The construction of chiral fused azabicycles using a Pd-catalyzed allylic substitution cascade and asymmetric desymmetrization strategy

Article abstract of DOI:10.1021/acs.orglett.6b03529

A highly enantioselective Pd-catalyzed asymmetric allylic substitution cascade of cyclic N-sulfonylimines with an accompanying asymmetric desymmetrization has been developed for the construction of fused tetrahydroindole derivatives bearing two chiral centers. Mechanistic studies confirmed that the cascade reaction proceeds by initial allylic alkylation and subsequent allylic amination. The first alkylation is a chirality-control step and represents an asymmetric desymmetrization of ciscyclic allyl diacetates. The reaction has been performed on a gram scale, and the desired products can take part in several transformations.

Full text of DOI:10.1021/acs.orglett.6b03529

Letter
pubs.acs.org/OrgLett
The Construction of Chiral Fused Azabicycles Using a Pd-Catalyzed
Allylic Substitution Cascade and Asymmetric Desymmetrization
Strategy
Qianjin An,† Delong Liu,*,† Jiefeng Shen,† Yangang Liu,† and Wanbin Zhang*,†,‡
‡
†School of Pharmacy and School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road,
Shanghai 200240, P. R. China
S
* Supporting Information
ABSTRACT: A highly enantioselective Pd-catalyzed asymmetric allylic substitution cascade of cyclic N-sulfonylimines with an
accompanying asymmetric desymmetrization has been developed for the construction of fused tetrahydroindole derivatives
bearing two chiral centers. Mechanistic studies confirmed that the cascade reaction proceeds by initial allylic alkylation and
subsequent allylic amination. The first alkylation is a chirality-control step and represents an asymmetric desymmetrization of cis-
cyclic allyl diacetates. The reaction has been performed on a gram scale, and the desired products can take part in several
transformations.
hiral fused azabicycles represent an important class of
compounds because of their prevalence in bioactive
reactions such as hydrogenations,8 nucleophilic additions,9 and
cycloadditions.10 N-Sulfonylimines have also been used as
binucleophiles for the construction of chiral piperidine rings in
metal-free asymmetric cascade reactions (sequential C- and N-
alkylations) (Scheme 1a).11 On the basis of our previous work
concerning metal-catalyzed asymmetric allylic substitutions12
and the use of N-sulfonylimines as binucleophiles in organo-
catalyzed asymmetric cascade reactions,13 we herein report the
efficient construction of fused chiral octahydroindole deriva-
tives bearing multiple chiral centers. The strategy employs N-
sulfonylimines as binucleophiles and cis-cyclic allyl diacetates as
allylic substrates14 in a Pd-catalyzed asymmetric allylic
substitution cascade with an accompanying asymmetric
desymmetrization (Scheme 1b).
C
natural products and chiral drugs.1 Among these, azabicycles
possessing octahydroindole skeletons are some of the most
important.2 For example, such compounds can serve as key
intermediates in the synthesis of angiotensin-converting
enzyme inhibitors (ACEIs) such as trandolapril and perindo-
pril.3 Several strategies for the synthesis of this skeleton have
been reported,4 but these methodologies can only be used for
the synthesis of a small variety of azabicyclic products using a
limited number of substrates. Therefore, an efficient pathway
toward the synthesis of diverse chiral fused azabicyclic
octahydroindoles from simple and abundant starting materials
remains challenging.
Initially, we carried out the Pd-catalyzed asymmetric allylic
substitution cascade of cyclic N-sulfonylimine 1a and cis-
cyclohex-2-ene-1,4-diyl diacetate (2a) for screening of the
reaction conditions. After examination of several chiral ligands,
we found that our previously developed tBu-RuPHOX15 ligand
is the best one for the reaction.16 The effects of different Pd
sources, solvents, and bases on the cascade reaction were then
examined.16 The optimal reaction conditions were found to be
Pd-catalyzed allylic substitution is a powerful synthetic tool
for the formation of C−C and C−X bonds (X = N, O, S, etc.).5
Asymmetric cascade reactions that utilize allylic substitutions
provide an efficient pathway for the construction of hetero-
cycles.6,7 Two strategies utilizing asymmetric cascade reactions
based on Pd-catalyzed allylic substitutions have been adopted
for the construction of heterocycles. One utilizes a double
asymmetric allylic substitution, which allows only the synthesis
of single heterocycles bearing a few chiral centers, with
somewhat modest catalytic behavior in most cases.6 A different
methodology utilizes an asymmetric conjugate addition−allylic
substitution cascade, which also gives single heterocyclic
products but with multiple chiral centers and excellent
stereoselectivities.7
t
a catalyst system consisting of [Pd(η3-C3H5)Cl]2 and Bu-
RuPHOX in the presence of DBU in 1,4-dioxane under a N2
atmosphere at 25 °C (Scheme 2).
