Recent advances in ring transformations of five-membered heterocycles and their fused derivatives

Ring transformations of five-membered heterocycles and their benzologues fo r the recent 10 years are discussed and reviewed. These transformations are classified into four groups: (a) "classical ring transformations", where t he starting and resulting ring system is of the same size, but the heteroat oms and/or their positions have been changed; (b) "degenerate ring transfor mations", where during the course of the transformation the starting compou nd and product have the same ring system, but the reaction proceeds by a ri ng opening and subsequent ring closure process; (c) "ring contractions" and "ring enlargements", where the sizes of the product rings are smaller or l arger, respectively, than those of the starting compound; and (d) "pseudo r ing transformation" or "ring-chain transfer", where the process is formally a ring transformation, but is realised by a ring closure of a side chain o f the starting heterocycle and opening of the original ring. Reaction mecha nisms of the most interesting ring transformations are also discussed.