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Topic: Base-catalyzed (?) pictet-spengler condensation  (Read 7264 times)

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Offline g_orbital

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Base-catalyzed (?) pictet-spengler condensation
« on: May 02, 2006, 08:35:40 AM »
The Pictet-Spengler reaction, in which beta-arylethylamine is heated in the presence of an aldehyde to give an isoquinoline, is implented in synthesis of beta-carbolines, where an indole reacts with carbonyl compounds.

The reaction is seem to be catalyzed almost always in acid. However, I found a reaction (taken from a paper, see the image file) in which tryptamine reacts with glyoxylic acid in the presence of potssium hydroxide (KOH) to give a beta-carboline with an acid functional group, without any decarboxylation.

I have two questions:
1) What are the differences bewteen the acid-catalyzed mechanism and the base-catalyzed one? I wonder whether the base is added here only for pH-balancing (because the glyoxylic acid decreases pH, of course...)
2) Why a decarboxylation doesn't occur in this incident?

Enclosed herewith the acid-catalyzed mechanism which I've proposed...I would be very pleased if you find the base-catalyzed one.

Thank you very much indeed!
« Last Edit: May 02, 2006, 08:42:52 AM by g_orbital »

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Re: Base-catalyzed (?) pictet-spengler condensation
« Reply #1 on: May 02, 2006, 03:29:56 PM »
The mechanism will be really similar to the acid catalyzed one.  In this case base might be better because if you deprotonate the acid in your starting material you will make hydration of the aldehyde less favorable.  In the presence of aqueous acid I bet that aldehyde is almost completely hydrated.

As for decarboxylation, you generally need an electron withdrawing group to make the decarboxylation go fast.  Without this decarboxylation often takes pretty forcing conditions because the anion you generate is higher in energy.

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