Reinforced cyclam derivatives functionalized on the bridging unit

Article abstract of DOI:10.1039/c5nj03488d

A new synthetic method has been developed for the preparation of reinforced cyclams (1,4,8,11-tetraazacyclotetradecane) C-functionalized on the bridging unit, by using a "one pot" reaction starting from the appropriate bis-aminal cyclam intermediate. The high reactivity of quaternized aminal moiety toward nucleophilic agent has been used to elaborate a new class of cross-bridged and side-bridged cyclam derivatives containing cyanide group on the ethylene bridge. Several chelators and corresponding copper(ii) complexes have been prepared and characterized by X-ray diffraction. These new constrained polyazamacrocycles are valuable precursors of bifunctional chelating agents for the preparation of PET radiotracers.

Full text of DOI:10.1039/c5nj03488d

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Reinforced cyclam derivatives functionalized on
the bridging unit†
Cite this: New J. Chem., 2016,
40, 5829
Nicolas Sok,*a Isabelle Baglin,b Yoann Rousselin,b Frederic Boschetti,c
Claire Bernhard,b Christine Gozeb and Franck Denat*b
A new synthetic method has been developed for the preparation of reinforced cyclams (1,4,8,11-tetraaza-
cyclotetradecane) C-functionalized on the bridging unit, by using a ‘‘one pot’’ reaction starting from the
appropriate bis-aminal cyclam intermediate. The high reactivity of quaternized aminal moiety toward
nucleophilic agent has been used to elaborate a new class of cross-bridged and side-bridged cyclam
derivatives containing cyanide group on the ethylene bridge. Several chelators and corresponding
copper(II) complexes have been prepared and characterized by X-ray diffraction. These new constrained
polyazamacrocycles are valuable precursors of bifunctional chelating agents for the preparation of PET
radiotracers.
Received (in Montpellier, France)
8th December 2015,
Accepted 29th March 2016
DOI: 10.1039/c5nj03488d
www.rsc.org/njc
biological vector. To this aim, several BiFunctional Chelating
agents (BFCs) have been prepared by using well-established
Introduction
Cyclam based chelators are well known to form highly thermo- functionalization methods.5 The functionalization of a carbon
dynamically and kinetically stable complexes with various in beta position of the macrocycle is particularly relevant, since
metal ions, especially with copper(II) ions.1 Several strategies the nitrogen atoms remain available for further introduction of
based on the functionalization of the cyclam framework have coordinating arms.6 However, in major cases the synthetic
been investigated for the preparation of 64Cu chelator-mediated routes suffer from shortcomings such as multi-step processes,
radiopharmaceuticals (radioconjugates) for Positron Emission the preparation of sophisticated precursors as well as low overall
Tomography (PET) imaging applications. Indeed, the increasing yields.
use of copper radioisotopes (64Cu, 67Cu) in radiopharmaceutical
In this work we describe a new powerful route for the preparation
science has led to extensive research in this field. A new class of of original C-functionalized constrained cyclams by taking advantage
topologically constrained cyclams (‘‘side-bridged’’ and ‘‘cross- of the high reactivity of quaternized aminal intermediates toward
bridged’’ cyclams) has been developed, appearing to be promising nucleophilic agents. We have recently reported the synthesis of
candidates for the preparation of stable radioconjugates.2 The C-functionalized [13]aneN4 and triazacyclononanes derivatives
remarkable in vivo kinetic inertness of these compounds is due by reacting sodium cyanide with an aminal intermediate.7
to the presence of an ethylene bridge linking two non-adjacent Inspired by our previous work, we decided to combine our
(‘‘cross-bridged’’) or adjacent (‘‘side bridged’’) nitrogen atoms, ‘‘cyanide strategy’’ with the procedure commonly used for the
which increases the rigidity of the macrocycle.3 The corres- synthesis of side- or cross-bridged cyclams, for the preparation of
ponding radiolabeled compounds show remarkable in vivo the first generation of reinforced cyclams functionalized on the
stability in comparison to the ‘‘non bridged’’ H4teta (1,4,8,11- bridge. More precisely, we present in this paper a powerful and
tetrazacyclotetradecane-1,4,8,11-tetraacetic acid) derivatives.4
efficient route to obtain constrained cyclams in one step from
The preparation of 64Cu radioconjugates containing a con- bisaminal salts, carrying one or two nitrile groups on the bridge.
