Synthesis of a bifunctional monophosphinate DOTA derivative having a free carboxylate group in the phosphorus side chain

Article abstract of DOI:10.1055/s-2008-1072571

A new bifunctional cyclen-based ligand, 3-[hydroxy({4,7,10-tris[(tert- butoxycarbonyl)methyl]-1,4,7,10-tetraazacyclododecan-1-yl}methyl)phosphoryl] propanoic acid, was synthesized by alkylation of tri-tert-butyl 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate (t-Bu₃DO3A) by ethyl 3-{ethoxy[(mesyloxy)methyl]phosphoryl}propanoate [MsOCH₂P(O)(OEt) CH₂CH₂CO₂Et]. The ethyl carboxylate group in the side chain was selectively deprotected to obtain the esterified bifunctional ligand. More efficient syntheses of some phosphinopropanoic acid derivatives were devised and the phosphorus alkylation reagent was prepared starting from hypophosphorus acid or its salt. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1072571

PAPER
1431
Synthesis of a Bifunctional Monophosphinate DOTA Derivative Having a Free
Carboxylate Group in the Phosphorus Side Chain
Bifunctional
M
a
onophosph
v
inate
D
OTA
e
Derivativel Řezanka, Vojtěch Kubíček, Petr Hermann,* Ivan Lukeš
Department of Inorganic Chemistry, Universita Karlova (Charles University), Hlavova 2030, 128 40 Prague 2, Czech Republic
Fax +420(2)21951253; E-mail: petrh@natur.cuni.cz
Received 25 October 2007; revised 4 February 2008
O
Abstract: A new bifunctional cyclen-based ligand, 3-[hy-
droxy({4,7,10-tris[(tert-butoxycarbonyl)methyl]-1,4,7,10-tetraaza-
cyclododecan-1-yl}methyl)phosphoryl]propanoic acid, was synthe-
sized by alkylation of tri-tert-butyl 1,4,7,10-tetraazacyclododecane-
1,4,7-triacetate (t-Bu3DO3A) by ethyl 3-{ethoxy[(mesyloxy)meth-
yl]phosphoryl}propanoate [MsOCH2P(O)(OEt)CH2CH2CO2Et]. The
ethyl carboxylate group in the side chain was selectively deprotect-
ed to obtain the esterified bifunctional ligand. More efficient syn-
theses of some phosphinopropanoic acid derivatives were devised
and the phosphorus alkylation reagent was prepared starting from
hypophosphorus acid or its salt.
HO2C
HO2C
R
HO2C
HO2C
P
R
OH
N
N
N
N
N
N
N
N
CO2H
CO2H
R = CH2CO2H DOTA (1)
R = OH, H, alkyl, aryl
R = CH2CH2CO2H
R = H
DO3A
2
Figure 1
ilar to those of the DOTA complexes.8,9 As a bifunctional
ligand, the 4-aminobenzyl5 derivative was prepared and
its isothiocyanate was conjugated to polyamidoamine
(PAMAM) dendrimers to obtain macromolecular MRI
contrast agents (as the Gd3+ complexes).10 However, such
amino/isothiocyanate derivatives are not suitable for pep-
tide synthesis (e.g., for conjugation to oligopeptides). Bio-
distribution data for the complexes (90Y, 111In) of the
monophosphinic acid ligands in rats are comparable with
those of DOTA 1.11
Key words: macrocycles, MRI contrast agents, phosphinate
ligands, phosphorus, radiopharmaceuticals
Complexes of macrocyclic ligands with metal ions have
been investigated as contrast agents for magnetic reso-
nance imaging (MRI)1 or as radiopharmaceuticals.2 The
most common macrocyclic ligand, which is also utilized
in clinical practice, is 1,4,7,10-tetraazacyclododecane-
1,4,7,10-tetraacetic acid (DOTA, 1, Figure 1). To im-
prove chemical and pharmacological properties of the In this paper, we describe the synthesis of a DOTA mono-
complexes many DOTA derivatives have been synthe- phosphinic acid derivative 12 which contains tert-butyl-
sized.1,3 For the next generation of MRI contrast agents protected azacycle carboxylates and a free carboxylate
and for most of radiopharmaceuticals, targeting a specific group in the phosphorus side chain that can be exploited
organ or tissue is desired. Thus, the ligands or complexes for oligopeptide modifications. The fully deprotected
are conjugated to biovectors such as oligopeptides or ligand9 2 (Figure 1) was also prepared.
