Highly α-position regioselective ring-opening of epoxides catalyzed by halohydrin dehalogenase from: Ilumatobacter coccineus: A biocatalytic approach to 2-azido-2-aryl-1-ols

Article abstract of DOI:10.1039/c9ra03774h

Halohydrin dehalogenases are usually recognized as strict β-position regioselective enzymes in the nucleophile-mediated ring-opening of epoxides. Here we found the HheG from Ilumatobacter coccineus exhibited excellent α-position regioselectivity in the az

Full text of DOI:10.1039/c9ra03774h

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Highly a-position regioselective ring-opening of
epoxides catalyzed by halohydrin dehalogenase
from Ilumatobacter coccineus: a biocatalytic
approach to 2-azido-2-aryl-1-ols†
Cite this: RSC Adv., 2019, 9, 16418
Miao An, Wanyi Liu, Xiaoying Zhou, Ran Ma, Huihui Wang, Baodong Cui,
*
*
Wenyong Han, Nanwei Wan
and Yongzheng Chen
Halohydrin dehalogenases are usually recognized as strict b-position regioselective enzymes in the
nucleophile-mediated ring-opening of epoxides. Here we found the HheG from Ilumatobacter
coccineus exhibited excellent a-position regioselectivity in the azide-mediated ring-opening of styrene
oxide derivatives 1a–1k, producing the corresponding 2-azido-2-aryl-1-ols 2a–2k with the yields up to
96%.
Received 20th May 2019
Accepted 20th May 2019
DOI: 10.1039/c9ra03774h
rsc.li/rsc-advances
functionalized alcohols, which are useful biocatalytic routes to
prepare some important pharmaceuticals and biologically active
compounds.12 Azide can be processed by HHDHs as one of the
Introduction
1,2-Azido alcohols are versatile precursors in organic synthesis
for a wide variety of target compounds such as triazoles,1
triazole-fused dihydrooxazinones,2 2-oxazolidinones,3 1,4-oxa-
zepines and 1,3-oxazines,4 oxazaborolidines,5 and 1,3-oxazoli-
dines.6 More importantly, they have been widely employed for
the regioselective preparation of 1,2-amino alcohols and highly
oxygenated compounds such as carbohydrates and
nucleosides.7
The epoxy group is a highly reactive moiety, which makes it
easily react with a broad range of nucleophiles and provides
a convenient access to diverse highly functionalized molecules.8
Thus among the transformations used for the preparation of
1,2-azido alcohols, the epoxide ring-opening is straightforward
and effective method which has been widely studied. Though
trimethylsilyl azide and other metal azides have been used to
introduce the azide group,9 the sodium azide is generally
employed under various reaction conditions.10
Halohydrin dehalogenases (HHDHs) are valuable enzymes
from short dehydrogenases/reductases superfamily, which
exhibit promiscuous catalytic functions in the synthesis and
conversion of epoxides.11 HHDHs are not only able to catalyze the
dehalogenation of vicinal haloalcohols to produce epoxides, but
also can accept several anionic nucleophiles in epoxide ring-
opening reactions with the formation of a broad range of b-
nucleophiles to produce b-azido alcohols. However, all the re-
ported HHDHs have been described to transform the azide group
to the epoxides at b-position (Scheme 1, red), generating the 2-
azido-1-phenylethan-1-ols (20).12a,12h,12j In this study, we explored
the HheG from I. coccineus for highly a-position regioselective
ring-opening of styrene oxide derivatives 1 with azide (Scheme 1,
blue), which gave the 2-azido-2-aryl-1-ols (2) with the yields up to
96%.
Results and discussion
Recently Schallmey and coworkers have reported that the
HHDH from I. coccineus, namely HheG, exhibited signicant
activity in the azidolysis of cyclohexene oxide and limonene
oxide.13 Analysis of the HheG's structure revealed a large open
cle harboring the active site to accept these cyclic epoxide
substrates, which was in sharp contrast to other known HHDH
structures. Attracted by these results, we inferred the HheG
might possess a special regioselectivity in the ring-opening
process.
With this conjecture in mind, we rst investigated the HheG-
catalyzed ring-opening of styrene oxide 1a with azide on an
Key Laboratory of Biocatalysis & Chiral Drug Synthesis of Guizhou Province, Generic
Drug Research Center of Guizhou Province, Green Pharmaceuticals Engineering
Research Center of Guizhou Province, School of Pharmacy, Zunyi Medical
University, Zunyi, 563000, China. E-mail: nanweiwan@zmc.edu.cn; yzchen@zmc.
edu.cn
† Electronic supplementary information (ESI) available: Experiment procedures
and NMR data. See DOI: 10.1039/c9ra03774h
Scheme 1 HHDH-catalyzed a-position and b-position regioselective
ring-opening of epoxides with azide.
