From [11C]CO2 to [11C]amides: A rapid one-pot synthesis: Via the Mitsunobu reaction

Article abstract of DOI:10.1039/c7cc01407d

A novel amide synthesis methodology is described using amines, CO₂ and Grignard reagents and Mitsunobu reagents. The method was applied to carbon-11 radiochemistry to label amides using cyclotron-produced [¹¹C]CO₂. The synthetic utility of the one-pot labelling methodology was demonstrated by producing [¹¹C]melatonin. The incorporation of [¹¹C]CO₂ into [¹¹C]melatonin was 36%-determined by radioHPLC 2 min post [¹¹C]CO₂ delivery.

Full text of DOI:10.1039/c7cc01407d

ChemComm
View Article Online
View Journal
COMMUNICATION
From [11C]CO2 to [11C]amides: a rapid one-pot
synthesis via the Mitsunobu reaction†
Cite this:DOI: 10.1039/c7cc01407d
S. Bongarzone, * A. Runser, C. Taddei, A. K. Haji Dheere and A. D. Gee*
Received 22nd February 2017,
Accepted 24th March 2017
DOI: 10.1039/c7cc01407d
rsc.li/chemcomm
A novel amide synthesis methodology is described using amines, the fixation and trapping of [11C]CO2:4 (1) trapping of [11C]CO2
CO2 and Grignard reagents and Mitsunobu reagents. The method using highly reactive organometallic reagents, such as Grignard
was applied to carbon-11 radiochemistry to label amides using or organolithium reagents to produce [11C]carboxylic acids and
cyclotron-produced [11C]CO2. The synthetic utility of the one-pot derivatives (e.g. acid chlorides);4 (2) utilisation of trapping
labelling methodology was demonstrated by producing [11C]melatonin. agents such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)5 and
The incorporation of [11C]CO2 into [11C]melatonin was 36% – 2-tertbutylimino-2-diethylamino-1,3-dimethyl-perhydro-1,3,2-
determined by radioHPLC 2 min post [11C]CO2 delivery.
diazaphosphorine (BEMP)6 to form a labile bond with [11C]CO2.
These approaches have been applied to radiolabel [11C]carboxylic
Positron emission tomography (PET) is a non-invasive imaging acids,7
[
11C]amides,8
11C]ureas,5,11
Current methods for the preparation of 11C-labelled amides
[ [
11C]amines,9 11C]acyl chlorides,10
technology used for in vivo molecular imaging.1 Carbon-11 (11C)
[
[
11C]carbamates6,12 and [11C]isocyanates.13
is a short-lived positron-emitting radionuclide widely used in
labelling radiopharmaceuticals for medical research, diagnosis, utilise the coupling between [11C]carboxylic acids and amines
staging and monitoring treatments. Its physical and nuclear at high temperatures,8 assisted by microwave irradiation14 or
characteristics (radioactive half-life = 20.4 min) and its orthology through activation to [11C]acyl chlorides using thionyl choride15
with carbon-12 make 11C an attractive radionuclide for labelling or phthaloyl chloride16 (Scheme 1). These methodologies often
and molecular imaging. The development of rapid and reliable lead to low molar radioactivities since the reagents used readily
chemical methods for incorporating carbon-11 into organic mole- react with atmospheric CO2 and have a synthesis time ranging
cules is required to expand the repertoire of available radiotracers from 5–35 minutes.8,14–16 In order to minimise this isotopic
for in vivo imaging studies. Cyclotron-produced [11C]CO2 is dilution, extreme efforts are required to control reagent stoichio-
obtained by the proton bombardment of nitrogen-14 gas via the metry, stability and exclusion of moisture and atmospheric CO2
14N(p,a)11C nuclear reaction. Because of its low reactivity and from the reagents and reaction system.
solubility in organic solvents, [11C]CO2 is usually converted into a
Boronic esters have higher stability to air and moisture
more reactive secondary precursor (e.g. [11C]iodomethane2) in compared with Grignard reagents and are able to react with
order to assemble a radiolabelled molecule of interest.3 The [11C]CO2 to form [11C]amides in two steps within 10–15 min
production of a secondary precursor results in significant losses from [11C]CO2. However, this method is limited to the produc-
due to non-quantitative yields and additional processing times. tion of aromatic [11C]amides (R = Ar, Scheme 1).7
Methods for the direct incorporation of [11C]CO2 into molecules of
interest would therefore have significant advantages over tradi-
tional carbon-11 labelling methodologies, since the processing
times and losses due to technical handling could be minimised.
