Ni-Catalyzed asymmetric reduction of α-keto-β-lactams: via DKR enabled by proton shuttling

Article abstract of DOI:10.1039/d0cc05599a

Chiral α-hydroxy-β-lactams are key fragments of many bioactive compounds and antibiotics, and the development of efficient synthetic methods for these compounds is of great value. The highly enantioselective dynamic kinetic resolution (DKR) of α-keto-β-lactams was realized via a novel proton shuttling strategy. A wide range of α-keto-β-lactams were reduced efficiently and enantioselectively by Ni-catalyzed asymmetric hydrogenation, providing the corresponding α-hydroxy-β-lactam derivatives with high yields and enantioselectivities (up to 92% yield, up to 94% ee). Deuterium-labelling experiments indicate that phenylphosphinic acid plays a pivotal role in the DKR of α-keto-β-lactams by promoting the enolization process. The synthetic potential of this protocol was demonstrated by its application in the synthesis of a key intermediate of Taxol and (+)-epi-Cytoxazone. This journal is

Full text of DOI:10.1039/d0cc05599a

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Ni-Catalyzed asymmetric reduction of a-keto-b-
lactams via DKR enabled by proton shuttling†
Cite this:DOI: 10.1039/d0cc05599a
Fangyuan Wang,‡ab Xuefeng Tan,‡b Ting Wu,c Long-Sheng Zheng,b
b
Gen-Qiang Chen *d and Xumu Zhang
*
Received 17th August 2020,
Accepted 17th November 2020
DOI: 10.1039/d0cc05599a
rsc.li/chemcomm
Chiral a-hydroxy-b-lactams are key fragments of many bioactive of Taxol9 and (ꢀ)-epi-cytoxazone10 (Scheme 1a). Thus, the efficient
compounds and antibiotics, and the development of efficient syn- synthesis of chiral a-hydroxy-b-lactams is of great value. The
thetic methods for these compounds is of great value. The highly asymmetric hydrogenation of a-keto-b-lactams is one of the most
enantioselective dynamic kinetic resolution (DKR) of a-keto-b- straightforward approaches to access these compounds and the
lactams was realized via a novel proton shuttling strategy. A wide corresponding a-keto-b-lactams can be easily synthesized via [2+2]
range of a-keto-b-lactams were reduced efficiently and enantio- cycloaddition between imines and in situ generated ketenes
selectively by Ni-catalyzed asymmetric hydrogenation, providing (Staudinger reaction).11
the corresponding a-hydroxy-b-lactam derivatives with high yields
Dynamic kinetic resolution (DKR) is an exceedingly useful
and enantioselectivities (up to 92% yield, up to 94% ee). Deuterium- tool in asymmetric catalysis because it allows the full conver-
labelling experiments indicate that phenylphosphinic acid plays sion of a racemic reactant into a chiral product.12 This is in
a pivotal role in the DKR of a-keto-b-lactams by promoting the sharp contrast to the standard kinetic resolution, which neces-
enolization process. The synthetic potential of this protocol was sarily has a maximum yield of 50%, and rapid interconversion
demonstrated by its application in the synthesis of a key inter- between the two enantiomers of the reactant was necessary to
mediate of Taxol and (+)-epi-Cytoxazone.
achieve effective DKR.13 The DKR of carbonyl compounds has
been a well-established protocol in asymmetric catalysis, and
Chiral b-lactams (2-azetidinone) are widely found in many the addition of a strong base is usually necessary to promote
bioactive compounds, especially in antibiotics.1 Ever since the the enolization process.14 Due to their low stability under basic
first synthesis of b-lactams in 1907 by Staudinger,2 b-lactam
compounds have attracted the attention of many organic
chemists during the last few decades.3 What’s more, chiral
a-hydroxy-b-lactams are the key fragment of many pharmaceu-
tically active molecules such as glycosides,4 anti-microbial
drugs5 and anti-cancer reagents6 (Scheme 1a). Chiral a-hydroxyl-
b-amino acid derivatives,7 the ring-opening products of
a-hydroxy-b-lactams, are ubiquitously found in many medicinal
targets and bioactive compounds,8 for example, the side chain
a School of Chemistry and Chemical Engineering, Harbin Institute of Technology,
Harbin 150001, People’s Republic of China
b Shenzhen Grubbs Institute and Department of Chemistry, Guangdong Provincial
Key Laboratory of Catalysis, Southern University of Science and Technology,
Shenzhen, 518055, China. E-mail: zhangxm@sustech.edu.cn
c College of Innovation and Entrepreneurship, Southern University of Science and
Technology, 1088 Xueyuan Road, Shenzhen, 518055, China
d Academy for Advanced Interdisciplinary Studies, Southern University of Science
and Technology, Shenzhen 518000, People’s Republic of China.
E-mail: chengq@sustech.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 (a) Examples of biologically active compounds derived from
a-hydroxy-b-lactams. (b) Previous work. (c) This work.
d0cc05599a
‡ These authors contributed equally to this work.
