Ligands derived from C-aryl substituted derivatives of cyclen: Formation of kinetically unstable complexes with lanthanide(III) ions

Article abstract of DOI:10.1039/a805231j

The synthesis of C-aryl monosubstituted derivatives of 1,4,7,10-tetraazacyclododecane (cyclen) by metal templated reductive amination of triethylenetetramine with aryl glyoxals has been developed and shown to be widely applicable. Octadentate ligands deri

Full text of DOI:10.1039/a805231j

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Ligands derived from C-aryl substituted derivatives of cyclen:
formation of kinetically unstable complexes with lanthanide(III) ions
Christopher D. Edlin,a Stephen Faulkner,a David Parker,*,a Matthew P. Wilkinson,a
Mark Woods,a Jin Lin,b Elisabeth Lasri,b Francoise Nethb and Marc Portb
a Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3L E
b Groupe Guerbet, 95943 Roissy Charles de Gaulle, France
The synthesis of C-aryl monosubstituted derivatives of 1,4,7,10-tetraazacyclododecane (cyclen) by metal
templated reductive amination of triethylenetetramine with aryl glyoxals has been developed and shown to be
widely applicable. Octadentate ligands derived from these C-substituted macrocycles form relatively labile eight-
coordinate complexes, which dissociate when challenged with EDTA in aqueous solution at pH 5.5, suggesting
their use in metal-transport processes rather than for applications in vivo.
In recent years, lanthanide complexes of ligands derived from
the aza-macrocycle tetraazacyclododecane (cyclen), 1, have
found a variety of diagnostic applications, particularly as
luminescent probes1,2 and as contrast agents for magnetic
resonance imaging (MRI).3,4 The use of lanthanide complexes
in vivo imposes certain restrictions on the properties of the
complexes. The complexes formed must be kinetically robust
and physiologically well tolerated, owing to the inherent tox-
icity of free lanthanide ions, which tend to accrete in skeletal
tissue and bone.5 1,4,7,10-Tetraazacyclododecane, or “cyclenÏ,
provides a rigid skeleton to which pendent donor groups are
readily attached. Examples of such ligands include the tetra-
acetate “dotaÏ (1,4,7,10-tetraazacyclododecane tetraacetate)
2,6 and the phosphinate analogue, 3.7 In both of these
systems, the lanthanide is held in an eight-coordinate square
antiprismatic environment and the ring adopts a regular
square [3333] conformation. The lanthanide complexes of
such octadentate ligands are kinetically as well as thermody-
namically stable owing to the cooperative binding of the ring
nitrogen and pendent arm oxygen donors.8
The published syntheses of substituted derivatives of cyclen
are all rather lengthy and relatively inefficient,9 rendering
diagnostics based on such systems very expensive. We have
recently published a single-step synthesis of C-aryl substituted
azamacrocycles,10 which relies on a metal-templated reaction
between triethylenetetramine and aryl glyoxals (Scheme 1).
This synthesis proceeds in good yield (40È70%) for a wide
range of aryl glyoxals. We report herein details of the ring
synthesis and the results of studies on the solution complex-
ation behaviour of selected octadentate ligands.
moisture, and it was found that the isolated yields of the
macrocycles were signiÐcantly improved when the complex
was not isolated, and reduction was carried out in situ using
sodium borohydride. Reduction of the diimine complex also
results in the reduction of the metal centre from FeIII to FeII,
associated with a change in colour from deep orange to pale
green. After acidic work-up to remove the coordinated iron(III)
ion, extraction of the free base into chloroform was e†ected at
pH 14. Subsequent Ðltration through a short plug of activated
alumina, allowed the 2-phenyl cyclen, 4, to be isolated in good
yield. The use of other transition-metal ions as templates, spe-
ciÐcally nickel(II) and chromium(III), was also investigated.
However, although the cyclisation processes proceeded effi-
ciently in all cases, removal of the metal ion proved consider-
ably more difficult with these ions and accordingly their use
was not investigated further. In theory, cyclisation of an aryl
glyoxal with triethylenetetraamine may yield four possible
products, including the 12-ring cycle 4 (Fig. 1). Each of the
other three possibilities is a primary amine. Reaction of 4 and
13 with acetyl chlorideÈEt NÈCH Cl yielded an amide which
3
2 2
was puriÐed by reverse phase high performance liquid chro-
matography (HPLC). Analysis by mass spectrometry (e.g.,
Results and Discussion
2-Phenyl-1,4,7,10-tetraazacyclododecane, 4, was prepared
using the general procedure outlined in Scheme 1. Reaction of
phenyl glyoxal with the iron(III) complex of tri-
ethylenetetraamine, 5, prepared in situ by addition of tri-
ethylenetetraamine to
a methanolic solution of iron(III)
chloride, yielded the diimine, 6. The cyclisation step is likely to
proceed via the cis-dichloro complex, 7.11 In this conÐgu-
ration, the two terminal nitrogen atoms are situated close to
one another, ideally placed to form the macrocyclic diimine.
In early experiments, the diimine was isolated and its existence
conÐrmed by IR (m
electrospray mass spectrometry (m/z 245.24). However, the
1636 cm~1), UV (k \ 254 nm) and
C/N
max
diimine complex tends to degrade on exposure to air and
Scheme 1
New J. Chem., 1998, Pages 1359È1364
1359

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Scheme 2
Fig. 1
the pentaalkylation product as the major product. This
problem was overcome by slow addition of only 3 equiv. of
tert-butyl bromoacetate to the macrocycle. Following ester
cleavage with triÑuoroacetic acid, the desired phenyl-dota
analogue 18 was isolated in moderate yield.
presence of molecular ion in ESMS at 417.15 for [HAc ]`)
44
and IR (absence of amide NH band at 3250 cm~1) conÐrmed
that a tetraamide had been formed , consistent only with the
formation of the 12-ring isomer .
