Enantioselective Access to Chiral 2-Substituted 2,3-Dihydrobenzo[1,4]dioxane Derivatives through Rh-Catalyzed Asymmetric Hydrogenation

Article abstract of DOI:10.1021/acs.orglett.8b01469

Rh-catalyzed asymmetric hydrogenation of various benzo[b][1,4]dioxine derivatives was successfully developed to prepare chiral 2-substituted 2,3-dihydrobenzo[1,4]dioxane derivatives using ZhaoPhos and N-methylation of ZhaoPhos ligands with high yields and excellent enantioselectivities (up to 99% yield, >99% enantiomeric excess (ee), turnover number (TON) = 24 000). Moreover, this asymmetric hydrogenation methodology, as the key step with up to 10 000 TON, was successfully applied to develop highly efficient synthetic routes for the construction of some important biologically active molecules, such as MKC-242, WB4101, BSF-190555, and (R)-doxazosin·HCl.

Full text of DOI:10.1021/acs.orglett.8b01469

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Enantioselective Access to Chiral 2‑Substituted 2,3-
Dihydrobenzo[1,4]dioxane Derivatives through Rh-Catalyzed
Asymmetric Hydrogenation
Xuguang Yin,† Yi Huang,† Ziyi Chen,† Yang Hu,† Lin Tao,† Qingyang Zhao,# Xiu-Qin Dong,*,†
and Xumu Zhang*,§,†
†Key Laboratory of Biomedical Polymers, Engineering Research Center of Organosilicon Compounds & Materials, Ministry of
Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, People’s Republic of China
§Department of Chemistry, Southern University of Science and Technology, Shenzhen, Guangdong 518055, People’s Republic of
China
#Key Laboratory of Synthetic and Natural Functional Molecule, Chemistry of Ministry of Education, College of Chemistry and
Materials Science, Northwest University, Xi’an, People’s Republic of China
S
* Supporting Information
ABSTRACT: Rh-catalyzed asymmetric hydrogenation of
various benzo[b][1,4]dioxine derivatives was successfully
developed to prepare chiral 2-substituted 2,3-dihydrobenzo-
[1,4]dioxane derivatives using ZhaoPhos and N-methylation
of ZhaoPhos ligands with high yields and excellent
enantioselectivities (up to 99% yield, >99% enantiomeric
excess (ee), turnover number (TON) = 24 000). Moreover, this asymmetric hydrogenation methodology, as the key step with
up to 10 000 TON, was successfully applied to develop highly efficient synthetic routes for the construction of some important
biologically active molecules, such as MKC-242, WB4101, BSF-190555, and (R)-doxazosin·HCl.
(R)-doxazosin,3 selective α2C adrenergic receptor antagonist,4
hiral 2-substituted 2,3-dihydro-1,4-benzodioxane motifs
the antidepressant MKC-242,5 the potent α1D-adrenergic
Chave wide application in a variety of bioactive natural
antagonist WB4101,6 the antihypertensive agent IDR-16084,7
products and therapeutic agents with important biological
the 5-HT1A receptor agonist BSF-190555,8 the flesinoxan
hydrochloride,9 and the anticonvulsant JNJ-26489112.10
activities (see Figure 1),1−10 such as the antihypertensive drug
In the past decades, several approaches were developed to
construct the important chiral 2,3-dihydro-1,4-benzodioxane
frameworks. Among them, the enzymatic resolution of racemic
1,4-benzodioxan-2-carboxylic acids and derivatives is a common
method.11,2a Other methods involved the chiral pool12 and
diastereomeric crystallization with chiral amines.13 Buchwald
and co-workers prepared chiral 1,4-benzodioxanes and deriva-
tives through Pd-catalyzed intramolecular etherification of aryl
halides with a chiral alcohol group.14 Cai and co-workers
developed Pd-catalyzed asymmetric intramolecular O-arylation
coupling reaction via asymmetric desymmetrization to obtain
chiral 1,4-benzodioxanes with moderate to excellent enantiose-
lectivities.15 Zhang and co-workers realized Ir-catalyzed
asymmetric hydrogenation of 2-substituted 1,4-benzodioxines
to prepare chiral 1,4-benzodioxanes.16 Because of the great
importance of the chiral 1,4-benzodioxane unit, it is necessary to
develop a highly reactive and enantioselective catalytic system to
construct chiral 1,4-benzodioxanes and derivatives. Transition-
metal-catalyzed asymmetric hydrogenation of prochiral unsatu-
rated compounds has been considered to be a direct and atom-
Figure 1. Examples of biologically active molecules containing the
chiral 2,3-dihydro-1,4-benzodioxane framework.
