A general procedure for carbon isotope labeling of linear urea derivatives with carbon dioxide

Article abstract of DOI:10.1039/d1cc02665h

Carbon isotope labeling is a traceless technology, which allows tracking the fate of organic compounds either in the environment or in living organisms. This article reports on a general approach to label urea derivatives with all carbon isotopes, including¹⁴C and¹¹C, based on a Staudinger aza-Wittig sequence. It provides access to all aliphatic/aromatic urea combinations.

Full text of DOI:10.1039/d1cc02665h

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A general procedure for carbon isotope labeling
of linear urea derivatives with carbon dioxide†
Cite this: Chem. Commun., 2021,
57, 6680
b
Victor Babin,a Antoine Sallustrau,a Olivier Loreau,a Fabien Caille,
´
Amelie Goudet,a Heloıse Cahuzac,c Antonio Del Vecchio,a Frederic Taran
and
a
Received 21st May 2021,
Accepted 26th May 2021
´
´
´ ´
¨
a
Davide Audisio
*
DOI: 10.1039/d1cc02665h
rsc.li/chemcomm
Carbon isotope labeling is a traceless technology, which allows
Two main procedures have been established for the labeling
tracking the fate of organic compounds either in the environment of linear ureas directly from [11/14]CO2. The first is the Mitsu-
or in living organisms. This article reports on a general approach to nobu approach using a base (BEMP or DBU) to trap CO2 and a
label urea derivatives with all carbon isotopes, including 14C and dehydrating agent (POCl3) to afford unsymmetrical ureas.12
11C, based on a Staudinger aza-Wittig sequence. It provides access This two-step procedure has been utilized both for 11C and
to all aliphatic/aromatic urea combinations.
14C labeling, but shows moderate functional group compat-
ibility. In 2006, van Tilburg published an aza-Wittig approach
Radioisotope labeling has multiple implications in drug design starting from commercial triphenylphosphine phenylimide
and development.1–3 Two radioisotopes of carbon are pharma- and amines affording unsymmetrical ureas with moderate
ceutically relevant: carbon-11 (11C), a positon (b+) emitter with a yields.13 This methodology was applied only to four amine
remarkably short half-life (t1/2 = 20.4 minutes), and carbon-14 substrates. Importantly, it was unsuccessful for drug radiola-
(14C), a long-lived electron (bÀ) emitter (t1/2 = 5730 years).4
beling despite different attempts to modify and reverse the
The urea substructure is found in numerous FDA approved nature of the triphenylphosphinimine/amine partners.14 It is
drugs for a variety of human diseases and pharmaceutically worth noting that all previous procedures are limited to ureas
relevant compounds (Scheme 1A).5 This functional group repre-
sents an attractive target for isotope labeling. Three main
reagents are known for the radiocarbonylation of ureas.6
[
11/14C]Phosgene was used due to its high reactivity, but its
complex chemical production requiring hazardous reactants,
such as C-labeled CO and Cl2, and the need for specialized
equipment have reduced its utilization.7,8 11/14C]Carbon mon-
[
oxide is suitable for installing the carbonyl group of ureas,
through metal-catalyzed procedures or selenium mediated
synthesis.9 Nevertheless, its utilization is not trivial due to the
toxicity and the time-consuming production.10 In addition,
[14C]CO was described to undergo radiolysis and its limited
stability prevents storage.11 11/14C]Carbon dioxide fulfills the
[
handiest criteria: it is the primary radioactive source from
which all 11C- and 14C-labeled compounds derive, and it has a
remarkable stability to radiolysis and a suitable safety profile.4
a
´
Universite Paris Saclay, CEA Service de Chimie Bio-organique et Marquage, DMTS,
Gif-sur-Yvette, F-91191, France. E-mail: Davide.Audisio@cea.fr
b
c
´
´
Universite Paris-Saclay, Inserm, CNRS, CEA, Laboratoire d’Imagerie Biomedicale
Multimodale Paris-Saclay, 91401, Orsay, France
´
´
´
´
Universite Paris-Saclay, Departement Medicaments et Technologies pour la sante
Scheme 1 (A) Examples of pharmaceutically relevant ureas; (B) synthetic
strategies to access ureas from radiolabeled CO2; bottom: Staudinger
Aza-Wittig procedure.
