Química Orgânica 2 · Aminoácidos e conectividade dos peptídeos
Boc-Gly-Ala-OMe from Boc-glycine and alanine methyl ester with DCC
Condições: DCC, CH2Cl2
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DCC takes the carboxyl proton
A carboxylic acid and an amine just form a salt, so the acid must be activated. The basic nitrogen of DCC (dicyclohexylcarbodiimide) removes the carboxyl proton, giving a carboxylate and a protonated carbodiimide.
As setas curvas
- A carbodiimide nitrogen lone pair takes the carboxyl proton.
- The O–H pair stays on oxygen as carboxylate.
The carboxylate adds to the carbodiimide
The carboxylate oxygen attacks the central, electron-poor carbon of the protonated carbodiimide and the C=N+ π pair moves onto nitrogen. The O-acylisourea formed has a very good leaving group on the acyl carbon.
As setas curvas
- A carboxylate oxygen lone pair attacks the central carbodiimide carbon.
- The C=N+ π pair moves onto nitrogen, neutralizing it.
Purpose of DCC
Activation converts OH into a leaving group.
The amine attacks the activated acyl carbon
The free NH2 of alanine methyl ester adds to the O-acylisourea carbonyl carbon and the C=O π pair moves onto oxygen, giving a tetrahedral intermediate.
As setas curvas
- The amine lone pair attacks the acyl carbon.
- The C=O π pair moves onto oxygen.
The isourea oxygen leaves
The alkoxide re-forms C=O and the C–O bond to the isourea breaks, releasing the urea-type anion as the leaving group.
As setas curvas
- An oxygen lone pair re-forms the C=O π bond.
- The C–O pair leaves with the isourea oxygen.
Proton transfer gives the amide and DCU
The basic imine nitrogen of the leaving anion takes the N–H proton of the protonated amide; at the same time the oxygen anion forms C=O and the C=N π pair moves onto nitrogen, giving neutral dicyclohexylurea (DCU).
As setas curvas
- The imine nitrogen lone pair takes the N–H proton.
- The N–H pair stays on the amide nitrogen.
- The oxygen lone pair forms the urea C=O π bond.
- The C=N π pair moves onto nitrogen.
Boc-Gly-Ala-OMe
The products are the protected dipeptide Boc-Gly-Ala-OMe and dicyclohexylurea, which precipitates from CH2Cl2. The alanine stereocentre is untouched, so its configuration is retained.
O que você vai praticar
Draw DCC activation of Boc-glycine as the O-acylisourea and its aminolysis by alanine methyl ester to give the dipeptide and dicyclohexylurea.
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