Novel Azo Compounds Derived from Thiazolidine-4-one: Synthesis, Metal Complexation, Coloristic Properties, and Biological Activity

Five novel azo compounds containing a fragment of 2-cyanomethylene-4-thiazolidinone suitable as potential permanent hair dyes were isolated and studied by a set of independent physicochemical methods. The compounds act as weak monobasic acids with p<i>K</i> _a values ranging from 7.83 to 11.87, depending on the substituents in the diazo component. In ethanol solutions, the compounds react with transition 3d metal cations to form moderately stable ML₂-type complexes. Dyeing tests on bleached natural hair were conducted using ethanol solutions of the dyes and their mixtures with metal salts and by incorporating the dye into the base of a permanent cream dye commonly used in industrial applications. The colored samples are resistant to 6 washes with shampoo and to irradiation with an ultraviolet lamp with a wavelength of 255 nm for 1 h. The isolated azo compounds showed medium activity against Gram-negative microorganisms. In silico and in vivo assessments indicate that the resulting dyes belong to classes 4 and 5 of acute toxicity when administered orally and subcutaneously.