monoclonal antibodies. For such applications, the useable
The most common method for introduction of the N–
ligand derivatives are called bifunctional and must con-
CH2–P moiety is the Mannich reaction of a primary/sec-
tain another suitable functional group (e.g., isothiocyan-
ondary amine, formaldehyde, and an organophosphorus
ate, carboxylate, or amino groups) to achieve the
compound containing a P–H bond. If the reaction is per-
conjugation. In the case of DOTA, its skeleton can be
formed under anhydrous conditions and with heating, the
modified on the macrocycle itself or on the pendant arms.
product mixture is generally difficult to separate and
Currently, the most common modification of DOTA is the
yields are variable, but low; the simplest (organo)phos-
formation of an amide bond on one pendant acetate arm.
phorus esters (e.g., dialkyl phosphites) are used for the re-
Another possible modification of the DOTA skeleton is action in organic solvents. The Mannich reaction in water
at highly acidic pH (generally ~20% aq HCl) and with
heating is more efficient. Unfortunately, these aqueous
the replacement of the pendant acetate arm(s) by meth-
ylphosphonic/phosphinic acid group(s). We have synthe-
sized a series of monophosphonic/phosphinic acid conditions are not compatible with the esters used as pro-
derivatives of DOTA bearing different substituents on the tecting groups of the acid functions. To obtain the target
phosphorus atom (Figure 1) and showed that the ligands bifunctional ligand, we decided to use an alkylation ap-
and their complexes show some improved properties as proach. In the chemistry of azacycles, alkylation with or-
possible MRI contrast agents.4–7 In addition, the thermo- ganophosphorus reagents of the X–CH2–P type
dynamic and kinetic properties of the complexes are sim- (X = halogen, OMs, OTf) has not been used frequently.12
The phosphinic acid ester used for alkylation was pre-
pared from hypophosphorus acid (Scheme 1). Initially, re-
SYNTHESIS 2008, No. 9, pp 1431–1435
Advanced online publication: 27.03.2008
DOI: 10.1055/s-2008-1072571; Art ID: T16707SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.
x
x
.2
0
0
8
action of hypophosphorus acid with excess
ethoxytrimethylsilane gave the known,13 unstable ethyl

1432
P. Řezanka et al.
PAPER
O
P
O
P
EtO
H
CO2t-Bu
HO
H
CO2H
EtOH–HCl (1:1)
CO2t-Bu
EtOSiMe3
EtOP(O)H2
H3PO2
reflux, 16 h
quant.
THF, r.t.
DIPEA, r.t., 6 d
71% (2 steps)
3
4
5
concd HCl, 105 °C
3.5 d, 76%
(CH2O)n
CO2H
O
P
O
O
EtO
MsOCH2
CO2Et
EtO
CO2Et
HO
MsCl, DIPEA
EtOH, DCC
P
P
CH2Cl2, 16 h, r.t.
97%
4.5 d, r.t., 48%
HOCH2
HOCH2
6
8
7
Scheme 1
Scheme 2
1.
CO2t-Bu
CH2Cl2, r.t., 12 h HO
O
CO2t-Bu
NH(SiMe3)2
110 °C, 12 h
(Me3SiO)2PH
NH4(H2PO2)
P
2. EtOH
(86%, 2 steps)
H
9
hypophosphinate (3). It was immediately used in nucleo- decided to test aqueous conditions for addition of the P–H
philic addition to tert-butyl acrylate to give phosphinopro- bond to formaldehyde. The ester 4 was hydrolyzed to the
panoate 4. Using a strong base (NaOEt)14 accelerated the known acid 59,17 and this free acid was reacted with form-
addition, but it also generated a much more complicated aldehyde in aqueous hydrochloric acid to give hydroxy-
reaction mixture. tert-Butyl acrylate was used to facilitate methyl acid 6. Here, the phosphinopropanoic acid 5 was
aqueous treatment to remove the unreacted H3PO2 (the prepared by a much simple procedure than those previous-
major phosphorus-containing impurity); the correspond- ly published (see also below).9,17 Again, the acid 6 is un-
ing phosphinopropanoate diethyl ester is partially soluble stable at room temperature as it slowly forms the lactone
in water. The ester 4 should be stored for extended periods and/or polymerizes with formation of hydroxymethyl-
at 4 °C; it slowly decomposes (over a number of weeks) at carboxylate esters (giving multiple peaks in the 31P NMR
room temperature. After extractive treatment, the only spectrum in a not fully reproducible way); the acid is ob-
byproduct detected was the bis(alkylated) product (t- tained quantitatively after hydrolysis of such a mixture in
BuO2CCH2CH2)2P(O)(OEt) (31P NMR: d = 55.1, always refluxing ethanol–aqueous concentrated hydrochloric
<5%) arising from addition to both P–H bonds of 3 acid 1:1. Acid 6 was esterified in anhydrous ethanol in
(despite using excess of the hypophosphinate ester). The presence of N,N¢-dicyclohexylcarbodiimide to give dieth-
bis(alkylated) byproduct was fully eliminated by silica gel yl ester 7. Mesylation with mesyl chloride gave the target
chromatography.
