Gadolinium DOTA chelates featuring alkyne groups directly grafted on the tetraaza macrocyclic ring: Synthesis, relaxation properties, "click" reaction, and high-relaxivity micelles

Article abstract of DOI:10.1021/ic2010997

This paper reports on the synthesis and relaxivity properties of tetraacetic DOTA-type chelating agents featuring one or two alkyne groups directly grafted on the tetraaza macrocyclic ring and available for "click" reactions with azide-bearing substrates. The racemic DOTAma ligand bearing one alkyne group was obtained by a bisaminal template route. The same approach was used to prepare ligand DOTAda substituted by two alkyne groups located on two adjacent carbon atoms. The S,S enantiomer of DOTAda was also prepared by a "crab-like" condensation. This ligand is the first example of a DOTA derivative featuring two reactive functions adjacent to each other on the macrocyclic ring. A triacetic monoalkyne ligand (DO3ma) was also synthesized for comparison purposes. NMR studies indicate that the Yb(III) chelates of DOTAma and DOTAda adopt two conformations in solutions in which the tetraaza ring is rigidified. The hydration state of the Eu(III) chelates was determined by luminescence spectroscopy, and the water exchange time of the Gd(III) complexes was measured by ¹⁷O NMR. Ring substitution accelerates the water exchange. These data were used to interpret nuclear magnetic relaxation dispersion curves of the Gd(III) chelates. Two long aliphatic chains have been added to DOTAda by a "click" procedure to form the (C18)₂DOTAda ligand. The corresponding Gd(III) complex forms micelles of unusually high relaxivity presumably because of the close proximity of the aliphatic chains on the macrocyclic ring that ensures a rigid double anchoring into the micelles.

Full text of DOI:10.1021/ic2010997

ARTICLE
pubs.acs.org/IC
Gadolinium DOTA Chelates Featuring Alkyne Groups Directly Grafted
on the Tetraaza Macrocyclic Ring: Synthesis, Relaxation Properties,
“Click” Reaction, and High-Relaxivity Micelles
Christian Vanasschen, Nouri Bouslimani, David Thonon, and Jean F. Desreux*
Coordination and Radiochemistry, University of Liꢀege, Sart Tilman B16, B4000 Liꢀege, Belgium
ABSTRACT: This paper reports on the synthesis and relaxivity properties of tetraacetic
DOTA-type chelating agents featuring one or two alkyne groups directly grafted on the
tetraaza macrocyclic ring and available for “click” reactions with azide-bearing substrates.
The racemic DOTAma ligand bearing one alkyne group was obtained by a bisaminal
template route. The same approach was used to prepare ligand DOTAda substituted by
two alkyne groups located on two adjacent carbon atoms. The S,S enantiomer of DOTAda
was also prepared by a “crab-like” condensation. This ligand is the first example of a DOTA
derivative featuring two reactive functions adjacent to each other on the macrocyclic ring. A
triacetic monoalkyne ligand (DO3ma) was also synthesized for comparison purposes.
NMR studies indicate that the Yb(III) chelates of DOTAma and DOTAda adopt two
conformations in solutions in which the tetraaza ring is rigidified. The hydration state of the
Eu(III) chelates was determined by luminescence spectroscopy, and the water exchange
time of the Gd(III) complexes was measured by 17O NMR. Ring substitution accelerates
the water exchange. These data were used to interpret nuclear magnetic relaxation dispersion curves of the Gd(III) chelates. Two
long aliphatic chains have been added to DOTAda by a “click” procedure to form the (C18)2DOTAda ligand. The corresponding
Gd(III) complex forms micelles of unusually high relaxivity presumably because of the close proximity of the aliphatic chains on the
macrocyclic ring that ensures a rigid double anchoring into the micelles.
grafting Gd-DOTA monoamide chelates to viral particles16 or
1. INTRODUCTION
β-cyclodextrins.17 However, it is well known that complexes
Magnetic resonance imaging (MRI) using gadolinium-containing
bearing amide chelating units are less stable and exhibit slower
contrast agents is now a commonly used high-resolution clinical
water exchange rates that are detrimental to the relaxivity.18,19
imaging modality.1 Gadolinium complexes drastically increase
Gd(III) diethylenetriaminetetraacetate (GdDTTA) has also
the longitudinal relaxation rate 1/T1 of the water protons, thus
generating bright spots in T1-weighted MRI images. The efficacy
of a contrast agent depends on several parameters, most
importantly the water exchange time τm, the electronic relaxation
time τs, and the rotational correlation time τr. High increases in
relaxation rate per millimolar of metal ion (or relaxivity) can be
been coupled to β-cyclodextrin, but this chelate is again less
stable than the clinically approved GdꢀDOTA and GdꢀDTPA.20
In the present work, we report on the synthesis of two novel alkyne-
bearing macrocyclic ligands as well as on the physical properties
of their lanthanide chelates that are of interest for a relaxometric
evaluation. Even if the synthesis of macrocyclic chelates is more
obtained by slowing down the tumbling rate of a contrast agent
difficult and time consuming than the preparation of their
thanks to a coupling with a macromolecule or nanoparticle.
Moreover, attachment of a large number of paramagnetic com-
noncyclic analogues, we preferred to prepare DOTA derivatives
because of their well-recognized kinetic inertness and thermo-
plexes to one macromolecular substrate increases the local
dynamic stability.21 Only one “clickable” tetraacetic DOTA
Gd(III) concentration and leads to a better sensitivity. The chelates
should be rigidly bonded to a high molecular weight entity and
ligand has been reported so far.22 This ligand features an alkyne
group on one of its acetate arms, and it was clicked to an azide-
also exhibit fast water exchange rates. A variety of macromole-
bearing somatostatin analogue. We chose to graft one or two alkyne
cular contrast agents have been obtained by grafting paramag-
moieties directly on the tetraaza ring of DOTA (compounds 1
netic DTPA- or DOTA-type chelates through amide or thiourea
and 2) to avoid steric crowding, Scheme 1. We also investigated
bonds to polymers2ꢀ4 and dendrimers5ꢀ9 or through noncova-
the Gd(III) chelate of the less strongly complexing triacetic derivative
lent interaction of lipophilic Gd(III) complexes with proteins
3that has already been prepared recently by Lowe et al.23,24 and which
such as albumin.10ꢀ12
is included here for comparison purposes. The GdDO3ma
complex is an easily obtainable metal complex that can be used
for testing various reaction conditions in a ”click” condensation.
The excellent yield reported for the copper-catalyzed 1,3-
dipolar Huisgen cycloaddition (“click” reaction) between an azide
and a terminal alkyne13,14 as well as its fast rate, robustness, and
bio-orthogonality15 make this transformation an excellent candidate
for coupling high payloads of Gd(III) complexes to a particular
macromolecule. “Click” chemistry has already been used for
Received: May 24, 2011
Published: August 22, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Scheme 1
Scheme 2. Synthesis of DO3ma, 3
Despite the added difficulties entailed by the synthesis of
DOTAda, 2, we wanted to obtain this ligand because it features
two alkyne substituents on the same ethylenediamine unit. These
closely spaced reacting groups could be particularly interesting
for firmly anchoring a Gd(III) complex on a macromolecule or
grafting on chemically modified proteins25 thanks to two attach-
ment points. In the present paper, ligand 2 is converted into an
amphiphilic gadolinium complex that self-assembles into para-
magnetic micelles of very high relaxivities.
in acetonitrile (20 mL) and placed in a 100 mL round-bottomed flask
under argon atmosphere. Potassium carbonate (293 mg, 2.1 mmol)
and propargyl bromide (126 μL of an 80% solution in toluene, 1.16
mmol) were added to the solution. After 4 h at room temperature,
potassium carbonate was filtered off and washed with dichloro-
methane (100 mL). The filtrate was washed with water (20 mL) and brine
(20 mL) and dried over MgSO4. After solvent evaporation under
reduced pressure, a yellowish oil remained (504 mg, yield 95%). MS
(ESI+): m/z 553 (M + H)+, 575 (M + Na)+. ESI-TOF-MS: calcd for
1
C29H52N4NaO6 (M + Na)+ 575.3779, found 575.3788. H NMR
(CDCl3) δ: 3.31 (CHtCCH2, s, 2H), 3.16 (m, 6H), 2.8ꢀ2.65 (m,
12H), 2.58 (m, 4H), 2.05 (CHtCCH2, s, 1H), 1.33 and 1.34 (2s,
27H) ppm. 13C NMR (CDCl3) δ: 173.5 and 172.5 (CdO), 82.0,
81.8, 79.1 (CHtCCH2), 72.5 (CHtCCH2), 56.5, 55.7, 52.1 (2C),
51.9, 51.7, 43.0 (CHtCCH2), 27.7, 27.8 ppm.
2. EXPERIMENTAL SECTION
1H and 13C NMR spectra were recorded on a Bruker DRX 400
spectrometer, and nuclear magnetic relaxation dispersion (NMRD) data
were collected from 0.01 to 70 MHz on an upgraded Stelar Spinmaster
FFC2000 relaxometer (Stelar, Mede, PV, Italy) to which a Bruker 2T
permanent magnet had been connected. All experimental T1 relaxation
times were corrected for the T1 value of pure water. 17O NMR mea-
surements were carried out on 4.6ꢀ7.6 mM (pH = 5.7) solutions of
Gd(III) complexes. 17O-Enriched water (10% H217O, Cortecnet, Paris)
was added to solutions of metal complexes in order to reach a 2% 17O
enrichment. The 17O transverse relaxation times were measured using
the standard CarrꢀPurcellꢀMeiboomꢀGill spinꢀecho pulse sequence
on a Bruker Avance AM250 spectrometer. Luminescence lifetimes were
measured on a Photon Technology International (PTI, Birmingham,
NJ) spectrometer equipped with a GL-3300 nitrogen laser using a PBBO
dye to excite the Eu3+ ion at 394 nm. The recorded luminescence decay
curves (emission monitored at 592 nm) were fitted to a monoexponen-
tional function using the PTI Time Master software. Quantitative
interpretations of the 17O, NMRD, and luminescence data were performed
with the Scientist software program (Micromath, USA). Electrospray
mass spectra were obtained on Fisons VG platform (ESI-MS) and Bruker
Daltonics MicrOTOF (TOF-ESI-MS) spectrometers. Melting or
decomposition points were measured on an Electrothermal IA
9100 apparatus.