strained cyclam scaffold requires the presence of an additional
pendant arm for covalent attachment of the chelator to the
Results and discussion
Synthesis and characterization of reinforced cyclams
a AgroSupDijon, UMR PAM, PCAV team, 1 Esplanade Erasme, 21000 Dijon, France.
E-mail: nicolas.sok@agrosupdijon.fr
b
´
´
ICMUB, UMR CNRS 6302, Universite de Bourgogne Franche-Comte,
9 avenue Alain Savary, 21000 Dijon, France. E-mail: franck.denat@u-bourgogne.fr
c Chematech, 9 Avenue Alain Savary, 21000 Dijon, France
Constrained cyclams (1,4,8,11-tetraazacyclotetradecane) were
prepared from bisaminal glyoxal cyclam 1. Di- and mono-
quaternized bisaminal salts with two or one benzyl groups were
obtained by reacting 1 and alkyl halide in excess in acetonitrile
or in toluene respectively, to give selectively compounds 2a, 2b
† Electronic supplementary information (ESI) available. CCDC 1439077–1439082.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5nj03488d
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Scheme 1 Synthesis of bisaminal salts 2a, 2b, 6, and constrained cyclams
3, 4, 5a, 5b and 7.
and 6 as shown in Scheme 1. This selectivity is due to the
precipitation of the monoquaternized salt 6 in toluene, which
prevents further alkylation.
The formation of the side bridged compound 7 could be
explained by (i) the nucleophilic attack of cyanide ion on the
aminal carbon atom of 6, which is rendered highly electrophilic
by the quaternization of the adjacent nitrogen atom, resulting
in the cleavage of the C–N bond, (ii) the removal of the proton
on the same carbon atom by CNÀ or OHÀ, the consequent
formation of the double bond and cleavage of the bond
between the second aminal carbon and an adjacent nitrogen
Fig. 1 ORTEP view of compound 3 (top) and 4 (down) showing thermal
ellipsoids at the 50% probability level.
atom, driven by the formation of a thermodynamically favored temperature, the elimination step is slowed and a double
six-membered ring and the conjugated group –N–CQC–CN. nucleophilic substitution mechanism is favored, resulting in
The formation of the cross bridged cyclams 3 or 4 by reacting the formation of two dinitrile compounds 5a and 5b. These two
NaCN with the diquaternized bisaminal compound 2a or 2b compounds could be easily separated, since 5a crystallized in
can be explained by a similar mechanism: the nucleophilic the reaction mixture. Monocrystals of 5a and 5b have been
substitution on one of the two equivalents aminal carbon atom obtained and their X-ray structure are shown in Fig. 2. These
as described above (step i) followed by an elimination step, in two compounds appeared as two diastereoisomers, the chirality
this case facilitated by the quaternization of a second nitrogen of the nitrogen atoms resulting from the rigidity of the macro-
atom in trans position, which imposes the cleavage of the bond bicyclic scaffold, which hampers the independent inversion of
between the second aminal atom and the adjacent N+ atom.
the two nitrogen atoms.
Compounds 3 and 4 were isolated and monocrystals have
The compound 5a has both benzyl groups on the same side
been obtained by recrystallization in diethyl ether. The chemical of the bridge bearing nitrile groups, while in the compound 5b
structure of compounds 3 and 4 were obtained by X-ray diffrac- benzyl groups were found in an opposite way to the bridge. The
tion and are shown in Fig. 1.
