Design and synthesis of a novel family of semi-rigid ligands: Versatile compounds for the preparation of 99mTc radiopharmaceuticals

Article abstract of DOI:10.1039/b313996d

Synthetic pathways to a range of novel semi-rigid tetradentate ligands, with sulfur, amido or amino donor groups, designed to coordinate technetium 99m have been developed. The technetium-99m complexes have been prepared and their stabilities in serum and

Full text of DOI:10.1039/b313996d

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Design and synthesis of a novel family of semi-rigid ligands:
versatile compounds for the preparation of 99mTc
radiopharmaceuticals
Julien Le Gal,a Laure Latapie,a Marie Gressier,a Yvon Coulais,b Michèle Dartiguenavea and
Eric Benoist*a
a Laboratoire de Chimie Inorganique, EA 807, Université Paul Sabatier, Bat. IIR1, 118,
route de Narbonne, 31062 Toulouse, France
b Laboratoire “Traceurs et traitement de l’image”, Faculté de Médecine de Purpan, EA 3033,
133, route de Narbonne, 31062 Toulouse, France
Received 4th November 2003, Accepted 22nd January 2004
First published as an Advance Article on the web 18th February 2004
Synthetic pathways to a range of novel semi-rigid tetradentate ligands, with sulfur, amido or amino donor groups,
designed to coordinate technetium 99m have been developed. The technetium-99m complexes have been prepared
and their stabilities in serum and by metathesis reaction via cysteine exchange reactions were compared. HPLC
comparison of two 99mTc-complexes and their rhenium analogues led to the first proof of the nature of our
radioactive complexes.
Introduction
Technetium chemistry attracts considerable research attention
owing to the prominent use of the metastable 99mTc isomer (half
life of 6.02 h, γ emitter with energy 140 KeV and convenient
availability from the 99Mo/99mTc generator) in diagnostic
nuclear medicine.1 The efforts made in this field in recent years
have led to the synthesis of new and more efficient radio-
pharmaceuticals.2–4 Several types of ligand frameworks have
been developed. Among them, N2S2 or N3S tetradentate ligands
are known to form stable technetium() complexes, exemplified
by compounds (1–4) shown in Fig. 1.5–8
Fig. 2 Design features of novel semi-rigid 99mTc-ligand.
enhances the stability of the technetium complex compared to
the compound bearing a methoxy moiety. To rationalize these
first results, we decided to introduce at the 2-position a non-,
partially- or fully-substituted amino group in order to check the
influence of the aromatic group substitution on the stability of
their corresponding 99mTc complexes and to determine the best
candidates for a radiopharmaceutical use. In this paper, we
describe the synthesis and characterisation of a range of new
unsymmetrical tetradentate ligands, the 99mTc-labelling pro-
cedure and the stability of the different radioactive complexes.
HPLC comparison of two 99mTc complexes and their rhenium
analogues led to the first proof of the nature of our radioactive
complexes.
Our research has focused on the development of a novel
family of 99mTc-specific ligands whose design features, illus-
trated in Fig. 2, may be summarised as follows: (i) an aromatic
ring to enhance the sp2 character of the nitrogen atom bonded
to the metal and promote rigidity of the square pyramidal base,
to favour and stabilise the chelate ring by an entropic effect
(even if in some cases the stiffening of the ligand should give an
unfavourable energetic contribution for the process of complex
formation); (ii) amide and thiol functions for 99mTc complex-
ation; (iii) ortho-N,O,S-substitution to compare the stability of
the corresponding technetium complexes. Moreover, another
advantage of including an aromatic cycle in the chelate ring
compared to the semi-rigid cyclohexyl moiety like in CDTA
derivatives9 is its ease of functionalisation in order to prepare
bifunctional chelating agents.
Results and discussion
If it has been shown that the use of semi-rigid tetradentate
ligands improved the stability of the resulting complexes,11 only
In a previous communication,10 we noticed that the presence
of an hydroxyl group at the 2-position of the ring greatly
Fig. 1 Representative tetradentate N2S2– or N3S–technetium() oxo complexes.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 8 7 6 – 8 8 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
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Scheme 1 ortho Substituted aniline. Reagents and conditions: (i) TrtOH, TFA, CH2Cl2, r.t., 1 h, 86%; (ii) BOC-ON, DMF, 55 ЊC, 16 h, 66%; (iii) 37%
aq. CH2O, B10H14, MeOH, r.t., 4 h, 70%; (iv) CF3COOH, CH2Cl2, r.t., 1 h, 68%.
a few examples of such systems have been described in the
design of Tc() and Re() ligands. To our knowledge, only N2S2
ligands bearing an aromatic ring in a central position have been
developed.12,13 Our synthetic strategy offers a simple method for
including an aromatic cycle in the chelate ring and generating a
range of new unsymmetrical N2S2, N2SO and N3S ligands.
For N3S or N2SO ligands, a three-step route was investigated
(Scheme 2). The first step involved a conventional carbodiimide
amide coupling of N-carbobenzyloxyglycine and ortho non-
sulfur substituted aniline in THF, followed by a classical
catalytic hydrogenation using 10% palladium on charcoal as
catalyst.19 The intermediates 10a–e were obtained in excellent
yields (70–94% for the two steps). The synthetic key step was
the acylation reaction of 10a–e with the activated ester 11, this
latter compound being first prepared in two steps (protection of
the thiol functionality with a trityl group followed by activation
of the carboxylic acid with N-hydroxysuccinimide (NHS) in the
presence of EDCI).20
While acylation of 10a–d with 11 afforded the corresponding
tetradentate ligands 12a–d in 55–88% yield, a mixture of several
compounds was obtained for 12e using the same reaction
conditions. To circumvent this problem, we adopted a different
synthetic route, carrying out the reaction at room temperature
instead of 60 ЊC and in the presence of Et3N instead of DMAP.
The ligand 12e was obtained in 76% yield. In these conditions,
no condensation between 11 and the aromatic amino group of
10e was observed.
The preparation of the N2S2 ligands followed a four-step
procedure as shown in Scheme 3. Peptidic coupling of 11 with
ethyl glycinate in DMF, in the presence of Et3N, gave 13 with an
excellent yield (98%). Basic hydrolysis of the ethyl ester of 13
followed by the activation of the acid function of 14 with NHS–
DCC afforded the corresponding N-hydroxysuccinimidyl ester
15 (83% for the two steps). The final step involved an amide
coupling reaction. The compound 15 reacted with the com-
mercial o-thiomethoxyaniline or the compound 5 in acetonitrile
to give respectively the ligands 12f (60%) or 12g (50%).
