Controlling the chair conformation of a mannopyranose in a large-amplitude [2]rotaxane molecular machine

Article abstract of DOI:10.1002/chem.200900076

Large-amplitude mannosyl [2]rotaxane molecular machines based on an anilinium and mono- or disubstituted pyridinium amide stations were synthesized. Dibenzo[24]crown-8 (DB24C8) initially resides around the anilinium station in both the cases. The mannosyl [2]rotaxane molecular machines 3a,b were successfully obtained from the initially prepared mannoside azide 1a,b containing a pyridinium amide moiety and the alkyne ammonium 2, by using the copper(I)-catalyzed Huisgen alkyne-azide 1,3-dipolar cycloaddition, also called CuAAC click chemistry. The interlocked architecture of glycorotaxanes 3a,b and 4a,b and the localization of the macrocycle at different pH were studied by 1H NMR spectroscopy. It was observed that the DB24C8 moved toward the pyridinium amide station upon deprotonation. However, with the disubstituted amide, DB24C8 forms hydrogen bonds with the pyridinium H7 atoms, which are located near the cationic nitrogen atom, and interacts by ion dipole contacts with the cationic charge.

Full text of DOI:10.1002/chem.200900076

DOI: 10.1002/chem.200900076
Controlling the Chair Conformation of a Mannopyranose in a Large-
Amplitude [2]Rotaxane Molecular Machine
[
a]
Frꢀdꢀric Coutrot* and Eric Busseron
Rotaxanes have received much attention during the past
decade, especially because they can be used as molecular
[1]
machines. As we are involved in the field of structure–ac-
tivity relationships between glucides and lectines, we recent-
[
2]
ly described very efficient preparations of glycorotaxanes,
[3]
pH-sensitive glycorotaxane molecular machines, and glyco-
[4]
syl molecular muscles containing an ammonium and a tria-
zolium station. Here, we report the synthesis of two large-
amplitude mannosyl [2]rotaxane molecular machines, con-
taining dibenzo[24]crown-8 (DB24C8), an anilinium station,
and either a new mono- or disubstituted pyridinium amide
station, both derived from isonicotinamide moiety
(
Scheme 1). Since Busch et al. reported the ability for the
[5]
DB24C8 to interact strongly with ammonium cations,
a
wide variety of other template moieties, such as benzylic
Scheme 1. The synthesis of the large-amplitude mannosyl [2]rotaxane
molecular machines 3a–b.
[6]
[7]
ammonium, N-benzylic anilinium, N,N’-dialkyl-4,4’-bipyr-
[8]
[9]
idinium, and 1,2-bis-(pyridinium)ethane cations, have
been investigated. However, the use of a monopyridinium
amide station for DB24C8 in a molecular machine has never
monosubstituted amide. This difference of localization
causes the impressive switching of the glucidic conformation
[10]
been described so far. As expected, anilinium was found
to be a much better station for the macrocycle than the pyri-
dinium amide station. However, upon deprotonation of the
anilinium, the macrocycle shuttles toward the pyridinium
amide station. More interestingly, the interactions of the
pyridinium amide station with DB24C8 proved to be very
different depending on the mono- or disubstituted conjugat-
ed amide, thus allowing different localizations of the macro-
cycle around the pyridinium amide station. Whereas the
DB24C8 resides around the positive charge of the disubsti-
tuted pyridinium amide, it is localized around the NH of the
1
4
from C to C upon deprotonation of the anilinium, only in
4
1
the rotaxane containing the disubstituted pyridinium amide
station.
