Organocatalytic oxyamination of azlactones: Kinetic resolution of oxaziridines and asymmetric synthesis of oxazolin-4-ones

Article abstract of DOI:10.1021/ja404379n

The first example of oxyamination of azlactones with oxaziridines was realized using a chiral bisguanidinium salt. Efficient catalytic asymmetric oxyamination and kinetic resolution of oxaziridines occurred simultaneously. Various chiral oxazolin-4-one derivatives with potential biological activity were obtained (up to 92% ee). Meanwhile, a series of optically pure oxaziridines were recovered with up to 99% ee and successfully used in the asymmetric oxyamination of 3-methyl-1H-indole and styrene. The triple stereodifferentiation process was also studied via control experiments.

Full text of DOI:10.1021/ja404379n

Communication
pubs.acs.org/JACS
Organocatalytic Oxyamination of Azlactones: Kinetic Resolution of
Oxaziridines and Asymmetric Synthesis of Oxazolin-4-ones
Shunxi Dong, Xiaohua Liu,* Yin Zhu, Peng He, Lili Lin, and Xiaoming Feng*
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu
610064, P. R. China
S
* Supporting Information
catalytic DKR of α-substituted azlactones by alcoholysis to
synthesize α-amino acids.8 To explore the ambiphilic nature of
ABSTRACT: The first example of oxyamination of
azlactones with oxaziridines was realized using a chiral
bisguanidinium salt. Efficient catalytic asymmetric oxy-
amination and kinetic resolution of oxaziridines occurred
simultaneously. Various chiral oxazolin-4-one derivatives
with potential biological activity were obtained (up to 92%
ee). Meanwhile, a series of optically pure oxaziridines were
recovered with up to 99% ee and successfully used in the
asymmetric oxyamination of 3-methyl-1H-indole and
styrene. The triple stereodifferentiation process was also
studied via control experiments.
this type of substrate, we envisioned that α-substituted
azlactones could be subjected to oxaziridine-mediated oxy-
amination5a−c (Scheme 1). The N-sulfonyl oxaziridine electro-
Scheme 1. Asymmetric Oxyamination of Azlactones and KR
of Oxaziridines as a Consequence of Triple
Diastereoselection
inetic resolution (KR)1 of racemic compounds is one of
Kthe most powerful tools to obtain enantiopure molecules
in both academia and industry, complementing approaches
such as asymmetric synthesis and classic resolution. With
racemates and a chiral agent (reagent, catalyst, solvent, etc.), a
substantial difference in matched and mismatched reactions
provides the underpinnings for KR. Catalytic nonenzymatic
KR1b−f and some modified versions such as parallel KR2 and
dynamic KR (DKR)3 have been actively investigated. The
major types of functional compounds that have been resolved
via KR processes mostly involve alcohols, epoxides, amines,
alkenes, carbonyl derivatives, sulfur compounds, and ferrocenes.
N-sulfonyl oxaziridines, as a class of aprotic and neutral
oxidizing reagents, are versatile reagents in organic synthesis.4,5
In particular, optically active oxaziridines have been used in a
variety of oxidation reactions.4 Nevertheless, the catalytic
enantioselective synthesis6 of N-sulfonyl oxaziridines was first
described only recently, by Jørgensen6a and Yamamoto6b in
2011. In addition, the phenomenon of KR was observed in
asymmetric reactions of racemic oxaziridines,5c,d which provides
another route to chiral oxaziridines. Ye’s group reported a
catalytic formal [3 + 2] cycloaddition of ketenes and
oxaziridines in which a special oxaziridine was recovered via
KR.5d Therefore, it was intriguing to explore useful reactions to
achieve two objectives: asymmetric catalytic reaction and KR of
oxaziridines.
phile would attach to the azlactone at first. Ring closure of the
sulfonamide onto the carbonyl group would then result in ring
opening of the azlactone9 to generate an oxazolin-4-one.
