Highly efficient synthesis of amine-substituted diazoacetoacetates from aldimines and vinyldiazoacetate catalysed by MgI2 etherate

Article abstract of DOI:10.3184/174751916X14547697518372

A mild and efficient procedure is described for the synthesis of amine-substituted diazoacetoacetates by the addition of ethyl 3-(tertbutyldimethylsilanoxy)-2-diazobut-3-enoate to aldimines catalysed by MgI₂ etherate (MgI₂· (OEt

Full text of DOI:10.3184/174751916X14547697518372

160 RESEARCH PAPER
VOL. 40 MARCH, 160–163
JOURNAL OF CHEMICAL RESEARCH 2016
Highly efficient synthesis of amine-substituted diazoacetoacetates from
aldimines and vinyldiazoacetate catalysed by MgI2 etherate
Tianpeng Wu, Weipeng Qi, Senyang Mi and Xingxian Zhang*
College of Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou, Zhejiang 310032, P.R. China
A mild and efficient procedure is described for the synthesis of amine-substituted diazoacetoacetates by the addition of ethyl 3-(tert-
butyldimethylsilanoxy)-2-diazobut-3-enoate to aldimines catalysed by MgI2 etherate (MgI2·(OEt2)n) in MeCN. The reaction produces good
to excellent yield at room temperature.
Keywords: aldimine, silyl enol ether, diazoacetoacetates, MgI2 etherate
Results and discussion
The chemistry of α-diazocarbonyl compounds has received
considerable attention because of the diverse range of
synthetically useful transformations that these compounds
can undergo.1–2 Diazoacetoacetates are widely used reactants
for metal carbene transformations in the synthesis of natural
products and compounds of pharmaceutical interest.3–6
Catalytic reactions of diazoacetoacetate involving addition
or insertion that produce cyclopropanes, cycloalkanones and
b-lactams are well known because of their higher thermal
stability and unique reactivity compared to other classes of
diazo compounds.7–11 A Mukaiyama–Mannich type reaction
of methyl 3-(tert-butyldimethylsilanoxy)-2-diazobut-3-enoate
with imines catalysed by scandium(III) triflate was reported
by Doyle’s research group.12 In addition, Mukaiyama–
Mannich type reaction of a silyl enol of α-diazoacetate and an
N-(benzylidene) derivative was investigated in the presence
of Lewis acids, such as BF3·Et2O, TiCl4, Cu(OTf)2, La(OTf)3
and TBSOTf. However, only TBSOTf showed reaction
and gave a low yield of the desired product. Typically, the
nucleophilic addition product was obtained by reactions of the
diazoacetate enolate with the imine in the presence of a strong
base LiHMDS [lithium bis(trimethylsilyl)-amide] in good
yield.13 Although the anionic species which was generated
can efficiently add to carbonyl compounds, some of these
methods clearly require strongly basic reagents and anhydrous
conditions. To the best of our knowledge, there are few reports
of Mukaiyama–Mannich addition reactions of the silyl enol
ethers of diazo compounds with imines.
We have recently reported an efficient and facile method
for the synthesis of α-diazo-b- hydroxy esters by coupling
aldehydes with ethyl diazoacetate (EDA) and promoted by
MgI2 etherate (MgI2•(OEt2)n) in the presence of Et3N at
room temperature under normal atmospheric conditions.14 In
continuation of our interest on the catalytic applications of
MgI2 etherate for various organic transformations, we wish to
describe a simple and efficient protocol for the Mukaiyama–
Mannich addition of ethyl 3-(tertbutyldimethylsilanoxy)-2-
diazobut-3-enoate 2 with imines catalysed by MgI2 etherate
under mild and neutral reaction conditions in good to excellent
yields (Scheme 1).
Initially, we prepared a variety of imines from aromatic
aldehydes and aromatic amines. To optimise the reaction
(Scheme 1), we chose N-(4-nitrobenzylidene)aniline as a
model substrate and we determined the yield of product 3b
using various solvents (Table 1).. Amongst the various solvents
that were screened, an excellent yield was obtained in MeCN
as the reaction media in a short time. A good yield was also
obtained in CH2Cl2, ClCH2CH2Cl and CHCl3. Moderate yields
were produced in toluene. The reaction was very sluggish in the
coordinative polar solvents, such as Et2O and THF. The reaction
was not observed in more polar solvents, such as DMF, dioxane,
DMSO and MeOH. In order to examine the halide anion effect,
halogen analogs of MgI2 etherate, MgBr2 etherate and MgCl2
etherate, were compared under parallel reaction conditions
in MeCN (5 mol% of promoters). MgCl2 etherate was almost
inactive. MgBr2 etherate is less effective in terms of substrate
conversion and yield.