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conditions, the DKR of a-keto-b-lactams is rarely reported, and with (Rc,Sp)-DuanPhos (L7) as a ligand. The enantioselectivity
a-keto-b-lactams are usually reduced via chemical or enzymatic could be further increased to 95% using (S)-Binapine (L8)23 as a
kinetic resolutions (Scheme 1b).15 In 2005, Kayser and co- ligand, albeit with only 35% conversion.
workers reported an example of enzymatic DKR with high
Encouraged by these promising results, we further screened
enantioselectivity, albeit with low diastereoselectivity and nar- various solvents, temperatures and additives, and the results
row substrate scope.16 To date, the dynamic kinetic reduction are summarized in Table 1. When the temperature was
of a-keto-b-lactams is still a significant challenge, and the key decreased from 80 1C to 65 1C (entry 1), only 17% conversion
to realizing the DKR is to accelerate the enolization process was observed. 48% conversion and 95% ee were achieved at
(a proton transfer process). In 2019, Dong and co-workers 75 1C after 48 h (entry 2). The conversion was increased to 58%
realized the DKR of aldehyde by hydroacylation, and an amine with the retention of enantioselectivity when the reaction
was utilized to promote the enolization–racemization process was conducted at 100 1C for 24 h, while the ee value dropped
via an enamine intermediate,17 which inspired us to explore sharply after further extension of the reaction time (entries 3–5).
some additives that are rarely used in the DKR process. We Various solvents such as THF, toluene, hexane and methanol were
envisioned that interconversion between the two enantiomers also screened at 80 1C, and no improvement in conversion and
((R)-1 and (S)-1) of a-keto-b-lactams 1 could be promoted by enantioselectivity was observed (entries 6–9). Next, we
an acidic proton shuttling catalyst.18 Herein, we report the focused our attention on the evaluation of the effect of addi-
highly efficient and enantioselective dynamic kinetic asym- tives on the current reaction. In the presence of K2CO3, the
metric hydrogenation of a-keto-b-lactams, wherein, a novel reaction became messy (entry 10). 84% conversion and 45% ee
proton shuttling strategy was utilized to promote the enoliza- were achieved when NaBARF was added (entry 11). 8% and
tion–racemization process (Scheme 1c).
45% ee were observed for the additive TsOH and AcOH,
We initiated our investigation with the asymmetric hydro- respectively (entries 12 and 13). No desired product was
genation of compound 1a by the protocols previously developed detected when p-methoxy aniline (A1) was utilized as an additive
in our group. The enantioselectivity of product 2a was very low (entry 14).17 75% conversion and 65% ee were achieved for the
for the Ir(I) catalytic system,19 the Ru(II)/diphosphine system20 additive diphenylphosphinic acid (A2) (entry 15). Only 7%
and the transfer hydrogenation system21 (for details see conversion and 25% ee were achieved for phosphoric acid (A3)
Tables S1–S3 in the ESI†), probably due to the fact that the (entry 16). To our great delight, product 2a was produced in
interconversion between the two enantiomers was very slow 495% conversion and 81% ee with phenylphosphinic acid (A4)
and the two enantiomers of 1a were both rapidly reduced. Thus,
we focused our attention on the less reactive earth-abuntant
Table 1 Optimization of the reaction conditionsa
metal Ni(II).22 The efficacy of several diphosphine ligands was
evaluated in the Ni-catalyzed asymmetric reduction using DCM
as a solvent, and the results are depicted in Scheme 2. When
BINAP (L1) or SEGPHOS (L2) was used as a ligand, the conversion
was very low even though the temperature was raised to 80 1C.
18% conversion, 23% ee and 5 : 1 dr were achieved with (R,S)-
JosiPhos (L3). Electron-donating ligands such as (S,S)-Ph-BPE
(L4) and (S,S)-Me-DuPhos (L5) were not suitable for the current
reaction, and o5% conversion was observed in both cases.
To our great delight, with (R, R)-Quinoxp* (L6) as a ligand, 19%
conversion and 77% ee were observed. The conversion and
enantioselectivity were improved to 41% and 79% respectively
Entry Solvent
Additive t (h) Conv.b (%) drc
eed (%)
1e
2f
DCM
DCM
DCM
DCM
DCM
THF
—
36
48
24
36
72
24
24
24
24
24
24
24
24
24
24
24
24
36
17
48
420 : 1 90
—
—
420 : 1 95
420 : 1 95
420 : 1 37
420 : 1 12
420 : 1 93
420 : 1 96
3g
4g
5g
6
58
—
—
—
90
495
35
7
8
9
Toluene
Hexane
MeOH
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
—
—
—
K2CO3
NaBArF
TsOH
AcOH
A1
23
o5
Messy
Messy
84
495
56
—
—
—
3 : 1
420 : 1
420 : 1 45
—
—
—
—
45
8
10
11
12
13
14
15
16
17
18
—
—
A2
A3
A4
75
7
495
495
420 : 1 65
420 : 1 25
420 : 1 81
420 : 1 91
Toluene A4
a
b
All reactions were carried out on 0.1 mmol scale. Determined via
1H NMR spectroscopy. Determined via 1H NMR spectroscopy. The
c
d
ees were determined by HPLC analysis using a chiral stationary phase.
e
f
g
65 1C. 75 1C. 100 1C.
