Hydroxymethylation of α-substituted nitroacetates

Article abstract of DOI:10.1016/j.tetlet.2011.09.020

Only 1 mol % of K₃PO₄ is efficient enough to catalyze the hydroxymethylation of α-substituted nitroacetates in good to excellent yield. Both aliphatic and aryl substituted nitroacetates work well under this reaction. The first cataly

Full text of DOI:10.1016/j.tetlet.2011.09.020

Tetrahedron Letters 52 (2011) 6118–6121
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Hydroxymethylation of a-substituted nitroacetates
Cong-Bin Ji a, Yun-Lin Liu a, Zhong-Yan Cao a, Yi-Yu Zhang b, Jian Zhou a,
⇑
a Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, 3663 N. Zhongshan Road, Shanghai 200062, PR China
b College of Chemistry and Material Sciences, Sichuan Normal University, Chengdu, Sichuan 610066, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2011
Revised 29 August 2011
Accepted 6 September 2011
Available online 10 September 2011
Only 1 mol % of K3PO4 is efficient enough to catalyze the hydroxymethylation of a-substituted nitroacetates
in good to excellent yield. Both aliphatic and aryl substituted nitroacetates work well under this reaction.
The first catalytic asymmetric version of this reaction also reported that 10 mol % of cupreidine could cat-
alyze this reaction up to 71% ee and 89% yield. Paraformaldehyde and formalin could both serve as the
hydroxymethylation C1 unit. The synthetic application of products is also demonstrated.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Tetrasubstituted carbon
Hydroxymethylation
a-Substituted nitroacetates
The efficient construction of tetrasubstituted carbon centers is a
a highly efficient hydroxymethylation of both a-aryl and a-alkyl
very important task in organic chemistry.1 In this context, the depro-
tonative activation of carbon nucleophiles, with three disparate car-
bon substituents, to react with different electrophiles is a fruitful
nitroacetates8 catalyzed by K3PO4 and the first example of asym-
metric version.
The reaction of
a-methyl nitroacetate 1a and para-formalde-
synthetic strategy.1,2 For example,
a
-substituted
a
-cyanoacetate,
hyde 2 was chosen to evaluate different bases, using CH2Cl2 as
the solvent at room temperature. Of all the examined organic bases
we first examined, DBU and DBACO could both afford the desired
product 3a in 80% yield (entries 1–4, Table 1). Inorganic bases were
b-keto esters,2a,b and 3-substituted oxindoles2c,d had been widely
used for the creation of tetrasubstituted carbon centers. In contrast,
the functionalization of
less studied judged by reaction type and substrate scope.3 For exam-
ple, although -aliphatic substituted nitroacetates had been used in
Mannich,3a–f Michael addition,3g–m alkylation,3n–r and aldol reac-
a-monosubstituted nitroacetates has been
9
then examined (entries 5–9), and K3PO4 was identified as a pow-
a
erful catalyst for this reaction, which could promote the reaction to
finish within 30 min, affording the product in 81% yield (entry 9).
The following evaluation of the solvent effects was carried out
using 5 mol % of the catalyst (entries 9–13), and Et2O turned out
to be the best (entry 13). Lowering the catalyst loading to only
1 mol %, the reaction could still finish in 4 h and gave product 3a
in 87% yield (entry 15). We also found that the reaction could pro-
ceed very slowly in the absence of catalyst, and product 3a was ob-
tained in 74% yield after 102 h (entry 16). This result suggested
that the use of catalyst was necessary. The employment of K3PO4
as the catalyst is remarkable, not only for its high efficiency, but
for its environmental benign nature. Because K3PO4 is a common
fertilizer containing K and P, there is no need to worry about the
environmental pollution by the remaining catalyst.
Based on the above results, the evaluation of substrate scope
was carried out by running the reaction in Et2O at room tempera-
ture, with 1 mol % of K3PO4 as the catalyst (Table 2). The ester
group of the nitroacetates was first examined. Isopropyl, methyl,
and ethyl esters worked well to afford the corresponding product
3a–c in high yield (entries 1–3), but isopropyl esters 1a seemed
to be more reactive. Bulky tert-butyl ester 1d could also afford
the corresponding product 3d in 69% yield (entry 4), and the
reaction proceeded obviously slowly. The different aliphatic
tion,3s
a-aryl substituted nitroacetates were rarely employed for
reaction development.3m
With our efforts in the preparation of compounds containing a
tetrasubstituted carbon center for biological evaluation,4 we are
interested in the hydroxymethylation5 of
a-substituted nitroace-
tates. The corresponding products are versatile synthons for the
synthesis of aziridines, amino alcohols, diamines, and quaternary
a
-substituted
been systematically studied. To prepare
compounds, Ono and Kaji found that the hydroxymethylation of
-alkyl nitroacetates and formaldehyde (37% aq) worked well in
a
-amino acids.6 Surprisingly, this reaction has not
a
-methylene carbonyl
a
the presence of 10 mol % of 10% NaOH, with five substrates
examined (60–82% yield).7a Recently, Chen et al. found that the
use of 5 mol % DBU at 70 °C could facilitate the reaction of
indolyl)methyl nitroacetates and paraformaldehyde to provide
hydroxymethylated tryptophan precursors.7b As far as we know,
the hydroxymethylation of -aryl nitroacetates was not reported,
a-(3-
a
-
a
and no asymmetric version was reported. Here, we wish to report
⇑
Corresponding author. Tel./fax: +86 21 62234560.
E-mail address: jzhou@chem.ecnu.edu.cn (J. Zhou).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.020

