Chiral Br?nsted acid-catalyzed dynamic kinetic resolution of atropisomeric: Ortho -formyl naphthamides

Article abstract of DOI:10.1039/d0cc02380a

Despite the widespread use of naphthamide atropisomers in biologically active compounds and asymmetric catalysis, few catalytic methods have succeeded in the enantioselective synthesis of these compounds. Herein, a chiral Br?nsted acid (CBA) catalysis strategy was developed for readily scalable dynamic kinetic resolution of challenging ortho-formyl naphthamides with pyrrolylanilines. The chiral axis of the atropisomeric amide and a stereogenic center were simultaneously established for a new family of potential biologically active pyrrolopyrazine compounds with high enantio- and diastereoselectivities (up to >20 : 1 d.r. and 98 : 2 e.r.). Epimerization experiments of its derivatives reveal that the N-substitution of the nearby stereogenic center could affect the configurational stability of the axially chiral aromatic amides. These results might be useful for the construction of other kinds of novel axially chiral molecules with a low rotational barrier.

Full text of DOI:10.1039/d0cc02380a

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Chiral Brønsted Acid-Catalyzed Dynamic Kinetic Resolution of
Atropisomeric ortho-Formyl Naphthamides
Zeng Gao,ab Jinlong Qian,a Huameng Yang,a Xiao-Chun Hang,*c Jinlong Zhang,*a and Gaoxi Jiang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
economy since a pair of racemic compounds could be converted
into a single enantiomer in quantitative yield theoretically,
although success in the case of direct DKR are very sparse
(Scheme 1, b). In 2002, Walsh realized the first kinetic resolution
of atropisomeric 2-vinyl-1-aryl amides in moderate results by
Sharpless asymmetric dihydroxylation.6 After that, Sakamoto
reported a conglomerate crystallization process for DKR of (2-
Despite the widespread use of naphthamide atropisomers in the
biologically active compounds and asymmetric catalysis, few cata-
lytic methods have succeeded in the enantioselective synthesis of
these compounds. Herein, a chiral Brønsted acid (CBA) catalysis
strategy was developed for readily scalable dynamic kinetic
resolution of challenging ortho-formyl naphthamides with pyrrol-
ylanilines. The atropisomeric amide’s chiral axis and a stereogenic
center were simultaneously established for a new family of po-
tential biologically active pyrrolopyrazine compounds with high
enantio- and diastereoselectivities (up to >20:1 d.r. and 98:2 e.r.).
Epimerization experiments of its derivatives reveal that the N-
substitution of nearby stereogenic center could affect the
configurational stability of axially chiral aromatic amides. These
results might be useful for the construction of other kinds of novel
axially chiral molecules with a low rotational barrier.
methoxynaphthalen-1-yl)(piperidin-1-yl)methanone
by
asymmetric photocycloaddition with 9-cyanoanthracene (9-
CNAN).7 Breakthrough came from Miller’s group in 2013. They
exploited an interesting peptide-based catalysis for effective
Axially chiral aromatic amide is one of the most important
scaffold in numerous in biologically active skeletons and
asymmetric catalysis (Scheme 1, a).1,2 Nevertheless, in a sharp
contrast with the blossoming of axially chiral biaryls,3,4 the
catalytic enantioselective construction of these compounds is
more challenging due to the lower barrier to amide
atropisomerization and difficult structural modification.
Compared with the elegant asymmetric cycloaddition to
construct aromatic rings although that always suffers from
unavoidable multistep synthetic dilemmas of starting
materials,5 directly and efficiently catalytic dynamic kinetic
resolution (DKR) of atropisomeric amides might represent the
cutting-edge technologies from the point of atom/step
a.State Key Laboratory for Oxo Synthesis and Selective Oxidation, Center for
Excellence in Molecular Synthesis, Su-zhou Research Institute of LICP, Lanzhou
Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lan-zhou 730000,
P. R. China. E-mail: zhangjl@licp.cas.cn, gxjiang@licp.cas.cn
b.University of Chinese Academy of Sciences, Beijing 100049, P. R. China
c.Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials
(IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing
211800, P. R. China. E-mail: iamxchhang@njtech.edu.cn
Scheme 1. Representative axially chiral amides and the dynamic kinetic
resolution synthesis.
