# Acid catalyst decarboxylation of 1-carboxyl-tryptoline

One of my labs had the following reaction:

I understand the Pictet Spengler mechanism. However, I am confused about the decarboxylation. I cannot come up with a way in which this decarboxylation would be acid catalyzed. Here is what I have for the Pictet mechanism:

I have been trying to figure out the acid catalyzed decarboxylation for several days now but I cannot. This is the closest I can come up with, but I'm not sure that this is correct:

Thanks!

• If you protonate the beta position of the indole, the resultant immonium salt is isoelectronic with a a beta-keto acid, which readily decarboxylate. – user55119 Apr 22 at 0:39

Your suggested mechanism should be acceptable (need two more steps, protonation and deprotonation, see last two steps of my scheme below), if you have isolated the carboxylic acid product (compound $$\bf{1}$$ in my scheme). If compound $$\bf{1}$$ is not isolable, I suggest following path would be taken to decarboxylation:
Your reaction is an analog to Pictet Spengler reaction (Wikipedia). When the intermediate $$\bf{I}$$ is formed during the reaction as depicted in the mechanism, 1,2-hydride transfer is possible to give more stable carbocation $$\bf{II}$$, which is resonance stabilized $$(\bf{\ce{II <-> III}})$$. The intermediate $$\bf{III}$$ can easily undergo decarboxylation to give some what stable enamine product, $$\bf{2}$$. In acidic medium, this enamine $$\bf{2}$$ would undergo protonation to give the intermediate $$\bf{IV}$$, which would then deprotonated to gain aromaticity and give thermodynamically stable indole product, $$\bf{3}$$ (the intended product).