Facile Access to the 12-Membered Macrocyclic Ligand PCTA and Its Derivatives with Carboxylate, Amide, and Phosphinate Ligating Functionalities

Article abstract of DOI:10.1002/ejoc.201800066

We describe a convenient synthetic pathway to access the 12-membered PCTA macrocycle, a polyaminocarboxylate ligand for which its M²⁺ and M³⁺ complexes are commonly associated with applications in biomedical diagnostics and radiother

Full text of DOI:10.1002/ejoc.201800066

A Journal of
Accepted Article
Title: Facile access to 12-membered macrocyclic ligand PCTA and
novel derivatives including carboxylate, amide and phosphinate
ligating functionalities.
Authors: Claude Picard, Morgane Enel, Nadine Leygue, Nathalie
Saffon, and Chantal Galaup
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800066
Link to VoR: http://dx.doi.org/10.1002/ejoc.201800066
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
Facile access to 12-membered macrocyclic ligand PCTA and novel derivatives including
carboxylate, amide and phosphinate ligating functionalities.
Morgane Enel,a,b Nadine Leygue,a,b Nathalie Saffon,c Chantal Galaup,*a,b and Claude
Picard*a,b
Keywords: Macrocyclic ligands – template synthesis - lanthanides
Abstract
We describe a convenient synthetic pathway for the access of the 12-membered PCTA
macrocycle, a polyaminocarboxylate ligand whose M2+ and M3+ complexes are commonly
associated with applications in biomedical diagnostic and radiotherapy. The synthetic
pathway is based on the use of a linear tri-N-alkylated triamine synthon incorporating masked
acetate arms and an efficient sodium template ion effect for the crucial macrocyclization step
(87 % macrocyclization yield). This approach was then successfully applied to the access of
three new PCTA[12] derivatives containing either a 4-pyridol unit either amide or
phosphinate coordinating functions in the central side arm. In all cases macrocyclization
reactions are controlled by a sodium template ion effect and display macrocyclization yields
in the range 60 – 75 %. The luminescence or relaxometric properties behaviour of Eu(III),
Tb(III) or Gd(III) complexes derived from mixed coordinating arms PCTA derivatives are
also investigated, making these complexes as possible alternative candidates for PCTA[12]-
Ln(III) complexes.
aCNRS; Laboratoire de Synthèse et Physico-Chimie de Molécules d’Intérêt Biologique,
SPCMIB, UMR-5068; 118 Route de Narbonne, F-31062 Toulouse cedex 9, France
E-mail : picard@chimie.ups-tlse.fr galaup@chimie.ups-tlse.fr
http://spcmib.univ-tlse3.fr
bUniversité de Toulouse; UPS; Laboratoire de Synthèse et Physico-Chimie de Molécules
d’Intérêt Biologique, SPCMIB; 118 route de Narbonne, F-31062 Toulouse cedex 9, France
C Institut de Chimie de Toulouse (FR 2599), 118 Route de Narbonne, F-31062 Toulouse cedex
9, France
Supporting information and ORCID(s) from the authors(s) for this article are available on the
WWW under https://
1
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
Introduction
Polyazamacrocyclic ligands continue to receive considerable attention due to the widespread
utility of their metal complexes. In particular, metal complexes derived from acetate
functionalized polyaza macrocycles are commonly associated with applications in the general
area of biomedical imaging and radiotherapy. In these fields, most of the investigated
macrocyclic ligands are built on a limited number of coordinating core structures, such as
DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) or NOTA (1,4,7-
triazacyclononane-1,4,7-triacetate).[1] The popularity of these chelators is such that several
precursors are now marketed. PCTA[12] (3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-
1(15),11,13-triene-3,6,9,-triacetic acid) is another noticeable example of this family of
macrocyclic ligands. It was described for the first time by Stetter et al..[2] More recently, it
was assessed as an efficient M2+/3+ chelator in various metal complexes (Figure 1).
