Biocatalytic α-Oxidation of Cyclic Amines and N-Methylanilines for the Synthesis of Lactams and Formamides

Article abstract of DOI:10.1002/cctc.201601703

An environmentally friendly method for the synthesis of lactams and formamides through the biocatalytic α-oxidation of amines was developed by employing Pseudomonas plecoglossicida ZMU-T04 as a biocatalyst. In this biocatalytic process, the α-oxidation of cyclic amines and N-methylanilines proceeded smoothly to give the corresponding amides in low to high yields. Furthermore, it was demonstrated that synthetic 3,4-dihydroquinolin-2(1H)-one could be used as a key precursor of antidepressant bioactive molecules. The mechanism of this biocatalytic α-oxidation process was investigated by isotope- labeling experiments.

Full text of DOI:10.1002/cctc.201601703

Accepted Article
Title: Biocatalytic α-Oxidation of Cyclic Amines and N-Methylanilines
for the Synthesis of Lactams and Formamides
Authors: Daijun Zheng, Xiaojian Zhou, Baodong Cui, Wenyong Han,
Nanwei Wan, and Yongzheng Chen
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To be cited as: ChemCatChem 10.1002/cctc.201601703
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10.1002/cctc.201601703
ChemCatChem
Communication
Biocatalytic α-Oxidation of Cyclic Amines and N-Methylanilines
for the Synthesis of Lactams and Formamides
Daijun Zheng+, Xiaojian Zhou+, Baodong Cui, Wenyong Han, Nanwei Wan, Yongzheng Chen*
Dedicated to Professor Min-Zhang Qian for her 70th birthday
Abstract: An environmentally friendly method for the synthesis of
lactams and formamides through the biocatalytic α-oxidation of
amines has been developed by employing Pseudomonas
plecoglossicida ZMU-T04 as biocatalyst. In this biocatalytic process,
the α-oxidation of cyclic amines and N-methylanilines proceed
smoothly to give the corresponding amides in low to high yields.
Furthermore, it is demonstrated that the synthetic 1,2,3,4-
tetrahydroquinoline-2-one can be used as the key precursor of
antidepressant bioactive molecules. In addition, the mechanism of
Scheme 1. Chemical α-oxidation of amines to amides.
this biocatalytic α-oxidation process is investigated by the isotope-
labeled experiments.
The microbial oxidation process has been intensely studied
and became a useful and green alternative method in the
Introduction
organic synthesis.9 These achievable examples mainly include
Bayer-Village oxidation,9b,c hydroxylation,10 epoxidation,9d,11
sulfoxidation12 and oxidative resolution.13 However, the
enzymatic oxidation of amine to amides is still challengable.14
In our previous report, we demonstrated an enantioselective
benzylic oxidation of 1,2,3,4-tetrahydroquinoline with P.
plecoglossicida ZMU-T04 as the biocatalyst.10a It was found that
the benzylic hydroxylation process generated not only the major
(R)-1,2,3,4-tetrahydroquinoline-4-ol but also a small amount of
1,2,3,4-tetrahydroquinoline-2-one. This discovery prompted us
to envision that the selectivity of this bio-oxidation might be
controlled towards the generation of 1,2,3,4-tetrahydroquinoline-
2-ones via modification and reconstruction of the microorganism.
In that case, it would provide an important complementary route
to access lactams. By altering the inducer from toluene to p-
xylene in the cultivation of P. plecoglossicida ZMU-T04,
excellent α-selectivity for the oxidation of cyclic amines to
lactams were detected with no benzylic hydroxylation products.
Noticeably, this strategy is also applicable to the α-oxidation of
N-methylanilines, giving the corresponding formamides (Scheme
2).