organophosphorus reagent 8. The mesyl derivative 8
should be used directly in the next step or can be stored
below 4 °C. The multistep procedure was easily scaled up
to a gram scale.
In the recent years, the chemistry of the esters of hypo-
phosphorus acid has been greatly developed by Mont-
champ et al.15 For the production of (RO)P(O)H2 esters,
they used other silicon reagents, (R1O)4–nSiR2 (n = 0–2; We realized that acid 5 has to be used in the synthetic se-
n
R1, R2 = alkyl, aryl), under analogous mild conditions.16 quence, hence we looked for an easier and faster proce-
Their silicon reagents gave the hypophosphorus acid es- dure to give the pure intermediate 5. Such a route
ters similarly in almost quantitative yields, but the remov- (Scheme 2) can be based on the very easy addition of
al of excess organosilicon reagents and/or reaction bis(trimethylsiloxy)phoshine18 to an activated double
mixture development can be problematic due to the for- bond. The siloxyphosphine was easily prepared from am-
mation of silica-like polymers. Our procedure is much monium hypophosphite and its addition to tert-butyl acry-
easier to perform as excess monoalkoxytrimethylsilane late (used again to facilitate the aqueous workup) led to a
and hydroxytrimethylsilane (the reaction product) is easi- high yield of a pure monoester 9.18b Using an excess of the
ly removed and we obtained a reasonable yield of ester 4. very reactive siloxyphosphine completely eliminated the
In addition, preliminary results with other alkoxytrimeth- formation of any bis(alkylated) byproduct. The ester 9
ylsilanes (ROSiMe3, R = alkyl) confirmed the generality was quantitatively hydrolyzed to acid 5.
of our reaction for the preparation of esters of hypophos-
phorus acid.
Synthesis of the macrocyclic derivatives 12 and 2 fol-
lowed Scheme 3. The reaction of excess mesylate 8 with
The reaction of ester 4 with paraformaldehyde under vari- t-Bu3DO3A 10 gave quantitative conversion of the mac-
able reaction conditions (various organic solvents, various rocyclic reagent into intermediate 11. The full ester 11
bases/catalysts etc.) did not lead to a reasonable yield of was not obtained in its pure form as it partially decom-
the hydroxymethylated derivative and resulted in the for- posed during chromatography (hydrolysis of the ethyl
mation of complicated reaction mixtures. Thus, we phoshinate ester bond) and, thus, the crude intermediate
Synthesis 2008, No. 9, 1431–1435 © Thieme Stuttgart · New York

PAPER
Bifunctional Monophosphinate DOTA Derivative
1433
O
CO2Et
t-BuO2C
t-BuO2C
t-BuO2C
t-BuO2C
P
N
N
HN
N
N
N
N
N
OEt
8 (2 equiv)
K2CO3, MeCN, reflux, 4 d
CO2t-Bu
CO2t-Bu
11
t-Bu3DO3A (10)
MeOH–H2O, r.t., 16 h
49% (2 steps)
Cs2CO3
O
O
CO2H
CO2H
HO2C
P
t-BuO2C
N
P
N
N
N
N
OH
N
N
OH
TFA–CH2Cl2 (4:1)
r.t., 22 h
quant.
N
HO2C
CO2H
t-BuO2C
CO2t-Bu
2
12
Scheme 3
CDCl3 (d = 77.0); D2O: t-BuOH (d = 32.8). External reference for
31P NMR: 85% H3PO4 (d = 0.0). LR-MS (ESI) spectra were record-
ed on a Bruker Esquire 3000 with an ion-trap detector in positive or
negative modes; HRMS (FAB) spectra were measured in positive
mode on a ZAB-EQ (VG Analytical) in thioglycerol–glycerol ma-
trix.
