2. Protecting groups - URI Department of Chemistry

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This is a useful protecting group because the side products (carbon dioxide, toluene) are so easilyseparable.2. Boc protecting group – you protect the amine by reacting it with Boc-anhydride:And when it’s done reacting, the Boc group is removed with trifluoroacetic acid (TFA) (relatively strongacid):Mechanism of deprotection:3. F-moc protecting group – abbreviation for 9-fluoromethoxycarbonylThe free amine is protected by reacting with Fmoc-chloride or Fmoc-OSU:The mechanism for F-moc protection is pretty straightforward:Nucleophilic attack of the amino group, followed by displacement of the succinimide leaving group, togenerate a protected amino acid.Fmoc is deprotected by treating it with a secondary amine base like piperidine.
The mechanism of F-moc deprotection is shown below:The base attacks the fluorenyl proton – this proton is slightly acidic because when you form the anion,that negative charge can be stabilized by the entire aromatic system. The anion then kicks off thecarbamate, which decomposes to lose carbon dioxide and form the desired free amine product.So that concludes the protecting groups for amines. The carboxylic acid also needs to be protected (if itis not the part of the amino acid that you want to react). This is typically done with a tert-butyl ester (oranother mild ester).Let’s take a sample peptide and think about how to synthesize it both retrosynthetically and in theforward direction.Retrosynthesis:Forward direction:
Page 1: Lecture 182. Protecting groups: Let

2. Protecting groups - URI Department of Chemistry

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