Tachibana et al.
CHART 1
application for rotaxanes, it is necessary to control the relative
conformation between the wheel and the axle components. This
can be attained by controlling the intercomponent interaction.
Sauvage et al. first demonstrated the conformation control of
an interlocked compound based on metal-ligand interaction.4
By elimination of the metal ion carrying the intercomponent
interaction, a certain conformation change was successfully
induced. Further, the redox reaction-induced change of coor-
dination numbers of copper ion was also used for conformation
control.5 Stoddart et al. prepared various interlocked compounds
based on charge-transfer interaction or hydrogen-bonding
interaction of ammonium salt and crown ether and demonstrated
that intercomponent conformation could be precisely controlled
by deprotonation or redox reaction of the ammonium group via
control of the intercomponent charge-transfer interaction.6
Further, Nakashima et al. reported rotaxanes based on the
hydrophobic interaction between the trans-azobenzene group
and cyclodextrin and showed that intercomponent conformation
could be controlled by photoinduced isomerization of the
azobenzene group to the cis form.7 Leigh et al. reported
rotaxanes based on the hydrogen-bonding interaction between
fumaramide and macrolactam with secondary amide moieties
and demonstrated that the photoinduced isomerization of
fumaramide to maleamide caused a change in intercomponent
conformation.8 In these systems, conformation control was
attained by some external stimuli that removed the intercom-
ponent interaction preintroduced in the preparation step. Since
the intercomponent interaction in interlocked systems is insepa-
rable from the combination of components used for the
construction of the system, there are scant choices of methods
to control the conformation. If intercomponent interaction is
independent of the combination of components, not only weak
and labile interaction but also a combination of different types
of interactions can be used to control conformation. Therefore,
the development of a general method to introduce desired
functional groups into the rotaxane after the elimination of
intercomponent interaction preintroduced is highly desirable.
Such a method, if developed, can make a variety of function-
alized rotaxanes possible.
We previously reported one of the most efficient methods to
synthesize rotaxanes consisting of crown ethers and secondary
ammonium salts (Chart 1).9 A pseudorotaxane possessing a
hydroxy group at the axle terminus was quantitatively acylated
by bulky acid anhydride in the presence of tributylphosphane
as catalyst10 to afford the corresponding rotaxane in up to 90%
yield. In the resulting rotaxane, the intercomponent interaction
between the ammonium group and crown ether was so strong
that the acidity of the secondary ammonium group was unusually
low.11 The N-acylation of the ammonium group proceeded
slowly in the presence of excess triethylamine to give the
corresponding N-acylated rotaxanes quantitatively (Chart 1). By
N-acylation, the intercomponent interaction was expected to be
eliminated. However, the intercomponent interaction working
in the resulting rotaxane was unclear.
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5094 J. Org. Chem., Vol. 71, No. 14, 2006