Oxazolin-4-ones are biologically useful intermediates.10
In the realization of the asymmetric version of the above
reaction, a further interesting issue arose: the rare occurrence of
triple diastereoselection.11 In this case, three chiral components
(racemic oxaziridine, racemic azlactone, and chiral catalyst)
participate in the reaction, and one new stereogenic unit is
generated in the product. Since each component can exert
some control on stereochemical events, the permutations of
matched/mismatched reactions and chiral-catalyst-induced KR
are more complex. Herein we disclose our preliminary results
on the catalytic asymmetric oxyamination of azlactones and KR
of oxaziridines. This method not only provides an effective
alternative route to a series of optically active oxaziridines (up
to 99% ee) but also affords various oxazolin-4-ones with good
results (up to 98:2 dr and 92% ee).
In our preliminary study, we chose racemic N-sulfonyl
oxaziridine ( )-1a and 0.5 equiv of phenylalanine-derived
azlactone 2a as the model substrates. Chiral guanidines12 were
chosen as the organocatalysts in view of their good perform-
ance in our previous study.13 The initial results were far from
encouraging, as the oxyamination reaction proceeded smoothly
with a 2.5 mol % loading of chiral bisguanidine 4 to give
racemic oxazolin-4-one derivative 3a in 45% yield with 64:36 dr
Azlactones contain multiple reactive sites and thus exhibit
diverse and interesting reactivity in organic synthesis.7 Racemic
azlactones can react with a variety of electrophiles as a result of
the acidity of the α-hydrogen. On the other hand, the
electrophilic nature of the carbonyl group allows for
nucleophilic attack to give the corresponding ring-opened
compounds. One representative example is the successful
Received: May 8, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja404379n | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Table 1. Optimization of the Reaction Conditions
3a
1a
b
c
c
b
c
entry
cat.
T (°C)
t (h)
yield (%)
cis:trans
ee (%)
yield (%)
ee (%)
S
1
2
3
4
−20
−20
−20
−20
−45
−78
−78
−78
−78
24
24
24
24
48
48
48
46
46
45
40
41
48
41
33
47
52
52
64:36
66:34
75:25
86:14
90:10
0/0
0/0
51
54
55
50
49
65
49
43
41
0
0
−
−
2
4·HCl
4·HBF4
4·HBArF
4·HBArF
4·HBArF
4·HBArF
4·HBArF
20/11
55/37
63/53
81
8
d
4
40
50
40
76
95
−96
5
4
4
4
4
4
5
6
7
94.5:5.5
95:5
14
32
43
45
e
7
87
ef
,
8
9
95:5
84
ef
,
ent-4·HBArF
95:5
−84
4
a
Unless otherwise noted, the reaction was carried out with catalyst 4 or 4·HX (5 × 10−3 mmol, 2.5 mol %), 1a (0.20 mmol), and 2a (0.10 mmol) in
b
c
d
e
THF (1.5 mL). Isolated yields according to the amount of 1a. Determined by chiral HPLC analysis. HBArF4 = HB[3,5-(CF3)2C6H3]4. A 1:2 (v/
f
v) THF/t-BuOMe mixed solvent (1.5 mL) and 4 Å MS (10 mg) were used. 2a (0.125 mmol) was used.