Indeed, this aminocarboxylic acid derivative of Pyclen is known to form very stable M2+ / M3+
chelates in aqueous solutions (log KCuL = 18.79, log KGdL = 20.39, log KInL = 24.7) and its
complexes display promising properties in the field of diagnostic imaging.[3] The initial use of
PCTA involved the gadolinium ion and was dedicated to magnetic resonance imaging
(MRI).[4] Due to the presence of two water molecules in the first coordination sphere of the
310K
metal, Gd-PCTA chelate has higher water relaxivity (r1
= 5.4 s-1 mM-1 at 20 MHz) than
310K
other clinically approved magnetic resonance imaging contrast agents (r1
= 3.5 – 4.8 s-1
mM-1).[5] Other lanthanide complexes, Eu-PCTA and Tb-PCTA, are characterized by the
presence of an antenna/sensitizer effect of the pyridine chromophore and act as long-lifetime
luminophores compatible with the use of time resolved luminescence technique.[6] More
64
68
recently, PCTA was investigated with two β+ emitters used for PET imaging, Cu and Ga,
and these complexes displayed much faster radiolabeling kinetics at room temperature than
DOTA.[7] PCTA and its derivatives have also been successfully used with other radiometals
90
such as 111In, a γ emitter used for SPECT imaging, as well as Y and 177Lu, two β- emitters
used for targeted radiotherapy.[8,9] These data show that PCTA[12] has a significant impact on
all major imaging modalities based on metallic complexes, thus making it an additional tool
of value to traditional ligands such as DOTA or NOTA.
Since the pioneering work of Stetter et al.,[2] thirty-five years ago, the access to PCTA[12]
and its derivatives is only reported using on the Richman-Atkins methodology which affords
high yields in the crucial macrocyclization step without high dilution conditions.[10] In our
2
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
earlier works, we developed a new friendly synthetic methodology which allows the
preparation of 15-21 membered macrocyclic chelators derived from tri- or tetra-aminoacetate
moieties and a (poly)pyridine intracyclic unit in good yields.[11] In this report, we wish to
demonstrate that this synthetic methodology is also very efficient to synthesize 12-membered
PCTA ligand and provides a facile access to novel PCTA derivatives containing either a 4-
pyridol unit either amide or phosphinate functions in the central side arm (Figure 1).
Figure 1. Structure of DOTA and NOTA ligands and PCTA[12] ligands studied in this work
Results and discussion
Currently, the route for the synthesis of PCTA is based on the preparation of Pyclen, a cyclen
derivative incorporating a diethylenetriamine backbone and an intracyclic pyridine unit.
(Scheme 1, route A). The access of this tetraazamacrocycle is based on the classical and time-
tested Richman-Atkins macrocyclization methodology which requires the use of
diethylenetriamine bearing arylsulfonamide (tosyl or nosyl) groups. In the crucial
macrocyclization step, these bulky protecting groups are expected to restrict the rotational
freedom of the open-chain intermediate and to shield it from intermolecular processes,
promoting the cyclization reaction.[12] Although this methodology leads to high
macrocyclisation yield for N,N',N"-trinosyl or tritosyl Pyclen derivative (76 and 94%,
respectively), the three/four-step overall yield for the synthesis of PCTA[12] following this
strategy is modest (27 or 36%).[13] This is a consequence of the two postmacrocyclization
3
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
steps yields: firstly, the cleavage of arylsulfonyl groups, and secondly, the per N-
carboxyalkylation reaction, a tricky step which may require optimization of reaction
conditions. However, this strategy allows easy variations in the nature of the pendant
coordinating arms (amide, phosphonic acid,…).
In our strategy (Scheme 1, route B), the key macrocyclization step involves a
diethylenetriamine derivative incorporating three masked acetate arms as tert-butyl ester and
two terminal secondary amine groups. So only a single simple reaction (deprotection of ester
functions) has to be conducted from the macrocyclic material. Besides, the presence of
coordinating atoms in this triamine may control the formation of the 12-membered ring by a
"metal-template" ion effect, thus avoiding the use of high dilution technique in the
macrocyclization step. This approach allows variation on the pyridine unit without the
reengineering of the whole macrocyclic molecule.
Scheme 1. General methodologies for the synthesis of PCTA[12]
In route B, the key triamine bearing tert-butyl ester groups may be prepared according two
independent synthetic pathways as depicted in Scheme 2. Our previously reported synthetic
pathway involves diethylenetriamine as starting material and affords the desired compound in
two steps and 74% overall yield.[14] Briefly, reaction of diethylenetriamine with benzaldehyde
and further reductive amination provided the N,N″-benzyl derivative 2. Trialkylation of 2 with
tert-butyl bromoacetate provided the targeted compound 6. In the present work, we have
investigated an alternative synthetic pathway in a three-step sequence starting from
commercially available N-benzyl ethanolamine 3. After N-alkylation of 3 with tert-butyl
bromoacetate in the presence of an inorganic base (Na2CO3 or K2CO3), the hydroxyl group of
4 was then replaced by the bromine group with the NBS/PPh3 system, using standard
procedure. Reacting bromide 5 in a 2 fold excess with glycine tert-butyl ester in CH3CN at
4
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
reflux and in the presence of K2CO3 afforded triamine compound 6 in 82% yield after
purification by column chromatography. This three-step sequence affords 6 in a slightly lower
overall yield (70% vs. 74%) but allows to introduce two different coordinating groups within
the diethylenetriamine backbone (vide infra). Debenzylation of 6 was readily achieved in a
quantitative yield by catalytic hydrogenation at room temperature under hydrogen pressure (6
bars), in methanol and using Pd/C (10 wt %) as catalyst. With these experimental conditions
of hydrogenolysis, we did not observe any cleavage of the diethylenetriamine backbone as
reported by Price et al. that use a Pd/C (5 wt %) / glacial acetic acid protocol.[15]
Scheme 2. Synthesis of triamine 7. Reagents and conditions: (a) (i) C6H5CHO, EtOH, 50 °C, 2h; (ii) NaBH4,
EtOH, 50 °C, 2h, then rt, 15h; (iii) HCl (aq, 37 %). (b) BrCH2CO2tBu, DIPEA, DMF, rt, 48h. (c) BrCH2CO2tBu,
K2CO3, CH3CN, rt, 24h. (d) N-bromosuccinimide, PPh3, CH2Cl2, rt 16h. (e) NH2CH2CO2tBu Hydrochloride salt,
K2CO3, CH3CN, reflux, 24h. (f) Pd/C (10%), H2 gas (6 bars), MeOH, rt, 12h.
