Cascade dearomatization of N-substituted tryptophols via Lewis acid-catalyzed Michael reactions

Article abstract of DOI:10.1039/c2ob26139a

Lewis acid-catalyzed cascade dearomatization of N-substituted tryptophols via Michael addition reaction was developed. The generality of the method has been demonstrated by the synthesis of versatile furoindoline derivatives with a quaternary carbon center in good yields.

Full text of DOI:10.1039/c2ob26139a

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PAPER
Cascade dearomatization of N-substituted tryptophols via Lewis
acid-catalyzed Michael reactions†
Chuan Liu, Wei Zhang, Li-Xin Dai and Shu-Li You*
Received 14th June 2012, Accepted 5th July 2012
DOI: 10.1039/c2ob26139a
Lewis acid-catalyzed cascade dearomatization of N-substituted tryptophols via Michael addition reaction
was developed. The generality of the method has been demonstrated by the synthesis of versatile
furoindoline derivatives with a quaternary carbon center in good yields.
Introduction
Furoindolines with a quaternary carbon center at the C-3 position
represents structural moieties widely existed in natural products
(Fig. 1). However, current synthetic methods for this important
type of fused indoline rings are rare. In many cases, the synthesis
still relied on Fischer indole synthesis.1 Recently, dearomatiza-
tion strategy2 has achieved significant progress for the synthesis
of pyrrolidinoindolines with a quaternary carbon center at the
C-3 position utilizing various cascade sequences. These reac-
tions generally proceed via the generation of indolenine that can
be further trapped by an intramolecular nucleophilic side chain.3
To our surprise, this straightforward protocol has been mainly
Scheme 1 Proposed cascade dearomatization of N-substituted
applied on dearomatization of tryptamines but much less
explored for constructing furoindolines4 via the corresponding
cascade dearomatization of tryptophols.3c,d,5 Lewis acid-cata-
lyzed Michael reactions have been extensively studied6 and
shown compatible with Friedel–Crafts alkylation reaction with
indoles.7
tryptophols.
As part of our ongoing efforts to explore dearomatization reac-
tions,8 we envisioned that a Lewis acid-catalyzed cascade de-
aromatization of N-substituted tryptophols including Michael
addition reaction and a subsequent iminium ion trapping would
afford the furoindoline core (Scheme 1). Herein, we report a
Lewis acid-catalyzed cascade dearomatization of N-substituted
tryptophols for the synthesis of furoindoline derivatives with a
quaternary carbon center.
Results and discussion
To test our proposed reaction sequence, N-methyl tryptophol 1a
was selected as the model substrate to react with 3-acryloyloxa-
zolidin-2-one 29 in CH2Cl2 in the presence of 10 mol% of Lewis
acid. The results are summarized in Table 1.
Fig. 1 Furoindoline core in selected natural products.
To our great delight, all the tested Lewis acids could catalyze
the cascade reaction smoothly to deliver the desired furoindoline
3a (Table 1). Sc(OTf)3 was found to be the optimal one, afford-
ing 3a in 74% yield (entry 1, Table 1). Notably, with 20 mol%
of Cu(OTf)2, the reaction could proceed smoothly to afford the
dearomatization product 3a in 73% yield (entry 3, Table 1).
State Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu,
Shanghai 200032, P.R. China. E-mail: slyou@sioc.ac.cn;
Fax: (+86) 21-54925087
†Electronic supplementary information (ESI) available: Experimental
procedures and analysis data for new compounds. See DOI: 10.1039/
c2ob26139a
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Table 1 Evaluation of Lewis acids
Entrya
Lewis acid
Time (d)
Yieldb (%)
1
Sc(OTf)3
Cu(OTf)2
Cu(OTf)2
Zn(OTf)2
Yb(OTf)3
SnCl4
AlCl3
TiCl4
ZrCl3
InBr3
3
3
3
3
3
3
3
3
3
3
7
74
33
73
12
50
55
43
39
18
22
64
2
3c
4
5
6
7
8
9
10
11d
BF3·Et2O
a Reaction conditions: 1a (0.3 mmol), 2 (0.36 mmol), Lewis acid
(0.03 mmol) in dichloromethane (3 mL) at rt. b Isolated yield. c With
20 mol% of Lewis acid. d With 100 mol% of Lewis acid.