All the purified ligands were characterised by MS, elemental
analysis, proton and carbon NMR spectroscopies with assign-
ments based on 2-dimensional spectra. This kind of ligand
exhibits enhanced deshielding for the proton ortho to the acyl-
amine group called the “ortho effect”.21 ortho-Substituents
Synthesis and characterisation of ligands
The ligands reported herein have all been synthesised in the
thiol-protected form. Triphenylmethyl (trityl) groups were pre-
ferred over other sulfur-protected groups (such as benzoyl,
ethylethoxyethyl. . .) as they combine a good stability during the
ligand synthesis with ease of removal during the 99mTc radio-
labelling reaction.
Our different synthetic pathways need commercial or syn-
thesised aromatic diamines as starting materials. To avoid oxid-
ation problems, the thiol function of the 2-aminothiophenol
was protected by a trityl group according to the conditions
described by Noveron14 to give 5 in excellent yield. The prepar-
ation of 8 follows a three-step procedure as shown in Scheme 1.
The monoprotection of the phenylenediamine, by BOC-ON
in DMF, gave 6 with 66% yield.15 The crucial step was the
dimethylation of the aromatic nitrogen of 6. Reductive methyl-
ation is known as one of the powerful methods for the methyl-
ation of amine. We tried different systems like sodium boro-
hydride–trifluoroacetic acid16 or sodium borohydride–zinc
chloride–paraformaldehyde17 but with limited success. The
best results were obtained by reductive methylation using 37%
aqueous formaldehyde and decaborane in methanol.18 The
corresponding tertiary amine 7 was synthesised in 70% yield.
Deprotection of the t-butyl carbamate by acid hydrolysis
yielded the amino precursor 8 in modest yield (after conversion
of the amino salt into the free amine).
The ligands were prepared according to two different path-
ways, depending on the ortho-substitution of the aromatic ring.
Scheme 2 Synthetic route for ligands 12a–e. Reagents and conditions: (i) CbZ-glycine, DCC, 4 h, r.t., THF (71–97%); (ii) H2, 10% Pd/C, 2 h, r.t.,
EtOH (81–100%); (iii) (a) TrtOH, AcOH, 2 h, r.t., CH2Cl2, 87% (b) EDCI, NHS, 4 h, r.t., ACN, 85%; (iv) 11, DMAP or Et3N, 3 h, 60 ЊC or r.t., ACN
(55–88%).
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Table 1 Retention time of 99mTc-complexes
Complex
99mTc-12a
99mTc-12b
99mTc-12c
99mTc-12d
99mTc-12e
99mTc-12f
99mTc-12g
13.60
tr (min)a
5.10
5.60
4.98
—
14.10
4.10
6.80
7.19
a MeOH–H2O–TFA 50 : 50 : 0.1.
Scheme 3 Synthesis of ligands 12f,g. Reagents and conditions: (i) HClؒNH2CH2COOEt, Et3N, 3 h, r.t., DMF, 98%; (ii) NaOH, 2 h, 40 ЊC, MeOH,
100%; (iii) NHS–DCC, 4 h, r.t., THF, 83%; (iv) 5 or o-C6H4(SMe)NH2, DMAP, 6 h, 60 ЊC, ACN (50–60%).
capable of hydrogen bonding with the amide hydrogen are
expected to stabilise planar conformations in which anisotropic
deshielding of the remaining ortho hydrogen is maximised and
in which the nitrogen, carbonyl carbon, oxygen, ortho proton,
and ring carbon are locked into a six -membered ring through
incipient H-bonding as illustrated in Fig. 3. The Hα shift
values are consistent with the values obtained for similar
compounds.22
After 12 hours (two half-lives of technetium 99m), more than
50% of dissociated 99mTc was observed for 99mTc-12a,f and 40%
for 99mTc-12g while 99mTc-12e exhibited a good stability (less
than 15% of 99mTc dissociated) and 99mTc-12b,d a great stability
(less than 5% of 99mTc dissociated). The excellent behaviour for
the two latter compounds was confirmed by a serum stability
study. After 12 h of incubation in fresh human serum, only
minor decomposition of the complexes or back oxidation of
the metal center to 99mTcO4Ϫ was observed (< 5%). For an even-
tual application in diagnostic nuclear medicine, this stability
would be sufficient.
Rhenium chemistry
In order to determine the structure of our 99mTc-complexes, we
synthesised the oxorhenium complexes of 12b and 12g using a
ligand exchange reaction with ReO(PPh3)2Cl3 24 in the presence
of sodium acetate (Scheme 4).
Fig. 3 Hydrogen bonding stabilisation of planar conformation.
99mTc radiolabelling
Trityl deprotection and 99mTc-labelling of the pure ligands were
performed in situ in a methanol–buffer solution pH 8.6 (1 : 4
v/v) by direct reduction of sodium pertechnetate in the presence
of an excess of tin chloride at 80 ЊC during 30 minutes.
The resultant rhenium complexes ReO-12b and ReO-12g
were obtained by silica gel chromatography as the sodium
salt respectively in 97% and 83% yield. In the free ligand, the
methylene group next to the sulfur appears as a unique
singlet (3.21 ppm for 12b and 3.10 for 12g). After complex-
ation with ReO(PPh3)2Cl3, the singlet splits into two doublets
forming the pattern of two AB-spin systems at 3.81 and 4.15
ppm (J = 17.2 Hz) for ReO-12b and 3.71 and 3.90 ppm
(J = 17.4 Hz) for ReO-12g. The two protons next to the
chelator amides are also non-equivalent. Negative-ion electro-
spray of each complex presents two prominent ion peaks with
an isotope distribution pattern consistent with the mono-
meric anion. The complexes were sodium salts as depicted by
elemental analysis. The IR spectra showed an absorption at
968 or 964 cmϪ1 indicating the presence of a rhenium–oxo
bond.
For the tetraanionic ligands 12b,d,e,g, under these condi-
tions, the labelling reaction was quantitative and only neg-
atively charged 99mTc-product was observed. The two analogous
complexes 99mTc-12b and 99mTc-12g were obtained with a large
difference in the retention time (Table 1). In a previous com-
munication,10 we suggest this difference results in the formation
of an L2M species for 99mTc-12g. As reported by Archer et al.,23
it may be that the thiolate sulfurs are slightly better donors than
the nitrogen for technetium in this complex. So the 2 : 1 (ligand :
metal) complex may form with each ligand bound only through
the sulfur atom when the ligand is present in large excess. The
ligands 12a,12f did not lead to a single complex as published
before4 but to a mixture of two complexes of technetium in an
approximate 2 : 1 ratio. These two products seemed to be the
anti and syn isomers with respect to the technetium oxo and the
methyl of the methoxy group. Attempts to prepare a 99mTc
complex with 12c remained unsuccessful; the latter reaction
afforded a mixture of several 99mTc-complexes.