In addition, methyl ester was tentatively used as protection for the acetic acid chains in the
key tri-substituted diethylenetriamine core (Scheme 3, compound 13). The alkylation of N-
benzyl ethanolamine with methyl bromoacetate in the presence of K2CO3 as a base and in
CH3CN at room temperature provided exclusively the lactonization product, 4-benzyl-
morpholin-2-one 8. This result was not unexpected because this side product was also
observed for alkylation of 3 with benzylbromoacetate.[16] Consequently, a set of general
conditions (solvent, base) was established and the main facets are gathered in Table 1. The
presence of N,N-diisopropylethylamine (DIPEA) as base and DMF as solvent gave the best
1
results, with the exclusive formation of the targeted product 9 as determined by H NMR
analyse of the crude reaction mixture. Attempted purification by chromatography led to a
mixture of alcohol 9 and lactone 8, as a result of silica-mediated cyclization. However, the
crude material was of sufficient purity for further derivatization of the alcohol function of 9
into bromide by using the NBS/PPh3 protocol, with a yield of 70% over the two steps. The
dialkylation of glycine methyl ester by bromide 10 was carried out in DMF and in the
5
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
presence of DIPEA and provided the desired product 11 in low yield (28%), due to the
extensive formation of a side product. During the reaction, intramolecular attack of the
secondary amine issued from the partially alkylated reaction intermediate on methyl ester
bond led to cyclization with the formation of piperazin-2-one derivative 12. Owing to this side
reaction, debenzylation of 11 by hydrogenolysis was not successfully met, leading to a
complex, difficult to purify, mixture.
C6H5
N
C6H5
a
C6H5
N
O
O
e
OH 100%
N
C6H5
8
N
N
HN
MeO2C
13
N
NH
H
3
MeO2C
CO2Me CO2Me
CO2Me CO2Me
+
(28%)
11
OH
COOMe
Br
Br
N
b
C6H5
c
d
N
C6H5
+
100%
70%
CO2Me
9
COOMe
C6H5
(48%)
CO2Me
10
N
N
O
12
Scheme 3. Synthesis of triamine 11. Reagents and conditions: (a) BrCH2CO2Me, K2CO3, CH3CN, rt, 24h. (b)
BrCH2CO2Me, DIPEA, DMF, rt, 24h. (c) N-bromosuccinimide, PPh3, CH2Cl2, rt 16h. (d) NH2CH2CO2Me,
DIPEA, DMF, rt, 16h. (e) Pd/C (10%), H2 gas (6 bars), MeOH, rt, 12h.
Table 1. Screening of bases and solvents[a]
Base
Solvent
Compound 8
Compound 9
(%)[b]
(%)[b]
K2CO3
K2CO3
Et3N
CH3CN
DMF
100
87
100
32
16
0
0
13
0
CH3CN
DMF
Et3N
68
86
100
DIPEA
DIPEA
CH3CN
DMF
[a] Reactions were carried out for 24 h at room temperature
with equimolar reagents and base. [b] Percentages obtained
from 1H NMR spectra (300 MHz, CDCl3) of the crude reaction
mixtures. No starting material was observed in the crude
reaction mixture.