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Subsequently, we tested the conditions of the HheG-catalyzed
azide-mediated ring-opening (ARO) reaction using epoxide 1a as
the model substrate. All the reactions were performed on
analytical scale, and the relative yields of 2a at different condi-
tions were determined by HPLC analysis to screen the optimal
conditions. First the reaction pH/buffer and temperature were
examined and the results were summarized in Table 1. It could
be found that the optimal pH/buffer was 7.0/PBS (entry 4).
Reaction in 5.0/HAc–NaAc gave a very low yield (entry 1). Reaction
in the higher pH/PBS caused a dramatic decrease of the yield
(entries 5 and 6 vs. entry 4). A good yield was obtained in 8.0/Tris–
H2SO4, which was dramatic higher than that obtained in 8.0/PBS
(entry 6 vs. entry 8). Temperature study revealed the optimal
reaction temperature was 30 ꢀC (entry 4). An excellent yield was
also obtained at 25 ꢀC (entry 10). Increasing reaction temperature
to 35–45 ꢀC resulted in a sharp decrease of the yield (entries 11–
13 vs. entry 4), which corresponded to the previous study of the
thermostability of the HheG.15
We further investigated the ratio of azide to substrate 1a.
From the results of Table 2 (entries 1–4), we could nd that the
optimal ratio for the ARO reaction was 1.5 : 1 (entry 2).
Increasing the ratio to 2 : 1 and 3 : 1 did not improve the yield
(entries 3 and 4 vs. entry 2). The recombinant cell density was
subsequent investigated at the ratio of 1.5 : 1 (entries 5–7). A
higher yield was obtained by increasing cell density (entries 5–7
vs. 2). However, the yields generated by 15 and 20 g cdw Lꢁ1 of
cells were almost equivalent (entry 6 vs. entry 7), and 15 g cdw
Lꢁ1 of cells was selected for the ARO reaction in the following
experiments.
Fig. 1 Ring-opening of 1a catalyzed by HheG (blue line), HheC (red
line), and no enzyme (pink line).
analytical scale. The HheC-catalyzed and non-enzymatic reac-
tions were also carried out as controls. The HPLC analysis
results were depicted in Fig. 1. In the absence of enzyme, ring-
opening of 1a preferred a-position regioselectivity (Fig. 1, pink
line) and the reaction rate was pH-dependent.14 Previous study
has revealed HheC was a R-enantioselective and b-regioselective
enzyme for ring-opening of epoxides by several nucleophiles.
HheC catalyzed azide ring-opening of 1a to produced 2-azido-1-
phenylethan-1-ol (2a0) with 79% b-position regioselectivity and
98% ee (R).12a Herein, the azide was excessive and we found that
the HheC showed about 100% conversion of 1a with about 50%
b-position regioselectivity (Fig. 1, red line). Formation of 2-
azido-2-phenylethan-1-ol (2a) might be contributed by the
chemical reaction via a-position ring-opening of 1a. Interest-
ingly, the HheG showed highly a-position regioselectivity in the
ring-opening process (Fig. 1, blue line) without b-position
selectivity, which was different from the other reported HHDH
regioselectivity.
Aer obtaining the optimum reaction conditions, the
reaction courses at the substrate concentrations from 5 to
60 mM were investigated. The yields were analyzed by HPLC
at 6, 12 and 24 h for each reaction. The highest yield was set
Table 1 Investigation of the pH/buffer and temperature of ARO reaction
Entry
pH/buffera
T (oC)
NaN3 : 1a
Conc. 1a (mM)
Cell conc. (g cdw Lꢁ1
)
Time (h)
Relative yield 2a b (%)
1
2
3
4
5
6
7
8
5.0/HAc–NaAc
6.0/HAc–NaAc
6.0/PBS
7.0/PBS
7.5/PBS
30
30
30
30
30
30
30
30
30
25
35
40
45
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
6
6
6
6
6
11
76
32
100
86
82
88
96
72
98
53
44
49
8.0/PBS
7.0/Tris–H2SO4
8.0/Tris–H2SO4
9.0/Gly–NaOH
7.0/PBS
7.0/PBS
7.0/PBS
9
10
11
12
13
7.0/PBS
a
Buffer concentration: 50 mM; PBS ¼ Na2HPO4–KH2PO4. b The relative yields were calculated by HPLC analysis. The highest peak area of 2a was set
to 100% yield. Conc. ¼ concentration.