The tetra tert-butyl esters, 17 and 19 were prepared simi-
larly by alkylation with tert-butyl bromoacetate in acetonitrile
solution. Once again, formation of the quaternary salt
occurred readily and the addition of 3 equiv. of alkylating
agent was found to be a reasonable compromise between a
lower conversion and the avoidance of over alkylation. PuriÐ-
cation of the tetra-esters was achieved by column chromatog-
raphy on alumina, and subsequent hydrolysis using
triÑuoroacetic acid in dichloromethane yielded the desired
octadentate ligands in an acceptable yield.¤
A series of substituted C-aryl cyclens never prepared pre-
viously was also synthesised by this pathway with a view to
simplifying conjugation of the ligand to various targeting
vectors. Nitro-, cyano- and dibenzylamino-phenyl substituted
cyclens were chosen for this purpose, as they present readily
modiÐed functionalities which can be used to attach the
macrocycle to a suitable targeting vector. In these cases, the
precursor substituted aryl glyoxals 8È11 were synthesised by
oxidation of the corresponding acetophenones using selenium
dioxide, according to literature procedures.12 Owing to the
equilibria that exist between the keto, enol and hydrate forms
of these compounds, characterisation and puriÐcation of these
compounds proved rather difficult. The desired glyoxals even-
tually crystallised in a partially hydrated form from aqueous
solution. Templated cyclisation reactions of 8È11 with tri-
ethylene tetramine yielded the macrocycles 12 to 15 in good
yields. Indeed, the cyclisation reaction would appear to be
widely applicable to aryl glyoxal systems. However, the 4-
substituted nitrophenyl cyclen 12 was found to be rather
unstable in air and in aqueous solution. This instability was
ascribed to the enhanced lability of the 2¡ benzylic hydrogen
leading to ring opening of the macrocycle assisted by the elec-
tron accepting capacity of the nitro group. By contrast, each
of the meta-substituted macrocycles (13, 14), were stable in air
and aqueous solution, consistent with this hypothesis.
Extension of this cyclisation procedure to the preparation
of cyclen itself was then investigated. However, attempted
cyclisations using either glyoxal itself, its hydrate or its sulÐte
addition compound did not yield cyclen in any signiÐcant
quantity. No clear evidence for the formation of the diimine
intermediate could be obtained by 1H NMR or electrospray
mass spectrometry. It is likely that the aryl group is necessary
to stabilise the intermediate diimine: the increased conjuga-
tion resulting from formation of a planar array may play a
part, as the glyoxals themselves exist mainly as their hydrates.
This hypothesis is supported, in part, by the fact that after
borohydride reduction, ESMS showed a small peak corre-
sponding to the hemiaminal 16 (m/z \ 189, [M ] H]`).
The complexation properties of ligand 18 were then investi-
gated. The europium and terbium complexes were prepared
by treatment of an aqueous solution of the ligand with the
lanthanide(III) nitrate salt while maintaining the pH around
5.5. Subsequent trituration with acetone yielded the complex,
which was identiÐed by electrospray mass spectrometry.
Luminescence measurements in H O and D O were used to
2
2
probe the hydration state of the complexes. Quenching of the
excited state of a bound lanthanide ion by proximate OwD
oscillators is much less efficient than by OwH oscillators
owing to the reduced FranckÈCondon overlap of the higher
vibrational harmonics of the OwD oscillator with the energy
levels of the metal centre. Hence, assuming all other factors
remain constant, the number of bound water molecules in
solution may be related to the observed rate constants for the
decay of the metal-based luminescence in H O and D O (k
2
2
H2O
and k
respectively), according to:
D2O
q \ A(k [ k
)
(1)
H2O
D2O
where A is an empirically determined proportionality constant
(A \ 1.05 s, A \ 4.2 s).13
Eu
Tb
The observed luminescent rate constants are shown in
Table 1. They demonstrate that the solution state behaviour
of [Ln É 18]~ is analogous to that of dota: each lanthanide ion
is 9-coordinate, and possesses one bound water molecule.
Deviations from this integral number can by explained by
invoking contributions from outer-sphere water molecules,
which also contribute to quenching.14
However, when the europium complexes were challenged
with a tenfold excess of EDTA at pH 5.5, their behaviour
diverged considerably from that which was expected. The
observed lifetimes [s (Eu) \ 0.1 ms and s (Eu) 0.5 ms]
Synthesis of octadentate ligands
Formation of C-substituted analogues of dota was investi-
gated using procedures similar to those employed with the
parent compound. Treatment of the C-phenyl derivative 4,
with an aqueous solution containing 4 molar equiv. of sodium
chloroacetate, using sodium hydroxide solution (1 M) to
maintain the pH in the range 9È10, resulted in formation of
H2O
D2O
¤ In the case of compound 20 a biphasic alkylation (CH Cl ÈNaOH)
2
2
yielded 19 with a yield of 35%, after puriÐcation by silica chromatog-
raphy. Subsequent hydrolysis using triÑuoroacetic acid in CH Cl
gave 20 in quantitative yield.
2
2
1360
New J. Chem., 1998, Pages 1359È1364

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Table 1 Radiative rate constants for decay of the metal-based emission in H O and D O for europium and terbium complexes (293 K, pH 5.5,
2
2
k
\ 250 nm)a
exe
Complex
k
/ms~1
k
/ms~1
0.63
0.41
1.61
0.49
Dk/ms~1
q@b
H20
D2O
[Eu É 18]~
1.75
1.56
1.89
0.71
1.12
1.15
0.28
0.22
1.09
1.12
1.15
0.92
[Eu É dota]~
[Tb É 18]~
[Tb É dota]~
a Errors on k values are ^10%. b A subtraction of 0.25 ms~1 has been applied, in the case of the Eu complexes only, to account for the e†ect of
outer-sphere (closely di†using) OH oscillators, followed by multiplication by 1.25 (A@).
were consistent with dissociation of the macrocyclic complex,
suggesting that the [Ln EDTA]~ complex was being formed
preferentially. This behaviour clearly indicates that these com-
plexes do not have the kinetic stability associated with their
dota analogues, and implies that their thermodynamic stabil-
for a further 4 h. Sodium borohydride (10 equiv. 1000 mg, 27.3
mmol) was added and the mixture heated to 50 ¡C under
reÑux with stirring (2 h). The solvent was then removed under
reduced pressure and the resulting orange/brown foam was
dissolved in water (50 ml) and the pH adjusted to 1 (conc.