HPLC comparison by co-injection of the two complexes,
ReO-12b and the corresponding 99mTc-12b, has shown only a
minor difference of retention times (4.50 versus 4.98), which
could be explained by the different polarities of the [Tc᎐O]3ϩ
᎐
and [Re᎐O]3ϩ groups25 (Fig. 4). These results indicate that
᎐
ligand 12b forms similar 1 : 1 complexes with [Tc᎐O]3ϩ and
᎐
Purification of 99mTc-complexes was accomplished by C-18
reverse phase HPLC and resulted in only one component (two
isomers in the case of 99mTc-12a,f ) with an excellent radio-
chemical yield.
[Re᎐O]3ϩ groups.
᎐
A short difference of retention time for 99mTc-12g and ReO-
12g was observed as well (13.60 versus 13.00). This surprising
result proved that the two complexes are similar. 99mTc-12g
seems to be a monomeric complex and not a dimeric struc-
ture as suggested previously. The reasons to explain the large
difference of retention time between the two metallic complex
analogues (Tc or Re) which differ only by one atom (an oxygen
Stability
The radiolabelled chelators were subject to metathesis in the
presence of an excess of cysteine as an assay of label stability.
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Scheme 4 ReO and TcO complex synthesis.
Fig. 4 HPLC comparison of ReO- and TcO-complexes of 12b.
instead of a sulfur) are not entirely clear yet. To better
Experimental
understand this point and confirm the structure of our metallic
complexes, 99Tc coordination chemistry with the two ligands is
under investigation.
All chemicals were of the highest purity commercially available.
Solvents were purified by standard methods before use and
stored over 0.3 nm molecular sieves. Silica gel (0.060–0.200 nm)
was purchased from Acros. TLC was performed using pre-
coated Kieselgel 60 plates F254 (TLC plates, Merck) and was
visualized by UV or iodine. Column chromatography was
carried out using “gravity” silica (Merck). NMR spectra were
recorded on a Bruker AC 200 (50.323 MHz for 13C and
200.133 MHz for 1H), 250 apparatus (62.896 MHz for 13C and
250.133 MHz for 1H) or 400 (100.63 MHz for 13C and 400.133
Conclusion
In an effort to develop efficient 99mTc-radiopharmaceuticals
based on tetradentate ligands, a high yielding route to a new
class of semi-rigid tetradentate ligands, with sulfur, amido or
amino donor groups has been developed. Their corresponding
99mTcO-complexes were achieved with a good radiochemical
yield except for one of them. Tests with technetium 99m
(in vitro behavior and stability) showed the major influence of
the ligand ortho substituent on the complex stability. Tetra-
anionic ligands formed more stable complexes than trianionic
ones with the following order for the ortho substituent: OH ∼
NHMe > NH2 > Strt >> OMe, SMe.
1
MHz for H). Chemical shifts are indicated in δ values (ppm)
downfield from internal TMS, and coupling constants (J) are
given in hertz (Hz). Multiplicities were recorded as s (singlet),
d (doublet), t (triplet), q (quadruplet) and m (multiplet). Infra-
red spectra were recorded as KBr pellets on a Bruker Vector 22
spectrophotometer in the range 4000–400 cmϪ1. Negative elec-
trospray or DCI-Mass spectra were obtained on a NERMAG
R 10–10 mass spectrometer. Microanalysis was performed by
the Microanalytical Department of the Ecole Nationale
Supérieure de Chimie de Toulouse. HPLC purifications were
achieved on a Waters 600E gradient chromatograph with a
Waters Lambda Max UV detector, an SAIP radioactivity
detector and an ICS dual integrator for effluent monitoring.
Radiochemical purity was assessed by thin layer chromato-
In addition, comparative HPLC analysis with prototypic
TcO- and ReO-complexes confirms the TcO nature of the
radiocomplexes.
Finally, the three ligands 12b, 12d and 12e seem promising
for a radiopharmaceutical use because they lead to pure, stable
and unique radiocomplexes. Biodistribution studies and co-
ordination chemistry with 99gTc and 185/187Re will be addressed
in ongoing investigations.
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graphy on Nano-sil C18 plates (Macherey-Nagel) with an LB
2832 linear analyser (Berthold).
1H NMR (250 MHz, CDCl3) δH(ppm): 3.83 (3H, s, OCH3);
4.04 (2H, d, J = 5.1 Hz, NCH2); 5.16 (2H, s, OCH2); 5.60 (1H,
br s, NH); 6.95 (3H, m, ArH); 7.36 (5H, m, ArHBz); 8.21
(1H, br s, NH); 8.31 (1H, d, J = 7.9 Hz, ArH); 13C {1H} NMR
(50.3 MHz, CDCl3) δC(ppm): 45.6 (NCH2); 55.8 (OCH3); 67.3
(OCH2); 110.1, 120.0, 124.3, 128.1 (4 × CHAr); 121.1, 128.3,
128.6 (3 × CHAr(Bz)); 127.0, 148.1 (2 × CAr); 136.3 (CAr(Bz));
156.8, 167.1 (2 × CO); MS (DCI/NH3): 315 (MϩHϩ), 332
(MϩNH4ϩ); elemental analysis found C, 65.21; H, 5.68; N,
9.05; C17H18N2O4 requires C, 64.96; H, 5.77; N, 8.91%.
Literature methods were used to prepare N-hydroxysuccin-
imidyl 2-(triphenylmethylthio)ethanoate 11,20 monoprotected
phenylenediamine 615 and ReO(PPh3)2Cl3.24 Preparation of
the intermediate ligands 9b and 9e, 10e has been described
previously.19
Synthesis of non-commercial o-substituted anilines
2-(Triphenylmethylthio)aniline 5. The reagent was prepared
following a published procedure14 using 2-aminothiophenol
and triphenylmethanol. A little improvement of yield (86% vs.
78%) was obtained and the mass spectra was done.
N-[2-(N,N-Dimethylamino)phenyl]-2-[(phenylmethoxy)-
carbonylamino]ethanamide 9c. 1.36 g of 8 gave 9c as a white
powder (2.32 g, 71%).
1H NMR (200 MHz, CDCl3) δH(ppm): 3.64 (2H, br s, NH2);
6.44 (2H, m, ArH); 7.00 (2H, m, ArH); 7.24 (9H, m, ArHTrt);
7.34 (6H, m, ArHTrt); 13C {1H} NMR (50.3 MHz, CDCl3)
δC(ppm): 70.9 (CPh3); 115.2, 118.0, 130.8, 137.9 (4 × CHAr);
116.3, 151.4 (2 × CAr); 126.8, 127.7, 129.2 (15 × CHTrt); 144.3
(3 × CTrt); MS (DCI/NH3): 368 (MϩHϩ).
