Recent Advances in Ring Transformations of Five-Membered Heterocycles and Their Fused Derivatives

Ring transformations of five-membered heterocycles and their benzologues for the recent 10 years are discussed and reviewed. These transformations are classified into four groups: (a) “classical ring transformations”, where the starting and resulting ring system is of the same size, but the heteroatoms and/or their positions have been changed; (b) “degenerate ring transformations”, where during the course of the transformation the starting compound and product have the same ring system, but the reaction proceeds by a ring opening and subsequent ring closure process; (c) “ring contractions” and “ring enlargements”, where the sizes of the product rings are smaller or larger, respectively, than those of the starting compound; and (d) “pseudo ring transformation” or “ring-chain transfer”, where the process is formally a ring transformation, but is realised by a ring closure of a side chain of the starting heterocycle and opening of the original ring. Reaction mechanisms of the most interesting ring transformations are also discussed.