Synthesis of 1-(Prop-2-ynyl)-4,7,10-tris(carboxymethyl)-
1,4,7,10-tetraazacyclododecane, 3, DO3ma (Scheme 2). Trie-
ster 5 (647 mg, 1.2 mmol) was dissolved in trifluoroacetic acid (10 mL),
and the solution was stirred at room temperature overnight. Evaporation
of the solvent under reduced pressure yielded a viscous brown oil. This
residue was dissolved in water (3 mL, pH ≈ 2) and loaded onto a Dowex
50x2-200 (H+ form) cation-exchange resin. The column was washed
with water (500 mL), and the sought product was recovered by elution
with 0.5 M aqueous NH3. The eluate was evaporated under reduced
pressure, and the remaining light brown oil was dissolved in decarbo-
nated water (3 mL). The pH was brought to 10 with aqueous NH3, and
the solution was transferred onto a Dowex (OHꢀ form) 1x2-200 anion-
exchange resin. The column was washed with decarbonated water
(500 mL), and the sought product was eluted using a formic acid
gradient (0.005, 0.04, 0.08, 0.15, 0.3, 0.5, 1 M). The fractions containing
the desired product (HCOOH 0.08 and 0.15 M) were combined and
evaporated to dryness in vacuo. Repetitive evaporations after addition of
water were performed in order to remove formic acid. Finally, 293 mg of
compound 3 was obtained as a golden oil. Yield 65%. MS (ESI+): m/z
385 (M + H)+. ESI-TOF-MS: calcd for C17H29N4O6 (M + H)+
1
385.2082, found 385.2078. H NMR (D2O) δ: 3.27 (CHtCCH2, s,
Synthesis of 1-(Prop-2-ynyl)-4,7,10-tris(tert-butoxycarbony-
lmethyl)-1,4,7,10-tetraazacyclododecane, 5 (Scheme 2).
(t-Bu)3DO3A, 4 (496 mg, 0.96 mmol, CheMatech, USA), was dissolved
2H), 3.23 (m, 6H), 2.9ꢀ2.6 (m, 12H), 2.56 (m, 4H), 2.12 (CHtCCH2, s,
1H) ppm. 13C NMR (D2O) δ: 177.7 and 170.3 (CdO), 77.8
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ARTICLE
Scheme 3. Synthesis of DOTAma, 1
(CHtCCH2), 75.5 (CHtCCH2), 57.2, 55.9, 51.9, 51.0, 48.1, 47.5, 42.3
(CHtCCH2) ppm.
173.2, 172.9, 172.4, and 172.2 (CdO), 82.1 (4C), 58.8, 58.2, 55.9, 55.4,
55.2, 55.0, 54.5, 53.7, 52.3, 52.1, 50.4, 49.9, 48.7, 47.5, and 27.4 (4C) ppm.
Synthesis of 2-(Prop-2-ynyloxymethyl)-1,4,7,10-tetra(tert-
butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane, 9
(Scheme 3). Tetraester 8 (598 mg, ∼0.91 mmol) was dissolved in
dichloromethane (20 mL), and this solution was cooled to 0 °C.
Propargylbromide in toluene (80% solution, 292 μL, 2.71 mmol,
Aldrich), tetrabutylammonium iodide (catalytic amount), and cesium
hydroxide monohydrate (457 mg, 2.72 mmol) were successively added
to the reaction mixture. After 30 min of vigorous stirring, the ice bath was
removed and the reaction mixture was left for 1 h at room temperature.
After allowing the cesium base to settle at the bottom of the flask, the
supernatant was taken up and washed with 15 mL of water. The organic
phase was then dried over MgSO4 and evaporated to dryness, and
the remaining residue was chromatographed over a silica gel column
(elution gradient: CH2Cl2ꢀEtOAc 50:50 to EtOAcꢀMeOH 85:15)
to yield 443 mg of a yellowish oil after evaporation of the solvents
(yield ≈ 70%) . MS (ESI+): m/z 719 (M + Na)+. ESI-TOF-MS: calcd
Synthesis of 2-Hydroxymethyl-1,4,7,10-tetraazacyclodo-
decane, 7 (Scheme 3). The synthesis of 7 was performed according
to the experimental procedure described by Boschetti et al.26 with some
modifications. Triethylenetetramine (6 g, 41 mmol) was dissolved in
acetonitrile (180 mL) and cooled in an ice bath. A solution of butanedione
(3.53 g, 41 mmol in 18 mL of acetonitrile) was added dropwise over 30
min, and the ice bath was removed after 4 h of vigorous stirring. The
yellow reaction mixture was heated to 60 °C, and potassium carbonate
(28.4 g, 205 mmol) and a solution of 2,3-dibromo-1-propanol (9.02 g,
41 mmol in 18 mL of acetonitrile) were added. After 48 h at this
temperature, potassium carbonate was removed by filtration over Celite,
and the filtrate was evaporated to dryness in vacuo. The residue was
transferred onto an alumina column and eluted with a dichlorometha-
neꢀmethanol 97:3 mixture. After removal of the solvents under reduced
pressure, the red oil was dissolved in acetonitrile (20 mL) and left at
ꢀ20 °C for 24 h. The formed crystals were collected by filtration and
washed with cold acetonitrile, hexane, and diethyl ether. This afforded a
purple solid, which was found to contain a mixture of the R,S,R and R,S,S
isomers of compound 6, which could not be separated. The two diaster-
eoisomers were then dissolved in a mixture of ethanol (540 mL) and
concentrated HCl (180 mL). After heating this solution at 60 °C for 48
h, the precipitate was filtered off and washed with cold ethanol and
diethyl ether to obtain 1.83 g of compound 7 as a beige powder. Yield
22.3%. MS (ESI+): m/z 203 (M + H)+. ESI-TOF-MS: calcd for C9H23-
N4O (M + H)+ 203.1866, found 203.1870. 1H NMR (D2O) δ:
3.78ꢀ3.05 (m, 17H) ppm. 13C NMR (D2O) δ: 59.0 (CH2OH), 56.2
(CHCH2OH), 46.7, 44.6, 44.0, 43.6, 43.5, 43.0, 42.1 ppm.
Synthesis of 2-Hydroxymethyl-1,4,7,10-tetra(tert-butoxycar-
bonylmethyl)-1,4,7,10-tetraazacyclododecane, 8 (Scheme 3).
Potassium carbonate (1.136 g, 8.22 mmol) and tert-butyl bromoacetate
(1.604 g, 8.22 mmol) were successively added to a solution of 7 (416 mg,
2.06 mmol) in 20 mL of dimethylformamide at 50 °C. After 3 h, ethyl
acetate (50 mL) was added to the reaction mixture and potassium
carbonate was filtered off. The filtrate was washed three times with brine,
dried over MgSO4, and evaporated under reduced pressure. Compound 8
was partially purified by eluting the crude mixture through a silica gel
column first with ethyl acetate and finally with methanol in order to
recover the sought product 8 (979 mg of an orange oil, yield ≈ 72%). MS
(ESI+): m/z 681.4 (M + Na)+. ESI-TOF-MS: calcd for C33H62N4NaO9
(M + Na)+ 681.4409, found 681.4398. 1H NMR (CDCl3) δ: 3.85ꢀ1.8
(m, 25H), 1.30 (s, 36H), 1.18 (OH, m, 1H) ppm. 13C NMR (CDCl3) δ:
1
for C36H64N4NaO9 (M + Na)+ 719.4566, found 719.4568. H NMR
(CDCl3) δ: 4.11 (CHtCCH2, m, 2H), 4.06ꢀ1.61 (m, 26H), 1.30
(s, 36H) ppm. 13C NMR (CDCl3) δ: 173.4, 173.2, 172.5, and 172.4
(CdO), 81.5 (4C), 79.1 and 79.3 (CHtCCH2), 74.9 and 74.8
(CHtCCH2), 67.4 and 66.3 (CHCH2OCH2), 58.5 and 58.0
(CHtCCH2), 55.9, 54.8, 53.9, 53.4, 51.8, 51.2, 50.8, 50.4, 49.4, 49.0,
47.3, 47.1, 27.4 ppm (t-But) ppm.
Synthesis of 2-(Prop-2-ynyloxymethyl)-1,4,7,10-tetra-
(carboxymethyl)-1,4,7,10-tetraazacyclododecane, 1, DOTA-
ma (Scheme 3). A solution of tetraester monoalkyne 9 (774 mg,
∼1.11 mmol) in trifluoroacetic acid (15 mL) was stirred at room
temperature overnight. Evaporation of the solvent under reduced
pressure yielded a viscous brownish oil. This residue was dissolved in
water (3 mL, pH ≈ 2) and loaded onto a Dowex 50x2-200 (H+ form)
cation-exchange resin. The column was washed with water (500 mL),
and the sought product as an ammonium salt was recovered by elution
with 0.5 M aqueous NH3. The eluate was evaporated under reduced
pressure, and the light brown oil was dissolved in decarbonated water
(3 mL). The pH was brought to 10 with aqueous NH3, and the solution
was transferred onto a Dowex (OHꢀ form) 1x2-200 anion-exchange
resin. The column was washed with decarbonated water (500 mL), and
the sought product was eluted using a formic acid gradient (0.005, 0.04,
0.08, 0.15, 0.3, 0.5, 1 M). The fractions containing the desired product
were combined and evaporated to dryness in vacuo. Formic acid was
eliminated by repetitive evaporations after addition of water. Compound
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Scheme 4. Synthesis of 1,4,7,10-Tetraazacyclododecane-2,3-diyl)dimethanol 15a and Its Dibenzylated Derivative 15b
1 (331 mg) was obtained as a yellowish oil (yield ≈ 63%). MS (ESI+):
m/z 473 (M + H)+, 495 (M + Na)+, 511 (M + K)+. ESI-TOF-MS: calcd
74.6 (benzylic CH2), 70.4 (CHCH2O), 58.0 (CHNH), 48.0, 47.6,
44.5 ppm.
Synthesis of rel-((2S,3S)-1,4,7,10-Tetraazacyclododecane-
2,3-diyl)dimethanol, 15a (Scheme 4). Although 15a could be
obtained by a one-pot procedure, we opted for a step by step approach in
which each intermediate was isolated. Preparation of 11 has been
reported28 without experimental details.
1
for C20H32N4NaO9 (M + Na)+ 495.2061, found 495.2075. H NMR
(D2O) δ: 4.16 (CHtCCH2, m, 2H), 4.10ꢀ1.56 (m, 26H) ppm. 13C
NMR (D2O) δ: 173.0 (2C) and 170.6 (2C) (CdO), 79.6 (CHtCCH2),
76.5 (CHtCCH2), 65.4 (CHCH2OCH2), 58.2 (CHtCCH2), 57.5ꢀ46.4
(br) ppm.
Synthesis of (5S,6S)-5,6-Bis(benzyloxymethyl)-1,4,7,10-
tetraazacyclododecane-2,9-dione, 14 (Scheme 4). Sodium
carbonate (3.76 g, 35.5 mmol) and N,N0-bis(chloroacetyl)ethylenediamine
(418 mg, 1.97 mmol) were added to a solution of (2S, 3S)-1,4-bis-
(benzyloxy)butane-2,3-diamine27 and 13 (590 mg, 1.97 mmol) in acetoni-
trile (62 mL), and this mixture was heated to reflux for 24 h under
vigorous stirring. After cooling to room temperature, the reaction
mixture was filtered and the filtrate was evaporated to dryness in vacuo.
