Synthesis of bifunctional chelating agents based on mono and diphosphonic derivatives of diethylenetriaminepentaacetic acid

Article abstract of DOI:10.1016/j.tet.2014.05.030

Bifunctional chelating agents (BFCAs) are small molecules containing a chelating unit, able to strongly coordinate a metal ion, and a reactive functional group, devised to form a stable covalent bond with another molecule. BFCAs are widely employed since their conjugation to a suitable biomolecule (e.g., a peptide or an antibody) allows the synthesis of diagnostic or therapeutic agents that specifically target diseased tissue with metals or radiometals. For this reason, BFCAs find application in diagnostic imaging, molecular imaging, and radiotherapy of cancer. The synthesis of new BFCAs based on a diethylenetriaminepentaacetic acid (DTPA) structure in which one or two carboxylic groups are replaced with phosphonic units is described. The phosphonic group, aside from being a classical isostere of the carboxylic acid in coordination chemistry, allows to modulate the physico-chemical properties of the ligands and of the corresponding complexes.

Full text of DOI:10.1016/j.tet.2014.05.030

Tetrahedron 70 (2014) 4809e4813
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of bifunctional chelating agents based on mono and
diphosphonic derivatives of diethylenetriaminepentaacetic acid
,
a
b
b,
Giovanni B. Giovenzana a , Claudia Guanci , Silvia Demattio , Luciano Lattuada
*
*
,
Veronica Vincenzi b
a
ꢀ
Dipartimento di Scienze del Farmaco, Universita degli Studi del Piemonte Orientale “A. Avogadro”, Largo Donegani 2/3, Novara 28100, Italy
b Bracco Imaging Spa, Bracco Research Centre, Via Ribes 5, Colleretto Giacosa, TO 10010, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Bifunctional chelating agents (BFCAs) are small molecules containing a chelating unit, able to strongly
coordinate a metal ion, and a reactive functional group, devised to form a stable covalent bond with
another molecule. BFCAs are widely employed since their conjugation to a suitable biomolecule (e.g.,
a peptide or an antibody) allows the synthesis of diagnostic or therapeutic agents that specifically target
diseased tissue with metals or radiometals. For this reason, BFCAs find application in diagnostic imaging,
molecular imaging, and radiotherapy of cancer. The synthesis of new BFCAs based on a diethylene-
triaminepentaacetic acid (DTPA) structure in which one or two carboxylic groups are replaced with
phosphonic units is described. The phosphonic group, aside from being a classical isostere of the car-
boxylic acid in coordination chemistry, allows to modulate the physico-chemical properties of the ligands
and of the corresponding complexes.
Received 9 January 2014
Received in revised form 24 April 2014
Accepted 8 May 2014
Available online 20 May 2014
Keywords:
Bifunctional chelating agents
Contrast agents
Conjugation
Metal complexation
DTPA
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
R
HOOC
HOOC
COOH
COOH
N
N
N
N
N
The use of metal or radiometal complexes in medicine as ther-
apeutic or diagnostic agents is an area of growing interest.1e3 For
diagnostic purposes, gadolinium complexes are employed as con-
trast agents in magnetic resonance imaging (MRI)4e6 while com-
plexes of 111In or 68Ga, for example, find application in single
photon emission computed tomography (SPECT) or positron
emission tomography (PET), respectively.7e10 For therapy, and in
particular in the treatment of some kind of tumors and metastases,
HOOC
HOOC
N
N
COOH
COOH
1a R = COOH
1b R = PO3H2
2
HOOC
COOtBu
N
n
N
tBuOOC
N
COOtBu
COOtBu
complexes of
b or a
emitters, such as 90Y, 177Lu or 225Ac, are usually
studied and administered to patients.7e12
tBuOOC
3
4
n = 1
n = 0
All these metal ions need to be strongly coordinated with
a suitable ligand to guarantee a safe administration in vivo and
prevent an undesirable deposition of toxic metal ions in tissues or
organs different from the desired target. Polyaminopolycarboxylic
derivatives, such as diethylenetriaminepentaacetic acid 1a (DTPA)
Fig. 1. Common chelating agents and bifunctional derivatives.
or 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid
2
a specific delivery to particular type of cells by a recognition pro-
cess. The easiest way of linking a vector to a metal complex is by
direct conjugation to a bifunctional chelating agent (BFCA) fol-
lowed by deprotection of eventual protective groups and final
complexation of the metal ion of choice.19e21
(DOTA) (Fig. 1), are the ligands of choice and are widely employed
because they provide very stable complexes with a wide variety of
metal ions.13
The metal complex is usually covalently bound to a targeting
vector (e.g., peptides14e16 or monoclonal antibodies17,18) that allows
Among the huge number of polyaminopolycarboxylic bi-
functional chelating agents reported in literature,22 BFCA 323 and
424 (Fig. 1) found a broad spectrum of applications. For example,
* Corresponding authors. E-mail addresses: giovanni.giovenzana@unipmn.it,
giovenzana@pharm.unipmn.it (G.B. Giovenzana).
