η6-Arene CH?O Interaction Directed Dynamic Kinetic Resolution – Asymmetric Transfer Hydrogenation (DKR-ATH) of α-Keto/enol-Lactams

Article abstract of DOI:10.1002/adsc.202100288

A dynamic kinetic resolution – asymmetric transfer hydrogenation (DKR-ATH) methodology of α-keto/enol-lactams was developed. We also propose a possible catalytic mechanism evolving a transition state stabilized by η⁶-arene CH?O interaction. The efficient approach can be applied to a wide range of substrates including non-aryl ones which would be difficult to prepare by other asymmetric reduction methods. (Figure presented.).

Full text of DOI:10.1002/adsc.202100288

Title: <i>η</i>⁶-arene CH-O Interaction Directed Dynamic
Kinetic Resolution – Asymmetric Transfer Hydrogenation (DKR-
ATH) of α-keto/enol-lactams
Authors: Zhonghua Luo, Guodong Sun, Shuming Wu, Yong Chen,
Yicao Lin, Lei Zhang, and Zhongqing Wang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202100288
Link to VoR: https://doi.org/10.1002/adsc.202100288

10.1002/adsc.202100288
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
6-Arene CH-O Interaction Directed Dynamic Kinetic
Resolution – Asymmetric Transfer Hydrogenation (DKR-ATH)
of α-keto/enol-lactams
‖
‖
Zhonghua Luo,a,b, Guodong Sun,b,c, Shuming Wu,b,c Yong Chen,c,d Yicao Lin,c Lei
Zhang, a* Zhongqing Wangb,c,d
*
a
School of Biology and biological Engineering, South China University of Technology, Guangzhou 510640, P. R.
China
Email: lzhangce@scut.edu.cn;
b
State Key Laboratory of Anti-Infective Drug Development (NO. 2015DQ780357), Sunshine Lake Pharma Co., Ltd,
Dongguan 523871, P. R. China
Fax: (+86)-0769-85370223
Phone: (+86)-18665178690
Email: Wangzhongqing@hec.cn;
Department of Process Research and Development, HEC Pharm Group, Dongguan 523871, P. R. China
School of Pharmacy, Xiangnan University, Chenzhou 423000 Hunan, China
c
d
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A dynamic kinetic resolution – asymmetric
transfer hydrogenation (DKR-ATH) methodology of -
keto/enol-lactams was developed. We also propose a
possible catalytic mechanism evolving a transition state
stabilized by 6-arene CH-O interaction. The efficient
approach can be applied to a wide range of substrates
including non-aryl ones which would be difficult to prepare
by other asymmetric reduction methods.
Keywords: Dynamic kinetic resolution-asymmetric
transfer hydrogenation; enatioselectivity; lactams; CH-O
interaction; mechanism
Chiral substituted lactams are an important
structural scaffold in both bioactive natural
products and pharmaceutical ingredients, such as
anti-viral hepatitis and anti-Alzheimer’s disease
Figure 1. Chiral disubstituted lactams and cyclic
secondary amines in natural products and pharmaceutical
ingredients
natural
product
(-)-clausenamide[1]
and
naucleamide A isolated from the bark and wood
of Nauclea latifolia (Figure 1).[2] Their
Therefore, tremendous effort has been devoted to
developing methodologies for enantioselective
corresponding
secondary amines with two contiguous
stereogenic centers, are also effective
reduction
products,
cyclic
construction of lactams. Take
-lactam for
example, these two stereogenic centers can be
constructed sequentially or simultaneously.
Reported methods for synthesizing one chiral
center included asymmetric conjugated addition[5]
substructures in drug molecules such as purine
nucleoside phosphorylase inhibitor BCX-4208[3]
and commercialized JAK inhibitor tofacitinib.[4]
of
asymmetric hydrogenation (AH) of
-unsaturated
-lactams[6] (Scheme 1B). The