HCl). The aqueous layer was extracted with dichloromethane
(3 ] 50 ml) and the pH readjusted to 14 using solid NaOH.
The basiÐed aqueous layer was then exhaustively extracted
with chloroform, the organic extracts dried (K CO ) and Ðl-
ity is considerably reduced (b \ 1010, cf. 1025 for dota) since
ML
EDTA is able to extract the metal. These results were con-
Ðrmed with [Gd É 21]~ following a challenge with a ten-fold
excess of EDTA at pH 5.5 followed by ESMS mass spectro-
metric analysis. Only [Gd EDTA]~ was observed in the
negative-ion mass spectral analysis, after 30 min. In fact in the
positive-ion mode the appearance of the free ligand [21 É H]`
was evident. This clearly precludes the use of such complexes
for in vivo applications as diagnostic agents, but suggests that
they may Ðnd application in the transport of lanthanide ions,
given that complex lipophilicity may be readily tuned by
C-aryl substitution.
By considering the crystal structures of the lanthanide com-
plexes of dota,14 it is possible to hypothesise that the lability
of the complex may arise from unfavourable steric interactions
between the aryl group attached to the macrocyclic ring and
the adjacent methylene protons on the ring. These interactions
may prevent the complex from taking up the favourable
square antiprismatic geometry and will inhibit ring inversion.
In an attempt to verify this hypothesis, complexation reac-
tions were carried out under conditions of high temperature
and pressure (up to 180 ¡C in a sealed tube for 72 h). However,
no formation of the putative thermodynamically stable
complex was observed, and it is possible that the activation
barrier to formation of such a complex is too high to be
readily overcome.
2
3
tered. Removal of the solvent under reduced pressure gave a
yellow oil (612 mg, 90%). This was further puriÐed by Ðl-
tration through a plug of basic alumina (MeOHÈCH Cl ,
2
2
15 : 100). 1H NMR (CDCl , 400 MHz) d 7.3 (2H, d, 3J \ 7.2
3
Hz), 7.24 (2H, t, 3J \ 7.6 Hz), 7.17 (1H, t, 3J \ 7.2 Hz), 3.77
(1H, dd, 3J \ 10.4 Hz, 3J \ 2.8 Hz, H-2), 2.99 (1H, J \ 11.6
Hz, 3J \ 2.8 Hz), 2.94 (1H, dt, J \ 11.2 Hz, 3J \ 2.80 Hz),
2.80 (1H, br t, 3J \ 10.8 Hz, H-3), 2.80 (1H, t, J \ 10.8 Hz),
2.73 (2H, t, 3J \ 5.60 Hz), 2.65 (2H, t, 3J \ 6.4 Hz), 2.6 (2H, t,
2J \ 5.6 Hz), 2.44 (2H, t, 3J \ 5.6 Hz), 2.09 (1H, dt, J \ 11.2
Hz, 3J \ 3.3 Hz), 1.97 (1H, t, 3J \ 10.4 Hz, H-3@), 1.9 (4H, s,
NH). 13C NMR (CDCl ) : 142.32 (C-l@), 128.19 (C-3@), 127.28
3
(C-4@), 126.81 (C-2@), 61.37 (C-3), 60.15 (C-2), 57.99, 53.17, 52.30,
46.02, 45.99, 41.44. m/z (ESMS, ]) 249.14 (100%, [M ] H]`).
4-Nitrophenyl glyoxal, 8
To a solution of 1,4-dioxane (18 ml) and water (2.4 ml) was
added para-nitroacetophenone (5 g, 30.2 mmol) and selenium
dioxide (3.35 g, 30.2 mmol). The reaction mixture was heated
under reÑux for 5 h. The mixture was allowed to cool and left
at room temperature overnight. The resulting mixture was
redissolved in dioxane (20 cm3), Ðltered and the residue
washed with hot dioxane. Solvent was then removed under
reduced pressure to produce a viscous orange oil. The oil was
dissolved in the minimum volume of boiling water and left to
cool, yielding pale-yellow crystals. The supernatant was then
concentrated and the process repeated. This procedure was
repeated until no further crystals were obtained (3.63 g, 67%).
NMR analysis revealed a mixture of the pure compound and
Conclusion
Lanthanide complexes formed by C-aryl functionalised cyclen
derivatives are prevented from adopting a stable square
[3333] ring conformation by the large steric bulk of the aryl
group. Though the complexes are unsuitable for use as
imaging and contrast agents, they may have applications in
lanthanide extraction and transport, which could merit further
investigation.
its monohydrate. m/z 151.96 (31%, MH` [ CO ), 179.47
2
(2.5%, LH`). 1H NMR (CDCl ) d 9.67 (0.24H, s, CHO), 8.35
3
(5.24H, m, aromatic), 5.96 [1H, s, broad, C(OH) H], 4.0 [2H,
2
s, broad, C(OH) H]. 13C NMR (CD OD) : 195.53, 152.84,
150.09, 148.71, 140.72, 132.82, 131.42, 125.47, 124.50, 102.03,
93.53.