(DMTS), CEA, INRAE, SIMoS, Gif-sur-Yvette 91191, France
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cc02665h
6680 | Chem. Commun., 2021, 57, 6680–6683
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Scheme 2 Reaction scope. Green colored circles denote the position of the carbon atoms labeled. Reaction conditions: a70 1C instead of 25 1C; bazide
(0.1 mmol, 2 equiv.), amine (0.05 mmol, 1 equiv.), PPhMe2 (0.1 mmol, 2 equiv.), [13C]CO2 (2.2 equiv.); cDIPEA (2 equiv.) was added; dDIPEA (1 equiv.) was
added. e2 hours instead of 10 min; f5 equivalents of cyclohexylamine; g0.5 mmol scale. Am: molar activity. For all products, isolated yields are indicated.
bearing carbon-substituents on positions 1 and 3. Sulfonylur- carbamates from stoichiometric [11/14]CO2.16 The close proxi-
eas, hydroxyl ureas, semicarbazides or terminal ureas still mity of the nucleophile to the azide was fundamental for a
constitute a challenge for radiolabeling.15
facile intramolecular attack onto the isocyanate generated
The current state-of-the-art highlights the lack of a robust in situ. We now look to develop an intermolecular SAW proce-
and general method to efficiently label unsymmetrical ureas dure to offer straightforward access to unsymmetrical carbon
and their derivatives. We reported an intramolecular Staudin- labeled ureas including hydroxyl ureas, terminal ureas, semi-
ger Aza-Wittig (SAW) approach to label cyclic ureas and carbazides and sulfonylureas.
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At first, we explored the SAW procedure with a combination amines. Interestingly, the SAW exhibited high chemoselectivity
of aliphatic azides and aliphatic amines (Scheme 2A). The toward aniline in respect to phenol: [13C]26 was isolated in 54%
reaction was performed under previous optimized experi- yield and no traces of carbamate were detected. Linear urea
mental conditions16 with benzylazide and s-butylamine in labeling was also possible with poor electron withdrawing
acetonitrile, in the presence of one equivalent of PPhMe2. aniline leading to aromatic ureas [13C]28 and [13C]29. However,
[13C]CO2 was utilized as 14C-surrogate, and its precise addition in the presence of strong electron-withdrawing anilines, the
was monitored utilizing an RC Tritec carboxylation manifold. reaction did not occur and [13C]30 was not observed.
The introduction of [13C]CO2 resulted in rapid formation of
We next turned our attention to the labeling of urea derivatives
expected urea [13C]1 at room temperature in less than (Scheme 2E). Semicarbazides, a precursor to semicarbazones, was
5 minutes in 84% isolated yield. On a 0.5 mmol scale, [13C]1 labeled through this procedure with the use of both aromatic and
was isolated in 62% yield. The procedure is tolerant towards aliphatic hydrazines [13C]31–33. To the best of our knowledge,
sterically bulky dicyclohexylamine ([13C]2) and chemoselective no radiochemical procedure has been described for radio-
for aliphatic amine in respect to alcohols ([13C]4). The use of a carbonylation of this family of derivatives. To access 1-hydroxy-
dipeptide and varying the nature of the azide did not affect the 3-(naphthalen-1-yl)urea [13C]34, hydroxylamine hydrochloride was
reaction. Secondary cycloalkyl azide provided [13C]5 in 55% utilized in the presence of DIPEA. Pleasingly, enrichment was
yield. In all the reactions performed, no parasite formation of observed in 13C-NMR corresponding to the expected hydroxyurea,
symmetrical urea was observed in 13C-NMR of the crude which was next isolated in 45% yield. The utilization of a
mixture. We subsequently explored the procedure with aro- commercially available solution of ammonia in methanol,
matic azides (Scheme 2B). The presence of electron donating assisted by N-methylimidazole at 80 1C, allowed isolating the
groups on the azide partner successfully leads to the unsymme- desired terminal urea in 43% yield. Though not optimized, this
trical ureas [13C]7, 12–13, and 15 in 53% to 77% yield. The procedure allows access to terminal ureas in one single operation
presence of electron withdrawing groups (EWG) on the aro- from CO2 with no need of protecting groups. Next, a series of
matic azide negatively impacted the outcome of the reaction representative pharmaceuticals were labeled starting from appro-
and 4-trifluoromethyl and 4-methylbenzoate ureas [13C]8–9 priate combinations of azides and amines (Scheme 2F). [13C]36,
were isolated in moderate yields. The anilines corresponding the methyl ester of A1120, an inhibitor of Retinol Binding Protein
to the hydrolysis of the iminophosphorane (IP) intermediates 4 (RBP4),17 was isolated in 66% yield. The structure of beta-
were observed as major by-products. In the presence of stronger blocker talinolol 37 was particularly interesting as its labeling
EWG such as cyano or nitro, only the aniline by-products were would provide a challenge using phosgene or the Mitsunobu/
recovered and no traces of the desired ureas were observed. To BEMP strategies. Pleasingly, the SAW enabled [13C]37 to be
prevent hydrolysis of the IP, the reactions were performed obtained in a fully chemo-selective and high yield. [13C]38, a
under strict anhydrous conditions in deuterated solvent. By primaquine analogue,18 and [13C]39, a positional isomer of
13C-NMR, the formation of the expected urea was observed, but JNJ-40355003, an inhibitor of the Fatty Acid Amide Hydrolase
the major product detected was the IP intermediate (31P-NMR (FAAH)19 were synthesized in 32% to 71% yield, respectively.