†
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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J. Name., 2013, 00, 1-3 | 1
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atropisomer-selective tribromination of simple atropisomeric
benzamides although the substrates have strong dependence
of meta-phenolic hydroxyl group.8a Recently, Smith9 and Li10
groups developed independently an asymmetric DKR of amide
naphthols by bulky biscinchona alkaloid-based catalysts via
atroposelective O-benzylation and allylation. For the former
reaction, the substituent at peri-position of the substrate was
very essential.9 Despite these impressive advances, the DKR of
simple atropisomeric ortho-formyl naphthamides is still
challenging due to its finite reaction modes and lower rotational
barrier.11 Compared with other naphthamide counterparts, its
intrinsic trigonal CHO group might be unfavourable for barrier
to rotation about the Aryl-CO bond.11a The interaction of the
Scheme 2. Substrate Scope of 2-(1H-pyrrol-1-yl)aniline.a
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DOI: 10.1039/D0CC02380A
transient pyramidalised nitrogen's lone pair with *(C=O) could
decrease the barrier energy via a five-membered ring transition
state.11a Two decades ago, Clayden and Lai achieved the first
DKR of atropisomeric ortho-formyl naphthamides by using
stoichiometric amount of four-step proline-based diamine
(Scheme 1, c-(i)).12a-b The pioneering work on catalytic DKR of
amide aldehyde with acetone was reported by Walsh group in
the presence of L-proline, giving products ranged from 77% to
95% with the diastereoselectivities from 1.6:1 to 8.0:1 (Scheme
1, c-(ii)).12c Herein, we reported an efficient CBA strategy for its
catalytic DKR with pyrrolylanilines in high enantio- and
diastereoselectivities (up to >20:1 d.r. and 98:2 e.r.), which
established a new reaction model to assemble potentially
biological active molecules bearing chiral axis, stereogenic
center and heterocycle (Scheme 1, c-(iii)).
Unless indicated otherwise, the reaction was carried out on 0.1 mmol
a
under the reaction conditions of entry 16 in Table 1.
Amide
naphthaldehyde 1a (0.1 mmol), 2-(1H-pyrrol-1-yl)aniline
2
(0.15 mmol,
1.5 equiv.) and chiral acid catalyst A5 (5.0 mol %) were stirred in n-
hexane (1.0 mL, 0.1 M) at room temperature under N2 for 36 h. b Yield
of isolated product. c d.r. value was determined by 1H NMR spectroscopy.
d ee value was determined by chiral HPLC analysis.
The investigation of 2-(1H-pyrrol-1-yl)aniline scope reveals
that the approach proceeded readily to deliver the
corresponding compounds 3ab-an in good outcomes no matter
Nitrogen-containing heterocycles like pyrrole and indole are
ubiquitous and privileged motifs in myriad of natural products
and pharmaceuticals.13 By considering the formation of new
potential bioactive molecules containing axially chiral aromatic
amide and another microstructure of centers and heterocyclic
rings, the catalytic DKR of naphthamides with a reactive
heterocycle should be an efficient and practical methodology
through only one step. As contrasted with indole, the
nucleophilicity of inert N-substituted pyrrole is much weaker
leading to its immanent low reactivity and the huge difficulties
to control the stereoselectivity. In our recent efforts, (1H-pyrrol-
1-yl)anilines have been proved efficient and successful for
asymmetric cascade cyclization reaction by CBA catalysis.14
Such compounds possess two reaction sites, similar to the (S)-
N-(pyrrolidin-2-ylmethyl)aniline,12 but it is more rigid. We
anticipate that the kind of substrates should be more
favourable control of enantio- and diastereoselectivities for
catalytic DKR of challenging ortho-formyl naphthamides.
After careful optimization of the reaction conditions (for
details, see ESI), the reaction between amide naphthaldehyde
1a (0.1 mmol) and 2-(1H-pyrrol-1-yl)aniline 2a (0.15 mmol) with
a 5.0 mol % of CBA catalyst A5 at room temperature in n-hexane
provided the desired product 3aa in 98% yield with >20:1 d.r.
and 93:7 e.r. Upon the result, we first amplified the reaction up
to gram-scale to evaluate the practicability of the catalytic DKR
process. Gratifyingly, 1.50 g of 3aa was smoothly isolated in 96%
yield with >20:1 diastereoselectivity and 90:10 e.r. (Scheme 2).
The enantioselectivity could be increased up to >99.5:0.5 after
once recrystallization. The structure of 3aa was determined by
X-ray crystal analysis. And according to the X-ray single crystal
structure of 3aj, its absolute configuration was assigned as aS,S.
with electron-deficient or electron-rich groups on the aromatic
rings (Scheme 2). Substituents at the 2-position of the aryl rings
near with the pyrrole were well compatible with current
reaction conditions to furnish the desired products 3ab-3af in
high enantioselectivities and diastereoselectivities ranging from
6:1 to >20:1 d.r. Substrates with a substituent at the 3-position
were also tolerated to produce adducts 3ag-ak with excellent
results in terms of both conversion and chirality control.
Fortunately, the e.r. value of the methoxyl substituted 2-(1H-
pyrrol-1-yl)aniline 2l could be increased to more than 99.5:0.5
via a simple recrystallization although original enantioselectivity
of the product 3an is moderate. Then, multi-substituted
reactants 2m
resulted in products 3am
We then investigated the scope of amide naphthaldehydes
with 2-(1H-pyrrol-1-yl)anilines 2a and 2i . As demonstrated in
-
n
were employed into this reaction system and
-
an in good results, respectively.
