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a building block to create helices [25]. Introduction of chiral amino
acid into this scaffold can be designed to afford a chiral molecular
arrangement [26]. Histidine has the advantage of forming an
ordered structure on the basis of the chiral center and an additional
hydrogen bonding site of the imidazolyl group. The single-crystal
X-ray structure determination of the 2,6-pyridinedicarboxamide
11
bearing the histidyl moieties (-L-His-OMe) reveals a left-handed
helical conformation through chirality of the histidyl moieties
and intramolecular hydrogen bonding between NH (amide) and N
(pyridine) to give five-membered hydrogen-bonded rings as depicted
in Figs. 3.14 and 3.15a [27]. It should be noted that a right-handed
helical conformation is formed in the crystal structure of the 2,6-
pyridinedicarboxamide
bearing the histidyl moieties (-D-His-
OMe) (Figs. 3.14 and 3.15b) [27]. Similar intramolecular hydrogen
bondings between NH (amide) and N (pyridine) are observed in
this helical structure (Fig. 3.15b). The molecular structures of
12
11
and
are in a good mirror-image relationship, indicating the
conformational enantiomers. The propensity to form the chiral
helicity appears to be controlled by the configuration of the histidyl
a
12
-carbon atoms. The mirror image of the CD signals between
11
and
is obtained, suggesting that a chiral molecular conformation
based on an ordered structure through intramolecular hydrogen
bondings and chirality of the histidyl moieties is present even in the
solution. Another interesting feature is that each molecule of
12
is
connected by continuous intermolecular hydrogen bonds to give a
left-handed helix (
11
-form) as shown in Fig. 3.15a. Noteworthy is that
an opposite helically ordered molecular assembly, a right-handed
helix (
M
(Fig. 3.15b). The
histidyl moieties of the pyridinedicarboxamide scaffold are likely to
play an important role in creating the helical arrangement.
P
-form), is formed in the crystal packing of
12
O
O
O
O
N
N
N
HH
N
N
HH
N
MeO 2 C
CO 2 Me
MeO 2 C
CO 2 Me
N
N
N
N
H
H
H
H
Py(- L -His-OMe) 2 11
Py(- D -His-OMe) 2 12
Figure 3.14
The 2,6-pyridinedicarboxamide
11
and
12
the bearing
histidyl moieties (-His-OMe).
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