Pyrazoles have been studied for over a century as an important class of heterocyclic compounds and continue to attract considerable interest due to the broad range of biological activities they possess. The electrophilic cyclization of the acetylenic hydrazones initiated by molecular iodine could provide new ways of synthesizing biologically active 4-iodopyrazole derivatives, which are important precursors for the synthesis of highly substituted pyrazole derivatives. For this reason, we investigated the synthesis of 4-iodopyrazole derivatives, such as 1-aryl- 5-alkyl/aryl-4-iodopyrazoles, starting from phenylhydrazine and ,-acetylenic aldehyde derivatives. Initially, ,-acetylenic aldehydes were synthesized by formylation reaction of corresponding alkynes with DMF. Then, hydrazone derivatives of these aldehydes were prepared by heating them with phenylhydrazine in a neat manner at 55 &deg / C for 5 h. Finally, acetylenic phenyl hydrazone derivatives were subjected to electrophilic cyclization by treating with excess molecular iodine at 80 &deg / C for 3 h. Although electrophilic cyclization is commonly used in organic chemistry, it has not been employed for the cyclization of acetylenic phenyl hydrazones to pyrazole derivatives. Under optimized conditions, these reactions afforded 1-aryl-5-alkyl/aryl-4-iodopyrazole derivatives in moderate to good yields as the single or the major product of the reactions. In some cases, 1-aryl-5-alkyl/arylpyrazole derivatives resulted from these reactions as minor products. In conclusion, 4-iodopyrazole derivatives were synthesized for the first time directly from acyclic starting materials, ,-acetylenic phenylhydrazones and iodine, via electrophilic cyclization.

Synthesis of five-membered heteroaromatic compounds such as pyrazoles, isoxazoles and 1,2,4-oxadiazoles are important for pharmaceutical industry and material science due to their applications. Although there are many methods to prepare such compounds, new variants continue to appear since they exhibit a wide range of biological and medicinal activities. In this thesis, new methods were developed for the synthesis of 4-iodopyrazoles, pyrazoles, isoxazoles, 1,2,4-oxadiazoles and/or 1,2,4-oxadiazepines. In the first part of the study, electrophilic cyclization of &alpha / ,&beta / -alkynic hydrazones by molecular iodine and copper iodide were investigated as new ways for the synthesis of 4-iodopyrazoles and pyrazoles, respectively. Initially, &alpha / ,&beta / -alkynic hydrazones were prepared by the reactions of propargyl aldehydes and ketones with hydrazines. Then &alpha / ,&beta / -alkynic hydrazones were treated with molecular iodine in the presence of NaHCO3, which afforded 4-iodopyrazoles in good to excellent yields. Subsequently, the same reactions were carried out with CuI in the presence of NEt3, which furnished corresponding pyrazoles in good yields. Moreover, ferrocenyl-substituted 4-iodopyrazoles and pyrazole derivatives were synthesized from corresponding