Thiol vs carboxylic acid reactivity to acyl chlorides?

I want to form a thioester between some acyl chlorides and NBoc-cysteine. Is the free thiol so much more reactive than the carboxylic acid in cysteine that the side product formation of carboxylic anhydrides is negligible?




I think so that's the case according to Ref.1. For example:


[![S-acylation of ceisteine](https://i.stack.imgur.com/EsLgm.png)](https://i.stack.imgur.com/EsLgm.png)


Your $\ce{R^1}$-group can be one you wish to have. A typical procedure to prepare *S*-Acylation of *N*-protected-cysteines is given in the reference as follows:



> 
> To a solution of *N*-protected-L-cysteine ($\bf{4a-c}$; $\pu{1 mmol}$) in THF ($\pu{5 mL}$) was added a solution of acyl 1-benzotriazolyl $\bf{5}$ ($\pu{1 mmol}$) in THF ($\pu{5 mL}$) in the presence of $\ce{KHCO3}$ ($\pu{1 mmol}$). The heterogeneous mixture was then stirred at room temperature for $\pu{12 h}$. THF was evaporated, and the solution was acidified with $\ce{HCl}$, and the solid formed was taken in ethyl acetate and washed with $\ce{HCl}$ ($\pu{1N}$, 3×) and brine (3×) and then dried over sodium sulfate. Evaporation of ethyl acetate under reduced pressure afforded *N*-protected *S*-acyl cysteine.
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**References:**


1. Tarek S. Ibrahim, Srinivasa R. Tala, Said A. El-Feky, Zakaria K. Abdel-Samii, and Alan R. Katritzky, "Cysteinoyl- and Cysteine-containing Dipeptidoylbenzotriazoles with Free Sulfhydryl Groups: Easy Access to *N*-terminal and Internal Cysteine Peptides," *Chemical Biology & Drug Design* **2012**, *80(2)*, 194-202 (DOI: <https://doi.org/10.1111/j.1747-0285.2011.01303.x>).


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