Synthesis and characterization of DOTA-mono-adamantan-1-ylamide

Article abstract of DOI:10.1007/s11164-014-1615-8

A novel chelator of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) functionalized by adamantane was synthesized by nucleophilic substituted reaction of 1,4,7-Tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane (5) and N-(adamantan-1-yl) bromoacetamide (2). The intermediates and target compound (7) were characterized by fourier transform infrared spectroscopy (FT-IR), ¹H nuclear magnetic resonance (NMR), ¹³C NMR, atmospheric-pressure ionization electrospray mass spectrometry (API-ES-MS), and elemental analysis.

Full text of DOI:10.1007/s11164-014-1615-8

Res Chem Intermed
DOI 10.1007/s11164-014-1615-8
Synthesis and characterization of DOTA-mono-
adamantan-1-ylamide
Fuxian Wan
•
Mingjie Liu
•
Junzheng Zhang
•
Ying Li Lin Jiang
•
Received: 28 November 2013 / Accepted: 12 March 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract A novel chelator of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic
acid (DOTA) functionalized by adamantane was synthesized by nucleophilic
substituted reaction of 1,4,7-Tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacy-
clododecane (5) and N-(adamantan-1-yl) bromoacetamide (2). The intermediates and
target compound (7) were characterized by fourier transform infrared spectroscopy
1
13
(FT-IR), H nuclear magnetic resonance (NMR), C NMR, atmospheric-pressure
ionization electrospray mass spectrometry (API–ES–MS), and elemental analysis.
Keywords Tetraazacyclododecane ꢀ Macrocycle ꢀ DOTA ꢀ Adamantane ꢀ
Synthesis
Introduction
1
,4,7,10-Tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) is one of the
most commonly used macrocyclic chelators and has received special attention
among the derivatives of cyclen (1,4,7,10-tetraazacyclododecane, first synthesized
in 1961 by the German chemists Stetter and Mayer [1]). Due to their strong
coordination ability towards a wide range of cations including transition metals and
lanthanides [2, 3], DOTA and its derivatives are widely used in the construction of a
wide variety of functional molecules for a spectrum of applications in biomedicine,
as therapeutic radiopharmaceuticals, luminescent probes for biochemical analysis,
or magnetic resonance imaging (MRI) contrast agents [4–7].
F. Wan (&) ꢀ M. Liu ꢀ J. Zhang ꢀ Y. Li ꢀ L. Jiang (&)
College of Chemistry and Material Science, Shan Dong Agricultural University, Taian 271018,
People’s Republic of China
e-mail: wfx@sdau.edu.cn
L. Jiang
e-mail: jiangl@sdau.edu.cn
123

F. Wan et al.
Replacement of functional groups (e.g., CO H) represents one strategy used in
2
medicinal chemistry for rational modification of a lead compound to yield safer and
more effective agents [8]. Fortunately, the replacement of one carboxylate with one
carboxyamido group does not reduce the thermodynamic formation constant [9] as
much as in Gd-DO3A, and the kinetic inertness remains excellent [10, 11]. For
about 20 years, several research groups have been engaged in synthesis of
monofunctionalized DOTA derivatives [12–14].
Adamantane and its derivatives have been extensively investigated due to their
high biological activity. What attracts us here is that their hydrophobicity and
lypophilicity favor transport of adamantane compounds across biological mem-
branes [15–17].
The above observations prompted us to design and synthesize a novel chelator of
0
00
DOTA functionalized by adamantane. As a result, 2,2 ,2 -[10-(adamantan-1-
ylcarbamoylmethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl]triacetic was pre-
1
13
pared, and characterized by FT-IR, H NMR, C NMR, MS, and elemental
analysis. The synthetic route of the target molecule is shown in Schemes 1 and 2.
Experimental
General instruments and materials
Melting points (mps) were determined on a digital mp apparatus. Thin layer
chromatography (TLC) analysis was performed on glass sheets coated with Merck
silica gel 60 F254 (0.25 mm). Compounds were visualized by I steam or ultraviolet–
2
1
visible spectroscopy (UV–Vis). H NMR spectra were recorded on Bruker AV-400
spectrometers. Chemical shifts (d) are reported in parts per million (ppm) from
internal standard tetramethylsilane. Coupling constants (J) were measured in hertz.
Multiplicity is reported as follows: s (singlet), d (doublet), t (triplet), m (multiplet),
and combination of these signals. Column chromatographies were performed on
silica gel 60 (200–300 mesh). IR spectrometry was recorded with a Nicolet 380 FT-
IR. Mass spectra (MS) were obtained on an Agilent 6110 (Agilent Technologies,
USA). Elemental analyses were carried out using a Vario EL III (Elementar).
1,4,7,10-Tetraazacyclododecane (cyclen, 3) was synthesized according to the
reference method [18]. All reagents and solvents were of analytical grade from
commercial suppliers and used without further treatment unless otherwise stated.
Reactions were carried out under N atmosphere unless otherwise noted.
2
Syntheses
Synthesis of bromoacetyl chloride (1)
A stirred solution of bromoacetic acid (13.9 g, 100 mmol) in thionyl chloride
(10 mL) was refluxed for 2 h. The reaction mixture was concentrated in vacuo to
remove excess thionyl chloride. The residue was evaporated under reduced
pressure, and the distillation cut between 48 and 49 °C/1.6 kPa was collected as a
1
23