BFCA 3
has been conjugated to bile acids,25,26 oxytocin,27
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.05.030

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G.B. Giovenzana et al. / Tetrahedron 70 (2014) 4809e4813
aminoacids,28 CCK8 peptides,29 RGD (Arg-Gly-Asp) peptidomi-
metics,30,31 while BFCA 4 has been exploited in the synthesis of
a potential MRI contrast agent,32 linked to a tamoxifen derivative,33
and conjugated to a cyclic peptide for fibrin targeting.34
Efforts directed towards improvements of MRI contrast agents
rely on different strategies involving structural variations of the
ligand; among them the substitution of carboxylic groups with
different acidic functionalities was extensively investigated, leading
to the preparation of several derivatives, embodying more or less
classical isosteres, such as phosphonic acid,35,36 phosphinic acid,37
acylsulphonamide,38 and tetrazole.39
Br
N
N
N
COOtBu
COOtBu
Ph
NH2
+
2
5
K2CO3, MeCN
Ph
N
tBuOOC
tBuOOC
N
COOtBu
COOtBu
The phosphonic acid is particularly appealing as its size and
acidic properties closely resemble the behavior of carboxylic
groups.40 Its tetrahedral deprotonated form (ePO23ꢀ) occurring at
physiological pH values offers multiple coordination modes, and
residual anionic charges after complexation provide a beneficial
effect on the solubility of the corresponding chelate, due to the
establishment of an extended hydrogen bond network leading to
improved solvation. Several examples of polydentate ligands em-
bodying one36 or more41,42 phosphonic groups were reported for
different purposes ranging from water corrosion inhibitors to di-
agnostic and therapeutic applications.
Phosphonic derivatives of the octadentate ligand DTPA (dieth-
ylenetriaminepentaacetic acid) are already known: the per-
phosphonated derivative (DTPMP)43 is commercially available
(DequestÒ 2060), while the monophosphonic DTPA-analog 1b44 is
reported to form thermodynamically very stable metal complexes,
with stability constants comparable with the parent DTPA ligand.45
Despite the growing interest in phosphonic derivatives of
polyaminocarboxylic ligands, it is surprising to observe only a few
examples of bifunctional derivatives. A perphosphonic derivative
(DOTP) of the macrocyclic ligand DOTA was decorated with dif-
ferent appendages, among them a p-aminophenyl residue ame-
nable to use for conjugation purposes.46 Moreover, two different
cyclohexane-1,2-diamine tetraphosphonic ligands bearing ready-
to-use isothiocyanate groups were designed for application in
153Sm-based radioimmunotherapy.47 To the best of our knowl-
edge, the only BFCA with carboxylic and phosphonic moieties
reported so far, is a triethylenetetraaminehexaacetic acid (TTHA)
analog.48
6
H2, Pd/C
AcOH/MeOH
H
N
tBuOOC
tBuOOC
BnOOC
N
COOtBu
7
COOtBu
P(OtBu)3
MeCN, 80-90°C
9
CHO
8
BnOOC
tBuOOC
tBuOOC
PO(OtBu)2
N
N
N
COOtBu
COOtBu
10
H2, Pd/C
THF
HOOC
tBuOOC
tBuOOC
PO(OtBu)2
N
N
N
COOtBu
We describe here the synthesis of two novel BFCA based on the
DTPA structure but having one (Scheme 1, compound 11) or two
(Scheme 2, compound 18) phosphonic group replacing the corre-
sponding carboxylic moieties.
COOtBu
11
Scheme 1. Synthesis of BFCA 11.
The synthesis of diphosphonic BFCA 18 relied on a similar
strategy, necessarily implying the modification of the N-( -bro-
2. Results and discussion
b
moethyl)iminodiacetate ester alkylating agent to include a (pro-
tected) phosphonate group. The novel bromoderivative 14 (Scheme
2) was synthesized by phosphonomethylation of N-(2-
hydroxyethyl)glycine 1,1-dimethylethyl ester 1253 with para-
formaldehyde and tri-t-butylphosphite 952 to give the amino-
alcohol 13. Mesylation of the primary alcohol with the combination
methanesulfonyl chloride/triethylamine in THF and a prompt
treatment with lithium bromide generated the desired ‘mixed’
alkylating agent 14.
The assembly of the BFCA continues with the bis-alkylation of
the nitrogen atom of a suitable protected aminoacid. Esterification
of aspartic acid 4-phenylmethyl ester 15 into its t-butyl ester de-
rivative 16 was easily performed in t-BuOAc with perchloric acid, as
reported by Taschner.54 Treatment of 16 with two molar equiva-
lents of the bromoderivative 14 in a buffered system gave com-
pound 17. As for the previous example, selective hydrogenation of
the benzyl ester located on the side chain released the corre-
sponding free carboxylic acid completing the synthesis of the
diphosphonic BFCA 18 (Scheme 3).
The synthesis of the phosphonic derivatives 3 and 4 was realized
adapting the original protocol of Rapoport,49 widely applied for the
easy assembly of DTPA derivatives and implying the double N-al-
kylation of a primary amine (representing the ‘central’ nitrogen
atom of DTPA) with two molar equivalents of an N-(b-bromoethyl)
iminodiacetate ester (representing the ‘left’ and ‘right’ wings of the
DTPA).
The synthesis of monophosphonic BFCA 11 is shown in Scheme
1. Benzylamine was bisalkylated with bromoderivative 549 in ace-
tonitrile and in the presence of micronized K2CO3 as base. Com-
pound 6 was then hydrogenated to give in good yield the
symmetrical diethylenetriaminetetraacetic acid tetra-t-butyl ester
7,50 with the central secondary amine available for further func-
tionalization. Reaction of 7 with the aldehydeeester 851 and tri-t-
butylphosphite 952 in refluxing acetonitrile produced the mono-
phosphonic derivative 10. Selective deprotection of the carboxylic
ester located on the side chain by hydrogenolysis afforded the
protected BFCA 11.