Recrystallization of the remaining residue in boiling ethanol afforded
174 mg of cycle 14 as a white solid (mp 164 °C, yield 20%). MS (ESI+):
m/z 441 (M + H)+. ESI-TOF-MS: calcd for C24H33N4O4 (M + H)+
441.2496, found 441.2511. 1H NMR (CD3OD) δ: 7.45ꢀ7.30 (Ar, m,
10H), 4.48 (benzylic CH2, s, 4H), 3.50ꢀ3.02 (m, 14H) ppm. 13C NMR
(CD3OD) δ: 175.2 (CdO), 139.9, 129.7, 129.4, and 129.0 (Ar), 74.6
(benzylic CH2), 70.5 (CHCH2O), 58.7 (CHNH), 51.8 (CH2CdO),
39.0 (CH2NHCdO) ppm.
Synthesis of (2S,3S)-2,3-Bis(benzyloxymethyl)-1,4,7,10-
tetraazacyclododecane, 15b (Scheme 4). Cyclic compound 14
(920 mg, 2.10 mmol) was dissolved in dry toluene (20 mL), placed
under argon atmosphere, and cooled to 0 °C. Red-Al (5 mL of a 3.5 M
solution in toluene, Aldrich) was added to this solution, and the resulting
viscous mixture was maintained at 0 °C for 30 min and then heated at
80 °C for 3 h. After dropwise addition of water (5 mL) at 0 °C, the
reaction mixture was stirred for 16 h at room temperature. The solvents
were evaporated in vacuo; the residue was dissolved in methanol and
purified by dry flash chromatography using a large silica gel column
(40 cm length, 5 cm diameter). Elution with methanol followed by
methanol saturated with NH3 allowed recovering tetraamine 15b as a
colorless oil (680 mg, yield 78%). MS (ESI+): m/z 413 (M + H)+. ESI-
TOF-MS: calcd for C24H37N4O2 (M + H)+ 413.2911, found 413.2906.
1H NMR (CD3OD) δ: 7.45ꢀ7.20 (Ar, m, 10H), 4.43 (benzylic CH2, m,
4H), 3.60ꢀ3.30 (CH2CHNH and CHCH2O, m, 6H), 2.80ꢀ2.55 (m,
12H) ppm. 13C NMR (CD3OD) δ: 139.9, 129.7, 129.4, and 129.0 (Ar),
An ethanolic solution of butanedione (1.77 g, 20.5 mmol in 30 mL)
was added dropwise to a solution of triethylenetetraamine, 10, in ethanol
(3 g, 20.5 mmol in 100 mL) at 0 °C. After 4 h of stirring, the solvent was
removed under reduced pressure without heating. The remaining yellow
solid was taken up in hexane (80 mL) to obtain a yellow solution with an
insoluble residue. The latter is highly soluble in methanol and was
eliminated by treating the mother solution of hexane with small amounts
of methanol (3 ꢁ 5 mL). The pale yellow hexane fraction was separated,
and the solvent was removed under reduced pressure to yield a white
solid (1.2 g, 21%). Note: In our hands, the ethanol/hexane procedure
reported above was more convenient than synthesis in acetonitrile
because bisaminal 11 crystallized more readily. However, solid 11
undergoes a relatively rapid alteration in contact with air at ambient
temperature; its color changes from white to yellow in less than 5 h.
Colorless solutions (CHCl3, MeOH) change to orange within 1 day
even in anhydrous conditions, indicating complete degradation of 11 as
evidenced by mass spectrometry. 1H NMR (CD3OD, 298 K) δ: 2.5ꢀ
3.05 (m, 12H, NꢀCH2), 1.16 (s, 3 H, CH3), 1.34 (s, 3 H, CH3), 13
C
NMR (CD3OD, 298 K) δ: 77.02, 68.05 (NCN), 51.26, 48.37, 40.53
(NCH2), 22.41, 10.91 (NCCH3). MS (ESI+): [M + Na]+ (m/z) found,
219.1616; calcd, 219.1586.
A solution of rel-(2R,3R)-2,3-dibromo-1,4-butanediol (1.51 g, 6.1
mmol) in 10 mL of acetonitrile was added to a mixture of the bisaminal
intermediate 11 (1.2 g, 6.1 mmol) and Cs2CO3 (10 g, 30.5 mmol) in
acetonitrile (40 mL), and the resulting mixture was refluxed for 48 h
under argon. The reaction mixture was filtered through a pad of Celite to
obtain a colorless to pale yellow solution, and the solvent was removed
to yield the protected macrocycle 12 as a yellowish oil (1.4 g, 81%). The
product can be used as such in the next synthetic step or better be
recrystallized from cold acetonitrile. 1H NMR (CD3OD, 233 K) δ: 4.1
(m, 1 H, NCHCH2OH), 3.56ꢀ3.76 (m, 2 H, NCHCH2OH), 3.5 (m, 1
H, NCHCH2OH), 3.07ꢀ3.27 (m, 2 H, NCHCH2OH), 2.35ꢀ3.07 (m,
12 H, NCH2), 13C NMR (CD3OD, 233K) δ: 79.84, 74.97 (NCN),
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Scheme 5. Synthesis of DOTAda, 2
78.20, 74.07 (NCHCH2OH), 56.87, 52,81 (NCHCH2OH), 50.52,
49.65, 49.50, 47.30, 45.87, 45.44 (NCH2). MS (ESI+): [M+H]+ (m/z)
found, 283.2129; calcd, 283.2129.
(Ar, m, 10 H), 4.33 (benzylic CH2, m, 4 H), 4.15ꢀ1.78 (m, 26 H), 1.22
(s, 36 H) ppm. 13C NMR (CDCl3) δ: 174.6, 174.2, 173.7, and 173.4
(CdO), 138.5 (2 C), 129.1 (2 C), 128.8 (2 C), and 128.5 (2 C) (Ar),
82.8, 82.6, 82.5, 82.3, 74.0, and 73.9 (benzylic CH2), 67.4, 65.8, 59.7,
56.5 (2 C), 55.6, 54.4, 54.0, 53.4, 53.2, 50.2, 49.0, 48.9, 45.6, 28.7 (4
C) ppm.
The bridged macrocycle 12 obtained in the preceding step was
dissolved in ethanol (50 mL), and 10 mL of concentrated HCl (37%)
was added carefully. The solution was refluxed for 48 h under argon. The
solid hydrochloride was collected by filtration and rinsed with small
amounts of cold ethanol and ether. Attempts to recrystallize the remaining
beige solid were unsuccessful. After dissolution in 30 mL of water, the
product was purified by adjusting the pH to 12 with a 4 M NaOH
solution followed by evaporating the solvent under vacuum. Acetonitrile
(150 mL) was added to the residual solid, and the resulting suspension
was stirred overnight at 45 °C. The insoluble material was filtered off,
and the solvent was evaporated to yield 15a as an orange oil (0.7 g, 61%).
The NMR spectrum of 15a is very sensitive to pH, particularly the
NCHCH2OH resonances. The best-resolved 1H and 13C NMR spectra
were obtained between pH 3 and 4. ESI-TOF-MS: [M + H]+ (m/z)
found, 233.1969; calcd, 233.1972. 1H NMR (D2O, 298 K) δ: 4.57 (m, 2
H, NCHCH2OH, assigned checked with the HSQC spectrum), 3.28ꢀ
3.81 (m, 16 H, NCH2 and CH2OH). 13C NMR (D2O, 298 K) δ: 68.43
(NCHCH2OH), 48.72 (CH2OH), 45.97, 45.48, 43.30 (NCH2).
Synthesis of (2S,3S)-2,3-Bis(benzyloxymethyl)-1,4,7,10-
tetra(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclo-
dodecane, 16 (Scheme 5). Potassium carbonate (280 mg, 2.04
mmol) and tert-butyl bromoacetate (395 mg, 2.04 mmol) were added to
tetraamine 15b (210 mg, 0.51 mmol) dissolved in DMF (10 mL). After 3
h of stirring at 50 °C, the reaction mixture was diluted with ethyl acetate
(50 mL) and washed three times with brine. The organic layer was dried
over MgSO4, and the solvents were removed by evaporation under
reduced pressure. The remaining residue was purified by silica gel column
chromatography, eluting first with EtOAc and finally with a EtOAcꢀ
MeOH 3:1 mixture. The fractions containing the sought product were
evaporated in vacuo to give 408 mg of 16 as an orange oil; yield 91%. MS
(ESI+): m/z 869 (M + H)+. ESI-TOF-MS: calcd for C48H77N4O10
(M + H)+ 869.5634, found 869.5649. 1H NMR (CDCl3) δ: 7.10ꢀ7.00
Synthesis of (2S,3S)-2,3-Bis(hydroxymethyl)-1,4,7,10-
tetra(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclo-
dodecane, 17 (Scheme 5). Compound 16 (200 mg, 0.23 mmol)
was dissolved in ethanol (15 mL) and placed in a reactor (Miniclave,
B€uchi). Concentrated HCl (192 μL) and Pd/C 10% catalyst (57.5 mg,
Degussa, E101 NE/W, Aldrich) were added, and the reaction mixture
was hydrogenated for 72 h (5 bar, RT). After filtration and abundant
washing of the catalyst with methanol, the combined organic fractions
were eluted on a column of Amberlyst A-21 resin (free base form,
ACROS) to eliminate HCl. Evaporation of the solvents under reduced
pressure afforded diol 17 as a green-yellow oil (95 mg, yield 60%). MS
(ESI+): m/z 690 (M + H)+, 711.4 (M + Na)+. ESI-TOF-MS: calcd for
C34H65N4O10 (M + H)+ 689.4695, found 689.4683. 1H NMR (CDCl3)
δ: 3.83ꢀ1.85 (m, 26H), 1.33 (s, 36H), 1.16 (OH, m, 2 H) ppm. 13C
NMR (CDCl3) δ: 174.7, 174.1, 173.7, and 173.5 (CdO), 82.9, 82.8,
82.7, 82.6, 62.8, 58.2, 58.0, 56.6 (2C), 56.3, 53.6, 53.5 (2C), 53.4, 49.9,
49.1, 48.9, 45.7, 28.8 (4C) ppm.