a
Table 2. Substrate Scope of Oxaziridines 1 for Asymmetric Oxyamination of Azlactone 2a
3
1
b
c
c
b
c
entry
R1
R2
C6H5
t (h)
yield (%)
cis:trans
ee (%)
yield (%)
ee (%)
S
1
2
3
4-MeC6H4
C6H5
46
46
52 (3a)
47 (3b)
54 (3c)
67 (3d)
48 (3e)
53 (3f)
51 (3g)
54 (3h)
58 (3i)
54 (3j)
43 (3k)
49 (3l)
95:5
94:6
94:6
85:15
94:6
>95:5
97:3
97:3
97:3
95:5
95:5
95:5
95:5
90:10
95:5
94:6
84
80
80
43 (1a)
39 (1b)
41 (1c)
29 (1d)
44 (1e)
46 (1f)
45 (1g)
41 (1h)
30 (1i)
39 (1j)
36 (1k)
45 (1l)
95
93
90
98
93
87
91
91
97
98
85
98
85
82
82
99
43
31
28
10
32
31
42
39
37
41
34
49
32
15
40
49
C6H5
4-ClC6H4
4-O2NC6H4
4-iPrC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
C6H5
48
de
,
f
4
5
6
7
8
9
C6H5
80
43 (99)
81
C6H5
48
2-MeC6H4
3-MeC6H4
4-MeC6H4
3-MeOC6H4
3-FC6H4
3-furyl
48
83
48
86
48
85
48
80
10
11
12
13
14
15
16
48
80
120
96
85
3-thienyl
1-naphthyl
Bn
83
d
g
96
43 (3m)
48 (3n)
49 (3o)
57 (3a)
82
33 (1m)
40 (1n)
39 (1o)
36 (1a)
120
120
48
71
Et
88
C6H5
81
a
Unless otherwise noted, the reaction was carried out with 4·HBArF (2.5 mol %), 1 (0.20 mmol), and 2a (0.125 mmol) in 1:2 (v/v) THF/t-
4
b
c
d
e
BuOMe (1.5 mL) at −78 °C. Isolated yields. Determined by chiral HPLC analysis. THF (1.5 mL) was used as the solvent without 4 Å MS. 2a
(0.14 mmol) was used. After recrystallization (21% yield). 1a (6.0 mmol) and 2a (3.75 mmol) were used.
f
g
(Table 1, entry 1). On the basis of our previous finding that the
identity of the counteranion affects the outcome in reactions
next focused on the effect of temperature, solvent, and
additives.16 Lowering the reaction temperature to −78 °C led
to a significant improvement in both the diastereo- and
enantioselectivity of the product 3a, but at the expense of the
reactivity (entries 5 and 6). It should be noted that the
unreacted azlactone was recovered as a racemate. Delightfully,
using a 1:2 (v/v) THF/t-BuOMe mixed solvent and adding 4 Å
molecular sieves (MS) resulted in a better S factor of 32 related
to the higher yield (entry 7). We next studied the influence of
promoted by chiral bisguanidinium salts,13,14 we examined a
−
series of counteranions, such as Cl−, BF4 , and BArF − [ArF =
4
3,5-bis(trifluoromethyl)phenyl].15 To our delight, a distinct
increase in diastereo- and enantioselectivity was observed. The
chiral catalyst 4·HBArF was promising (55% ee, 86:14 dr for
4
3a), and oxaziridine 1a was recovered in 50% yield with 40% ee
(entries 2−4). Attempts to improve the reaction parameters
B
dx.doi.org/10.1021/ja404379n | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Table 3. Substrate Scope of Azlactones 2 for Asymmetric Oxyamination with Oxaziridine 1a
3
1a
b
c
c
b
c
entry
R3
R4
t (h)
yield (%)
cis:trans
ee (%)
yield (%)
ee (%)
S
d
1
2-ClC6H4
3-ClC6H4
4-MeC6H4
3,5-Me2C6H3
4-O2NC6H4
C6H5
Bn
Bn
Bn
Bn
Bn
48
48
49 (3p)
55 (3q)
53 (3r)
49 (3s)
53 (3t)
36 (3u)
54 (3v)
98:2
97:3
95:5
97:3
95:5
95:5
98:2
81
82
80
88
72
92
88
41
41
40
39
38
60
45
98
93
90
92
99
55
82
43
34
28
52
31
42
40
d
2
d
3
60
d
4
60
d
5
48
6
4-HOC6H4CH2
3-indolylmethyl
96
7
C6H5
118
d
a−cSee Table 2, footnotes a−c. DMAP (0.5 mg, 2 mol %) was added.
the ratio of oxaziradine ( )-1a and azlactone 2a. Remarkably,
the use of 0.625 equiv of 2a was the best choice, giving rise to
an S factor of 43 (52% yield with 84% ee for 3a; 43% yield with
95% ee for the recovered 1a) after 46 h (entry 8).16 The use of
structure analysis.17 The absolute configuration of the
recovered oxaziridine 1a was determined to be the (R,R) by
comparison of the optical rotation with that reported in the
literature.6a The results also convincingly demonstrated that
(S,S)-1 is more favorable in the oxyamination process using
ent-4·HBArF furnished 3a and recovered 1a with comparable
4
4·HBArF as the catalyst.
results but the opposite configuration (entry 9).
