ion.3,8 This unfavorable nitroxides feature has established the
need for development of new structures with enhanced
persistency to obtain strongly and clearly resolved EPR
signals. It has been reported that cyclodextrins (CDs) play
an important role in stabilizing nitroxides, leading to EPR
signal enhancement and in increasing the partial resistance
of the radical against biomimetic reductive conditions.9
However, the need for high CD concentrations can constitute
an important disadvantage for in ViVo EPR experiments.
Moreover, once the complex is dissolved in a biological
environment, the higher affinity of biomolecules toward the
hydrophobic compartment of CD, can result in separation
of the radical guest from the host. These limitations can be
overcome if the nitroxide functionality is mechanically
trapped inside the cavity of CD by a covalent link with one
of the CD rims, that is, by formation of [1]rotaxane.10 CD-
labeled nitroxides have already been described in the
literature.11 In all cases, however, the paramagnetic label was
found to be located outside the ring cavity. Presumably, this
behavior is due to the fact that in all cases the reaction
between the CD and the nitroxide moiety were carried out
in organic solvents which are known to strongly reduce the
affinity of an organic guest for the CD cavity, if compared
to water. Actually, [1]rotaxanation reaction with CD has been
recently shown to be very effective when performed in water
respect to DMSO.12
Scheme 1
analysis they were quantitatively converted into the analo-
gous N-hydroxy amines (4-OH, 5-OH) by adding directly
inside the NMR tube a stoichiometric amount of phenylhy-
drazine.13
The presence of the nitroxide moiety inside the ꢀ-CD cavity
in 4 has been confirmed using 2D ROESY H NMR both in
DMSO (Figure 1) and water (see Supporting Information). The
1
Based on this, we decided to prepare a nitroxide-based
[1]rotaxane by reacting nitroxide 1 or 2 with 6-mercapto-
ꢀ-cyclodextrin (3, ꢀ-CD-SH) using alkaline water as reaction
medium. After the reaction, the target molecules 4 and 5
were purified by using reverse phase and exclusion chro-
matographies (see Supporting Information).
As indicated in Scheme 1, different results were obtained
depending on the nature of paramagnetic unit that has been
reacted with ꢀ-CD-SH. While with 2 only the product with
the paramagnetic arm located outside the cavity was obtained
(5), with nitroxide 1 the reaction afforded [1]rotaxane 4 in
7.6% yield.
The fortuitous formation of 5 and the comparison of its
NMR and EPR spectra with those of 4 notably helped us to
the structurally assignment of the latter one as [1]rotaxane.
To render the paramagnetic nitroxides suitable to NMR
Figure 1.
Partial 2D ROESY 1H NMR spectrum of 4-OH (1 mM)
(7) Swartz, H. M.; Timmins, G. S. In Toxicology of the human en-
Vironment: the critical role of free radicals; Rhodes, C. J., Ed.; Taylor &
in DMSO-d6. The sample recorded in D2O shows similar interac-
Francis: New York, 2000; pp 91-111
.
tions (see Supporting Information).
(8) Samuni, A.; Goldstein, S.; Russo, A.; Mitchell, J. B.; Krishna, M. C.;
Neta, P. J. Am. Chem. Soc. 2002, 124, 8719–8724
.
(9) (a) Okazaki, M.; Kuwata, K. J. Phys. Chem. 1985, 89, 4437–4440.
(b) Ebel, C.; Ingold, K. U.; Michon, J.; Rassat, A. Tetrahedron Lett. 1985,
26, 741–744. (c) Karoui, H.; Rockenbauer, A.; Pietri, S.; Tordo, P. Chem.
Comm. 2002, 3030–3031. (d) Karoui, H.; Tordo, P. Tetrahedron Lett. 2004,
45, 1043–1045.
NOEs between the Hax, Heq, and CH3 protons and the H3 and
H5 protons on the interior of ꢀ-CD showed that the heterocycle
is embedded in the ꢀ-CD cavity to form [1]rotaxane 4-OH.
On the basis of the results mentioned above, it is concluded
that the nitroxide moiety should be included with its symmetry
axis nearly parallel to that of the CD and with the geminal
methyl groups pointing toward the larger rim of the CD (see
Scheme 2).
(10) Very few example of 1[rotaxane] has been reported in the literature:
Hiratani, K.; Kaneyama, M.; Nagawa, Y.; Koyama, E.; Kanesato, M. J. Am.
Chem. Soc. 2004, 126, 13568–13569.
(11) (a) Paton, R. M.; Kaiser, E. T. J. Am. Chem. Soc. 1970, 92, 4723–
4725. (b) Ionita, G.; Chechik, V. Org. Biomol. Chem. 2005, 3, 3096–3098.
(c) Bardelang, D.; Rockenbauer, A.; Jicsinszky, L.; Finet, J.-P.; Karoui,
H.; Lambert, S.; Marque, S. R. A.; Tordo, P. J. Org. Chem. 2006, 71, 7657–
7667. (d) Bardelang, D.; Charles, L.; Finet, J.-P.; Jicsinszky, L.; Karoui,
H.; Marque, S. R. A.; Monnier, V.; Rockenbauer, A.; Rosas, R.; Tordo, P.
Chem.-Eur. J. 2007, 13, 9344–9354.
The 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) frag-
ment has an approximately cylindrical shape whose diameter
is about 8.5 Å,1 well above the internal diameter of the ꢀ-CD
(12) Ma, X.; Qu, D.; Ji, F.; Wang, Q.; Zhy, L.; Xu, Y.; Tian, H. Chem.
Comm. 2007, 1409–1411.
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Org. Lett., Vol. 10, No. 10, 2008