Enantioselective Synthesis of β-Aminotetralins via Chiral Phosphoric Acid-catalyzed Reductive Amination of β-Tetralones

Article abstract of DOI:10.1002/adsc.201701198

A new protocol for the synthesis of chiral β-aminotetralins has been developed via chiral phosphoric acid-catalyzed asymmetric reductive amination of β-tetralones using a Hantzsch ester as an organic hydride donor. Various chiral β-aminotetralins were obtained in good yields with good to high enantioselectivities. Furthermore, the utility of our new protocol was successfully demonstrated in the enantioselective synthesis of rotigotine. (Figure presented.).

Full text of DOI:10.1002/adsc.201701198

Title: Enantioselective Synthesis of β-Aminotetralins via Chiral
Phosphoric Acid-catalyzed Reductive Amination of β-Tetralones
Authors: Do Young Park, Kyung-Hee Kim, and Cheol-Hong Cheon
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10.1002/adsc.201701198
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Enantioselective Synthesis of β-Aminotetralins via Chiral
Phosphoric Acid-catalyzed Reductive Amination of β-
Tetralones
Do Young Parka,†, Kyung-Hee Kima,† and Cheol-Hong Cheona*
a
Department of Chemistry, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea
Tel: (+82)-2-3290-3147; Fax: (+82)-2-3290-3121; e-mail: cheon@korea.ac.kr
These authors contributed equally to this work.
†
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))
number of efforts has been made to control the
aminotetralins has been developed via chiral phosphoric stereochemistry of the amine moiety to date.[2,3]
Abstract. A new protocol for the synthesis of chiral β-
acid-catalyzed asymmetric reductive amination of β-
tetralones using a Hantzsch ester as an organic hydride
donor. Various chiral β-aminotetralins were obtained in
good yields with good to high enantioselectivities.
Furthermore, the utility of our new protocol was
successfully demonstrated in the enantioselective synthesis
of rotigotine.
The majority of reported syntheses of chiral β-
aminotetralins involve the classical diastereomeric
resolution of racemic β-aminotetralins with a chiral
auxiliary.[2] However, diastereomeric resolution
requires a stoichiometric amount of a chiral auxiliary
and the maximum yield of the desired β-
aminotetralins cannot exceed 50% (Scheme 1a).
Keywords: β-Aminotetralins; Asymmetric reductive
amination; Chiral phosphoric acid catalysis; Rotigotine; β-
Tetralones
Chiral β-aminotetralin scaffolds have been found in
various biologically active compounds and
pharmaceuticals, several of which are currently
marketed or are under development (Figure 1).[1]
Scheme 1. General strategies for the synthesis of chiral β-
aminotetralins.
To overcome these limitations, new catalytic
asymmetric protocols to access these chiral β-
aminotetralins have been developed. Most of the
previous catalytic protocols have relied on the
asymmetric hydrogenation of cyclic enamides derived
from β-tetralones in the presence of transition metals,
such as iridium, ruthenium, and rhodium (Scheme
1b).[3] However, these methods involve expensive and
toxic transition metals, which can often cause
additional problems associated with the residual
transition metal catalysts.[4] Thus, the development of
Figure 1. Representative examples of pharmaceutical
products bearing a chiral β-aminotetralin scaffold.
For example, rotigotine is a dopamine agonist of
the non-ergoline class of medication used for the
treatment of Parkinson’s disease and restless legs
syndrome.[1a-1d] Thus, the development of efficient
methods for the synthesis of chiral β-aminotetralins a protocol to access these building blocks without
has been considered the research of importance and a
1
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10.1002/adsc.201701198
Advanced Synthesis & Catalysis
using toxic transition metal catalysts is highly derivatives, bearing different aryl substituents at the
desirable. 3,3’-position on the BINOL scaffold, were
During the past decade, chiral phosphoric acid investigated. Interestingly, the introduction of bulky
catalysis has become one of the most rapidly growing substituents on the 2,6-positions of the aryl group
fields in asymmetric catalysis.[5] Among the various turned out to have a beneficial effect on the
asymmetric transformations developed with chiral enantioselectivity of this transformation (entries 1-4).