The mannosyl [2]rotaxane molecular machines 3a,b were
successfully obtained from the initially prepared mannoside
azide 1a,b containing a pyridinium amide moiety and the
alkyne ammonium 2, by using the copper(I)-catalyzed Huis-
[
11]
gen
alkyne-azide 1,3-dipolar cycloaddition, also called
[12]
“CuAAC click chemistry”. (Scheme 1)
The reaction was carried out in dichloromethane, at ambi-
ent temperature, over a period of 24 h, in the presence of
DB24C8 (2 equiv), [Cu ACHTUNGTRNEGNU( MeCN) ]PF (1 equiv) and 2,6-luti-
4 6
[
a] Dr. F. Coutrot, E. Busseron
dine (0.1 equiv). In these experimental conditions, only the
[2]rotaxanes 3a,b were isolated, as a result of the exclusive
very good complexation of the anilinium template with the
macrocycle, and no thread or [3]rotaxane were detected.
This last observation evidently suggests that mono- and dis-
ubstituted pyridinium amides are very weak templates for
the macrocycle in comparison with anilinium. The further
shuttling of the macrocycle in rotaxanes 3a and 3b was car-
Institut des Biomolꢀcules Max Mousseron (IBMM)
UMR 5247CNRS-Universitꢀs Montpellier 2 et 1
Bꢁtiment de Recherche Max Mousseron
Ecole Nationale Supꢀrieure de Chimie de Montpellier
8
, rue de l’Ecole Normale, 34296 Montpellier Cedex 5 (France)
Fax : (+33)467-14-43-44
E-mail: frederic.coutrot@univ-montp2.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900076.
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COMMUNICATION
Scheme 2. Shuttling of the macrocycle in the large-amplitude mannosyl
2]rotaxane molecular machines 3–4 by variation of pH.
[
ried out by deprotonation of the anilinium station.
(
Scheme 2)
The interlocked architecture of glycorotaxanes 3a,b and
1
3
Figure 1. H NMR spectra (400 MHz, CD CN, 298 K) of a) the uncom-
4
a,b and the localization of the macrocycle at different pH
plexed dumbbell-shaped thread 3au, b) the rotaxane 3a, c) the rotaxane
4a, and d) the uncomplexed dumbbell-shaped thread 4au. The lettering
and numbering correspond to the proton assignments indicated in
Scheme 1.
1
were studied by H NMR spectroscopy (Figures 1 and 2).
Deprotonation of 3a was realized by using NaOH and af-
forded the rotaxane 4a. As expected, the DB24C8 moved
toward the pyridinium amide station upon deprotonation.
The localization of the DB24C8 around either the anilinium
station in 3a or around the pyridinium amide station in 4a
2
5
fectively, the chemical shift of H is shifted upfield as a
result of both the deprotonation of the anilinium and the
shuttling of the macrocycle, whereas pyridinium H atoms
and to a lesser extent the amide hydrogen atom H are si-
multaneously shifted downfield (Dd=0.96 and 0.34 ppm, re-
spectively), due to hydrogen bonding with the DB24C8.
This is corroborated by a slight upfield shift of H –H ,
which now undergo the shielding effect of the aromatic ring
1
8
was confirmed by the analyses of the H NMR spectra of
11
both rotaxanes 3a and 4a, and uncomplexed threads 3au
and 4au. (Figure 1)
Comparison between the spectra of the uncomplexed
thread 3au and the rotaxane 3a reveals the presence and
the localization of the macrocycle. (Figure 1a,b) Apart from
the evident appearance of the signals for the macrocycle hy-
12
16
2
0
23
of the DB24C8 to the detriment of H –H , which experi-
ence a downfield shift. Interestingly, no variation of the pyri-
dinium hydrogen atoms H is observed, which indicates the
A
E
25
drogen atoms H –H , the H atoms experience a downfield
shift (Dd=0.95 ppm), while no chemical shift variations of
the hydrogen atoms belonging to the pyridinium station
7
7
absence of any kind of interaction between H and the
(
blue signals) are noticed, indicating that the DB24C8 ring
DB24C8. No ion–dipole interactions occur between the cat-
ionic charge of the pyridinium and the oxygen atoms of the
macrocycle. Eventually, a tremendous upfield shift is ob-
binds exclusively with the monosubstituted N-alkylanilinium
20
23
18
center. The H –H atoms and to a lesser extent H are
more or less shielded in the rotaxane, because they undergo
the shielding effect of the aromatic ring of the macrocycle.