Next, the applicability of the asymmetric allylic substitution
cascade reaction of 1 bearing different R1 and R2 substituents
Saccharin-derived cyclic N-sulfonylimines are an intriguing
class of synthons and have been used in several asymmetric
Received: November 26, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03529
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Organic Letters
Letter
a
Scheme 1. Use of N-Sulfonylimines in Asymmetric Catalysis
Scheme 3. Substrate Scope
Scheme 2. Screening of the Reaction Conditions
was evaluated (Scheme 3). N-Sulfonylimines with an electron-
withdrawing group located on the phenyl ring were first
investigated. Similar to 1a, all of these substrates (1b−h)
provided the desired products in high yields with excellent
diastereo- and enantioselectivities. We were pleased to discover
that the asymmetric catalytic results for the above reactions
were unaffected by steric hindrance. The reaction of 1i
containing two F atoms at the 3- and 4-positions of the phenyl
ring also gave excellent catalytic results. N-Sulfonylimines 1j−n
bearing electron-donating substituents were also examined. The
desired products were obtained in yields in excess of 90% with
excellent enantioselectivities. When the phenyl ring was
replaced by a naphthalene (1o), excellent catalytic results
were also observed. Finally, the aromatic ring of the N-
sulfonylimines was replaced by aliphatic substituents (1p−r).
To our delight, these substrates all provided excellent
enantioselectivities, albeit in moderate yields, with 3r bearing
a Ph group being obtained with 99.8% ee. We also carried out
the asymmetric cascade reaction by using a racemic trans-
cycloalkenediol diacetate instead of cis-isomer 2a.17 Full
conversion but complicated products were obtained even
after 72 h.16
a
t
Using the optimal reaction conditions shown in Scheme 2 with Bu-
RuPHOX as the ligand in 1,4-dioxane. All of the reactions afforded
>20:1 dr. Isolated yields are shown. The ee values were determined by
HPLC using chiral Daicel columns.
3a was obtained in 92% yield with 94% ee. 3a has the potential
to participate in a variety of transformations. First, the SO2
group of 3a and the double bond of the enamide were removed
and reduced simultaneously using sodium naphthalide (4,
pathway a).10a The double bond of the cyclohexene ring can
also be hydrogenated using RuCl2(PPh3)3 as a catalyst under 40
bar H2 at room temperature (5, pathway b). When 3a is treated
with H2 and a Pd/C catalyst (pathway c), the above two double
bonds can be reduced simultaneously to afford octahydroindole
6 in 99% yield with 1:1 dr. Alternatively, the double bond of the
enamide can be reduced in the presence of Et3SiH/BF3·Et2O to
give 7 (pathway d),13c which can then be subjected to
hydrogenation (pathway c) for 2 h to give octahydroindole 8 in
80% yield with 95% ee after a simple recrystallization. Finally,
the double bond of the enamide can be oxidized regioselec-
tively by treating 3a with m-CPBA, affording the oxidized
product 9 in high yield and ee; 9 is a key intermediate for the
synthesis of chiral alicyclic amino alcohols that possess
pharmacological properties.18
Substrates 1s and 1t with different R2 substituents (Me and t-
Bu, respectively) were next examined. High yields and excellent
enantioselectivities were still obtained for the cascade reaction.
In addition, a high yield but moderate enantioselectivity was
obtained when a five-membered cyclic diacetate (2u) was used.
However, the use of a seven-membered cyclic diacetate (2v)
gave the corresponding product with high enantioselectivity but
low yield.