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Table 2 Investigation of the ratio of NaN3 to 1a and cell density of ARO reaction
Entry
pH/buffera
T (oC)
NaN3 : 1a
Conc. 1a (mM)
Cell conc. (g cdw Lꢁ1
)
Time (h)
Relative yield 2a b (%)
1
2
3
4
5
6
7
7.0/PBS
7.0/PBS
7.0/PBS
7.0/PBS
7.0/PBS
7.0/PBS
7.0/PBS
30
30
30
30
30
30
30
1 : 1
1.5 : 1
2 : 1
5
5
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
70
83
80
72
84
3 : 1
5
1.5 : 1
1.5 : 1
1.5 : 1
10
15
20
99
100
a
Buffer concentration: 50 mM; PBS ¼ Na2HPO4–KH2PO4. b The relative yields were calculated by HPLC analysis. The highest peak area of 2a was set
to 100%. Conc. ¼ concentration.
to 100% to calculate the relative yields. The results depicted group at p-position generated relative higher yield than that
in Fig. 2 indicated the reactions at the substrate concentra- with substituent group at m-position (entries 6–9 vs. entries 2–
tion of 5–40 mM were almost nished at 12 h. Prolonging the 5). The azidolysis of p-cyanophenyl-substituted epoxide 1j and
reaction time to 24 h did not caused notable increase in p-tert-butyl-substituted epoxide 1k also gave the correspond-
yields. For a higher substrate concentration of 60 mM, the ing azidoalcohols with moderate yields of 65% and 76%
yield obtained at 6 h was as same as that obtained from the respectively (entry 10 and entry 11). All the isolated yields were
lower concentrations. It should be noted that the yield didn't obtained from 40 mM of epoxides aer reaction for 12 h,
increase by extending reaction time, which might be caused which pointed to a good enzyme activity in such biocatalytic
by product inhibition at high substrate concentration. The ring-opening process. More importantly, the ARO reaction
ARO reaction was established to react on 40 mM of epoxides exhibited highly a-position regioselectivity in the ring-opening
for 12 h.
of epoxides 1a–1k, which indicated the HheG was an excellent
Finally, several structurally different aromatic epoxides 1a– a-position regioselective HHDH. To our knowledge, this was
1k were selected as substrates to investigate the substrate the rst time that 2-azido-2-aryl-1-ols 2a–2k were synthesized
scope of the ARO reaction. All the reactions were carried out on by an enzymatic catalysis.
the optimal conditions with 40 mM of epoxides 1. DMSO
(1 vol%) was used as co-solvent to solubilize the epoxides in
the aqueous reaction medium. From the results of Table 3, we
Table 3 Substrate scope of the ARO reaction
could nd that the HheG showed a broad substrate range in
the ARO reaction. The 2-azido-2-aryl-1-ols 2a–2k were synthe-
sized from the corresponding epoxides 1a–1k with the high
yields up to 96%. The epoxide substrates with substituent
Entrya
Substrate
R
Product
Yieldb (%)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
H
m-F
m-Cl
m-Br
m-CH3
p-F
p-Cl
p-Br
p-CH3
p-CN
p-C(CH3)3
2a
2b
2c
2d
2e
2f
2g
2h
2i
96
73
69
66
76
80
91
76
95
65
76
9
10
11
1j
1k
2j
2k
a
Reaction condition: epoxide 1 conc. 40 mM in PBS (50 mM, pH 7.0),
NaN3 conc. 60 mM, cells conc. 15 g cdw Lꢁ1, 30 ꢀC, 12 h. Isolated
b
Fig.
2 Investigation of ARO reaction courses at the substrate
yields.
concentrations from 5 to 60 mM.
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1H), 7.13 (m, 3H), 4.62 (t, J ¼ 6.4 Hz, 1H), 3.72 (d, J ¼ 6.5 Hz, 2H),
2.36 (s, 3H), 1.24 (s, 1H); 13C NMR (100 MHz, CDCl3) d 138.9,
136.3, 129.6, 129.0, 127.9, 124.3, 68.0, 66.6, 21.6.
