Recent Developments in the Ring‐Chain Tautomerism of 1,3‐Heterocycles

Some important results from the past seven years on the ring‐chain tautomerism of 1,3‐heterocycles are reviewed, including substituent effects on the tautomeric equilibria and synthetic applications of this phenomenon. The structures and reactivities of numerous five‐ and six‐membered, saturated, N‐unsubstituted 1,3‐X,N‐heterocycles (X = O, S, NR) can be characterized by the ring‐chain tautomeric equilibria of the 1,3‐X,N‐heterocycles and the corresponding Schiff bases; this is often exploited advantageously in different areas of organic synthesis, and also in physical, medicinal and peptide chemistry. The selectivity of certain transformations can be explained on the basis of the ring‐chain tautomeric equilibration of the intermediates, followed by a shift in the equilibrium. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)