2
3
Experimental
Details of the experimental techniques used in this work,
together with the instrumentation employed for product char-
acterisation have been described previously.15,16
3-Nitrophenyl glyoxal, 9
Selenium(IV) oxide (3.35 g, 30.2 mmol) and 3-nitro-
acetophenone (3.71 g, 27.5 mmol) were dissolved in a 1,4-
dioxane(18 ml)Èwater(2.4 ml) mixture. The solution was
heated under reÑux for 18 h and then allowed to cool to room
temperature. The selenous salts were removed by Ðltration
through celite and the solvents removed under vacuum. The
brown residue was puriÐed by column chromatography (20%
tetrahydrofuran in dichloromethane). The solvents were
removed in vacuo and the orange residue distilled twice
(Kugelrohr) (120 ¡C, 0.5 mbar) to yield a bright yellow waxy
2-phenyl-1,4,7,10-tetraazacyclododecane, 4
A solution of triethylenetetraamine, 5 (400 mg, 2.73 mmol) in
methanol (20 ml) was added dropwise over 10 min to a solu-
tion of iron(III) chloride (739 mg, 2.73 mmol) in methanol (40
ml). The mixture was stirred at room temperature for 2 h. A
solution of phenyl glyoxal (366 mg, 2.73 mmol) in methanol
(45 ml) was then added to the reaction and the mixture stirred
New J. Chem., 1998, Pages 1359È1364
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solid (2.1 g, 39%); m/z (EI`) 179 (M`, 6%), 150
extracts were combined, dried (K CO ) and the dichloro-
2
3
([M [ HCO]`, 100%), 133 ([M [ NO ]`, 28%), 104 ([M
methane removed under reduced pressure to give a pale-
brown oil (560 mg). This was further puriÐed by reverse phase
HPLC using Hypersil ODS semi-preparative column, k \ 258
2
[ NO [ HCO]`, 52%). R \ 0.3 (5% tetrahydrofuran in
2
f
dichloromethane, SiO ). 1H NMR (CDCl , 250 MHz) d 7.75
2
3
(1H, t, 3J \ 8.0 Hz, H-5), 8.52 (2H, m, H-4 and 6), 9.00 (1H, t,
nm, t \ 0 min, A \ 90% H O (0.1% triÑuoroacetic acid),
2
4J \ 1.5 Hz, H-2), 9.64 (1H, s, CHO). 13C NMR (CDCl , 50
B \ 10% MeCN (0.1% triÑuoroacetic acid); t \ 20 min,
3
MHz) : 123.8 (d), 127.4 (d), 128.7 (s), 128.8 (d), 134.4 (d), 147.1
A \ 100% MeCN (0.1% triÑuoroacetic acid); t \ 25 min end;
t \ 5.81 min. 1H NMR (CDCl , 400 MHz) d 8.10 (2H, d,
(s, C-N), 183.8 (s, Ar-CxO), 197.2 (d, CHO); m
(Ðlm)/cm~1
max
R
3
3452 br (H O), 3089, 1705 (CxO), 1614, 1531 (NO ), 1480,
3J \ 8.8 Hz), 7.51 (2H, d, 3J \ 8.8 Hz), 3.92 (1H, dd, 3J \ 10.4
Hz, 3J \ 2.8 Hz, H-2), 3.04 (1H, tt, J \ 8.8 Hz, 3J \ 2.4 Hz),
2.95 (1H, dt, J \ 11.2 Hz, 3J \ 2.80 Hz), 2.81 (1H, t, J \ 10.8
Hz), 2.81 (1H, t, J \ 10.8 Hz, H-3), 2.75 (2H, t, 3J \ 5.60 Hz),
2.65 (2H, t, 3J \ 6.4 Hz), 2.63 (2H, t, 3J \ 6.0 Hz), 2.47 (2H, t,
3J \ 6.4 Hz), 2.11 (1H, dt, 2J \ 14.4 Hz, 3J \ 3.2 Hz), 1.95
(1H, br t, J \ 10.4 Hz, H-3@), 1.83 (4H, s, NH). 13C NMR
2
2
1439, 1351 (NO ), 1086, 1000, 910, 811, 728. Anal. calcd for
2
C H NO É 0.2H O: C, 52.6; H, 3.0, N, 7.7. Found: C, 52.5;
8
5
4
2
H, 3.4; N, 7.3%.
3-Cyanophenyl glyoxal, 10
Selenium(IV) oxide (3.35 g, 30.2 mmol) and 3-acetylbenzonitrile
(3.98 g, 27.5 mmol) were dissolved in a 1,4-dioxane(18 ml)È
water(2.4 ml) mixture. The solution was heated under reÑux
for 18 h and then allowed to cool to room temperature. The
selenous salts were removed by Ðltration through celite and
the solvents removed under vacuum. The brown residue was
eluted over silica gel with 5% tetrahydrofuran in dichloro-
methane. The solvents were removed in vacuo and the orange
residue distilled twice (Kugelrohr; 140 ¡C, 0.5 mbar) to yield a
(CDCl , 75 MHz): 149.94, 147.24, 127.84, 123.61, 61.26, 59.71
(CHN), 58.07, 53.01, 52.49, 46.11, 45.87, 41.61. m/z (ESMS, ]),
294.16 (100%, LH`).
3
2-(3’-Nitrophenyl)-1,4,7,10-tetraazacyclododecane, 13
This was prepared using a procedure analogous to that
reported for 12, using iron(III) chloride (760 mg, 2.8 mmol.),
triethylenetetraamine
(400
mg,
2.73
mmol.),
m-
nitrophenylglyoxal (490 mg, 2.73 mmol) and sodium boro-
hydride (800 mg, 21.8 mmol). The product was puriÐed by
chromatography on silica gel (83% CH Cl , 15% MeOH, 2%
bright-yellow waxy solid (3.32 g, 76%). mp, 65È73 ¡C. R \ 0.4
f
(SiO , 5% tetrahydrofuran in dichloromethane). 1H NMR
2
(CDCl , 300 MHz) d 7.66 (1H, t, 3J \ 7.8 Hz, H-5), 7.92 (1H,
2
2
3
NH OH) to yield a pale-yellow oil (180 mg, 26%); m/z (ES])
dt, 3J \ 7.8 Hz, 4J \ 1.5 Hz, H-4), 8.43 (1H, dt, 3J \ 7.8 Hz,
4
294.13 (MH`, 100%), 251, 246. 1H NMR (CDCl , 300 MHz)
4J \ 1.5 Hz, H-6), 8.47 (1H, t, 4J \ 1.5 Hz, H-2), 9.60 (1H, s,
3
d 8.28 (1H, s, H-2@), 8.10 (1H, d, J \ 7.5 Hz, H-4@), 7.71 (1H, d,
aldehyde). 13C NMR (CDCl , 75 MHz) : 113.8 (nitrile), 117.7
3
H-6@), 7.46 (1H, t, H-5@), 3.95 (1H, dd, J \ 11.5, 2.6 Hz, CHN),
(aromatic-CN), 130.3 (C-5), 131.8 (C-1), 134.4 (C-4), 134.5 (C-6),
3.12È2.71 (14H, m, ring CH N), 1.90 (4H, br s, NH). 13C
137.8 (C-2), 185.5 (CO), 188.8 (CHO). m
3078, 2979, 2936, 2234 (CN), 1700 (br, CxO), 1599, 1579,
1479, 1421, 1291, 1269, 1102, 1047, 984, 912, 786, 732, 678. m/z
(thin Ðlm)/cm~1
2
max
NMR (CDCl , 75 MHz) 148.9, 145.4, 133.9, 129.9, 123.1,
3
122.7; 62.0 (CH N); 60.1 (CHN); 58.8, 53.8, 53.3, 46.7, 46.6,
2
42.4 (ring CH N).