Synthesis and characterization of DOTA-mono-adamantan-1-ylamide
O
BrCH C NH
2
(
i)
(ii)
BrCH2COOH
BrCH2COCl
1
2
Scheme 1 Synthetic route of bromoacetyl chloride and N-(adamantan-1-yl) bromoacetamide. Reagents
and conditions: (i) SOCl , reflux; (ii) 1-aminoadamantane hydrochloride, NEt , CHCl
2
3
3
COOC(CH3)3
(
H C) COOC
COOC(CH3)3
ii)
(
H C) COOC
3
3
3
3
N
N
NH HN
NH HN
N
N
N
(
i)
(
HBr
HN
N
HN
(H3C)3COOC
(H3C)3COOC
3
Cyclen
4 DO3A-t-Bu ester HBr
5 DO3A-t-Bu ester
(
H3C)3COOC
COOC(CH3)
3
COOH
HOOC
HOOC
N
N
N
N
N
N
N
N
(
iii)
(iv)
3
CF COOH
O
3
O
CH C NH
2
(
H3C)3COOC
NH
CH2C
6
7
Scheme 2 Synthetic route of DOTA-mono-adamantan-1-ylamide. Reagents and conditions: (i) DMAC,
CO CO , CH CN, reflux; (iii) 2, K CO
, rt; (ii) Method A: 10 % aqueous KOH, 40 °C; Method B: K
CH CN, reflux; (iv) CF COOH, CH Cl
K
2
3
2
3
3
2
3
,
3
3
2
2
slightly yellow transparent liquid with yield of 15 g (95 mmol, 95.23 %).
-1
2
D
0
n
¼ 1:4950; FT-IR (KBr, cm ) 3,011, 2,955, 1,808, 1,415, 1,391, 1,248, 1,215,
1
,181, 523.
Synthesis of N-(adamantan-1-yl) bromoacetamide (2)
To a solution of 1-aminoadamantane hydrochloride (1.87 g, 10 mmol) and
triethylamine (3.06 g, 30 mmol) in dry chloroform (60 mL) at 0 °C was added a
solution of freshly prepared bromoacetyl chloride (1.73 g, 11 mmol) in dry
chloroform (20 mL) dropwise over a period of 0.5 h. The temperature was
maintained at 0 °C during the addition, after which the reaction mixture was
allowed to warm to room temperature (rt). After 3 h of vigorous stirring, 1 mL of
water was added slowly to remove excess bromoacetyl chloride.
The organic layer was washed with 5 % hydrochloric acid solution (3 9 30 mL),
% sodium bicarbonate solution (3 9 30 mL), and water (3 9 30 mL), dried
5
(
Na SO ), filtered, and concentrated under reduced pressure, after which N-(1-
2
4
123

F. Wan et al.
1
Fig. 1 H NMR spectrum of 2
1
Fig. 2 H NMR spectrum of 4
adamantyl)bromoacetamide was recrystallized from 75 % ethanol as a white
needle-like solid with yield of 2.17 g (8.00 mmol, 80 %). Mp 44–47 °C (Fig. 1);
FT-IR (KBr, cm ) 3,271, 2,910, 2,858, 1,659, 1,454, 1,211; H NMR (400 MHz,
-
1
1
3
3
= 3.2 Hz, 6 H), 2.01 (s, 6 H), 2.10 (d, J
CDCl , d/ppm) 1.69 (t, J
= 3.2 Hz,
3
H,H
H,H
3H), 3.78 (s, 2 H), 6.14 (brs, 1 H); Anal. calcd. for C H BrNO: C, 52.95; H, 6.67;
12 18
N, 5.15. Found: C, 52.86; H, 6.69; N, 5.24.
1
23