Table 2 Optimization of the reaction conditions
Scheme 2 Substrate scope.
and the reaction went to completion within 11 h in refluxed
CH2Cl2 (entry 8, Table 2). However, when the reaction was
carried out in refluxed toluene, the yield of 3a was decreased
likely due to the harsh reaction conditions (entry 9, Table 2).
Notably, varying the catalyst loadings from 2 mol% to 20 mol%
resulted in no obvious change of the yield of 3a (entries 10–12,
Table 2). Slightly higher yield was given upon the increase of
catalyst loading (entry 12, Table 2).
Under the standard conditions, various substituted tryptophols
were examined to test the generality of this Lewis acid-catalyzed
cascade dearomatization reaction (Scheme 2). On the indole
core, substituents varying electronic properties and positions
(4-Me, 4-Br, 5-OMe, 5-Br, 5-Cl, 6-F, 6-Cl, 7-Me, 7-Br) could all
be tolerated. In all cases, the corresponding furoindolines were
obtained in good yields (64–84%, 3a–3j, Scheme 2). It should
be noted that the N-allyl tryptaphol was also a suitable substrate,
and the corresponding furoindoline was afforded in 67% yield
under the optimized reaction conditions (3k, Scheme 2).
To demonstrate the synthetic value of the present catalytic
reaction, we carried out the reaction of N-methyl tryptophol 1a
with 3-acryloyloxazolidin-2-one 2 on a gram scale with 5 mol%
of catalyst (Scheme 3). To our great delight, the desired furo-
indoline 3a could be obtained without notable loss of yield
following the same reaction conditions and work-up procedure
(69% yield).
Entrya
x (mol%)
Solvent
Time
Yieldb (%)
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
10
2
CH2Cl2
CHCl3
DCE
Et2O
THF
Toluene
Hexane
CH2Cl2
Toluene
CH2Cl2
CH2Cl2
CH2Cl2
3 d
3 d
3 d
3 d
3 d
3 d
3 d
11 h
3 d
74
73
54
55
23
68
57
76
41
61
71
76
9c
10c
11c
12c
20 h
20 h
11 h
5
20
a Reaction conditions: 1a (0.3 mmol), 2 (0.36 mmol), Sc(OTf)3
(0.03 mmol) in solvent unless otherwise noted. b Isolated yield.
c Refluxed.
With Sc(OTf)3 as the catalyst, further optimization of reaction
conditions including solvents, temperature and catalyst loadings
has been carried out. The results are summarized in Table 2. A
variety of commonly used solvents such as CHCl3, DCE, Et2O,
toluene, and hexane could be utilized in the cascade dearomati-
zation reaction of 1a (54–73% yields, entries 2–4, 6–7, Table 2).
THF gave a relatively poor yield (23% yield, entry 5, Table 2).
Increasing temperature significantly accelerated the reaction rate,
Transformations of the oxazolidinone moiety of 3a into some
useful functionalities were also carried out to explore the
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The 3-acyloyl-1,3-oxazolidin-2-one10a and trytophol deriva-
tes10b,c were prepared according to the reported procedures.
Scandium(III) trifluoromethanesulfonate was purchased from
Alfa Aesar, dried under vacuum at 80 °C and stored under nitro-
gen before use.
Scheme 3 Gram-scale preparation of 3a.
General procedures for synthesis of the N-methyl tryptophol 1
tert-Butyldimethylsilylchloride (331.5 mg, 2.2 mmol, 1.1 equiv)
was added to a solution of tryptophol (2.0 mmol, 1.0 equiv) and
imidazole (272.3 mg, 4.0 mmol, 2.0 equiv) in DMF (30 mL) at
0 °C. The ice bath was then removed and the reaction mixture
was stirred at room temperature for
was quenched with water (20 mL) and extracted with EtOAc
(3 30 mL), then the combined organic layers were
3 h. The mixture
×
washed with water, brine, separated, and dried with Na2SO4,
then filtered and concentrated under reduced pressure. The
residue was used directly for the next step without further
purification.