1H NMR (250 MHz, CDCl3) δH(ppm): 2.57 (6H, s, 2 ×
NCH3); 4.05 (2H, d, J = 5.8 Hz, NCH2); 5.16 (2H, s, OCH2);
5.65 (1H, br s, NH); 7.03–7.18 (3H, m, ArH); 7.37 (5H, m,
ArHBz); 8.32 (1H, d, J = 7.3 Hz, ArH); 8.93 (1H, s, NH); 13C
{1H} NMR (100.6 MHz, CDCl3) δC(ppm): 44.9 (2 × NCH3);
45.8 (NCH2); 67.5 (OCH2); 119.6, 120.2, 124.4, 125.3 (4 ×
CHAr); 132.9, 136.5 (2 × CAr); 128.4, 128.5, 128.8 (5 × CHAr(Bz));
143.1 (CAr(Bz)); 167.0, 171.5 (2 × CO); MS (DCI/NH3): 328
(MϩHϩ); elemental analysis found C, 66.42; H, 6.60; N, 12.72;
C18H21N3O3 requires C, 66.04; H, 6.47; N, 12.84%.
N,N-Dimethyl-NЈ-(tert-butyloxycarbonyl)-1,2-diamino-
benzene 7.
A solution of N-(tert-butyloxycarbonyl)-1,2-
diaminobenzene 6 (2.08 g, 10 mmol) and 37% aqueous form-
aldehyde (1.73 mL, 60 mmol) in MeOH (40 mL) was stirred for
2 hours at room temperature. To the solution was added
decaborane (732 mg, 6 mmol) then the resulting solution was
stirred for 2 more hours. Solvents were concentrated and the
residue was purified by column chromatography on silica gel
(eluent: CH2Cl2) to give 7 as a pale yellow oil (1.65 g, 70%).
1H NMR (250 MHz, CDCl3) δH(ppm): 1.54 (9H, s, CH3);
2.63 (6H, s, NCH3); 6.93–7.16 (3H, m, ArH); 7.71 (1H, br s,
NH); 8.07 (1H, d, J = 8.2 Hz, ArH); 13C {1H} NMR (62.9 MHz,
CDCl3) δC(ppm): 28.4 (3 × CH3); 44.8 (2 × NCH3); 71.8
(CMe3); 117.7, 119.9, 122.3, 125.0 (4 × CHAr); 134.1, 142.3 (2 ×
CAr); 153.3 (CO); MS (DCI/NH3): 237 (MϩHϩ); elemental
analysis found C, 66.30; H, 8.82; N, 11.53; C13H20N2O2 requires
C, 66.07; H, 8.53; N, 11.85%.
N-[2-(N-Methylamino)phenyl]-2-[(phenylmethoxy)carbonyl-
amino]ethanamide 9d. 1.13 mL of N-methyl-1,2-phenylene-
diamine gave 9d as a white powder (3.04 g, 97%).
1H NMR (250 MHz, DMSO d6) δH(ppm): 2.71 (3H, d,
J = 5.0 Hz, NCH3); 3.86 (2H, d, J = 5.9 Hz, NCH2); 5.07 (3H,
br s, NH ϩ OCH2); 6.58 (2H, m, ArH); 7.10 (2H, m, ArH); 7.38
(5H, m, ArHBz); 7.56 (1H, br s, NH); 9.13 (1H, s, NH); 13C {1H}
NMR (100.6 MHz, DMSO d6) δC(ppm): 30.5 (NCH3); 44.5
(NCH2); 66.2 (OCH2); 110.7, 115.9, 126.7, 127.4 (4 × CHAr);
123.5, 137.7 (2 × CAr); 128.4, 128.5, 129.0 (5 × CHAr(Bz)); 144.6
(CAr(Bz)); 157.3, 169.0 (2 × CO); MS (DCI/NH3): 314 (MϩHϩ)
331 (MϩNH4ϩ); elemental analysis found C, 65.38; H, 6.22; N,
13.09; C17H19N3O3 requires C, 65.16; H, 6.11; N, 13.41%.
N,N-Dimethyl-1,2-diaminobenzene 8. Trifluoroacetic acid
(4.46 mL, 60 mmol) was added to a solution of 7 (1.00 g, 4.23
mmol) in CH2Cl2 (3 mL). The mixture was stirred at room tem-
perature for one hour. The solution was concentrated to dry-
ness and the residue was washed with 1 M HCl solution–EtOAc
mixture (20 mL, 50 : 50 v/v). The pH of the aqueous layer was
raised to 12 and the product was extracted using CHCl3 (2 × 10
mL). The organic layer was dried, concentrated to dryness and
the residue was purified by column chromatography on silica
gel (eluent: CH2Cl2 then CH2Cl2–AcOEt 8 : 2) to give 8 as a
yellow oil (0.40 g, 68%).
2-Amino-N-(o-substituted phenyl)ethanamides
General procedure. Catalytic hydrogenation of N-(o-substi-
tuted phenyl)-2-[(phenylmethoxy)carbonylamino]ethanamides
(10 mmol) in methanol (100 mL) over 10% Pd/C (20% w/w) was
carried out at atmospheric pressure. The mixture was stirred
for 2 h, then the catalyst was filtered off through Celite and the
solvent was removed under reduced pressure to give the depro-
tected amine as an oil or a solid.
1H NMR (400 MHz, CDCl3) δH(ppm): 2.71 (6H, s, NCH3);
3.98 (2H, br s, NH2); 6.77 (2H, m, ArH); 6.95 (1H, m, ArH);
7.06 (1H, m, ArH); 13C {1H} NMR (100.6 MHz, CDCl3)
δC(ppm): 43.9 (2 × NCH3); 115.4, 118.7, 119.5, 124.4 (4 ×
CHAr); 140.9, 141.6 (2 × CAr); MS (DCI/NH3): 137 (MϩHϩ).
2-Amino-N-(2-methoxyphenyl)ethanamide 10a. 3.14 g of 9a
gave 10a as white crystals (1.76 g, 98%).
1H NMR (250 MHz, CDCl3) δH(ppm): 1.61 (2H, br s, NH2);
3.50 (2H, s, NCH2); 3.89 (3H, s, OCH3); 6.95 (3H, m, ArH);
8.42 (1H, dd, J ؍ 1.8 and 7.8 Hz, ArH); 9.71 (1H, br s, NH); 13
C
{1H} NMR (50.3 MHz, CDCl3) δC(ppm): 45.7 (NCH2); 55.8
(OCH3); 110.0, 119.6, 121.0, 123.8 (4 × CHAr); 127.4, 148.4
(2 × CAr); 170.9 (CO); MS (DCI/NH3): 181 (MϩHϩ).
