Introduction to Unnatural Foldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 57

O

COOH

(a) ClSO 2 NCO

aq. HCl

NH

NH 2 HCl

(b) hydrolysis

β -Lactam route to β -amino acids

O

COOH

O

lipase

+ n

n

NH

n

NH

NH 2

Lipase-catalyzed ring opening of cycloalkane-fused β -lactam

COOMe COOMe COOH

COOH CON 3 NH 2

COOMe COOH NH 2

COO t Bu COO t Bu COOH

Desymmetrization followed by Curtius degradation

COOEt

COOEt

Ru-catalysts

NHCOMe

NHCOMe

Ruthenium-catalysed enantioselective hydrogenation

Figure 2.6 Synthetic methods for the synthesis of enantiomerically pure cyclic b-amino

acids.

(MeOH, H 2 O) with chain lengths as short as four residues and without restricted
backbone rotation, as in oligomers containing 2-amino-cyclopentane- and 2-amino-
cyclohexane-carboxylic acid moieties [34].
The conformations of b-peptides can be analyzed in terms of the main chain torsional
angles, which are assigned the angles v, w, u and c (Figure2.7)intheconventionof
Balaram [35].
Folded helical or turnlike conformations of b-peptides require a gauche conformation
about the u torsion angle defined by the C2-C3 bond. A trans rotamer leads to a fully
extended conformation, provided the values of w and c are appropriate. The effects of
substituents on the local conformation of a b-amino acid are summarized in Figure 2.8.
b-Alanine is highly flexible, analogous to Gly in the a-amino acids. Alkyl substituents
at positions 2 and 3 favor a gauche conformation about the C2-C3 bond [36].