Catalytic enantioselective intermolecular desymmetrization of 3-substituted oxetanes

Article abstract of DOI:10.1002/anie.201300188

Wring it out: The title reaction proceeds in the presence of chiral Bronsted acid catalysts. This efficient ring-opening process features low catalyst loading, mild reaction conditions, broad functional group compatibility, high enantioselectivity, and the capability to generate chiral quaternary centers. The highly functionalized desymmetrization products are versatile chiral building blocks in organic synthesis. Copyright

Full text of DOI:10.1002/anie.201300188

Angewandte
Chemie
Communications
Asymmetric Catalysis
Z. Wang, Z. Chen, J. Sun*
&&&& — &&&&
Catalytic Enantioselective Intermolecular
Desymmetrization of 3-Substituted
Oxetanes
Wring it out: The title reaction proceeds
in the presence of chiral Brønsted acid
catalysts. This efficient ring-opening pro-
cess features low catalyst loading, mild
reaction conditions, broad functional
group compatibility, high enantioselec-
tivity, and the capability to generate chiral
quaternary centers. The highly function-
alized desymmetrization products are
versatile chiral building blocks in organic
synthesis.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
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1
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Communications
DOI: 10.1002/anie.201300188
Asymmetric Catalysis
Catalytic Enantioselective Intermolecular Desymmetrization of
-Substituted Oxetanes**
3
Zhaobin Wang, Zhilong Chen, and Jianwei Sun*
Dedicated to Professor Gregory C. Fu on the occasion of his 50th birthday
Enantioselective desymmetrization represents a powerful
[1]
method to access chiral building blocks. For example, the
enantioselective ring opening of meso epoxides has been
[1,2]
a topic of intense investigations.
In contrast, oxetanes, the
immediate homologue of epoxides, have been much less
[
3–5]
studied in terms of enantioselective ring openings,
[
6]
[7]
although studies on their preparation and application
have been well-documented. Oxetanes substituted at the 3-
position are prochiral, and can lead to various useful three-
carbon chiral building blocks upon enantioselective ring
opening by nucleophiles. However, there have been only two
reports to date. In 1996, Tomioka and co-workers reported the
first (and only) example of intermolecular desymmetrization
of 3-substituted oxetanes by organolithium reagents, but the
reaction requires a stoichiometric amount of a chiral boron
reagent and proceeds with low enantioselectivity (< 48% ee;
[
4a]
Scheme 1a).
Recently, Loy and Jacobsen reported an
Scheme 1. Enantioselective desymmetrization of 3-substituted oxe-
III
intramolecular desymmetrization catalyzed by [(salen)Co ]
complexes with good to excellent enantioselectivity (Sche-
me 1b). However, the same catalytic system could not be
tanes. a) First and only intermolecular example (noncatalytic, low
[4a]
ee values). b) First intramolecular example (catalytic, good ee va-
[4c]
[4c]
lues). c) First intermolecular catalytic example (good ee values).
[
4c]
extended to an intermolecular reaction. Herein we describe
the first catalytic enantioselective intermolecular desymmet-
rization of 3-substituted oxetanes leading to the efficient
synthesis of useful chiral building blocks, bearing tertiary or
quaternary chiral centers, with high enantioselectivity (Sche-
me 1c).
[
11]
for subsequent nucleophilic attack. Meanwhile, in view of
the relatively weak acidity of chiral phosphoric acids, we also
anticipated a significant challenge in search for a suitable
nucleophile.
The limited success in the intermolecular enantioselective
ring opening of oxetanes is partly due to the decreased ring
strain relative to epoxides, as well as the increased difficulty in
We began to test our hypothesis with 3-phenyloxetane
(1a). Initial evaluation of different types of nucleophiles, such
as alcohols and amines, resulted in essentially no conversion,
presumably because of either low nucleophilicity or compet-
ing binding with the catalyst.
[
8]
controlling chirality while maintaining good reactivity.
Therefore, realization of the process entails the proper
choice of not only an excellent catalyst for both sufficient
activation and excellent chiral induction, but also a suitable
nucleophile. Intrigued by the superior basicity of oxetanes
After considerable effort, we were pleased to identify 2-
[
12]
mercaptobenzothiazole (2)
as a nucleophile of choice
(Table 1), and other typical thiols, such as PhSH and BnSH,
remain unreactive even at an elevated temperature. Thus, in
the presence of 2.5 mol% of the catalyst (R)-A1, the reaction
between 1a and 2 in CH Cl proceeds smoothly at room
[9]
relative to epoxides and other ethers and the recent success
[
10]
of chiral Brønsted acid catalysis, we hypothesized that the
use of a suitable chiral phosphoric acid may provide both
good oxetane activation and an excellent chiral environment
2
2
temperature to form the desired product 3a, albeit with
a disappointingly low enantioselectivity (entry 1). Further
screening of phosphoric acids having different chiral back-
bones indicated that the spinol-derived catalyst (R)-C2
exhibited superior catalytic capability regarding both product
[*] Z. Wang, Z. Chen, Prof. Dr. J. Sun
Department of Chemistry
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong SAR (China)
E-mail: sunjw@ust.hk
[
13]
yield and enantioselectivity (entries 2–7). Subsequent opti-
mization suggested that the use of Et O as a solvent provides
2
[14]
both excellent yield and enantioselectivity (entry 10).