To a solution of the above TBS-tryptophol in THF (30 mL)
was added NaH (80.0 mg, 2.0 mmol, 1.0 equiv, 60% dispersion
in mineral oil) at 0 °C. After stirring at 0 °C for 15 min and then
at rt for 1 h, the reaction mixture was cooled to 0 °C, treated
with MeI (140 μl, 2.2 mmol, 1.1 equiv), and then allowed to stir
at rt for 6–12 h. After the reaction was complete (monitored by
TLC), 1 M HCl (5 mL) was added. After stirring for 30 min,
aqueous saturated NaHCO3 (30 mL) was added slowly. The
organic layer was separated and the aqueous layer was extracted
with EtOAc (3 × 20 mL). The combined organic layers were
washed with brine (30 mL), separated, dried with Na2SO4,
filtered and concentrated under reduced pressure. The crude
product was purified by column chromatography on silica gel
(2 : 1, PE–EtOAc) to afford the desired product 1.
Scheme 4 Transformations of product 3a.
potential application of the current methodology. As shown in
Scheme 4, conversions of oxazolidinone 3a into the correspond-
ing acid 4, ester 5 and alcohol 7 could proceed smoothly in
excellent yields (93–99%). In addition, aldehyde 6 could also be
obtained under the reduction of DIBAL-H, albeit with a moder-
ate yield (38%).
1a. Light yellow oil, 76% yield.5d 1H NMR (400 MHz,
CDCl3) δ 7.59 (d, J = 8.0 Hz, 1H), 7.28 (d, J = 8.0 Hz, 1H),
7.22 (t, J = 7.6 Hz, 1H), 7.10 (t, J = 7.6 Hz, 1H), 6.89 (s, 1H),
3.85 (t, J = 6.4 Hz, 2H), 3.71 (s, 3H), 2.99 (t, J = 6.4 Hz, 2H),
1.76 (brs, 1H); 13C NMR (100 MHz, CDCl3) δ 137.0, 127.8,
127.2, 121.6, 118.8, 118.8, 110.6, 109.2, 62.6, 32.5, 28.5.
1b. Light yellow oil, 42% yield. 1H NMR (400 MHz, CDCl3)
δ 7.06 (d, J = 3.6 Hz, 2H), 6.80 (t, J = 4.0 Hz, 1H), 6.76 (s, 1H),
3.78 (t, J = 6.4 Hz, 2H), 3.59 (s, 3H), 3.09 (t, J = 6.4 Hz, 2H),
2.65 (s, 3H), 2.25 (s, 1H); 13C NMR (100 MHz, CDCl3)
δ 137.3, 130.6, 127.3, 126.0, 121.4, 120.5, 111.0, 107.0, 63.4,
32.4, 30.0, 20.2; IR (film): vmax (cm−1) = 3354, 2943, 1609,
1574, 1548, 1497, 1465, 1446, 1417, 1371, 1323, 1248, 1154,
1070, 1042, 912, 802, 774, 744, 669; MS (EI, m/z, rel. intensity)
189 ([M]+, 17), 158 (100), 159 (13); HRMS (EI) calcd for
C12H15NO ([M]+): 189.1154. Found: 189.1157.
Conclusions
In conclusion, an unprecedented Lewis acid-catalyzed cascade
dearomatization of N-substituted tryptophols was developed. In
the presence of 10 mol% of Sc(OTf)3, versatile furoindoline
derivatives with a quaternary carbon center were obtained in
good yields. Further development of an enantioselective reaction
and application into the total synthesis of natural products are
currently under way in our lab.
Experimental
General methods
Unless stated otherwise, all reactions were carried out in flame-
dried glassware under a dry argon atmosphere. All solvents were
purified and dried according to standard methods prior to use.
1H and 13C NMR spectra were recorded on Varian instruments
(300 MHz and 75 MHz or 400 MHz and 100 MHz, respectively)
and internally referenced to tetramethylsilane signal or residual
protio solvent signals. Data for 1H NMR are recorded as follows:
chemical shift (δ, ppm), multiplicity (s = singlet, d = doublet, t =
triplet, q = quartet, dd = doublet of doublets, m = multiplet or
unresolved, coupling constant(s) in Hz, integration). Data for
13C NMR are reported in terms of chemical shift (δ, ppm).