Consequently, the macrocyclization reaction was attempted between tert-butyl acetate
functionalized triamine 7 and 2,6-dibromo pyridine. This reaction was carried out under a
batchwise procedure (reactants concentration: 2 × 10-3 M) and under heterogeneous
6
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
1
conditions in refluxing acetonitrile (48 h) with sodium carbonate as a base (Scheme 4). H,
13C NMR and MS analyses of the crude reaction product evidenced the presence of a major
.
species, characterized as the sodium monomeric complex 14 Na. Purification by column
.
chromatography led to the isolation of a mixture of 14 Na and free ligand as a result of
alumina-mediated dissociation. This could be expected because of the soft character of N-
donor atoms towards alkali metal cations. Finally, treatment of this mixture with saturated
EDTA aqueous solution cleanly afforded the free ligand 14 in 87% isolated yield. Besides,
.
treatment of free ligand 14 by Na2CO3 in CH3CN at reflux afforded quantitatively 14 Na
complex. It can also be noticed that the macrocyclization reaction with dibromo derivative
took place within 2 h under microwave irradiation (300 W, 90°C), affording isolated 14 in
1
.
77% yield. It is worth noting that 14 and 14 Na species can be readily distinguished by H
NMR spectroscopy, reflecting a fluxional macrocyclic system for the free ligand 14 and a
.
more rigid system for 14 Na because of the presence of sodium ion.(Figure 2). On the other
hand, Complexation by Na+ was also accompanied by a reduction in the frequency of the
carbonyl infrared band (8 cm-1), suggesting that the ester groups are involved in complexation.
.
In order to confirm the presence of the 14 Na complex, we attempted to get single crystals
+
.
suitable for X-ray diffraction analysis. A single crystal of [14 Na] was obtained by the
crystallization from a mixture of free ligand 14 and NaClO4 in methanol and the molecular
structure of the complex is shown in Fig 3.[17] The metal ion is coordinated by the four
nitrogen atoms of the tetraazamacrocyclic core and the three carbonyl oxygen atoms provided
by the ester side arms. The Na-O and Na-N bond distances spanning in the range of 2.382(4) -
2.460(5) and 2.421(6) - 2.641(5) Å, respectively, are comparable with those previously
reported for an analogous Na+ complex derived from a cyclen derivative.[18]
Scheme 4. Synthesis of ligand H3L1 (PCTA). Reagents and conditions: (a) [reagents] = 2×10-3 M, Na2CO3,
CH3CN, reflux, 48 h; (b) aqueous 6N HCl, reflux, 12 h.
7
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
H8
H7
H6
*
H9
H2
H3b
H3a
H1
7.7 7.6 7.5 7.4 7.3 7.2 4.1 3.9 3.7 3.5 3.3 3.1
1.55 1.50 1.45
H5
H4 H4
8.0
7.5
7.0
6.5
6.0
5.5
5.0
δ
4.5
4.0
3.5
3.0
2.5
2.0
1.5
(ppm
H8
H9
H6,7
H2
H3
H1
*
H4,5
1
.
Figure 2. H NMR spectra of 14 and 14 Na in CDCl3 at 300 MHz (* = solvent peak)
.
Figure 3. Complex [14 Na]ClO4 structure. All hydrogen atoms were omitted for clarity.
The role of the respective nature of the leaving groups and of the base was then examined for
the ring closure step by using the same experimental conditions (Table 2). A change in the
8
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
leaving group has a marked effect upon the yields of the macrocyclization reaction (entries 1,
3, 4). Tosylate and chloride gave product of comparable purity but in lower yields. The role of
the nature of the base was examined during nucleophilic displacement of the bromide for the
ring closure. The replacement of sodium carbonate by the organic base DIPEA does not
inhibit the macrocyclization reaction as one might expect in the case of a metal ion template
effect but decreased the yield of 14 to 53% (entries 1, 5). The difference in yields between 53
and 87%, whereas regarded as rather moderate, may reflect a template effect of the sodium
cation. Changing the counter-cation of the inorganic base also affected intramolecular
cyclization (entries 1, 6-8). In acetonitrile, sodium carbonate was the best catalyst followed by
lithium, potassium and cesium carbonates, which gave decreasing yields of the 12-membered
ring. We can note that the so-called “cesium effect” which has been found to be beneficial in
Richman-Atkins cyclisation did not operate here.[19] In these reactions, no detectable 2:2
cyclization product was observed by MS analyses of the crude reaction mixture.
Table 2. Screening of leaving groups and bases in the
macrocyclisation reaction[a]
Entry
X
Base
14 (isolated
yields %)
1
2[b]
Br
Br
Na2CO3
Na2CO3
Na2CO3
Na2CO3
DIPEA
Li2CO3
K2CO3
87
77
35
56
53
59
48
26
3
Cl
4
OTs
Br
5
6
Br
7
Br
8
Br
Cs2CO3
[a] Reactions conditions: [reactants] = 2 × 10-3 M, base 1
equiv, CH3CN, reflux, 48 h. [b] Reaction under microwave
irradiation.