The other hydrogen atoms of the west part of the molecule
E
served in 4a for the signals of H , probably because they ex-
perience the shielding effect of the aromatic pyridinium
ring. The localization of the DB24C8 around the pyridinium
amide station is corroborated by the direct comparison of
the rotaxane 4a with the uncomplexed thread 4au. (Fig-
1
17
(
H –H ) stay unchanged as they are not surrounded by the
macrocycle. Upon deprotonation, the DB24C8 moved to the
pyridinium amide station. This was confirmed by comparing
25
AHCTUNGTRENNUNGu re 1c,d) In the rotaxane 4a, the chemical shift of H stays
1
11
8
H NMR spectra of rotaxanes 3a and 4a. (Figure 1b,c) Ef-
unchanged, while the signals for H and H are simulta-
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5187

F. Coutrot and E. Busseron
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
neously shifted downfield due to hydrogen bonding with the
spectra of 3b and 4b (Figure 2b,c), which shows a downfield
1
2
7
DB24C8 and that for H is shifted slightly upfield, as they
experience the shielding effect of the aromatic ring of the
DB24C8.
Deprotonation of the rotaxane 3b was carried out with
diisopropylethylamine (DIEA) and the resulted shuttling of
shift of Dd=0.96 ppm for H , whereas no variations of
8
signal are noted for H , indicating hydrogen bonding be-
7
tween the oxygen atoms of the DB24C8 and H . This is con-
7
firmed by the upfield shift of H when comparing rotaxane
4b and thread 4bu (Figure 2c,d). Furthermore, and in con-
trast to the deprotonation of rotaxane 3a, very important
changes in both the chemical shifts and the vicinal coupling
constants of the mannosyl hydrogen atoms are noted after
deprotonation of 3b (Figure 3, Table 1) These important
variations of chemical shifts and vicinal coupling constants
indicate a conformational change of the chair-like pyranose
1
the macrocycle was studied by H NMR spectroscopy
(
Figure 2). The interlocked architecture of rotaxane 3b was
1
4
from the C to a C conformation upon deprotonation.
4
1
1
3
Figure 3. Partial H NMR spectra (400 MHz, CD CN, 298 K) showing the
conformational change of the mannoside: a) the rotaxane 3b; b) rotax-
ane 3b+DIEA (0.5 equiv); c) the rotaxane 4b after the addition DIEA
(
2.5 equiv). The numbering corresponds to the proton assignments indi-
cated in Scheme 1.
1
4
Table 1. Vicinal coupling constants of mannosyl hydrogens H –H in ro-
taxanes 3b and 4b.
3
1
2
[a]
3
2
3
[a]
3
3
4
[a]
3
4
5 [a]
J
A
T
N
T
E
N
N
(H ,H )
J
A
T
N
T
E
N
N
(H ,H )
J
A
T
N
T
E
N
N
(H ,H )
J ACHTUNGTRENNUNG( H ,H )
rotaxane 3b
rotaxane 4b
9.4
2.3/3.0
3.2
– /3.0
3.2
8.4/8.4
3.2
8.4/8.4
[
b]
[c]
1
Figure 2. H NMR spectra (400 MHz, CD
3
CN, 298 K) of a) the uncom-
plexed dumbbell-shaped thread 3bu, b) the rotaxane 3b, c) the rotaxane
b, and d) the uncomplexed dumbbell-shaped thread 4bu. The lettering
and numbering correspond to the proton assignments indicated in
Scheme 1.
[a] The coupling constant is given in Hz. [b] Due to the cis/trans isomer-
ism of the amide bond, rotaxane 4b exists as a mixture of two diastereo-
mers, hence a value of coupling constant for each diastereomer is report-
ed. [c] Coupling constant could be measured only for one stereoisomer
because of broad multiplet.