Synthesis of (2S,3S)-2,3-Bis(prop-2-ynyloxymethyl)-1,4,7,
10-tetra(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclo-
dodecane, 18 (Scheme 5). A solution of propargyl bromide in
toluene (385 μL of a 80% solution, 3.58 mmol), a catalytic amount of
tetrabutylammonium iodide, and cesium hydroxide monohydrate (600
mg, 3.58 mmol) were added successively to a solution of diol 17 (410
mg, 0.595 mmol) in dichloromethane (20 mL) at 0 °C. After 30 min of
vigorous stirring, the ice bath was removed and the reaction mixture was
left for 1 h at room temperature. After allowing the cesium base to settle
at the bottom of the flask, the supernatant was collected and washed with
15 mL of water. The organic phase was dried over MgSO4 and
evaporated to dryness, and the obtained residue was purified by elution
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Scheme 6. Synthesis of (C18)2DOTAda, 20
on a silica gel column (elution gradient CH2Cl2ꢀEtOAc 50:50 to
EtOAcꢀMeOH 85:15). After evaporation of the fractions containing
the desired compound (ESI-MS), dialkyne 18 was recovered as a
yellowish oil (409 mg, yield 90%). MS (ESI+): m/z 765 (M + H)+,
787 (M + Na)+. ESI-TOF-MS: calcd for C40H68N4NaO10 (M + Na)+
787.4828, found 787.4832. 1H NMR (CDCl3) δ: 3.98ꢀ1.58 (m, 28H),
4.03 (CHtCCH2, m, 4 H), 1.28 (s, 36H) ppm. 13C NMR (CDCl3) δ:
173.7, 173.1, 172.7, and 172.4 (CdO), 81.8, 81.7, 81.6, 81.5, 79.1, and
79.2 (CHtCCH2), 75.1 and 74.9 (CHtCCH2), 66.3 and 64.6
(CHCH2O) 58.0 (2C, CHtCCH2), 55.6, 55.5, 54.3, 53.6, 53.4, 52.9,
52.5, 52.3, 49.2, 48.0, 47.9, 44.7, 27.8 (4C) ppm.
Synthesis of (2S,3S)-2,3-Bis(prop-2-ynyloxymethyl)-1,4,7,
10-tetra(carboxymethyl)-1,4,7,10-tetraazacyclododecane,
2, DOTAda (Scheme 5). Dialkyne 18 (660 mg, 0.86 mmol) was
dissolved in trifluoroacetic acid (15 mL), and the obtained solution was
stirred at room temperature overnight. Evaporation of the solvent under
reduced pressure yielded a viscous brownish oil. This residue was dissolved
in water (3 mL, pH ≈ 2) and loaded onto a Dowex 50x2-200 (H+ form)
cation-exchange resin. The column was washed with water (500 mL),
and the ammonium salt of DOTAda was recovered by elution with 0.5 M
aqueous NH3. The eluate was evaporated under reduced pressure, and
the light brown oil was dissolved in decarbonated water (3 mL). The pH
was brought to 10 with aqueous NH3, and the solution was transferred
onto a Dowex (OHꢀ form) 1x2-200 anion-exchange resin. The column
was washed with decarbonated water (500 mL), and the sought product
was eluted using a formic acid gradient (0.005, 0.04, 0.08, 0.15, 0.3, 0.5, 1
M). The fractions containing the desired product (HCOOH 0.08 and
0.15 M) were combined and evaporated to dryness in vacuo. Repetitive
evaporations after addition of water were performed in order to remove
formic acid. Finally, 313 mg of ligand 2 was obtained as a yellowish oil in
67% yield. MS (ESI+): m/z 541 (M + H)+, 563 (M + Na)+, 579
(M + K)+. ESI-TOF-MS: calcd for C24H36N4NaO10 (M + Na)+ 563.2324,
found 563.2341. 1H NMR (D2O) δ: 4.07 (CHtCCH2, m, 4 H),
3.97ꢀ1.60 (m, 28 H) ppm. 13C NMR (D2O) δ: 176.5, 176.4, 171.6,
and 171.2 (CdO), 79.5 and 79.2 (CHtCCH2), 76.5 and 76.4
(CHtCCH2), 65.1 and 63.7 (CHCH2O), 62.4, 58.0 (2 C, CHtCCH2),
56.5, 56.2, 55.9 (2 C), 52.8, 51.0 (2 C), 49.8, 49.3, 48.7, 46.0 ppm.
Synthesis of (2S,3S)-2,3-Bis(1-octadecyl-1H-[1,2,3]triazol-
4-ylmethoxymethyl)-1,4,7,10-tetra(tert-butoxycarbonylme-
thyl)-1,4,7,10-tetraazacyclododecane, 19 (Scheme 6).
1-Azidooctadecane29 (69.7 mg, 0.236 mmol), 27 μL of 2,6-lutidine
(0.236 mmol), 41 μL of diisopropylethylamine (0.236 mmol), and
25 mg of CuI (0.13 mmol) were added successively to a solution of
tetraester dialkyne 18 (90 mg, 0.118 mmol) in dry dichloromethane
(10 mL) under argon atmosphere. After 3 h, a 1 M aqueous DTPA
solution was added (10 mL), and the resulting mixture was stirred
vigorously overnight. The blue aqueous phase was discarded, and the
organic phase was dried over MgSO4 and evaporated under reduced
pressure. The remaining residue was purified by chromatography on a
silica gel column, eluting with dichloromethane. Product 19 was
recovered as a yellowish oil (109 mg, yield, 68%). MS (ESI+): m/z
1378 (M + Na)+. ESI-TOF-MS: calcd for C76H142N10NaO10 (M + Na)+
1378.0803, found 1378.0817. 1H NMR (CDCl3) δ: 8.25 and 8.13 (ss,
triazole ring protons), 4.43 (m, 4 H, OCH2CdC), 3.8ꢀ1.75 (m, 30 H),
1.58 (m, 4 H, CH2CH2N), 1.42 (s, 36 H), 1.54ꢀ1.03 (m, 60 H, aliphatic
CH2), 0.85 (t, 6 H, CH3) ppm. 13C NMR (CDCl3) δ: 174.0, 173.3, 172.7,
and 172.5 (CdO), 143.5 and 143.4 (OCH2CdC), 124.5 and 124.3
(OCH2CdC), 81.8, 81.7 (2 C), 81.5, 63.9, and 63.7 (OCH2CdC),
66.3, 64.9, 58.8, 55.7, 55.6, 54.4, 53.4, 53.0, 52.6, 52.3, 49.2, 48.3, 48.1,
44.8, 50.4 (2 C, CH2NꢀNdN), 31.8 (2 C, CH2CH2CH3), 30.4, 30.3,
30ꢀ28.5, 26.6, and 26.5 (28 C, aliphatic CH2), 27.8 (4 C, tert-butylic CH3),
22.6 (2 C, CH2CH3), 14.0 (2 C, CH2CH3) ppm.
Synthesis of (2S,3S)-2,3-Bis(1-octadecyl-1H-[1,2,3]triazol-
4-ylmethoxymethyl)-1,4,7,10-tetra(carboxymethyl)-1,4,7,10-
tetraazacyclododecane, 20, (C18)2DOTAda (Scheme 6). Com-
pound 19 (94 mg, 0.07 mmol) was dissolved in a 1:1 mixture of
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trifluoroacetic acid and dichloromethane and vigorously stirred overnight
at room temperature. After evaporation of the solvents in vacuo, the
remaining brownish oil was dissolved in dichloromethane (5 mL) and
diethyl ether (30 mL) was added to this solution. The formed precipitate
was isolated by centrifugation, washed with diethyl ether, and dried under
reduced pressure to obtain ligand 20 as a pale yellow oil in 67% yield (52 mg).
not always reproducible.28 This is the route selected by Boschetti
et al.26 to obtain 1,4,7,10-tetraazacyclododecan-2-yl)methanol,
7, in a one-pot procedure starting from triethylenetetramine and
butanedione to form a bisaminal that is directly reacted with 2,3-
dibromopropan-1-ol or 1,3-ditosylpropan-1-ol. In our hands,
simple filtration of crude 6 through an alumina column (97:3
CH2Cl2ꢀMeOH) as suggested by these authors26 led to only
partial purification, and we had to isolate the bisaminal 6 by
recrystallization in ice-cold acetonitrile in order to obtain pure
macrocyclic derivative 7 as a mixture of two diastereoisomers
(R,S,R and R,S,S) displaying 26 carbon NMR resonances as also
observed for its amino analogue33 (yield from triethylenetetra-
mine 22.3%).
tert-Butyl ester arms were added to 7 in basic medium, and a
Williamson-type O-alkylation using propargyl bromide and
cesium hydroxide in dichloromethane led to the monoalkyne 9
in less than 2 h (yield 95%). It is interesting to note that all carbon
atoms of the prop-2-ynyloxymethyl substituent give rise to two
resonances in the 13C NMR spectrum of 9. Partial differentiation
of the R and S enantiomers appears to be due to a rigidification
effect by the tert-butyl ester groups that forces the alkyne
substituents to be located in two slightly different environments.
Cleavage of the esters by trifluoroacetic acid followed by ion-
exchange chromatography afforded pure DOTAma ligand, 1
(yield 65%). Since this molecule no longer experiences the
rigidifying effect of the tert-butyl ester arms, the carbon atoms
of the prop-2-ynyloxymethyl substituent now give rise to single
NMR peaks. Monoalkyne ligand 1 could be obtained in 6 steps
with an overall yield of 7% starting from triethylenetetraamine.
The tetraaza cycle featuring two alcohol groups was prepared
by the bisaminal template route as a racemic mixture and by the
“crab-like” technique as the S,S isomer. Caution was needed to
ensure that the two alcohol substituents would not prevent the
tetraazacyclododecane cycle from adopting its preferred [3.3.3.3]
square conformation34 in which the lone pairs of all nitrogen
atoms are pointing toward the same side of the macrocyclic unit
and are thus available for binding a metal ion. This implies that
the substituents in the tetraaza cycle be located in the two
equatorial positions rather than in the axial positions where the
steric crowding would become so large that complexing could
not take place. The diol unit in the macrocyclic ring thus has to be
in the S,S or R,R conformation.
MS (ESI+): m/z 1132 (M + H)+. ESI-TOF-MS: calcd for C60H111
-
N10O10 (M + H)+ 1131.8479, found 1131.8468. 1H NMR (CDCl3) δ:
7.77 (b, triazole ring protons), 5.0 ꢀ 0.8 (br, cycle and aliphatic chains)
ppm. 13C NMR (CDCl3) δ: 171.4 (2 C) and 171.2 (2 C) (CdO), 143.0
and 142.8 (OCH2CdC), 123.8 and 123.6 (OCH2CdC), 67ꢀ44 (18 C,
br), 31.9 (2 C, CH2CH2CH3), 29.7, 29.4, 30ꢀ28.5, 26.6, and 26.5 (28 C,
aliphatic CH2), 22.7 (2 C, CH2CH3), 14.0 (2 C, CH2CH3) ppm.
Preparation of Metal Complexes. Typically, 1 equiv of DO-
TAma, 1, DOTAda, 2, or DO3ma, 3, was dissolved in water (5 mL), and
a slight stoichiometric excess of a lanthanide trichloride solution (1.1
equiv) was added dropwise. The solution was stirred at 50 °C while the
pH was continuously maintained at pH 5.5 by adding 1 M KOH until no
more pH variation was observed. After stirring 24 h at this temperature,
the solution was passed through a column of Chelex 100 resin (Aldrich)
that was eluted with water to remove the excess of metal ions.