Finally, subsequent hydrolysis under acid conditions (aqueous 6N HCl) of the tert-butyl
functions of 14 afforded H3L1 (PCTA) as its tetra hydrochloride salt in a quantitative yield.
9
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
By using this approach, our currently obtained better macrocyclization yield (87%) can be
favourably compared with those previously reported for the macrocyclization step leading to
N,N′,N″-arylsulfonamide Pyclen derivative ( 70%).[13] Moreover PCTA can be obtained in a
better overall yield (60-65%) starting from diethylenetriamine or N-benzyl ethanol amine.
As a first illustration of this approach, we investigated the preparation of a PCTA platform
derived from a 4-hydroxy pyridine moiety. The synthesis of this compound (L2) starting from
chelidamic acid is depicted in Scheme 5.
Scheme 5. Synthesis of ligand L2. Reagents and conditions: (a) SOCl2, MeOH, reflux, 6h (b) 3,4-dihydro-
2Hpyran, pyridinium p-toluenesulfonate, CH2Cl2, rt, 16h then 50 °C, 4h. (c) NaBH4, CaCl2, EtOH, 0 °C, 2h then
rt, 1h. (d) TsCl, KOHaq, CH2Cl2, rt 16h. (e) compound 7, Na2CO3, CH3CN, reflux, 24h.
Esterification of chelidamic acid provided dimethylester 15 which upon protection of the
hydroxyl group with dihydropyran (DHP) gave the protected diester 16. Although NaBH4 was
effective for reducing this diester, a mixture of NaBH4 and CaCl2 produced diol 17 in a better
yield (80 vs. 60 %). Our trials for conversion of alcohol groups of 17 to bromo groups in
neutral conditions (NBS / PPh3 system) were not successful. Hence we turned our efforts to
the use of the corresponding bis sulfonate ester derivative 18 for the macrocyclization step.
Intermolecular cyclization of triamine 5 and ditosylate 18 was accomplished under batchwise
procedure and in the presence of Na2CO3, as described for 14. After working up under mild
acidic conditions and purification by column chromatography on alumina, free ligand L2 was
obtained in 59 % yield. This result is in line with that observed for macrocycle 14 using
tosylate as a leaving group (Table 2, entry 4). In comparison, Lincoln et al. reported the
synthesis of a pyridol-based Pyclen in a three step-sequence and in analogous overall yield,
starting from nosylated diethylenetriamine and a benzyl protected 4-pyridol derivative.[20] The
10
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
1
presence of the pyridone tautomer in chloroform was assessed by NMR analyses. The H
NMR spectrum displays a singlet at 6.07 ppm, characteristic of two aromatic protons in a
pyridone ring and the 13C NMR spectrum displays a signal at 180 ppm, attributed to C4 atom
of the pyridine ring and indicative of a carbonyl function.[21]
Then, we investigated the flexibility of this synthetic approach, studying the possibility to mix
the nature of coordinating functions. In this further goal, we planned the access to PCTA[12]
derivatives where amide and phosphinate functions are substituted for the carboxylic acid
function in the central side arm (H3L3 and H2L4, scheme 6).
CO2tBu
O
N
tBuO2C
O
N
O
b
a
c
P
N
N
N
H2N
O
H5C6
77%
37%
C6H5
98%
Me
Br
OEt
P
O
O
OEt
19
20
PO(OEt)Me
21
Me
d
100%
CO2tBu
tBuO2C
N
N
N
N
f
e
HN
N
NH
CO2H
CO2tBu
N
HO2C
N
tBuO2C
N
N
N
+
62%
100%
Br
Br
H3L3
23
22
PO(OEt)Me
PO(OH)Me
PO(OEt)Me
O
CO2tBu
tBuO2C
g
h
C6H5CH2O
CONMe2
COOH
H2N
N
N
N
N
H
90%
67%
H5C6
C6H5
25
24
CONMe2
i
100%
CO2tBu
tBuO2C
HN
26
N
N
j
k
N
NH
CO2H
CO2tBu
N
HO2C
H2L4
N
tBuO2C
N
N
N
+
100%
73%
N
N
Br
Br
27
CONMe2
CONMe2
CONMe2
Scheme 6. Synthesis of ligand H3L3 and H2L4. Reagents and conditions: (a) diethyl methylphosphonite, xylene,
300 mBar, 80 °C, 2h. (b) Hydrazine, EtOH, rt, 16h. (c) compound 5, K2CO3, CH3CN, reflux, 24h. (d) Pd/C
(10%), H2 gas (6 bars), MeOH, rt, 12h. (e) Na2CO3, CH3CN, reflux, 48h. (f) aqueous 6N HCl, reflux, 12 h. (g)
according to25, (i) Me2NH.HCl, EDCI, HOBt, DIPEA, DMF, rt, 16h, (ii) Pd/C (10%), H2 gas , MeOH, rt, 16h.