Synthesis and characterization of DOTA-mono-adamantan-1-ylamide
1
Fig. 3 H NMR spectrum of 5
Synthesis of 1,4,7-Tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane
hydrobromide (4) [19]
To a suspension of cyclen (5.00 g, 29 mmol) and sodium acetate (7.86 g, 96 mmol)
in N,N-dimethylacetamide (DMAC, 60 mL) at -20 °C was added a solution of t-
butyl bromoacetate (18.7 g, 14.1 mL, 96 mmol) in DMAC (20 mL) dropwise over a
period of 0.5 h. The temperature was maintained at -20 °C during the addition,
after which the reaction mixture was allowed to warm to rt. After 24 h of vigorous
stirring, the reaction mixture was poured into water (300 mL) to give a clear
solution. Solid KHCO3 (15 g, 150 mmol) was added portionwise, and 1,4,7-
Tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane (4) precipitated as
a white solid. The precipitate was collected by filtration and dissolved in CHCl3
(250 mL). The solution was washed with water (100 mL), dried (Na SO ), filtered,
2 4
and concentrated to about 20–30 mL. Diethyl ether (250 mL) was added, and 4
precipitated as a white fluffy solid with yield of 13.6 g (23 mmol, 79 %). Mp
1
IR (KBr, cm ) 3,470, 2,978, 2,860, 3,000–2,453 (brs), 1,728, 1,590, 1,467, 1,393,
76.8–178.3 °C; R = 0.80 (DCM/MeOH on silica gel 60 F254, V:V = 100:15); FT-
f
-1
1
1
,257, 1,155 (Fig. 2); H NMR (400 MHz, CDCl , d/ppm) 1.45 (s, 9 H, –CH ), 1.46
3
3
(
s, 18 H, –CH ), 2.81–2.85 (m, 12 H, –CH N–), 3.00 (s, 4 H, –CH N–), 3.30 (s, 2 H,
CH CO–), 3.35 (s, 4 H, –CH CO–); Anal. calcd. for C H BrN O : C, 52.43; H,
2 2 26 51 4 6
3 2 2
–
8
.63; N, 9.41. Found: C, 52.36; H, 8.69; N, 9.45.
123

F. Wan et al.
1 13
Fig. 4 a H NMR spectrum of 6, b C NMR spectrum of 6, and c API–ES–MS spectrum of 6
1
23

Synthesis and characterization of DOTA-mono-adamantan-1-ylamide
1 13
Fig. 5 a H NMR spectrum of 7, b C NMR spectrum of 7, and c API–ES–MS spectrum of 7
123