G.B. Giovenzana et al. / Tetrahedron 70 (2014) 4809e4813
4811
4. Experimental section
H
N
1. (CH2O)n, 50°C
2. P(OtBu)3 9, 20°C
COOtBu
HO
4.1. General
12
Reagent-grade chemicals and solvents were obtained from
commercial sources and directly used without further purifica-
tion. N-(2-Bromoethyl)-N-[2-(1,1-dimethylethoxy)-2-oxoethyl]
glycine 1,1-dimethylethyl ester 5,49 4-oxobutanoic acid phenyl-
methyl ester 8,51 tri-t-butylphosphite 9,52 and N-(2-hydroxyethyl)
glycine 1,1-dimethylethyl ester 1253 were synthesized as reported
in literature. TLC was performed on Merck silica gel 60 TLC plates
F254 and visualized by using UV or 1% KMnO4 in 1 M NaOH. Col-
umn chromatography was performed by using silica gel 60
(70e230 mesh) while flash chromatography was carried out on
silica gel 60 (230e400 mesh). The 1H, 13C, and 31P spectra were
recorded on a Bruker Avance 400 instrument and using CDCl3 as
solvent. Mass spectra were recorded with a ThermoFinnigan
TSQ700 triple-quadrupole instrument equipped with an electro-
spray ionization source. Analytical HPLC was performed on
a Merck KGaA apparatus with the following method: stationary
3. MsCl
Et3N, THF
PO(OtBu)2
COOtBu
PO(OtBu)2
COOtBu
N
N
HO
Br
4. LiBr
13
14
Scheme 2. Synthesis of bromoderivative 14.
COOH
BnOOC
NH2
15
tBuOAc
HClO4
phase: Lichrosorb RP-Select B 5
m
m, 250ꢁ4 mm column packed by
Merck KGaA; mobile phase: eluent A¼0.01 M KH2PO4 and 0.017 M
H3PO4 in H2O, eluent B¼MeCN, gradient elution: t¼0 min (5% B),
t¼45 min (80% B); T¼45 ꢂC; flow rate: 1 mL minꢀ1; UV detection:
210 nm.
COOtBu
BnOOC
NH2
16
4.2. N,N0-[[(Phenylmethyl)imino]di-2,1-ethanediyl]bis[N-[2-
(1,1-dimethylethoxy)-2-oxoethyl]glycine bis(1,1-
dimethylethyl) ester (6)
14
MeCN
Phosphate buffer
Micronized K2CO3 (13.82 g; 100 mmol) was added to a solution
of benzylamine (5.46 mL; 50 mmol) and N-(2-bromoethyl)-N-[2-
(1,1-dimethylethoxy)-2-oxoethyl]glycine 1,1-dimethylethyl ester
549 (37.2 g; 100 mmol) in MeCN (100 mL). The mixture was stirred
at room temperature and in a nitrogen atmosphere. After 72 h the
mixture was filtered and the solvent evaporated. The residue was
dissolved in CH2Cl2 (150 mL) and the solution was washed with
water (3ꢁ100 mL). The organic phase was dried (Na2SO4) and
evaporated. The residue was purified by column chromatography
(EtOAc/petroleum ether, 1:1) to give 6 (28.0 g; 86%) as a yellow oil.
Found: C, 64.4; H, 9.1; N, 6.4. C35H59N3O8 requires C, 64.69; H,
9.15; N, 6.47%; HPLC Rt¼28 min, 97% (area %); 1H NMR (CDCl3,
COOtBu
BnOOC
N
tBuOOC
N
N
COOtBu
(tBuO)2PO
17
PO(OtBu)2
H2, Pd/C
THF
COOtBu
N
400 MHz, 298 K): d 7.32e7.20 (5H, m, Ph), 3.63 (2H, s, CH2Ph), 3.41
HOOC
N
(8H, s, CH2COOtBu), 2.86e2.82 (4H, m, NCH2), 2.64e2.60 (4H, m,
N
t
NCH2), 1.46 (36H, s, Bu); 13C NMR (CDCl3, 100.6 MHz, 298 K):
tBuOOC
COOtBu
d
171.1 (CO), 140.0 (C), 129.2 (CH), 128.5 (CH), 127.2 (CH), 81.1 (C),
59.5 (CH2), 56.5 (CH2), 53.2 (CH2), 52.4 (CH2), 28.6 (CH3); nmax
(KBr): 3086, 3063, 2978, 2933, 1741, 1603, 1455, 1393, 1255, 1219,
1152, 738, 700; ESI/MS m/z calcd for: [C35H59N3O8þH]þ 650.87;
found: 650.65.
18
(tBuO)2PO
PO(OtBu)2
Scheme 3. Synthesis of BFCA 18.
3. Conclusions
4.3. N,N0-(Iminodi-2,1-ethanediyl)bis[N-[2-(1,1-
dimethylethoxy)-2-oxoethyl]glycine bis(1,1-dimethylethyl)
ester (7)
The preparation of two original bifunctional chelating agents
formally based on mono and diphosphonic derivatives of
diethylenetriaminepentaacetic acid, was designed and realized.
For the first time, DTPA ligands with a mixed combination of
carboxylic and phosphonic coordinating group are endowed
with an additional remote carboxylic group, devised for direct
conjugation to biomolecules or to be coupled to terminal amino
groups or lysine ε-amine during solid phase synthesis.55 The
availability of a multigram synthesis of these BFCA will pave the
way to the preparation of several conjugated derivative of the
corresponding metal complexes, exploiting the potential of
these mixed ligands in their diagnostic and therapeutic
applications.