With the optimal conditions in hand, a variety of aldimines
were subjected to this reaction. There was no need to
exclude moisture and oxygen from the reaction system. The
experimental results are summarised in Table 2. As shown in
Table 2, the reaction proceeded smoothly at room temperature
and gave good to excellent yields. The Mukaiyama–Mannich
addition of the aldimines 1a–1d, derived from benzaldehyde,
p-nitrobenzaldehyde,p-anisaldehydeandp-methylbenzaldehyde
with aniline, proceeded very smoothly and provided the
Mannich adduct 3a–3d in excellent yield, (Table 2, entries
1–4). The use of p-nitroaniline, p-methoxyaniline and
p-methylaniline as the imine component was examined and the
Mannich reaction products 3e–3g was obtained in good yields
(Table 2, entries 5–7). Significantly, the reaction of aldimine,
prepared from p-nitrobenzaldehyde and p-methoxyaniline, also
gave the desired product 3h in nearly quantitative yield (Table
2, entry 8). The Mannich addition of the imine 1i derived
from cinnamaldehyde and aniline produced the adduct 3i in
90% yield without any decomposition or polymerisation under
the present reaction conditions (Table 2, entry 9). Moreover,
the reaction of heteroaromatic imine 1j and 1k derived from
2-thiophenealdehyde and furfural also proceeded in good
R'
O
R'
TBSO
O
HN
O
(Et2O)n
MgI2
N
+
OEt
R
OEt
MeCN, r.t.
Normal atmospheric
condition
R
H
N2
N2
1
3
2
Scheme 1 MgI2 etherate-promoted Mukaiyama–Mannich type addition of aldimines with vinyldiazoacetate 2.
* Correspondent. E-mail: zhangxx@ zjut.edu.cn

JOURNAL OF CHEMICAL RESEARCH 2016 161
Table 1 The effect of solvents on the addition of the aldimines of vinyldiazoacetate 2 catalysed by MgI2·(Et2O)na
TBSO
O
N
NH
O
O
(OEt2)n
MgI2
+
OEt
H
OEt
solvent, r.t.
N2
N2
O2N
O2N
2
1b
3b
Entry
1
2
Solvent
CH2Cl2
ClCH2CH2Cl
CHCl3
Time/h
3
3
Yield/%b
85
86
3
3
89
4
5
6
CH3CN
PhCH3
THF
0.5
5
5
98
77
62
7
9
10
11
12
Et2O
DMF
MeOH
Dioxane
DMSO
5
58
12
12
12
12
NRc
NR
NR
NR
aReaction conditions: A stirred solution of aldimine 1b (1.0 mmol) and vinyldiazoacetate 2 (1.2 mmol) in the various solvents was treated with 5 mol % MgI2·(Et2O)n and the reaction mixture
was stirred at room temperature for various lengths of time.
bYields after silica gel column chromatography.
cNR denotes no reaction.
Table 2 The Mukaiyama–Mannich addition to aldimines of vinyldiazoacetate 2 catalysed by MgI2•(OEt2)na
R''
R''
TBSO
O
NH
R'
O
O
(Et2O)n
MgI2
N
+
OEt
R
OEt
MeCN, r.t.
Normal atmospheric
condition
R
R'
N2
2
N2
3a–3p
1a–1p
Entry
1
2
3
4
5
6
7
8
R
C6H5
4-NO2C6H4
4-CH3OC6H4
4-CH3C6H4
C6H5
R’
H
H
H
H
H
H
H
H
H
H
H
H
H
R’’
Time/h
Product
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
NAc
NA
NA
NA
NA
Yield/%b
92
C6H5
C6H5
C6H5
C6H5
4-NO2C6H4
4-CH3OC6H4
4-CH3C6H4
4-CH3OC6H4
C6H5
3
0.5
1
3
2
2
3
1
3
3
2
12
12
8
96
97
95
93
91
90
98
90
C6H5
C6H5
4-NO2C6H4
(E)-C6H5CH=CH
2-thienyl
2-furfuryl
C6H5
C6H5
C6H5
4-NO2C6H4
2-thienyl
9
10
11
12
13
14
15
16
C6H5
C6H5
89
90
C6H5CH2
(S)-C6H5CH(CH3)
C6H5
4-CH3OC6H4
4-FC6H4
NRd
NR
NR
NR
NR
CH3
CH3
CH3
8
8
aReaction conditions: A solution of aldimine 1 (1.0 mmol) and vinyldiazoacetate 2 (1.2 mmol) in MeCN was treated with 5 mol% of MgI2·(Et2O)n and the reaction mixture was stirred at
room temperature for various lengths of time.