In order to determine the reaction pathway, the asymmetric
allylic substitution cascade of 1a and 2a was carried out using
the above optimal reaction conditions at −30 °C for 24 h, with
the exception that THF was used in place of 1,4-dioxane
To evaluate the practicality of this catalytic process, a gram-
scale reaction of 1a with 2a was carried out under the optimal
reaction conditions. As shown in Scheme 4, the desired product
B
DOI: 10.1021/acs.orglett.6b03529
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Organic Letters
Letter
Scheme 4. Large-Scale Synthesis of 3a and Its
Transformations
Scheme 6. Proposed Reaction Mechanism
a
in high yields with up to 99.8% ee. The reaction was performed
on a gram scale, and the corresponding products allowed for
several transformations. Mechanistic studies confirmed that the
cascade reaction undergoes an allylic alkylation followed by an
allylic amination. The first alkylation is the chirality-control step
and represents an asymmetric desymmetrization of cis-2. The
alkylated intermediates determine the configuration of the
terminal cascade products 3 via a subsequent amination step.
a
Conditions: (a) Sodium naphthalide, DME, −78 °C to rt, 2 h; Ac2O,
DMAP, DCM, 4 h. (b) RuCl2(PPh3)3, K2CO3, CH3OH, 40 bar H2, 12
h. (c) Pd/C, CH3OH, H2 balloon, 12 h. (d) Et3SiH/BF3·Et2O, DCM,
−20 °C, 24 h. (e) m-CPBA, DCM, −20 °C, 8 h.
(Scheme 5). A mixture of diastereomers 10 was obtained as the
major product with only a trace amount of 3a and recovery of
ASSOCIATED CONTENT
■
S
* Supporting Information
Scheme 5. Reaction Pathway
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b03529.
Crystallographic data for 3a (CIF)
Crystallographic data for 3q (CIF)
Crystallographic data for 6-2 (CIF)
Crystallographic data for 8 (CIF)
Crystallographic data for 9 (CIF)
Experimental procedures and characterization data for all
reactions and products, including 1H and 13C NMR
spectra, HPLC data, and crystal data (PDF)
the remaining starting material. Without further isolation, the
mixture of 10 was subjected to further transformations using
racemic BINAP as a ligand, providing the desired product 3a in
98% yield with 96% ee.
The above results suggested that the fused azacycle 3a is
constructed via an allylic alkylation followed by an allylic
amination. The reaction pathway is envisaged to proceed as
follows (Scheme 6). First, combination of cis-2a with L2Pd0
provides allyl-Pd complex A. As expected, the process
represents an asymmetric desymmetrization of cis-2a because
L2Pd0 prefers to attack the R-chiral carbon of 2a. Complex A
then reacts with nucleophile 1a to give alkylated intermediate
B. Finally, B takes part in the next allylic amination, giving the
terminal fused heterocycle 3a in high yield and enantiose-
lectivity via allyl-Pd complex C. The original catalyst system
L2Pd0 is subsequently regenerated. It is clear that the
asymmetric desymmetrization is the chirality-control step and
that the chirality of 3a is determined by B.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: dlliu@sjtu.edu.cn.
*E-mail: wanbin@sjtu.edu.cn.
ORCID
Wanbin Zhang: 0000-0002-4788-4195
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
In summary, we have developed an efficient pathway for the
construction of chiral fused azabicycles using a Pd-catalyzed
allylic substitution cascade and asymmetric desymmetrization
strategy. Under the optimal reaction conditions, a series of
cyclic N-sulfonylimines can be used to give the desired products
This work was partially supported by the National Natural
Science Foundation of China (21232004, 21372152, 21402117,
21472123, and 21672142), the Program of Shanghai Subject
Chief Scientists (14XD1402300), and the Instrumental
Analysis Center of SJTU for characterization.
C
DOI: 10.1021/acs.orglett.6b03529
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Organic Letters
Letter
Chem., Int. Ed. 2014, 53, 9936. (f) Chen, Y.-J.; Chen, Y.-H.; Feng, C.-
G.; Lin, G.-Q. Org. Lett. 2014, 16, 3400.
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