,

-unsaturated

-lactams (Scheme 1A) and

-substituted-
,


introduction of the second chiral center might still
be problematic. Very few methods for the
1
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10.1002/adsc.202100288
Advanced Synthesis & Catalysis
synthesis of two adjacent chiral centers
concomitantly have been reported, except for the
Au-catalyzed cycloisomerizative hydroalkylation,
which required an optical active allene
substrate (Scheme 1C).
Whereas the directing sulfone group does not
exist in our target molecules, here we designed to
utilize the intrinsic amide group for directing. In
[7]
this case, an unusual
instead of the traditional CH-

6-arene CH-O interaction

interaction[11,14]
might be requisite as the origin of the
enantioselectivity in our DKR-ATH reactions. In
2016, Bhanage et al. reported an application of
CH-/CH-O interactions co-driven asymmetric
transfer hydrogenation for the reduction of
dibenzo[b,e]azepin-6-11-dione.[15]
DKR-ATH reactions directed by sole
However,

6-arene
CH-O interaction have not yet been disclosed.
Encouraged by the possibility from mechanistic
analysis and our previous researches on DKR-
ATH reactions,[16] we then started a detailed
investigation for the oxo-tethered Ru (II)
catalyzed[17] DKR-ATH of this type of
keto/enol lactams.[18]
-
Catalyst screening. To test this hypothesis,
racemic 1-benzyl-4-methylpyrrolidine-2,3-dione
1a was selected as the model substrate to estimate
the catalytic ability for a number of Ru catalysts
((R,R)-CAT1~9, Table 1). A loading of 1.0
mol % of Ru complexes was initially applied with
formic acid (HCOOH) as the organic reductant
and triethylamine (Et3N) as the base in
dichloromethane (DCM) at reflux for 12 h.[19] All
catalysts, except for tether-Ru catalyst (R,R)-
CAT8 and (R,R)-CAT9, were prepared in situ
from precatalyst [RuCl2(p-cymene)]2 and
Scheme 1. Previous Works for Asymmetric Synthesis of
Substituted -lactams
Since its first adoption, asymmetric reduction
coupled with dynamic kinetic resolution (DKR)
has gained significant attention for its atom-
economy with respect to the rubric of green
chemistry.[8] With the milestone improvements
made by Noyori and coworkers in the 1980-90s,
corresponding
1,2-diphenylethylenediamine
(DPEN) ligands. To our delight, all catalysts
provided the desired product 2a with excellent
conversion and diastereoselectivity (Table 1,
entries 1-9). Luckily, excellent selectivity was
achieved on our first attempt using Ru catalyst
(R,R)-CAT1. By subjecting 1a to the classic
Noyori’s catalyst (R,R)-CAT2 and other catalysts
with modification on the sulfonamide ((R,R)-
CAT3~6), an increase in enantioselectivity was
observed (up to 95.5% ee). However, the
enantiomeric excess gained from catalyst (R,R)-
CAT7 was as low as 67.8%, and it was
rationalized that the sterically hindered isopropyl
(iPr) groups possibly decreased the efficiency. To
achieve even better results, we assessed tethered-
Ru catalyst (R,R)-CAT8 and (R,R)-CAT9 with
more rigid ligand structures. Carbo-tethered-Ru
catalyst (R,R)-CAT9 gave a good result (>99%
conversion, 92.7% ee and 98:2 dr, entry 9, Table
1) and to our delight, oxo-tethered-Ru catalyst
(R,R)-CAT8 displayed a fantastic diastereo and
enantio selectivity (>99% conversion, 98% ee
and >99:1 dr, entry 8, Table 1). Further attempts
to reduce the catalyst loading from 1.0 mol % to
0.5 mol % were proven successful, a better
enantioselectivity (98.7% ee, Table 1, entry 11)
was obtained and the conversion was maintained
up above 99%.
[9]