11 was directly used in the next step. Removal of ethyl
carboxylate ester together with very the labile ethyl phos-
phinate ester group was carried out with cesium carbonate
according to a known procedure.19 The protected bifunc-
tional ligand 12 was obtained after simple chromatogra-
phy. The ethyl phosphinate group in 11 is rather labile,
however, unprotected phosphinates/phosphonates can be
used in, e.g., peptide synthesis.20 To test the stability of the
protected ligand during a possible deprotection reaction,
the acid 2 was prepared from ester 12. Deprotection with
trifluoroacetic acid was smooth giving a quantitative yield
of 2. The physical data of 2 prepared here were identical
to those of ligand 2 previously prepared by the aqueous
Mannich reaction between DO3A, formaldehyde, and
acid 5.9 The multistep synthesis of ligand 2 presented here
is not so inconvenient as it may appear, as the previously
published procedure includes a tedious, time-consuming
purification of ligand 2.9
Safety note: (Me3SiO)2PH is pyrophoric.
Ethyl Hypophosphinate (3)
Dry crystalline H3PO2 (10.0 g, 152 mmol), anhyd THF (10 mL), and
EtOSiMe3 (50 mL, 320 mmol) were stirred at r.t. for 1 h to form 3
in ca. 75% yield [based on 31P NMR: d = 15.8 (t, 1JPH = 571 Hz)].
The mixture was used in the next reaction step without purification
due to the instability of 3.
tert-Butyl 3-[Ethoxy(hydro)phosphoryl]propanoate (4)
To the mixture containing 3 from the previous step, tert-butyl acry-
late (15.9 g, 124 mmol) was added and the mixture was stirred at r.t.
Then DIPEA (19.6 g, 152 mmol) was added dropwise to the mixture
over 2 h. When the addition was complete, the mixture was stirred
for a further 6 d. The mixture was diluted with CH2Cl2 (100 mL) and
successively washed with H2O (200 mL), 1.5% aq HCl (200 mL),
and sat. aq NaHCO3 (200 mL). The organic phase was dried (anhyd
Na2SO4) and evaporated under reduced pressure. After chromatog-
raphy (silica gel, EtOAc), ester 4 was obtained as a colorless oil
(16.7 g, 71% over 2 steps).
1H NMR (CDCl3): d = 1.38 (t, 3JHH = 7.3 Hz, 3 H,CH2CH3), 1.46 [s,
9 H, C(CH3)3], 2.04 (dtd, 2JHP = 15.3 Hz, 3JHH = 7.9 Hz, 3JHH = 1.8
Hz, 2 H, PCH2), 2.57 (m, 2 H, CH2CO2), 4.09 (m, 1 H, CH2CH3),
4.19 (m, 1 H, CH2CH3), 7.21 (dt, 1JHP = 545 Hz, 3JHH = 1.8 Hz, 1 H,
PH).
In conclusion, we describe a procedure for the preparation
of a protected bifunctional DOTA derivative that is poten-
tially useful in peptide syntheses. The hydrophilic phos-
phinic acid group should conveniently change
physicochemical and pharmacological properties of pos-
sible conjugates similarly to the other free monophosphi-
nate DOTA analogues, their conjugates and complexes as
we have shown before. The syntheses of some organo-
phosphorus intermediates were simplified and scaled up.
13C NMR (CDCl3): d = 16.1 (d, JCP = 6.1 Hz, CH2CH3), 24.0 (d,
3
2
1JCP = 96.1 Hz, PCH2), 27.0 (d, JCP = 3.1 Hz, CH2CO2), 27.9 [s,
Paraformaldehyde was obtained by filtration of aged formaldehyde
solns and dried over P2O5. t-Bu3DO3A·HBr, 10·HBr, was synthe-
sized according to the published procedure.21 Other chemicals were
available from commercial sources. Crystalline H3PO2 was ob-
tained by a careful vacuum evaporation (<30 °C bath temp.) of a
commercial 50% aq soln followed by drying in a vacuum desiccator
over P2O5 at 5 °C. Solvents were dried by established procedures.22
TLC was performed on silica gel sheets (Merck TLC aluminum
sheets silica gel 60 F254) and column chromatography was per-
formed on silica gel (60–230 mesh, Merck). Elemental analyses
were arranged at the Institute of Macromolecular Chemistry, Acad-
emy of Sciences of the Czech Republic. NMR spectra were record-
ed on a Varian Unity Inova 400 at 399.95 MHz for 1H, 100.58 MHz
C(CH3)3], 62.4 (d, 2JCP = 6.5 Hz, CH2CH3), 81.2 [s, C(CH3)3], 171.0
(d, 3JCP = 13.0 Hz, CO2).