Synthesis of N2SO or N3S tetradentate ligands
N-(o-substituted phenyl)-2-[(phenylmethoxy)carbonylamino]-
ethanamides
General procedure. To a solution of carbobenzyloxyglycine
(2.09 g, 10 mmol) in THF (40 mL) were added o-substituted
aniline (10 mmol) and a slight excess of dicyclohexylcarbo-
diimide (2.26 g, 11 mmol). The mixture was stirred under N2 at
room temperature for 6 hours. The insoluble dicyclohexylurea
was removed by filtration and the solvent concentrated to dry-
ness. Pure products were obtained after crystallisation from
ethyl acetate–petroleum ether 40–60 ЊC 2 : 1.
2-Amino-N-(2-hydroxyphenyl)ethanamide 10b. This reagent
was first prepared by Bermejo et al.19 using cyclohexene as
hydrogen source but a quantitative yield was obtained with our
method.
3.00 g of 9b gave 10b as green powder (1.66 g, 100%).
1H NMR (250 MHz, DMSO d6) δH(ppm): 3.25 (2H, s,
NCH2); 6.75 (1H, m, ArH); 6.87 (2H, m, ArH); 8.17 (1H, d,
J = 8.0 Hz, ArH); MS (DCI/NH3): 167 (MϩHϩ).
N-(2-Methoxyphenyl)-2-[(phenylmethoxy)carbonylamino]-
ethanamide 9a. 1.23 g of o-anisidine gave 9a as a white powder
(2.95 g, 94%).
2-Amino-N-[2-(N,N-dimethylamino)phenyl]ethanamide 10c.
3.27 g of 9c gave 10c as a white powder (1.56 g, 81%).
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 8 7 6 – 8 8 3
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1H NMR (250 MHz, DMSO d6) δH(ppm): 2.61 (6H, s,
2 × NCH3); 3.26 (2H, s, NCH2); 7.02 (2H, m, ArH); 7.19 (1H,
m, ArH); 8.28 (1H, m, ArH); 13C {1H} NMR (50.3 MHz,
DMSO d6) δC(ppm): 44.6 (2 × NCH3); 45.8 (NCH2); 119.2,
120.2, 124.0, 124.4 (4 × CHAr); 133.0, 144.6 (2 × CAr); 171.9
(CO); MS (DCI/NH3): 194 (MϩHϩ).
(DCI NH3): 483 (MϩHϩ), 500 (MϩNH4ϩ); elemental analysis
found C, 71.90; H, 5.67; N, 5.71; C29H26N2O3S requires C,
72.17; H, 5.43; N, 5.80%.
N-[2-(N,N-Dimethylamino)phenyl]-2-[(triphenylmethylthio)-
methylcarbonylamino]ethanamide 12c
Method A: 1.93 g of 10c gave 12c as a yellow powder (4.12 g,
81%).
2-Amino-N-[2-(N-methylamino)phenyl]ethanamide 10d. 3.13 g
of 9d gave 10d as a white powder (1.79 g, 100%).
1H NMR (400 MHz, CDCl3) δH(ppm): 2.61 (6H, s, NCH3);
3.22 (2H, s, SCH2); 3.80 (2H, d, J = 5.3 Hz, CH2N); 6.65 (1H, br
s, NH), 7.10–7.34 (12H, m, 3ArH ϩ 9ArHTrt); 7.46–7.49 (6H,
m, ArHTrt); 8.31 (1H, d, J = 8.0 Hz, ArH); 8.75 (1H, br s, NH);
13C {1H} NMR (100.6 MHz) δC(ppm): 35.9 (CH2S); 44.6
(CH2N); 45.0 (2 × NCH3); 68.2 (CPh3); 119.6, 120.3, 124.3,
125.3 (4 × CHAr); 127.3, 128.4, 129.7 (15 × CHTrt); 133.0, 143.1
(2 × CAr); 144.0 (3 × CTrt); 166.2, 168.8 (2 × CO); νmax/cmϪ1
(KBr): 3330, 3287 (NH), 1691, 1640 (CO); m/z (DCI NH3): 510
(MϩHϩ), elemental analysis found C, 72.76; H, 6.24; N, 8.04%;
C31H31N3O2S requires C, 73.05; H, 6.13; N, 8.24%.
1H NMR (250 MHz, DMSO d6) δH(ppm): 2.70 (3H, d,
J = 4.8 Hz, NCH3); 3.28 (2H, s, NCH2); 6.60 (2H, m, ArH); 7.05
(1H, m, ArH); 7.24 (1H, m, ArH); 13C {1H} NMR (50.3 MHz,
DMSO d6) δC(ppm): 30.6 (NCH3); 46.0 (NCH2); 111.1, 116.4,
125.6, 126.9 (4 × CHAr); 124.3, 144.1 (2 × CAr); 172.8 (CO); MS
(DCI/NH3): 180 (MϩHϩ).
N-(o-substituted phenyl)-2-[(triphenylmethylthio)methyl-
carbonylamino]ethanamides; general procedure
Method A. To a solution of 2-amino-N-(o-substituted phenyl)-
ethanamide (10 mmol) in acetonitrile (100 mL) were added 11
(4.31 g, 10 mmol) and DMAP (1.22 g, 10 mmol). The solution
was heated at 60 ЊC, under nitrogen, for 3 hours. After cooling,
the product was left to precipitate overnight at Ϫ4 ЊC. After
filtration, the precipitate was washed with cold acetonitrile to
lead to pure crystals.
N-[2-(N-Methylamino)phenyl]-2-[(triphenylmethylthio)methyl-
carbonylamino]ethanamide 12d
Method A: 1.79 g of 10d gave 12d as a grey powder (2.77 g,
56%).
1H NMR (250 MHz, CDCl3) δH(ppm): 2.73 (3H, s, NCH3);
3.15 (2H, s, SCH2); 3.72 (2H, d, J = 5.2 Hz, CH2N); 6.69 (2H,
m, ArH); 6.80 (1H, br s, NH); 7.11–7.30 (12H, m, NH ϩ 2ArH
Method B. As method A except the purification procedure.
After cooling, the solution was concentrated to dryness and the
crude was purified by column chromatography on silica gel.