Received: May 9, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01469
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Scope Study for the Rh-Catalyzed Asymmetric
Hydrogenation of Methyl Benzo[b][1,4]dioxine-2-
a
carboxylates
Figure 2. Chiral biphosphine ligands for the Rh-catalyzed asymmetric
hydrogenation of methyl benzo[b][1,4]dioxine-2-carboxylate 1a.
Table 1. Screening Ligands for the Rh-Catalyzed Asymmetric
a
Condition A: 0.1 mmol substrate 1, substrate/ [Rh(NBD)2]BF4/
Hydrogenation of Methyl Benzo[b][1,4]dioxine-2-
ZhaoPhos L1 = 1/0.01/0.011 at 25 °C under 10 atm H2 in 1.0 mL
CH2Cl2 for 12 h. Condition B: 0.1 mmol substrate 1, substrate/
[Rh(NBD)2]BF4/L2 = 1/0.01/0.011 at 25 °C under 10 atm H2 in 1.0
mL CH2Cl2 for 12 h. Conversion was determined by 1H NMR
analysis. Isolated yields. The ee value was determined by HPLC on a
chiral column.
a
carboxylate 1a
b
c
entry
ligand
(S)-Binap
conversion (%)
ee (%)
1
2
3
4
5
6
7
8
9
15
20
98
65
67
>99
>99
97
55
62
70
77
84
98
98
50
96
carboxylate 1a as a model substrate to optimize the reaction
conditions. Some important bisphosphine ligands such as (S)-
Binap, (RC,SP)-Duanphos, (S,S)-Me-Duphos, (S)-Binapine,
(S,S)-f-Binaphane, and ZhaoPhos (Figure 2) were evaluated in
the presence of 1.0 mol % of catalyst under 30 atm H2 in MeOH
at room temperature for 12 h. When ZhaoPhos L1 was
employed, the product 2a was obtained with full conversion and
98% ee (Table 1, entry 6). Other chiral bisphosphine ligands
provided poor to moderate reaction results (15%−98%
conversions, 55%−84% ee; Table 1, entries 1−5). Other chiral
bisphosphine-(thio)urea ligands (L2−L4) were also inves-
tigated in this transformation. Except for ligand L3, which
changed from a thiourea motif to urea motif and displayed poor
enantioselectivity (50% ee; Table 1, entry 8), others showed
satisfactory results (96%−98% ee; Table 1, entries 7 and 9). To
our surprise, the N-methylation of ZhaoPhos L2 displayed
excellent enantioselectivity, which is the same with ZhaoPhos
L1 (98% ee; Table 1, entry 7); it is possible that one hydrogen
bond between the substrate and ligand displayed nearly the same
effect with two hydrogen bonds in this asymmetric reaction. The
solvent effect was then inspected; the results are summarized in
Table S1 in the Supporting Information, and CH2Cl2 was
selected as the best solvent.