1c. Light yellow oil, 54% yield.10d 1H NMR (400 MHz,
CDCl3) δ 7.22 (dd, J = 7.6, 0.8 Hz, 1H), 7.16 (dd, J = 8.4, 0.8
Hz, 1H), 6.98 (t, J = 8.0 Hz, 1H), 6.88 (s, 1H), 3.88 (t, J = 6.4
Hz, 2H), 3.63 (s, 3H), 3.20 (t, J = 6.8 Hz, 2H), 2.11 (s, 1H). 13
C
NMR (100 MHz, CDCl3) δ 138.2, 129.2, 125.5, 123.2, 122.1,
114.2, 111.2, 108.5, 63.5, 32.6, 29.1.
1d. Light yellow oil, 75% yield.5d 1H NMR (400 MHz,
CDCl3) δ 7.10 (d, J = 8.8 Hz, 1H), 7.01 (d, J = 2.4 Hz, 1H),
6.85 (dd, J = 8.8, 2.4 Hz, 1H), 6.79 (s, 1H), 3.81–3.78 (m, 5H),
This journal is © The Royal Society of Chemistry 2012
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3.58 (s, 3H), 2.91 (t, J = 6.8 Hz, 2H), 2.42 (s, 1H); 13C NMR
(100 MHz, CDCl3) δ 153.4, 132.2, 127.9, 127.6, 111.5, 109.9,
109.8, 100.5, 62.4, 55.7, 32.4, 28.4.
(100), 222 (39); HRMS (EI) calcd for C11H12BrNO ([M]+):
253.0102. Found: 253.0097.
1k. Light yellow oil, 71% yield.5d 1H NMR (400 MHz,
CDCl3) δ 7.52 (t, J = 6.8 Hz, 1H), 7.19 (d, J = 7.6 Hz, 1H),
7.14–7.10 (m, 1H), 7.06–7.02 (m, 1H), 6.83 (s, 1H), 5.89–5.79
(m, 1H), 5.07 (d, J = 10.4 Hz, 1H), 4.97 (d, J = 17.2 Hz, 1H),
4.51 (s, 2H), 3.75 (t, J = 6.4 Hz, 2H), 2.89 (t, J = 6.4 Hz, 2H),
2.10 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 136.3, 133.4,
127.9, 126.0, 121.5, 118.8, 117.0, 111.0, 109.5, 62.5, 48.4, 28.5.
1e. Yellow oil, 55% yield. 1H NMR (400 MHz, CDCl3)
δ 7.68 (d, J = 1.6 Hz, 1H), 7.23 (dd, J = 8.8, 2.0 Hz, 1H), 7.07
(d, J = 8.4 Hz, 1H), 6.84 (s, 1H), 3.78 (t, J = 6.4 Hz, 2H), 3.63
(s, 3H), 2.87 (t, J = 6.4 Hz, 2H), 2.16 (s, 1H); 13C NMR
(100 MHz, CDCl3) δ 135.5, 129.4, 128.3, 124.2, 121.2, 112.0,
110.6, 110.4, 62.4, 32.3, 28.0; IR (film): vmax (cm−1) = 3354,
2925, 1611, 1477, 1422, 1377, 1286, 1144, 1046, 865, 790, 746,
610; MS (EI, m/z, rel. intensity) 253 ([M]+, 25), 144 (100), 222
(95); HRMS (EI) calcd for C11H12BrNO ([M]+): 253.0102.
Found: 253.0106.