ϩ 9ArHTrt); 7.40–7.44 (6H, m, ArHTrt); 7.93 (1H, br s, NH); 13
C
{1H} NMR (62.9 MHz) δC(ppm): 30.5 (NCH3); 35.6 (CH2S);
44.5 (CH2N); 67.9 (CPh3); 111.4, 117.1, 125.3, 127.5 (4 × CHAr);
127.1, 128.2, 129.5 (15 × CHTrt); 122.5, 143.2 (2 × CAr); 143.8
(3 × CTrt); 167.2, 169.7 (2 × CO); νmax/cmϪ1 (KBr): 3409, 3390,
3260 (NH), 1673, 1645 (CO); m/z (DCI NH3): 496 (MϩHϩ),
elemental analysis found C, 72.61; H, 5.87; N, 8.40; C30H29-
N3O2S requires C, 72.70; H, 5.90; N, 8.48%.
Method C. To a solution of 2-amino-N-(o-substituted phenyl)-
ethanamide (10 mmol) in acetonitrile (100 mL) were added 11
(4.31 g, 10 mmol) and Et3N (1.01 g, 10 mmol). The solution was
stirred at room temperature, under nitrogen, for 3 hours. After
cooling, the product was left to precipitate overnight at Ϫ4 ЊC.
After filtration, the precipitate was washed with cold aceto-
nitrile to lead to pure crystals.
N-(2-Aminophenyl)-2-[(triphenylmethylthio)methylcarbonyl-
amino]ethanamide 12e
N-(2-Methoxyphenyl)-2-[(triphenylmethylthio)methylcarbonyl-
amino]ethanamide 12a
Method C: 1.65 g of 10e gave 12e as a white powder (3.66 g,
76%).
Method A: 1.80 g of 10a gave 12a as pure white crystals (4.36 g,
88%).
1H NMR (250 MHz, DMSO d6) δH(ppm): 2.89 (2H, s, CH2S);
3.80 (2H, d, J = 5.6 Hz, CH2N); 4.88 (2H, br s, NH2); 6.52 (1H,
m, ArH); 6.70 (1H, m, ArH); 6.91 (1H, m, ArH); 7.07 (1H, m,
ArH); 7.22–7.35 (15H, m, ArHTr); 8.27 (1H, br s, NH); 9.12
(1H, s, NH); 13C {1H} NMR (100.6 MHz, DMSO d6) δC(ppm):
35.8 (SCH2); 42.7 (NCH2); 65.9 (CPh3); 115.4, 115.9, 125.8,
126.2 (4 × CHAr); 126.7, 128.0, 129.0 (15 × CHTrt); 122.5, 142.4
(2 × CAr); 143.9 (3 × CTrt); 167.4, 167.8 (2 × CO); νmax/cmϪ1
(KBr): 3454, 3365, 3275 (NH), 1736, 1672 (CO); m/z (DCI
NH3): 482 (MϩHϩ), elemental analysis found C, 72.00; H, 5.69;
N, 9.02%; C29H27N3O2S requires C, 72.32; H, 5.65; N, 8.72%.
1H NMR (250 MHz, CDCl3) δH(ppm): 3.20 (2H, s, SCH2);
3.74 (2H, d, J = 5.5 Hz, CH2N); 3.82 (3H, s, OCH3); 6.60 (1H,
br s, NH); 6.84–7.10 (3H, m, ArH); 7.20–7.32 (9H, m, ArHTrt);
7.43–7.47 (6H, m, ArHTrt); 8.27 (1H, br s, NH); 8.30 (1H, dd,
J = 1.5 and 8.0 Hz, ArH); 13C {1H} NMR (50.3 MHz) δC(ppm):
35.7 (CH2S); 44.5 (CH2N); 55.7 (OCH3); 68.0 (CPh3); 110.1,
119.9, 121.1, 124.2 (4 × CHAr); 127.2, 128.3, 129.6 (15 × CHTrt);
143.9 (3 × CTrt); 125.0, 148.2 (2 × CAr); 166.2, 168.9 (2 × CO);
νmax/cmϪ1 (KBr): 3409, 3280 (NH) 1643 (CO); m/z (DCI NH3):
497 (MϩHϩ), 514 (MϩNH4ϩ); elemental analysis found C,
72.31; H, 5.94; N, 5.77; C30H28N2O3S requires C, 72.55; H, 5.68;
N, 5.64%.
Synthesis of N2S2 tetradentate ligands
Ethyl 2-[(triphenylmethylthio)methylcarbonylamino]ethanoate
13. A solution of N-hydroxysuccinimidyl 2-(triphenylmethyl-
thio)ethanoate 11 (7.46 g, 17.3 mmol) in dry THF (25 mL) was
added dropwise, using a syringe, to a stirred solution of ethyl
glycinate hydrochloride (2.67 g, 19 mmol) and dry triethylamine
(2.67 mL, 19 mmol) in fresh distillated DMF (25 mL). The
mixture was stirred at room temperature for 3 h. Solvents were
removed under pressure and the residue was washed with 1 M
HCl–CHCl3 mixture (100 mL, 50 : 50 v/v). The organic layer
was separated, dried over sodium sulfate, filtered off and con-
centrated to dryness under reduced pressure to give 13 as a pure
white powder (7.10 g, 98%).
N-(2-Hydroxyphenyl)-2-[(triphenylmethylthio)methylcarbonyl-
amino]ethanamide 12b
Method B: after purification by column chromatography on
silica gel (eluent: CH2Cl2–AcOEt 9 : 1) 1.66 g of 10b gave 12b as
a pale yellow powder (2.65 g, 55%).
1H NMR (250 MHz, CDCl3) δH(ppm): 3.21 (2H, s, SCH2);
3.76 (2H, d, J = 5.2 Hz, CH2N); 6.72 (1H, br s, NH); 6.80–7.31
(13H, m, 4ArH ϩ 9ArHTrt); 7.41–7.44 (6H, m, ArHTrt); 8.68
(1H, s, OH); 8.83 (1H, br s, NH); 13C {1H} NMR (62.9 MHz)
δC(ppm): 35.5 (CH2S); 44.4 (CH2N); 68.1 (CPh3); 118.9, 120.4,
122.3, 126.8 (4 × CHAr); 125.3, 148.1 (2 × CAr); 127.2, 128.3,
129.5 (15 × CHTrt); 143.7 (3 × CTrt); 167.6, 169.9 (2 × CO);
νmax/cmϪ1 (KBr): 3409, 3347, 3292 (NH), 1700, 1627 (CO); m/z
1H NMR (250 MHz, CDCl3) δH(ppm): 1.26 (3H, t, J =
7.0 Hz, CH3); 3.15 (2H, s, CH2S); 3.69 (2H, d, J = 5.1 Hz,
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CH2N); 4.18 (2H, q, J = 7.0 Hz, CH2O); 6.51 (1H, br s, NH);
7.18–7.31 (9H, m, ArH); 7.41–7.44 (6H, m, ArH); 13C {1H}
NMR (50.3 MHz, CDCl3) δC(ppm): 14.2 (CH3); 35.7 (CH2S);
41.6 (CH2N); 61.5 (CH2O); 67.8 (CPh3); 127.1, 128.2, 129.5 (15
× CHTrt); 143.9 (3 × CTrt); 168.1, 169.4 (2 × CO); MS (DCI/
NH3): 420 (MϩHϩ), 437 (MϩNH4ϩ); elemental analysis found
C, 71.86; H, 6.15; C25H25O3S requires C, 71.57; H, 6.01%.
