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O
R
R
N
O
N
H
H
X
O
O
S
N
aza
alkyl
depsi
H
thio
O
S
retro-inverso
O
reduced
OH
H
keto-
methylene
O
B
n
n = 1, 2
S
bora
hydroxethylene
O
(E)-alkene
P
HO
carba
OH
O
B
P
OH
Figure 2.2 Some examples of chemical functions that have been introduced in the peptide
skeleton for the synthesis of new foldamers.
2,3
-amino
acids depending on the position of the side chain at the 3-aminopropionic acid core (Fig-
ure 2.3). In addition, cyclic amino acids have the amino group integrated in a ring, as is
the case in b-proline.
A general synthesis of b
2
-, b
3
- and b
available till a few years ago. b-Amino acids are subdivided into b
3
h
Xaa-type b-amino acids was obtained by Seebach via
Arndt-Eistert a-amino acid homologation and now most of these compounds are com-
mercially available [18]. The b
3
-derivatives by
enolate alkylation [19] or from enolate esters by the Davies method [20]. For the
b
2,3
-amino acids were prepared from the b
2
-homoamino acids, the overall enantioselective Mannich reaction or benzyloxycarbo-
nylmethylation/Curtius degradation was applied using a modified Evans auxiliary [21].
Besides the methods that utilize the chiral pool, several syntheses of b-amino acids rely
on classical resolution or a stoichiometric use of chiral auxiliaries (Figure 2.4) [22].
Recently, several methods that utilize the catalytic asymmetric synthesis have been devel-
oped, including transition metal catalysis, organocatalysis, biocatalysis and important
synthetic methods, such as hydrogenation,
the Mannich reaction and conjugate
R
N
R
H
2
N
CO
2
H
H
2
N
H
2
N
CO
2
H
CO
2
H
CO
2
H
R
R
2
3
β
-proline
β
−
2,3
β
−
β
−
Figure 2.3 Chemical structure of different kinds of b-amino acids.
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