bYields after silica gel column chromatography.
cNA denotes not formed.
dNR denotes no reaction.
yields (Table 2, entries 10 and 11). Apparently, the aldimines
derived from aromatic aldehydes or anilines bearing electron-
donating and electron-withdrawing groups in the aromatic ring
reacted smoothly to afford the desired Mannich adducts in
good to excellent yields. Furthermore, we have observed that
the use of aromatic amine as the imine component gave, in
general, good results whereas aliphatic amine did not yield any
product even after prolonging the reaction time in the presence
of MgI2 etherate (Table 2, entries 12 and 13). In addition,
various ketoimine substrates were also unreactive toward to
vinyldiazoacetate 2 in the presence of MgI2 etherate (Table 2,
entries 14–16).
In conclusion, we have found that a facile and efficient
Mukaiyama–Mannich addition to aldimines of TBSO-
vinyldiazoacetate derivatives occurs in the presence of a MgI2
etherate system. This catalytic reaction permits the Mukaiyama–

162 JOURNAL OF CHEMICAL RESEARCH 2016
(dd, 2H, J = 3.0, 12.2 Hz). dC 14.32, 29.69, 46.82, 54.85, 61.94, 112.12,
126.09, 125.14, 127.87, 129.02, 138.41, 140.81, 152.06, 161.50, 190.29.
m/z (ESI): 383.14 [M + H]+; HRMS (ESI) calcd. for C19H19N4O5:
383.1355; found for [M + H]+: 383.1336.
Ethyl 2-diazo-5-phenyl-3-oxo-5-((4-methoxyphenyl)amino)pentanoate
(3f)16: white solid. m.p. 103–104 ºC. Rf = 0.19 (PE:EA = 7:1). dH 1.37 (t, 3H,
J = 7.1 Hz), 3.28 (dd, 1H, J = 9.3, 14.2 Hz), 3.39 (dd, 1H, J = 4.5, 14.3 Hz),
3.79 (s, 1H), 4.35 (q, 2H, J = 7.1 Hz), 4.72 (s, 1H), 4.88 (q, 1H, J = 4.5 Hz),
6.55 (d, 2H, J = 7.9 Hz), 6.66 (t, 1H, J = 7.3 Hz), 6.87 (d, 2H, J = 8.7 Hz),
7.10 (t, 2H, J = 7.8 Hz), 7.38 (d, 2H, J = 8.6 Hz).
Mannich reaction to be carried out under essentially neutral
conditions at room temperature in good to excellent yields. We
are examining other carbon–carbon bond forming reactions
based on TBSO-vinyldiazoacetate derivatives catalysed by
MgI2 etherate. Enantioselective variants of this reaction will be
our next target in this area.
Experimental
The purification of the products by flash column chromatography,
utilised silica gel (200 ~ 300 mesh) and light petroleum ether (PE, b.p.
o
1
Ethyl 2-diazo-5-phenyl-3-oxo-5-(p-tolylamino)pentanoate (3g):
yellowish solid. m.p. 101–102 ºC. Rf = 0.33 (PE:EA = 7:1). IR (film)
ν (cm–1) 3364 (N–H), 2997, 2140 (C=N=N), 1716 (C=O), 1645 (C=O),
1522, 1454, 1317, 1280, 702. dH 1.35 (t, 3H, J = 7.2 Hz), 2.19 (s, 3H),
3.26 (dd, 1H, J = 9.6, 14.2 Hz), 3.38–3.41 (m, 1H), 4.33 (q, 2H, J = 7.2
Hz), 4.88 (dd, 1H, J = 4.4, 9.6 Hz), 6.45 (d, 2H, J = 8.5 Hz), 6.90 (d, 2H,
J = 8.3 Hz), 7.23 (t, 1H, J = 7.4 Hz), 7.33 (t, 2H, J = 7.5 Hz), 7.45 (d, 2H,
J = 7.4 Hz). dC 14.35, 20.32, 47.57, 55.33, 61.61, 113.75, 126.31, 126.73,
127.17, 128.66, 129.56, 142.81, 144.57, 161.44, 190.83. m/z (ESI):
352.16 [M + H]+; HRMS (ESI) calcd. for C20H22N3O3: 352.1656; found
for [M + H]+: 352.1645.