m, ArH); 7.22–7.29 (26H, m, ArH); 7.42–7.45 (6H, m, ArH);
8.09 (1H, br s, NH); 8.17 (1H, d, J = 7.5 Hz, ArH); 13C {1H}
NMR (50.3 MHz) δC(ppm): 35.5 (CH2S); 43.7 (CH2N); 67.8,
71.7 (2 × CPh3); 119.4, 123.6, 131.2, 137.4 (4 × CHAr); 127.1,
127.2, 127.9, 128.3, 129.5, 129.6 (30 × CHTrt); 120.4, 141.5 (2 ×
CAr); 143.6, 143.9 (6 × CTrt); 165.5, 168.4 (2 × CO); νmax/cmϪ1
(KBr): 3325 (NH), 1679 (CO); m/z (DCI NH3): 741 (MϩHϩ),
758 (MϩNH4ϩ); elemental analysis found C, 77.00; H, 5.32; N,
3.50; C48H40N2O2S2 requires C, 77.80; H, 5.44; N, 3.78%.
2-[(Triphenylmethylthio)methylcarbonylamino]ethanoic acid
14. A solution of 13 (10.20 g, 24.3 mmol) in a 1 M NaOH–
MeOH mixture (225 mL, 67 : 33, v/v) was boiled at 40 ЊC for
2 h. The solution was then cooled and acidified with 6 M HCl
(15 mL). The obtained white precipitate was filtered off, washed
with cooled water and dried under low pressure to give 14 as a
white powder (9.85 g, 100%).
Rhenium complex synthesis
N-(2-Hydroxyphenyl)-2-(thiomethylcarbonylamino)ethan-
amide oxorhenate(V), sodium salt: ReO-12b. To 12b (144.6 mg,
0.3 mmol) and sodium acetate (163.2 mg, 1.2 mmol) dissolved
in 40 mL of dry methanol, was added ReO(PPh3)2Cl3 (324.5
mg, 0.39 mmol). After refluxing for 4 hours, the solution turned
brown and clear. After cooling, the solution was filtered and
evaporated to dryness. The residue was purified by column
chromatography on silica gel (eluent: CH2Cl2–MeOH: 9 : 1) to
yield the complex as an orange-red powder (134.2 mg, 97%).
1H NMR (400 MHz, DMSO d6) δH(ppm): 3.81 (1H, d,
J = 17.2 Hz, CH2S); 4.15 (1H, d, J = 17.2 Hz, CH2S); 4.54 (1H,
d, J = 18.3 Hz, CH2N); 5.46 (1H, d, J = 18.3 Hz, CH2N); 6.78
(1H, t, J = 7.7 Hz, CHAr); 6.90 (1H, t, J = 7.7 Hz, CHAr); 7.06
(1H, d, J = 7.7 Hz, CHAr); 8.25 (1H, d, J = 7.9 Hz, CHAr); 13C
{1H} NMR (100.63 MHz; MeOD) δC(ppm): 40.3 (1C, CH2S);
61.0 (1C, CH2N); 115.1, 118.1, 119.5, 123.4 (4C, CHAr); 140.2
(1C, CAr); 169.5 (1C, CAr); 187.3, 194.5 (2C, C=O); νmax/cmϪ1
(KBr): 1624 (CO) 968 (ReO); m/z (ESϪ): 437 (60), 439 (100)
[MϪ]; elemental analysis found C, 25.74; H, 2.06; N, 5.62;
C10H8N2O4SReNa requires C, 26.00; H, 1.73; N, 6.07%.
1H NMR (250 MHz, CDCl3) δH(ppm): 3.20 (2H, s, CH2S);
3.68 (2H, d, J = 4.8 Hz, CH2N); 6.51 (1H, br s, NH); 7.24–7.28
(9H, m, ArH); 7.41–7.45 (6H, m, ArH); 10.49 (1H, br s, OH);
13C {1H} NMR (50.3 MHz, CDCl3) δC(ppm): 35.4 (CH2S); 41.6
(CH2N); 67.9 (CPh3); 127.1, 128.2, 129.5 (15 × CHTrt); 143.9
(3 × CTrt); 169.1, 172.7 (2 × CO); MS (DCI/NH3): 409
(MϩNH4ϩ), 391 (MϩNH4ϩϪH2O).
N-Hydroxysuccinimidyl
2-[(triphenylmethylthio)methyl-
carbonylamino]ethanoate 15. 14 (4.00 g, 10.22 mmol) and
N-hydroxysuccinimide (1.18 g, 10.22 mmol) were dissolved in
dry THF (50 mL). Dicyclohexylcarbodiimide (1.32 g, 11.26
mmol) was added and the mixture was stirred at room temper-
ature under nitrogen for 4 h. The reaction mixture was then
filtered and the filter cake extracted twice with THF. All filtrates
were combined and evaporated to give a solid which was
recrystallised from ethyl acetate–petroleum ether (v/v: 2 : 1) to
afford 15 as a white powder (4.14 g, 83%).
1H NMR (250 MHz, CDCl3) δH(ppm): 2.81 (4H, s, CH2);
3.20 (2H, s, CH2S); 4.01 (2H, d, J = 5.5 Hz, CH2N); 6.45 (1H, br
s, NH); 7.23–7.29 (9H, m, ArH); 7.42–7.44 (6H, m, ArH); 13C
{1H} NMR (50.3 MHz, CDCl3) δC(ppm): 25.6 (2 × CH2);
35.3 (CH2S); 39.2 (CH2N); 69.2 (CPh3); 127.1, 128.3, 129.4
(15 × CHTrt); 143.8 (3 × CTrt); 165.4, 168.3 (2 × CO); 168.5 (2 ×
CONHS); MS (DCI/NH3): 506 (MϩNH4ϩ); elemental analysis
found C, 66.50; H, 5.06; N, 5.45; C27H24N2O5S requires C,
66.38; H, 4.95; N, 5.73%.
N-(2-Thiophenyl)-2-(thiomethylcarbonylamino)ethanamide
oxorhenate(V), sodium salt: ReO-12g. The same procedure
described for compound ReO-12b was used (with 12g instead
of 12b). After purification by column chromatography on silica
gel (eluent: CH2Cl2 then CH2Cl2–MeOH: 9 : 1) ReO-12g was
obtained as an orange-red powder (197.5 mg, 83%).