[**] Financial support was provided by the HKUSTand Hong Kong RGC
(
GRF-604411).
With the established standard reaction conditions, we next
examined the scope of the desymmetrization reaction. As
shown in Table 2, a wide array of oxetanes having different
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300188.
2
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Angew. Chem. Int. Ed. 2013, 52, 1 – 5
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Chemie
Table 1: Optimization of reaction parameters.
Table 2: Scope study for tertiary chiral center formation.
[
a]
[b]
Entry
Catalyst
Solvent
Yield [%]
ee [%]
[a]
Entry
R
3
Yield [%]
ee [%]
[
c]
1
2
3
4
5
6
7
8
9
(R)-A1
(R)-A2
(R)-A3
(R)-B
(R)-C1
(R)-C2
(R)-C3
(R)-C2
(R)-C2
(R)-C2
CH Cl2
51
90
94
92
90
97
93
92
9
À8
À44
À55
18
55
88
75
88
92
93
2
1
2
3
4
5
6
7
Ph
4-MeC H
3-ClC H
3-BrC H
4-CNC H
3-CF C H
4
4-CF OC H
4
1a
1b
1c
1d
1e
1 f
3a
3b
3c
3d
3e
3 f
3g
91
91
90
92
96
96
97
92
92
92
92
90
92
91
2 2
CH Cl
6
4
CH Cl2
2
6
4
CH Cl2
2
6
4
CH Cl2
2
6
4
CH Cl2
2
3
6
CH Cl2
2
1g
3
6
toluene
THF
Et O
2
[
c]
8
1h
3h
91
92
1
0
92
9
2-Np
1i
1j
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
91
90
96
87
87
97
74
94
92
94
94
86
92
93
96
91
96
96
94
96
91
96
92
92
[
[
a] Determined by GC analysis with decane as an internal standard.
b] Determined by HPLC analysis using a chiral stationary phase. [c] 1a
10
11
12
13
14
15
16
17
18
19
20
OTBS
OBn
OTs
O(CH ) OBn
O(CH ) OBn
O(CH ) CHO
O(4-MeOC H )
O(3,4,5-(MeO) C H ) 1q
O(1-Np)
1k
1l
1m
1n
1o
1p
was the mass balance. THF=tetrahydrofuran.
2
2
2
4
[
b]
2
3
6
4
3
6
2
1r
1s
1t
S(4-MeOC H )
S(2-Np)
3s
3t
6
4
1
u
2
1
2
3u
3v
94
97
96
98
(R’=H)
2
1v
(
1w
(
1x
R’=2-Me)
substituents in the 3-position all participate smoothly in the
2
3
4
3w
3x
96
96
96
96
CÀS bond-formation process, thus furnishing the correspond-
R’=5-OMe)
ing alcohols 3 with high efficiency. The desymmetrization
process is not limited to the formation of tertiary chiral
centers. As shown in Table 3, chiral quaternary centers can
also be efficiently formed with good to excellent enantiose-
2
(R’=6-Br)
25
1y
3y
93
97
[15]
lectivity from the corresponding 3,3-disubstitued oxetanes.
2
2
6
7
1z
3z
85
73
92
77
tBuSO NH
1aa
3aa
2
Table 3: Scope study for quaternary chiral center formation.
[
a] Yield of purified product. [b] Run with CH Cl as the solvent.
2 2
Np=naphthyl, TBS=tert-butyldimethylsilyl, Ts=4-toluenesulfonyl.
It is also noteworthy that the method is capable of generating
all-carbon chiral quaternary centers (Table 3, entries 11 and
12), which is a well-known challenge in organic synthesis.
[15c]
1
2
[a]
Entry
R
R
4
Yield [%]
ee [%]
Finally, the mild reaction conditions tolerate a diverse set of
functional groups, including halides, ethers, silyl ethers,
aldehydes, sulfonamides, sulfonates, free alcohols, alkenes,
and alkynes.
1
2
3
4
5
6
7
8
Ph
2-BrC H4
4-H C
2-H C=CHC H
OH
OH
OH
OH
OH
OH
OH
OH
OH
4a
4b
4c
4d
4e
4 f
4g
4h
4i
91
86
80
81
86
93
96
93
93
82
94
93
97
99
97
98
89
94
87
97
97
78
71
77
6
=
CHC H4
2
6
2
6
4
2-TMSCꢀCC H
6
4
We have also evaluated the effect of substituents on the
nucleophile (Table 4). 2-Mercaptobenzothiazoles substituted
with an electron-withdrawing or electron-donating group all
participate smoothly in the efficient desymmetrization reac-
tions, thus furnishing the corresponding alcohol products 4m–
o with both excellent yield and enantioselectivity. In addition,
1,3,4-thiadiazole-2-thiol is also a suitable nucleophile (4p). It
is worth noting that the benzothiazole/thiadiazole thioether
moiety in the desymmetrization products is not only an
important structural unit found in many biologically active
allyl
CꢀCH
CꢀCPh
[
c]
9
1
1
1
CF
3
[
[
[
c]
d]
c]
0
1
2
Ph
CH OTBS
Ph
tBuSO NH
Me
Me
4j
4k
4l
2
2
[a] Yield of purified product. [b] Run with Et O as the solvent. [c] Run at
2
4
08C with 5 mol% of (R)-C2. [d] Run at 408C with CPME as the solvent.