General procedure for cascade dearomatization of N-substituted
tryptophols
1
1f. Light yellow oil, 51% yield. H NMR (400 MHz, CDCl3)
δ 7.51 (t, J = 0.8 Hz, 1H), 7.10 (d, J = 1.2 Hz, 2H), 6.84 (s, 1H),
3.76 (t, J = 6.4 Hz, 2H), 3.62 (s, 3H), 2.86 (t, J = 6.4 Hz, 2H),
2.29 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 135.2, 128.7,
128.4, 124.4, 121.6, 118.1, 110.4, 110.2, 62.4, 32.5, 28.2; IR
(film): vmax (cm−1) = 3356, 2926, 1612, 1479, 1422, 1379,
1287, 1046, 864, 791, 623; MS (EI, m/z, rel. intensity) 209
([M]+), 178 (100); HRMS (EI) calcd for C11H12 ClNO ([M]+):
209.0607. Found: 209.0609.
To a flame-dried Schlenk tube under argon was added 3-acyloyl-
1,3-oxazolidin-2-one (50.8 mg, 0.36 mmol, 120 mol%), scan-
dium trifluoromethanesulfonate (14.8 mg, 0.03 mmol, 10 mol%)
and DCM (1.0 mL). The reaction mixture was stirred at rt for
30 min then 2-(1-methyl-1H-indol-3-yl)ethanol (0.30 mmol, dis-
solved in 1.0 mL DCM) was added and another 1 mL DCM was
added via syringe. The reaction mixture was then heated at
50 °C for 11 h. After the reaction was complete (monitored by
TLC), the reaction mixture was loaded directly on silica gel and
purified by column chromatography on silica gel (PE/EA = 2/5)
to afford the desired product 3.
1
1g. Light yellow oil, 55% yield. H NMR (400 MHz, CDCl3)
δ 7.42 (dd, J = 8.8, 5.2 Hz, 1H), 6.87 (dd, J = 10.0, 2.0 Hz, 1H),
6.84–6.78 (m, 2H), 3.77 (t, J = 6.4 Hz, 2H), 3.54 (s, 3H), 2.89
(t, J = 6.4 Hz, 2H), 2.51 (s, 1H); 13C NMR (100 MHz, CDCl3) δ
159.7 (d, J = 235.8 Hz), 136.8 (d, J = 12.0 Hz), 127.2 (d, J =
3.6 Hz), 124.2, 119.4 (d, J = 10.1 Hz), 110.9, 107.2 (d, J = 24.4
Hz), 95.4 (d, J = 35.9 Hz), 62.4, 32.3, 28.3; IR (film): vmax
(cm−1) = 3354, 2935, 1623, 1557,1479, 1422, 1382, 1335,
1242, 1106, 1045, 917, 828, 799; MS (EI, m/z, rel. intensity)
3a. White solid, 76% yield, M.p. 143–145 °C. 1H NMR
(300 MHz, CDCl3) δ 7.09 (t, J = 7.5 Hz, 1H), 7.02 (d, J =
7.2 Hz, 1H), 6.64 (t, J = 7.2 Hz, 1H), 6.35 (d, J = 7.8 Hz, 1H),
5.18 (s, 1H), 4.34–4.22 (m, 2H), 3.96–3.83 (m, 3H), 3.44–3.39
(m, 1H), 2.92 (s, 3H), 2.85 (t, J = 7.8 Hz, 2H), 2.41–2.31 (m,
1H), 2.17–1.98 (m, 3H); 13C NMR (75 MHz, CDCl3) δ 172.9,
153.1, 151.2, 131.0, 128.3, 123.3, 116.9, 104.8, 102.7, 66.9,
61.8, 55.7, 42.3, 40.2, 32.5, 31.0, 30.6; IR (thin film): νmax
(cm−1) = 2924, 2870, 1779, 1698, 1607, 1494, 1388, 1362,
1304, 1224, 1122, 1039, 1019, 747, 666; MS (ESI) 317
([M + H]+); HRMS (MALDI) calcd for C17H21N2O4 ([M+H]+):
317.1496. Found: 317.1504.
+
193 ([M] , 21), 162 (100); HRMS (EI) calcd for C11H12FNO
([M]+): 193.0903. Found: 193.0904.