phosphoric acids, the enantioselective synthesis of The reaction with phosphoric acid 3c bearing 2,4,6-
chiral amines via the asymmetric reduction of tri(isopropyl)phenyl groups at the 3,3’-positions of
ketimines and/or the asymmetric reductive amination the BINOL scaffold provided 4aa in 84% yield and
of ketones has been considered one of the most 72% ee (entry 3).
popular methods.[6] However, the previously reported
chiral
phosphoric
acid-catalyzed
asymmetric
Table 1. Optimization of reaction conditions.
reduction of ketimines and/or reductive amination of
ketones have been limited to the preparation of chiral
acyclic amines from acyclic ketones, and the chiral
phosphoric acid-catalyzed asymmetric reductive
amination of cyclic ketones has remained
unexplored.[7] Considering the biological and
pharmaceutical importance of chiral cyclic amines
and the lack of protocols to access these important
building blocks using chiral phosphoric acids, we
strongly felt the need to develop a new method for the
enantioselective synthesis of cyclic amines via chiral
phosphoric acid-catalyzed asymmetric reductive
amination of cyclic ketones as an alternative to
asymmetric hydrogenation.
We herein report the enantioselective synthesis of
β-aminotetralins via chiral phosphoric acid-catalyzed
asymmetric reductive amination of β-tetralones with
anilines using a Hantzsch ester as an organic hydride
donor (Scheme 1c).[8] This protocol can be applied to
various β-tetralones and anilines, providing the
corresponding β-aminotetralins in good yields with
good to high enantioselectivities. Furthermore, the
usefulness of this protocol was demonstrated in the
enantioselective synthesis of rotigotine, one of the
representative pharmaceuticals bearing
aminotetralin scaffold.
a
β-
Recently, our group has initiated a program for the
enantioselective synthesis of chiral β-arylamines via
chiral phosphoric acid-catalyzed reductive amination
of benzyl methyl ketones.[9,10,11] As a continuing effort
to develop protocols to access chiral β-arylamines via
chiral
phosphoric
acid-catalyzed
asymmetric
[a] Isolated yield of 4aa.
reductive amination, we further attempted to develop
a protocol for the synthesis of chiral cyclic β-
arylamines by reductive amination of benzofused
cyclic β-ketones.
[b] Enantiomeric excess (ee) was determined by a chiral
HPLC analysis.
Based on this idea, we began our research with the
investigation of the reaction conditions required for
the synthesis of chiral cyclic β-arylamines via chiral
phosphoric acid-catalyzed asymmetric reductive
Then, the effect of a substituent at the para-
position of the aryl substituent was investigated and
the size of the substituent was found to have a
considerable influence on the enantioselectivity
(entries 3, 5-6); enantioselectivity increased with
increasing size of the substituent (entries 3 and 5),
whereas the introduction of a bulky phenyl substituent
at the para-position was found to have a detrimental
effect on the enantioselectivity (entries 3 and 6).
Since 3c gave the best result among the chiral
phosphoric acids tested, 3c was chosen as the optimal
catalyst.
amination, using β-tetralone 1a as
a
model
benzofused cyclic β-ketone in the presence of a chiral
phosphoric acid 3 (Table 1). When β-tetralone 1a and
para-anisidine 2a were subjected to asymmetric
reductive amination with a Hantzsch ester as an
organic hydride donor in the presence of the chiral
phosphoric acid 3a, the desired β-aminotetralin 4aa
was obtained in 71% yield but with low
enantioselectivity (entry 1). With this result in hand,
other chiral phosphoric acids 3 derived from BINOL
We next explored the effect of reaction media. It
was found that the choice of the solvent had only a
2
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10.1002/adsc.201701198
Advanced Synthesis & Catalysis
slight influence on the outcome of this transformation anisidines
provided
the
corresponding
β-
(entries 3, 7-11). Reactions in aromatic hydrocarbons aminotetralins (4aa and 4ah) with similar efficiencies
and non-polar etherated solvents afforded the desired (entries 1 and 8), while ortho-anisidine was not
product 4aa in similar enantioselectivities (entries 3, suitable for this transformation and no formation of β-
7-10), while the enantioselectivity slightly decreased aminotetralin 4ai was observed with ortho-anisidine
in the reaction using a halogenated solvent (entry 11). (entry 9). Furthermore, the more electron-rich 3,4,5-
Among the solvents tested, toluene was chosen as the trimethoxyaniline 2j was applicable to this protocol
optimal solvent. Next, screening the reaction and the desired product 4aj was obtained in 82% ee
temperature (entries 3, 12-15) revealed that (entry 10). Among the aniline derivatives 2 tested,
enantioselectivity increased when the reaction was para-anisidine 2a was chosen as the optimal protected
carried out at lower temperatures. Since the reaction ammonia surrogate because 1) the reaction with 2a
at room temperature yielded 4aa in the best provided the best results in terms of the yield and
enantioselectivity (entry 15), we decided to perform enantioselectivity of the desired β-aminotetralin, and
this transformation at room temperature.[12]
2) the para-methoxyphenyl (PMP) group could be
removed under oxidative conditions to generate a free
Table 2. Substrate scope of aniline derivatives 2 in amine.[13]
reductive amination of β-tetralone 1a
Table 3. Scope of β-tetralone derivatives 1 in reductive
amination of para-anisidine 2a
[a] Isolated yield of 4.
