Synthesis of 3,4-Fused Tricyclic Indoles through Cascade Carbopalladation and C-H Amination: Development and Total Synthesis of Rucaparib

Article abstract of DOI:10.1021/acs.orglett.0c01513

3,4-Fused tricyclic indole scaffolds are ubiquitous in bioactive natural products and pharmaceuticals. A new protocol for the synthesis of 3,4-fused tricyclic indoles has been developed through cascade carbopalladation and C-H amination with N,N-di-tert-butyldiaziridinone. The protocol allows access to a range of 3,4-fused tricyclic indoles, including those containing various linkers and fused with medium-sized rings. Rucaparib can be synthesized via this reaction, providing an advantageous synthetic method for the FDA-approved cancer medicine.

Full text of DOI:10.1021/acs.orglett.0c01513

pubs.acs.org/OrgLett
Letter
Synthesis of 3,4-Fused Tricyclic Indoles through Cascade
Carbopalladation and C−H Amination: Development and Total
Synthesis of Rucaparib
Cang Cheng, Xiang Zuo, Dongdong Tu, Bin Wan, and Yanghui Zhang*
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ABSTRACT: 3,4-Fused tricyclic indole scaffolds are ubiquitous in
bioactive natural products and pharmaceuticals. A new protocol for
the synthesis of 3,4-fused tricyclic indoles has been developed
through cascade carbopalladation and C−H amination with N,N-
di-tert-butyldiaziridinone. The protocol allows access to a range of
3,4-fused tricyclic indoles, including those containing various
linkers and fused with medium-sized rings. Rucaparib can be
synthesized via this reaction, providing an advantageous synthetic
method for the FDA-approved cancer medicine.
3,4-Fused tricyclic indoles are essential core structures of many
bioactive natural products and pharmaceuticals and are
attractive synthetic targets in the fields of medicinal chemistry
and organic synthesis (Figure 1)1 The synthesis of these
3,4-fused tricyclic indoles using alkyne-tethered ortho-iodoani-
lines via palladium-catalyzed intramolecular Larock indole
synthesis.4 Replacing the alkyne moiety with an allene group
can also give 3,4-fused tricyclic indoles.5 The intramolecular
Larock indole syntheses require the presynthesis of multi-
substituted haloanilines as starting materials. Notably, an
innovative method through C−H alkenylation using anilines
tethered to an alkyne at the meta position has been
independently developed by the groups of Jia, Xu and Liu,
Zhou and Li, and Nemoto.6 However, to activate more
hindered C−H bonds, the reactions are restricted to indoles
bearing an alkoxy group.2a The group of Miura and Murakami
disclosed an elegant dearomatizing annulation reaction from
1,2,3-triazole-tethered arenes.7 However, this method requires
an additional oxidation reaction to form indole products and
the presynthesis of 1,2,3-triazole-containing substrates. In all of
these reactions, 3,4-fused tricyclic indoles are directly formed
from the intramolecular cyclization of the substrates, and all of
the substrates preinstalled with a nitrogen-containing group are
used.8 Considerable efforts should still be devoted to
developing efficient and general methods for the synthesis of
3,4-fused tricyclic indoles.
Figure 1. Selected examples of 3,4-fused tricyclic indoles.
complex indole molecules is challenging and has been the
subject of numerous synthetic studies.2 Traditionally, 3,4-fused
tricyclic indoles can be synthesized by the intramolecular
cyclization of 4-substituted or 3,4-disubstituted indoles.
However, the functionalization of the four-positions of indoles
is challenging and usually requires multistep synthesis.
Recently, the construction of 3,4-fused tricyclic indoles via
indole ring formation has gained considerable interest.
Compared with the traditional methods, this innovative
approach avoids the laborious synthesis of four-substituted
indole precursors and has great advantages. An elegant
example is the method based on an intramolecular Fischer
indole synthesis, which was developed by Cho and coworkers.3
In 2013, the groups of Boger and Jia reported the synthesis of
Over the past several decades, transition-metal-catalyzed C−
H functionalization underwent explosive growth.9 C−H
Received: May 2, 2020
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© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
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amination has also gained considerable interest and emerged as
an innovative and powerful strategy for the formation of C−N
bonds.10 In this context, N,N-di-tert-butyldiaziridinone repre-
sents a particularly intriguing C−H aminating reagent.11 N,N-
Di-tert-butyldiaziridinone can diaminate C,C-palladacycles
formed by palladium-catalyzed C−H activation, which
provides an innovative method for the synthesis of N-
heterocycles. We envisioned that a method for the
construction of a 3,4-fused tricyclic indole skeleton could be
developed via the C−H amination reaction with N,N-di-tert-
butyldiaziridinone. It should be mentioned that the indole
moieties of bioactive 3,4-fused tricyclic indoles are bridged
with different ring sizes, and many compounds contain a
medium-sized ring. The formation of medium-sized rings is
often challenging due to entropic factors and transannular
interactions.12 The reaction for the synthesis of 3,4-fused
tricyclic indoles via C−H amination with N,N-di-tert-
butyldiaziridinone is a cascade process and involves several
Pd(II) species during the catalytic cycle. Achieving selective
cyclization and amination in different stages of the catalytic
cycle is crucial for the development of the reaction and is
expected to be challenging, in particular, in the formation of
medium-sized rings.
