Dienediolates of carboxylic acids in synthesis. Recent advances

Resonance-stabilized carboxylic acids dienediolates result from O,C-deprotonations of alpha,beta-unsaturated or beta,gamma-unsaturated carboxylic acids, the second deprotonation occurring easily, at an allylic position, than on saturated congeners. This pi-extended enolates can react through their alpha or gamma carbon atoms, leading to single or predominant compounds when allowed to react with electrophiles under adequate conditions. The case of deprotonation of carboxylic acids permits for a range of bases and countercations to be employed, both factors altering the reactivity of ensuing dianions, and new conditions are described continuously. This versatile building blocks have this quality as an additional advantage, from an experimental point of view, that the products are easily isolated from the reaction mixture with high purity by means of a pH control in the reaction work up. This review tries to focus on the effort to control the regioselectivity, diastereoselectivity and enantioselectivity of the addition of dienediolates of carboxylic acids to different electrophiles. 2-Alkylbenzoic acids and their heterocyclic congeners have also been included as they may be considered as pi-extended dianions, the second deprotonation occurring at a benzylic site. The aim of this review is to make the reader up to date with the recent and numerous developments in the use of dienediolates of carboxylic acids in synthesis. The period up to and including 1992 has been previously reviewed [1-4], most recently by Thompson [5, 6], and will not be covered in detail in this review.