d = 11.9 ppm), suggesting that the isocyanate formation is the Aromatic urea [13C]40, a tau aggregation inhibitor,20 was obtained
rate determining step (see the ESI† for details). To favor the aza- in 36% yield, while kinase inhibitor sorafenib [13C]41 was
Wittig step, the procedure was reproduced in the glove box and obtained in 47% yield in the presence of N-methylimidazole.
the reaction heated at 70 1C for 10 minutes. Gratifyingly, it was Semicarbazide [13C]42, recently described as a full agonist of the
possible to isolate the corresponding linear ureas [13C]10 and D4 receptor,21 was labeled with 66% yield. Finally, antidiabetic
[
13C]11 in 73% and 68% yield, respectively (see the ESI† for sulfonylurea glyburide [13C]43 was labeled for the first time on the
more details). In the presence of piperazine, a double SAW took carbonyl position. After deprotonation of the corresponding sul-
place in the presence of two equivalents of phosphine, azide fonamide with sodium hydride, the sequence successfully pro-
and [13C]CO2 and bis-urea [13C]16 was isolated in 34% yield. vided the labeled urea with 40% yield using cyclohexylazide as a
Though this result was not further optimized, it paves the way partner. Finally, we applied this protocol to 14C-radiolabeling
for the development of high molar activity 14C-derivatives. using high molar activity (Am) [14C]CO2 (Am: 2.17 GBq mmolÀ1).
Finally, the direct diversification of drugs was performed. The In agreement with the results obtained with 13C, four drugs were
three commercially available pharmaceuticals trimetazidine, labeled with carbon 14 in high molar activities. Talinolol [14C]37
paroxetine and duloxetine were easily functionalized was labeled with a Am of 2.05 GBq mmolÀ1 and 82% yield. [14C]38
(Scheme 2C). We next explored the SAW in the presence of less and FAAH inhibitor [14C]39 were respectively labeled with Am of
nucleophilic aniline derivatives (Scheme 2D). As expected in the 1.80 GBq mmolÀ1 and 1.94 GBq mmolÀ1 in 60% and 76% yields.
presence of m-toluidine, the corresponding urea [13C]22 was Sorafenib [14C]41 was obtained with Am of 2.07 GBq mmolÀ1
.
obtained in poor yield (see the ESI†). After careful optimization, Based on the short reaction time associated with the SAW
N-methylimidazole was identified as the most suitable additive, procedure, application to 11C-labeling seemed promising. None-
affording the expected compound in ten minutes at 80 1C in theless, a series of challenges have to be faced: the high energy b+
62% isolated yield (see the ESI† for details). Electron-donating emission that requires the use of specialized automated systems,
anilines reacted smoothly affording the labeled unsymmetrical the minute concentrations of [11C]CO2 produced in the cyclotron
urea [13C]22–25 in the presence of both primary and secondary and its short half-life (20.4 min).22 Despite the challenges, [11C]1
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We thank the European Union’s Horizon 2020 research and
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Conflicts of interest
There are no conflicts to declare.
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