Significant efforts have been made to improve the solubility
of [11C]CO2 in organic solvents, resulting in two approaches for
Division of Imaging Sciences and Biomedical Engineering, King’s College London,
King’s Health Partners, St. Thomas’ Hospital, London, SE1 7EH, UK.
E-mail: antony.gee@kcl.ac.uk, salvatore.bongarzone@kcl.ac.uk
Scheme 1 Different approaches available for the preparation of [11C]amides
† Electronic supplementary information (ESI) available: Experimental details and
using Grignard reagents or boronic esters.
NMR spectra. See DOI: 10.1039/c7cc01407d
This journal is ©The Royal Society of Chemistry 2017
Chem. Commun.

View Article Online
Communication
ChemComm
formation and stabilisation of the carbamate anion. Mitsunobu
reagents were subsequently added and the reaction stirred for
10 min before adding an excess of 1-propynylmagnesium
bromide (A – as a 0.5 M solution in THF, 7.2 equiv.). The
reaction was quenched after 30 min and the conversion to
compound 1A was evaluated. Using the reaction conditions
previously used for the synthesis of ureas, led to no observable
quantities of 1A (Table 1, entry 1). Increasing the concentration
of Mitsunobu reagents or DBU had no beneficial effect on the
reaction yield (Table 1, entries 2 and 3). As next step, lowering the
amount of DBU from 0.1 to 0.05 equivalents different Mitsunobu
reagent concentrations were tested (Table 1, entries 4–7). 1A was
obtained in 46% and 44% yield using 2 and 3.8 equivalents of
Mitsunobu reagent, respectively (Table 1, entries 5 and 6). Using
an equimolar or an excess of Mitsunobu reagents did not yield 1A
(Fig. S4, ESI†).
The synthesis of primary, secondary and aromatic amides
was explored to test the scope of the reaction (Table 2). When
using the secondary amine N-methylbenzylamine (2), amide 2A
was not obtained (Table 2). The poorly nucleophilic aromatic
amine (3) did not form the target product, probably due to its
inability to form an isocyanate as reported previously.18 The
activated aromatic amine (4) however gave 4A in good yield
Scheme 2 Radiosynthetic approach to radiolabelled
[
[
11C]ureas5 and
11C]amides (this work) from cyclotron-produced [11C]CO2.
To address the shortcomings of current [11C]amide labelling
methodologies, we sought to develop an efficient method for
producing [11C]amides in short synthesis times, high molar
radioactivities and with applicability to a range of amide
derivatives bearing alkyl and aryl groups on R and R1 positions
(Scheme 2). The synthetic strategy was inspired by our previous
work on the labelling of [11C]ureas,5 which involved: (1) reac-
tion of an amine with [11C]CO2 to form a [11C]carbamate anion
in the presence of DBU and acetonitrile (MeCN) as a solvent
(Scheme 2); (2) conversion from [11C]carbamate anion to an
[11C]isocyanate or an [11C]oxyphosphonium intermediate using
Mitsunobu reagents (tri-n-butyl phosphine and di-tert-butyl azo-
dicarboxylate, Bu3P and DBAD); (3) reaction of the [11C]isocyanate
intermediate with another molecule of amine to yield the corres-
ponding [11C]urea derivative. Based on this pathway, it was
hypothesised that the synthesis of [11C]amides might be achieved
by coupling the intermediate [11C]isocyanate with a Grignard
reagent. To test this hypothesis, the formation of amides via
the Mitsunobu reaction was tested using both macroscopic
amounts of non-radioactive CO2 and tracer amounts of radio-
labelled [11C]CO2.
Table 2 Synthesis of 1A, 4A, 1B, 1C, 5C and 6C
Amine
Grignard reagent Solvent Product
MeCN
Yield (%)
46
The synthesis of 1A was chosen as model reaction (Table 1).