[b] Ee was determined by a chiral HPLC analysis.
[c] 1.5 mmol scale.
[a] Isolated yield of 4.
[d] 3,4,5-Trimethoxyaniline 2j was used.
[b] Ee was determined by a chiral HPLC analysis.
[c] N.R. means no reaction.
[d] N.D. means not determined.
Next, we investigated the substrate scope of β-
tetralone derivatives 1 in this transformation with
para-anisidine 2a (Table 3). First, the electronic effect
Under these optimized conditions, the substrate of the phenyl ring in β-tetralones 1 was investigated
scope of aniline derivatives 2 was explored in this and found to exert little influence on the efficiency of
transformation with β-tetralone 1a (Table 2). The this
asymmetric
transformation;
β-tetralone
electronic effect of the aniline derivatives had derivatives bearing either an electron-donating or an
negligible effect on the enantioselectivity (entries 1-6). electron-withdrawing substituent on the phenyl ring
Generally, aniline derivatives 2 carrying either an provided the desired β-aminotetralin derivatives 4
electron-donating or an electron-withdrawing with similar enantioselectivities (entries 1-3).
substituent provided β-aminotetralins 4 in similar However, the position of a substituent on the β-
enantioselectivities. However, aniline 2g bearing a tetralone scaffold affected the efficiency of this
free hydroxy group provided the desired product 4ag transformation. β-Tetralones bearing a methoxy group
in much lower yield and enantioselectivity (entry 7). at either 5-, 6-, or 7-position provided the β-tetralin
Next, the steric bulk of the aniline derivatives was products 4 in similar yields and enantioselectivities
found to have a considerable influence on the regardless of the position of the substituent (entries 2,
outcome of this transformation. Para- and meta- 4 and 5), while β-tetralone 1f bearing a methoxy
3
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10.1002/adsc.201701198
Advanced Synthesis & Catalysis
group at the 8-position provided the desired product the desired tertiary amine 6a in 86% yield. Treatment
4fa with much lower efficiency (entry 6). of 6a with CAN successfully provided the desired
Furthermore, this transformation could be carried out secondary amine 7 in 70% yield through oxidative
on a relatively large scale (1.5 mmol) without any loss removal of the PMP moiety.[13] Deprotection of the
of efficiency. In addition, the asymmetric reductive methoxy group with BBr3, followed by reaction with
amination of 1d with 3,4,5-trimethoxyaniline 2j 2-thienylethanol tosylate provided rotigotine (5) in
proceeded smoothly to provide β-aminotetralin 4dj in 61% yield and with 81% ee over two steps. It should
85% yield and 83% ee. Unfortunately, however, our be noted that all these transformations could be
attempts to extend this protocol to the synthesis of accomplished without any loss in enantiopurity.
other cyclic amines, such as α-aminotetralins, via Furthermore, the absolute stereochemistry of
enantioselective reductive amination of α-tetralones rotigotine 5 was assigned as (S) by comparing the
was unsuccessful.[14]
optical rotation of each HCl salt of compounds 5 and
With these results in hand, we further attempted to 7, respectively, with the reported value in the
demonstrate the utility of this protocol in the literature.[18]
enantioselective synthesis of rotigotine (5) (Scheme
In conclusion, we have reported the first chiral
phosphoric acid-catalyzed reductive amination of β-
tetralones leading to chiral β-aminotetralins. This
protocol could be applied to various β-tetralones and
aniline derivatives and the desired β-aminotetralins
were obtained in good yields with good to high
enantioselectivities. Furthermore, the utility of this
transformation was successfully demonstrated by the
enantioselective synthesis of rotigotine. The absolute
stereochemistry of the resulting β-aminotetralins
could be assigned as (S) by the comparison of the
optical rotation with the reported value in the
literature. Further application of this method to the
preparation of other biologically important
compounds and detailed mechanistic studies are
currently underway in our laboratory and will be
reported in due course.