We commenced the studies by investigating the reaction of
model substrate 1a with N,N-di-tert-butyldiaziridinone (2). As
shown in Table 1, 3,4-fused six-membered tricyclic indole 3a
Table 1. Survey of the Reaction Conditions
Pd(OAc)2
(mol %)
a
entry
solvent
ligand (mol %)
yield (%)
b
1
2
3
4
5
6
7
8
9
10
10
10
10
10
10
10
5
5
2
1
DMF
DMF
toluene
THF
MeCN
DMA
DMF
DMF
DMF
DMF
90
c
98 (96 )
52
38
90
78
P(o-tol)3 (10) 95
P(o-tol)3 (4)
P(o-tol)3 (2)
91
d
56 (78% )
(SM = 41%)
a
1
Rucaparib is a poly(ADP-ribose) polymerase-1 (PARP-1)
inhibitor.13 It was approved by the FDA as Rubraca (rucaparib
camsylate) for the treatment of ovarian cancer in 2016.14 Only
several syntheses of rucaparib have been reported, and they
generally follow two strategies (Figure 2): (1) The 3,4-fused
Yields were determined by H NMR analysis of the crude reaction
b
mixture using CHCl2CHCl2 as the internal standard. No KOPiv.
c
d
Isolated yield. 36 h.
was formed in 90% yield using 10 mol % of Pd(OAc)2 as the
catalyst and Cs2CO3 as the base (entry 1). The yield was
improved to 98% by adding 0.5 equiv of KOPiv (entry 2). 3a
was not observed or was obtained in a lower yield when the
reaction was carried out in other solvents (entries 3−6).
Decreasing the amount of Pd(OAc)2 led to a lower yield
(entry 7). However, an excellent yield was obtained by using
ligand P(o-tol)3 (entry 8). Notably, even 2 mol % of Pd(OAc)2
gave a yield of 91% (entry 9), which indicates the high
efficiency and practical utility of the protocol. The yield
dramatically decreased when the catalyst loading was reduced
to 1 mol % (entry 10). In this reaction, most of 1a was
recovered, and no other side products were observed. The
yield was improved to 78% when the reaction time was
prolonged to 36 h.
Figure 2. Strategies for the synthesis of rucaparib.
Having developed an efficient protocol for the construction
of 3,4-fused tricyclic indoles, we then studied the substrate
scope of the reaction. The compatibility of various iodoaryl
groups was first investigated. As shown in Scheme 1, phenyl
groups bearing an electron-donating substituent (methyl and
methoxyl) were compatible (3b and 3c). It should be noted
that the yields decreased only slightly when 2 mol % of
Pd(OAc)2 was used. Although the presence of a withdrawing
substituent (trifluoromethyl) led to a moderate yield, it could
be enhanced to 85% by adding ligand P(o-tol)3 (3d). The
chloro and fluoro groups were well-tolerated, and the desired
products were formed in good or excellent yield (3e and 3f).
Even the pyridine moiety-containing substrates were suitable,
and the reaction provides a new method for the synthesis
tricyclic 6-azaindoles (3g−i).
Next, the performance of diverse substituents on the alkynyl
groups was studied. A range of phenyl groups bearing different
substituents were first examined. As shown in Scheme 2, the
phenyl groups bearing various functionalities at the para
positions were compatible, and high yields could be achieved
by choosing suitable conditions (3j−m). The meta- and ortho-
tricyclic indoles are constructed from 6-fluoro indole
derivatives, and the 4-((methylamino)methyl)phenyl group is
introduced at a late stage.1g,15 The synthesis of 6-fluoro indole
derivatives requires multiple steps, and the installation of a
functional group to the three- or four-positions of the indoles is
often low-yielding. Furthermore, an additional brominating
reaction is needed to introduce the 4-((methylamino)methyl)-
phenyl group. (2) Multisubstituted indole derivatives bearing a
4-((methylamino)methyl)phenyl group are first synthesized,
and rucaparib is formed via the late-stage cyclization. However,
the synthesis of multisubstituted indole derivatives requires
quite a number of steps.13,16 It is still highly desirable to
develop facile and efficient synthetic methods for rucaparib.
Herein we report a general synthetic protocol for 3,4-fused
tricyclic indoles through palladium-catalyzed C−H amination
with N,N-di-tert-butyldiaziridinone. The reaction was success-
fully applied to the synthesis of rucaparib, which represents an
advantageous method for the synthesis of this cancer medicine.
B
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Scheme 1. Substrate Scope with the Respect to the Iodoaryl
Groups
Scheme 3. Substrate Scope with the Respect to the Linkers
a
b
Pd(OAc)2 (10 mol %); Pd(OAc)2 (10 mol %), P(o-tol)3 (20 mol
c
%). Pd(OAc)2 (2 mol %), P(o-tol)3 (4 mol %).
a
b
Pd(OAc)2 (10 mol %). Pd(OAc)2 (10 mol %), P(o-tol)3 (20 mol
c
%). Pd(OAc)2 (2 mol %), P(o-tol)3 (4 mol %).
the reaction was far less efficient in the presence of an ester
linker (5h).