(h) compound 5, K2CO3, CH3CN, reflux, 24h. (i) Pd/C (10%), H2 gas (6 bars), MeOH, rt, 12h. (j) Na2CO3,
CH3CN, reflux, 48h. (k) aqueous 6N HCl, reflux, 12 h.
The replacement of one acetic arm of polyaminocarboxylic macrocyclic ligands by
methyleneamide or phosphinate groups has often been achieved and investigated.[22,23] This
substitution may clearly influence several properties of the corresponding metallic complexes
11
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
such as thermodynamic stability, kinetic stability by hindering the access to the coordination
sites of the metal ion, as well as overall electric charge of the complex, hydration number and
lipophilicity. Amide and phosphinate groups can also act as starting points for the access of
bifunctional chelates (BFCs) in the context of biomaterials or nanoparticles derivatization.[24]
The synthetic pathway for ligands H3L3 and H2L4 requires two starting materials,
ethyl(aminomethyl)(methyl) phosphinate 20 and glycinedimethylamide 24 (Scheme 6).
Amine 20 was prepared in a two-step procedure. According to the method of Popoff et al.,
diethyl methylphosphonite and bromomethyl phtalimide were mixed in xylene and heated at
80°C for 2 h under reduced pressure (300 mBar).[25] In these conditions, the ethyl bromide
that generated during the reaction was removed and 19 was obtained in 77% yield after
column chromatography. Subsequent deprotection of the amine function was realized in the
presence of hydrazine in ethanol. Varying both the quantity of hydrazine and time reaction
leads to an optimal value of 1.7 equivalents of hydrazine and a 5 h heating at reflux for which
only trace of starting materials remained. Glycinedimethylamide was also prepared in two
steps from the Z-protected amino acid glycine by following the procedure of Guan et al..[26]
Triamines 22 and 26 were then obtained upon bisalkylation of amine precursors 20 and 24
with 2 equiv of bromoderivative 5 in refluxing acetonitrile and in the presence of K2CO3 as
base, followed by debenzylation of intermediates by catalytic hydrogenation. Condensation of
triamines 22 and 26 with 2,6-dibromomethyl pyridine afforded the targeted free macrocycles
23 and 27 in 62 and 73 % yields, respectively. As previously observed for 14 and L2, these
macrocyclization reactions were controlled by a sodium template effect. Subsequent acid
hydrolysis of tert-butyl ester and phosphinate ester functionalities afforded H3L3 and H2L4 as
their hydrochloride salts in quantitative yields.
To test the effectiveness of ligands H3L3 and H2L4 to act as luminophores or paramagnetic
contrast agents, their Ln(III) complexes (Ln = Eu, Tb, Gd) were prepared by mixing a slight
excess of LnCl3 salts and ligands in aqueous solution. The purity of the isolated complexes
was checked by UPLC analysis and their structure determined by HRMS spectrometry.
Interestingly, the Ln(III) complexes showed identical UPLC retention times within their
respective series (H3L3 and H2L4), suggesting that the lanthanide ions share the same
coordination sphere for each complex series. Some luminescence and relaxometry properties
.
.
in aqueous solution of these L3 Ln and L4 Ln complexes are gathered in table 3 and table 4.
12
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
Table 3. Photophysical data of Eu(III) and Tb(III) complexes derived from H3L1(PCTA), H3L3 and H2L4 ligands
in aerated Tris-buffer (pH 7.4) at 298 K
[b]
I 7F2 /
qH2O
λmax
ε
τH2O
τD2O
I 7F1[a]
(nm)
(M-1
(ms)
(ms)
cm-1)
.
270
270
270
269
269
269
270
270
270
3900
4100
4200
3600
3700
3600
3800
4000
3700
2.34
0.37
1.24
-
2.08
2.91
-
2.12
2.02
-
L1 Eu
.
-
L1 Tb
.
-
L1 Gd
.
2.49
0.38
1.25
-
2.06
3.09
-
2.04
2.08
-
L3 Eu
.
-
L3 Tb
.
-
L3 Gd
.
2.44
0.42
1.31
-
2.03
3.12
-
1.75
1.91
-
L4 Eu
.
-
-
L4 Tb
.
L4 Gd
[a] integrated intensity ratio of 5D0→ 7F2 and 5D0→ 7F1 transitions.
[b] Number of metal coordinated water molecules at 298K,
-1
calculated using the following equations: qH2O(Eu) = 1.11[(τH2O
)
-
-1
-1
-1
(
τD2O
)
– 0.31][28c] and qH2O(Tb) = 5[(τH2O – 0.06][28b]
) - (τD2O)
with
τ
in ms.