1h. Yellow oil, 90% yield.5d 1H NMR (400 MHz, CDCl3) δ
7.36 (d, J = 8.4 Hz, 1H), 7.14 (d, J = 2.0 Hz, 1H), 6.97 (dd, J =
8.4, 2.0 Hz, 1H), 6.69 (s, 1H), 3.71 (t, J = 6.8 Hz, 2H), 3.45 (s,
3H), 2.97 (s, 1H), 2.83 (t, J = 6.8 Hz, 2H); 13C NMR
(100 MHz, CDCl3) δ 137.0, 127.5, 127.1, 126.2, 119.4, 119.0,
110.8, 108.9, 62.2, 32.1, 28.0.
3b. Viscous colorless oil, 84% yield. 1H NMR (400 MHz,
CDCl3) δ 7.00 (t, J = 7.6 Hz, 1H), 6.40 (d, J = 7.6 Hz, 1H), 6.19
(d, J = 7.6 Hz, 1H), 5.18 (s, 1H), 4.29–4.20 (m, 2H), 3.92–3.90
(m, 1H), 3.84–3.81 (m, 1H), 3.76–3.74 (m, 1H), 3.47–3.46
(m, 1H), 2.90 (s, 3H), 2.83–2.80 (m, 1H), 2.68–2.66 (m, 2H),
2.34–2.28 (m, 4H), 2.07–1.96 (m, 2H); 13C NMR (100 MHz,
CDCl3) δ 172.8, 153.0, 151.7, 134.2, 128.3, 127.2, 119.6, 103.2,
102.5, 66.5, 61.7, 56.0, 42.4, 38.8, 32.0, 30.7, 30.7, 18.1;
IR (thin film): νmax (cm−1) = 2923, 2868, 1780, 1697, 1596,
1479, 1448, 1388, 1362, 1298, 1223, 1153, 1117, 1037,
1014, 967, 772, 759; MS (ESI) 331 ([M + H]+); HRMS
(MALDI) calcd for C18H23N2O4 ([M + H]+): 331.1652. Found:
331.1649.
1
1i. Light yellow solid, 51% yield. M.p. 65–68 °C. H NMR
(400 MHz, CDCl3) δ 7.36 (d, J = 8.4 Hz, 1H), 6.92 (t, J =
7.6 Hz, 1H), 6.85 (d, J = 7.2 Hz, 1H), 6.66 (s, 1H), 3.84 (s, 3H),
3.76 (t, J = 6.4 Hz, 2H), 2.88 (t, J = 6.4 Hz, 2H), 2.65 (s, 3H),
2.33 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 135.4, 128.6,
124.0, 121.0, 118.8, 116.6, 110.0, 62.3, 36.1, 28.1, 19.3; IR
(film): vmax (cm−1) = 3346, 2927, 2875, 1605, 1584, 1494,
1460, 1407, 1319, 1252, 1212, 1135, 1105, 1045, 781, 743, 669,
580; MS (EI, m/z, rel. intensity) 189 ([M]+, 20), 158 (100), 115
(15); HRMS (EI) calcd for C12H15NO ([M]+): 189.1154. Found:
189.1151.
1j. White solid, 81% yield. M.p. 68–70 °C. 1H NMR
(400 MHz, CDCl3) δ 7.42 (dd, J = 8.0, 0.8 Hz, 1H), 7.28 (dd,
J = 7.6, 0.8 Hz, 1H), 6.83 (t, J = 7.6 Hz, 1H), 6.72 (s, 1H), 3.97
(s, 3H), 3.78–3.72 (m, 2H), 2.85 (t, J = 6.4 Hz, 2H), 2.45 (t, J =
5.2 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 133.1, 130.8,
130.0, 126.3, 119.8, 118.0, 110.6, 103.7, 62.2, 36.2, 28.1; IR
(film): vmax (cm−1) = 3346, 2945, 1610, 1566, 1486, 1455,
1409, 1361, 1305, 1216, 1199, 1128, 1072, 1046, 1013, 809,
778, 733, 609; MS (EI, m/z, rel. intensity) 253 ([M]+, 11), 144
3c. Viscous colorless oil, 83% yield. 1H NMR (400 MHz,
CDCl3) δ 6.95 (t, J = 8.0 Hz, 1H), 6.74 (d, J = 8.8 Hz, 1H), 6.28
(d, J = 8.0 Hz, 1H), 5.21 (s, 1H), 4.37–4.26 (m, 2H), 3.94–3.86
(m, 3H), 3.45–3.44 (m, 1H), 2.91–2.80 (m, 5H), 2.65–2.61 (m,
2H), 2.05–1.94 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 172.5,
153.2, 153.1, 130.1, 127.7, 121.0, 119.1, 103.8, 102.4, 66.7,
61.8, 57.5, 42.4, 38.1, 31.1, 30.6, 30.4; IR (thin film): νmax
(cm−1) = 2924, 2867, 1778, 1697, 1597, 1478, 1387, 1362,
1328, 1298, 1224, 1106, 1016, 760, 736; MS (ESI) 397
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([M + H]+); HRMS (MALDI) calcd for C17H20BrN2O4
([M + H]+): 395.0610. Found: 395.0610.
