Five-membered Ring Heterocycles by Reacting Enolates with Dipoles

The synthetic utility of enolates in the heterocycles synthesis constitutes the main topic of this review. Appropriate enolates, in a number of synthetic approaches, are easily formed in situ from a variety of carbonyl compounds (aldehydes, ketones, amides, etc.) and used for the ring construction of (poly)substituted heterocycles. Often, the synthesis of five-membered rings such as isoxazoles, triazoles, pyrazoles, etc. has been performed by using these intermediates through a two-step or even by one pot procedures. The selectivity and the versatility of enolates intermediates will be discussed, with particular attention to those methodologies useful for preparation of heterocycle containing pharmacologically active molecules.