4
To highlight the utility of the recovered enantiomeric pure
oxaziridine, we carried out the oxyaminations5a,b of 3-methyl-
1H-indole 5 and styrene 7 using chiral oxaziridine (+)-1a
(Scheme 2). The oxyamination of 5 proceeded smoothly in the
With the optimized reaction conditions established, the
scope of oxaziridines 1 in the KR via asymmetric oxyamination
of azlactone 2a was studied (Table 2). Comparatively, N-nosyl-
substituted oxazolin-4-one 1d gave the corresponding product
with lower stereoselectivities than other substituents (entries
1−5), although the starting substrate was recovered with a high
ee of 98% but in low yield. A broad range of N-tosyl-substituted
oxaziridines with different aryl substituents on the C atom
proceeded smoothly, affording the desired oxazolin-4-one
derivatives 3f−m with high diastereo- and enantioselectivities
(43−58% yield, 95:5−97:3 dr, and 80−86% ee), while the
unreacted oxaziridines 1f−m were recovered in 30−46% yield
with 85−98% ee (entries 6−13). Remarkably, chiral oxazir-
idines 1k and 1l were obtained for the first time with 85% and
98% ee, respectively (entries 11 and 12). Oxaziridines 1n and
1o with aliphatic substituents also underwent the asymmetric
oxyamination and KR processes with S factors of 15 and 40,
respectively, although a longer reaction time was required
(entries 14 and 15). To evaluate further the synthetic potential
of the catalytic system, the reaction was conducted on a gram
scale, and 3a and recovered 1a were obtained with similar
results (entry 16).16 Enantiomerically pure product 3a (>99%
ee) was obtained via recrystallization.16
Scheme 2. Application of Chiral Oxaziridine (+)-1a
presence of CuCl2 and NBu4Cl, affording the corresponding
product 6 in 84% yield with 2:1 dr and >99% ee. Only one
isomer of 8 was obtained in the asymmetric oxyamination of 7.
To identify the “matched/mismatched” effect of the
oxaziridine clearly, we investigated the reaction of azlactone
2a and (R,R)-oxaziridine 1a with both enantiomeric forms of
the bisguanidinium salt catalyst. The comparative experiments
gave very impressive stereodifferentiation.16 The matched
Having investigated the scope of oxaziridines, we tried to
expand the oxyamination to other azlactones. A representative
selection of azlactones with different aromatic substituents at
the C2 position were used in the reaction. The bisguanidinium
reaction of 1a with 2a catalyzed by ent-4·HBArF gave the
4
optically pure product (2R,5R)-3a (>99% ee, >99:1 dr) in
salt 4·HBArF promoted the stereoselective formation of the
4
nearly quantitative yield. In contrast, the mismatched reaction
with 4·HBArF as the catalyst afforded 3a in poor yield (36%)
oxyaminated products (72−88% ee) when 4-dimethylamino-
pyridine (DMAP) was added to increase the conversion of the
reaction, and the unreacted oxaziridine 1a was recovered in
38−41% yield with 90−99% ee (Table 3, entries 1−5).
Moreover, azlactones bearing free OH and NH groups derived
from tyrosine and tryptophan also participated successfully in
the asymmetric oxyamination reactions, giving S factors of 42
and 40, respectively (entries 6 and 7). In some cases, small
amounts of byproducts were detected.16 The stereochemistry of
product 3q was established to be (2S,5S) by X-ray crystal
4
and diastereoselectivity [(2R,5R)-3a:(2R,5S)-3a = 67:33] along
with consistently excellent ee. Moreover, the reaction catalyzed
by DMAP gave 3a in moderate yield (41.5%) with 85:15 dr.
