Esters of 4-formylpyrazol-3-carboxylic acids

The scientific publications devoted to chemistry of the synthetically promising bifunctional derivatives of pyrazole – esters of 4-formylepyrazole-3-carbonic acids have been generalized and systematized. A special attention has been given to description of two main synthetic approaches: 1) processes of the conjugated formation of aldehyde and ester groups, on one hand, and the pyrazolic system, on the other; 2) synthetic strategies targeted on functionalization of the pyrazolic nucleus. It has been shown that the first approach is performed usually by the scheme [3+2] meaning in situ cyclojoining of the generated nitrilimines or esters of diazoacetic acid with formylacetylenes or their derivatives followed by cyclization with simultaneous formylation of hydrazones and semicarbazones of pyruvic acid by Wilsmaer-Haack reagent. The second approach deals with formylation of esters of pyrazole-3-carbonic acids according to Wilsmaer-Haack reaction. As seen from the analysis of chemical properties of esters of 4-formylpyrazole-3-carbonic acids, the aldehyde group is the most typical target of their chemical transformations. Such transformations are reactions with nitrogen-containing nucleophiles, alkenylation, oxidation, reduction and other multicomponent reactions that can be considered as usual approaches to structural functionalization of the pyrazolic nucleus. Cyclocondensations are also important in this context since these processes involve formyl, ester and other functional groups obtained from them. These substances appeared to be effective substrates for construction of some biologically active pyrazolo[4,3-d]pyrimidine and pyrazolo [3,4-d]pyridazynic systems.