I was told that the ethyl ester of L-proline may not be synthesized from L-proline and ethyl alcohol in refluxing toluene, with an acid catalyst such as p-toluensulfonic acid for example, because toluene and ethanol and in this case water as well form an azeotrope. Is this true?

I do not have a supply of pure thionyl chloride, and that's why I am trying to use this route. I appreciate if anyone may answer this question.

  • $\begingroup$ Hello and welcome to Chemistry.CE. I always recommend taking the short tour to better familiarize yourself with the site. Regardless, good luck with you problem! $\endgroup$
    – airhuff
    Feb 24, 2017 at 2:12
  • $\begingroup$ I would check the literature to be sure, but the rationale seems reasonable. $\endgroup$
    – Zhe
    Feb 24, 2017 at 2:32
  • $\begingroup$ I checked in SciFinder the preparation of the ethyl ester of L-proline and to my surprise didn't find what I was looking for, but anyway, I will try to see if mixing pure toluene, ethanol and water in the proportions they are found in the azeotrope yields a two phase liquid in the side arm of a Dean-Stark trap. $\endgroup$ Feb 24, 2017 at 4:22
  • $\begingroup$ I doubt (neither Scifinder, nor Reaxys at hand), an N-protection "simply by a Boc group" survives the PTSA-catalysed esterification until intentionally set to be cleaved off again. But maybe Kociensky's or Theodora Greene's compilations indeed know N-protections more suitable here. $\endgroup$
    – Buttonwood
    Feb 25, 2017 at 1:17

1 Answer 1


I think this is unlikely to work under the conditions described as the catalytic PTSA is tied up as its proline salt. If you reflux under D-S conditions you will just remove the EtOH due to the azeotrope with Toluene.


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