Ethyl 2-diazo-5-(4-nitrophenyl)-3-oxo-5-((4-methoxyphenyl)amino)
pentanoate (3h): yellowish solid. m.p. 126–127 ºC. Rf = 0.26 (PE:EA =
10:1). IR (film) ν (cm–1) 3400 (N–H), 2952, 2907, 2145 (C=N=N), 1713
(C=O), 1655 (C=O), 1515, 1343, 1242, 1032, 743. dH 1.37 (t, 3H, J =
7.2 Hz), 3.28 (dd, 1H, J = 9.4, 14.5 Hz), 3.41 (dd, 1H, J = 4.2, 14.6 Hz),
3.69 (s, 1H), 4.35 (q, 2H, J = 7.2 Hz), 4.56 (s, 1H), 4.92 (dd, 2H, J = 4.1,
9.3 Hz), 6.44 (d, 2H, J = 8.9 Hz), 6.68 (d, 2H, J = 8.7 Hz), 7.64 (d, 2H,
J = 8.7 Hz), 8.18 (d, 2H, J = 8.7 Hz). dC 14.31, 47.02, 55.44, 55.66, 61.81,
77.29, 114.83, 114.98, 123.98, 127.39, 140.26, 147.24, 150.71, 152.57,
161.35, 190.01 ppm. m/z (ESI): 413.15 [M + H]+; HRMS (ESI) calcd. for
C20H21N4O6: 413.1456; found for [M + H]+: 413.1464.
(E)-ethyl 2-diazo-7-phenyl-3-oxo-5-(phenylamino)hept-6-enoate (3i):
yellowish oil. Rf = 0.29 (PE:EA = 7:1). IR (film) ν (cm–1) 3393 (N–H),
3025, 2137 (C=N=N), 1713 (C=O), 1650 (C=O), 1602, 1504, 1374, 1312,
1179, 967. dH 1.37 (t, 3H, J = 7.2 Hz), 3.28 (dd, 1H, J = 5.4, 15 Hz), 3.39 (dd,
1H, J = 7.5, 15.0 Hz), 4.35 (q, 2H, J = 7.1 Hz), 4.62–4.65 (m, 1H), 6.30 (dd,
1H, J = 6.0, 16.0 Hz), 6.67–6.77 (m, 4H), 7.20–7.23 (m, 2H), 7.27 (d, 1H,
J = 5.8 Hz), 7.32–7.35 (m, 2H), 7.38 (d, 2H, J = 7.2 Hz). dC 14.21, 44.93,
52.65, 61.48, 77.28, 113.64, 117.61, 126.35, 127.40, 128.37, 129.08, 130.28,
130.50, 136.59, 146.86, 161.31, 190.63. m/z (ESI): 364.19 [M + H]+; HRMS
(ESI) calcd. for C21H22N3O3: 364.1656; found for [M + H]+: 364.1638.
Ethyl 2-diazo-5-(thiophen-2-yl)-3-oxo-5-(phenylamino)pentanoate
(3j): yellowish solid. m.p. 80–82 ºC. Rf = 0.33 (PE:EA = 5:1). IR (film)
ν (cm–1) 3373 (N–H), 2985, 2134 (C=N=N), 1713 (C=O), 1653 (C=O),
1602, 1507, 1377, 1222, 747. dH 1.35 (t, 3H, J = 7.1 Hz), 3.48 (dd, 1H,
J = 5.2, 15.3 Hz), 3.56 (dd, 1H, J = 8.4, 15.3 Hz), 4.33 (q, 2H, J = 7.1
Hz), 5.25 (dd, 1H, J = 5.2, 8.0 Hz), 6.66 (d, 2H, J = 7.8 Hz), 6.74 (t,
1H, J = 7.4 Hz), 6.94 (dd, 1H, J = 3.6, 5.0 Hz), 7.04 (d, 1H, J = 3.5 Hz),
7.13 (t, 1H, J = 1.7 Hz), 7.15–7.18 (m, 3H). dC 14.31, 47.19, 51.01, 61.63,
77.28, 113.83, 118.17, 123.83, 124.08, 126.81, 129.13, 146.54, 147.48,
161.34, 190.17. m/z (ESI): 344.11 [M + H]+; HRMS (ESI) calcd. for
C17H18N3O3S: 344.1063; found for [M + H]+: 344.1069.
