| Summary: | Synthesis of the first representative of a 5-formyl-1<i>H</i>-1,2,3-triazole-4-carboxylic acids series – 1-(4-ethoxyphenyl)-5-formyl-1<i>H</i>-1,2,3-triazole-4-carboxylic acid was performed. The 1-azido-4-ethoxybenzene was chosen as a starting reagent in a two-step synthesis, which reacted with the ethyl 4,4-diethoxy-3-oxobutanoate under base catalysis to form ethyl 5-(diethoxymethyl)-1-(4-ethoxyphenyl)-1H-1,2,3-triazole-4-carboxylate with protected formyl and acid groups. By the subsequent saponification of the ester group and removing of acetal protection, the target 1-(4-ethoxyphenyl)-5-formyl-1H-1,2,3-triazole-4-carboxylic acid was obtained. It has been found that the free acid form predominated in the solution under its cyclic 6-hydroxy-1,6-dihydro-4<i>H</i>-furo[3,4-<i>d</i>][1,2,3]triazol-4-one tautomer. According to <sup>1</sup>H NMR, cyclic hemiacetal is about 20%.
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