Unfortunately, it seemed that the mismatched reaction gave
different diastereo- and enantiomeric mixtures of the product
isomers than the matched reaction. The diastereo- and
enantioselectivity observed in the products are a combination
of the KR and the diastereomer differentiation induced by the
chiral catalyst and oxaziridine. No perceptible DKR was
C
dx.doi.org/10.1021/ja404379n | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Chem. 1996, 61, 430. (e) Ruble, J. C.; Latham, H. A.; Fu, G. C. J. Am.
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observed in these cases, which precludes the major
intermediacy of an epoxide as proposed in Ye’s work.5d
To elucidate the stereodifferentiation of the catalytic system,
the structure of the bisguanidinium salt was pursued.
Disappointingly, a series of attempts to get the crystal structure
of bisguanidinium hemisalt 4·HX failed. Instead, crystals of
4·2HBF417 were obtained from solutions of either 4·2HBF4 or
4·HBF4. It occurred to us that an equilibrium among 4, 4·HX,
and 4·2HX might exist. Therefore, comparison experiments
were performed to confirm this presumption. A 1:1 mixture of
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4 and 4·2HBArF gave an outcome comparable to that for
4
4·HBArF . However, catalyst 4 resulted in a racemic product,
4
and its salt 4·2HBArF4 gave no reaction at all. This observation
provided a direct method to utilize bisguanidinium hemisalt
catalyst 4·HBArF . On the basis of the above results, we
4
considered the possibility that the chiral bisguanidinium
hemisalt serves as a bifunctional catalyst. The guanidine section
of the catalyst might activate the azlactone. On the other hand,
the guanidinium section in the catalyst could preferentially
recognize and activate the (S,S)-oxaziridine. The spatial
arrangement of 4·2HBF4 seemed to make a great deal of
difference in comparison with bisguanidine 4.13a The intra-
molecular hydrogen bonds between the amide and guanidine
moieties disappear after protonation of the guanidine. The
flexibility of the diamine backbone and the surrounding
counterion appear to create a chiral environment suitable for
the asymmetric reaction.
In conclusion, we have developed an efficient asymmetric
oxyamination of azlactones catalyzed by a chiral bisguanidinium
hemisalt that allows the generation of a variety of optically
active oxazolin-4-one derivatives with potential biological
activity. Furthermore, the reaction displays remarkable triple
stereodifferentiation and kinetic resolution. Chiral nonracemic
oxaziridines were readily recovered with good S factors.
Efficient asymmetric oxyaminations of 3-methylindole and
styrene were accomplished using the recovered chiral
oxaziridine. Further exploration of the use of this catalyst in
other reactions is underway.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and analytical data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
(15) For a review, see: (a) Krossing, I.; Raabe, I. Angew. Chem., Int.
Ed. 2004, 43, 2066. For selected examples using BArF − as the
4
AUTHOR INFORMATION
Corresponding Author
liuxh@scu.edu.cn; xmfeng@scu.edu.cn
■
counterion, see: (b) Shi, W.-J.; Zhang, Q.; Xie, J.-H.; Zhu, S.-F.; Hou,
G.-H.; Zhou, Q.-L. J. Am. Chem. Soc. 2006, 128, 2780. (c) Ding, Z.-Y.;
Chen, F.; Qin, J.; He, Y.-M.; Fan, Q.-H. Angew. Chem., Int. Ed. 2012,
51, 5706.
Notes
(16) See the Supporting Information for details.
The authors declare no competing financial interest.
(17) CCDC 931982 (cis-3q) and CCDC 936308 (4·2HBF4) contain
the supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
ACKNOWLEDGMENTS
■
We thank the National Basic Research Program of China (973
Program, 2010CB833300), the National Natural Science
Foundation of China (21021001, 21072133, and 21222206),
and the Ministry of Education (NCET-11-0345) for financial
support.
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̈
D
dx.doi.org/10.1021/ja404379n | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX