Enantioselective organocatalytic synthesis of oxazolidine derivatives through a one-pot cascade reaction

Article abstract of DOI:10.1002/adsc.201000647

An asymmetric organocatalytic cascade reaction which can afford a series of oxazolidine derivatives has been developed. The one-pot reaction reported here can produce an oxazolidine derivative in a highly enantioselective manner and good yield with good to excellent diastereomeric ratio.

Full text of DOI:10.1002/adsc.201000647

COMMUNICATIONS
DOI: 10.1002/adsc.201000647
Enantioselective Organocatalytic Synthesis of Oxazolidine
Derivatives through a One-Pot Cascade Reaction
Zhichao Jin,a Huicai Huang,a Wenjun Li,a Xiaoyan Luo,a Xinmiao Liang,a
and Jinxing Yea,*
a
Engineering Research Center of Pharmaceutical Process Chemistry, Ministry of Education, School of Pharmacy, East
China University of Science and Technology, 130 Meilong Road, Shanghai 200237, Peopleꢀs Republic of China
Fax : (+86)-21-6425-1830; phone: (+86)-21-6425-1830; e-mail: yejx@ecust.edu.cn
Received: August 18, 2010; Published online: February 4, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000647.
Abstract: An asymmetric organocatalytic cascade
reaction which can afford a series of oxazolidine de-
rivatives has been developed. The one-pot reaction
reported here can produce an oxazolidine deriva-
tive in a highly enantioselective manner and good
yield with good to excellent diastereomeric ratio.
lyzed conjugate addition have been accomplished by
Franzꢁn[8] and Zhao,[9] respectively. These strategies
were well established and the results were good. Nev-
À
ertheless, both of the strategies resulted in a C C
bond formation between an electron-enriched aryl
ring and an activated iminium intermediate in the an-
nulation step. Herein, we report an efficient protocol
by which a series of oxazolidine derivatives can be
Keywords: asymmetric catalysis; cascade reactions;
one-pot process; oxazolidines; vinyl aldehydes
À
prepared through a C O bond formation in a highly
enantioselective manner by means of an easy one-pot
operation. Notably, this oxazolidine structure can be
found in a number of biologically active natural and
non-natural compounds and is of great potential im-
The development of asymmetric organocatalytic reac- portance in the future exploitations in drug discover-
tions has provided an efficient way in which scientists ies.[10]
are able to synthesize highly enantioenriched com-
Initially, the cascade reaction of the rationally de-
pounds.[1] With its simple operation, low toxicity and signed aryl amide 1a[11] and cinnamyl aldehyde 2a was
ready availability, this method is attractive both to carried out at room temperature by using the Jørgen-
natural product synthesis and pharmaceutical engi- sen catalyst A[12], which was derived from R-proline,
neering.[2] Of these well developed reactions, the for the first step of Michael addition and then adding
enantioselective organocatalytic cascade reaction has aqueous HBr solution to the reaction system for the
become one of the most efficient strategies from annulation step. To our surprise and great delight, the
which complex products with multiple chiral centres Michael addition of 1a and 2a proceeded much faster
and bond formations can be obtained in a highly ste- than the results reported before,[8,9] probably due to
reoselective manner and a simple one-pot operation.[3] the formation of an intramolecular hydrogen bond in
Up to now, large numbers of excellent works have the aryl amide,[13] and the sequence proved to go on
been reported with both ingenious designs and good smoothly. However, only moderate enantioseletivity
to excellent results.[4] For example, Enders and co- was observed. Assuming that the ee value was pre-
workers have reported a three-component domino re- dominately determined by the Michael addition step
action in the synthesis of enantioenriched substituted and that the annulation step would only effect the
cyclohexenes as one of the most outstanding starting total yield and the diastereomeric ratio of the final
principles in this field,[5] and a number of delicately product,[8,9] the optimization of the Michael addition
designed two-component multistep Michael–Henry was firstly investigated based on the ee value of 3a
sequences for the synthesis of similar structures have and the formation of the isolable intermediate 4a
been performed by Hayashi[6] and Zhong[7] et al. Re- (Table 1). The results showed that both the tempera-
cently, several highly efficient protocols for the syn- ture and the solvent had significant effects on the con-
thesis of a series of optically active quinolizidine de- version of the Michael addition and the enantio
rivatives by an enantioselective secondary amine-cata- purity of the final product. After screening a number
Adv. Synth. Catal. 2011, 353, 343 – 348
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
343

