An Fmoc protecting group can be removed from an amino acid by treatment with the amine base pipe

step 1 All right, so here I've got drawn in ethanol -protected amino acid. And the little B you see wit

An Fmoc protecting group can be removed from an amino acid...

Key Concepts

Protecting Groups

Protecting groups are chemical modifications used to temporarily block reactive sites in molecules during multi-step syntheses. In the context of peptide synthesis, protecting groups prevent unwanted side reactions on the amino and carboxyl groups, facilitating selective reactions and allowing the sequential addition of building blocks.

Fmoc Protecting Group

The Fmoc (fluorenylmethyloxycarbonyl) group is a base-labile protecting group commonly used to mask the amino functionality of amino acids during peptide synthesis. It is stable under acidic conditions but can be selectively removed under mild basic conditions, making it particularly useful in methods that require orthogonal deprotection strategies.

Base-Induced Beta-Elimination Mechanism

The deprotection of the Fmoc group proceeds via a beta-elimination mechanism. In this process, a strong base, such as piperidine, abstracts a proton from the methylene group adjacent to the carbonyl of the Fmoc moiety. This deprotonation forms a carbanion intermediate that undergoes a beta-elimination to yield a stable, conjugated olefin (typically dibenzofulvene) and the free amine of the amino acid.

Role of Piperidine

Piperidine acts as the nucleophilic base in the Fmoc deprotection reaction. It is responsible for the initial deprotonation step that triggers the beta-elimination mechanism. Its use ensures efficient and selective removal of the Fmoc group while minimizing side reactions, allowing for clean generation of the free amine group necessary for subsequent coupling reactions in peptide synthesis.

Transcript

00:01 All right, so here i've got drawn in ethanol -protected amino acid.

00:04 And the little b you see with the two dots i wrote is just a general base.

00:08 So in proposing a mechanism for the removal of this eflon group, it's said that we're going to use papyridine and water.

00:16 So this b with the two dots is going to act as both papyridine and water when it needs to be, just in both cases.

00:22 So an important thing to point out about this mechanism is that this hydrogen right here is incredibly acidic, considering where it is.

00:30 Is having a pca of around 20.

00:32 So it's definitely acidic enough to be abstracted or to be removed.

00:37 So i believe that the first step we should do is remove that hydrogen.

00:41 I'm going to have this base attacking the hydrogen there, and then this bond kind of shifting over one.

00:48 So what that's going to do is produce a pretty similar set of products, varying only really in one bond.

01:04 And that's the fact that that's a double bond now.

01:07 And oxygen, and then the rest of the molecule stays roughly the same, or i guess exactly the same.

01:25 So another important thing is that this carbon now probably has some sort of pseudo -positive charge on it, or maybe just a normal positive charge, because there were two protons on that carbon prior to the deprotonation, which means there's probably still two protons right there now, meaning that that definitely has a positive.

01:48 Charge.

01:49 So since we can kind of have that positive charge on that double bond, it's not super favorable.

01:56 What we can do is we can have this bond kind of just move onto the oxygen.

02:00 And there's two reasons we can do that.

02:02 The first is that the whole right side of this molecule with an amino acid with the additional carboxyl group is going to be a relatively good leaving group because you have a lot of carboxyl groups and you have a lot of hetero atoms that an electron charge can move on to.

02:16 The second part is that the bottom part is also going to be a good leaving group because it's a highly substituted kind of multi -ring structure.

02:24 Alright, so with that being said, we'll have the leaving group and we'll form kind of two big products out of that.

02:29 The first is going to be this ring structure, which forms there...

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