Thiocarboxylic S-esters

Acylation of a simple thiol with an alkyl carboxylate is not a very suitable method for preparation of S-alkyl thiocarboxylates. Transesterification is, however, possible if either the thiol or the carboxylic ester is activated. The enhanced reactivity of boron, aluminum and silicon thiolates has been utilized for the synthesis of a large variety of thiocarboxylic S-esters, including hydroxy derivatives (from lactones). a,P-Unsaturated thiol esters, e.g. cinnamoyl or 2-butenoyl derivatives, are also accessible. Michael addition, an undesirable side reaction of thiols, is completely avoided if alkyl trimethylsilyl sulfides ortris(arylthio)boranes are applied. ... 

Diethylaluminium cyanide is a valuable reagent for the conjugate addition of HCN to o , 3-unsaturated carboxylic acid derivatives [120]. Steroidal conjugate esters are somewhat resistant to HCN addition even with this powerful reagent and optimum yields of only 36-50% were obtained. With aj3-unsaturated acid chlorides and acid cyanides, however, the reaction proceeds smoothly in toluene at room temperature and, after hydrolysis the 0-cyanoacid was obtained in 71—73% yield. Conjugated steroidal thiocarboxylic S-esters react under similar conditions to give the product in yields of 85-87%. 

Although acylation reactions rank among the best-known methods for the synthesis of thiocarboxylic S-esters (1), there exist other important and convenient routes, - e.g. the alkylation of thiocarboxylate anions (equation 1) or radical addition of thiocarboxylic acids to carbon-carbon double bonds (equation 2). These methods are not dealt with here cf. Volume 4, Chapter 1.5). 

---

See also in sourсe #XX -- [ , ]

---