Peptides are linear polyamides formed by end to end linkage of –aminoacids most frequently of the L– — Analytical Chemistry Chemistry Question

Which dipeptides could result from condensing L–alanine and L–phenylalanine? Use stereo representations in your answer.

The stepwise elongation of the peptide chain almost invariably starts from the C terminal aminoacid of the desired sequence (employed in the form of ester) to which each successive aminoacid unit (employed in the form of N–protected aminoacid derivative) is linked, followed by removal of the N–substituent (protecting group) before the next unit is added. The substituent most often employed is an alkoxycarbonyl group ROCO– and the derivatives are then called carbamates. Why does the presence of such a substituent on the amine nitrogen impede that amine from forming an amide linkage with a carboxyl group?
a) Because the nitrogen has only one H.
b) Because the group lowers the electron density on nitrogen.
c) Because the group hinders the approach of the carboxyl.
d) Because of electrostatic repulsion.
e) Because it is already an amide.

Draw the resonance structures for an amide moiety. Use stereo representations and curved arrows to show the flow of electron density.

Which of the following reagents would you use to prepare the benzyl carbamate of an amine (Bergmann–Zervas protecting group)? Write the reaction
1. C₆H₅CH₂OCONH₂, 2. C₆H₅CH₂OCO₂CH₃, 3. C₆H₅CH₂OCO₂C(CH₃)₃, 4. C₆H₅CH₂OCOCl, 5. C₆H₅OCOCl

The removal of an alkoxycarbonyl protecting group is often accomplished by the action of acid that triggers a fragmentation represented schematically below:

R^-OCO^-NH-peptide + H⁺ → R⁺ + CO₂ + H₂N-peptide

Rank the following carbamates according to increasing lability under acidic conditions:
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A.
B.
C.
D.