X-ray diffraction analysis clearly shows that the two different
Both structures crystallize in a centrosymetric space group crystalline compounds correspond to the meso and erythro form
%
(P1), therefore having both enantiomers in the unit cell. In both of 5a and 5b respectively. They crystallize in a centrosymetric
compounds, the bridge connecting the two nitrogen atoms N2 space group C2/c and P21/c, which means that both enantiomers
and N4 imposes a strong constraint to the macrocycle besides to are present in each crystal.
the presence of a double bond between C41–C42 and C11A–C12A
1H and 13C NMR experiments were also performed to confirm
atoms with a distance of 1.361(3) Å and 1.358(2) Å respectively. the formation of the two diastereoisomers. Fig. 3 shows the
1
The orientation of the nitrogen atoms N1 and N3 are different in superimposition of H NMR spectra of compound 5a and 5b.
both structures. The benzyl groups are on the opposite side of Different signals corresponding to the diastereotopic geminal
the handle, whereas for the compound 4 the methyl group can protons Ha and Hb of the CH2 of benzyl groups can be observed.
be found sometimes on the same side and sometimes on the These protons appear as an AB system centered around
1
opposite side of the handle.
d = 3.50 ppm in the H spectrum of 5a (black line), while they
When the reaction of two equivalents of NaCN with the give rise to two doublets at d = 3.10 ppm and d = 3.70 ppm in the
diquaternized bisaminal compound is performed at room spectrum of compound 5b (red line). A significant difference
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Fig. 2 ORTEP view of compound 5a (top) and 5b (down) showing thermal
ellipsoids at the 50% probability level. Only H-atoms on the bridge were
shown for clarity.
Fig. 4 ORTEP view of compound [Cu(4)(H2O)2]2+ (top) showing thermal
ellipsoids at the 50% probability level. H-atoms and nitrate counterion are
omitted for clarity. View of coordination polyhedron (down). Selected bond
lengths (Å): Cu1–O2 = 2.339(5), Cu1–O1 = 1.989(5), Cu1–N3 = 2.255(6),
Cu1–N1 = 2.056(6), Cu1–N2 = 2.107(7), Cu1–N4 = 2.133(7).
Copper complex [Cu(4)(H2O)2](NO3)2 (Fig. 4) adopts an Oh
symmetry, for which the four nitrogen atoms of the ligand and
two water molecules complete the coordination sphere of the
metal. The equatorial plane is determined by the three nitrogen
donor atoms N1, N2, N4 and one oxygen donor atom O1. In the
literature, analog complexes are available without the ethylenic
bridge and form a square-based pyramid geometry where the
copper coordination sphere was completed by one halogen.9
One notices octahedron elongation in the N3–Cu1–O2 axis. This
deformation is related to the d9 copper(II) electronic configuration
and reflects an energy stabilization of the complex by lowering of
symmetry, according to the Jahn–Teller effect. Intermolecular
hydrogen bonds between coordinated water molecules and nitrate
counterion provide crystalline stability.
Fig. 3 Superimposition of partial 1H NMR spectra (600 MHz) of com-
pound 5a (black) and 5b (red) in CDCl3 at 298 K.
could also be observed on the chemical shift of the signal of
hydrogen on the carbon atom bearing the cyanide group. This
singlet signal appears at d = 5.60 ppm in compound 5a and at
d = 5.00 ppm in compound 5b.
Characterization of copper(II) complexes
In the [Cu(5)(NO3)](NO3)(CH3OH) complex (Fig. 5) the coordina-
Complexation of copper(II) by different constrained cyclams tion sphere is completed by a nitrate counterion disordered over two
was performed and crystals of Cu(II) complex from compounds positions (52%/48%), instead of water molecules, and the pendant
4 and 5a have been obtained. This Cu(II) cross-bridged cyclams arms are directed away from the macrocycle. As a consequence, the
were prepared from Cu(NO3)2 and the ligand in reflux of coordination polyhedron undergone most distorded than previously.
methanol. Both complexes adopt a folded cis-V configuration The continuous shape measure (CShM) criterion SQ(P), introduced
with the copper(II) positioning within the macrocyclic cavity.8
by Avnir and co-workers,10 reflects the minimal normalized square
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Fig. 6 View of coordination geometry of compound [Cu(5)(NO3)]+ with
major disordered part.
structure of these complexes showed an interesting variation
in the coordination environment and the potential for distor-
tion in the Cu–N bonds of the macrocyclic chelator.