Scheme 2. Substrate Scope with the Respect to the Alkynyl
Groups
Many bioactive 3,4-fused tricyclic indoles contain a medium-
sized ring. The construction of medium-sized rings still
remains a challenge in organic synthesis. Notably, our reaction
is applicable to the formation of medium-sized rings and
therefore provides a facile method for the synthesis of indole-
annulated medium-sized ring compounds (Scheme 4). A
Scheme 4. Substrate Scope with the Respect to the Ring
Sizes
a
b
Pd(OAc)2 (10 mol %). Pd(OAc)2 (10 mol %), P(o-tol)3 (20 mol
c
%). Pd(OAc)2 (2 mol %), P(o-tol)3 (4 mol %).
substituted phenyl groups also gave high yields for both
electron-donating and -withdrawing substituents (3n−q). The
naphthyl and thiophenyl groups were suitable, and the
corresponding products were formed in a good or moderate
yield (3r and 3s). The suitability of an alkyl group was also
investigated (3t). Although the desired product was not
generated in the case of a methyl group under the conditions
without a ligand, it was obtained in a 76% yield by using P(o-
tol)3. The unsubstituted alkyne failed to form the desired
product (3u).
The compatibility of different linkers was also investigated.
As shown in Scheme 3, the amides bearing an alkyl or phenyl
group were compatible, and the reactions were high-yielding
(5a−c). The presence of a methyl group on the methylene
carbon resulted in a yield of 80% (5d). An amide linker that
links the iodobenzene and the alkyne in an opposite way gave a
much lower yield (5e). It is notable that excellent yields were
obtained for amine and ether linkages (5f and 5g). However,
seven-membered indole product was formed in 88% yield
under the standard conditions consisting of ligand P(o-tol)3
(5i). An ether linkage was also compatible, albeit in a lower
yield (5j). Eight- and even nine-membered rings could be
constructed, and various linkers were tolerated (5k−o).
The synthesis of rucaparib started with the Sonogashira
coupling of commercially available compounds 6 and 7. The
reaction afforded compound 8 in high yield. The resulting
compound 8 was converted to p-methoxybenzyl (PMB)-
protected amine compound 9. The condensation of 9 and 10,
which is cheap, yielded the key intermediate 11. 11 was
subjected to the standard conditions to afford compound 12 in
87% yield. A yield of 61% was still obtained, even using 2 mol
% of Pd(OAc)2. Finally, treating 12 with trifluoroacetic acid
C
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(TFA) provided rucaparib in 88% yield. Notably, the synthesis
was carried out on a gram scale, which demonstrated the
practical utility of this method (Scheme 5). This new method
has advantages, including readily available starting materials,
few steps, and a high overall yield.
FDA-approved cancer medicine, providing an advantageous
synthetic method for it.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01513.
Scheme 5. Synthesis of Rucaparib
General information, general procedure for the synthesis
of the substrates, general procedure for the synthesis of
3,4-fused tricyclic indoles, procedure for the synthesis of
rucaparib, characterization of the substrates, character-
ization of the products, NMR spectra, and references-
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
Yanghui Zhang − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China;
orcid.org/0000-0003-2189-3496; Email: zhangyanghui@
tongji.edu.cn
a
b
Pd(OAc)2 (5 mol %), P(o-tol)3 (10 mol %). Pd(OAc)2 (2 mol %),
P(o-tol)3 (4 mol %).
Authors
A tentative mechanism is proposed for the tricyclic indole-
forming reaction by using compound 1a as the model substrate
(Scheme 6).11 The reaction is initiated by the oxidative
Cang Cheng − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China
Xiang Zuo − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China
Dongdong Tu − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China
Bin Wan − School of Chemical Science and Engineering,
Shanghai Key Laboratory of Chemical Assessment and
Sustainability, Tongji University, Shanghai 200092, China
Scheme 6. Plausible Mechanism
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01513
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was supported by the National Natural Science
Foundation of China (no. 21971196 and 21672162)
addition of 1a to Pd(0), affording aryl Pd(II) species A. The
subsequent intramolecular carbopalladation yields vinyl Pd(II)
species B. The Pd(II) species cleaves the aryl C−H bonds to
form C,C-palladacycle C as the key intermediate. The resulting
C,C-palladacycle undergoes oxidative addition to N,N-di-tert-
butyldiaziridinone to generate pallada(IV)cycle D. Intermedi-
ate D is converted into tricyclic indole product 3a with the
release of tert-butyl isocyanate (tBuNCO) and Pd(0).
In conclusion, we have developed a general protocol for the
synthesis of 3,4-fused tricyclic indoles through palladium-
catalyzed C−H amination with N,N-di-tert-butyldiaziridinone.
A wide range of 3,4-fused tricyclic indoles, including those
containing various linkers and fused with medium-sized rings,
can be synthesized by this new protocol. Notably, the reaction
has been successfully applied to the synthesis of rucaparib, an
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