Scheme 2 Evaluation of the effect of various ligands.
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as an additive (entry 17). The enantioselectivity was further electron-withdrawing groups on the phenyl group and with
improved to 91% when toluene was used as a solvent (entry 18). normal alkyl groups on the b position was problematic.
With the optimal reaction conditions in hand, we investi-
A probable mechanism for the dynamic kinetic resolution
gated the substrate scope of the current reaction. First, the was proposed and depicted in Scheme S2 in the ESI.† After the
effect of N-protecting groups on the current reaction was introduction of phenylphosphinic acid (A4), (R)-1a could be
evaluated and the results are summarized in Scheme 3. For transformed to the enol intermediate 1a0 via an eight-
substrates with electron-donating or electron-withdrawing membered transition state, and the enol intermediate 1a0 could
groups on the phenyl ring of the N-protecting groups (2b–2i), isomerize to compound (S)-1a, which was reduced much faster
high yields and moderate to high enantioselectivities were than its enantiomer (R)-1a. Deuterium-labelling experiments
achieved (81% to 90% yield, 84% to 91% ee). The reaction also were also conducted to elucidate the role of the additive A4 (for
worked well for substrates with benzyl or alkyl N-protecting details see Scheme S2 in the ESI†).
groups (2j–2m, 68% to 77% yield, 88% to 94% ee). In addition,
To demonstrate the synthetic potential of the current meth-
the effect of substituents on the g-position was also evaluated. odology, the reduction of 1a was conducted on 1.0 mmol scale,
With PMP as the N-protecting group, 87% yield and 91% ee and product 2a was obtained in 85% yield and 91% ee under
were achieved (2a), which were comparable with the results standard conditions. After protection with TIPSCl, oxidative
of substrates 2b and 2j. PMP was selected as the standard removal of the PMP group and acylation with benzoyl bromide,
N-protecting group because it can be easily removed in the the side chain of Taxol could be efficiently synthesized
subsequent step. For substrates with an electron-donating (Scheme 4a).9 The reduction of 1n could also be conducted
group (2n–2o), an electron-withdrawing group (2p) or halogens on 1.0 mmol scale, and product 2n was produced in 91% yield
(2q–2s) on the para position of the phenyl ring, the reaction and 92% ee. In the presence of TMSCl and MeOH, compound
proceeded smoothly to deliver the desired products in high 5n, the key intermediate of (+)-epi-cytoxazone, was synthesized
yields and enantioselectivities (81% to 91% yield, 87% to 94% in 90% yield and 92% ee (Scheme 4b).10
ee). Moderate yield and enantioselectivity can be achieved
In conclusion, we have developed a Ni-catalyzed highly
for the substrate with an amide group on the para position efficient methodology for the dynamic kinetic asymmetric
of the phenyl ring (2t, 45% yield, 78% ee). The reaction also reduction of a-keto-b-lactams, providing the a-keto-b-lactam
tolerates substrates with meta-(2u, 85% yield, 87% ee) or ortho- products in high enantioselectivity and diastereoselectivity.
substituents (2v, 80% yield, 82% ee) on the phenyl ring. 92% The dynamic kinetic resolution (DKR) of a-keto-b-lactams is
yield and 88% ee were achieved for 2-naphthyl substituted realized via a novel proton shuttling strategy. Deuterium-
substrate 2w. The current reaction also worked well for thienyl labelling experiments indicate that phenylphosphinic acid
and furyl substituted substrates (2x and 2y, 85% and 88% yield, plays a pivotal role in the enolization of a-keto-b-lactams.
82% and 87% ee, respectively) and alkyl substrates (2z and 2aa, Further applications of this strategy in the DKR of carbonyl
70% and 91% yield, 85% and 90% ee, respectively). The compounds are underway in our group and the results will be
diastereoselectivities were high for all the substrates tested reported in due course.
(1a–1aa, 420 : 1 dr). The main limitation of this reaction is
This work was sponsored by the Guangdong Provincial Key
that low yield was generally achieved for substrates with Laboratory of Catalysis (No. 2020B121201002), the Shenzhen
electron-withdrawing groups on the phenyl group, in the case Nobel Prize Scientists Laboratory Project (C17213101), South-
of 1t, a considerable amount of the ring-opening product ern University of Science and Technology (start-up fund),
was detected. The substrate scope of the current reaction is the Shenzhen Science and Technology Innovation Committee
also restricted by the fact that the synthesis of substrates with
Scheme 3 Evaluation of the substrate scope.
Scheme 4 Synthetic elaborations of the Ni-catalyzed DKR.
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