60 ~ 90 C) were used. H NMR spectra were obtained on a Bruker
AM-500 spectrometer with TMS as the internal standard and CDCl3
as the solvent. The reactions were monitored by TLC on silica-gel
polygram SILG/UV 254 plates. Melting points were measured on
a BUCHI B-540 and uncorrected. FTIR were recorded on a Bruker
Tensor 27 spectrometer. HRMS were determined on a Waters GCT
Premier spectrometer. All compounds were identified by 1H NMR and
the data are in good agreement with those reported.
The typical procedure for the synthesis of ethyl 2-diazo-5-
phenyl-3-oxo-5-(phenylamino)pentanoate:
A
stirred solution
of N-benzylideneaniline 1a (1.0 mmol) in MeCN (5 mL) was
15
treated with freshly prepared MgI2 etherate (0.05 mmol) at room
temperature, followed by the addition of the vinyldiazoacetate 2
(1.2 mmol). The resulting homogeneous reaction mixture was stirred
at room temperature for 3 h and quenched by saturated NaHCO3
aqueous solution. The product was recovered with ethyl acetate.
Chromatographic purification of the crude product on silica gel gave
ethyl 2-diazo-5-phenyl-3-oxo-5-(phenylamino) pentanoate 3a in 92%
yield.
Spectroscopic data for products 3a–3k (Table 2, entries 1–11)
Ethyl 2-diazo-5-phenyl-3-oxo-5-(phenylamino)pentanoate (3a)16:
white solid. m.p. 108–109 ºC. Rf = 0.11 (PE:EA = 5:1). dH 1.53 (t, 3H,
J = 7.1 Hz), 3.28 (dd, 1H, J = 4.6, 14.4 Hz), 3.65 (dd, 1H, J = 9.6, 14.4
Hz), 4.49 (dd, 1H, J = 4.6, 9.4 Hz), 4.59 (q, 2H, J = 7.1 Hz), 5.03 (s, 1H),
6.33 (d, 2H, J = 8.0 Hz), 6.61 (t, 1H, J = 7.3 Hz), 6.90 (d, 2H, J = 8.3
Hz), 7.23 (t, 1H, J = 7.5 Hz), 7.33 (t, 2H, J = 7.6 Hz), 7.45 (d, 2H, J = 7.5
Hz).
Ethyl 2-diazo-5-(4-nitrophenyl)-3-oxo-5-(phenylamino)pentanoate
(3b): yellowish oil. Rf = 0.21 (PE:EA = 5:1). IR (film) ν (cm–1) 3397
(N–H), 3054, 2141 (C=N=N), 1713 (C=O), 1650 (C=O), 1603, 1519,
1313, 1016, 856, 751, 695. dH 1.37 (t, 3H, J = 7.2 Hz), 3.26 (dd, 1H,
J = 9.4, 14.45 Hz), 3.67 (dd, 1H, J = 4.2, 14.5 Hz), 4.35 (q, 2H, J = 7.1
Hz), 4.94(s, 1H), 4.99 (dd, 1H, J = 4.2, 9.3 Hz), 6.49 (d, 2H, J = 7.8
Hz), 6.69 (t, 1H, J = 7.3 Hz), 7.10 (dd, 2H, J = 7.6, 8.3 Hz), 7.66 (d,
2H, J = 8.7 Hz), 8.18 (dd, 2H, J = 1.7, 7.1 Hz). dC 14.21, 46.92, 54.46,
61.75, 113.49, 118.07, 123.89, 127.26, 129.10, 146.09, 147.13, 150.42,
161.27, 189.79. m/z (ESI): 383.14 [M + H]+; HRMS (ESI) calcd. for
C19H19N4O5: 383.1355; found for [M + H]+: 383.1365.