31P NMR (CDCl3): d = 37.4 (dm, 1JPH = 545 Hz).
3-[Hydro(hydroxy)phosphoryl]propanoic Acid (5)
Ester 4 (16.7 g, 75 mmol), EtOH (150 mL), and concd aq HCl (150
mL) were refluxed for 16 h. After evaporation and further co-evap-
oration with H2O under reduced pressure (3 ×), acid 5 was obtain as
a colorless oil (10.4 g, 100%). If ester 4 was used without column
chromatography, the product of this reaction, acid 5, also contained
a byproduct, the bis(derivative) [HO2CCH2CH2)2PO2H (31P NMR:
d = 59.1, <5%).
1
for 13C and 161.9 MHz for 31P. Internal references for H NMR:
CDCl3: TMS (d = 0.00); D2O: t-BuOH (d = 1.25); 13C NMR: CDCl3:
Synthesis 2008, No. 9, 1431–1435 © Thieme Stuttgart · New York

1434
P. Řezanka et al.
PAPER
2
3
1H NMR (D2O): d = 2.07 (dtd, JHP = 15.3 Hz, JHH = 7.9 Hz,
3JHH = 1.8 Hz, 2 H, PCH2), 2.68 (dt, 3JHP = 15.3 Hz, 3JHH = 7.9 Hz,
2 H, CH2CO2), 7.14 (dt, 1JHP = 557 Hz, 3JHH = 1.8 Hz, 1 H, PH).
2.68 (m, 2 H, CH2CH2CO2), 3.14 (s, 3 H, SO3CH3), 4.17 (q,
3JHH = 7.2 Hz, 2 H, CO2CH2CH3), 4.18 (m, 2 H, PCH2O), 4.44 (m,
2 H, POCH2CH3).
1
3
13C NMR (D2O): d = 30.4 (d, JCP = 91.6 Hz, PCH2), 32.0 (s,
13C NMR (CDCl3): d = 14.1 (s, CO2CH2CH3), 16.6 (d, JCP = 6.0
CH2CO2), 182.6 (d, 3JCP = 13.0 Hz, CO2).
31P NMR (D2O): d = 38.2 (dquint, 1JPH = 557 Hz, 2JPH = 3JPH = 15.3
Hz, POCH2CH3), 21.7 (d, JCP = 99.6 Hz, PCH2CH2), 26.1 (d,
1
2JCP = 4.6 Hz, CH2CH2CO2), 37.7 (s, SO3CH3), 61.1 (s,
2
CO2CH2CH3), 61.8 (d, JCP = 6.5 Hz, POCH2CH3), 62.2 (d,
Hz).
1JCP = 104.5 Hz, PCH2O), 171.8 (d, 3JCP = 14.1 Hz, CO2).
31P NMR (CDCl3): d = 44.9 (m).
MS (ESI): m/z = 138.9 [M + H]+.
3-[Hydroxy(hydroxymethyl)phosphoryl]propanoic Acid (6)
Acid 5 (10.4 g, 75 mmol), H2O (160 mL), and concd aq HCl (80
mL) were refluxed 30 min. To the refluxing mixture, paraformalde-
hyde (7.2 g, 240 mmol) was added and mixture was stirred at 105
°C for 16 h in a bath. A further portion of paraformaldehyde (7.2 g,
240 mmol) was added and mixture was stirred at 105 °C for 3 d. Af-
ter evaporation and further co-evaporation with H2O under reduced
pressure (3 ×) and chromatography (Dowex 1×8, 200–400 mesh,
H2O then 3% aq HCl) followed by further chromatography (silica
gel, i-PrOH–concd aq NH3, 3:2), the pure acid 6 was obtained as a
colorless oil (9.6 g, 76%).