Initial experiments were performed adapting previously estab-
lished conditions for the synthesis of urea derivatives.17 CO2
was bubbled into a solution of benzylamine (1) and DBU in
MeCN at room temperature (r.t.) for 40 min. DBU facilitates the
MeCN
MeCN
0
0
Table 1 Optimization of substrates and reaction conditions for the
synthesis of 1A
MeCN
37
MeCN
Et2O
57
28
19
Entrya DBU (equiv.) DBAD (equiv.) Bu3P (equiv.) Yield of 1Ab (%)
1
2
3
4
5
6
7
0.1
0.1
0.2
0.05
0.05
0.05
0.05
2
2
0
0
0
0
3.8
3.8
1
2
3.8
7.2
3.8
3.8
1
2
3.8
7.2
Et2O
46 Æ 8c
44d
0
Et2O
5
a
Reaction conditions: CO2 was bubbled in a solution of 1 (138.6 mmol, Reaction conditions: (i) 1–6 (1 equiv.) and DBU (0.05 equiv.) in MeCN or
1.0 equiv.), DBU (0.05–0.2 equiv.) in MeCN (1 mL), r.t. for 40 min. Et2O, CO2 (1 atm) r.t. for 40 min; (ii) DBAD (2 equiv.) and Bu3P (2 equiv.)
Mitsunobu reagents (7.2–1 equiv.) in MeCN (0.5 mL) were added and in MeCN or Et2O, 10 min; (iii) A–C (as a 0.5–1.0 M solution in THF,
the solution stirred for 10 min. A (7.2 equiv. of a 0.5 M solution in THF) 7.2 equiv.) and quenching after 30 min. Yield of isolated compound
b
was added and quenched after 30 min. Yield of isolated 1A calculated calculated from the corresponding amine derivative (N = 4 for 1A and
c
d
from compound 1. N = 4. N = 1.
N = 1 for 4A, 1B, 1C, 5C, 6C).
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2017

View Article Online
ChemComm
Communication
(37%, Table 2) due to its ability to form an isocyanate inter- the radiochemical yield (RCY) of [11C]1A, the influence of chan-
mediate in agreement with the results obtained by others.18
ging reagent concentrations (amine, DBU and Mitsunobu
The reactivity of different Grignard reagents in MeCN or reagents), temperature and solvent were studied.
Et2O was also investigated. Reaction of 1 with ethylmagnesium Initial experiments were performed with higher amounts of
bromide (B – as a 1.0 M solution in THF, 7.2 equiv.) in MeCN DBU. A low RCY20 (2%, Table 3, entry 2) of [11C]1A was observed
gave 1B in a good yield (57%, Table 2), whereas when using using 1 equiv. of DBU with [11C]dibenzylurea as a major
phenylmagnesium bromide (C – as a 1.0 M solution in THF, by-product. When the reaction was carried out at a lower amine
7.2 equiv.) in MeCN, the target amide was not obtained. Amide concentration, a slight increase in RCY was observed (5%, Table 3,
formation between isocyanates and bulky Grignard reagents entry 3). Interestingly, increasing the DBU concentration further
such as C has been successfully observed using Et2O as solvent to 1, 2.5, 5 and 10 equiv. led to a RCY of 10% (Table 3, entries 3–6).
at 0 1C.19 Encouragingly, substituting the MeCN with diethyl
We have previously reported that the RCY of [11C]ureas is
ether (Et2O) under these conditions we obtained 1C, 5C and 6C dependent on reaction temperature.5 A similar trend was
in yields of 5–28% (Table 2). These results indicate that primary observed for the synthesis of [11C]amides. Indeed, varying the
and activated aromatic amines are able to form isocyanate temperature from 20 to 30, 40, 50 and 80 1C improved the RCY
intermediates and subsequently react with a broad range of (Table 3, entries 6–13). The RCY increased from 9% at 20 1C to 27%
Grignard reagents forming amides.
at 80 1C (Table 3, entries 6–10, Fig. S5, ESI†). Similarly by adding
Our amide preparation methodology developed using non- 2 equiv. of Mitsunobu reagents the RCY increased from 10% at
radioactive CO2 was subsequently applied to the synthesis of 20 1C to 45% at 80 1C (Table 3, entries 11–13, Fig. S5, ESI†).
