Kinetic Resolution of Tertiary 2-Alkoxycarboxamido-Substituted Allylic Alcohols by Chiral Phosphoric Acid Catalyzed Intramolecular Transesterification

Article abstract of DOI:10.1002/anie.201905034

A highly enantioselective kinetic resolution of tertiary 2-alkoxycarboxamido allylic alcohols has been achieved through a chiral phosphoric acid catalyzed intramolecular transesterification reaction. Both alkyl,aryl- and dialkyl-substituted tertiary allylic alcohols were resolved with excellent efficiencies, affording both the recovered tertiary alcohols and the carbamate products with high enantioselectivities (with s factors up to 164.6). A gram-scale reaction with 1 mol % catalyst loading and the facile conversion of the enantioenriched products into useful chiral building blocks, such as chiral oxazolidinones and β-amino alcohols, demonstrate the value of this reaction.

Full text of DOI:10.1002/anie.201905034

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Accepted Article
Title: Kinetic Resolution of Tertiary 2-Alkoxycarboxamido-allylic
Alcohols by Chiral Phosphoric Acid Catalyzed Intramolecular
Transesterification
Authors: Subramani Rajkumar, Shunlong He, and Xiaoyu Yang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201905034
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10.1002/anie.201905034
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Kinetic Resolution of Tertiary 2-Alkoxycarboxamido-allylic
Alcohols by Chiral Phosphoric Acid Catalyzed Intramolecular
Transesterification
Subramani Rajkumar‡, Shunlong He‡ and Xiaoyu Yang*
Abstract: A highly enantioselective kinetic resolution of tertiary 2-
alkoxycarboxamido-allylic alcohols has been achieved through chiral
phosphoric acid catalyzed intramolecular transesterification reaction.
Both alkyl,aryl- and dialkyl-substituted tertiary allylic alcohols can be
well-resolved by the optimal conditions with excellent efficiencies,
affording both recovered tertiary alcohols and transesterification
products with high enantioselectivities (with s factor up to 164.6).
The gram-scale reaction with 1 mol % catalyst loading and facile
conversion of chiral products into useful chiral building blocks, such
as chiral oxazolidinones and β-amino alcohols, well demonstrated
the value of this reaction.
catalysis (Figure 1, a). On the other hand, Chan group reported
dehydrative Nazarov-type electrocyclization of α-hydroxy
enamides to provide access to chiral 1H-indene derivatives
under chiral Brønsted acid catalysis7 (Figure 1, b). In these
examples, the chiral anion-paired 2-amidoallyl cations (INT A)
were proposed as the key intermediates, in which the chiral
center from the starting material was eliminated. During our
continuous study on asymmetric reactions of α-hydroxy
enamides under chiral Brønsted acid catalysis, we recently
found that no 2-amidoallyl cation intermediate was generated by
treatment of α-hydroxy encarbamates 1 with phosphoric acid
catalyst; instead an intramolecular transesterification reaction8
proceeded smoothly to give the oxazolidinone derivatives 2
(Figure 1, c). Herein, we reported a highly efficient kinetic
resolution of tertiary 2-alkoxycarboxamido-allylic alcohols (α-
hydroxy encarbamates) by utilizing this reaction under the
catalysis of chiral phosphoric acid catalyst9. A range of racemic
tertiary 2-alkoxycarboxamido-allylic alcohols (including both
aryl,alkyl- and alkyl,alkyl-substituted allylic alcohols) could be
well resolved by the optimal conditions, generating both
transesterification products and unreacted tertiary alcohols with
high enantioselectivities. The obtained chiral products could be
readily converted into useful chiral building blocks, such as
oxazolidinones10 and β-amino alcohols11, which have wide
applications as chiral auxiliaries, chiral ligands and biologically
active molecules.
