Squaric acid bioisostere

Scheme 33 Squaric acid bioisostere synthesis of amino squaric acid [133-135]...

In a smdy undertaken on antagonists of the dihydropyridine neuropeptide Yi receptor, common bioisosteres such as thiourea and cyanoguanidine were examinated as urea replacement (Figure 15.37) and also more uncommon derivatives like squaric acid and thiadiazole oxide. 

Keywords Bioisostere Cyclobutenone Electrocyclic reaction Reactive intermediate Squaric acid... 

Amino squaric acids 186 and 188, the relatives of natural amino acids, represent an intuitive example of bioisostere, although the heterocycle is absent in these molecules. These were obtained by radical reaction of 2-stannylcyclobutenedione [133] and carbanion addition to squarate ester [134,135] (Scheme 33). 

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