Evaporation under reduced pressure yielded the complexes as pale
yellow solids. The absence of free metal ions was checked with xylenol
orange.30
Micelles of (C18)2DOTAda, 20, were obtained by adding very slowly
(portions of 20 μL) a THF solution of the ligand (10 mg in 400 μL
THF) to 5 mL of water under vigorous stirring. The stirring was
maintained overnight in a recipient that was not covered to let the
organic solvent evaporate. The remaining solution was sonicated for 5
min to obtain a stable micellar suspension to which a GdCl3 solution (1.1
equiv) was added dropwise. The reaction mixture was stirred at 50 °C,
while the pH was continuously maintained at 5.5 by adding 1 M KOH
until no more pH variation was observed. After stirring 24 h at 50 °C,
500 mg of Chelex 100 resin was added and the suspension was stirred for
3 h at room temperature. The supernatant was collected and lyophilized
to afford Gd(C18)2DOTAda as a colorless material. The Gd content
was checked by ICP atomic emissions spectroscopy and/or measuring
the relaxation after mineralization in concentrated acid. Ligand solutions
were prepared by weight and successive dilutions. Three samples of
Gd(C18)2DOTAda micelles yielded the same relaxivity curves within
the errors limits.
3. RESULTS AND DISCUSSION
In the template approach to preparation of the basic unit of
DOTAda (Scheme 4), bisaminal 11 obtained from triethylene-
tetramine and 2,3-butanedione28 was reacted with dl-2,3-dibro-
mobutane-1,4-diol35 in the presence of Cs2CO3. Removal of the
central bridging unit of 12 with concentrated HCl led to diol 15a.
The latter could not be obtained in a pure state without isolat-
ing intermediate 12. Moreover, we could not obtain this com-
pound by reacting 11 with a ditosylated reagent rather than a
dibromide. Unsuccessful attempts at cyclizing tricyclic bisaminal
have already been reported and assigned to an improper orienta-
tion of the secondary nitrogen atoms for a cyclocondensation.28
In the present case, it is not clear why steric requirements could
prevent a reaction between a tricyclic bisaminal and 1,4-bis-
(benzyloxy)butane-2,3-diol dimethanesulfonate.
The “crab-like” approach to preparation of the optically pure S,
S isomer of diol 15b with the alcohol groups protected by benzyl
functions is also illustrated in Scheme 4. It should be noted here
that an enantiomer of 15b could not be prepared by the template
pathway reported above because optically pure enantiomers of
2,3-dibromobutane-1,4-diol have not been prepared so far. Our
Syntheses of Alkyne-Substitued Ligands. Chelating agent
DO3ma, 3, was readily synthesized starting from commercially
available DO3A(t-Bu)3, 4, and following the procedure recently
reported by Lowe et al.23 with minor modifications (Scheme 2).
Alkylation of the free amino group using propargyl bromide
afforded DO3ma(t-Bu)3, 5, in 95% yield. A purified sample of the
DO3ma ligand, 3, was obtained with a 62% global yield after
deprotection of the triester with TFA followed by cationic and
anionic ion-exchange chromatography.
Synthesis of ligands substituted with one or two alkyne groups
grafted on the tetraaza cyclododecane ring requires full con-
struction of the macrocyclic tetraaza unit. Essentially two
approaches can be followed to achieve this goal: (a) a reaction
between triethylenetetramine and butanedione to form a gem-
inal-cis bisaminal possessing the requisite configuration to under-
go a (1 + 1) cyclocondensation with a 1,2-disubstituted ethylene
derivative featuring two leaving groups26,28,31 or (b) a “crab-like”
synthesis between a bis-chloroacetamide and a diamine.32 The
bisaminal route is more direct, but in our hands, the yields were
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synthesis started from (2S,3S)-1,4-bis(benzyloxy)-2,3-diamino-
butane, 13, which was obtained in three steps as reported by
Scheurer et al.27 from commercially available 1,4-di-O-benzyl-L-
threitol, which was submitted to the classical transformation into
a dimesylate, a diazide, and finally a diamine by hydrogenation in
dry THF rather than in methanol as reported in a more recent
publication by the same authors.36 The “crab-like” cyclization
with N,N0-(ethane-1,2-diyl)bis(2-chloroacetamide)37 followed
by recrystallization in boiling ethanol afforded pure 5,6-bis-
(benzyloxymethyl)-1,4,7,10-tetraazacyclododecane-2,9-dione, 14.
The poor yield obtained for this reaction (20%) is in agreement
with previous works38 and with the results obtained for similar
cyclizations undertaken in our laboratory.39 The reduction of amide
groups in a macrocycle is usually carried out with a BH3ꢀTHF
complex, but as expected, the drastic conditions needed to convert
the intermediate amineꢀborane complexes into amines (6 M
HCl) caused partial deprotection of the alcohol groups. The mild
reducing agent Red-Al was thus preferred as it is known to leave
the benzyl ether groups untouched.40 After flash chromatogra-
phy purification using silica gel, tetraamine 15b was obtained in
78% yield. The latter was reacted with tert-butyl bromoacetate to
yield tetraester 16 (Scheme 5), which must adopt a preferred
conformation as the 2-fold symmetry observed in the 13C NMR
spectra of compounds 14 and 15b is no longer observed.
Debenzylation of 16 proved to be a challenging task. Use of
common reaction conditions41 (3 atm of H2, 25 °C, Pd/C 10%)
in different organic solvents failed even in the presence of large
amounts of catalyst. The lack of reaction in the presence of
amines has already been reported42 and could be avoided by
addition of a small amount of acid. With large amounts of
Pearlman’s catalyst (Pd(OH)2/C) and acetic acid, diol 17 was
observed in mass spectrometry but the reaction yield remained
lower than 30%. The use of acids such as TFA and CCl3COOH,
of Raney nickel as catalyst, or of several deprotection methods
not based on catalytic hydrogenation41 led to hydrolysis of the
tert-butyl ester to slow kinetics or to formation of large amounts
of side products. Finally, addition of a small amount of concen-
trated HCl43 to an ethanol solution of 16 followed by catalytic
hydrogenation for 3 days afforded the target diol 17 in 60% yield
while preserving the integrity of the ester functions. Two alkyne
units were then introduced by a Williamson-type O-alkylation. In
the case of aliphatic alcohols, this reaction is usually carried out in
the presence of NaH and propargyl bromide.44,45 Compound 17
proved to be unstable when sodium hydride was added to a
toluene or THF solution: mass spectrometry indicated that the
diol underwent a double intramolecular transesterification. Dif-
ferent conditions of phase transfer catalysis (PTC) were tested as
the synthesis of propargylic ethers from allylic,46 benzylic,47 and
propargylic48 alcohols have already been reported by this meth-
od. In our hands, O-alkylation of 17 using NaOH or KOH (as
50% aqueous solution or as pellets), tetrabutylammonium iodide
in dichloromethane and propargyl bromide (80% solution in
toluene) went to completion only after 14 days. The reaction was
even slower in pure propargyl bromide. Dueno et al.49 reported
that cesium bases generate highly nucleophilic cesium alkoxides
able to accelerate O-alkylation reactions of aliphatic alcohols.
This was confirmed by the observation of a mass peak for
dialkyne 18 after 10 h when a 50% aqueous solution of cesium
hydroxide was used. The accelerating effect of cesium was even
compared to sodium hydroxide. Dialkyne 18 was then converted
to DOTAda, 2, by cleaving the tert-butyl esters using TFA
followed by purification by ion-exchange chromatography. It is
worthwhile to note that the two alkyne groups of 2 give rise to
separate 13C peaks, although they are far removed from the
macrocyclic unit.
The template procedure illustrated in Scheme 4 is obviously
much more effective than the “crab-like” approach (three steps
instead of five, higher overall yield). A well-defined isomer can
only be synthesized thanks to a cyclization with a diamide
dichloride as in Scheme 4, but three steps are required for
obtaining the starting derivative 13.
Solution Structures. Solutions of the lanthanide complexes
with ligands 1ꢀ3 were prepared by adding a 10% excess of metal
ion to a solution of ligand and adjusting the pH to 5.5 at 50 °C.
Once the pH was constant, the reaction mixtures were eluted on
a Chelex 100 resin to eliminate the metal ions in excess.
It is now well established34,50,51 that the tetraaza cycle of the
DOTA-like complexes adopts either a δδδδ or a λλλλ arrange-
ment depending on the NꢀCꢀCꢀN torsion angles in the
ethylenediamine units that all have either the same positive (δ)
or negative (λ) values. Moreover, the pendant acetate arms form
either a clockwise (Λ) or a counterclockwise (Δ) four-bladed
propeller. This leads to four stereoisomers, two of which are in a
square antiprismatic (SAP) arrangement (Λ δδδδ and Δ λλλλ)
while the other two adopt a twisted square antiprism (TSAP)
geometry (Δ δδδδ and Λ λλλλ). These four stereoisomers are
related as two pairs of enantiomers that give rise to separate peaks
because of a slow intermolecular exchange on the NMR time
scale. The resonances most shifted to low fields of the Yb3+
chelates are known34,50,51 to correspond to the axial ring protons
located on the carbon atom on the side of the [3.3.3.3] square
52
conformation in the SAP geometry (also called Hax up
,
see
Figure 1). In the TSAP geometry, these resonances are also
found at low fields; they are less shifted and easily distinguishable
from the SAP peaks. Figure 2 presents the low-field part of NMR
spectra of the Yb3+ complexes with DOTAda and DOTAma.
A chiral carbon atom in the tetraaza ring transforms the four
potential isomers of a DOTA complex into diastereoisomers
whose number is increased to eight if this complex is a racemic
mixture as in the case of YbDOTAma. Instead of observing the
expected eight sets of four resonances for the axial proton shifted
to low field (Hax,up between 150 and 60 ppm), only four series of
four peaks are visible (four resonances are overlapping to form
the resonance at 148 ppm). Molecular modeling calculations
confirmed by analyses of the dipolar NMR shifts indicate that
ring substituents cannot occupy axial positions because of steric
crowding and that the “eq up” orientation is preferred to the “eq
down” position.52ꢀ54 This preference is sufficiently strong to
prevent the inversion of the ethylenediamine groups in the
tetraaza ring as already found in the EXSY spectra of an Yb3+
complex with a DOTA ligand substituted with one methyl group
(MDOTA).52 With a methyl52 or benzyl55 ring substituent in an
S enantiomer, this necessarily leads to a δδδδ arrangement that
will also be found for the R enantiomer of DOTAma. Conversely,
the S enantiomer will adopt a λλλλ conformation. Both
conformations are found in the spectrum of racemic YbDO-
TAma with a “frozen” cycle and either Δ or Λ arrangements of
the acetate arms. Only four sets of four peaks are thus
observed in the NMR spectrum of YbDOTAma, and the
SAP form is the major species in solution (TSAP/SAP = 1:2
for YbDOTAma).
more impressive when solid CsOH H2O was directly added to
3
the reaction mixture in dichloromethane: 18 could be isolated
after 90 min. This corresponds to a 200-fold increase in rate
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Figure 1. (Left) Conformation of an ethylenediamine group in a lanthanide DOTA complex. (Right) [3.3.3.3] Conformation of the tetraaza ring in a
lanthanide DOTA complex with the proton nomenclature.
Figure 2. Downfield part of the NMR spectra of the Yb(III) complexes with DOTAma, 1 (left), and DOTada, 2 (right).
The Yb3+ complex with the enantiopure DOTAda ligand, 2, is
the first known example of a lanthanide chelate with two adjacent
asymmetric carbon atoms in the tetraaza cycle. The two C
substituents have no choice but to occupy the two equatorial
positions that have been shown to be the least crowded by
molecular modeling.52 As DOTAda is an S,S-enantiomer, the
tetraaza cycle must thus again be in a δδδδ arrangement but the
TSAP geometry is now dominant (TSAP/SAP ratio close to 2:1)
presumably because of the higher steric crowding. A 9:1 SAP/
TSAP ratio was found in the case of the Yb(III) (S-2-nitrobenzyl)-
DOTA complex.53
Relaxivity Properties. The paramagnetic lanthanide ion Gd-
(III) induces a remarkable increase in the experimental relaxation
rate r1exp of the water protons because of its long electronic
relaxation time T1e. The dipolar interaction taking place in the
first coordination sphere is accounted for by the Solomonꢀ
BloembergenꢀMorgan (SBM) equations56,57 that are summar-
ized below for a complex GdL
The other parameters are the hydration number of the encapsulated
metal ion qH2O, the metalꢀproton distance r, the correlation
time of the modulation of the zero-field splitting τv, and the
mean-square zero-field splitting energy Δ2. The solvent mol-
ecules outside the first coordination sphere also make a con-
tribution to the relaxivity that depends on the metalꢀproton
56,57
distance router and the diffusion coefficient Ddiff
.
Small
complexes such as GdDO3ma, GdDOTAma, and GdDOTAda
are tumbling rapidly in solution, and one can safely assume that τr
is the predominant factor in the above equations at most fields B0
because it is the smallest correlation time in eq 3. For such
chelates, the nuclear magnetic relaxation dispersion (NMRD)
curve between 0.01 and 80 MHz is an S-shaped curve as found in
Figure 3.
The main difficulty when using the SBM eqs 1ꢀ3 is that they
depend on a large number of unknown parameters. A reliable and
quantitative interpretation of the NMRD curves in Figure 3
requires determination of as many parameters as possible by
independent methods. The hydration number qH2O was deduced
from the measurements of the fluorescence lifetimes τH2O and
τD2O of the Eu(III) complexes in water and D2O using the
modified Horrocks equation58
ꢂ
qH2O ½GdLꢃ
r1exp
¼
þ R1outer
ð1Þ
ð2Þ
ð3Þ
ꢂ
55:56 ðT1M þ τmÞ
ꢂꢀ
ꢁ
ꢃ
1
¼ functionðrꢀ6, τc, Δ2, B0Þ
1
1
qH2O ¼ 1:2
ꢀ
ꢀ 0:25
ð4Þ
T1M
τH2O τD2O
The qH2O value determined for EuDOTAma and EuDOTAda
was 1.1 ( 0.3, and both species are assumed to be monohydrated
as are all tetraacetic DOTA-type chelates.59 The measure-
ments carried out on EuDO3ma indicate that this species is bis-
hydrated (qH2O = 2.1 ( 0.3), in keeping with the hydration
1
1
1
1
¼
þ
þ
τc
τr
τm
T1e
where τr and τm are the rotational and water exchange correlation
times and where the relaxivity r1exp is expressed in sꢀ1 mMꢀ1
.
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ARTICLE
Table 1. Relaxivity Parameters Obtained by Best Fittings of
the 17O Data
ΔH‡
complex
(kJ molꢀ1
)
Δ2 (1019 sꢀ2
)
τv298K (ps) τm298K (ns)
3
GdDO3ma
GdDOTAma
GdDOTAda
43
52
48
7.9
5.8
3.6
12
89
12.5
14
183
122
Table 2. Relaxivity Parameters Obtained by Best Fittings of
the NMRD Data
complex
Δ2 (1019 sꢀ2 τv298K (ps) τm298K (ns) τr298K (ps)
)
Gd-DO3ma
9.9
13.7
14.8
14.4
26c
89a
60
Gd-DOTAma
Gd-DOTAda
Gd-(C18)2DOTAb
3.6
183a
122a
323c
333d
80
2.0
107
Figure 3. Nuclear magnetic relaxation curves (NMRD) of GdDOTA-
ma (b), GdDOTAda (O), and GdDO3ma (2) at 25 °C.
0.52c
0.81d
3950c
135d,e
5206d,e
36d
a Deduced from the 17O measurements. b Above cmc. c Bertiniꢀ
KowalewskiꢀLuchinat (BKL) approach 66,67 with an axial component
of the zero-field splitting D = 0.017 cmꢀ1
.
d SBM and LipariꢀSzabo
approach68 with second sphere.69 e Rotational correlation times for fast
and slow motion with rigidity factor S2 = 0.78.
one 4-nitrobenzyl group in the tetraaza ring and one methyl
substituent on each acetate arm.55 The experimental values
reported in Table 1 follow this trend even if it should be
remembered that GdDOTAma is a racemic mixture. The water
exchange is faster for GdDOTAma than in the case of the
unsubstituted GdDOTA (τm = 243 ns), a chelate which adopts
essentially an SAP geometry, and it is still faster for GdDOTAda
for which the TSAP geometry is the major contributor. In the
latter case, the exchange water time τm becomes closer to the
theoretically “ideal” value of about 20 ns at 0.47 T for slowly
rotating Gd(III) complexes. This value could probably be
reached if the acetate arms of the alkyne chelates were
substituted by methyl groups so as to increase the steric
crowding of the complex. As expected, the water exchange is
much faster in the case of the bishydrated electrically neutral
triacetate complex GdDO3ma and is very close to the value
measured for DO3A, its analogue with an unsubstituted second-
ary amine (τm = 80 ns).64
Figure 4. Temperature dependence of the paramagnetic contribution
R2p to the transverse 17O water relaxation rate of GdDO3ma (2),
GdDOTAma (b), and GdDOTAda (O) at 5.87 T, pH 5.7, [Gd] = 4.65,
7.04, and 7.61 mM, respectively. Experimental data and calculated
curves.
numbers reported for DO3A-type complexes bearing only three
acetate arms.60,61
The dispersion of the relaxivity of Gd-DO3ma, Gd-DOTAma,
and Gd-DOTAda with the frequency at 298 K (Figure 3) was
fitted using the SBM equations and experimental values of the
water exchange time τm. Several parameters were fixed prior to
the calculations: the metalꢀwater proton distances in the first
coordination sphere and in the outer sphere were set at 3.1 and
3.6 Å,10 whereas the relative diffusion constant of the complexes
The rate of water exchange 1/τm of GdDO3ma, GdDOTAma,
and GdDOTAda is another parameter in the SBM equations that
can be determined independently. The 1/τm values were de-
duced from the paramagnetic contribution of the Gd3+ com-
plexes to the 17O transverse relaxation rate of water (R2p)
between 278 and 353 K using the Swift and Connick equations62
(see Figure 4).
The hyperfine coupling constant (A/p) was fixed at ꢀ3.5 ꢁ
106 rad sꢀ157, the activation energy of the modulation of the zero
field splitting was fixed at 1 kJ/mol, and we also assumed an
Eyring temperature dependence of the water exchange time.57,63
The parameters giving the best agreement between the experi-
mental and the computed transverse relaxation times of the three
Gd(III) complexes are given in Table 1.
was fixed at 2.5 ꢁ 10ꢀ5 cm2 s
.
ꢀ1 65 The calculated parameters are
reported in Table 2. The low values of the rotational correlation
times τr are in good agreement with those reported for similar
rapidly rotating chelates.18,57,60 Variations between the Δ2 values
in Tables 1 and 2 are probably not physically significant. Indeed,
the parameters deduced from the NMRD curves always have to
be considered with some caution since several factors, especially
τv and Δ2, are strongly correlated. Nonetheless, τm and τr are
considered to be reliable, as the influence of the other parameters
on these factors is more limited.
It has been shown that the water exchange time is about 10
times smaller in the more open TSAP than in the SAP form in the
case of the optically pure isomers of GdDOTA chelates bearing
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Micellar Assembly Based on the GdDOTAda Complex. As a
proof of the versatility of the click reaction with alkyne DOTA
derivatives and also of the interest of having two closely spaced
alkyne moieties, we prepared a gadolinium complex able to self-
assemble into micelles in aqueous media. Forming micelles is a
well-known procedure for slowing down the tumbling rate τr of
contrast agents and hence to obtain higher relaxivities. Amphi-
philic chelates have been obtained by appending aliphatic
substituents to DOTA or DTPA-type ligands70ꢀ74 and to other
polyaminopolyacetic derivatives.64,75 The ligand (C18)2DOTAda is
a new addition to this series of amphiphilic Gd(III) chelates
from which it differs by having two long aliphatic chains
directly grafted onto the tetraaza macrocycle on two adjacent
carbon atoms. Accardo et al.74 reported another rare example
of this approach in the case of a DTPA chelate that also
features two aliphatic chains, but they are far removed from
the chelating unit.
Figure 5. Plot of the variation of the water 1H longitudinal relaxation
rate 1/T1 versus the concentration of (C18)2GdDOTAda at 20 MHz
and 25 °C.
Ligand (C18)2DOTAda, 20, was synthesized as shown in
Scheme 6. The reaction conditions of Scheurer et al.27 were used
to convert 1-bromooctadecane into 1-azidooctadecane 16 that
was “click” coupled with tetraester dialkyne 18 using copper
iodide as catalyst in dichloromethane. Complicating side reac-
tions were minimized by using two equivalents of two nitrogen
bases, diisopropylethylamine and 2,6-lutidine,76 per macrocycle
18 and by performing the reaction in the absence of oxygen. In
our hands, 1.1 equiv of CuI was needed for obtaining 19,
presumably because the DOTA unit of ester 18 forms a stable
Cu(II) complex and totally consumes one equivalent of this
reagent. Compound 18 had completely reacted after 3 h as
shown by the ES-MS. After removal of the copper ions by
extracting the reaction mixture with a 1 M DTPA solution overnight
followed by silica gel purification, bisaliphatic compound 19 was
recovered in 68% yield. This compound features two 1H NMR
resonances at 8.13 and 8.25 ppm that correspond to the two
triazole substituents that are nonequivalent as are the two alkyne
groups of ligand 2 (vide supra). Cleavage of the tert-butyl esters
of 19 with TFA led to the target ligand (C18)2DOTAda, 20. The
Gd(C18)2DOTAda complex was obtained by adding gadoli-
nium trichloride to an aqueous micellar suspension of ligand 20
(see Experimental Section). Preparation of Gd(III)-containing
micelles was usually carried out by preparing first the metal
complex and then proceeding to formation of micelles. In our
hands, several variations of this procedure (synthesis in pyridine73b
or in water74 with or without sonication) led to nonreproducible
results including sedimentation and degradation. The compact-
ness of (C18)2DOTAda, the substitution of its macrocyclic ring
by two long aliphatic chains rather than one, and the low cmc of
its Gd complex could be at the origin of these difficulties.