The kinetic resolution of racemic secondary alcohols has
been extensively investigated in the past few decades and
demonstrated as one of the most efficient and practical
approaches to obtain chiral secondary alcohols1. In contrast, the
corresponding kinetic resolution of tertiary alcohols has been
less studied and only a handful of catalytic examples have been
reported, including both biocatalytic processes2 and
nonenzymatic methods3. However, due to the relative
challenging demands for spatial differentiation among three R
groups3a, 3h, 3i and the intrinsic reaction requirements3b, 3c, kinetic
resolution of tertiary alcohols with high efficiencies that can
provide both unreacted starting material and products with high
enantioselectivities are still quite limited. Among them, List and
co-workers reported chiral Brønsted acid catalyzed kinetic
resolution of homoaldols and diols possessing tertiary hydroxyl
groups via intra- and intermolecular acetalizations with excellent
performance, which could generate both tertiary alcohol starting
materials and acetal products with high enantioselectivities3d, 3f
.
Zhao group3e and Smith group3g reported highly efficient kinetic
resolution of tertiary heterocyclic alcohols via asymmetric
acylation reactions enabled by NHC catalyst and isothiourea
catalyst, respectively. Recently, Oestreich and co-workers
developed kinetic resolution of tertiary progargylic alcohols by
enantioselective Cu-H-catalyzed Si-O coupling with a broad
substrate scope3j.
Recently, the combination of 2-amidoallylic alcohol
substrates (could also be termed as α-hydroxy enamides) with
chiral Brønsted acid catalysis has become a useful strategy for
asymmetric synthesis of functionalized enamide derivatives4.
Our group5 and Kartika group6 independently reported the
asymmetric synthesis of β-indolyl cyclopentenamides from α-
hydroxy cyclopentenamides upon chiral phosphoric acid
[*]
Dr. S. Rajkumar, S. He, Prof. Dr. X. Yang
School of Physical Science and Technology
ShanghaiTech University, Shanghai 201210, China
E-mail: yangxy1@shanghaitech.edu.cn
These authors contribute equally.
Figure 1. a) Asymmetric addition of indoles to α-hydroxy cyclopentenamides
under chiral Brønsted acid catalysis. b) Asymmetric Nazarov-type
electrocyclization of α-hydroxy enamides under chiral Brønsted acid catalysis.
c) Kinetic resolution of tertiary 2-alkoxycarboxamido-allylic alcohols via chiral
phosphoric acid catalyzed intramolecular transesterification.
[‡]
Supporting information for this article is given via a link at the end of
the document.
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Table 1. Optimization of reaction conditions.a
conversionb ee (S)-1c ee 2ac
entry
R
T.
cat.
solvents time
sd
(%)
(%)
(%)
1
2
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
Bn
25 oC
25 oC
25 oC
25 oC
25 oC
25 oC
25 oC
25 oC
25 oC
25 oC
25 oC
25 oC
25 oC
25 oC
10 oC
0 oC
A1
A2
A3
A4
A5
A6
A7
A6
A6
A6
A6
A6
A6
A6
A6
A6
toluene 64 h
toluene 16 h
toluene 16 h
toluene 16 h
toluene 16 h
toluene 64 h
toluene 64 h
50
60
60
7.2
3.8
62
45
72
63
54
47
42
7
61
34
97
61
91
70
63
6
37
42
38
36
79
79
87
78
75
86
36
66
82
89
89
3
3.4
4
8.1
5
3.8
6
26.6
17.7
27.4
8.6
7
8
DCM
Et2O
CCl4
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
48 h
48 h
48 h
48 h
5 h
9
10
11
12
13
14
15
16
50
50
64
58
54
52
52
76
87
65
90
97
95
98
15.8
37.5
3.9
Et
2 h
14.5
41.8
63.7
78.4
iPr
5 h
iPr
24 h
48 h
iPr
aReactions were run with 1 (0.1 mmol) with catalyst (0.01 mmol, 10 mol %) in solvent (1 mL) with 5 Å molecular sieves (30 mg). bConversion (C) = ees/(ees+eep).
cDetermined by HPLC analysis on a chiral stationary phase. ds = ln[(1-C)(1-eep)]/ln[(1-C)(1+eep)].
