Synthesis of15N-labeled heterocyclesviathe cleavage of C-N bonds of anilines and glycine-15N

Article abstract of DOI:10.1039/d1cc01734a

A nitrogen replacement process that directly incorporates the¹⁵N atom of glycine-¹⁵N into anilines was reported. The process involves a Csp²-N bond cleavage of anilines driven by dearomatization and a Csp³-N bond cleavage of glycine-¹⁵N driven by aromatization. A variety of¹⁵N-labeled aromatic heterocycles can be preparedviathis process.

Full text of DOI:10.1039/d1cc01734a

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Synthesis of N-labeled heterocycles via
the cleavage of C–N bonds of anilines and
glycine-15N†
Cite this: DOI: 10.1039/d1cc01734a
Received 1st April 2021,
Accepted 28th April 2021
b
ac
c
Jiwen He,a Xingguo Zhang,
Qiuqin He,c Hao Guo
*
and Renhua Fan
*
DOI: 10.1039/d1cc01734a
rsc.li/chemcomm
A nitrogen replacement process that directly incorporates the metabolism of compounds at various stages of drug develop-
15N atom of glycine-15N into anilines was reported. The process ment.13 Simultaneously 15N labels is required to enhance the
involves a Csp2–N bond cleavage of anilines driven by dearomati- MRI signals14 by hyperpolarization techniques15 and has been
zation and a Csp3–N bond cleavage of glycine-15N driven by employed for synthesis of a wide range of small biomolecules
aromatization. A variety of 15N-labeled aromatic heterocycles can used in vivo to probe metabolism and function.16 Thus it is of
be prepared via this process.
great significance to develop effective method to introduce a
15N atom into pharmaceutical molecules and biomolecules.
The cleavage of C–N bonds is one of the most important topics 15N-labeled anilines are the key precursors to 15N-labeled
in chemistry because it provides a convenient approach to aromatic heterocycles. 15N-labeled anilines are normally pre-
generate nitrogen or/and carbon sources to facilitate the synthesis pared by the reduction of 15N-labeled nitrobenzene using
of the desired compounds from simple and available starting H15NO3 as nitrogen source,17 or the Hofmann rearrangement
materials. Compared with the cleavage of the activated C–N bonds of 15N-labeled benzamide using 15NH4NO3 or 15NH4Cl as nitro-
such as diazonium salts,1 ammonium salts,2 and strained gen source.18 The need for harsh conditions and the use of
azaheterocycles,3–5 the cleavage of unactivated C–N bonds of explosive reagents make many of these methods unsuitable for
aromatic or aliphatic amines is not easily achievable due to the practical applications. Glycine-15N, an important isotopic labeling
high C–N bond dissociation energy. In recent years, oxidative amino acid, is widely used as tracer for detecting metabolism or
cleavage of unactivated C–N bonds has drawn particular attention as precursor to other 15N-labeled amino acids. In connection with
and made great progress. For example, the groups of Huang,6 Su,7 our recent research on the dearomatization of anilines,19 we
Li,8 Jiang,9 and Yin10 have reported the successful examples of the originally conceived that glycine-15N might be ready to undergo
transition metal catalysed oxidative cleavage of C–N bonds via the an imine exchange reaction with the dearomatized intermediate
similar imine/iminium intermediates (Scheme 1a).11 But notwith- of anilines. Subsequent aromatization to restore the aromaticity of
standing these advances, the development of a metal-free oxida- the imine exchange product might drive a rearrangement to form
tive cleavage method for the unactivated C–N bonds under mild 15N-labeled anilines.
conditions is seldom explored, especially for the C–N bonds of
primary amines. In the present metal-free cleavage process, the
iminium intermediates are normally formed from the radical C–H
activation and oxidation (Scheme 1b).12
Incorporation of a 15N atom in biologically active molecules
is an important method in studying the pharmacokinetics and
a Academy for Engineering and Technology, Fudan University, 220 Handan Road,
Shanghai 200433, China
b College of Chemistry and Materials Engineering, Wenzhou University,
Wenzhou 325035, China
c Department of Chemistry, Fudan University, 2005 Songhu Road,
Shanghai 200438, China
† Electronic supplementary information (ESI) available: General experimental
procedures, characterization data, and 1H and 13C NMR spectra of new com-
Scheme 1 Oxidative cleavage of C–N bonds.
pounds. See DOI: 10.1039/d1cc01734a
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Herein, we report a nitrogen replacement process that
directly incorporates the 15N atom of glycine-15N into anilines
via a Csp2–N bond cleavage of anilines driven by dearomatiza-
tion and a Csp3–N bond cleavage of glycine-15N driven by
aromatization. A variety of 15N-labeled aromatic heterocycles
can be prepared by using glycine-15N as the nitrogen source
(Scheme 1c).
The main challenge to implementation of this tactic was the
realization of an efficient imine exchange reaction between the
dearomatized intermediate of anilines with glycine-15N because