Zhichao Jin et al.
Table 1. Screening of the reaction conditions for the Michael addition of the aryl amide 1a to cinnamyl aldehyde 2a.[a]
COMMUNICATIONS
Entry
Catalyst
Solvent
Temperature [oC][b]
Conversion[c] [%]
ee[d] [%]
1
2
3
4
5
6
7
8
9
A
B
CH2Cl2
CH2Cl2
CH2Cl2
THF
toluene
Et2O
CH3CN
DMF
EtOAc
2-propanol
r.t.
r.t.
full (1.5 h)
full (12 h)
full (4.5 h)
21 (4.5 h)
73 (4.5 h)
46 (4.5 h)
44 (4.5 h)
<10 (4.5 h)
31 (4.5 h)
36 (4.5 h)
77
59
96
90
91
82
71
nd
55
A
A
A
A
A
A
A
A
À20
À20
À20
À20
À20
À20
À20
À20
[e]
10
–
[a]
All reactions were carried out using 1.0 equiv. of 1a (0.20 mmol), 1.5 equiv. of 2a, 0.10 equiv. of catalyst, 0.4 mL solvent
and 2.0 equiv. of HBr (40% in water).
This represents the reaction temperature of the first step of the tandem reaction.
[b]
[c]
1
The conversion of the Michael addition which was determined by H NMR on the crude reaction mixture based on 4a
and 1a after the certain time given within the parentheses.
Enantiomeric excess of the major diastereoisomer, which was determined by chiral HPLC.
No final product was isolated after 12 h since the acid aqueous HBr had been added.
[d]
[e]
Scheme 1. Process of the one-pot cascade reaction.
of different conditions, the results proved best using low conversions compared with that in CH2Cl2 (en-
catalyst A under À208C for the first step of the cas- tries 4 to 6). However, strong polar solvents presented
cade reaction in CH2Cl2 (entry 3). The reaction pro- poor results either in reaction rates or in enantioselec-
ceeded well in several non-polar solvents with good tivities (entries 8 to 10).
to excellent enantioselectivities, albeit with relatively
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Adv. Synth. Catal. 2011, 353, 343 – 348

Enantioselective Organocatalytic Synthesis of Oxazolidine Derivatives
Table 2. Screening of the acid additives for the annulation
step of the tandem reaction.[a]
Entry Acid additive
Equiv. t[b]
Conv.[c] dr[d]
[%]
1
2
3
4
5
6
7
8
37% aqueous HCl
40% aqueous HBr
TFA
PhCOOH
CH3COOH
2.0
2.0
2.0
2.0
2.0
12 h 66
12 h 79
95:5
90:10
86:14
–
12 h 81
12 h NR
12 h NR
12 h 76
12 h 61
12 h 95
12 h trace
12 h 69
12 h 82
12 h 87
–
diphenyl phosphate 2.0
91:9
90:10
93:7
nd
nd
nd
Amberlyst 15
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
TsOH·H2O
2.0
2.0
0.1
0.2
0.5
1.0
2.0
2.0
2.0
2.0
9
10
11
12
13
14
15
16
nd
1 h
2 h
4 h
8 h
83
83
86
89
93:7
93:7
93:7
93:7
Figure 1. X-ray crystal structure of 3m.
[a]
All reactions were carried out using 1.0 equiv. of 1a
(0.20 mmol), 1.5 equiv. of 2a, 0.10 equiv. of catalyst,
0.4 mL CH2Cl2 and the stated amount of the acid addi-
tives.
Time t represents the reaction time of the annulation
step.
Having established the most effective reaction con-
ditions for the Michael addition step, the annulation
procedure promoted by acid additives was subse-
quently investigated. After adding excess aqueous
HBr solution to the reaction system, we observed that
the intermediate 4a had diminished within 1 h to
afford a small amount of the isolable intermediate 5a
and the main product 3a (Scheme 1). Therefore, a va-
riety of acid additives were screened at first mainly
based on the conversion of 3a from 5a and the diaste-
reomeric ratio of the final product (Table 2). Results
[b]
[c]
The conversion of the cyclization step of the tandem re-
1
action which was determined by H NMR on the crude
reaction mixture based on 3a and 5a.
[d]
1
Determined by H NMR on the crude reaction mixture
after stirring for a certain time since the acid had been
added.
showed that the annulation could not be promoted by using a strong acid, the procedure proceeded well
acids that were relatively weak (entries 4 to 5). When with generally good to excellent diastereomeric ratio
Figure 2. Prposed model of the cascade reaction.
Adv. Synth. Catal. 2011, 353, 343 – 348
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
345