MS (ESI): m/z = 302.9 [M]+.
tert-Butyl 3-[Hydro(hydroxy)phosphoryl]propanoate (9)
Under an argon atmosphere, dry NH4H2PO2 (15.0 g, 178 mmol) was
suspended in (Me3Si)2NH (90 mL) and the mixture was heated at
110 °C (bath temperature) under a small flow of argon overnight.
The mixture containing pure (Me3SiO)2PH was cooled to r.t. and
anhyd CH2Cl2 (150 mL) was added. The soln of tert-butyl acrylate
(10.5 g, 82 mmol) in anhyd CH2Cl2 (60 mL) was added dropwise
and the mixture was stirred at r.t. overnight. The resulting soln was
added dropwise to EtOH (900 mL) to hydrolyze the silyl com-
pounds. The volatiles were removed under reduced pressure and the
crude product was dissolved in CHCl3 (300 mL) and washed with
1% aq HCl (300 mL). The aqueous layer was re-extracted with
CHCl3 (2 × 300 mL) and the combined organic layers were dried
(anhyd Na2SO4). The volatiles were removed under reduced pres-
sure to give the pure ester 9 (13.5 g, 86%) as a colorless oil. The es-
ter 9 was hydrolyzed under identical conditions as ester 4 to give
acid 5 in a quantitative yield.
1H NMR (D2O): d = 1.85 (m, 2 H, PCH2), 2.44 (m, 2 H, CH2CO2),
3.63 (m, 2 H, CH2OH), 7.22 (br s, 1 H, CH2OH).
1
13C NMR (D2O): d = 26.7 (d, JCP = 90.0 Hz, PCH2), 32.0 (s,
CH2CO2), 62.3 (d, 1JCP = 108.7 Hz, CH2OH), 183.6 (d, 3JCP = 16.0
Hz, CO2).
31P NMR (D2O): d = 43.0 (m).
MS (ESI): m/z = 168.8 [M + H]+.
1H NMR (CDCl3): d = 1.45 [s, 9 H, C(CH3)3], 2.02 (dt, 3JHH = 8.0
Hz, 2JPH = 16.0 Hz, 2 H, CH2P), 2.55 (dt, 3JPH = 16.0 Hz, 3JHH = 8.0
Hz, 2 H, CH2CO2), 7.18 (d, 1JPH = 557.0 Hz, 1 H, PH).
Ethyl 3-[Ethoxy(hydroxymethyl)phosphoryl]propanoate (7)
To the soln of acid 6 (9.6 g, 57 mmol) in anhyd EtOH (250 mL) was
added DCC (13.5 g, 65 mmol) and mixture was stirred at r.t. Further
portions of DCC were added after 16 h (20 g, 97 mmol) and 3 d (20
g, 97 mmol). Finally, mixture was stirred at r.t. for 4 d. H2O (100
mL) was added and the mixture was filtered. The filtrate was evap-
orated under reduced pressure and chromatography (silica gel,
CH2Cl2–MeOH, 96:4) gave the ester 7 (6.2 g, 48%) as a colorless
oil.
13C{1H} NMR (CDCl3): d = 24.6 (d, 1JPC = 95.4 Hz, CH2P), 27.5 (d,
2JPC = 84.7 Hz, CH2CO2), 28.0 [s, C(CH3)3)], 81.3 [s, C(CH3)3)],
171.1 (d, 3JPC = 13.4 Hz, CO2).
31P NMR (CDCl3):
d
=
35.5 (dquint, JPH = 557.0 Hz,
1
2JPH = 3JPH = 16.0 Hz).
Tri-tert-butyl 10-({Ethoxy[2-(ethoxycarbonyl)ethyl]phosphor-
yl}methyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetate (11)
Compound 8 (8.1 g, 27 mmol), anhyd MeCN (500 mL), macrocy-
clic ester 10·HBr (7.8 g, 13.5 mmol) and K2CO3 (annealed, 22 g,
159 mmol) were refluxed for 4 d to form ester 11 in 100% yield (by
31P NMR: d = 54.5). The mixture was filtered, the filtrate was evap-
orated under reduced pressure and used directly in the next reaction
step without purification due to the instability of the ester 11.
