Cesium thioacetate

The anions of such cesium salts as cesium thioacetate or cesium benzoate often undergo extremely clean Sn2 substitution reactions. Optically active thiols, especially those sensitive to racemization, can be prepared using cesium thiocarboxylates as nucleophiles. The reaction of ethyl (i )-0-mesylmandelate (64), readily prepared from the commercially available... 

A modified Mitsunobu procedure in which 63 is first treated with the preformed complex 68 (prepared by reaction of triphenylphosphine and diisopropyl azodicarboxylate) and then cesium thioacetate leads to significant racemization [17]. However, if the free acid is reacted instead with an appropriate thioacid (rather than the ester and a cesium salt), optical yields improve significantly. Thus, thioacetylation of (S)-l can be accomplished by treating it with 68 followed by the addition of thioacetic acid in THF to provide in 48% yield (5)-2-(acet-ylthio)-2-phenylacetic acid (69) with 84% ee after recrystallization. The low yield is due in part to the unavoidable formation to the extent of at least 50% of a viscous, polymeric material. The reaction is complete in minutes, however, and proceeds with retention of configuration. Presumably this is a result of a double inversion mechanism that passes through an a-lactone. Interestingly, the corresponding reaction with lactic acid does occur with inversion [18]. 

Preparative Methods the synthesis of 1 was achieved by protection of the amino alcohol 2 with B0C2O to give the carbamate 3. Conversion to the mesylate 4 was followed by reaction with cesium thioacetate to give the thiol acetate 5. Finally, a selective reduction of the thioacetate group afforded 1 (eq 1). ... 

The cesium salt of thioacetic acid undergoes clean Sn2 substitution with 156b to afford (/ )-2-acetylthiopropionic acid ethyl ester (168) in nearly quantitative yield and with 100% ee [63]. Acid hydrolysis furnishes ethyl (7 )-2-mercaptopropionate (169) (92% ee). 

Mesylate 191, prepared from 3 (MsCl, Et3N, ether, —20 °C), undergoes clean 8 2 displacement with cesium thiolacetate to give (i )-2-(acetylthio)succinate (192) with 100% ee [68]. The cesium salt of thioacetic acid is readily prepared by treatment with cesium carbonate in methanol. 

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