Construction of amides is one of the most significant
conversions in organic synthesis and medicinal chemistry due to
their structurally versatile motifs for the synthesis of natural
products, drug candidates and bioactive molecules.1
Conventionally, synthesis of amides mainly relies on the
amidation reactions of free carboxylic acids/prior activated
carboxylic acids with amines.2 In recent years, some other direct
amidation strategies have been developed, including the
oxidative amidation of methylarenes3/aldehydes4/alcohols5 with
amines, tandem reactions6 and Beckmann rearrangement of
ketoximes.7 Despite these achievements are reached, the direct
α-oxidation of amines to amides is still a challenge, which is
limited to the reaction selectivity.8 Additionally, the reported
chemical protocols to amides through a α-oxidation usually
require
rigorous
reaction
conditions
involving
extra
stoichiometric oxidants8c,d or expensive metals8a,b,e (Scheme 1).
In this context, highly selective and environmentally friendly
methods for direct synthesis of amides remain desirable goals.
[*]
D. Zheng, X. Zhou, Dr. B. Cui, Dr.W. Han, Dr. N. Wan, Prof. Dr. Y.
Chen
School of Pharmacy
Zunyi Medical University
Zunyi, 563000, P. R (China)
E-mail: yzchen@zmc.edu.cn
These authors contributed equally.
[+]
Scheme 2. Our previous work and the current study.
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10.1002/cctc.201601703
ChemCatChem
Communication
Table 2: Substrate scope of the whole cells of P. plecoglossicida ZMU-T04
catalyzed α-oxidation of N-methylanilines 3.[a]
Results and Discussion
Initially,
the
biocatalytic
α-oxidation
of
1,2,3,4-
tetrahydroquinoline-2-ones 1a using the wholes of P.
plecoglossicida strain was investigated and the optimal reaction
conditions were outlined as follows: the reaction was performed
with substrate 1a (4 mM) in KH2PO4-Na2HPO4 buffer (5.0 mL,
50 mM, pH 7) at 30 °C and 300 rpm for 18 h (For details,
see the Supporting Information).
Entry
Substrate 3
R1 = H, R2 = H, R3 = H/3a
R1 = Cl, R2 = H, R3 = H/3b
R1 = H, R2 = Cl, R3 = H/3c
R1 = H, R2 = H, R3 = Cl/3d
R1 = H, R2 = H, R3 = NO2/3e
R1 = Me, R2 = H, R3 = H/3f
R1 = H, R2 = Me, R3 = H/3g
R1 = H, R2 = H, R3 = Me/3h
R1 = H, R2 = H, R3 = OMe/3i
Yieldb(%)/4
4a/46
4b/44
4c/14
4d/21
4e/n.d.c
4f/55
4g/33
4h/48
4i/13
1
2
3
4
5
6
7
8
9
After establishment of the optimal reaction conditions,
substrate scope of P. plecoglossicida ZMU-T04 catalyzed α-
oxidation of cyclic amines 1 was explored (Table 1). The
experimental results indicated that 1,2,3,4-tetrahydroquinolines
1b-d with electron-withdrawing substituent on the phenyl ring
were viable under the optimal reaction conditions, giving the
corresponding lactams 2b-d in up to 83% yield. Exceptionally,
substrate 1e merely provided 4% yield of product 2e.
Furthermore, 6-methoxyl substituted 1,2,3,4-tetrahydroquinoline
1f was also applicable to this biotransformation process and
afforded the product 2f in 65% yield. When the behavior of cyclic
amines 1g and 1h with methyl substituent on N1-position was
investigated, the formamides 2g and 2h were interestingly
observed. It may be attributed to the reason that the α-methyl is
more reactive than α-methylene.
[a]Unless otherwise noted, whole cells of P. plecoglossicida ZMU-T04 (50 g
cdw/L) were suspended in a solution of substrate 3 (10 mM) and KH2PO4-
Na2HPO4 buffer (15.0 mL, 50 mM, pH 7). The mixture was incubated at 30 °C
and 300 rpm for 18 h. [b]Isolated yields. [c]n.d. = not detected.
as the position of substituent on the phenyl ring of N-
methylanilines 3, all reactions proceeded smoothly to give the
corresponding formamides 4 in acceptable isolated yields (13-
55%) under the standard reaction conditions. However, 4-NO2
substituted N-methylaniline 3e was not applicable to the
biocatalytic α-oxidation process (Table 2, entry 5).
