Why is S-alanine acetylated in the synthesis of S-cathinone?

[![enter image description here](https://i.stack.imgur.com/LJtSi.png)](https://i.stack.imgur.com/LJtSi.png)I came across a method of synthesizing enantiomerically pure S-cathinone using S-alanine which is chlorinated to the corresponding acid chloride which is used in a Friedel-Crafts acylation of benzene. In the first step, the S-alanine is N-acetylated to "protect" the amino group. I was wondering if the amino group will be replaced by chlorine by reaction with the PCl5 hence requiring protection with an acetyl group. This of course will bring about the problem of causing dehydration of the amide to a nitrile when chlorinating the carboxylic acid to an acid chloride. So why acetylate the amino group in S-alanine?




The treatment of N-acetyl-S-alanine with PCl5 gives the acid chloride of the carboxylic acid and converts the N-acetyl group to the imidoyl chloride. This species undergoes classical F-C acylation with benzene and then on aqueous work-up the N-acetyl group is regenerated by hydrolysis.


Contrast this with the reaction of S-alanine with PCl5. This paper [here](https://arpi.unipi.it/retrieve/handle/11568/659267/124762/99%20ICA.pdf) shows that L-alanine forms the acyl chloride hydrochloride which will be unable to give the acylium cation on treatment with AlCl3 as it is already a charged species.


 3