3
1H NMR (CDCl3): d = 1.27 (t, JHH = 7.2 Hz, 3 H, CO2CH2CH3),
1.33 (t, 3JHH = 7.0 Hz, 3 H, POCH2CH3), 2.15 (m, 2 H, PCH2CH2),
2.67 (m, 2 H, CH2CH2CO2), 3.89 (m, 2 H, PCH2OH), 4.12 (m, 2 H,
POCH2CH3), 4.16 (q, 3JHH = 7.2 Hz, 2 H, CO2CH2CH3), 4.64 (br s,
1 H, CH2OH).
13C NMR (CDCl3): d = 14.1 (s, CO2CH2CH3), 16.6 (d, JCP = 5.3
3
1
Hz, POCH2CH3), 21.0 (d, JCP = 90.7 Hz, PCH2CH2), 26.5 (d,
2JCP = 2.6 Hz, CH2CH2CO2), 58.7 (d, JCP = 106.5 Hz, PCH2OH),
1
2
3-[Hydroxy({4,7,10-tris[(tert-butoxycarbonyl)methyl]-1,4,7,10-
tetraazacyclododecan-1-yl}methyl)phosphoryl]propanoic Acid
(12)
Above mixture containing compound 11, MeOH (200 mL), H2O
(100 mL), and Cs2CO3 (7.1 g, 22 mmol) was stirred at r.t. for 16 h.
The mixture was evaporated under reduced pressure and purified by
chromatography (silica gel, i-PrOH–MeOH–concd aq NH3,
15:10:2) to obtain the acid 12 as a yellow oil (4.3 g, 49% over 2
steps based on 10·HBr).
1H NMR (CDCl3): d = 1.37 [s, 18 H, C(CH3)3], 1.39 [s, 9 H,
C(CH3)3], 1.80–4.20 (m, 28 H, ring CH2, NCH2CO2,
NCH2PCH2CH2CO2).
13C NMR (CDCl3): d = 25.1 (d, 1JCP = 88.1 Hz, PCH2CH2), 27.8 [s,
C(CH3)3], 27.9 (s, 2 × C(CH3)3], 29.7 (d, 2JCP = 4.6 Hz, PCH2CH2),
48.8 (s, 2 × ring CH2), 51.9 (s, 6 × ring CH2), 52.9 (d, 1JCP = 103.8
Hz, NCH2P), 55.5 (s, 2 × NCH2CO2), 55.6 (s, NCH2CO2), 81.5 [s,
C(CH3)3], 81.8 [s, 2 × C(CH3)3], 172.1 (s, CH2CH2CO2), 175.9 (s, 3
× NCH2CO2).
61.1 (d, JCP = 7.6 Hz, POCH2CH3), 61.2 (s, CO2CH2CH3), 172.5
(d, 3JCP = 13.8 Hz, CO2).
31P NMR (CDCl3): d = 52.7 (m).
MS (ESI): m/z = 246.9 [M + Na]+.
Ethyl 3-{Ethoxy[(mesyloxy)methyl]phosphoryl}propanoate (8)
Ester 7 (6.2 g, 27 mmol) and DIPEA (4.2 g, 32.5 mmol) in anhyd
CH2Cl2 (180 mL) were stirred at –5 °C. A soln of MsCl (3.4 g, 30
mmol) in anhyd CH2Cl2 (120 mL) was added dropwise over 1 h.
When the addition was complete, the mixture was allowed to reach
r.t. and stirred under argon for 16 h. The mixture was evaporated un-
der reduced pressure and the solid residue was dissolved in CH2Cl2
(50 mL) and the soln was washed with 1.5% aq HCl (2 × 100 mL)
and sat. aq NaHCO3 (2 × 100 mL). The organic phase was dried (an-
hyd Na2SO4) and volatiles were evaporated under reduced pressure
to give ester 8 (8.1 g, 97%) as a yellow oil.
3
1H NMR (CDCl3): d = 1.28 (t, JHH = 7.2 Hz, 3 H, CO2CH2CH3),
1.37 (t, 3JHH = 7.2 Hz, 3 H, POCH2CH3), 2.19 (m, 2 H, PCH2CH2),
Synthesis 2008, No. 9, 1431–1435 © Thieme Stuttgart · New York

PAPER
Bifunctional Monophosphinate DOTA Derivative
1435
31P NMR (CDCl3): d = 34.6 (m).
MS (ESI): m/z = 665.0 [M]+.
HRMS (FAB): m/z [M + H]+ calcd for C30H58N4O10P: 665.38906;
found: 665.38957; m/z [M + Na]+ calcd for C30H57N4NaO10P:
687.37100; found: 687.37061.