We initiated our studies by using racemic N-Boc protected
tertiary 2-alkoxycarboxamido-allylic alcohol 1a’ as model
substrate and chiral phosphoric acid as catalyst. While using 10
mol % of TRIP catalyst (cat A1) in toluene at ambient
temperature, the kinetic resolution of tertiary alcohol 1a’
proceeded with a promising selectivity factor (s)12 of 7.2, giving
the transesterification product 2a with 60% enantiomeric excess
(ee) and recovered starting material 1a’ with 60% ee (Table 1,
entry 1). Subsequently, a variety of BINOL- and H8-BINOL-
derived chiral phosphoric acid catalysts were examined (entries
2-7), and the TCYP catalyst (cat A6) turned out to be the optimal
catalyst, giving the best s factor of 26.6 (entry 6). Next, a range
of solvents were also explored (entries 8-11), and PhCl was
found to be the optimal one, in which the s factor could be
further improved to 37.5 (entry 11). The leaving groups on the
carbamate moiety were also investigated, and we observed that
replacing of the tBu group by either Et and Bn substituents led to
faster reaction rate, however, with poor kinetic resolution
performance as well (entries 12-13). Interestingly, using an iPr
group instead of tBu group provided an improved s factor of 41.8
and also reduced reaction time (entry 14). With the faster
reaction rate found for substrate 1a at ambient temperature, we
further decreased the reaction temperature to 10 C and 0 C,
and found the s factor could be further improved to 63.7 and
78.4 respectively, albeit with longer reaction time (entries 15-16).
o
o
With the optimal conditions established, we next sought to
investigate the substrate scope of this kinetic resolution reaction.
A range of different substitutions at the 3-position of allylic
alcohols were firstly evaluated (Table 2). A variety of substituted
phenyl groups (bearing different substitutions at the para-, meta-
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and ortho-positions) were well tolerated under the optimal
conditions, providing the recovered 2-alkoxycarboxamido-allylic
alkoxycarboxamido-allylic alcohol substrates (1l-1r). It’s
interesting to find that an electron-donating –OMe substitution at
different sites of the phenyl groups all gave excellent kinetic
resolution performance (1n, 1p and 1r), with s factors > 98.
Other aryl and heteroaryl tertiary alcohols were also well
tolerated with the optimal conditions (1s and 1t). Other alkyl
groups than methyl group were also evaluated. The Et,Ph-
substituted substrate (1u) still gave high selectivity factor, while
the iPr,Ph-substituted substrate (1v) gave a decreased s factor,
probably due to the small size differentiation between these two
groups. Tertiary alcohols with two alkyl groups were also
investigated under the standard conditions and a range of
Me,alkyl-substituted 2-amidoallylic alcohol substrates provided
excellent performance under the optimal kinetic resolution
conditions. The Me,tBu-substituted allylic alcohol gave the best s
factor of 164.6 (1w), affording both recovered alcohol and
cyclized product with > 95% ee. The Me,iPr- and Me,Cy-
substituted allylic alcohols were also amenable with the optimal
kinetic resolution conditions (1x and 1y). It’s surprising to find
alcohols
1
and oxazolidinone derivatives
2
with high
enantioselectivities (1a-1i), as well as a 2-naphthyl group (1j, s
factor = 133.4). The absolute configurations of the products were
determined as (R) by analogy to product 2c, whose structure
was unambiguously confirmed by X-ray crystallography13. A
linear alkyl group at the terminal alkene carbon of the allylic
alcohol substrate was also compatible with the optimal
conditions, albeit providing the products and recovered SM with
slightly diminished enantioselectivities (1k).
Table 2. Substrate scope regarding to the 3-position of 2-alkoxycarboxamido-
allylic alcohols.a
Table 3. Substrate scope regarding to the tertiary alcohol moiety of 2-
alkoxycarboxamido-allylic alcohols. a
aUnless otherwise noted, reactions were performed with 0.2 mmol rac-1, 0.01
mmol (S)-cat A6 catalyst and 60 mg 5 Å MS in 2 ml PhCl at 0 oC. The
reactions were monitored by HPLC using a chiral stationary phase. bThe yields
are isolated yields. cThe ee values were determined by HPLC analysis on a
chiral stationary phase. dThese reactions were performed at 10 oC. eThis
reaction was performed at 25 oC.
With the well compatibility of various substitutions at the 3-
position of the 2-alkoxycarboxamido-allylic alcohol substrates,
we moved our attention to the scope of tertiary alcohol moiety of
the substrates (Table 3). Replacing the phenyl group of the
tertiary alcohols in 1a with a range of substituted phenyl groups
(with various different substitutions at the para-, meta- and
ortho-positions) gave highly efficient kinetic resolution of the 2-
aThe same conditions as indicated in Table 2. bThese reactions were
performed at 10 oC. cThese reactions were performed at 25 oC.