2
(CI`) \ 336 (100%, 2M ] NH `). Anal. calcd for
4
C H NO É 0.2H O: C, 66.7; H, 3.3; N, 8.6. Found: C, 66.7;
H, 3.4; N, 8.6%.
2-(3’-cyanophenyl)-1,4,7,10-tetraazacyclododecane, 14
9
5
2
2
Iron(III) chloride hexahydrate (0.74 g, 2.7 mmol) was dissolved
in methanol (20 ml). Under argon
a solution of tri-
N,N-Dibenzyl-p-aminophenyl glyoxal, 11
ethylenetetraamine (0.40 g, 2.7 mmol) in methanol (10 ml) was
added dropwise over 30 min, resulting in an orange-brown
precipitate. meta-Cyanophenyl glyoxal, 10, (0.40 g, 2.7 mmol)
was dissolved in methanol (20 ml) and added to the tri-
ethylenetetraamine iron(III) complex. The reaction was left to
Selenium(IV) oxide (27 g, 0.063 mol) and N,N-dibenzyl-p-
aminoacetophenone (20 g, 0.063 mol) were dissolved in a 1,4-
dioxane(130 ml)Èwater(7 ml) mixture. The solution was heated
under reÑux for 18 h and then allowed to cool to room tem-
perature. The selenous salts were removed by Ðltration
through celite and the solvents removed under reduced pres-
stir at room temperature for 4 h under argon before NaBH
(3 ] 0.4 g pellets, 32 mmol) was added over 30 min. The reac-
4
sure. The residue was puriÐed by Ðltration through SiO with
tion was stirred for a further 2 h after which 2M HCl (ca. 10
ml) was added until a clear-yellow solution was obtained. The
volatiles were removed under vacuum and the remaining solu-
tion was washed with dichloromethane (50 ml). The pH was
then raised to 14 by addition of NaOH pellets and extracted
2
CH Cl Èheptane (70 : 30) and Ðnally CH Cl , in order to
2
2
2 2
obtain the unsolvated 1,2-dicarbonyl as a yellow-orange oil
(12.5 g, 60%). 1H NMR (CDCl , 200 MHz) d 9.63 (1H, s,
3
CHO), 6.77È7.98 (14H, m, ArH), 4.73 (4H, s, CH ). 13C NMR
2
(CDCl , 75 MHz): 190.2 (CO), 190.0 (CHO), 153.7 (CwN);
3
with CHCl (3 ] 200 ml). The organics were combined, dried
3
136.7, 133.4, 131.6, 129.6, 129.0, 128.1, 120.8, 112.0, 53.9
over K CO and the solvents removed. The residue was puri-
2
3
(CH N).
Ðed by column chromatography over silica gel (10% meth-
anol, 10% ammonia, 80% tetrahydrofuran) to give
2
a
2-(4’-Nitrophenyl)-1,4,7,10-tetraazacyclododecane, 12
pale-brown oil, (0.4 g, 54%). 1H NMR (CDCl , 250 MHz) d
3
2.0È3.2 (14H, br m, CH N), 2.37 (4H, br s, NH), 3.90 (1H, dd,
2
To a stirring solution of iron(III) chloride (739 mg, 2.73 mmol)
in methanol (40 ml) was added dropwise over 10 min a solu-
tion of triethylenetetramine (400 mg, 2.73 mmol) in methanol
(20 ml) which was left to stir at room temperature for 2 h. To
the reaction mixture was added a solution of para-nitrophenyl
glyoxal (489 mg, 2.73 mmol) in methanol (45 ml) and the
mixture was left for a further 4 h. Sodium borohydride (10
equiv. 1000 mg, 27.3 mmol) was added and the mixture heated
under reÑux (16 h). The methanol was removed under reduced
pressure, the resulting brown foam was dissolved in water (50
ml) and the pH adjusted to 1 (conc. HCl). The aqueous layer
was extracted with dichloromethane (3 ] 50 ml), and the pH
was readjusted to pH 14 with solid NaOH. The aqueous layer
was extracted exhaustively with dichloromethane, the organic
CHN), 7.45 (1H, t, 3J \ 7.8 Hz, H-5@), 7.56 (1H, d, 3J \ 7.8
Hz, H-6@) 7.64 (1H, d, 3J \ 7.8 Hz, H-4@), 7.73 (1H, s, H-2@).
13C NMR (CDCl , 50 MHz) : 41.5 (CH N), 45.8 (CH N), 46.0
3
2
2
(CH N), 52.4 (CH N), 53.0 (CH N), 58.0 (CH N), 59.4 (CHN),
2
2
2
2
61.2 (CH N), 112.3 (CN), 118.7 (C-1@), 129.1 (C-6@), 130.7 (C-5@),
2
131.1 (C-2@), 131.6 ( C-4@), 143.9 (C-3@). m
(Ðlm)/cmv1; 3540
max
(NH), 3018, 2844 (CH), 2236 (CN), 1434, 1202, 1136, 904, 837,
806, 718, 694. m/z (ES]): 274 (100% , M ] H`).