5% Palladium on charcoal (2.85 g) was added to a solution of 6
(28.49 g; 43.8 mmol) in AcOH (10 mL; 175.4 mmol) and MeOH
(1250 mL) and the mixture was stirred under hydrogen atmosphere
at room temperature. After 2.5 h the catalyst was filtered and the
solvent was evaporated. The residue was dissolved in CH2Cl2
(150 mL) and the solution was washed with saturated solution of
NaHCO3 (3ꢁ100 mL) then with water (2ꢁ100 mL). The organic
phase was dried (Na2SO4) and evaporated to give 7 (23.48 g; 96%) as
a yellow oil. Found: C, 59.8; H, 9.4; N, 7.4. C28H53N3O8 requires C,
60.08; H, 9.54; N, 7.51; Rf¼0.24 (MeOH/EtOAc, 1:4); 1H NMR (CDCl3,
400 MHz, 298 K):
d
3.35 (8H, s, CH2COOtBu), 2.79 (4H, t, J¼6.3 Hz,

4812
G.B. Giovenzana et al. / Tetrahedron 70 (2014) 4809e4813
t
NCH2), 2.61 (4H, t, J¼6.3 Hz, NCH2), 1.37 (36H, s, Bu); 13C NMR
57.07 mmol) was stirred at 50 ꢂC. After 16 h CH2Cl2 (150 mL) was
added, the solution was dried (Na2SO4) and the solvent was re-
moved under reduced pressure. Tri-t-butylphosphite 951 (14.28 g;
57.07 mmol) was added to the residue and the mixture was stirred
overnight at room temperature, then evaporated in vacuo. The
residue was purified by flash chromatography (Et2O/CH2Cl2, 45:55)
to give 13 (5.21 g; 24%) as a pale yellow oil. Rf¼0.19 (Et2O/CH2Cl2,
(CDCl3, 100.6 MHz, 298 K):
d 171.2 (CO), 81.1 (C), 56.7 (CH2), 54.7
(CH2), 48.4 (CH2), 28.4 (CH3); nmax (KBr): 3313, 2978, 2933, 1736,
1457, 1393, 1368, 1254, 1220, 1151; ESI/MS m/z calcd for:
[C28H53N3O8þH]þ 560.75; found: 560.63.
4.4. 9-[1-[Bis(1,1-dimethylethoxy)phosphinyl]-4-oxo-4-(phe-
nyl methoxy)butyl]-6,12-bis[2-(1,1-dimethylethoxy)-2-
oxoethyl]-2,2-dimethyl-4-oxo-3-oxa-6,9,12-triazatetradecan-
14-oic acid, 1,1-dimethylethyl ester (10)
45:55); 1H NMR (CDCl3, 300.0 MHz, 298 K):
d
3.52 (2H, t, J¼4.9 Hz,
CH2OH), 3.47 (2H, s, CH2COOtBu), 2.95 (2H, d, J¼8.3 Hz, CH2P), 2.84
(2H, t, J¼4.9 Hz, NCH2), 1.45 (18H, s, POtBu), 1.41 (9H, s, COOtBu); 13
C
NMR (CDCl3, 75.4 MHz, 298 K):
d
171.1 (CO), 82.7 (d, JCP¼9.2 Hz, C),
A solution of 4-oxobutanoic acid phenylmethyl ester 8 (7.7 g;
40 mmol)51 in MeCN (50 mL) was added dropwise in 30 min to
a solution of 7 (20.15 g; 36 mmol) in MeCN (200 mL) at 80 ꢂC. The
reaction mixture was stirred for further 30 min and then cooled to
room temperature. A solution of tri-t-butylphosphite 9 (14.62 g;
40 mmol) was added dropwise over 20 min, then the mixture was
stirred at room temperature for 4 days. After evaporation of the
solvent, the residue was purified by flash chromatography (EtOAc/
petroleum ether, 3:7/1:1) to give 10 (12.08 g, 36%) as a yellow oil.
Found: C, 60.7; H, 8.9; N, 4.4; P, 3.1. C47H82N3O13P requires C, 60.82;
H, 8.91; N, 4.53; P, 3.34; Rf¼0.45 (EtOAc/n-hexane, 1:1); HPLC
Rt¼34.5 min, 98% (area %); 1H NMR (CDCl3, 400 MHz, 298 K):
81.3 (C), 59.9 (CH2), 58.8 (d, JCP¼9.2 Hz, CH2), 57.2 (d, JCP¼3.4 Hz,
CH2), 52.3 (d, JCP¼165.7, CH2), 30.5 (CH3), 28.2 (CH3); 31P NMR
(CDCl3, 121.4 MHz, 298 K):
d 19.4 (s); nmax (KBr): 3367 (br), 2973,
2931, 2877,1732,1668,1457,1387,1368,1232,1155,1061; ESI/MS m/
z calcd for: [C17H36NO6PþNa]þ: 404.21; found: 404.30.
4.7. N-[[Bis(1,1-dimethylethoxy)phosphynyl]methyl]-N-(2-
bromoethyl)glycine 1,1-dimethylethyl ester (14)
Under
a nitrogen atmosphere, methanesulfonyl chloride
(2.8 mL; 36 mmol) was slowly added to a solution of compound 13
(12.72 g; 33.3 mmol) and triethylamine (6.5 mL; 46.6 mmol) in
dried THF (500 mL) cooled at ꢀ15 ꢂC. After 1.5 h lithium bromide
(25.0 g; 288 mmol) was added and the slurry was vigorously stirred
for 16 h allowing the temperature to rise spontaneously to 20 ꢂC.