(RC,SP)-Duanphos
(S,S)-Me-Duphos
(S)-Binapine
(S,S)-f-Binaphane
ZhaoPhos L1
L2
L3
L4
99
a
Unless otherwise mentioned, all reactions were carried out with a
[Rh(NBD)2]BF4/ligand/substrate ratio of 1:1.1:100 in MeOH, 0.1
b
mmol substrate 1a, 30 atm hydrogen, 25 °C, 12 h. Determined by
c
1H NMR analysis. Determined by HPLC on a chiral phase.
economic synthetic methodology in asymmetric synthesis.17
Previously, we have successfully applied a bifunctional bi-
sphosphine−thiourea ligand (ZhaoPhos) into Rh-catalyzed
asymmetric hydrogenation through the important interactions
between the thiourea group and the functional group of
substrates with excellent results.18 On the basis of this theory,
we herein successfully realized Rh/ZhaoPhos and N-methyl-
ation of ZhaoPhos-catalyzed asymmetric hydrogenation of 2-
substituted benzo[b][1,4]dioxine derivatives with up to 99%
yield, >99% enantiomeric excess (ee), and a turnover number
(TON) of 24 000.
With the optimized reaction conditions in hand, we started to
explore the substrate scope of this reaction catalyzed by
[Rh(NBD)2]BF4/L1 and L2 in CH2Cl2. As shown in Scheme
1, a series of methyl benzo[b][1,4]dioxine-2-carboxylates (1a−
We began the initial investigation with Rh-catalyzed
asymmetric hydrogenation of methyl benzo[b][1,4]dioxine-2-
B
DOI: 10.1021/acs.orglett.8b01469
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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Scheme 2. Scope Study for the Rh-Catalyzed Asymmetric
Hydrogenation of 2-Substituted Benzo[b][1,4]dioxins
Scheme 3. Synthetic Transformations of Hydrogenation
Products
a
a
Condition A: 0.1 mmol substrate 1, substrate/ [Rh(NBD)2]BF4/
ZhaoPhos L1 = 1/0.01/0.011 at 25 °C under 10 atm H2 in 1.0 mL
CH2Cl2 for 12 h. Condition B: 0.1 mmol substrate 1, substrate/
[Rh(NBD)2]BF4/L2 = 1/0.01/0.011 at 25 °C under 10 atm H2 in 1.0
mL CH2Cl2 for 12 h. Conversion was determined by 1H NMR
analysis. Isolated yields. The ee values were determined by HPLC on
b
a chiral column. Reaction temperature = 60 °C.
Table 2. TON Experiments of Rh-Catalyzed Asymmetric
Hydrogenation of Methyl Benzo[b][1,4]dioxine-2-
a
carboxylate 1a
b
c
d
H2
(atm)
time
(h)
conversion
(%)
yield
(%)
ee
entry
S/C
(%)
1
2
3
2000
5000
10 000
24 000
24 000
30
30
30
60
60
36
48
52
72
72
>99
>99
>99
>99
>99
98
98
99
99
99
98
98
98
97
98
e
4
ef
,
5
a
Unless otherwise noted, all reactions were performed with a
b
[Rh(NBD)2]BF4]/ZhaoPhos L1 = 1:1.1 in CH2Cl2. Determined
c
d
by 1H NMR analysis. Isolated yield. Determined by HPLC analysis
e
f
using a chiral stationary phase. Reaction temperature = 40 °C. Using
ligand L2.
1i)2b,19a with various substituents on the phenyl ring were
hydrogenated smoothly and were catalyzed by both [Rh-
(NBD)2]BF4/L1 and [Rh(NBD)2]BF4/L2 with excellent
results. The methyl benzo[b][1,4]dioxine-2-carboxylates bear-
ing electron-donating groups (1b and 1c) or electron-with-
drawing groups (1d−1i) on the phenyl ring performed well with
full conversions, 91%−99% yields, and 96%−>99% ee. The
substrate with a sterically hindered naphthyl group 1j is also
converted to the hydrogenation product 2j with excellent results
(99% yield, 99%−>99% ee).
