Synthesis of 4-(5-Nitrophenylfur-2-yl)-1,2,3-thia- and Selenadiazoles

Arylation of 2-acetylfuran with o-, m-, and p-nitrophenyldiazonium salts under the conditions of the Gomberg-Bachmann reaction has afforded the corresponding 5-(nitrophenyl)-2-acetylfurans. Their carbo-ethoxyhydrazones have undergone the cyclization into stable 4-(5-nitrophenylfur-2-yl)-1,2,3-thiadiazoles under the conditions of the Hurd-Mori reaction. Analogous semicarbazones have afforded the corresponding selenadiazoles upon oxidation with selenium dioxide. The analysis of electronic absorption spectra of the obtained hybrid heterocycles has shown that the conjugation of the phenyl and the furan ring in o-nitrophenyl derivatives is distorted due to steric hindrance, whereas the effect of direct polar conjugation leading to strong bathochromic shift of the absorption band has been observed in the case of the p-nitro derivatives. The position and intensity of the bands in the electronic absorption spectra of the studied compounds are determined by electronic as well as steric factors. The difference in the length of conjugation chain determined by the position of the nitro group in the phenyl fragment also contributes to the observed trend. The introduction of selenadiazole fragment instead of thiadiazole one has caused slight bathochromic shift of the band in the electron absorption spectra.