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Tetrahedron Letters
journal homepage: www.elsevier.com
Tris-ACE-6,6’-bipyridyl--Cyclodextrin : Synthesis and Chirality of the Tripodal
Metal Complexes
Guillaume Poisson a , Florence Dumarçay-Charbonnier a and Alain Marsura a*
a SRSMC, Nancy Université & CNRS, Faculté des Sciences et Techniques, B. 70239, F-54506 Vandœuvre-lès-Nancy Cedex, France.
Fax: +33 (0)3 83 68 23 45; Tel: +33 (0)3 83 68 49 55; E-mail: Alain.Marsura@pharma.uhp-nancy.fr
ARTICLE INFO ABSTRACT
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
tris- ACE --Cyclodextrin
metallocyclodextrins
Circular Dichroism
helicates
In the past decade, seminal work describe the preparation and
properties of new metallocyclodextrins (metallo-CyDs)
molecular receptors. 1 Metallo-CyDs architectures are potentially
able to act in a wide range of applications such as: biomimetic
catalysts, 2a-c photoactive devices, 3a-c chemical sensors 4a-b due to
their properties after coordination to metals and formation of
host-guest complexes with small invited molecules. A few years
ago one of our research has concern the synthesis and
complexation properties of a novel symmetrical ureido-5,5’bipyridyl--CyD
tripode. 5 As expected, selective dual metal
complexation and fluorescence properties controlled via HSAB
metal classification were detected and analyzed. Nevertheless,
considering the conformational aspects, the ureido-5,5’-bipyridyl
tethers were found unable to generate expected chiral triple
helices after coordination with appropriate metal cations. It is
well known that spontaneous folding into a helical secondary
structure is based on a general molecular self-organization
process enforced by the conformational information encoded
within the primary structure of the molecular strand itself. 6 Key
features which determine the outcome of the assembly process
are the ligand design, flexibility of the linker groups joining the
coordination sites and the stereochemical preferences of the
coordinating metal ion. 7 Failure in the helicate formation with
ureido-5,5’-bipyridyl--CyD tripod was certainly due to an
improper preorganization (orientation) and have motivated the
synthesis of novel C 3-symmetrical tris-ACE-6,6’-bisheterocylic--CyDs
in which the heterocyclic unit is anchored at
the 6-position. Surprisingly, this minor modification, induces a
dramatic change into the conformation of the ureido-bisheterocyclic
strands around the upper-rim of the rigid CyD core.
This letter describes the synthesis of a novel C3-symmetrical tris-ACE-6,6’-bis-heterocylic--
CyD via “one-pot” Staudinger-Aza-Wittig reaction (SAW). Metal complexation behavior was
investigated by Uv-Vis and circular dichroism. Coordination of 6,6’-bipyridyl units with Cu II
and Ni II cations, spontaneously generates a strong exciton coupling-type positive Cotton effect
proving the formation of a -helix with a left-handed screw propeller (M).
2009 Elsevier Ltd. All rights reserved.
iii
N N reflux, 90min, h N N N N
PPh 3/NH 4OH 10%
OH
6
HO
B, D, F
Br
A, C, E
AcO
B, D, F
i ii
OH
OH
5 6 7
12
12
AcO
B, D, F
B, D, F
1 2
AcO
8
9
NBS/AIBN/CCl 4
N N
4
OAc
12
N N
HN
N
H
OAc
12
H 2N
N3 A, C, E
C O
+
A, C, E
N N
Scheme 1 : Synthetic pathway of 9
4
Br
OAc
12
A, C, E
1
3
Br N3 H 2N
NaN 3/DMSO
SAW
i = NBS/PPh 3/DMF
ii = Ac 2O/pyridin; 7h, 80°C
iii = NaN 3/DMF; 72h, 60°C
SAW = PPh 3/DMF/CO 2 + 4