30.9, 30.4; IR (thin film): νmax (cm−1) = 2923, 2868, 1782,
1698, 1606, 1498, 1448, 1415, 1388, 1362, 1330, 1224, 1123,
1039, 1016, 964, 828, 758, 710, 607; MS (ESI) 351 ([M + H]+);
HRMS (MALDI) calcd for C17H20ClN2O4 ([M
351.1106. Found: 351.1113.
3d. Viscous colorless oil, 74% yield. 1H NMR (400 MHz,
CDCl3) δ 6.68–6.65 (m, 2H), 6.28 (d, J = 8.4 Hz, 1H), 5.16 (s,
1H), 4.37–4.27 (m, 2H), 3.94–3.92 (m, 3H), 3.74 (s, 3H),
3.47–3.43 (m, 1H), 2.88–2.77 (m, 5H), 2.36–2.31 (m, 1H),
2.16–2.00 (m, 3H); 13C NMR (100 MHz, CDCl3) δ 173.0,
153.2, 152.4, 145.8, 132.5, 112.9, 110.7, 105.5, 103.4, 67.0,
61.8, 56.0, 55.9, 42.4, 40.1, 32.3, 31.4, 31.2; IR (thin film): νmax
(cm−1) = 2926, 2872, 1780, 1698, 1596, 1498, 1448, 1388,
1362, 1325, 1280, 1226, 1156, 1120, 1035, 964, 802, 759, 697,
667; MS (ESI) 347 ([M + H]+); HRMS (MALDI) calcd for
C18H22N2O5 ([M]+): 346.1523. Found: 346.1519.
+
H]+):
3i. White solid, 67% yield, M.p. 119–122 °C. 1H NMR
(400 MHz, CDCl3) δ 6.86 (dd, J = 10.0, 7.6 Hz, 2H), 6.64 (t,
J = 7.6 Hz, 1H), 5.06 (s, 1H), 4.36–4.25 (m, 2H), 3.94–3.86 (m,
3H), 3. 50–3.44 (m, 1H), 3.18 (s, 3H), 2.88–2.75 (m, 2H), 2.39
(s, 3H), 2.37–2.34 (m, 1H), 2.15–1.96 (m, 3H); 13C NMR
(100 MHz, CDCl3) δ 173.0, 153.2, 149.6, 132.2, 131.4, 121.3,
118.6, 118.3, 105.2, 66.5, 61.8, 55.2, 42.4, 40.4, 36.0, 33.0,
31.2, 19.1; IR (thin film): νmax (cm−1) = 2924, 2866, 1779,
1698, 1600, 1567, 1478, 1417, 1388, 1362, 1330, 1224, 1110,
1017, 965, 759, 686, 619; MS (ESI) 331 ([M + H]+); HRMS
(MALDI) calcd for C18H23N2O4 ([M + H]+): 331.1652. Found:
331.1661.