As reported by different authors,72,77 the critical micellar con-
centration (cmc) of amphiphilic chelates can be determined by
Figure 6. Nuclear magnetic relaxation curve (NMRD) of a micellar
suspension of Gd(C18)2DOTAda at a concentration of 0.158 mM and
25 °C. Calculated NMRD curves by the BertiniꢀKowalewskiꢀLuchinat
(BKL) approach66,67(solid line) and the SBM and LipariꢀSzabo
approach68,69 (dashed line).
rotating species (see Figure 6). The 20 MHz relaxivity is close
to 35 sꢀ1 mMꢀ1, which is approximately six times higher than the
value obtained for the parent GdꢀDOTAda complex. The 20
MHz relaxivity is also higher than or comparable to the maxima
reported for high-generation Gd3+-containing dendrimers78 and
for the paramagnetic micellar systems that exhibit the highest
efficacy reported so far.77 We ascribe the high relaxivity at 20ꢀ40
MHz to the double anchoring of the Gd(III) complex into the
micellar structure and to the close spacing between the two
octadecyl chains that both prevent independent rotations of the
metal chelates and the micelles.
There are several approaches for interpreting relaxivity curves
of slowly rotating Gd(III) complexes. In the BertiniꢀKowalewskiꢀ
Luchinat (BKL) approach,66,67 the SBM equations have been
modified to take into account a static zero-field splitting (ZFS) in
addition to the transient ZFS that causes the electron spin
relaxation. In another approach, the SBM equations have been
adapted to better represent the partially independent motions of
1
measuring the H relaxation rates as a function of the Gd(III)
complex concentration. A relaxivity vs complex concentration
plot exhibits a marked break when a surfactant changes from its
monomeric state to a slowly tumbling micellar form. As shown in
Figure 5, Gd(C18)2DOTA possesses a very low cmc of about
4 ꢁ 10ꢀ3 mM. An even lower cmc value has been determined for
a DTPA amphiphilic complex with two long aliphatic chains
(cmc = 9.6 ꢁ 10ꢀ4 mM).74
At concentrations above the cmc (0.158 mM), the NMRD
profile of a solution of Gd(C18)2DOTAda micelles displays a
pronounced maximum at 20ꢀ40 MHz and high relaxivities at
low fields, i.e., two features that are characteristic of slowly
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ARTICLE
a chelate and of the macromolecule to which it is linked. The
LipariꢀSzabo theory that is applied for this purpose yields
different rotational correlation times for the slow and fast
motions as well as a rigidity factor that describes the internal
flexibility. Mathematical fits with the BKL computer program66,67
and by the BMSꢀLipariꢀSzabo approach led to the relaxivity
parameters listed in Table 2. Caution must again be advised in
accepting the values collected in this table because the number of
parameters involved in the fit procedures is even larger than in
the simple BMS treatment. In addition, several parameter sets
can give equally good fits, and the parameters τv and Δ2 in eq 2
are strongly correlated. We also neglected the relaxivity of the
monomeric complex as its contribution to relaxivity must be very
weak compared to the micelles because of its fast rotational time
and as its concentration is very small because of its low cmc.72
These problems have already been mentioned several times,79
and ranges of values are probably more reliable. This being taken
into account, there is a sizable increase in rotational correlation
times upon micelle formation and the high value of the rigidity
factor (S2 = 0.78) supports the assumption that the Gd-
(C18)2DOTAda complex has little mobility in the micelles
thanks to their two closely spaced aliphatic substituents. It
should also be noted that the computed water exchange time
τm of the micelles is two to three times higher than the one
found for the parent unsubstituted chelates complex and is
thus detrimental to a high relaxivity maximum at high fields.
This does not prevent the Gd(C18)2DOTAda micelles from
exhibiting very high relaxivities. Finally, it should be noted that
the changes undergone by the water exchange time τm upon
formation of micelles remains poorly understood. The τm
factors of Gd(III)-containing micelles have been reported to
be identical,72 larger,73 or smaller74 than that of the corre-
sponding monomers.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: jf.desreux@ulg.ac.be.
’ ACKNOWLEDGMENT
The authors gratefully acknowledge the Fonds de la
Recherche Scientifique (FNRS) and the Institut Interuniversitaire
des Sciences Nuclꢁeaires of Belgium for their financial support.
’ REFERENCES
(1) (a) Merbach, A. E.; Toth, E. The Chemistry of Contrast Agents in
Medical Magnetic Resonance Imaging; Wiley: Chichester, 2001. (b)
Caravan, P.; Ellison, J. J.; Mc.Murry, T. J.; Lauffer, R. B. Chem. Rev.
1999, 99, 2293–2352.
(2) Rebizak, R.; Schaefer, M.; Dellacherie, E. Eur. J. Pharm. Sci. 1999,
7, 243–248.
(3) Corsi, D. M.; Vander Elst, L.; Muller, R. N.; van Bekkum, H.;
Peters, J. A. Chem.—Eur. J. 2001, 7, 1383–1389.
(4) Lebduskova, P.; Kotek, J.; Hermann, P.; Vander Elst, L.; Muller,
R. N.; Likes, I.; Peeters, J. A. Bioconjugate Chem. 2004, 15, 881–889.
(5) Jaszberenyi, Z.; Moriggi, L.; Schmidt, P.; Weidensteiner, C.;
Kneuer, R.; Merbach, A. E.; Helm, L.; Toth, E. J. Biol. Inorg. Chem. 2007,
12, 406–420.
(6) Langereis, S.; Dirksen, A.; Hackeng, T. M.; van Genderen,
M. H. P.; Meijer, E. W. New J. Chem. 2007, 31, 1152–1160.
(7) Nicolle, G. M.; Toth, E.; Schmitt-Willich, H.; Raduchel, B. E.;
Merbach, A. E. Chem.—Eur. J. 2002, 8, 1040–1048.
(8) Xu, H.; Regino, C. A. S.; Bernardo, M.; Koyama, Y.; Kobayashi,
H.; Choyke, P. L.; Brechbiel, M. W. J. Med. Chem. 2007, 50, 3185–3193.
(9) Rudovsky, J.; Hermann, P.; Botta, M.; Aime, S.; Lukes, I. Chem.
Commun. 2005, 2390–2392.
(10) Caravan, P.; Cloutier, N. J.; Greenfield, M. T.; McDermid, S. A.;
Dunham, S. U.; Bulte, J. W. M.; Amedio, J. C. J.; Looby, R. J.; Supkowski,
R. M.; Horrocks, W. D., Jr.; McMurry, T. J.; Lauffer, R. B. J. Am. Chem.
Soc. 2002, 124, 3152–3162.
4. CONCLUSIONS
(11) Caravan, P.; Parigi, G.; Chasse, J. M.; Cloutier, N. J.; Ellison,
J. J.; Lauffer, R. B.; Luchinat, C.; McDermid, S. A.; Spiller, M.; McMurry,
T. J. Inorg. Chem. 2007, 46, 6632–6639.
(12) Henoumont, C.; Henrotte, V.; Laurent, S.; Vander Elst, L.;
Muller, R. N. J. Bioinorg. Biochem. 2008, 102, 721–730.
(13) Rostovsev, V. C.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
Angew. Chem., Int. Ed. Engl. 2002, 41, 2596–2599.
(14) Tornoe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002,
8, 1040–1048.
(15) Wu, P.; Fokin, V. V. Aldrichim. Acta 2007, 40, 7–17.
(16) Prasuhn, D. E.; Yeh, R. M.; Obenaus, A.; Manchester, M.; Finn,
M. G. Chem. Commun. 2007, 1269–1271.
(17) Song, Y.; Kohlmeir, E. K.; Meade, T. J. J. Am. Chem. Soc. 2008,
130, 6662–6663.
(18) Powell, D. H.; Ni Dhubhghaill, O. N.; Pubanz, D.; Helm, L.;
Lebedev, Y. S.; Schlaepfer, W.; Merbach, A. E. J. Am. Chem. Soc. 1996,
118, 9333–9346.
(19) Toth, E.; Burai, L.; Br€ucher, E.; Merbach, A. E.; Paight, E. S.
J. Chem. Soc., Dalton Trans. 1997, 1587–1594.
(20) Bryson, J. M.; Chu, W. J.; Lee, J. H.; Reineke, T. M. Bioconjugate
Chem. 2008, 19, 1505–1509.
(21) Morcos, S. K. Eur. Radiol. 2011, 496–500.
(22) Kn€or, S.; Modlinger, A.; Poethko, T.; Schottelius, M.; Wester,
H. J. Chem.—Eur. J. 2007, 13, 6082–6090.
(23) Stasiuk, G. J.; Lowe, M. P. Dalton Trans. 2010, 9725–9727.
(24) Jauregui, M.; Perry, W. S.; Allain, C.; Vidler, L. R.; Willis, M. C.;
Kenwright, A. M.; Snaith, J. S.; Stasiuk, G. J.; Lowe, M. P.; Faulkner, S.
Dalton Trans. 2009, 6283–6285.
(25) Keizers, P. H. J.; Desreux, J. F.; Overhand, M.; Ubbink, M.
J. Am. Chem. Soc. 2007, 129, 9292–9293.
Obtaining the very high relaxivities that could be reached
according to the SBM equations remains an elusive goal,
although much progress has been made. The easiest procedure
consists in slowing down the rotation of a Gd3+ chelate by
linking it covalently or noncovalently to a macromolecule
whether synthetic or biological. However, relaxivities remain
lower than expected because of a water exchange time that is too
long and because of the flexibility of the chelateꢀmacromolecule
link. We report here an attempt at solving this problem by a
double anchoring of the metal chelate. A DOTA ligand featuring
two alkyne moieties grafted on two adjacent carbon atoms in the
tetraaza ring was synthesized for this purpose and reacted by a
“click” approach to add long aliphatic chains. The relaxivity of
resulting micelles is particularly high presumably due to the
double anchoring. In a future extension of the present research
work, GdDOTAda will be reacted with polyazido ester
polymers.80 The monoalkyne ligands DOTAma and DO3ma
were synthesized for comparison purposes, but they are
interesting in their own right as reacting them with polyazido
polymers is more straightforward.81 Finally, the bisalkyne
ligand DOTAda can be substituted with a large variety of
chemical functions thanks to a “click” reaction with azide-featuring
substrates. The rigid association of DOTAda lanthanide chelates
with proteins for structure determination from the dipolar shifts
and from the residual dipolar couplings is one application of
potential interest.25
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ARTICLE
(26) Boschetti, F.; Denat, F.; Espinosa, E.; Lagrange, J.-M.; Guilard,
R. Chem. Commun. 2004, 588–589.
(27) Scheurer, A.; Mosset, P.; Saalfrank, R. W. Tetrahedron: Asym-
metry 1997, 8, 1243–1251.