The solvent was evaporated and EtOAc (300 mL), Et2O (300 mL),
and water (200 mL) were added to the residue. The organic phase
was separated, washed with water (200 mL), brine (2ꢁ100 mL), and
dried (Na2SO4). The solvents were evaporated and the residue was
purified by flash chromatography (2-PrOH/Et2O/n-hexane,
0.01:1:1) to give 14 (12.35 g; 83%) as a colorless oil, solidifying upon
storage at ꢀ18 ꢂC. Mp 50e51 ꢂC; Rf¼0.35 (EtOAc/iPr2O, 1:4); 1H
d
7.35e7.28 (5H, m, Ph), 5.10 (2H, s, CH2Ph), 3.39 (8H, s, CH2COOtBu),
2.94e2.86 (5H, m, NCH2 and CH), 2.74e2.69 (4H, m, NCH2), 2.56
(2H, t, J¼7.2 Hz, CH2COOBn), 1.95 (1H, m, CH2), 1.79 (1H, m, CH2),
1.49 (18H, s, POtBu), 1.42 (36H, s, COOtBu); 13C NMR (CDCl3,
100.6 MHz, 298 K):
d 174.0 (CO), 170.9 (CO), 136.6 (C), 128.8 (CH),
128.6 (CH), 128.4 (CH), 82.9 (d, JCP¼12.1 Hz, C), 82.2 (d, JCP¼9.3 Hz,
C), 81.0 (C), 66.3 (CH2), 60.8 (d, JCP¼138.8 Hz, CH), 56.0 (CH2), 54.2
(CH2), 50.6 (CH2), 31.3 (d, JCP¼12.4 Hz, CH3), 31.0 (d, JCP¼3.4 Hz, C),
30.9 (d, JCP¼3.3 Hz, CH3), 28.6 (CH3), 24.3 (d, JCP¼6.5 Hz, CH2); 31P
NMR (CDCl3, 162 MHz, 298 K):
d 19.9 (s); nmax (KBr): 3066, 2979,
2934, 2874,1738, 1457,1393,1369, 1257, 1155, 978, 752, 698; ESI/MS
NMR (CDCl3, 300 MHz, 298 K): d
3.55 (2H, s, CH2COOtBu), 3.41 (2H,
m/z calcd for: [C47H82N3O13PþNa]þ 951.14; found: 951.25.
t, J¼7.5 Hz, BrCH2), 3.15 (2H, t, J¼7.5 Hz, NCH2), 3.03 (2H, d, J¼9.6 Hz,
CH2P), 1.45 (18H, s, POtBu), 1.43 (9H, s, COOtBu); 13C NMR (CDCl3,
4.5. 9-[1-[Bis(1,1-dimethylethoxy)phosphinyl]-3-
carboxypropyl]-6,12-bis[2-(1,1-dimethyl ethoxy)-2-oxoethyl]-
2,2-dimethyl-4-oxo-3-oxa-6,9,12-triazatetradecan-14-oic acid,
14-(1,1-dimethylethyl) ester (11)
75.4 MHz, 298 K):
d
171.1 (CO), 82.4 (d, JCP¼9.0 Hz, C), 81.3 (C), 58.1
(d, JCP¼15.1 Hz, CH2), 56.7 (CH2), 54.2 (CH2), 52.0 (CH2), 30.6 (d,
JCP¼4.3 Hz, CH3), 28.3 (CH3); 31P NMR (CDCl3, 121.4 MHz, 298 K):
d
17.3 (s); nmax (KBr): 2974, 2933, 1735, 1454, 1389, 1370, 1250, 1150,
994, 731; ESI/MS m/z calcd for: [C17H35BrNO5PþNa]þ: 466.13;
5% Palladium on charcoal (3.6 g) was added to a solution of 10
(12 g; 13 mmol) in THF (75 mL) and the mixture was stirred
under hydrogen atmosphere at room temperature. After 18 h the
catalyst was filtered and the solution was evaporated to give 11
(10.2 g; 94%) as a viscous yellow oil. Found: C, 57.4; H, 9.1; N, 5.1;
P, 3.7. C40H76N3O13P requires C, 57.33; H, 9.14; N, 5.01; P, 3.70;
Rf¼0.41 (EtOAc/n-hexane, 4:1); 1H NMR (CDCl3, 400 MHz, 298 K):
found: 466.00.
4.8.
L-Aspartic acid 1-(1,1-dimethylethyl) 4-(phenylmethyl)
ester (16)
A 70% aqueous solution of HClO4 (0.930 mL; 10.75 mmol) was
slowly added to a suspension of aspartic acid 4-phenylmethyl
ester 15 (2.0 g; 8.95 mmol) in t-butyl acetate (50 mL). The mix-
ture was then stirred for 36 h at room temperature. The reaction
mixture was diluted with H2O (40 mL) and after separation the
aqueous phase was extracted with EtOAc (3ꢁ15 mL). The com-
bined organic phases were washed with 10% aq Na2CO3 (50 mL)
and H2O (50 mL), then dried (Na2SO4). The solvent was evapo-
rated under reduced pressure to give 16 (1.4 g; 56%) as a colorless
oil. Rf¼0.52 (EtOAc); HPLC Rt¼14.4 min, 97% (area %); 1H NMR
d
3.42 (8H, s, CH2COOtBu), 3.27 (1H, bt, CH), 3.09e3.00 (4H, m,
NCH2), 2.92e2.83 (4H, m, NCH2), 2.72e2.65 (2H, m, CH2COOH),
1.95 (1H, m, CH2), 1.85 (1H, m, CH2), 1.51 (9H, s, POtBu), 1.50 (9H,
s, POtBu), 1.44 (36H, s, COOtBu); 13C NMR (CDCl3, 100.6 MHz,
298 K):
d
176.2 (CO), 170.8 (CO), 83.8 (d, JCP¼12.1 Hz, C), 83.1 (d,
JCP¼9.0 Hz, C), 81.4 (C), 61.5 (d, JCP¼141.8 Hz, CH), 55.6 (CH2), 52.8
(CH2), 50.3 (CH2), 34.2 (d, JCP¼14.1 Hz, CH2), 31.0 (d, JCP¼4.0 Hz,
CH3), 30.9 (d, JCP¼3.0 Hz, CH3), 28.5 (CH3), 24.0 (d, JCP¼2.0 Hz,
CH2); 31P NMR (CDCl3, 162.0 MHz, 298 K):
d
17.4 (s); nmax (KBr):
(CDCl3, 300 MHz, 298 K): d 7.42e7.23 (5H, m, Ph), 5.12 (2H, br s,
3441 (br), 2978, 2934, 1732, 1457, 1394, 1369, 1256, 1220, 1151,
981; ESI/MS m/z calcd for: [C40H76N3O13PꢀH]ꢀ 836.50; found:
836.37.