dynamic kinetic resolution – asymmetric
hydrogenation (AH) / transfer hydrogenation
(ATH) has been reckoned one of the most
powerful approaches to build contiguous
stereocenters from racemic compounds.[10]
However, in typical asymmetric reduction models,
ketone substrates possessing aromatic rings,
heterocyclic rings, or alkynyl groups were
identified to afford the reduced products with
high enantioselectivities. Asymmetric reduction
of non-aryl substrates has been rarely reported
except for the synthesis of natural wine lactone
facilitated
by
DKR-ATH/lactonization.[11]
Recently, Zhou and co-workers has developed an
efficient catalyst for asymmetric hydrogenation
of dialkyl ketones.[12] Though, on account of
process safety, we prefer avoiding a direct usage
of hydrogen gas. Wills et al. also disclosed a
sulfone-directed
asymmetric
transfer
hydrogenation to convert simple ketone
substrates that could be applied with dynamic
kinetic resolution while the substrate scope of this
method was relatively narrow and all of them
comprised aryl groups.[13]
2
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10.1002/adsc.202100288
Advanced Synthesis & Catalysis
Table 1. Evaluation of Chiral catalysts(a)
crystallography (CCDC: 2034928, see supporting
information).[20] Interestingly, the products 2k
and 2p were both with very good ee values
(>99% ee), while the dr ratios (92:8 dr and 93:7
dr, respectively) of which were a little lower than
the others. Furthermore, substrates bearing
different substituted groups on the nitrogen atoms
were also evaluated with the catalyst system.
Substrates in which the benzyl group was
replaced with a methyl, isopropyl, n-butyl, or 4-
methoxyphenyl (PMP) were successfully reduced
with
enantioselectivities and diastereoselectivities (2q
2t). Successful acquirement of non-aryl products
2q 2s) which would be difficult to achieve by
full
conversions
and
excellent
-
(
-
other asymmetric reduction methods further ruled
out the influence of aromatic substituted group on
the nitrogen atom on the reaction performance of
the substrates. The phenomenon also provided an
evidence for our proposal of a different stereo
Entry Ru Cat.
Conv.(b) dr (cis:trans)(c) ee(c)
(%)
(%)
1
2
3
4
5
6
7
8
(R,R)-CAT1 >99
(R,R)-CAT2 >99
(R,R)-CAT3 >99
(R,R)-CAT4 >99
(R,R)-CAT5 >99
(R,R)-CAT6 >99
(R,R)-CAT7 >99
(R,R)-CAT8 >99
(R,R)-CAT9 >99
97:3
98:2
98:2
98:2
98:2
98:2
99:1
99:1
98:2
97:3
98:2
93.2
93.3
95.3
95.5
93.1
94.8
67.8
98.0
92.7
-93.4
98.7
origin compared to the existing CH-
 interaction
models.[11,14]
To further extend the substrate scope, two types
of substrates bearing larger ring size were
examined in the DKR-ATH reaction. To our
delight, under the standard catalyzed conditions,
the products with six- and seven-membered ring
9
(
2u-2w, Table 2) were obtained in high yields
10(d)
(S,S)-CAT1
>99
and with excellent enantioselectivities (>99% ee,
99.8% ee and 98.5% ee) with only a little
diastereoslectivity loss (88:12 dr, >93:7 dr and
89:11 dr). This expansion of substrate scope
would definitely add value to our methodology as
reports on such simple and concise construction
of enantiomerically-enriched piperidin-2-one and
azepan-2-one were rare.
11(e)
(R,R)-CAT8 >99
(a)
Reaction conditions: 1a (2.46 mmol), Ru Cat. (1.0
mol %), HCO2H/Et3N (5:2) (3 equiv), DCM (6 mL) at
(b)
(c)
reflux for 12 h. determined by HPLC. determined by
(d)
chiral HPLC using a DAICEL CHIRALPAK column.
(S,S)-CAT1 was used. 0.5 mol % of (R,R)-CAT8 was
(e)
used.
Mechanistic studies. Based on the experimental
results and possible

