Chemistry Reference
In-Depth Information
H
CHR
CO
H
OC
CHR
+ DMF
O
C
N
+
N
-
CO
-DMF
O
O
_
H
N
OC
CHR
CO
- Py
O
H
CHR'
CO
N
CHR
O
C
OH
OC
CO
O
-CO
2
H
CHR
H
2
N
CHR
CO
N
OC
CHR
x
OC
CO
CO
O
O
(-xCO
2
)
H-NH
CHR
CO
HN
x
CHR
CO
N
OC
CHR
CO
O
-CO
2
CHR
CO
H
2
N
x
+2
Formula 15.3
Surprisingly, it was found that also trialkylamines yield cyclic polypeptides,
although a zwitterionic mechanism is excluded, because a nucleophilic attack on
the carbonyl C-5 can for steric reasons not occur. As evidenced by model reactions
[
6
-
8
], trialkylamines are basic enough to deprotonated NCAs, and the resulting
NCA anion reacts with another NCA (see Formula
15.4
). The ensuing ROP may
proceed either by reaction of the amino end group with NCAs (amine mechanism)
or by reaction of NCA anions with the N-acyl NCA chain end (activated monomer
mechanism) [
8
-
10
]. Regardless which kind of propagation predominates, the
resulting difunctional chains can cyclize (see Formula
15.4
). Finally, it should be
mentioned that two types of cyclic polypeptides are also formed, when NCAs are
heated above their melting points, and initiation via zwitterions is highly probable,
but clear-cut evidence and a detailed study of the polymerization mechanism is