(58) Beeby, A.; Clarkson, I. M.; Dickins, R. S.; Faulkner, S.; Parker,
D.; Royle, L.; de Sousa, A. S.; Williams, J. A. G.; Woods, M. J. Chem. Soc.,
Perkin Trans. 2 1999, 493–503.
(59) Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B. Chem.
(28) Herve, G.; Bernard, H.; Le Bris, N.; Yaouanc, J.-J.; Handel, H.;
Toupet, L. Tetrahedron Lett. 1998, 39, 6861–6864.
(29) Marti, M. J.; Rico, I.; Ader, J. C.; De Savignac, A.; Lattes, A.
Tetrahedron Lett. 1989, 30, 1245–1248.
(30) Barge, A.; Cravotto, G.; Gianolio, E.; Fedeli, F. Contrast Media
Mol. Imaging 2006, 1, 184–188.
(31) Chuburu, F.; Tripier, R.; Le Baccon, M.; Handel, H. Eur. J. Org.
Chem. 2003, 1050–1055.
(32) Uggeri, F.; Aime, S.; Anelli, P. L.; Botta, M.; Brochetta, M.; de
Ha€en, C.; Ermondi, G.; Grandi, M.; Paoli, P. Inorg. Chem. 1995, 34,
633–642.
Rev. 1999, 99, 2293–2352.
(60) Aime, S.; Botta, M.; Geninatti Crich, S.; Giovenzana, G.;
Pagliarin, R.; Sisti, M.; Terreno, E. Magn. Reson. Chem. 1998, 36, 200–208.
(61) Botta, M.; Quici, S.; Pozzi, G.; Marzanni, G.; Pagliarin, R.;
Barra, S.; Crich, S. G. Org. Biol. Chem. 2004, 2, 570–577.
(62) Swift, T. J.; Connick, R. E. J. Chem. Phys. 1962, 37, 307–320.
(63) Paris, J.; Gameiro, C.; Humblet, V.; Mohapatra, P. K.; Jacques,
V.; Desreux, J. F. Inorg. Chem. 2006, 45, 5092–5102.
(64) Aime, S.; Botta, M.; Bruce, J. I.; Mainero, V.; Parker, D.;
Terreno, E. Chem. Commun. 2001, 1, 115–116.
(65) Vander Elst, L.; Sessoye, A.; Laurent, S.; Muller, R. N. Helv.
(33) Rousselin, Y.; Sok, N.; Boschetti, F.; Guilard, R.; Denat, F. Eur.
Chim. Acta 2005, 88, 574–587.
J. Org. Chem. 2010, 1688–1693.
(66) Bertini, I.; Galas, O.; Luchinat, C.; Parigi, G. J. Magn. Reson. A
1995, 113, 151–158.
(67) Bertini, I.; Kowalewski, J.; Luchinat, C.; Nilsson, T.; Parigi, G.
J. Chem. Phys. 1999, 111, 5795–5807.
(34) Desreux, J. F. Inorg. Chem. 1980, 19, 1319–1324.
(35) Chickos, J. S.; Bausch, M.; Alul, R. J. Org. Chem. 1981,
46, 3559–3562.
(36) Scheurer, A.; Mossett, P.; Saalfrank, R. W. Tetrahedron: Asym-
metry 1999, 10, 3559–3570.
(68) Dunand, F. A.; Toth, E.; Hollister, R.; Merbach, A. E. J. Biol.
Inorg. Chem. 2001, 6, 247–255.
(37) Lin, W. O.; Figueira, J. A. A.; Alt, H. G. Monatsh. Chem. 1985,
116, 217–221.
(38) Krakowiak, K. E.; Bradshaw, J. S.; Izatt, R. M. J. Heterocycl.
Chem. 1990, 27, 1585–1589.
(69) Rudovsky, J.; Botta, M.; Hermann, P.; Hardcastle, K. I.; Lukes,
I.; Aime, S. Bioconjugate Chem. 2006, 17, 975–987.
(70) Torres, S.; Martins, J. A.; Andrꢁe, J. P.; Geraldes, C. F. G. C.;
Merbach, A. E.; Toth, E. Chem.—Eur. J. 2006, 12, 940–948.
(71) Andrꢁe, J. P.; Toth, E.; Fischer, H.; Seelig, A.; M€acke, H. R.;
Merbach, A. E. Chem.—Eur. J. 1999, 5, 2977–2983.
(72) Nicolle, G. M.; Toth, E.; Eisenwiener, K. P.; M€acke, H. R.;
Merbach, A. E. J. Biol. Inorg. Chem. 2002, 7, 757–769.
(73) (a) Parac-Vogt, T. N.; Kimpe, K.; Laurent, S.; Piꢁerart, C.;
Vander Elst, L.; Muller, R. N.; Binnemans, K. Eur. J. Inorg. Chem.
2004, 3538–3543. (b) Kimpe, K.; Parac-Vogt, T. N.; Laurent, S.; Piꢁerart,
C.; Vander Elst, L.; Muller, R. N.; Binnemans, K. Eur. J. Inorg. Chem.
2003, 3021–3027.
(39) Desreux, J. F.; Tweedle, M. F.; Ratsep, P. C.; Wagler, T. R.;
Marinelli, E. R. U.S. Patent US 5358704, 1994; pp 1ꢀ22.
(40) Anelli, L.; Beltrami, A.; Franzini, M.; Paoli, P.; Rossi, P.; Uggeri,
F.; Virtuani, M. Inorg. Chim. Acta 2001, 317, 218–229.
(41) Greene, T. W.; Wuts, P. G. M. Protective groups in organic
synthesis, 3rd ed.; Wiley: New York, 1999.
(42) Czech, B. P.; Bartsch, R. A. J. Org. Chem. 1984, 49, 4076–4078.
(43) Sato, S.; Nemoto, M.; Kumazawa, T.; Matsuba, S.; Onodera,
J. I.; Aoyama, M.; Obarab, H.; Kamadab, H. Carbohydr. Res. 2004,
339, 2425–2432.
(74) Accardo, A.; Tesauro, D.; Morelli, G.; Gianolio, E.; Aime, S.;
Vaccaro, M.; Mangiapia, G.; Paduano, L.; Schillen, K. J. Biol. Inorg. Chem.
2007, 12, 267–276.
(44) Labadie, G. R.; Viswanathan, R.; Poulter, C. D. J. Org. Chem.
2007, 72, 9291–9297.
(45) Lucas, R.; Neto, V.; Bouazza, H.; Zerrouki, R.; Granet, R.;
Ktausz, P.; Champavier, Y. Tetrahedron Lett. 2008, 49, 1004–1007.
(46) Mikami, K.; Kataoka, S.; Yusa, Y.; Aikawa, K. Org. Lett. 2004,
6, 3699–3701.
(75) Gianolio, E.; Giovenzana, G.; Ciampa, A.; Lanzardo, S.; Imperio,
D.; Aime, S. ChemMedChem 2008, 3, 60–62.
(76) Horne, W. S.; Stout, C. D.; Ghadiri, M. R. J. Am. Chem. Soc.
2003, 125, 9372–9376.
(47) Lamberth, C.; Jeanguenat, A.; Cederbaum, F.; De Mesmaeker,
A.; Zeller, M.; Kempf, H. J.; Zeun, R. Bioorg. Med. Chem. 1990, 27,
1585–1589.
(48) Jones, J. B.; Wright, J. M.; Hybd, G.; Wyatt, J. K.; Warner, P. M.;
Huber, R. S.; Li, A.; Kilgore, M. W.; Sticca, R. P.; Pollenz, R. S. J. Org.
Chem. 2002, 67, 5727–5733.
(77) Gianolio, E.; Giovenzana, G. B.; Longo, D.; Longo, I.; Menegotto,
I.; Aime, S. Chem.—Eur. J. 2007, 13, 5785–5797.
(78) Laus, S.; Sour, A.; Ruloff, R.; Toth, E.; Merbach, A. E. Chem.—Eur.
J. 2005, 11, 3064–3076.
(79) Kruk, D.; Nilsson, T.; Kowalewski, J. Phys. Chem. Chem. Phys.
2001, 3, 4907–4917.
(49) Dueno, E. E.; Chu, F.; Kim, S. I.; Jung, K. W. Tetrahedron Lett.
1999, 40, 1843–1846.
(80) Riva, R.; Schmeits, S.; Jꢁer^ome, C.; Jꢁer^ome, R.; Lecomte, P.
Macromolecules 2007, 40, 796–803.
(50) Aime, S.; Botta, M.; Fasano, M.; Marques, M. P. M.; Geraldes,
C. F. G. C.; Pubanz, D.; Merbach, A. E. Inorg. Chem. 1997, 36, 2059–2068.
(51) Howard, J. A. K.; Kenwright, A. M.; Moloney, J. M.; Parker, D.;
Port, M.; Navet, M.; Rousseau, O.; Woods, M. J. Chem. Soc., Chem.
Commun. 1998, 1381–1382.
(81) Kimura, K.; Matsuba, T.; Tsujimura, Y.; Yokoyama, M. Anal.
Chem. 1992, 64, 2508–2511.
(52) Ranganathan, R. S.; Raju, N.; Fan, H.; Zhang, X.; Tweedle,
M. F.; Desreux, J. F.; Jacques, V. Inorg. Chem. 2002, 41, 6856–6866.
(53) Woods, M.; Kovacs, Z.; Kiraly, R.; Br€ucher, E.; Zhang, S.;
Sherry, A. D. Inorg. Chem. 2004, 43, 2845–2851.
(54) Di Bari, L.; Pescitelli, G.; Sherry, A. D.; Woods, M. Inorg. Chem.
2005, 44, 8391–8398.
(55) Woods, M.; Kovacs, Z.; Zhang, S.; Sherry, A. D. Angew. Chem.,
Int. Ed. Engl. 2003, 42, 5889–5892.
(56) Banci, L.; Bertini, I.; Luchinat, C. Nuclear and electron relaxa-
tion; VCH: Weinheim, 1991.
(57) Bruce, J. I.; Lowe, M. P.; Parker, D. Photophysical aspects of
lanthanide(III) complexes. In The chemistry of contrast agents in MRI;
Merbach, A. E., Toth, E., Ed.; Wiley: New York, 2001; p 1.
8958
dx.doi.org/10.1021/ic2010997 |Inorg. Chem. 2011, 50, 8946–8958