Isothiocyanates. XXX. Synthesis, Ultraviolet and Infrared Spectra of 3-Substituted Rhodanines

Our preceding papers [1 — 3] referring to the quantitative study of nucleophilic addi­ tion reactions of isothiocyanates (ITC) deal with the reactivity of alkyl and particu­ larly aryl isothiocyanates with —OH ions, amino groups (of various amines and amino acids), and thioglycolate. Results thus obtained together with further analogous ones from investigation of the reactivity of ITC with other thiols indicated the enormously high reactivity of ionized thiols when compared with amino groups of similar compounds (addition of amino groups in nonprotonized form). In this very case isothiocyanates gave with thioglycolate iV-substituted thiocarbamoylmercaptoacetic acids or corres­ ponding 3-substituted rhodanines as final products, depending on reaction conditions. N-Substituted thiocarbamoylmercaptoacetic acids could be quantitatively transformed into corresponding rhodanines (cyclization in acid medium). Similarly, isothiocyanates could be obtained from rhodanines upon alkaline hydrolysis of these five-membered rings to give i\-substituted thiocarbamoylmercaptoacetic acids, which are unstable under the given reaction conditions. In this connection it seemed useful to examine the some time ago described prepara­ tion of 3-substituted rhodanines from isothiocyanates and thioglycolic acid and investi­ gate the recently described types of aliphatic and particularly aromatic ITC. Physicochemical properties of rhodanines including their stability and reactions in various media in relation to their structure is important, because these compounds belong to the group of so-called synthetic producers of ITC, i.e. substances liberating biologically active ITC. This paper deals with the preparation of 3-substituted rhodanines and their i.r. and u.v. spectra.