CH2Ph), 3.73 (1H, dd, J1¼7.0 Hz, J2¼4.9 Hz, CHeN), 2.81 (1H, dd,
J1¼16.5 Hz, J2¼4.9 Hz, CH2), 2.73 (1H, dd, J1¼16.8 Hz, J2¼7.0 Hz,
CH2), 2.68 (2H, br s, NH2),1.40 (9H, s, tBu); 13C NMR (CDCl3,
75.6 MHz, 298 K):
d 173.1 (C), 171.2 (C), 135.7 (C), 128.6 (CH),
4.6. N-[[Bis(1,1-dimethylethoxy)phosphynyl]methyl]-N-(2-
hydroxyethyl)glycine 1,1-dimethylethyl ester (13)
128.4 (2ꢁCH), 81.7 (C), 66.6 (CH2), 51.8 (CH), 39.0 (CH2), 28.0
(CH3); [
a
]
20 þ16.0 (c 0.57, CHCl3) (lit.56 þ10.9, c 0.57, CHCl3); nmax
D
(KBr): 3350, 2974, 2942, 1728, 1736, 1667, 1368, 1360, 1250, 1151,
750, 736, 697; ESI/MS m/z calcd for: [C15H21NO4þH]þ: 280.15;
found: 280.05.
A mixture of paraformaldehyde (1.71 g; 57.07 mmol) and N-(2-
hydroxyethyl)glycine 1,1-dimethylethyl ester 1252 (10 g;

G.B. Giovenzana et al. / Tetrahedron 70 (2014) 4809e4813
4813
12. Brechbiel, M. W. Dalton Trans. 2007, 4918e4928.
13. Anderegg, G.; Arnaud-Neu, F.; Delgado, R.; Felcman, J.; Popov, K. Pure Appl.
Chem. 2005, 77, 1445e1495.
14. Correira, J. D. G.; Paulo, A.; Raposinho, P. D.; Santos, I. Dalton Trans. 2011, 40,
6144e6167.
4.9. N,N-Bis[2-[[[2-(1,1-dimethylethoxy)-2-oxoethyl][[bis(1,1-
dimethylethoxy)phosphynyl]methyl]amino]ethyl]]- -aspartic
acid 1-(1,1-dimethylethyl) 4-(phenylmethyl) ester (17)
L
15. Lee, S.; Xie, J.; Chen, X. Chem. Rev. 2010, 110, 3087e3111.
16. Ferro-Flores, G.; Ramírez, F. de M.; Melendez-Alafort, L.; Santos-Cuevas, C. L.
A mixture of 16 (0.947 g; 3.39 mmol) and 14 (3.32 g; 7.46 mmol)
in MeCN (8 mL) and 2 M phosphate buffer (pH 8.0, 8.0 mL) was
vigorously stirred at room temperature for 24 h.49 The aqueous
layer was separated and replaced with fresh buffer (8 mL) and the
mixture was stirred for further 18 h. The organic phase was sepa-
rated and the solvent was evaporated. The residue was dissolved in
EtOAc (50 mL), washed with water (50 mL), brine (2ꢁ30 mL), and
then dried (Na2SO4). The solvent was evaporated and the crude oil
was purified by flash chromatography (2-PrOH/Et2O/petroleum
ether, 5:15:75) to give 17 (1.87 g; 55%) as a pale yellow oil. Rf¼0.35
(2-PrOH/Et2O/petroleum ether, 5:15:75); 1H NMR (CDCl3, 400 MHz,
ꢁ
Mini Rev. Med. Chem. 2010, 10, 87e97.
17. Boswell, C. A.; Brechbiel, M. W. Nucl. Med. Biol. 2007, 34, 757e778.
18. Milenic, D. E.; Brady, E. D.; Brechbiel, M. W. Nat. Rev. Drug Discovery 2004, 3,
488e498.
19. Liu, S. Adv. Drug Delivery Rev. 2008, 60, 1347e1370.
€
20. Bartholoma, M. D. Inorg. Chim. Acta 2012, 389, 36e51.
21. Liu, S.; Edwards, D. S. Bioconjugate Chem. 2001, 12, 7e34.
22. Lattuada, L.; Barge, A.; Cravotto, G.; Giovenzana, G. B.; Tei, L. Chem. Soc. Rev.
2011, 40, 3019e3049.
23. Anelli, P. L.; Fedeli, F.; Gazzotti, O.; Lattuada, L.; Lux, G.; Rebasti, F. Bioconjugate
Chem. 1999, 10, 137e140.
24. Amedio, J. C., Jr.; Van Wagenen, G., Jr.; Zavlin, G. Synth. Commun. 2000, 30,
3755e3763.
25. Anelli, P. L.; Brocchetta, M.; Lattuada, L.; Manfredi, G.; Morosini, P.; Murru, M.;
Palano, D.; Sipioni, M.; Visigalli, M. Org. Process Res. Dev. 2009, 13, 739e746.
26. Anelli, P. L.; Lattuada, L.; Lorusso, V.; Lux, G.; Morisetti, A.; Morosini, P.; Serleti,
M.; Uggeri, U. J. Med. Chem. 2004, 47, 3629e3641.
298 K):
d
7.36e7.30 (5H, m, Ph), 5.13 (1H, d, J¼12.3 Hz, CH2Ph), 5.07
(1H, d, J¼12.3 Hz, CH2Ph), 3.78 (1H, m, CH), 3.54 (4H, s, CH2COOtBu),
2.98 (4H, d, J¼9.8 Hz, CH2P), 2.83 (1H, dd, J1¼15.9 Hz, J2¼8.5 Hz,
CH2COOBn), 2.75e2.67 (8H, m, NCH2), 2.53 (1H, dd, J1¼15.9 Hz,
27. Lattuada, L.; Cappelletti, E.; De Miranda, M.; Umbelli, C. Lett. Org. Chem. 2009, 6,
J2¼6.6 Hz, CH2COOBn), 1.49 (36H, s, POtBu), 1.45 (18H, s, Bu), 1.44
t
624e629.
