Recent advances in ring transformation of five-membered heterocycles

Abstract

Ring transformations of heterocyclic rings by temporary opening and subsequent closure to a new molecule are of particular interest both synthetically and theoretically. Only rearrangement of five-membered heterocycles 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 expansions" where the sizes of the product rings are smaller or larger, respectively, than those of the starting compound, and (d) "pseudo ring transformations" 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 orirignal ring. Reaction mechanism of the most interesting ring transformations are also discussed.