1
-j
Scheme 3, axially chiral products 3ba and 3bi were easily
synthesized with high enantiomeric excess and excellent
diastereoselectivities from 4-Me substituted naphthaldehyde
1b. High yields and acceptable e.r. values were detected for 3ca
and 3ci from naphthamide 1c having a fluorine atom at the4-
position, respectively. Delightedly, dihydroacenaphthylene (1d
and pyrene aldehydes (1e) were also amenable to the reaction
conditions, providing 3da 3di 3ei and 3ej in 94-98% yields with
)
,
,
good enantio- and diastereoselectivities. Finally, the
dicyclohexylamide 1f was subject to the reaction. The desired
product 3fa and 3fi were generated readily with high yields and
diastereoselectivities.
To highlight the synthetic potential of this asymmetric
catalytic DKR strategy, further transformations of the axially
2 | J. Name., 2012, 00, 1-3
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useful principle to the asymmetric synthesis of VnieowvAertliclaexOinallinlye
DOI: 10.1039/D0CC02380A
chiral molecules with feeble rotational barriers.
Scheme 3. Scope of the amide naphthaldehydes.a
Based on the previous reports,14,16 above DKR
transformation should undergo the imine formation with a
sequential nucleophilic addition to the pyrrole unit in the
presence of CBA. By using the imine intermediate as the starting
material under the standard reaction conditions, 3aa was
obtained in 26.4:73.6 e.r., lower than that of one-pot reaction.
The preliminary results imply that CPA catalyst might have chiral
recognition for each step in the cascade reaction and the
synergistic effect would benefit the final enantioselectivity (for
the detail, please see the ESI). We postulated the catalytic
transition state to reasonably explain the stereoselectivity
(Scheme 5). Theoretically, the TS-A should be more favourable
than the TS-B in which the larger steric impulsion is existed
between the N-isopropyl groups and the catalyst skeleton. On
the other hand, the addition of the pyrrole to the intermediate
imine would take place from the anti-face of the isopropyl
groups with lower steric hindrance, leading to the desired
product 3aj with the absolute configuration as aS,S.
a Amide naphthaldehyde 1 (0.1 mmol), 2-(1H-pyrrol-1-yl)aniline 2 (0.15 mmol,
1.5 equiv.) and chiral phosphoric acid catalyst A5 (5.0 mol %) were stirred in
n-hexane (1.0 mL, 0.1 M) at room temperature under N2 for 36 h. b Yield of
1
d
isolated product. c d.r. value was determined by H NMR spectroscopy. ee
value was determined by chiral HPLC analysis.
Scheme 5. Postulated reaction models rationalizing the observed
enantioselectivity
chiral product were performed. As shown in Scheme 4, 3aa was
oxidized into the derivative 4 by KMnO4 at 0 °C in acetone for 2
h with a slightly decreased e.r. value of 91.5:8.5. By acylation
with 4-(trifluoromethyl)benzoyl chloride at the presence of N-
ethyl-N-isopropylpropan-2-amine (DIPEA) at room temperature,
adduct
5 was isolated in almost quantitative yield without any
loss of chirality. In order to investigate the rotational stability of
these novel compounds, racemization experiments by Eyring
equation15 were carried out. To the free rotation around the Ar-
CO axially chiral bond, the higher shield effect of stereogenic
tetrahedral Csp3 amine
Scheme 4. Further transformation and epimerization investigation.a
In conclusion, we developed the first readily scalable CBA-
catalyzed enantioselective DKR between naphthamides and
pyrrolylanilines with high enantio- and diastereoselectivities
(up to >20:1 d.r. and 98:2 e.r.). This strategy can install easily
and simultaneously an atropisomeric amide’s chiral axis, a
stereogenic center and nitrogen-containing heterocycle into a
new family of valuable potential biologically active
pyrrolopyrazine compounds. Importantly, obvious barrier effect
of the chiral center with larger substituent was confirmed and
might be benefit to prepare other new kinds of axially chiral
compounds with lower rotational barrier. Examination of
biological activities of this kind of molecules was ongoing in our
lab.
a
About 1.0 mg of an enantioenriched sample was dissolved in 0.1 mL
toluene and heated at a specified temperature. The change in
enantiomeric excess over time was monitored by chiral HPLC. 3aa at
50C, 4 at 30 C, 5 at 60 C. For details, please see the ESI.
Financial support from the National Natural Science Founda-
tion of China (21602231), the Natural Science Foundation of
Jiangsu Province (BK20191197, BK20181373) and Major
Program of Natural Science Research of Jiangsu Higher
Education Institutions of China (No. 18KJA150005) is gratefully
acknowledged.
in 3aa than that of the planar Csp2 imine in
4 was affirmed by
an energy gap of about 6.5 kJ/mol, and the half-time dropped
from 3.57 days to 6.34 hours. As expected, the configurational
stability could be improved by increase the steric hindrance
through installing a bulky substituent group at the nitrogen site.
As a result, compound
5 has a longer half-time of 8 days than
that of 3aa. The free rotation of an axis would be prevented by
a chiral center with large steric hindrance, which should be a
This journal is © The Royal Society of Chemistry 20xx
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Conflicts of interest
There are no conflicts to declare.
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COMMUNICATION
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DOI: 10.1039/D0CC02380A
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