4
verified by comparison of spectra of uncomplexed thread
3
bu and rotaxane 3b (Figure 2a,b). Similar observations to
As already mentioned in the literature, with metals and
[14]
those seen for the monosubstituted amide 3au and 3a con-
cerning the downfield and upfield shifts of characteristic hy-
drogen atoms are evidenced. The only difference resides in
the more complicated spectrum for compounds b, which is
due to the cis/trans isomerism of the disubstituted amide
diamine glucidic derivatives, and with silylated protecting
[15]
group of glucidic hydroxyle, the pyranose moiety can be
4
1
flipped between C1 and C4 chair conformation. On the
other hand, it is also known that glycosyl pyridinium com-
pounds undergo the reverse anomeric effect (RAE), be-
cause a cationic charge, located at the anomeric position of
a glucide, forces the aglycone chain to sit in the energetical-
ly preferred equatorial position, even though more protect-
ed hydroxyl groups are in axial orientations on the pyra-
[13]
bond.
In comparison with the depro ACHTUNGTERNNUNGt on ACHTUNGTRNENUNGa tion of 3a, the
deprotonation of 3b makes the DB24C8 move toward the
pyridinium unit, but with a slightly different localization of
the macrocycle, which involves a tremendous change of the
conformation of the mannosyl extremity. This difference of
[16]
nose. If vicinal hydrogen atoms are both axial, the cou-
pling constant generally observed is 8–10 Hz; however it is
1
localization can be explained by the comparison of H NMR
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Chem. Eur. J. 2009, 15, 5186 – 5190

Molecular Machines
COMMUNICATION
usually between 2–4 Hz if at least one of the two vicinal hy-
Experimental Section
[17]
drogen atoms is equatorial. For the mannosyl rotaxanes
a,b and 4a and for all uncomplexed threads, the vicinal
3
3 4 6
Compound 3b: [Cu AHCTUNGTREUNNNG( CH CN) ]PF (81 mg, 0.22 mmol, 1 equiv) and 2,6-lu-
tidine (2.5 mL, 0.02 mmol, 0.1 equiv) were added successively to a solu-
tion of the mannosyl azide 1b (160 mg, 0.22 mmol, 1 equiv), alkyne 2
100 mg, 0.22 mmol, 1 equiv), and DB24C8 (194 mg, 0.44 mmol, 2 equiv)
in dry CH Cl (3 mL). The mixture was stirred for 24 h at RT, then the
coupling constants measured for the hydrogen atoms of the
glucidic scaffold unambiguously indicate that the glucide
(
1
4
adopts a C chair conformation, whereas they indicate a C
4
1
2
2
chair conformation for 4b (Figure 3, Table 1). It is worth
solvent was removed under vacuo. The crude mixture was directly puri-
fied by column chromatography (SiO : solvent gradient elution: acetone/
CH Cl 20:80, then 30:70) to afford the pure rotaxane 3b (266 mg, 74%)
as a white solid.
noting that the tremendous NMR downfield shift observed
2
2
2
2
for H (Dd=0.70 ppm) between rotaxanes 3b and 4b is con-
1
sistent with the flipping of the pyranose from the C to the
4
4
2
C conformation, since H can only experience the deshield-
1
[18]
ing effect of the aromatic pyridinium ring in 4b.
Keywords: chair conformation
·
click chemistry
·
·
4
The conformational change to C observed for 4b can
glycosides
rotaxanes
·
molecular devices
·
pyridinium amides
1
result from the following effects. Besides the interaction be-
7
4
tween the DB24C8 and H , the C chair is stabilized by one
1
more equatorial bond, which is energetically favorable.
Moreover, it can be assumed that the interactions of the
oxygen atoms of the DB24C8 with the pyridinium charge
would result in the masking of the cationic charge and, as a
consequence, the switching off of the reverse anomeric
effect. Such an induced conformational effect was not ob-
served with the rotaxane containing the monosubstituted
pyridinium amide 4a, in which the DB24C8 is located far-
ther from the mannopyranose and interacts preferentially
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[
[
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4
1
[
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