Conclusions
The work described herein reports the synthesis of a new family
of constrained cyclams bearing one or two nitrile groups on the
bridging unit. The reaction of two equivalents of sodium cyanide
on quaternized bisaminal derivatives in ethanol at 80 1C resulted
in the unexpected formation of an acrylonitrile bridge, either
linking two adjacent nitrogen atoms starting from the mono-
quaternized bisaminal salt, or two opposite nitrogens starting
from the diquaternized bisaminal derivative. The expected dinitrile
cross-bridged compound was obtained as two diastereoisomers
when the reaction was performed at room temperature. The
results were fully supported by X-ray structures and NMR analyses.
Two copper complexes were prepared and X-ray diffraction shows
that both of them adopt a folded cis-V conformation with the
copper atom lying within the cavity of the macrobicycle, as
described in the literature for crossbridged cyclams. Further
work is ongoing for the preparation of bifunctional chelating
agents (BFC) starting from these valuable constrained macrocyclic
precursors. This will imply one or several of the following steps:
(i) the removal of benzyl groups through catalytic hydrogenolysis,
(ii) the introduction of coordinating acetate arm(s) on the resulting
secondary amine(s), (iii) the reduction of nitrile group(s) and
introduction of a suitable function to allow further bioconjugation
of the BFC. Physicochemical studies will also be performed in
order to evaluate the stability of the complexes and the kinetic of
formation. The compounds thus obtained could represent a
promising alternative to the BFC previously used for the synthesis
of 64Cu radioconjugates.
Fig. 5 ORTEP view of compound [Cu(5)(NO3)]+ with major disordered
part (top) and minor disordered part (down) showing thermal ellipsoids at
the 50% probability level. H atoms, solvent and counterion are omitted for
clarity. Selected bond lengths (Å): Cu–N1 = 2.102(7), Cu1–N2 = 2.184(7),
Cu–N3 = 2.079 (7), Cu–N4 = 2.120(6), Cu–O1A = 1.968(11), Cu–O2A =
2.641(12), Cu–O1B = 2.715(16), Cu–O2B = 2.042(16).
deviation between the Cartesian coordinates of each vertex that
belongs to the actual coordination polyhedron Q and a perfect
reference geometry P. Accordingly, SQ(P) ranges between 0 and
100, a closer structural match that gives a lower SQ(P) value.
Casanova and co-workers reported shape maps and reference poly-
hedra for six-vertex structures. The lowest values of SQ(P) was
determined, for different shape with 5 or 6 vertices, by continuous
shape measure calculations using the SHAPE 2.1 Program.11 The
SQ(P) parameter for [Cu(4)(H2O)2](NO3)2 has a minimal value of
0.824 for OC-6 (Octahedron) geometry. In compound [Cu(5)(NO3)]-
(NO3)(CH3OH), the minimal SQ(P) value was found for vOC-5
(Johnson square Pyramid (J1)) geometry. Furthermore, the distance
Cu–O2A and Cu–O1B in [Cu(5)(NO3)](NO3)(CH3OH) are unusually
long that confirm geometry shown in Fig. 6.
Experimental section
General
Such a coordination environment was already described in
the literature by Archibald et al. for bis-benzyl cross-bridged NMR spectra were recorded with a Bruker 300, 500, or 600 spectro-
cyclam derivatives coordinated to copper(II).12 The X-ray meter (300, 500, or 600 MHz for 1H; 75, 125, or 150 MHz for 13C).