Table 4. Relaxometric data at 20 MHz of Gd(III) complexes derived from H3L1(PCTA), H3L3 and H2L4 ligands
in water at 20 MHz
298K
310K
310K
r1
r1
r2
(s-1 mM-1)
(s-1 mM-1)
(s-1 mM-1)
.
6.9
8.2
7.1
5.4
5.8
5.5
6.3
7.2
5.9
L1 Gd
.
L3 Gd
.
L4 Gd
Upon excitation to the absorption band of ligands (λexc = 270 nm), all Eu(III) and Tb(III)
complexes exhibited the narrow characteristic red and green luminescence due to 5D0→ 7FJ (J
5
7
= 0-4) and D4→ FJ (J = 3-6) transitions, respectively. In both cases, the hypersensitive
7
5
7
5D0→ F2 transition at 615 nm and the sensitive D4→ F5 transition at 545 nm are the most
prominent (Figure 4). Due to the generally higher sensitivity of europium bands with regards
5
7
to the coordination environment, the ratio between the intensity of the D0→ F2 transition
5
7
(electric dipole) and the intensity of the D0→ F1 transition (magnetic dipole) gave valuable
information about environment changes around the Eu(III) ion.[27] In comparison with
.
.
PCTA Eu complex, no major changes in the value of this ratio were observed in L3 Eu and
.
L4 Eu chelates. This implies that the symmetry around the Eu(III) ion, both induced by the
ligands and the solvent, is nearly constant. The temporal decays of the emission of both
complexes were rigorously monoexponential showing the presence of a single emissive
13
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
solution species. At room temperature, the emission lifetimes are in the millisecond range
compatible with the use of time resolved luminescence technique. These values are
comparable to those of the parent PCTA[12] complexes. Owing to the well-established
contribution of OH and OD oscillators to non-radiative decay rate and as well reported in the
literature, comparison of lifetimes in deuterated and non deuterated water allowed us to
determine quantitatively the q value (which provides the number of metal-bound water
molecules) of these complexes.[28] This analysis indicated that two water molecules are
present in the first coordination sphere of the metal in both complexes. These data clearly
indicate that these new chelates did not limit the accessibility of water molecules to the
lanthanide ion, in spite of the steric hindrance of amide and phosphinate groups. In addition,
the overall emission quantum yields were determined by excitation of the ligands (λexc = 270
nm) and are similar to those of PCTA complexes (ΦL(Eu) = 3 %, ΦL(Tb) = 8 %). These
results are not unexpected due to the same inner-sphere hydration (q = 2) and the same
antenna unit in these three complexes.
.
[L4 Eu]Cl
300
400
500
600
700
800
.
[L4 Tb]Cl
400
500
600
700
800
300
Wavelength (nm)
___
.
.
Figure 4. Normalized excitation (- - - ) and luminescence (
These spectra were measured at 298 K in Tris buffer (pH 7.4).
) spectra of [L4 Eu]Cl and [L4 Tb]Cl complexes.
.
.
The relaxometric properties of L3 Gd and L4 Gd were assessed by measuring their overall
relaxivity r1 which represents the increase in the longitudinal nuclear magnetic relaxation rate
of water protons promoted by the paramagnetic complex. At 20 MHz and 310 K, the r1 values
-1
-1
-1
-1
.
.
of L3 Gd (5.8 s mM ) and L4 Gd (5.5 s mM ) are larger than those found in mono-aqua
14
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
-1
-1
-1
-1
.
.
contrast agent DTPA Gd (3.8 s mM ) and DOTA Gd (3.5 s mM ) which are currently
used in clinical practice.[29] They are similar to that of PCTA Gd and in the range of those
.
reported for bis-hydrated Gd(III) complexes of small size and with no self-aggregation
-1
-1
properties.[30] It is also interesting to note that relaxivity r1 of L3 Gd at 298 K (8.2 s mM ) is
.
298K
= 8.3 s-1 mM-1 at 20 MHz), where phosphonate
.
close to that reported for PCP2A Gd (r1
function is substituted for the phosphinate function in the central arm.[31] These relaxometric
data confirm the number of hydration q = 2 established by luminescence experiments of the
corresponding Eu(III) and Tb(III) complexes. The temperature dependence of relaxivity r1 at
20 MHz for the two complexes was also measured in the range 278 – 318 K and is reported in
.