1H NMR (300 MHz, DMSO d6) δH(ppm): 3.71 (1H, d,
J = 17.4 Hz, CH2S); 3.90 (1H, d, J = 17.4 Hz, CH2S); 4.54 (1H,
d, J = 18.9 Hz, CH2N); 4.94 (1H, d, J = 18.9 Hz, CH2N); 6.88
(2H, m, CHAr); 7.54 (1H, m, CHAr); 8.72 (1H, m, CHAr); 13C
{1H} NMR (75.5 MHz; MeOD) δC(ppm): 40.7 (1C, CH2S); 61.2
(1C, CH2N); 126.1, 124.7, 125.3, 129.1 (4C, CHAr); 148.8 (1C,
CAr); 152.6 (1C, CAr); 190.5, 194.8 (2C, C=O); νmax/cmϪ1 (KBr):
1620 (CO), 964 (ReO); m/z (ESϪ): 453 (60), 455 (100) [MϪ];
elemental analysis found C, 25.08; H, 2.10; N, 5.48;
C10H8N2O3S2ReNa requires C, 25.10; H, 1.67; N, 5.85%.
N-(2-Thiomethoxyphenyl)-2-[(triphenylmethylthio)methyl-
carbonylamino]ethanamide 12f. To a solution of 2-(methylthio)-
aniline (1.00 g, 7.18 mmol) in acetonitrile (30 mL) were added
15 (3.51 g, 7.18 mmol) and DMAP (0.88 g, 7.18 mmol). The
mixture was heated at 60 ЊC for six hours then cooled to room
temperature and concentrated. Purification by column chrom-
atography on silica gel (eluent: CH2Cl2 then CH2Cl2–AcOEt:
95 : 5) gave 12f as a white powder (2.21 g, 60%).
99mTc labelling. Into a borosilicated vial containing buffer
(pH = 8.6; 200 µl), were added a solution of ligand in methanol
(1 mg mlϪ1, 100 µl) and Na99mTcO4 generator eluate (100 µl,
2 mCi). After addition of a fresh SnCl2 solution in MeOH
(75 µl, 2.25 mg mlϪ1), the vial was sealed with a Teflon-lined cap
and the mixture was heated at 80 ЊC for 30 minutes. After
cooling, the resulting complexes were purified by HPLC,
using a Satisfaction RP18AB column (eluent: MeOH–H2O–
1H NMR (250 MHz, CDCl3) δH(ppm): 2.31 (3H, s, SCH3);
3.25 (2H, s, SCH2); 3.75 (2H, d, J = 5.5 Hz, CH2N); 6.68 (1H, br
s, NH); 7.08–7.30 (12H, m, 3ArH ϩ 9ArHTrt); 7.43–7.48 (6H,
m, ArHTrt); 8.23 (1H, d, J = 8.0 Hz, ArH); 8.63 (1H, br s, NH);
13C {1H} NMR (50.3 MHz) δC(ppm): 18.9 (SCH3); 35.7 (CH2S);
44.6 (CH2N); 68.0 (CPh3); 120.8, 124.9, 128.8, 132.8 (4 × CHAr);
127.2, 128.3, 129.5 (15 × CHTrt); 126.1, 137.6 (2 × CAr); 143.9
(3 × CTrt); 166.7, 169.0 (2 × CO); νmax/cmϪ1 (KBr): 3356, 3267
(NH), 1699, 1651 (CO); m/z (DCI NH3): 513 (MϩHϩ), 530
(MϩNH4ϩ); elemental analysis found C, 69.85; H, 5.59; N,
5.72; C30H28N2O2S2 requires C, 70.28; H, 5.50; N, 5.46%.
TFA 50 : 50 : 0.1) at a flow rate of 1 ml minϪ1
.
Stability versus cysteine. The complex after purification by
HPLC under the conditions described above was incubated at
37 ЊC, under a nitrogen atmosphere with a freshly prepared
solution of -cysteine at 1 mg mlϪ1 in a 500 : 1 cysteine/complex
molar ratio. All solutions were purged with nitrogen prior to
use. Incubate aliquots at 2, 6 and 12 hours intervals were
analyzed by thin layer chromatography on nano-sil C18 plates
by elution with MeOH–CH3CN–H2O–TFA 20 : 15 : 65 : 0.1. Rf
(free technetium) = 1; Rf (99mTc-complex) = between 0.25 and
0.60.
N-[2-(Triphenylmethylthio)phenyl]-2-[(triphenylmethylthio)-
methylcarbonylamino]ethanamide 12g. The same procedure
described for compound 12f was used (with 5 instead of
2-(methylthio)aniline as starting material). 12g was obtained as
a white powder (2.60 g, 50%).
1H NMR (250 MHz, CDCl3) δH(ppm): 3.10 (2H, s, SCH2);
3.29 (2H, d, J = 5.2 Hz, CH2N); 6.42 (1H, br s, NH); 6.79 (1H,
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 8 7 6 – 8 8 3
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Sousa and R. D. Hancock, J. Chem. Soc., Chem. Commun., 1995,
415–416.
Acknowledgements
12 S. Kasina, A. R. Fritzberg, D. L. Johnson and D. Eshima, J. Med.
Chem., 1986, 29, 1933–1940.
We are grateful to the French Ministère de l’Education Nation-
ale, de la Recherche et de la Technologie.
13 (a) B. J. McBride, R. M. Baldwin, J. M. Kerr, J. L. Wu, L. M.
Schultze, N. E. Salazar, J. M. Chinitz and E. F. Byrne, J. Med.
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Rhenium and Other Metals in Chemistry and Nuclear Medicine Vol.
4, Eds M. Nicolini, G. Bandoli, U. Mazzi, SGEditoriali, Padova,
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14 J. C. Noveron, M. M. Olmstead and P. K. Mascharak, J. Am. Chem.
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15 C. T. Seto, J. P. Mathias and G. M. Whitesides, J. Am. Chem. Soc.,
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16 G. W. Gribble and C. F. Nutaitis, Synthesis, 1987, 709–711.
17 S. Bhattacharyya, Synth. Commun., 1995, 25, 2061–2069.
18 Y. J. Jung, J. W. Bae, C. O. M. Yoon, B. W. Yoo and C. M. Yoon,
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19 (a) M. R. Bermejo, A. M. Gonzalez, M. Fondo, A. Garcia-Deibe,
M. Maneiro, J. Sanmartin, O. L. Hoyos and M. Watkinson, New J.
Chem., 2000, 24, 235–241; (b) O. L. Hoyos, M. R. Bermejo,
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