The potential synthetic application of this biotransformation
was demonstrated by the feasible conversion of the α-oxidation
product 2a into the antidepressant 715 (Scheme 3).16 The
transformation was started with the N-alkylation of 1-(4-
fluorophenyl)piperazine 5. With 1,2-dibromoethane as the
alkylated reagent and DIEA as the base, the intermediate 6 was
obtained in 30% yield. Compound 6 was subsequently used to
react with 1,2,3,4-tetrahydroquinoline-2-one 2a in the presence
of potassium carbonate, giving the target product 7 in 50% yield.
Table 1. Substrate scope of the whole cells of P. plecoglossicida ZMU-T04
catalyzed α-oxidation of cyclic amines 1.[a]
R1
R1
n
n
P. plecoglossicida ZMU-T04
O
N
N
KH2PO4-Na2HPO4 buffer
pH 7, 30 C, 18 h
R3
R3
2a-h
R2
R2
Cl
1
Substrates 1
Products 2
Substrates 1
Products 2
Cl
Cl
O
N
H
N
H
O
N
H
N
H
1a
2a: 71%
1b
2b: 77%
Cl
Br
Br
O
N
N
N
O
O
N
H
H
H
H
Cl
Cl
1c
2c: 55%
1d
1f
2d: 83%
Br
Br
MeO
MeO
N
N
O
N
H
N
H
H
H
Br
Br
1e
2f: 65%
2e: 4%
Scheme 3. Transformation of 1,2,3,4-tetrahydroquinoline-2-one 2a into
antidepressant 7. Reagents and conditions: (a) DIEA (1 equiv.), 1,2-
dibromoethane, rt, overnight, 30% yield; (b) K2CO3 (1.2 equiv.), MeCN, 50 oC,
overnight, 50% yield.
N
Me
N
N
N
CHO
2h: 57%
Me
CHO
2g: 82%
1g
1h
[a]Unless otherwise noted, whole cells of P. plecoglossicida ZMU-T04 (50 g
cdw/L) were suspended in a solution of substrate 1 (4 mM) and KH2PO4-
Na2HPO4 buffer (5.0 mL, 50 mM, pH 7). The mixture was incubated at 30 °C
and 300 rpm for 18 h. All yields were determined by HPLC analysis of the
crude reaction mixture using p-xylene as internal standard.
To investigate the mechanism of this α-oxidation process that
mediated by P. plecoglossicida ZMU-T04, the 18O-labeled
experiments were performed, and the ratio of bioproducts was
determined by HRMS analysis.16 The α-oxidation of 1,2,3,4-
tetrahydroquinoline 1a in H218O, using air as the oxidant and dry
whole cells as catalyst, resulted in a 99% 18O incorporation in
the lactam carbonyl group. However, the α-oxidation of substrate
1a was performed in H2O with 18O2 instead of air, only 4% 18O-
labeled 2a was obtained (Scheme 4). These reaction results
indicated that the oxygen atom in the amide group was derived
The successful bio-oxidation of exocyclic α-methyl of
substrates 1g and 1h in Table 1 inspired us to explore the
oxidation of non-cyclic N-methylanilines 3 for producing another
class of formamides 4. As summarized in Table 2, regardless of
the electron-withdrawing or electro-donating substituent, as well
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10.1002/cctc.201601703
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Na2SO4. The mixture was filtered and concentrated. The residue was
diluted with iPrOH (5.0 mL of 0.04 mol/L residue p-xylene in i-PrOH,) to a
total volume of 5.0 mL. The crude reaction mixture was taken for the
HPLC analysis with p-xylene as the internal standard to determine the
yield of product 2. It was noted that 10 parallel experiments were
performed for collecting the pure compound 2 by flash chromatography
on silica gel (petroleum ether/ethyl acetate = 5:1~10:1).