2).[15]
Experimental Section
General procedure for the asymmetric reductive
amination of β-tetralones with anilines (Tables 2 and 3).
To a test tube equipped with a stirring bar were added β-
tetralone derivative 1 (0.10 mmol; 1.0 equiv), aniline
derivative 2 (0.12 mmol; 1.2 equiv), Hantzsch ester (0.15
mmol; 1.5 equiv), catalyst 3c (0.010 mmol; 10 mol%) and
4 Å molecular sieves. Then, toluene (1.0 mL) was added to
the above mixture. The reaction mixture was stirred at
room temperature and monitored by TLC. After complete
consumption of the tetralone derivative 1, the reaction
mixture was filtered to remove molecular sieves. The
filtrate was purified by column chromatography on silica to
provide the desired product 4.
Scheme 2. Synthesis of rotigotine (5)
Since β-aminotetralin 4dj bearing
trimethylphenyl group has slightly better
a
3,4,5-
a
enantioselectivity than 4da carrying a PMP group
(entry 4, Table 3), we first attempted to complete the
synthesis of rotigotine (5) with 4dj. Reductive
amination of 4dj with propanal afforded the tertiary
amine 6b in 85% yield. However, deprotection of the
3,4,5-methoxyphenyl group present in 6b was
Acknowledgements
challenging.
Deprotection
of
the
3,4,5-
This work was supported by the National Research Foundation of
Korea (NRF) grants funded by the Korean Government (NRF-
20100020209 and NRF-2015R1D1A1A01057200). C.-H.C. also
thanks for a financial support from an NRF grant funded by the
Korean Government (NRF-2014-011165, Center for New
Directions in Organic Synthesis). We also thank Professor
Jianwei Sun for providing us with SP-CPA.
trimethoxyphenyl group with cerium ammonium
nitrate (CAN)[16] provided a complex mixture, while
the reaction of 6b with trichloroisocyanuric acid
(TCCA) yielded the unwanted side product 7-Cl
containing one chlorine atom on the phenyl ring in a
low yield.[17]
Thus, we focused our efforts on the completion of
the asymmetric synthesis of rotigotine (5) with 4da.
The reductive amination of 4da with propanal yielded
References
4
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10.1002/adsc.201701198
Advanced Synthesis & Catalysis
[1] For examples of bioactive compounds bearing a chiral [7] The List group reported an excellent example of the
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[12] For further optimization of other reaction parameters,
see Supporting Information.
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[17] Based on the spectroscopic data, we confirmed that
the product 7-Cl has one chlorine atom either ortho or
5
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10.1002/adsc.201701198
Advanced Synthesis & Catalysis
para to the methoxy group. However, we did not fully
analyze the position of the chlorine atom in 7-Cl
because it was not the desired product and the yield was
poor.
[18] Optical rotation ([α]D20) values of HCl salts of
rotigotine (5) and 7 were found to be -38.3 (c = 1.0,
CH3OH) and -26.1 (c = 1.0, CH3OH), respectively.
Since these values have the same sign with the
reference values of HCl salts of (S)-5 {-54.6 (c = 1.0,
CH3OH)} and (S)-7 {-71.5 (c = 1.0, CH3OH)},
respectively, we assigned the absolute stereochemistry
at the C-2 in rotigotine (5) as (S), although there were
20
some discrepancies in [α]D between the measured
20
values and the reported ones. For the reported [α]D
values in literature, see ref 2d.
6
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10.1002/adsc.201701198
Advanced Synthesis & Catalysis
COMMUNICATION
Enantioselective Synthesis of β-Aminotetralins via
Chiral Phosphoric Acid-catalyzed Reductive
Amination of β-Tetralones
Adv. Synth. Catal. Year, Volume, Page – Page
Do Young Park, Kyung-Hee Kim, Cheol-Hong
Cheon*
7
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