2-(N,N-dibenzyl-p-aminophenyl)-1,4,7,10-tetraazacyclodo-
decane, 15
Iron(III) chloride hexahydrate (1.35 g, 4.99 mmol) was dis-
solved in MeOH (80 ml). A solution of triethylenetetraamine
1362
New J. Chem., 1998, Pages 1359È1364

View Article Online
(0.73 g, 5 mmol) in MeOH (40 ml) was added dropwise. The
mixture was stirred for 2 h and then a solution of N,N-
dibenzyl-p-aminophenyl glyoxal (1.65 g, 3 mmol) in THF(35
ml)/MeOH(55 ml) was added dropwise. The reaction was left
MHz) d 2.45È3.33 (16H, m, ring NCH and CH CO), 3.43 (4H,
2
s, CH CO), 3.65 (2H, s, CH CO), 3.83 (1H, dd overlapping,
2
2
3J \ 8.0 Hz, 3J \ 7.5 Hz, CHAr), 6.96 (1H, t, 3J \ 7.8 Hz,
H-5@), 7.08 (1H, br d, 3J \ 7.8 Hz, H-6@), 7.22 (1H, d, 3J \ 7.8
to stir at room temperature for 6 h under Ar before NaBH
(1.9 g, 50 mmol) was added. The mixture was heated under
Hz, H-4@), 7.31 (1H, s, H-2@); m
(Nujol mull)/cm~1 2975,
4
max
2820, 1715 (CO H), 1646 (CO ~), 1304, 1202, 978, 966, 918.
2
2
reÑux for 16 h and then allowed to cool to room temperature.
The salts were removed by Ðltration through charcoal. The
volatiles were removed under reduced pressure and the
residue was dissolved in water at pH 10. This solution was
extracted with CH Cl (3 ] 100 ml), the organic extracts were
m/z (ESMS) 481 ([M ] H]`), 504 ([M ] Na]`).
N,Nº,N/,NÓ-Tetrakis(tert-butoxycarbonylmethyl)-2-(N,N-di-
benzyl-p-aminophenyl)-1,4,7,10-tetraazacyclododecane, 19
2
2
To an emulsion of 20 g (34 mmol) 2-(N,N-dibenzyl-p-amino-
phenyl)-1,4,7,10-tetraazacyclododecane in water (110 ml) and
dichloromethane (100 ml), at 10 ¡C, an aqueous solution of
NaOH (90.5 ml, 3 mol dmh3) was added slowly. The mixture
was stirred at 15 ¡C for 15 min before a solution of tert-butyl
bromoacetate (27.14 g, 0.139 mol) in 100 ml CH Cl was
combined, dried over Na SO and the solvent removed. The
2
4
residue was dissolved in water and acidiÐed with HCl (6 M)
and then washed with CH Cl (3 ] 50 ml). Water was
2
2
removed under reduced pressure and the remaining solution
was added dropwise to 300 ml of EtOH to yield pale-brown
crystals of the tetrahydrochloride salt which were Ðltered o†
and dried in vacuo (0.9 g, 31%). m/z (ESMS, ]) 444
2
2
added dropwise. The reaction was stirred for a further 4 h at
the same temperature and then at room temperature for a
further hour. After decanting, the organic layer was washed
with water (2 ] 90 ml) and dried over Na SO and the
([M ] H`]`, 100%). 1H NMR (CDCl , 300 MHz) d 6.83
3
(14H, m), 4.61 (4H, s, CH Ph), 3.85 (1H, dd, H-2), 3.11 (14H,
2
br m, CH N), 2.4 (4H, br s, NH). t [C -5l-“LichrosphereÏ
2
4
2
R
18
solvent removed under reduced pressure. The residue was
puriÐed by column chromatography over silica gel
(CH Cl ÈMeOH, 96 : 4). A second puriÐcation step was neces-
(Merck); KH PO (10 mM)È(CH CN) 50 : 50; pH 2.7, 1 ml
2
4
3
min~1] 3.4 min.
2
2
sary involving column chromatography over silica gel
(AcOEtÈMeOH, 95 : 5) after which an orange oil was
obtained (11 g, 36%). m/z (ES]) 450.5 ([M ] 2H]2`, 100%);
N,Nº,N/,NÓ-Tetrakis(tert-butoxycarbonylmethyl)-2-phenyl-
1,4,7,10-tetraazacyclododecane, 17
900 ([M ] H]`, 18%). Anal. calcd for C
H
N O É H O: C,
2-Phenyl cyclen, 4, (0.37 g, 1.5 mmol) and caesium carbonate
(1.74 g, 5 mmol) were dissolved in acetonitrile (5 ml) under
argon. tert-Butyl bromoacetate (0.88 g, 4.5 mmol) was dis-
solved in acetonitrile (10 ml) under argon and added to the
solution over 10 min by cannula transfer at room tem-
perature. The reaction mixture was left stirred for 3 h and was
monitored by ESMS analysis, showing the extent of tetraester
formation and the onset of pentaalkylation. The solvents were
then removed under reduced pressure, the residue taken up
into dichloromethane (25 ml) and the inorganics removed by
Ðltration. The solvents were removed under reduced pressure
and the residue puriÐed by column chromatography over
silica gel. The column was washed with dichloromethane
before the title compound was eluted with 20% tetra-
hydrofuran in dichloromethane. Removal of the solvents
under reduced pressure yielded a colourless oil (0.26 g, 25%).
R \ 0.6 (SiO , 15% tetrahydrofuran in dichloromethane). 1H
52 77
5
8
2
68.2; H, 8.6; N, 7.6. Found: C, 68.1; H, 8.6; N, 7.6%. t [5l-
R
diol (Merck), 98 : 2,heptaneÈethanol; 1 ml min~1] 7.3 min.
N,Nº,N/,NÓ-Tetrakis(carboxymethyl)-2-(N,N-dibenzyl-p-
aminophenyl)-1,4,7,10-tetraazacyclododecane, 20
A solution of the tetra-tert-butyl ester, 19, (11 g, 0.012 mol) in
CF COOH (110 ml) was stirred overnight under Ar at room
3
temperature. The solvent was then removed and the residue
dissolved in water in order to be puriÐed by Ðltration on
reverse phase silica (R.P. 2, Merck) eluting with water(40%)È
MeOH(60%). The solvents were removed under reduced pres-
sure and acetone was added to the remaining oil. White
crystals were obtained after Ðltration (6.5 g, 79.3%). m/z
(ES]), 676 ([M ] H]` 100%); t [C -5 l, Lichrosphere
R
18
(Merck); KH PO (10 mM)ÈCH CN, 60 : 40; pH 3.1; Ñow 1
ml min~1] 5.1 min.