(5) Rudovský, J.; Kotek, J.; Hermann, P.; Lukeš, I.; Mainero,
V.; Aime, S. Org. Biomol. Chem. 2005, 3, 112.
(6) Lebdušková, P.; Hermann, P.; Helm, L.; Tóth, É.; Kotek, J.;
Binnemans, K.; Rudovský, J.; Lukeš, I.; Merbach, A. E.
Dalton Trans. 2007, 493.
(7) Rudovský, J.; Botta, M.; Hermann, P.; Koridze, A.; Aime, S.
Dalton Trans. 2006, 2323.
10-{[Hydroxy(2-carboxyethyl)phosphoryl]methyl}-1,4,7,10-tet-
raazacyclododecane-1,4,7-triacetic Acid (2)
Compound 12 (4.3 g, 6.5 mmol), anhyd CH2Cl2 (100 mL), and TFA
(400 mL) were stirred at r.t. for 22 h. The mixture was evaporated
and co-evaporated with H2O under reduced pressure (2 ×) to get the
free ligand 2 as a slightly yellow oil (3.2 g, 100%).
(8) Táborský, P.; Lubal, P.; Havel, J.; Kotek, J.; Hermann, P.;
Lukeš, I. Collect. Czech. Chem. Commun. 2005, 70, 1909.
(9) Försterová, M.; Svobodová, I.; Lubal, P.; Táborský, P.;
Kotek, J.; Hermann, P.; Lukeš, I. Dalton Trans. 2007, 535.
(10) (a) Rudovský, J.; Botta, M.; Hermann, P.; Hardcastle, K. I.;
Lukeš, I.; Aime, S. Bioconjugate Chem. 2006, 17, 975.
(b) Rudovský, J.; Hermann, P.; Botta, M.; Aime, S.; Lukeš,
I. Chem. Commun. 2005, 2390.
NMR analysis (D2O, 90 °C) was in full accordance with that of a
previously prepared sample of the ligand.9
(11) Försterová, M.; Petřík, M.; Lázničková, A.; Lázníček, M.;
Hermann, P.; Lukeš, I.; Melichar, F. Appl. Radiat. Isotop.
2008, submitted.
Acknowledgment
(12) (a) Broan, C. J.; Cole, E.; Jankowski, K. J.; Parker, D.;
Pulukkody, K.; Boyce, B. A.; Beeley, N. R. A.; Millar, K.;
Millican, A. T. Synthesis 1992, 63. (b) Li, X.; Zhang, S.;
Zhao, P.; Kovacs, Z.; Sherry, A. D. Inorg. Chem. 2001, 40,
6572. (c) Chavez, M. R.; Zhao, P.; Kovacs, Z.; Sherry, A. D.
Lett. Org. Chem. 2004, 1, 194.
(13) (a) Fitch, S. J. J. Am. Chem. Soc. 1964, 86, 61. (b) Pinnich,
H. W.; Reynolds, M. A. Synth. Commun. 1979, 9, 535.
(c) Stawinski, J.; Thelin, M.; Westman, E.; Zain, R. J. Org.
Chem. 1990, 55, 3505.
We thank Bracco SpA (Italy) for a kind gift of cyclen and Dr. J.
Cvačka (Institute of Organic Chemistry and Biochemistry, Acade-
my of Sciences of the Czech Republic, Prague) for HR-MS measu-
rements). Support of the Grant Agency of the Czech Republic (No.
203/06/0467), Academy of Science of the Czech Republic (KAN
201110651) and Ministry of Education of the Czech Republic
(MSM0021620857) is acknowledged. The work was done in the
frame of COST D38 and EC-FP6 projects EMIL (LSHC-2004-
503569) and DiMI (LSHB-2005-512146).
(14) Maier, L. Helv. Chim. Acta 1973, 56, 489.
(15) Montchamp, J.-L. J. Organomet. Chem. 2005, 690, 2388.
(16) Deprele, S.; Montchamp, J.-L. J. Organomet. Chem. 2002,
643-644, 154.
(17) Wróblewski, A. E.; Verkade, J. G. J. Am. Chem. Soc. 1996,
118, 10168.
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Chem. 1985, 530, 16. (b) Boyd, E. A.; Corless, M.; James,
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Synthesis 2008, No. 9, 1431–1435 © Thieme Stuttgart · New York