6-arene CH-O directing
Substrate scope of lactams. Based on the above
encouraging results, the substrate scope of the
Ru-catalyzed DKR-ATH for a series of 4-
interaction,[21] we suggest
a
full catalytic
mechanism for the Ru-catalyzed DKR-ATH
reaction based on catalyst (R,R)-CAT8 (Scheme
2). At first, enol substrate 1q tautomerizes to R-
1q or S-1q under the reaction conditions. Either
tautomer might attach to the hydrido Ru catalyst
(R,R)-CAT8 via hydrogen bonds to form 6-
membered cyclic transition states, such as TSR
and TSS. However, TSS is largely destabilized by
substituted
N-benzyl-pyrrolidine-2,3-diones
1a 1p (Table 2) was subsequently examined.
~
Substrates bearing alkyl groups of different
length at the 4-position on the pyrrolidine ring
were converted to their corresponding products in
high yields and with excellent stereoselectivities
(
2a-2e, Table 2), albeit the substrate 1d afforded
a repulsive interaction of the S-substituents at -
product 2d with slightly lower enantiomeric
excess (>96% ee, Table 2). Other substituted
groups, such as the azido group, alkenyl and ester,
were also tolerated in this reaction, and excellent
position of the lactam and the ligand of the
catalyst. As such, S-1q is not likely to transform
efficiently to 2q’ through the unfavored transition
state. In contrast, TSR may be stabilized through
results were achieved (2f-2h, Table 2). Similarly,
a CH-O interaction[22] between

6-arene C-H on
substrates bearing different arylmethylene groups,
such as the aryl of which were phenyl, substituted
phenyl, or even 1-naphthyl and 2-thienyl, could
also afford the corresponding products in good
Ru catalyst and the ketone/enol oxygen of the
substrate. As a result, the preference for TSR
finally leads to the formation of the desired cis-
product 2q after a kinetic resolution process.
yields and excellent stereoselectivities (2i-2p).
The absolute and relative stereochemistry of
product 2m was confirmed by X-ray
Table 2. Substrate Scope of DKR-ATH for
,-
Disubstituted-Lactams(a)
3
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10.1002/adsc.202100288
Advanced Synthesis & Catalysis
reaction can be greatly amplified in diversity
through simple manipulations (Scheme 3). For
example, sulfonylation of optical pure 2a with
methanesulfonyl chloride and tosyl chloride
afforded sulfonic esters 3a and 3b, which were
primed to undergo further SN2 substitutions with
anionic or enolate-based nucleophiles, yielding
azide
yield) and secondary alcohol
allowing viability of functionality of
4
(93% yield),