[11C]amides using [11C]CO2. Translating the reaction conditions
Interestingly, the increase of Mitsunobu reagents from 1 to
from synthetic chemistry to carbon-11 chemistry, we were unable 2, 4 and 8 equiv. (Table 3, entries 10 and 13–15), significantly
to obtain [11C]1A (Table 3, entry 1), with only [11C]dibenzylurea increased the RCY from 27% to 50%. Increasing DBU concen-
present as a by-product. The solution trapped the cyclotron- tration from 10 to 30 equivalents did not improve the RCY
produced [11C]CO2 in high efficiency (98%). As the [11C]CO2 further (28–30%, Table 3, entries 16–17). Good RCY’s of the desired
chemistry is performed on a nano-molar scale the [11C]amide [11C]amide were observed when MeCN was used as a solvent (50%)
reaction conditions were further investigated. In an effort to while DMF was detrimental for the RCY (0%, Table 3, entry 18).
reduce the amount of [11C]dibenzylurea by-product and optimise
To demonstrate the potential utility of this novel labelling
strategy, the endogenous neurotransmitter melatonin, was
radiolabeled with carbon-11 (Scheme 3). The incorporation of
Table 3 Radiosynthesis of [11C]1A
[11C]CO2 into [11C]melatonin was 36%
– determined by
radioHPLC 2 min from end of [11C]CO2 delivery with an
estimated molar radioactivity of 70–100 GBq mmolÀ1, consistent
with the molar radioactivities obtained for other 11C-labelled
tracers at our institution.21
In summary a rapid one-pot methodology for the synthesis
of amides has been successfully developed. Readily available
CO2 was incorporated into primary or activated aromatic
amines in the presence of Mitsunobu reagents to form the
isocyanate intermediate. Grignard reagents were then reacted
with the isocyanate to form the corresponding amides. The
approach was optimised for the radiolabelling of [11C]amides
using [11C]CO2 directly from the cyclotron. The synthesis time
from end of [11C]CO2 delivery was less than 2 minutes. Radio-
labelling of a biologically relevant biomolecule, [11C]melatonin,
was achieved. A limitation of the proposed technique is the
restricted number of commercially available Grignard reagents
Mitsunobu
reagents (equiv.)
RCY of
[
11C]1Ab (%)
Entrya
DBU (equiv.)
T (1C)
1c
0.05
1
1
2.5
5
2
1
1
1
1
1
1
1
1
1
2
2
2
4
8
2
4
8
20
20
20
20
20
20
30
40
50
80
20
50
80
80
80
80
80
80
0g
2c
2 f
3d
5 f
4d
4 f
5d
8 f
6d
10
10
10
10
10
10
10
10
10
10
20
30
10
9.5 Æ 7g
10 Æ 2g
20 Æ 4g
18 Æ 7g
27 Æ 8g
10 Æ 4g
36 Æ 4g
45 Æ 5g
50 Æ 9h
50 Æ 5h
28 Æ 5g
30 Æ 2g
0 f
7d
8d
9d
10d
11d
12d
13d
14d
15d
16d
17d
18e
a
Reaction conditions: [11C]CO2 was bubbled in a solution of 1 (32 mmol,
1 equiv.), DBU (0.05–30 equiv.) in MeCN (160 mL) at room temperature.
Then, the reaction mixture was heated (20–80 1C) for 30 seconds.
Mitsunobu reagents (1–8 equiv.) in MeCN (100 mL) were added and
stirred for 10 s. A (8 equiv. of a 0.5 M solution in THF) was added
Scheme 3 Reaction conditions: [11C]CO2 was bubbled in a solution of 7
(32 mmol, 1 equiv.), DBU (10 equiv.) in MeCN (160 mL) at room temperature.
Then, the reaction mixture was heated (50 1C) for 30 seconds. Mitsunobu
reagents (8 equiv.) in MeCN (100 mL) were added and stirred for 10 s. D
(8 equiv., 3.0 M in Et2O) was added. The reaction was quenched after 1 min.
b
and quenching after 1 min. RCY determined by radio-HPLC not
c
d
e
decay-corrected. 138.5 mmol of 1. 32 mmol of 1. 32 mmol of 1 in DMF.
f
g
h
N = 1. N = 2. N = 3.
This journal is ©The Royal Society of Chemistry 2017
Chem. Commun.

View Article Online
Communication
ChemComm
5 (a) A. K. Haji Dheere, N. Yusuf and A. Gee, Chem. Commun., 2013, 49,
8193–8195; (b) A. K. Dheere, S. Bongarzone, C. Taddei, R. Yan and
A. D. Gee, Synlett, 2015, 2257–2260.
6 A. A. Wilson, A. Garcia, S. Houle and N. Vasdev, Org. Biomol. Chem.,
2010, 8, 428–432.