C.-B. Ji et al. / Tetrahedron Letters 52 (2011) 6118–6121
6119
Table 1
Table 2
Reaction condition optimization
Substrate scope for hydroxymethylation of a-substituted nitroacetates
NO2
NO2
COOR1
OH
K3PO4 (1 mol%)
Et2O, rt
O2N
Me
OH
COOR1
O2N
R
Base (X mol%)
Solvent, rt
+
(CH2O)n
+
(CH2O)n
COOiPr
R
1
Me
COOiPr
1a
3a
2 (3.0 eq)
3
(1.0 eq)
2 (3.0 eq)
Entrya
Solvent
Base
X
Time
Yield (%)b
Entrya
R
R1
3
Time (h)
Yield (%)b
87 (86)c
82
86
69
88
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
Acetone
CH3CN
Et2O
Et2O
Et2O
DBU
Et3N
DMAP
10
10
10
10
10
10
10
10
10
5
5
5
5
2
1
–
1.5 h
3.0 h
3.0 h
3.0 h
0.5 h
0.5 h
0.5 h
3.0 h
80
59
76
80
48
68
72
74
81
55
59
34
90
91
87
74
1
2
3
4
5
6
7
Me (1a)
Me (1b)
Me (1c)
Me (1d)
Et (1e)
i-Pr
Me
Et
t-Bu
i-Pr
i-Pr
i-Pr
3a
3b
3c
3d
3e
3f
4.0
36.0
9.0
23.0
8.0
DBACO
NaOH
Na2CO3
K2CO3
KOAc
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
–
n-Bu (1f)
i-Bu (1g)
4.0
7.0
95
98
3g
0.5 h
8
i-Pr
3h
20.0
94
50 min
20 min
20 min
70 min
2.5 h
(1h)
9
iPrO2C
i-Pr
i-Pr
3i
3j
10.0
2.5
78
99
(1i)
10
Bn (1j)
4.0 h
102.0 h
11
12
13
i-Pr
i-Pr
i-Pr
3k
3l
8.0
20.0
20.0
97
92
62
Et2O
Cl
Cl
(1k)
a
MeO
0.5 mmol in 5 mL of solvent under N2.
Isolated yield.
b
MeO
O2N
(1l)
substituents of the nitroacetates were then checked. It turned out
that all the -aliphatic substituted nitroacetates provided the cor-
responding product 3e–o in high to excellent yield (entries 5–15).
To our delight, -aryl substituted nitroacetates also worked well
under this reaction conditions. The nature of the aryl substituents
had no big influence on the yield, and the corresponding products
3p–t were obtained in good to high yield (entries 16–20). However,
3m
(1m)
a
14
15
i-Pr
i-Pr
3n
3o
3.0
6.0
93
96
a
Br
(1n)
(1o)
16
17
18
19
20
21
Ph (1p)
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
3p
3q
3r
3s
3t
8.0
12.0
40.0
15.0
15.0
16.0
85
a
-aryl substituted nitroacetates with ortho substituent did not
p-FC6H4 (1q)
p-MeOC6H4 (1r)
p-ClC6H4 (1s)
m-BrC6H4 (1t)
o-ClC6H4 (1u)
89
work under this reaction condition, possibly because steric hin-
drance resulting from the ortho substituent prevented the ap-
proach of base for deprotonative activation. For example,
nitroacetate 1u could not react with paraformaldehyde even in
the presence of 100 mol % of K3PO4 (entry 21), and no reaction took
place even on running the reaction at 40 °C in a screw-capped pres-
sure tube with 20 mol % of K3PO4 catalyst.
77 (78)c
85
87
–
3u
a
b
c
Reaction scale: 0.5 mmol.
Isolated yield.
Reaction run in air.
Formalin5a–m could also be used as the hydroxymethylation C1
unit (Scheme 1). Four typical substrates 1a, 1j, 1p, and 1q were
examined.
sponding product 3a and 3j in lower yield than that obtained by
using paraformaldehyde (63% vs 87%, 47% vs 99%, respectively).
However, a-aryl substituted nitroacetates 1p and 1q afforded the
corresponding products 3p and 3q in high yield, comparable to
that obtained by using paraformaldehyde.
OH
COOiPr
NO2
K3PO4 (1 mol%)
Et2O, rt
O2N
R
a-Aliphatic nitroacetates 1a and 1j gave the corre-
+
HCHO (aq.)
2 (3.0 eq)
R
COOiPr
1a: R = Me
1j: R = Bn
1p: R = Ph
3a: R = Me
3j: R = Bn
3p: R = Ph
63%
47%
90%
1q: R = p-FC6H4
3q: R = p-FC6H4 82%
It should be noted that this hydroxymethylation reaction could
be carried out in air without erosion of the yield. For example, the
hydroxymethylation of nitroacetate 1a and 1r under atmosphere
afforded almost the same reactivity and the yield of products 3a
and 3r as those reactions that ran under an atmosphere of N2 (en-
tries 1 and 18).
Scheme 1. Formalin as the hydroxymethylation C1 unit.
room temperature, the model reaction of nitroacetate 1a with para-
formaldehyde was carried out at 0 °C, in the presence of 5 mol % of
chiral catalyst with CH2Cl2 as the solvent. Since K3PO4 was an effi-
cient base for this reaction, we first tried using chiral phase transfer
catalysts in combination with K3PO4 for reaction development. To
our great disappointment, when two commercially available chiral
phase transfer catalysts 9a and 9b were used, product 3a was
obtained in racemic form (entries 1 and 2, Table 3). Because nitro-
gen based organobase could efficiently catalyze this reaction, we
next examined cinchona alkaloid derivatives,10 and some typical
results were given below. Moderate ee was obtained for product
3a when using (QD)2PYR 10 (entry 3). After intensive screening,
bifunctional catalyst 13, which Deng developed for the Michael
addition of malonates to nitroalkenes,10a was found to be the most
enantioselective, affording product 3a in 38% ee (entry 6).
Thus the synthetic application of the obtained products was
demonstrated by the following transformations (Scheme 2). For
example,
a-phenyl substituted product 3p could be readily re-
duced to the corresponding amino alcohol 4p in 76% yield by Zn/
HCl, which was further converted to oxazolidinone 5 in 89% yield.
a-Methyl substituted product 3a was reduced to amino alcohol 4a
in 57% yield by hydrogenation. Aziridine 6 could be prepared from
4a in 24% yield in two steps. Triazole 7 was easily obtained from
product 3a in three steps with 32% yield, which was further con-
verted to a novel quaternary a-substituted a-amino acid derivative
8 in 90% yield.
We next tried to develop an enantioselective version of this reac-
tion. Because the background reaction could slowly take place at
We initially examined the influence of the catalyst structure on
the enantioselectivity. Without the phenol hydroxy group,