CPME=cyclopentylmethyl ether, TMS=trimethylsilyl.
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Table 4: Other suitable nucleophiles.
[
a]
Entry
1
Ar
Product
Yield [%]
ee [%]
4m
97
99
2
4n
97
96
Figure 1. Plausible transition states for activation of oxetanes with (a)
and without (b) a hydrogen-bond donor substituent.
3
4
4o
4p
96
73
>99
come of the intermolecular ring-opening process. We believe
that the hydrogen bond between the catalyst OH and the
oxetane oxygen atom is the primary substrate–catalyst
interaction. Additional interactions vary with substrates. For
oxetanes bearing a hydrogen-bond donor substituent (e.g.,
OH) at the 3-position, we propose a nine-membered cyclic
transition state, which involves another hydrogen bond
between the catalyst phosphoryl oxygen atom and the
substituent (Figure 1a). In contrast, oxetanes lacking such
a hydrogen-bond donor substituent may adopt a transition
90
[a] Yield of purified product.
[16]
molecules, but also a versatile functional group which can
be easily converted into other useful functionalities.
The enantioenriched products obtained by our catalytic
intermolecular desymmetrization reactions can be trans-
formed into other useful compounds (Scheme 2). The benzo-
[
17]
state with the larger substituent (R ) oriented opposite to the
L
catalyst pocket to minimize steric interactions (Figure 1b). In
both cases, the nucleophile approaches the reactive center
from the back side because the front side is blocked by the
bulky anthryl group in the catalyst. These rationalizations are
consistent with the absolute stereochemistry observed for the
product.
Although attempts to obtain a substrate/catalyst cocrystal
failed, we were able to observe an obvious substrate–catalyst
interaction by NMR experiments. As shown in Figure 2, the
Scheme 2. Representative transformations of the desymmetrization
products.
thiazole thioether can be easily oxidized to the corresponding
sulfone, which is widely used as a Julia olefination reagent
[17]
(
e.g., formation of 5). Moreover, the primary alcohols can
also be oxidized to carbonyl compounds with an a-tertiary or
[
18]
quaternary chiral center (e.g., aldehyde 6 and ester 7). The
conventional synthesis of these compounds through the
catalytic enantioselective a arylation or a heterofunctionali-
zation has been a long-standing topic in organic synthesis and
[
14]
Figure 2. NMR observation of the substrate–catalyst interaction.
Values on the horizontal axes: d( H) [ppm].
1
[
19]
some of these reactions are still challenging today. Finally,
enantioenriched 1,1-disubstituted epoxides (e.g., 8) can be
obtained from the corresponding 1,2-diols after two simple
chemical steps. It is worth noting that 1,1-disubstituted
terminal olefins are, in general, challenging substrates for
asymmetric epoxidation. It is also noteworthy that in all
these transformations no erosion in product enantiopurity
was observed, thus demonstrating that our present method is
versatile and complementary to known strategies for the
synthesis of useful chiral building blocks.
addition of one equivalent of the catalyst C2 to either the
substrate 1a or 1ab results in dramatic shifts of all the proton
signals in the oxetane ring towards high field. A higher
catalyst/substrate ratio causes a larger shift, thus indicating
a reversible interaction. In addition, more complicated peak
splitting patterns of the oxetane signals were observed, thus
suggesting that the interaction imposes a chiral environment
around the prochiral oxetane ring. These observations fully
agree with our proposed transition states.
[
20]
As shown in Figure 1, we have proposed possible tran-
sition states to rationalize the absolute stereochemical out-
4
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In summary, the first catalytic asymmetric intermolecular
desymmetrization of 3-substituted oxetanes has been devel-
oped. It represents a new addition to the small family of
asymmetric reactions of oxetanes. With the proper choice of
the catalyst and nucleophile, a range of readily available
oxetanes participate smoothly in the ring-opening process,
thus furnishing a diverse set of highly functionalized chiral
building blocks with remarkable efficiency and enantioselec-
tivity. This mild process, featuring low catalyst loading and
broad functional group compatibility, is also effective for the
formation of quaternary chiral centers, including all-carbon
quaternary stereocenters. Moreover, the enantioenriched
desymmetrization products are versatile precursors to other
useful chiral building blocks, thus suggesting that our present
method provides an attractive alternative to other strategies
in asymmetric synthesis. The development of other useful
processes of oxetanes is underway.
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Published online: && &&, &&&&
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Keywords: asymmetric catalysis · nucleophilic addition ·
organocatalysis · strained molecules · sulfur
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These are not the final page numbers!