F. Wan et al.
Synthesis of 1,4,7-Tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane
(
5)
Method A [19] 1,4,7-Tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclodode-
cane hydrobromide (4, 5.95 g, 10 mmol) was dissolved in water (250 mL) at 70 °C.
The solution was allowed to cool to 40 °C, after which 10 % aqueous KOH solution
(
11.2 mL, 20 mmol) was added. The reaction mixture was stirred at 40 °C for 30 min
and then extracted with hexanes (3 9 100 mL). The combined organic layers were
washed with water (3 9 100 mL), dried (Na SO ), filtered, and concentrated under
2
4
reduced pressure to give 5 as a colorless viscous oil which solidified upon storage at
0 °C (yield 4.10 g; 8.00 mmol, 95 %). Mp 44–47 °C.
2
Method B [2] Compound 4 (5.95 g, 10 mmol) was dissolved in 50 mL of dry
acetonitrile containing anhydrous K CO (20 mmol). The resulting mixture was
2
3
stirred at 50 °C for 24 h. After removal of the solids, the filtrate was concentrated
under reduced pressure. The residue was redissolved in CHCl and washed with
water thoroughly (3 9 100 mL). Evaporation of CHCl afforded 5 in quantitative
3
3
1
yield. Mp 43–45 °C (Fig. 3); H NMR (400 MHz, CDCl , d/ppm) 1.45 (s, 9 H),
3
1
Anal. calcd. for C H N O : C, 60.67; H, 9.79; N, 10.89. Found: C, 60.26; H,
.46 (s, 18 H), 2.56 (m, 4 H), 2.72–2.78 (m, 8 H), 2.81 (m, 4 H), 3.30 (s, 6 H);
26 50 4 6
9
.28; N, 10.80.
Synthesis of 1,4,7-Tris(tert-butoxycarbonylmethyl)-10-(adamantan-1-
ylcarbamoylmethyl)-1,4,7,10-tetraazacyclododecane (6)
To a suspension of 5 (2.57 g, 5 mmol) and anhydrous K CO (1.38 g, 10 mmol) in
2
3
6
0 mL of dry acetonitrile was added a solution of 2 (1.36 g, 5 mmol) in 20 mL of
acetonitrile. The resulting mixture was stirred vigorously at 80 °C under argon for
8 h. After removal of the solids and evaporation of the solvent, the residue was
4
redissolved in CHCl and washed thoroughly by water (3 9 30 mL). Evaporation of
3
CHCl afforded the crude product, which was purified by column chromatography
3
(
(
CHCl /CH OH, V:V = 10:1) to give 6 as yellow–white solid with yield of 2.68 g
3 3
-
1
7.6 mmol, 78 %). Mp 188.5–190.5 °C; FT-IR (KBr, cm ) 3,248, 2,980, 2,906,
,850, 2,827, 1,736, 1,668, 1,456, 1,423, 1,369, 1,309, 1,222, 1,171, 1,107, 756, 595
2
1
(
3
Fig. 4); H NMR (400 MHz, CDCl , d/ppm) 1.469 (s, 27 H, –CH ), 1.63 (t,
3
3
JH,H = 3.2 Hz, 4 H), 2.02 (s, 6 H), 2.07 (s, 3 H), 2.21–2.75 (m, 10 H, –CH N–),
2
2
3
.88–2.92 (s, 6 H, –CH N–), 3.10 (s, 2 H, –CH CONH–), 3.29 (s, 2 H, –CH COO–),
2 2 2
.34 (s, 2 H, –CH COO–), 3.38 (s, 2 H, –CH COO–), 7.01 (brs, 1 H, –CH CONH–).
2
2
2
1
3
C NMR (100 MHz, CDCl , d/ppm) 27.9, 28.1, 29.3, 29.4, 36.3, 36.4, 41.0, 41.1,
3
4
1
7
6
1.2, 41.4, 41.5, 52.0, 52.3, 52.6, 53.2, 56.3, 56.6, 58.3, 62.2, 81.6, 81.7, 81.8, 169.9,
?
70.6, 171.8, 172.3; MS (API, positive mode) m/z [M?H] calcd. 706.51, obsd.
06.30; Anal. calcd. for C H N O (w /%): C, 64.65; H, 9.57; N, 9.92. Found: C,
3
8
67
5
7
B
4.52; H, 9.65; N, 9.75.
1
23

Synthesis and characterization of DOTA-mono-adamantan-1-ylamide
0
00
Synthesis of 2,2 ,2 -[10-(adamantan-1-ylcarbamoylmethyl)-1,4,7,10-
tetraazacyclododecane-1,4,7-triyl]triacetic (7)
Trifluoroacetic acid (TFA, 5 mL) was added to a solution of 6 (1.34 g, 3.8 mmol) in
CH Cl (10 mL) under argon atmosphere. The reaction mixture was stirred
2
2
vigorously at rt overnight. After removal of bulk solvent under reduced pressure,
coevaporation with toluene (2 9 10 mL) and CH Cl (2 9 10 mL) and precipita-
2
2
0
00
tion with anhydrous ether (20 mL) synthesis of 2,2 ,2 -[10-(adamantan-1-ylcarba-
moylmethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl]triacetic (7) as a white
1
solid. H NMR spectroscopy confirmed successful removal of the tert-butyl ester
groups as indicated by the absence of signals at 1.46 ppm. Yield 2.68 g (4.75 mmol,
-
1
8
1
1
–
–
3
1
0 %). Mp 135–136.5 °C; FT-IR (KBr, cm ) 3,428, 2,913, 2,855, 1,682, 1,459,
1
,360, 1,183, 1,094, 799, 721, 518 (Fig. 5); H NMR (400 MHz, CDCl , d/ppm)
3
3
.51 (t, J
= 3.2 Hz, 6 H), 1.81 (s, 6 H), 1.89 (s, 3 H), 2.76–3.30 (m, 16 H,
CH N–), 3.47 (s, 2 H, –CH CONH–), 3.60 (s, 2 H, –CH COO–), 3.72 (s, 2 H,
H,H
2
2
2
1
3
CH COO–), 3.79 (s, 2 H, –CH COO–); C NMR (100 MHz, CDCl , d/ppm) 29.1,
5.6, 40.6, 42.2, 47.7, 48.4, 49.0, 50.7, 51.7, 52.9, 53.0, 55.5, 114.8, 117.7, 120.6,
?
62.7, 163.1, 169.4, 174.3; MS (API, positive mode) m/z [M?H] calcd. 538.32,
2
2
3
?
obsd. 538.20; m/z [M?Na] calcd. 560.31, obsd. 560.0; Anal. calcd. for
C H F N O (w /%): C, 43.69; H, 5.27; N, 7.96. Found: C, 43.52; H, 5.65; N,
3
2
46
9
5
13
B
7
.75.
Results and discussion
As mentioned above, because of the favorable pharmacological properties of
adamantane compounds, we have produced corresponding DOTA analogues
containing adamantane.
Synthesis of compounds 1 and 2
Bromoacetyl chloride (1) was synthesized in high yield (95.23 %) by reaction of
bromoacetic acid with thionyl chloride; it must be stored in desiccator. Acylation of
1 with 1-aminoadamantane hydrochloride in CHCl at rt (3–4 h) in the presence of
3
triethylamine, which act as catalyst to help promote the reaction and as base to
neutralize the resulting HCl, results in the formation of 2 in 80 % yield.
Synthesis of compound 4
1
,4,7-Tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane is widely
used as an intermediate in preparation of medically important DO3A and DOTA
metal chelators; despite its commercial availability, it is still very expensive. The
methods for synthesizing 1,4,7-Tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraaza-
cyclododecane were recently reviewed by Jagadish et al. [19]. After comparing
many synthetic methods, Jagadish et al. reported preparation of the hydrobromide
salt 4 in *80 % yield and high purity by Tris-alkylation of cyclen with tert-butyl
123