(9H, s, tBu); 13C NMR (CDCl3, 100.6 MHz, 298 K):
d 171.6 (CO), 171.3
28. Lattuada, L.; Demattio, S.; Vincenzi, V.; Cabella, C.; Visigalli, M.; Aime, S.;
GeninattiCrich, S.; Gianolio, E. Bioorg. Med. Chem. Lett. 2006, 16,
4111e4114.
(CO),171.1 (CO),136.3 (C), 128.9 (CH),128.7 (CH),128.5 (CH), 82.4 (d,
JCP¼10 Hz, C), 81.7 (C), 81.0 (C), 66.6 (CH2), 61.6 (CH), 56.3 (CH2),
53.1 (d, JCP¼166.0 Hz, CH2), 51.7 (CH2), 36.3 (CH2), 30.9 (d,
JCP¼3.0 Hz, CH3), 28.7 (CH3), 28.6 (CH3); 31P NMR (CDCl3, 162 MHz,
ꢀ
29. Aloj, L.; Caraco, C.; Panico, M.; Zannetti, A.; Del Vecchio, S.; Tesauro, D.; De Luca,
S.; Arra, C.; Pedone, C.; Morelli, G.; Salvatore, M. J. Nucl. Med. 2004, 45,
485e494.
30. Manzoni, L.; Belvisi, L.; Arosio, D.; Bartolomeo, M. P.; Bianchi, A.; Brioschi, C.;
Buonsanti, F.; Cabella, C.; Casagrande, C.; Civera, M.; De Matteo, M.; Fugazza, L.;
Lattuada, L.; Maisano, F.; Miragoli, L.; Neira, C.; Pilkington-Miksa, M.; Scolastico,
C. ChemMedChem 2012, 7, 1084e1093.
298 K):
d 18.8 (s); nmax (KBr): 2975, 2933, 2909, 2868, 1731, 1389,
1368, 1193, 1156, 1022, 951; ESI/MS m/z calcd for: [C49H89N3O14P2
þNa]þ: 1028.57; found: 1028.98.
31. Kim, J. H.; Lim, J. C.; Yun, K. C.; Choi, S. J.; Hong, Y. D. J. Labelled Compd. Ra-
diopharm. 2012, 55, 10e17.
32. Atkins, K. M.; Martínez, F. M.; Nazemi, A.; Scholl, T. J.; Gillies, E. R. Can. J. Chem.
2011, 89, 47e56.
33. Lashley, M. R.; Niedzinski, E. J.; Rogers, J. M.; Denison, M. S.; Nantz, M. H. Bioorg.
4.10. N,N-Bis[2-[[[2-(1,1-dimethylethoxy)-2-oxoethyl]
[[bis(1,1-dimethylethoxy)phosphynyl]methyl]amino]ethyl]]-
aspartic acid 1-(1,1-dimethylethyl) ester (18)
L-
Med. Chem. 2002, 10, 4075e4082.
34. Nair, S. A.; Kolodziej, A. F.; Bhole, G.; Greefield, M. T.; McMurry, T. J.; Caravan, P.
Angew. Chem., Int. Ed. 2008, 47, 4918e4921.
5% Palladium on charcoal (0.75 g) was added to a solution of 17
(1.87 g; 1.86 mmol) in THF (80 mL) and the mixture was stirred
under a hydrogen atmosphere at room temperature. After 1.5 h the
catalyst was filtered and the solution was evaporated to give 18
(1.57 g; 92%) as a yellowish oil. Rf¼0.14 (2-PrOH/Et2O/petroleum
ꢁ
ꢂ
ꢁ
35. Lebduskova, P.; Hermann, P.; Helm, L.; Toth, E.; Kotek, J.; Binnemans, K.;
ꢁ
ꢂ
Rudovsky, J.; Lukes, I.; Merbach, A. E. Dalton Trans. 2007, 493e501.
36. Aime, S.; Botta, M.; Frullano, L.; Geninatti Crich, S.; Giovenzana, G.; Pa-
gliarin, R.; Palmisano, G.; Riccardi Sirtori, F.; Sisti, M. J. Med. Chem. 2000,
43, 4017e4024.
ꢁ
ꢂ
37. Rudovsky, J.; Hermann, P.; Botta, M.; Aime, S.; Lukes, I. Chem. Commun. 2005,
2390e2392.
ether, 5:15:75); 1H NMR (CDCl3, 400 MHz, 298 K):
d 4.13 (1H, dd,
J1¼11.5 Hz, J2¼4.2 Hz, CH), 3.55 (2H, d, J¼17.7 Hz, CH2COOtBu), 3.50
(2H, d, J¼17.7 Hz, CH2COOtBu), 3.10e2.93 (4H, m, CH2P), 2.90e2.76
(9H, m, NCH2 and CH2COOH), 2.65 (1H, dd, J1¼15.8 Hz, J2¼4.1 Hz),
38. Aime, S.; Botta, M.; Cravotto, G.; Frullano, L.; Giovenzana, G. B.; Geninatti Crich,
S.; Palmisano, G.; Sisti, M. Helv. Chim. Acta 2005, 88, 588e603.
39. Aime, S.; Cravotto, G.; Geninatti Crich, S.; Giovenzana, G. B.; Ferrari, M.;
Palmisano, G.; Sisti, M. Tetrahedron Lett. 2002, 43, 783e786.