7 P. J. Riss, S. Lu, S. Telu, F. I. Aigbirhio and V. W. Pike, Angew. Chem.,
Int. Ed., 2012, 51, 2698–2702.
8 C. Aubert, C. Huard-Perrio and M.-C. Lasne, J. Chem. Soc., Perkin
Trans. 1, 1997, 2837–2842.
and their chemical stability. However these reagents can easily
be produced in-house upon demand. As some functional
groups are not compatible with Grignard reagents this should
be borne in mind when using this methodology. In conclusion,
this alternative approach offers new possibilities for the rapid
and efficient carbon-11 labelling of drugs containing amide
functional groups for PET imaging applications.
9 (a) C. Perrio-Huard, C. Aubert and M.-C. Lasne, J. Chem. Soc., Perkin Trans.
1, 2000, 311–316; (b) F. Liger, T. Eijsbouts, F. Cadarossanesaib,
C. Tourvieille, D. Le Bars and T. Billard, Eur. J. Org. Chem., 2015, 6434–6438.
10 D. Le Bars, S. K. Luthra, V. W. Pike and C. L. Duc, Int. J. Rad. Appl.
Instrum. A, 1987, 38, 1073–1077.
11 E. W. van Tilburg, A. D. Windhorst, M. van der Mey and J. D. M.
Herscheid, J. Labelled Compd. Radiopharm., 2006, 49, 321–330.
12 J. M. Hooker, A. T. Reibel, S. M. Hill, M. J. Schueller and J. S. Fowler,
Angew. Chem., Int. Ed., 2009, 48, 3482–3485.
13 A. A. Wilson, A. Garcia, S. Houle, O. Sadovski and N. Vasdev,
Chemistry, 2011, 17, 259–264.
14 S.-Y. Lu, J. Hong and V. W. Pike, J. Labelled Compd. Radiopharm.,
2003, 46, 1249–1259.
This work was supported by Medical Research Council (MRC,
MR/K022733/1) and European Commission, FP7-PEOPLE-2012-
ITN (316882, RADIOMI). The authors acknowledge financial
support from the Department of Health via the National Institute
for Health Research (NIHR) comprehensive Biomedical Research
Centre award to Guy’s & St Thomas’ NHS Foundation Trust in
partnership with King’s College London and King’s College
Hospital NHS Foundation Trust and the Centre of Excellence
in Medical Engineering funded by the Wellcome Trust and
EPSRC under grant number WT 088641/Z/09/Z.
15 D. R. Hwang, N. R. Simpson, J. Montoya, J. J. Man and M. Laruelle,
Nucl. Med. Biol., 1999, 26, 815–819.
16 S. K. Luthra, V. W. Pike and F. Brady, J. Chem. Soc., 1985, 1423–1425.
17 S. L. Peterson, S. M. Stucka and C. J. Dinsmore, Org. Lett., 2010, 12,
1340–1343.
Notes and references
1 (a) G. Antoni, J. Labelled Compd. Radiopharm., 2015, 58, 65–72; 18 D. Saylik, M. J. Horvath, P. S. Elmes, W. R. Jackson, C. G. Lovel and
(b) J. S. Fowler and A. P. Wolf, Acc. Chem. Res., 1997, 30, 181–188. K. Moody, J. Org. Chem., 1999, 64, 3940–3946.
2 (a) B. Langstrom and H. Lundqvist, Int. J. Appl. Radiat. Isot., 1976, 19 G. Schafer, C. Matthey and J. W. Bode, Angew. Chem., Int. Ed., 2012,
27, 357–363; (b) C. Marazano, M. Maziere, G. Berger and D. Comar,
Int. J. Appl. Radiat. Isot., 1977, 28, 49–52.
3 P. W. Miller, N. J. Long, R. Vilar and A. D. Gee, Angew. Chem., Int. Ed.,
2008, 47, 8998–9033.
4 B. H. Rotstein, S. H. Liang, J. P. Holland, T. L. Collier, J. M. Hooker,
A. A. Wilson and N. Vasdev, Chem. Commun., 2013, 49, 5621–5629.
51, 9173–9175.
20 The radiochemical yields (RCY) determined by radio-HPLC of the
crude product.
21 An example of clinal tracer produced in our instute is [11C]Methionine
produced by 11C-methylation using [11C]CH3I obtained via the ‘‘gas-
phase’’ method.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2017