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that primary-alkyl,Me-disubstituted allylic alcohols could also be
well-resolved by these conditions (1z and 1aa), providing a rare
access for the asymmetric synthesis of these tertiary alcohols.
To evaluate the practicability of this kinetic resolution
reaction, gram-scale reaction of rac-1a was conducted (Scheme
1, a). While performing this reaction at ambient temperature, the
catalyst loading of (R)-cat A6 could be further reduced to 1
mol %, still providing comparable
s
factor value and
enantioselectivities for (R)-1a and (S)-2a. The substrate scope
of this kinetic resolution reaction was further extended to
secondary
2-alkoxycarboxamido-allylic
alcohol
substrate
Scheme 2. Derivatizations of the chiral products.
(Scheme 1, b). With racemic secondary alcohol 3a used as
substrate, the kinetic resolution with the same conditions
proceeded smoothly to provide (S)-3a in 33% isolated yield with
85% ee without optimization, with selectivity factor of 10.
In conclusion, we have disclosed a highly efficient kinetic
resolution of tertiary 2-alkoxycarboxamido-allylic alcohols via
intramolecular transesterification reaction under the catalysis of
chiral phosphoric acid catalyst. A rang of different tertiary
alcohols, including aryl,alkyl- and alkyl,alkyl-disubstituted 2-
alkoxycarboxamido-allylic alcohols are well-compatible with the
optimal kinetic resolution conditions, providing access to both
unreacted tertiary alcohols and cyclized oxazolidinone products
with high enantioselectivities. The gram-scale reaction with
reduced catalyst loading and facile transformation of the chiral
products into useful chiral building blocks well demonstrated the
potential of this method.
However,
under
these
standard
conditions,
the
transesterification product underwent further isomerization to
give 2-oxazolone 4a as product, with the chiral center eliminated.
Acknowledgements
We gratefully acknowledge NSFC (Grant No. 21702138),
Shanghai Pujiang Program (Grant No. 17PJ1406300),
“Thousand Talents Plan” Youth program and ShanghaiTech
University start-up funding for financial support. Dr. Na Yu
(Analysis Center, School of Physical Science and Technology,
ShanghaiTech University) was acknowledged for assistance
with single-crystal crystallography. Prof. Baihua Ye was
acknowledged for manuscript proofreading and helpful
discussion.
Scheme 1. Gram-scale reaction and kinetic resolution of secondary 2-
alkoxycarboxamido-allylic alcohol.
To demonstrate the utility of this reaction, the derivatizations
of the chiral products were also studied. Base promoted
cyclization of (S)-1a readily yielded (S)-oxazolidinone 2a, thus
providing an enantiodivergent route for the asymmetric synthesis
of both enantiomers of 2a (Scheme 2, a). Hydrogenation of
oxazolidinone 2a with Raney Ni as catalyst afforded
oxazolidinone 5a in 80% yield with > 25:1 dr without erosion of
the enantioselectivity, which could be used as chiral auxiliaries
in asymmetric synthesis10. The relative configuration of
compound 5a was determined by NOE analysis13. Further
hydrolysis of 5a with aqueous NaOH solution cleanly removed
the protecting group, affording free β-amino alcohol 6a in high
yield (Scheme 2, b).
Conflict of interest
The authors declare no conflict of interest.
Keywords: kinetic resolution • tertiary alcohols • 2-
alkoxycarboxamido-allylic • chiral Brønsted acid•
transesterification
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Subramani Rajkumar, Shunlong He and
Xiaoyu Yang*
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Kinetic Resolution of Tertiary 2-
Alkoxycarboxamido-allylic Alcohols
by Chiral Phosphoric Acid Catalyzed
Intramolecular Transesterification
Text for Table of Contents
A highly enantioselective kinetic resolution of tertiary 2-alkoxycarboxamido-allylic alcohols has been achieved through chiral
phosphoric acid catalyzed intramolecular transesterification reaction. Both alkyl,aryl- and dialkyl-substituted tertiary allylic alcohols
can be resolved with excellent efficiencies, with s factor up to 164.6. This method affords expedient access to enantioenriched
oxazolidinones and β-amino alcohols.
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