Figure 5. Analogous data for PCTA Gd complex is also shown for comparison. For the two
complexes, relaxivity r1 increases with decreasing temperature, revealing that the exchange
between the coordinated water molecules and bulk water (τM) does not limit the relaxation
rate. On the other hand when the relaxivity is limited by the water exchange, either a plateau
or a decrease of the relaxivity is observed at low temperature.[32] Further work should be
undertaken about the evaluation of τM by oxygen-17 relaxometry, as well as inertness of these
neutral and cationic GdIII complexes towards di-anions present in human plasma such as
carbonate. Actually, the two inner-sphere water molecules of bis-hydrated GdIII complexes
may be displaced by di-anions leading to ternary adducts of lower hydration state and thus
lower relaxivity, as reported in the literature.[5,33]
16
14
12
10
8
6
4
275 280 285 290 295 300 305 310 315 320
Temperature (K)
.
Figure 5. Temperature (278 – 318 K) dependence of the proton longitudinal relaxivity, r1, of L1 Gd (closed
.
.
triangles, c = 0.536 mM), L3 Gd (closed circles, c = 0.262 mM) and [L4 Gd]Cl (open circles, c = 1.713 mM)
complexes in water at 20 MHz.
15
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
Conclusions
In conclusion, we describe a convenient synthetic pathway for the access of the 12-membered
PCTA macrocycle, whose metallic complexes possess a number of favourable properties in
the fields of biomedical imaging and radiotherapy. This procedure appears as an efficient
alternative compared to the nowadays preparation of PCTA[12] and its derivatives based on
the classical Richman-Atkins methodology. In addition, we demonstrate that such an
approach is general and can be successfully applied to PCTA derivatives containing either
substituted pyridine unit or mixed coordinating functions. Moreover, we reported the
luminescence and relaxometry properties of Ln(III) complexes of the two novel PCTA[12]
derivatives bearing amide or phosphinate functions in the central side arm. The photophysical
studies established that the Eu(III) and Tb(III) complexes are bis-hydrated and do not impair
the luminescence characteristics of PCTA europium and terbium complexes. The relaxometric
experiments on Gd(III) complexes confirm an enhanced relaxivity associated with the
presence of two coordinated water molecules and indicate that the relaxivity is not limited by
the water exchange rate but rather by the fast rotational motion of these low molecular weight
Gd-chelates. Finally, we believe that this synthetic pathway may open up a new way for the
synthesis of other PCTA[12] derivatives, especially bifunctional chelates (BFCs) suitable for
conjugation and applications
Experimental Section
For table of abbreviations, information concerning details of the experimental procedure and
characterization of compounds 4-12, 14 and H3L1, 15-18 and L2, 19-23 and H3L3 and 24- 27
1
13
.
and L2H4; selected bond lengths and angles for [14 Na]ClO4 complex; H, C NMR and MS
spectra of selected compounds; UPLC chromatograms and HRMS spectra of Ln(III)
complexes; luminescence emission spectra of Eu(III) complexes, please use our Supporting
Information.
Acknowledgements
This work was financially supported by the French Ministère de la Recherche (Ph.D. grant for
E M), the Centre National de la Recherche Scientifique (CNRS) and the Cancéropôle Grand-
Sud Ouest. We thank Dr Isabelle Nasso for preliminary work on the synthesis of PCTA[12],
16
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10.1002/ejoc.201800066
European Journal of Organic Chemistry
Isabelle Fabing for technical assistance on UPLC equipment provided by the Integrated
Screening Platform of Toulouse (PICT, IBiSA), Dr Rémy Brousses for X-ray diffraction
study and Pr Luce Vander Elst, Dr Sophie Laurent (University of Mons, Belgium) for
assistance in relaxometric measurements.
_________________
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European Journal of Organic Chemistry
Table of contents entry
Facile access to 12-membered macrocyclic ligand PCTA and novel derivatives including
carboxylate, amide and phosphinate ligating functionalities.
Morgane Enel, Nadine Leygue, Nathalie Saffon, Chantal Galaup, and Claude Picard
Macrocyclic ligands, lanthanides
R1 = H, R2 = CO2H (PCTA[12])
R1 = OH, R2 = CO2H
R1 = H, R2 = PO(OH)Me
R1 = H, R2 = CONMe2
Ln(III) complexes
q = 2H2O
Convenient synthesis
Macrocyclization
yields 60-90%
Gd3+ r1298K = 6,9-8,2 s-1 mM-1
Eu3+, Tb3+ Φ = 3-8 %
A convenient and efficient alternative to the classical Richman-Atkins methodology for
obtaining PCTA[12] and new derivatives is reported. The key macrocyclization step occurs in
good yields as a result of a sodium template effect. The potentiality of lanthanide (III)
complexes derived from these new PCTA[12] ligands to act as paramagnetic contrastophores
or luminophores is reported.
20
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