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Chemical shifts are reported in d ppm referenced to the CDCl3, 300 K) d (ppm): 1.42 (m, 2H), 1.62 (m, 2H), 2.20 (m, 2H),
residual peak of [D]chloroform (d = 7.25 or 77.00 ppm for 2.37 (m, 2H), 2.49–2.55 (m, 8H), 2.66 (m, 2H), 2.77 (m, 2H), 3.13
1H or 13C). The following abbreviations are used; s: singlet, (bs, 2H), 3.20 (m, 2H), 3.72 (m, 2H), 7.30 (m, 10H). 13C{1H} NMR
d: doublet, t: triplet, q: quartet, m: multiplet, br.: broad. (150 MHz, CDCl3, 300 K) d (ppm): 25.9 (CH2-b), 47.9, 52.6, 55.0,
Elemental analyses were performed at the PACSMUB at the 57.6, 58.9, 59.0, 59.1 (CH and CH2-a), 116.5 (CN), 127.5, 128.7,
University of Burgundy. Mass spectra were obtained by ESI 129.2, 138.8 (CH and C-Ar). ESI-TOF: m/z = 457.31 [M + H]+.
(electrospray ionization) with a Bruker MicroTOF-Q spectro- IR (cmÀ1): 2221 (CN). Elemental analysis: C28H36N6, calculated:
meter. Infrared spectra were recorded with a Bruker Vector 22 C (73.65%), H (7.95%), N (18.40%), found: C (73.61%), H (8.80%),
spectrometer in ATR mode.
N (18.27%). Mp = 193.8 Æ 0.5 1C.
Bisaminal cyclam 1 was purchased from Chematech SA.
4,11-Dibenzyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane-15,16-
Compound 2a, 2b and 6 were prepared using the procedure dicarbonitrile (5b). After isolation of compound 5a, filtrate was
detailed by Wong and Weisman.13 All other chemicals and concentrated. The residual oil was taken in methanol. The
solvents were of analytical reagent grade and were used as compound 5b was obtained as colorless crystals after slow
received. Caution: NaCN is highly toxic and should be handled evaporation of solvent (m = 1.06 g, 2.30 mmol, yield = 35%).
with care.
1H NMR (600 MHz, CDCl3, 300 K) d (ppm): 1.58 (m, 2H), 1.69
(m, 2H), 2.31 (m, 2H), 2.47 (m, 4H), 2.54 (m, 2H), 2.77 (m, 2H),
2.87 (m, 4H), 3.22 (m, 2H), 3.48 (m, 4H), 5.31 (s, 2H), 7.26 (m,
X-ray crystallography
CCDC deposition numbers 1439077–1439082 for compound 3, 6H), 7.35 (m, 4H). 13C{1H} NMR (150 MHz, CDCl3, 300 K) d
4, [Cu(4)(H2O)2](NO3)2, 5a, 5b and [Cu(5)(NO3)](NO3)(CH3OH). (ppm): 29.2 (CH2-b), 53.2, 53.6, 54.9, 56.7, 60.2, 60.7 (CH and
For each experimental structure details please refer to the ESI.† CH2-a), 116.9 (CN), 127.7, 128.7, 129.3, 138.8 (C-Ar). ESI-TOF:
Some B alerts (short intra HÁ Á ÁH contact) are generated for m/z = 457.31 [M + H]+.