2
4
3
f
2
NMR (CDCl , 200 MHz) d 2.44 (36H, s, But), 2.0È3.25 [16, m,
3
CH N (ring) ] CH N], 3.40 (2H, s, CH NCO), 3.53 (4H, s,
Tetrakis(carboxymethyl)-2-(p-aminophenyl)-1,4,7,10-tetraaza-
cyclododecane, 21
2
2
2
2
CH NCO), 3.70 (1H, dd, 3J \ 7.5 Hz, 3J \ 2.5 Hz, CHN),
7.1È7.3 (5H, m, arom. CH). 13C NMR (CDCl , 50 MHz) : 26.4
3
(CH ), 50.4 (ring CH N), 50.8 (CH N), 51.7 (CH N), 51.9
To a solution of the tetraacid (2.5 g, 3.7 mmol) in MeOH (105
ml) and water (45 ml), 0.5 g of Pd/C (10%) was added and the
mixture maintained at room temperature under a pressure of
hydrogen of 22 psi,” with vigorous stirring for 12 h. The cata-
lyst was removed by Ðltration through charcoal and the sol-
vents were removed under reduced pressure. A white solid was
obtained (1.8 g, 100%).
3
2
2
2
2
2
(CH N), 54.6 (CH N), 55.3 (CH N), 55.6 (CH N), 60.3
2
2
(NCH CO ] CHN), 63.5 (NCH CO), 79.0 (s, CwMe), 79.2 (s,
2
2
CwMe), 79.4 (s, CwMe), 126.9 (C-2@), 127.3 (C-4@), 128.6 (C-3@),
(thin
142.3 (s, C-13@), 168.4 (CO), 169.6 (CO), 169.8 (CO); m
max
Ðlm)/cm~1 2970, 2814, 1733 (CO But), 1475, 1455, 1390, 1367,
2
1152, 910. m/z (ES]) 704 (100%, M ] H`), 727 (95%,
M ] Na`). Anal. calcd for C
H
N O É 2H O: C, 63.2; H,
39 63
5
8
2
9.14, N, 7.76. Found: C, 62.8; H, 9.38; N, 7.51%.
[Gd.21]~
To a stirred solution of the aminoacid (1.7 g, 3.43 mmol) in
N,Nº,N/,NÓ-Tetrakis(carboxymethyl)-2-phenyl-1,4,7,10, tetra-
water (30 ml) was added GdCl É 6H O (0.8 g, 3.43 mmol). The
3
2
azacyclododecane, 18
pH was raised with NaOH (5 M) and adjusted to 6. The
mixture was heated at 60 ¡C for 5 h and left at room tem-
perature overnight. The pH was readjusted to 6 with NaOH
(1 M). To this solution 9 ml of the ion exchange resin
ÏCHELEXÏ was added. One hour later, the pH was raised to 7.
The mixture was concentrated under vacuum and the remain-
ing solution added dropwise to MeOH (100 ml). The product
was Ðltered and washed with MeOH (3 ] 20 ml). This pro-
The tetra-tert-butyl ester 17 (141 mg, 0.2 mmol) was dissolved
in 50% triÑuoroacetic acid in dichloromethane (3 ml) and the
solution was stirred for 24 h. The solvents were then removed
under vacuum and dichloromethane (3 ] 5 ml) added and
then removed under vacuum. Methanol (10 ml) was added
and removed under vacuum before the oily residue was trit-
urated twice from diethyl ether. After decanting the solvents
and drying under vacuum a colourless solid was obtained in
quantitative yield, mp 133È135 ¡C. 1H NMR (CD OD, 300
” 1 psi B 6.89 ] 103 Pa.
3
New J. Chem., 1998, Pages 1359È1364
1363

View Article Online
cedure was repeated twice. White crystals were obtained (1.3
g, 59%). m/z (ES[) 829 ([M [ H]~, 100%). [C
vents and drying under vacuum a colourless solid was
t
-
obtained in quantitative yield, mp 119È122 ¡C. 1H NMR
R
18
Lichrosphere, 5l (Merck); KH PO (10 mM)ÈCH CN ,
(CD OD, 300 MHz) 2.50È3.37 (16H, m, cyclen ring and
2
4
3
3
70 : 30; pH 2.8; Ñow 1 ml min~1] 27.8 min.
1 ] acetate), 3.43 (4H, s, 2 ] acetates), 3.65 (2H, s,
1 ] acetate), 3.89 (1H, dd overlapping, 3J \ 8.2 Hz, 3J \ 7.9
Hz, CHAr), 6.96 (1H, t, 3J \ 7.8 Hz, H-5@), 7.08 (1H, br d,
3J \ 7.8 Hz, H-6@), 7.22 (1H, d, 3J \ 7.8 Hz, H-4@), 7.31 (1H, s,
N,Nº,N/,NÓ-Tetrakis(tert-butoxycarbonylmethyl)-2-(3º-cyano-
phenyl)-1,4,7,10-tetraazacyclododecane, 22
H-2@). m
(Nujol mull)/cm~1 2234 (CN), 1718 (CO H), 1650
max
2
meta-Cyanophenyl cyclen, 14, (0.37 g, 1.3 mmol) and caesium
carbonate (1.74 g, 5 mmol) were dissolved in acetonitrile (5 ml)
under argon. tert-Butyl bromoacetate (0.78 g, 4 mmol) was
dissolved in acetonitrile (10 ml) under argon and added to the
solution over 10 min by cannula transfer at room tem-
perature. The reaction was stirred for 4 h and followed by
ESMS analysis. The solvents were then removed in vacuo and
the residue taken up into dichloromethane (20 ml), and the
inorganics were removed by Ðltration. The solvents were
removed under vacuum and the residue puriÐed by column
chromatography over silica gel. The column was washed with
dichloromethane before the title compound was eluted with
20% tetrahydrofuran in dichloromethane. Removal of the sol-
vents under reduced pressure yielded a colourless oil (0.21,
(CO ~), 1304, 1202, 978, 966, 918, 834, 800, 720; m/z (ESMS)
2
506 ([M ] H]`).