of the multifunctional structural features of the dearomatized
intermediate. In an initial test, MeOH was removed in vacuo
after the oxidative dearomatization of 4-methyl-2-(2-phenylethynyl)
benzenamine 1, and CH2Cl2 was used as the solvent for the reaction
of the crude dearomatization product with ethyl glycinate-15N. No
reaction occurred in the absence of catalyst. The addition of a variety
of Lewis acids did not promote the imine exchange reaction but
promote a [1,2] methyl group migration and cyclization of the
dearomatized intermediate leading to the formation of 4-methyl
substituted indoles.20 Because secondary amines have been widely
used to activate a,b-unsaturated compounds via an iminium
intermediate,21 1 equivalent of piperidine was added to pro-
mote the imine exchange reaction. To our delight, the formation
of 15N-labeled 4-methyl-2-(2-phenylethynyl)benzenamine
obtained in 45% yield. Screening of the ratio of reagents and
solvents indicated that, when 3 equivalents of piperidine was
employed, the commercial ethyl glycinate-15N hydrochloride could
2 was
Scheme 3 Tentative reaction pathway.
be directly used and the reaction in CH2Cl2 at room temperature glyoxylate generated from the hydrolysis of intermediate VI,
gave rise to 2 in 94% yield. A variety of secondary amines were then 0.6 equivalents of AuCl3 was added into the reaction mixture.
examined, piperidine proved to be the best promoter for the Compound 3 which is supposed to be formed by the reaction of
formation of 15N-labeled product 2 (Scheme 2).
two molecules of product 2 with the ethyl glyoxylate was
A tentative pathway for the piperidine-promoted reaction is obtained.
depicted in Scheme 3. The inert Csp2–N bond in 2-alkynylanilines
In the presence of 20 mol% of AgOTf, 15N-labeled 4-methyl-
is converted to the CQN bond in 2-alkynylcyclohexadienimine 2-(2-phenylethynyl)benzenamine was converted to 15N-labeled
intermediate I through oxidative dearomatization. Condensation indole 4 in an almost quantitative yield. Under the established
of piperidine with I forms an iminium intermediate III that then conditions, the scope for the transformation of 2-alkynylanilines
reacts with ethyl glycinate-15N to generate the second cyclohex- to 15N-labeled indoles was investigated (Scheme 4). The length of
adienimine intermediate V. Aromatization might be a great the linear alkyl group at the C4 position of 2-alkynylanilines had
driving force to drive a rearrangement to restore the aromaticity no influence on the transformation. For example, the reactions of
and convert the Csp3–N bond in ethyl glycinate-15N to the CQN 4-ethyl, 4-butyl and 4-dodecyl-2-(phenylethynyl)aniline gave rise to
bond in the N-aryl imine intermediate VI. Subsequent hydrolysis the corresponding products 5, 6 and 7 in 94%, 93% and 90%
of the imine group produces 15N-labeled 2-alkynylanilines.
yield, respectively. In addition to the linear alkyl group, isopropyl,
There is some evidence to support the hypothesized path- cyclohexyl, or phenyl groups can be the para-substituent in the
way. The analysis of the reaction mixture by LC–MS confirmed 2-alkynylanilines. The steric hindrance caused by the second
the generation of intermediates II and VI. To capture the ethyl methyl group or a phenyl group at the C6 or the C5 position of
2-alkynylanilines tended to diminish the yield of compounds 12,
13 and 14. A range of functional groups on the benzene ring of the
alkynyl moiety were compatible with the reaction conditions. An
electronic effect of the functional groups was observed. Reactions
with electron-rich substituents such as the methyl or the tert-butyl
group gave higher yields than those with electron-deficient groups
such as the nitro or the carbonyl group or the halogen atoms.
Reaction of substrates bearing a thienyl, alkyl, cycloalkyl, or TMS
group at the 2-alkynyl moiety proceeded smoothly leading to the
formation of the corresponding 2-substituted 15N-labeled indoles
Scheme 2 Evaluation of secondary amines as promoters.
in moderate to good yield.
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In summary, we have developed
a
nitrogen replace-
ment process that directly incorporates the 15N atom of
glycine-15N into anilines. The process involves a Csp2–N bond
cleavage of anilines driven by dearomatization and a Csp3–N
bond cleavage of glycine-15N driven by aromatization. A variety
of 15N-labeled aromatic heterocycles including indoles, acri-
dines, quinolines, carbazoles and phenanthridines can be
prepared via this process. The extension of this method to
other aniline system and the application of this strategy to the
synthesis of useful 15N-labeled molecules are in progress.
We are grateful to the National Natural Science Foundation
of China (21572033, 21971043) and the Science and Technology
Commission of Shanghai Municipality (18XD1400800, 19ZR140
3400) for support of this research.
Conflicts of interest
There are no conflicts to declare.
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Scheme 5 Synthesis of 15N-labeled heterocycles.
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