Experimental
Strains culture and enzymes preparation
2-Azido-2-(4-uorophenyl)ethanol (2f). Light yellow oil,
115 mg, 80% yield, 1H NMR (400 MHz, CDCl3) d 7.31 (dd, J ¼ 8.3,
5.4 Hz, 2H), 7.08 (t, J ¼ 8.6 Hz, 2H), 4.68–4.61 (m, 1H), 3.80–3.65
(m, 2H), 2.32 (s, 1H); 13C NMR (100 MHz, CDCl3) d 162.9 (d, J ¼
246.0 Hz), 132.3 (d, J ¼ 4.0 Hz), 129.0 (d, J ¼ 8.0 Hz), 116.0 (d, J ¼
22.0 Hz), 67.2, 66.5.
2-Azido-2-(4-chlorophenyl)ethanol (2g). White solid, 144 mg,
91% yield, 1H NMR (400 MHz, CDCl3) d 7.35 (d, J ¼ 8.4 Hz, 2H),
7.25 (d, J ¼ 8.4 Hz, 2H), 4.62 (dd, J ¼ 7.4, 5.1 Hz, 1H), 3.76–3.62
(m, 2H), 2.34 (s, 1H). 13C NMR (100 MHz, CDCl3) d 134.9, 134.6,
129.2, 128.6, 67.1, 66.4.
The HheC gene (GenBank accession number: AF397296.1) and
HheG gene (GenBank accession number: AP012057.1, 3979394–
3980182) were synthesized aer codon optimization and ligated
into pET-28a(+) plasmid using restricted enzymes NcoI and
XhoI. The recombinant plasmid pET-28-HheC and pET-28-
HheG were transformed into Escherichia coli BL21 (DE3),
respectively. The kanamycin antibiotic was used in whole
culture process to a nal concentration of 50 mg mLꢁ1
.
A single recombinant E. coli colony was taken from the agar
plate and inoculated in Luria-Bertani (LB) medium. The culture
ꢀ
was grown overnight at 37 C, 200 rpm, for 16 h. A large ask
2-Azido-2-(4-bromophenyl)ethanol (2h). Light yellow solid,
96 mg, 76% yield, 1H NMR (400 MHz, CDCl3) d 7.53 (d, J ¼
8.3 Hz, 2H), 7.21 (d, J ¼ 8.3 Hz, 2H), 4.64 (dd, J ¼ 7.3, 5.1 Hz,
1H), 3.78–3.66 (m, 2H), 1.99 (s, 1H); 13C NMR (100 MHz, CDCl3)
d 135.5, 132.3, 129.0, 122.9, 67.3, 66.5.
(1000 mL) containing 100 mL LB was inoculated with 5 mL of
the overnight culture. The ask culture was grown overnight at
37 ꢀC, 200 rpm, until the OD600 up to 0.6–0.8. Then, culture was
induced with IPTG (nal concentration 0.1 mM) and further
shaken at 200 rpm, 28 ꢀC, for 12–14 h. The cultures were
centrifuged and harvested to obtain recombinant E. coli cells.
2-Azido-2-(p-tolyl)ethanol (2i). Light yellow oil, 136 mg, 95%
yield, 1H NMR (400 MHz, CDCl3) d 7.28 (s, 4H), 4.70 (t, J ¼
6.4 Hz, 1H), 3.79 (d, J ¼ 6.5 Hz, 2H), 2.62 (s, 1H), 2.43 (s, 3H); 13
C
General procedure for the synthesis of 2a–2k
NMR (100 MHz, CDCl3) d 138.7, 133.7, 129.7, 127.2, 67.7, 66.47,
To a 20 mL suspension of 15 g cdw Lꢁ1 recombinant E. coli cells 21.2.
(HheG) in 50 mM PBS buffer (pH 7.0), NaN3 was added to
2-Azido-2-(4-cyanophenyl)ethanol (2j). Light yellow oil,
1
a concentration of 60 mM. Then the epoxides 1 was added to 90 mg, 65% yield, H NMR (400 MHz, CDCl3) d 7.71–7.69 (m,
a concentration of 40 mM using 200 mL of dimethyl sulfoxide 2H), 7.48–7.46 (m, 2H), 4.74 (dd, J ¼ 7.7, 4.5 Hz, 1H), 3.77 (dq, J
(DMSO) as co-solvent. The mixture was stirred at 250 rpm and ¼ 19.3, 11.5, 6.1 Hz, 2H), 1.79 (s, 1H); 13C NMR (100 MHz,