from water rather than the molecular oxygen.
dry whole cells
buffer (H2O18, Air)
18O-2a content: 99%
+
N
O18
N
O
N
H
H
H
dry whole cells
1a
buffer (H2O, 18O2)
18O-2a content: 4%
Procedure for the whole cell catalyzed α-oxidation of N-
Scheme 4. 18O-labeled experiments.
methylanilines 3:
Whole cells of P. plecoglossicida ZMU-T04 (50 g cdw/L) were suspended
in a solution of substrate 3 (10 mM) and KH2PO4-Na2HPO4 buffer (15.0
mL, 50 mM, pH 7). The mixture was incubated at 30 °C and 300 rpm for
18 h. It was noted that 10 parallel experiments were performed. After
completion of the reaction, the 10 parallel reaction mixtures were
combined together and extracted with EtOAc (120 mL × 5). The
combined organic layers were dried over anhydrous Na2SO4 and
concentrated. The residue was purified by flash chromatography on silica
gel (petroleum ether/ethyl acetate = 5:1~10:1) to give the corresponding
N-phenylformamides 4.
On the basis of the above isotope-labeled experiments and
the previous relevant reports,8a,8c,8e
proposed reaction
a
mechanism of the bio-mediated α-oxidation process is
demonstrated in Scheme 5. The first α-oxidation step of amines
leads to the production of imine intermediates. The second
hydration step of imines generats the hemiaminal intermediates.
Finally, the oxidation of the hemiaminal intermediates give the
corresponding amide products.
Acknowledgements
We are grateful for financial support from National Nature
Science Foundation of China (21562054), Science and
Technology Department of Guizhou Province (QKHRC-2016-
4029, QKHRCTD-2014-4002 and QKHPTRC-2016-5801), and
New Century Excellent Talent of the Ministry of Education
(NCET-13-1069).
Scheme 5. Proposed pathway for the biocatalytic α-oxidation of amines.
Keywords: region-selective • Pseudomonas plecoglossicida
ZMU-T04 •α-oxidation • lactams • N-phenylformamides
Conclusions
In summary, we have achieved an environmentally friendly α-
oxidation of amines by P. plecoglossicida ZMU-T04, prouding a
series of lactams and formamides in low to good yields.
Additionally, this reaction provides a potential application for the
synthesis of the antidepressant bioactive molecule 7.
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Experimental Section
Procedure for the whole cell catalyzed α-oxidation of cyclic amines
1:
Whole cells of P. plecoglossicida ZMU-T04 (50 g cdw/L) were suspended
in a solution of substrate 1 (4 mM) and KH2PO4-Na2HPO4 buffer (5.0 mL,
50 mM, pH 7). The mixture was incubated at 30 °C and 300 rpm for 18 h.
After completion of the reaction, the mixture was extracted with CH2Cl2 (4
mL × 5) and the combined organic layer was dried over anhydrous
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ChemCatChem
Communication
Communication
D. Zheng+, X. Zhou+, B. Cui, W. Han, N.
Wan, Y. Chen*
Biocatalytic α-Oxidation of Cyclic
Amines and N-Methylanilines for the
Synthesis of Lactams and N-
Phenylformamides
Text for Table of Contents
Biocatalytic α-oxidation: A highly regioselective and environment-friendly method for the synthesis of amides from amines was
achieved. With the biocatalytic α-oxidation protocol, various cyclic amines and N-methylanilines can be oxidized smoothly to give the
corresponding amides in moderate to high yields. In addition, the results of mechanistic studies suggest the oxygen atom in the
amide group of this biocatalytic α-oxidation process was derived from water.
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