40. Wermuth, C. G. The Practice of Medicinal Chemistry; Academic: Oxford, UK,
2011; pp 303e310.
t
t
t
1.49 (36H, d, J¼2.0 Hz, Bu), 1.46 (9H, s, Bu), 1.45 (18H, s, Bu); 13C
NMR (CDCl3, 100.6 MHz, 298 K): d 172.7 (CO), 171.8 (CO), 170.8 (CO),
ꢁ ꢁ
41. Lazar, I.; Hrncir, D. C.; Kim, W.-D.; Kiefer, G. E.; Sherry, A. D. Inorg. Chem. 1992,
83.0 (C), 82.2 (C), 81.3 (C), 61.4 (CH), 56.7 (d, JCP¼4.0 Hz, CH2), 55.6
(d, JCP¼11.1 Hz, CH2), 53.1 (d, JCP¼163.1 Hz, CH2), 50.9 (CH2), 35.8
(CH2), 30.8 (CH3), 28.7 (CH3), 28.4 (CH3); 31P NMR (CDCl3, 162 MHz,
31, 4422e4424.
42. Aime, S.; Botta, M.; Geninatti Crich, S.; Giovenzana, G.; Pagliarin, R.; Sisti, M.;
Terreno, E. Magn. Reson. Chem. 1998, 36, S200eS208.
43. Kan, A. T.; Oddo, J. E.; Tomson, M. B. Langmuir 1994, 10, 1450e1455.
298 K):
d 19.0; nmax (KBr): 3432 (br), 2976, 2934, 1733, 1456, 1392,
ꢂ
ꢁ
44. Kotek, J.; Lebduskova, P.; Hermann, P.; Vander Elst, L.; Muller, R. N.; Geraldes, C.
ꢂ
F. G. C.; Maschmeyer, T.; Lukes, I.; Peters, J. A. Chem.dEur. J. 2003, 9,
1369, 1275, 1221, 1153, 951; ESI/MS m/z calcd for:
5899e5915.
[C42H83N3O14P2þH]þ 916.54; found: 916.65.
ꢁ
ꢁ
€
ꢂ
45. Kotek, J.; Kalman, F. K.; Hermann, P.; Brucher, E.; Binnemans, K.; Lukes, I. Eur. J.
Inorg. Chem. 2006, 1976e1986.
46. Li, X.; Zhang, S.; Zhao, P.; Kovacs, Z.; Sherry, A. D. Inorg. Chem. 2001, 40,
6572e6579.
References and notes
47. Ouadi, A.; Bultel, K.; de France-Robert, A.; Loussouarn, A.; Morandeau, L.;
Gestin, J.-F. Tetrahedron Lett. 2004, 45, 1395e1397.
48. Loussouarn, A.; Duflos, M.; Benoist, E.; Chatal, J.-F.; Le Baut, G.; Gestin, J.-F. J.
Chem. Soc., Perkin Trans. 1 1998, 237e241.
49. Williams, M. A.; Rapoport, H. J. Org. Chem. 1993, 58, 1151e1158.
50. Quici, S.; Cavazzini, M.; Marzanni, G.; Accorsi, G.; Armaroli, N.; Ventura, B.;
Barigelletti, F. Inorg. Chem. 2005, 44, 529e537.
51. Couffignal, R.; Dardoize, F.; Goasdoue, C.; Goasdoue, N.; Laborit, H. M.; Topall, G.
Ann. Pharm. Fr. 1990, 48, 321e325.
1. Gaynor, D.; Griffith, D. M. Dalton Trans. 2012, 41, 13239e13257.
2. Biomedical Imaging. The Chemistry of Labels, Probes and Contrast Agents; Brad-
dock, M., Ed.; RSC: Cambridge, UK, 2012.
3. Bioinorganic Medicinal Chemistry; Alessio, E., Ed.; Wiley-VCH: Weinheim, Ger-
many, 2011.
4. Geraldes, C. F. G. C.; Laurent, S. Contrast Media Mol. Imaging 2009, 4, 1e23.
5. Contrast Agents I. Magnetic Resonance Imaging. Topics in Current Chemistry;
Krause, W., Ed.; Springer: Berlin, Germany, 2002; Vol. 221.
6. The Chemistry of Contrast Agents in Medical Magnetic Resonance Imaging; Mer-
52. Cox, J. R.; Newton, M. G. J. Org. Chem. 1969, 34, 2600e2605.
53. Breipohl, G.; Will, D. W.; Peyman, A.; Uhlmann, E. Tetrahedron 1997, 53,
14671e14686.
ꢁ
ꢁ
bach, A., Toth, E., Eds.; John Wiley and Sons Ltd: Chichester, UK, 2001.
7. Ramogida, C. F.; Orvig, C. Chem. Commun. 2013, 4720e4739.
8. Bhattacharyya, S.; Dixit, M. Dalton Trans. 2011, 40, 6112e6128.
9. Wadas, T. J.; Wong, E. H.; Weisman, G. R.; Anderson, C. J. Chem. Rev. 2010, 110,
2858e2902.
54. Taschner, E.; Chimiak, A.; Bator, B.; Sokolowska, T. Liebigs Ann. Chem. 1961, 646,
134e136.
€
55. Dirscherl, G.; Konig, B. Eur. J. Org. Chem. 2008, 597e634.
10. Liu, S. Chem. Soc. Rev. 2004, 33, 445e461.
11. Cutler, C. S.; Hennkens, H. M.; Sisay, N.; Huclier-Markai, S.; Jurisson, S. S. Chem.
56. Foley, D.; Pieri, M.; Pettecrew, R.; Price, R.; Miles, S.; Lam, H. K.; Bailey, P.;
Meredith, D. Org. Biomol. Chem. 2009, 7, 3652e3656.
Rev. 2013, 113, 858e883.