Ethyl 2-diazo-5-(4-methoxyphenyl)-3-oxo-5-(phenylamino)pentanoate
(3c)16: white solid. m.p. 103–104 ºC. Rf = 0.26 (PE:EA = 10:1). dH 1.37 (t,
3H, J = 7.1 Hz), 3.28 (dd, 1H, J = 9.6, 14.2 Hz), 3.39 (dd, 1H, J = 4.5, 14.3
Hz), 3.79 (s, 1H), 4.35 (q, 2H, J = 7.1 Hz), 4.72 (s, 1H), 4.88 (q, 1H, J = 4.5
Hz), 6.55 (d, 2H, J = 7.9 Hz), 6.66 (t, 1H, J = 7.3 Hz), 6.87 (d, 2H, J = 8.7
Hz), 7.10 (t, 2H, J = 8.0 Hz), 7.38 (d, 2H, J = 8.6 Hz).
Ethyl 2-diazo-5-(p-tolyl)-3-oxo-5-(phenylamino)pentanoate (3d)16:
white solid. m.p. 97–99 ºC. Rf = 0.4 (PE:EA = 10:1). dH 1.40 (t, J = 7.1
Hz, 3H), 2.38 (s, 1H), 3.34 (dd, J = 9.3, 14.3 Hz, 1H), 3.44 (dd, J = 4.6,
14.3 Hz, 1H), 4.37 (q, J = 7.2 Hz, 2H), 4.8 (s, 1H), 4.95 (q, J = 4.5, 1H),
6.59 (d, J = 7.9 Hz, 2H), 6.7 (t, J = 7.3 Hz, 1H), 7.14 (d, J = 7.6Hz, 2H),
7.17 (q, J = 7.9Hz, 2H), 7.27 (s, 2H), 7.40 (d, J = 8.0 Hz, 2H).
Ethyl 2-diazo-5-phenyl-3-oxo-5-((4-nitrophenyl)amino)pentanoate
(3e): yellowish solid. m.p. 139–140 ºC. Rf = 0.28 (PE:EA = 3:1).
IR (film) ν (cm–1) 3973 (N–H), 2138 (C=N=N), 1714 (C=O), 1650
(C=O), 1596, 1305, 1112, 706. dH 1.35 (t, 3H, J = 7.2 Hz), 3.25 (dd, 1H,
J = 4.6, 14.6 Hz), 3.42 (dd, 1H, J = 9.0, 14.5 Hz), 4.35 (q, 2H, J = 7.1
Hz), 4.95–4.99 (m, 1H), 5.76 (d, 1H, J = 6.2 Hz), 6.46–6.49 (m, 2H),
7.27–7.28 (m, 1H), 7.34–7.37 (m, 2H), 7.40 (d, 2H, J = 7.3 Hz), 7.99
Ethyl
2-diazo-5-(furan-2-yl)-3-oxo-5-(phenylamino)pentanoate
(3k): yellowish oil. Rf = 0.27 (PE:EA = 7:1). IR (film) ν (cm–1) 3366
(N–H), 3120, 2131 (N=N), 1720 (C=O), 1656 (C=O), 1507, 1379,
1305, 1024, 740. dH 1.35 (m, 3H), 3.41 (dd, 1H, J = 5.5, 15.7 Hz), 3.56
(dd, 1H, J = 7.8, 15.6 Hz), 4.33 (q, 2H, J = 7.1 Hz), 5.14 (dd, 1H, J = 5.6,
7.6 Hz), 6.22 (d, 1H, J = 3.2 Hz), 6.28 (dd, 1H, J = 1.8, 3.2 Hz), 6.67
(m, 2H), 6.73 (t, 1H, J = 7.4 Hz), 7.14–7.34 (m, 2H), 7.34 (s, 1H). dC
14.32, 43.83, 48.91. 61.61, 106.22, 110.23, 113.87, 118.23, 129.18, 141.71,
146.60, 154.88, 161.38, 190.27. m/z (ESI): 328.13 [M + H]+; HRMS
(ESI) calcd. for C17H18N3O4: 328.1292; found for [M + H]+: 328.1283.
We are grateful to the National Natural Science Foundation of
China (No. 21372203 and 21272076).

JOURNAL OF CHEMICAL RESEARCH 2016 163
7
M.P. Doyle, R. Duffy, M.O. Ratnikov and L. Zhou, Chem. Rev., 2010, 110,
704.
Received 29 November 2015; accepted 17 January 2016
Paper 1503744 doi: 10.3184/174751916X14547697518372
Published online: 23 February 2016
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