-alkylation lactam
(87% yield),
-lactams
6 (75%
7

with high stereospecifications. As concluded in
Table 2, substrates with larger ring size are also
applicable to this methodology. Piperidin-2-one
product 2v underwent a complete reduction upon
treatment of LiAlH4, resulting in piperidine
8
which can be further converted into intermediate
N
to afford the marketed drug substance
tofacitinib.[23] For the wide application and easy
implementation, we envision this methodology to
be of paramount importance in the synthesis of
both pharmaceutical ingredients and bioactive
natural products.
(a)
Reaction conditions: 1a~1w (2.00 mmol), (R,R)-CAT8
(0.5 mol %), HCO2H (2.60 mmol, 1.30 equiv), Et3N (6.50
mmol, 3.25 equiv), DCM (8 mL) at reflux for 15~18 h.
Isolated yields after column chromatography were reported,
dr ratios and ee values were determined by a DAICEL
CHIRALPAK column
Scheme 3. Synthetic Utility of Pyrrolidin-2-One and
Piperidine Products
Conclusions
In summary, we have successfully developed a
dynamic kinetic resolution – asymmetric transfer
hydrogenation methodology for the synthesis of
,-disubstituted-lactams mediated by 
6-arene
CH-O interaction. The highly efficient approach
can be applied to a wide range of substrates
including non-aryl ones which would be
extremely difficult to prepare by other
asymmetric reductions. This method can further
provide easy access to a diversity of synthetically
Scheme 2. Proposed catalytic mechanism
Synthetic applications. The α,
lactam structures obtained from the DKR-ATH
-disubstituted
4
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10.1002/adsc.202100288
Advanced Synthesis & Catalysis
[7] A. Boutier, C. Kammerer ‐ Pentier, N. Krause, G.
Prestat, G. Poli, Chem. Eur. J. 2012, 18, 3840–3844.
or pharmaceutically useful chiral lactams or
cyclic secondary amines that would be circuitous
to synthesize in high selectivity by other methods.
[8] A. Tai, H. Watanabe, T. Harada, Bull. Chem. Soc. Jpn.
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Experimental Section
[9] a) R. Noyori, T. Ikeda, T. Ohkuma, M. Widhalm, M.
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Dynamic kinetic resolution – asymmetric transfer
hydrogenation of racemic 1a using (R,R)-Ts-DENEB®:
To a solution of 1a (0.4 mmol, 1.0 equiv) in DCM (5.0
mL), (R,R)-Ts-DENEB® (0.002 mmol, 0.005 equiv) and
Et3N (119 mg, 1.2 mmol, 3.25 equiv) were added. After
inertization by N2, formic acid (HCOOH, 22 mg, 0.5 mmol,
1.3 equiv) was added by drops. Reaction temperature was
kept at reflux and the mixture was stirred until the reaction
was completed. Water (10 mL) was added to quench the
reaction. Separated the two phases, and the aqueous phase
was extracted with DCM (20 mL  3). The organic phases
were combined and washed by saturated aqueous NaCl
solution (20 mL). After being dried over MgSO4, the
organic solution was concentrated at 40°C. Flash column
chromatography on silica gel eluting with Hexane/EA gave
compound 2a as a white solid (364 mg, 88.76% yield, dr:
98:2). The enantiometric excess of the product was
determined to be 98.7% by chiral HPLC (DAICEL
CHIRALPAK® AS-3 (4.6150mm, 3μm), hexane/i-
propanol 75:25, flow rate 0.5 mL/min, tmajor =14.59 min,
tminor =13.91 min, tminor =18.93 min, λ = 210 nm).
[10] a) V. K. Vyas, B. M. Bhanage, Org. Lett. 2016, 18,
8436–6439; b) E. R. Ashley, E. C. Sherer, B. Pio, R.
K. Orr, R. T. Ruck, ACS Catal. 2017, 7, 1446–1451; c)
A. E. Cotman, B. Modec, B. Mohar, Org. Lett. 2018,
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Org. Lett. 2018, 20, 3006–3009; e) P.-G. Echeverria,
T. Ayad, P. Phansavath, V. Ratovelomanana-Vidal,
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Ludlow, P. Villo, P. Somfai, Chem—Eur. J. 2012, 18,
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Acknowledgements
We gratefully acknowledge the Key research and development
program of Guangdong Province, China (Grant No.
2019B02021002) and the grant from the State Key Laboratory of
Anti-Infective Drug Development (Sunshine Lake Pharma Co.,
Ltd), (N0. 2015DQ780357) for financial support. We thank
Baolei Luan (HEC pharm Co., Inc.) for HPLC and optical
rotation assistance.
[12] F. Zhang, F. Zhang, M. Li, J. Xie, Q. Zhou, Nat Catal.
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5
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10.1002/adsc.202100288
Advanced Synthesis & Catalysis
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6
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10.1002/adsc.202100288
Advanced Synthesis & Catalysis
COMMUNICATION
6-Arene CH-O Interaction Directed Dynamic
Kinetic Resolution – Asymmetric Transfer
Hydrogenation (DKR-ATH) of α-keto/enol-lactams
Adv. Synth. Catal. Year, Volume, Page – Page
Zhonghua Luo, Guodong Sun, Shuming Wu, Yong
Chen, Yichao Lin, Lei Zhang,* Zhongqing Wang*
7
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