In addition, in order to further investigate the tolerance of
substrate scope of this catalytic system, a series of different 2-
substituted benzo[b][1,4]dioxine derivatives 1k−1r, including
2-carboxyl, 2-amido, and 2-hydroxymethyl group were applied
in this asymmetric hydrogenation catalyzed by [Rh(NBD)2]-
BF4/L1 and L2. As shown in Scheme 2, these different 2-
substituted benzo[b][1,4]dioxins proceeded smoothly to afford
the hydrogenation products 2k−2r with excellent results (95%−
99% yields, 92%−>99% ee). It was worth noting that the
substrate benzo[b][1,4]dioxin-2-ylmethanol 1p reacted well to
C
DOI: 10.1021/acs.orglett.8b01469
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
produce compound 2p with high yield and excellent
enantioselectivity, which is the key intermediate for the synthesis
of some important biologically active molecules, such as MKC-
242 and WB4101.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: xiuqindong@whu.edu.cn (X.-Q. Dong).
*E-mail: zhangxm@sustc.edu.cn (X. Zhang).
Our Rh/ZhaoPhos L1 and N-methylation of ZhaoPhos L2
catalytic system are very highly efficient in this asymmetric
hydrogenation. The model substrate methyl benzo[b][1,4]-
dioxine-2-carboxylate 1a was hydrogenated well, catalyzed by
Rh/ZhaoPhos L1 with 0.05 mol % (S/C = 2000) catalyst, 0.02
mol % (S/C = 5000), or even with 0.01 mol % (S/C = 10 000).
Full conversion, high yield, and 98% ee can be obtained (Table
2, entries 1−3). 99% yield and 97% ee can be obtained when S/
C is 24 000 (Table 2, entry 4). The Rh/N-methylation of
ZhaoPhos L2 catalytic system also worked well; almost the same
excellent results can be afforded with S/C = 24 000 (Table 2,
entry 5).
This highly efficient asymmetric hydrogenation methodology
demonstrated powerful synthetic utility. As shown in Scheme 3,
several great and creative synthetic routes were described to
prepare some important biologically active compounds. Hydro-
genation product 2a is the key intermediate for the construction
of antidepressant MKC-242 and potent α1D-adrenergic
antagonist WB4101, which can be easily obtained through our
asymmetric catalytic hydrogenation system in 1.2 g scale with
0.01 mol % catalyst (S/C = 10 000, 99% yield, 98% ee; Scheme
3a). Compound 2a was reduced by diisobutylaluminum hydride
(DIBAL-H) to provide compound 2p, which was treated with p-
TsCl (p-toluene sulfonyl chloride) to afford compound 3 in 96%
yield and without a loss of ee. Finally, compound 3 can react with
amines 4 and 5 to give the corresponding MKC-242 and
WB4101.15b Compound 7 can be easily synthesized through a
similar approach, which could be worked as the important
intermediate for the construction of 5-HT1A receptor agonist
BSF-190555 (Scheme 3b).8 Compound 2o was easily obtained
in 99% yield and 99% ee through this asymmetric hydro-
genation. It then underwent deprotection to give intermediate 9
in 96% yield and 98% ee, which reacted with the amine 10 to
afford (R)-doxazosin·HCl (Scheme 3c).12a,19b
ORCID
Xumu Zhang: 0000-0001-5700-0608
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (Grant Nos. 21432007, 21502145,
21602172), the Natural Science Foundation of Hubei Province
(Grant No. 2016CFB449), the Wuhan Morning Light Plan of
Y ou t h S c i e n c e a n d T e c h n o l o g y ( G r a n t N o .
2017050304010307), and the Fundamental Research Funds
for Central Universities (Grant No. 2042018kf0202). The
Program of Introducing Talents of Discipline to Universities of
China (111 Project) is also appreciated.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b01469.
General remarks; general procedures for the synthesis of
1a−1j; general procedures for the prpearation of 1k−1s;
characterization of the 1 substrates; screening solvents for
the Rh-catalyzed asymmetric hydrogenation of 1a;
general procedure for the asymmetric hydrogenation of
1; general procedure regarding the high TON experiment
of 1a; NMR and HPLC spectra (PDF)
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