[Cu(4)(H2O)2](NO3)2 and [Cu(5)(NO3)](NO3)(CH3OH) because
5-Benzyl-1,5,8,12-tetraazabicyclo[10.2.2]hexadec-15-ene-15-
there is poor data quality or large amount of disorder in the carbonitrile (7). This compound was synthesized according to
structure. the procedure used for the synthesis of 3 starting from compound 6
4,11-Dibenzyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadec-15-ene-15- (1.94 g, 4.94 mmol) and NaCN (0.48 g, 10.0 mmol). The
carbonitrile (3). 0.30 g of sodium cyanide (7.80 mmol) was added compound 7 was obtained as a colorless oil (m = 1.32 g,
1
to a suspension of 2a (1.0 g, 3.9 mmol) in ethanol (20 mL) at 3.90 mmol, yield = 70%). H NMR (600 MHz, CDCl3, 300 K) d
reflux. The mixture was stirred during 2 hours. Then, the mixture (ppm): 1.72 (m, 3H), 1.91 (m, 1H), 2.22 (m, 1H), 2.37 (m, 1H),
was filtrated and the solvent was removed by evaporation. The 2.45 (m, 2H), 2.55 (m, 2H), 2.70 (m, 1H), 2.84–3.17 (m, 8H), 3.38
compound 3 was obtained as a white powder (m = 0.54 g, (m, 2H), 3.60 (m, 2H), 6.48 (s, 1H), 7.23 (m, 5H). 13C{1H} NMR
1
1.25 mmol, yield = 75%). H NMR (500 MHz, DMSO-d6, 373 K) (150 MHz, CDCl3, 300 K) d (ppm): 25.8, 28.2 (CH2-b), 43.9, 46.6,
d (ppm): 1.44–1.88 (m, 5H), 2.37–2.94 (m, 16H), 3.51 (m, 2H), 3.71 47.7, 48.2, 50.2, 51.7, 53.3, 56.5, 58.3 (CH2-a), 120.9 (CN), 126.8,
(m, 3H), 6.48 (s, 1H), 7.32 (m, 10H). 13C{1H} NMR (125 MHz, 128.1, 128.9, 135.7 (C alkene and C-Ar), 139.1 (CH-alkene). ESI-
DMSO-d6, 373 K) d (ppm): 26.3, 27.3 (CH2-b), 51.4, 51.5, 51.6, 52.7, TOF: m/z = 340.24 [M + H]+.
54.4, 55.6, 55.9, 59.0, 61.1 (CH2-a), 118.7 (CN), 127.1, 127.2, 128.4,
Preparation of complexes [Cu(4)(H2O)2](NO3)2 and [Cu(5)(NO3)]-
129.1, 129.8, 140.1 (C alkene and C-Ar), 148.4 (CH alkene).
(NO3)(CH3OH). Compounds [Cu(4)(H2O)2](NO3)2 and [Cu(5)-
4,11-Dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadec-15-ene- (NO3)](NO3)(CH3OH) were obtained as blue crystals after cooling
15-carbonitrile (4). This compound was synthesized according of a solution of Cu(NO3)2 (0.08 mmol) in 2 mL MeOH and of
to the procedure used for 3 starting from 2b (m = 0.5 g, compound 4 or 5a (0.08 mmol) in 15 mL of MeOH heated at
0.9 mmol) and NaCN (0.1 g, 1.9 mmol). Compound 4 was reflux during 30 min.
obtained as colorless crystals (m = 0.2 g, 0. 72 mmol, yield =
72%). 1H NMR (500 MHz, CDCl3, 300 K) d (ppm): 1.35–1.59
(m, 2H), 1.67–1.82 (m, 2H), 2.20 (s, 3H), 2.29 (s, 3H), 2.34–2.91
Acknowledgements
(m, 15H), 3.07 (m, 1H), 6.38 (s, 1H). 13C{1H} NMR (175 MHz,
Support was provided by the CNRS, the University of Burgundy and
CDCl3, 300 K) d (ppm): 23.7, 25.7 (CH3), 26.6 (*2) (CH2-b), 44.8,
´
the Conseil Regional de Bourgogne through the 3MIM Project. N. S.
47.8, 51.9, 52.2, 54.6, 55.0, 55.9, 58.1, 59.9 (CH2-a), 119.2 (CN), 140.3
(C alkene), 147.2 (CH-alkene). ESI-TOF: m/z = 300.2 [M + H]+.
4,11-Dibenzyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane-15,16-
dicarbonitrile (5a). A suspended solution of compound 2 (3.60 g,
6.40 mmol) and NaCN (0.62 g, 12.80 mmol) in ethanol (20 mL)
was stirred for 3 days at room temperature. The compound 5a
was obtained after filtration. The residual white solid was taken
in chloroform (20 mL). The impurities salts were eliminated by
filtration. After evaporation of solvent, the colorless oil was taken
in diethyl ether, the compound 5a was obtained as colorless
crystals (m = 1.06 g, 2.30 mmol, yield = 35%). 1H NMR (600 MHz,
and C. B. thank the French Ministry of Research for PhD Grant.
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