ꢀ
30 C. Aer reaction for 12 h, the mixture was extracted using CDCl3) d 138.7, 133.7, 129.7, 127.2, 67.7, 66.4, 21.2.
ethyl acetate (2 ꢂ 20 mL). The organic phase was separated,
2-Azido-2-(4-(tert-butyl)phenyl)ethanol (2k). Light yellow
dried over anhydrous Na2SO4, ltered, and concentrated by solid, 100 mg, 76% yield, 1H NMR (400 MHz, CDCl3) d 7.41 (d, J
rotary evaporation. The residue was puried by ash column ¼ 8.3 Hz, 2H), 7.25 (d, J ¼ 8.3 Hz, 2H), 4.64 (t, J ¼ 6.4 Hz, 1H),
chromatography on silica gel (petroleum ether/ethyl acetate ¼ 3.73 (d, J ¼ 6.4 Hz, 2H), 2.56 (s, 9H), 1.32 (s, 9H); 13C NMR (100
10 : 1–5 : 1) to afford the desired product 2.
MHz, CDCl3) d 151.8, 133.3, 126.9, 125.9, 67.7, 66.5, 34.7, 31.4.
2-Azido-2-phenylethanol (2a). Light yellow oil, 125 mg, 96%
yield, 1H NMR (400 MHz, CDCl3) d 7.44–7.31 (m, 5H), 4.68 (t, J ¼
Conclusions
6.4 Hz, 1H), 3.74 (t, J ¼ 6.0 Hz, 2H), 2.24 (t, J ¼ 6.1 Hz, 1H). 13
C
NMR (100 MHz, CDCl3) d 136.3, 129.1, 128.8, 127.3, 68.0, 66.6. In summary, we found the HheG exhibited highly a-position
2-Azido-2-(3-uorophenyl)ethanol (2b). Light yellow oil, regioselectivity for ring-opening of styrene oxide derivatives
82.9 mg, 73% yield, 1H NMR (400 MHz, CDCl3) d 7.39–7.32 (m, with azide and developed a biocatalytic approach to 2-azido-2-
1H), 7.13–7.01 (m, 3H), 4.66 (dd, J ¼ 7.9, 4.7 Hz, 1H), 3.81–3.66 aryl-1-ols 2a–2k with good yields. Our results brought
(m, 2H), 2.57 (s, 1H); 13C NMR (100 MHz, CDCl3) d 163.0 (d, J ¼ a complementary regioselective biotransformation for the
246.0 Hz), 138.9 (d, J ¼ 7.0 Hz), 130.6 (d, J ¼ 8.0 Hz), 122.9 (d, J ¼ HHDH-catalyzed ring-opening of epoxides. We expect that it
3.0 Hz), 115.7 (d, J ¼ 21.0 Hz), 114.2 (d, J ¼ 22.0 Hz), 67.2 (d, J ¼ would broaden the product spectrum of b-functional alcohols
1.0 Hz), 66.4.
synthesized by HHDHs.
2-Azido-2-(3-chlorophenyl)ethanol (2c). Light yellow oil,
56 mg, 69% yield, 1H NMR (400 MHz, CDCl3) d 7.33 (d, J ¼
4.9 Hz, 3H), 7.21–7.21 (m, 1H), 4.64 (dd, J ¼ 7.7, 4.8 Hz, 1H),
Conflicts of interest
3.74–3.70 (m, 2H), 2.30 (s, 1H); 13C NMR (100 MHz, CDCl3) There are no conicts of interest to declare.
d 138.5, 134.9, 130.3, 129.0, 127.4, 125.4, 67.2, 66.5.
2-Azido-2-(3-bromophenyl)ethanol (2d). Light yellow oil,
Acknowledgements
1
128 mg, 66% yield, H NMR (400 MHz, CDCl3) d 7.48–7.43 (m,
2H), 7.23 (dd, J ¼ 4.4, 0.8 Hz, 2H), 4.60 (dd, J ¼ 7.8, 4.7 Hz, 1H), We are grateful for nancial support from the National Natural
3.69 (qd, J ¼ 11.6, 6.3 Hz, 2H), 2.68 (s, 1H); 13C NMR (100 MHz, Science Foundation of China (No. 21662050 and 31800663),
CDCl3) d 138.7, 131.8, 130.5, 130.2, 125.8, 123.0, 67.0, 66.3.
Guizhou Science and Technology Department (QKHRC-2016-
2-Azido-2-(3-methphenyl)ethanol (2e). Light yellow oil, 4029, QKHPTRC-2016-5801, QKHJC-2019-1467 and QKHZC-
108 mg, 76%, yield, 1H NMR (400 MHz, CDCl3) d 7.31–7.22 (m, 2019-2579), Program for Outstanding Youth of Zunyi Medical
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