F. Wan et al.
bromoacetate in DMAC using sodium acetate as base, without the need for the
column chromatography separation and purification step, thus making it attractive
for large-scale preparation.
Synthesis of compound 5
There are two methods for neutralization of hydrobromide salt 4 to produce free
base 5: Methods A and B are suitable for manufacture of the product at laboratory
scale, but method A provides several advantages such as simple workup procedure,
shorter reaction time, milder conditions, and environmental friendliness.
Synthesis of 1,4,7-Tris(tert-butoxycarbonylmethyl)-10-(adamantan-1-
ylcarbamoylmethyl)-1,4,7,10-tetraazacyclododecane (6)
In our initial efforts to prepare 6 from the known building block DOTA-t-Bu ester with
the primary amine of 1-aminoadamantane directly, the isolated yield was relatively low
(about 20–30 %). In addition, the synthesis of DOTA-t-Bu ester from DO3A-t-Bu ester is
relatively low yield and more expensive. The shortcomings of this approach led us to
attempt alkylation of DO3A-t-Bu ester with N-(adamantan-1-yl) bromoacetamide (2);
0
00
this strategy for synthesis of 2,2 ,2 -[10-(adamantan-1-ylcarbamoylmethyl)-1,4,7,10-
tetraazacyclododecane-1,4,7-triyl]triacetic (7) is shown in Scheme 2. Alkylation of free
base 5 with 2 and an excess of K CO results in the formation of 1,4,7-Tris(tert-
2
3
butoxycarbonylmethyl)-10-(adamantan-1-ylcarbamoylmethyl)-1,4,7,10-tetraazacyclod-
odecane (6, 78 % yield).
0
00
Synthesis of 2,2 ,2 -[10-(adamantan-1-ylcarbamoylmethyl)-1,4,7,10-
tetraazacyclododecane-1,4,7-triyl]triacetic (7)
0
00
The tert-butyl esters of 6 were removed with TFA to give 2,2 ,2 -[10-(adamantan-1-
ylcarbamoylmethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl]triacetic (7) as a
white solid. Quantitative removal of the tert-butyl ester moieties was confirmed
1
by the disappearance of their –CH signals from the H NMR spectrum. Meanwhile,
3
the API–ES–MS data indicated the accurate peak of 7 molecular mass. In the FT-IR
-
1
-1
spectrum of 6, the peaks at 1,736 cm (A) and 1,668 cm (B) were attributed to
vCOOC(CH ) and vCONH–, respectively, while band A disappeared in 7, showing
the successful deprotection of tert-butyl.
3
3
Acknowledgments The authors thank Prof. Kenneth Curry and Dr. Ji-dong Zhang (University of
British Columbia) for good advice and hard work in the modification of this manuscript. Project
supported by the National Natural Science Foundation of China (No. 20675033) and the Science and
Technology Development of Shandong Province (No.2013CEX2019).
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