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C.-B. Ji et al. / Tetrahedron Letters 52 (2011) 6118–6121
O
importance of the alcoholic hydroxyl group. These results were
useful for the development of new chiral bifunctional Brønsted
acid–base catalysts for further improving the ee. The quinine de-
rived catalyst 15 could give the opposite enantiomer of product
3a in 37% ee (entry 8). We next used catalyst 13 to examine the sol-
vent effects at À10 °C (entries 10–15), and toluene turned out to be
more suitable which afforded product in 44% ee (entry 15).
Improving the catalyst loading to 10 mol % and lowering the tem-
perature to À25 °C, the ee of product 3a could be improved to 60%
(entry 16), but it took 6 days for the reaction to complete.
OH
triphosgene/Et3N
89%
O2N
Ph
OH
H2N
Ph
Zn/HCl
76%
NH
O
i
i
CO2 Pr
CO2 Pr
Ph
i
CO2 Pr
3p
5
4p
Ts
1) TsCl, Et3N
2) DEAD, Ph3P
Pd/C, NH4HCO2
MeOH, rt, 57%
OH
O2N
Me
OH
H2N
Me
N
Me
CO2 Pr
i
i
24% yield for 2 steps
CO2 Pr
CO2 Pr
i
6
3a
4a
1) TsCl, Et3N, DMAP
2) NaN3, DMF
MeO2C
The substrate scope with respect to different a-substituted nit-
roacetate was then examined by running the reaction in toluene at
CO2Me
OH
CO2i Pr
O2N
Me
7
R = NO2
Me
CO2 Pr
N
N
a
3) EtOH, reflux
N
i
8 R = NH2
À25 °C in the presence of 10 mol % of 13. As shown in Table 4, five
MeO2C
CO2Me
3a
R
a-aliphatic substituted nitroacetates worked well to afford the cor-
32% yield for 3 steps
a) Zn/HOAc, THF, rt. 90%
responding products in 51–71% ee with good to high yield (entries
1–5). However, when -phenyl nitroacetate 1p was used, the
a
Scheme 2. Product elaboration.
enantioselectivity decreased to 25%. Although there is ample room
for further improvement in the enantioselectivity, our results rep-
resented the first example of this useful hydroxymethylation.
In conclusion, we have developed a highly efficient hydroxy-
cinchonine 12 afforded product 3a in 23% ee (entry 5), and quini-
dine provided product 3a in 26% ee (entry 7). Both results sug-
gested the importance of the phenol group in an enantiofacial
control. We next tried cupreidine derived catalyst 16, and only
23% ee was obtained for product 3a (entry 9), suggesting the
methylation of both
a-alkyl and aryl substituted nitroacetates
catalyzed by only 1 mol % of K3PO4. Ether paraformaldehyde or for-
malin can serve as the electrophilic reaction partner. We also
Table 3
Condition optimization for asymmetric version
NO2
OH
O2N
Me
Cat. (X mol%)
(CH2O)n
+
*
Me
COOiPr
1a
COOiPr
Solvent
2
3a
(3.0 eq)
Br
N
Ph
N
N
O
N
O
O
O
HO
R
N
N
OMe
MeO
Ph
N
N
N
N
11
9a R = Ph
9b R = 9-anthryl
10 (QD)2PYR
N
N
N
N
N
HO
BnO
HO
HO
HO
HO
HO
MeO
13
HO
16
N
N
12
14
N
15
N
N
Entrya
Cat.
X
Solvent
T (°C)
Yield (%)b
ee (%)c
1d
2d
3
4
5
6
7
8
9
10
11
12
13
14
15
16
9a K3PO4
9b K3PO4
10
11
12
13
14
15
16
13
13
13
13
13
13
13
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
10
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
ClCH2CH2Cl
Et2O
MeOBu-t
THF
Toluene
Toluene
0
0
0
0
0
0
0
0
0
À10
À10
À10
À10
À10
À10
À25
51
63
74
89
92
84
89
95
79
89
87
68
87
Trace
97
83
0
0
14
2e
23e
38
26
37e
23
40
18
28
11
–
44
60
a
b
c
On a 0.20 mmol scale.
Isolated yield.
Determined by chiral HPLC analysis.
9a or 9b (5 mol %) and K3PO4 (5 mol %).
Opposite enantiomer.
d
e

