Thioamides from isothiocyanates

Subst. carboxylic acid thioamides from isothiocyanates... 

Subst. carboxylic acid thioamides from isothiocyanates N = C = S - NHCSR... 

Phenyl isothiocyanate reacts less readily with alkynyllithium than methyl isothiocyanate. The thioamide from the reaction of CHjC=CLi with PhN=C=S is not easy to obtain in a crystalline state. 

Imidothioesters are easily obtained through 5-alkylation of thioamides, and we have noted (see Section 2.8.3) a general preparation of such compounds from isothiocyanates via addition of an organometallic and 5-alkylation. 

Imidothioates in the neutral or ionic form are the most generally applied educts for the preparation of dithiocarboxylic esters. They are themselves prepared from nitriles, thioamides or isothiocyanates, and react with hydrogen sulfide under appropriate conditions to yield the dithioesters (Scheme 7) 6,14,26,27,29,141 Interesting examples (62)-<73) were described in the recent literature. 

For example, Dimroth treated thioamides, derived from isothiocyanates and malonates, with phosphorus pentachloride, and he obtained the corresponding imidoyl chlorides (III), which can be dehydro-chlorinated to produce the ketenimines IV. 

The oxidation of thioamides 63 with a wide variety of oxidizing agents is a well-employed method for the synthesis of 3,5-disubstituted-l,2,4-thiadiazoles 64 <1982AHC285>. However, this method is limited mainly to arylthioamides. The most common oxidizing agents tend to be halogens, hydrogen peroxide, dimethyl sulfoxide (DMSO), and nitrous acid. Yields from these reactions are variable and depend on the thioamide, oxidant, and conditions used (Equation 19). By-products such as nitriles and isothiocyanates are usually formed. 

The thiophene synthesis described herein is related to the synthesis in solution reported by Laliberte, and Medawar4 but differs in some aspects from the procedure in homogeneous phase. Laliberte and Medawar succeeded in obtaining aminothio-phenes in a one-pot reaction from acceptor-substituted acetonitriles, isothiocyanates, a-haloketones, and sodium ethoxide. In contrast to their procedure, solid-phase S-alkylation of the intermediate thioamides under basic conditions led to the formation of product mixtures. We obtained pure aminothio-phenes only when conducting the S-alkylation under neutral or slightly acidic conditions. 

Substituted 2-mercapto-4//-pyrido[l,2-a]pyrimidin-4-ones 121 were prepared by the cyclization of thioamides 120, prepared from 2-pyridyl-isothiocyanate and the appropriate CH acids, in boiling ethanol by the action of sodium ethylate (81CCC2428). 

The cyclization of several thiourea derivatives 122, which were obtained from the reactions of a number of isothiocyanates 123 and thioamides 124, with AS2O3, phenylarsine oxide, and triphenylarsine, afforded the corresponding 1,3,5,2-dithiazarsinine compounds 125 (Scheme 37) <2002PS497>. Tautomerism between the thiourea 122a and imine-thiol 122b forms has been observed by H and NMR. 

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