic compounds widely exist in bioactive natural products and drug molecules with unique bioactivity. In the past few decades, great efforts have been devoted to developing convenient and efficient strategies for the synthesis of these compounds. Pyridinium zwitterions with unique reactivity play a significant role in the construction of nitrogen- and sulfur-containing heterocycles. In recent years, pyridinium zwitterions have been extensively used in cyclization reactions for the synthesis of novel heterocyclic compounds. This review summarizes the recent advances in cyclization reactions involving pyridinium 1,n-zwitterions (n = 4 and 5) and highlights the applications of these zwitterions in the efficient construction of heterocycles.