C.-B. Ji et al. / Tetrahedron Letters 52 (2011) 6118–6121
6121
Table 4
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OH
OH
NO2
N
OH
O2N
R
+
(CH2O)n
(3.0 eq)
COOPri
(1.0 eq)
COOPri
R
N
13 (10 mol%)
*
1
2
3
Toluene, -25 °C
Entrya
R
3
Time (d)
Yield (%)b
ee (%)c
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1
2
3
4
Me (1a)
Et (1e)
n-Bu (1f)
i-Bu (1g)
3a
3e
3f
6
6
6
6
83
80
77
72
60
64
71
51
3g
5
6
3h
3p
6
7
89
82
52
25
(1h)
Ph (1p)
a
b
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0.2 mmol sacle.
Isolated yield.
Determined by chiral HPLC analysis.
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The financial support from the National Natural Science Founda-
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and the Fundamental Research Funds for the Central Universities
(East China Normal University 11043) are highly appreciated.
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7. (a) Ono, N.; Miyake, H.; Fujii, M.; Kaji, A. Tetrahedron Lett. 1983, 24, 3477–3480;
(b) Sui, Y.; Liu, L.; Zhao, J.-L.; Wang, D.; Chen, Y.-J. Tetrahedron Lett. 2007, 48,
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Supplementary data
Supplementary data associated (experimental details, IR, MS,
and NMR spectra) with this Letter can be found, in the online ver-
sion, at doi:10.1016/j.tetlet.2011.09.020.
8. Both a-aryl and a-aliphatic substituted nitroacetates can be easily synthesized
according to literature methods: (a) Kornblum, N.; Blackwood, R. K.; Powers, J.
W. J. Am. Chem. Soc. 1957, 79, 2507–2509; (b) Berry, J. P.; Isbell, A. F.; Hunt, G. E.
J. Org. Chem. 1972, 37, 4396–4399; (c) Mattingly, P. G.; Miller, M. J. J. Org. Chem.
1981, 46, 1557–1564; (d) Nakamura, S.; Sugimoto, H.; Ohwada, T. J. Am. Chem.
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References and notes
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