References
1 D. Parker and J. A. G. Williams, J. Chem. Soc., Dalton T rans.,
1996, 3613.
2 A. Beeby, D. Parker and J. A. G. Williams, J. Chem. Soc., Perkin
T rans. 2, 1996, 1565; D. Parker and J. A. G. Williams, J. Chem.
Soc., Perkin T rans. 2, 1995, 1305.
3 D. Parker, in Comprehensive Supramolecular Chemistry, eds. J.-M.
Lehn, D. D. Macnicol, J. L. Atwood, J. E. Davies, D. N. Rein-
houdt and F. Vogtle, Pergamon, Oxford, 1996, vol. 10, ch. 17.
4 S. Aime, M. Botta, E. Terreno, Chem. Soc. Rev., 1998, 27, 19.
5 D. Parker, Chem. Soc. Rev., 1990, 19, 271.
6 J. F. Desreux, Inorg. Chem., 1980, 19, 1319; S. Aime, M. Botta and
G. Ermondi, Inorg. Chem., 1992, 31, 4291.
7 C. J. Broan, K. J. Jankowski, R. Kataky and D. Parker, J. Chem.
Soc., Chem. Commun., 1990, 1739.
22%).
R \ 0.7
(SiO ,
20%
tetrahydrofuran
in
f
2
dichloromethane); 1H NMR (CDCl , 200 MHz) d 2.47 (36H,
3
s, But), 2.0È3.2 [16, m, CH N (ring) ] CH N], 3.42 (2H, s,
2
2
CH NCO), 3.51 (4H, s, CH NCO), 3.74 (1H, dd, 3J \ 7.5 Hz,
8 S. Aime, A. S. Batsanov, M. Botta, J. A. K. Howard, D. Parker, K.
Senanayake and J. A. G. Williams, Inorg. Chem., 1994, 33, 4696.
9 J. P. L. Cox, A. S. Craig, I. M. Helps, K. J. Jankowski, D. Parker,
M. A. W. Eaton, A. T. Millican, K. Millar, N. R. A. Beeley and B.
A. Boyce, J. Chem. Soc., Perkin T rans. 1, 1990, 2567; O. Renn and
C. F. Meares, Bioconjugate Chem., 1993, 3, 563; T. J. McMurry, M.
W. Brechbiel, K. Kumar and O. A. Gansow, Bioconjugate Chem.,
1992, 2, 108; M. L. Garrity, G. M. Brown, J. E. Elbert and R. A.
Sachleben, T etrahedron L ett., 1993, 34, 5531.
2
2
3J \ 2.5 Hz, CHN), 7.48 (1H, t, 3J \ 8.0 Hz, H-5@), 7.63 (1H,
d, 3J \ 8.0 Hz, H-6@), 7.70 (1H, d, 3J \ 8.0 Hz, H-2@), 7.77 (1H,
s, H-2@). 13C NMR (CDCl , 50 MHz) : 26.7 (CH ), 50.1 (ring
3
3
CH N), 50.7 (CH N), 51.7 (CH N), 51.9 (CH N), 54.8 (CH N),
2
2
2
2
2
55.1 (CH N), 55.3 (CH N), 60.5 (NCH CO ] CHN), 63.1
2
2
2
(NCH CO), 79.3 (s, CwMe), 79.4 (s, CwMe), 79.5 (s, CwMe),
2
111.2 (CN), 117.3 (C-1@), 127.9 (C-6@), 129.4 (C-6@), 130.3 (C-5@),
10 C. D. Edlin, S. Faulkner, D. Parker and M. P. Wilkinson, Chem.
Commun., 1996, 1249.
131.2 (C-4@), 141.3 (s, C-3@), 168.1 (CO), 169.2 (CO), 169.4 (CO).
m
(thin Ðlm)/cm~1 2977, 2817, 2229 (CN), 1731 (CO But),
11 G. H. Searle, M. Petkovic and F. R. Keene, Inorg. Chem., 1974, 13,
max
2
1479, 1455, 1392, 1367, 1156, 916, 851, 801, 735, 697. m/z
(ES]) 730 (100%, M ] H`), 752 (95%, M ] Na`); Anal.
calcd for C
399.
12 (a) W. L. Evans and E. J. Witzemann, J. Am. Chem. Soc., 1911, 33,
1772; (b) M. W. Goldberg and A. I. Rachlin, Ho†mann-La Roche,
US Patent 2641601, 1953.
13 W. D. Horrocks and D. R. Sudnick, Acc. Chem. Res., 1981, 14, 384.
14 R. S. Dickins, D. Parker, A. S. de Sousa and J. A. G. Williams,
Chem. Commun., 1996, 697.
15 S. Aime, A. S. Batsanov, M. Botta, R. S. Dickins, S. Faulkner, C.
E. Foster, A. Harrison, J. A. K. Howard, J. M. Moloney, T. J.
Norman, D. Parker, L. Royle and J. A. G. Williams, J. Chem. Soc.,
Dalton T rans., 1997, 3623.
16 T. J. Norman, D. Parker, K. Pulukkody, L. Royle and C. J. Broan,
J. Chem. Soc., Perkin T rans. 2, 1993, 605.
H
N O É 2H O: C, 61.1; H, 8.7, N, 9.15.
39 63
5
8
2
Found: C, 60.6; H, 8.30; N, 9.00%.
N,Nº,N/,NÓ-Tetrakis(carboxymethyl)-2-(3º-cyanophenyl)-
1,4,7,10-tetraazacyclododecane, 23
The tetra-tert-butyl ester 22 (112 mg, 0.15 mmol) was dis-
solved in 50% triÑuoroacetic acid in dichloromethane (3 ml)
and the solution was stirred for 24 h. The solvents were then
removed under vacuum and dichloromethane (2 ] 5 ml)
added and then removed under vacuum. Methanol (5 ml) was
added and removed under vacuum before the oily residue was
triturated twice from diethyl ether. After decanting the sol-
Received in Cambridge, UK, 2nd July 1998;
Paper 8/05231J
1364
New J. Chem., 1998, Pages 1359È1364