Zhichao Jin et al.
COMMUNICATIONS
Table 3. The scope of the tandem reaction of the aryl amide 1 and different vinyl aldehyde 2.[a]
Entry
1
R2
t[b]
Yield[c] [%]
Product
ee[d] [%]
dr[e]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1c
1c
1a
1a
1a
Ph
4.5 h
4.5 h
3 d[f]
9 h
90
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
96
94
98
90
90
92
95
97
97
96
85
98
92:8
93:7
95:5
92:8
85:15
85:15
96:4
93:7
96:4
4-NO2C6H4
4-FC6H4
55[g,h]
64
4-CH3C6H4
4-BrC6H4
4-ClC6H4
3-CH3C6H4
3-ClC6H4
2-ClC6H4
2-CH3OC6H4
4-CH3OC6H4
2-FC6H4
2-BrC6H4
CH3
CH3CH2
4-CH3C6H4
4-ClC6H4
3-CH3C6H4
2-ClC6H4
83
5.5 h
3.5 h
9 h
76[g,h]
53[g]
90
3 d[f]
24 h
14 h
2 d
65
70
87
82
89
69
93
60
86
81
54
53
50
72
27
9
10
11
12
13
14
15
16
17
18
19
20
21
22
93:7
92:8
93:7
3 d
14 h
11 h
3 d[f]
9.5 h
8.5 h
12 h
3 d
97
95:5
92:87
96:95
94
96
96
73:27[i]
67:33[i]
93:7
94:6
95:5
3s
3t
3u
3v
99
93:7
CH
(CH3)2CH
CH3A(CH2)6
(CH2)2
5 d[f]
5 d[f]
5 d[f]
97:95
99:99
98:98
69:31[i]
55:45[i]
54:46[i]
G
E
[a]
Unless otherwise noticed, all reactions were carried out using 1.0 equiv. of 1 (0.40 mmol), 1.5 equiv. of 2, 0.10 equiv. of
catalyst A, 0.8 mL CH2Cl2 and 2.0 equiv. of TsOH·H2O.
The time needed for the first step of Michael addition, which was determined by TLC.
[b]
[c]
[d]
[e]
[f]
Isolated yield of the product mixture 3 of both diastereoisomers.
Enantiomeric excess of the major diastereoisomer (except for those dr <4:1), which was determined by chiral HPLC.
1
Determined by H NMR on the product mixture.
Without full conversion of 1.
[g]
[h]
[i]
The reaction mixture was stirred for 3 days at room temperature after the acid additive had been added.
0.8 mmol 40% aqueous HBr in 0.7 mL CH2Cl2 was added instead of the solid TsOH·H2O.
Four different diastereoisomers were detected by chiral HPLC, and only the major two were recorded.
and moderate to good conversion (entries 1 to 3, 6 to 2.0 equivalents of TsOH·H2O after stirring for 12 h
8). To our surprise, the acid TsOH·H2O promoted the since the acid had been added.
annulation step with much higher conversion than
With the optimized reaction conditions for both the
any other acid that had been examined and afforded Michael addition and the annulation steps in hand,
excellent diastereoselectivity (entry 8). Then, a series the scope of the two-component, one-pot cascade re-
of control experiments was carried out focusing re- action was then expended using either aromatic or ali-
spectively on the amount of TsOH·H2O (entries 9 to phatic vinyl aldehydes under similar conditions
12) and the reaction time of the second step (en- (Table 3). Good to excellent enantio- and diaster-
tries 13 to 16) in order to find the best conditions for eoenriched products were obtained in moderate to
the annulation reaction. Experimental data showed good yields, with the exceptant of the low diastereo-
that the final product 3a could be afforded in 95% meric ratios observed when aliphatic vinyl aldehydes
conversion from 5a and with a 93:7 dr value using were employed (entries 14 and 15). It appeared that
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Adv. Synth. Catal. 2011, 353, 343 – 348

Enantioselective Organocatalytic Synthesis of Oxazolidine Derivatives
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the substituents on the phenyl ring of the cinnamyl al-
dehydes had little effect on the ee value but did
impact the total yields greatly. The electron-donating
group substituted cinnamyl aldehydes generally af-
forded highly yielded products while the electron-
withdrawing group often resulted in only moderate
yields. However, all the products were obtained with
high ee values, regardless of the properties of the sub-
stituents either on the vinyl aldehydes 2 or the aryl
amides 1.
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The absolute configuration of the isolated major
diastereoisomer 3m was determined by X-ray analysis
of a single crystal[14] (Figure 1), which was identical to
our predicted model (Figure 2) based on previous
work by Jørgensen,[15] Franzꢁn[8] and Rios.[16]
In summary, an easy one-pot cascade reaction
which afforded an oxazolidine derivative in a highly
enantioselective manner and moderate to good yield
with good to excellent diastereomeric ratio has been
developed. Further applications of this approach to-
wards the preparations of more complex enantioen-
riched compounds are currently being investigated.
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Experimental Section
Typical Procedure: (2R,3S,4aS)-Methyl 1-Oxo-3-
phenyl-1,2,3,4,4a-pentahydrobenzo[d]pyrido
ACHTUNGTNER[NUNG 2,1-
b]oxazole-2-carboxylate (3a)
A solution of methyl 3-(2-hydroxyphenylamino)-3-oxopro-
panoate 1a (0.4 mmol) in CH2Cl2 (0.4 mL) was stirred at
À208C for 5 min whereupon the solution of catalyst A
(0.04 mmol) and cinnamyl aldehyde (0.6 mmol) in CH2Cl2
(0.4 mL) was added. After that the reaction mixture had
been stirred at À208C until 1a disappeared (determined by
TLC), TsOH·H2O (0.8 mmol) was then added into the mix-
ture in one portion. After 10 min, the reaction mixture was
allowed to warm to room temperature and was stirred over-
night. Finally the reaction mixture was separated by column
chromatography with petroleum ether:ethyl acetate=10:1
as eluant to give the pure product 3a as white crystals;
yield: 90%. For more details, see Supporting Information.
Acknowledgements
We acknowledge National Natural Science Foundation of
China (20902018), the Fundamental Research Funds for the
Central
Universities,
Shanghai
Pujiang
Program
(08J1403300) and 111 project (B07023) for financial support.
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Adv. Synth. Catal. 2011, 353, 343 – 348