Ring transformations of heterocycles

The richness and complexity of the present section is considerable. Almost any heterocycle conveniently substituted can be transformed into another chosen ring system, and this is shown in the two excellent volumes of van der Plas ( Ring Transformations of Heterocycles ) (B-73MI4Q4Q2). The arrangement of the aforementioned book (from the starting heterocycle point of view) does not suit this section and for the purposes of this chapter an alternative classification has been selected. When no explicit references are given, the material has been taken from (B-73MI4Q4Q2). 

Ring transformations of heterocycles leading to isoxazoles have been briefly reviewed (79AHC(25)147). The heterocycles undergoing such transformations may be divided into three classes. 

The study on ring transformations of heterocycles is an attractive subject of research for many years. This great interest is due to the fact that these reactions are usually easily performed and that by these ring transformations heterocycles can be synthesized which are otherwise difficult to obtain. Moreover, unravelling the course of the ring transformation has always been a challenging problem and has attracted the interest of many chemists it requires studies on substituent and solvent effects, labeling and NMR studies, kinetic studies and quantum chemical calculations. In the course of... 

HC van der Plas, Ring Transformations of Heterocycles, Vol. 1. Academic Press, New York, 1973. 

B. Classification of Degenerate Ring Transformations of Heterocyclic Systems... 

Degenerate ring transformations of heterocyclic systems that have been discovered to date can be classified in two main groups (1) Nucleophilic... 

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