Recent advances in the Dimroth rearrangement: a valuable tool for the synthesis of heterocycles.

The Dimroth rearrangement (DR) is a translocation of two heteroatoms in a heterocyclic system, with or without changing its ring structure. The conversion of a heterocycle to a rearranged isomeric product can give one or two isomers via anintermediate, which selectively or specifically forms one of them as the major one. The stability of the rearranged product is the driving force for its formation, which may lead to the preference of one isomer. The heteroatoms which exchange positions in the rearrangement are S, N, O, and Se. DR can be divided into two types based on the positions of the translocating heteroatom(s): either both in the ring or one in the ring and the other located in an exocyclic position of that ring. This chapter discusses the translocation of heteroatoms between rings in fused heterocycles, and the translocation of exo-and endo-heteroatoms within a heterocyclic ring. In the process of rearrangement by translocation of heteroatoms between rings in fused heterocycles, the translocated heteroatoms are part of the ring. The translocation process changes the position of the heteroatom or the substituent on that ring leading to either a retained or a changed ring structure. The presence of a heteroatom within a five-membered ring and also at an exocyclic position of the adjacent ring is a promoting factor for the rearrangement. In the translocation of exo-and endocyclic heteroatoms in heterocyclic rings the translocation of heteroatom X (endocyclic in the ring) and Y (exocyclic to the ring) takes place during the rearrangement. All the processes are driven by the stability of the product, solvent, aromaticity of the ring, heteroatom valence, and bulkiness of the substituent.