3e. Viscous colorless oil, 75% yield, 1H NMR (400 MHz,
CDCl3) δ 7.17 (d, J = 10.0 Hz, 1H), 7.08 (s, 1H), 6.22 (d, J =
8.4 Hz, 1H), 5.18 (s, 1H), 4.38–4.26 (m, 2H), 3.96–3.88
(m, 3H), 3.45–3.38 (m, 1H), 2.89 (s, 3H), 2.82 (t, J = 7.6 Hz,
2H), 2.41–2.34 (m, 1H), 2.11–2.00 (m, 3H); 13C NMR
(100 MHz, CDCl3) δ 172.7, 153.1, 150.4, 133.4, 131.0, 126.2,
108.2, 106.2, 102.6, 66.8, 61.8, 55.9, 42.4, 40.3, 32.4, 30.9,
30.6; IR (thin film): νmax (cm−1) = 2925, 2872, 1780, 1698,
1600, 1494, 1388, 1362, 1325, 1272, 1224, 1108, 1039, 1016,
964, 804, 758, 667, 626, 523; MS (ESI) 395 ([M + H]+); HRMS
(MALDI) calcd for C17H20BrN2O4 ([M + H]+): 395.0601.
Found: 395.0607.
3j. White solid, 72% yield, M.p. 102–105 °C. 1H NMR
(400 MHz, CDCl3) δ 7.20 (d, J = 9.2 Hz, 1H), 6.95 (d, J =
7.2 Hz, 1H), 6.54 (t, J = 7.6 Hz, 1H), 5.11 (s, 1H), 4.40–4.30
(m, 2H), 3.96–3.91 (m, 3H), 3.50–3.44 (m, 1H), 3.31 (s, 3H),
2.89–2.74 (m, 2H), 2.37–2.29 (m, 1H), 2.12–1.98 (m, 3H); 13C
NMR (100 MHz, CDCl3) δ 172.7, 153.2, 147.7, 134.9, 133.7,
122.6, 119.2, 104.4, 100.4, 66.5, 61.9, 55.0, 42.4, 40.5, 35.0,
32.7, 31.1; IR (thin film): νmax (cm−1) = 2922, 2866, 1780,
1698, 1592, 1474, 1451, 1388, 1362, 1323, 1271, 1225, 1072,
1038, 1002, 965, 750, 700; MS (ESI) 395 ([M + H]+); HRMS
(MALDI) calcd for C17H20BrN2O4 ([M + H]+): 395.0601.
Found: 395.0613.
3f. Viscous colorless oil, 81% yield. 1H NMR (400 MHz,
CDCl3) δ 7.04 (dd, J = 8.4, 2.0 Hz, 1H), 6.96 (d, J = 2.0 Hz,
1H), 6.26 (d, J = 8.0 Hz, 1H), 5.19 (s, 1H), 4.38–4.27 (m, 2H),
3.94–3.88 (m, 3H), 3.45–3.39 (m, 1H), 2.90 (s, 3H), 2.82 (t, J =
8.0 Hz, 2H), 2.41–2.33 (m, 1H), 2.12–2.00 (m, 3H); 13C NMR
(100 MHz, CDCl3) δ 172.7, 153.2, 150.0, 132.9, 128.1, 123.5,
121.4, 105.6, 102.8, 66.9, 61.9, 55.9, 42.4, 40.3, 32.4, 30.9,
30.7; IR (thin film): νmax (cm−1) = 2924, 2872, 1779, 1698,
1604, 1495, 1387, 1362, 1325, 1271, 1223, 1108, 1086, 1039,
1016, 806, 758, 667, 525; MS (ESI) 351 ([M + H]+); HRMS
(MALDI) calcd for C17H20ClN2O4 ([M + H]+): 351.1106.
Found: 351.1114.
3k. Viscous colorless oil, 67% yield. 1H NMR (300 MHz,
CDCl3) δ 7.09–7.02 (m, 2H), 6.65 (t, J = 7.2 Hz, 1H), 6.37 (d,
J = 7.5 Hz, 1H), 5.96–5.83 (m, 1H), 5.28 (s, 1H), 5.27 (d, J =
17.1 Hz, 1H), 5.17 (d, J = 10.2 Hz, 1H), 4.33–4.26 (m, 2H),
3.96–3.84 (m, 5H), 3.50–3.41 (m, 1H), 2.95–2.71 (m, 2H),
2.42–2.32 (m, 1H), 2.19–1.98 (m, 3H); 13C NMR (75 MHz,
CDCl3) δ 172.9, 153.1, 150.4, 134.0, 130.9, 128.2, 123.3, 117.1,
116.5, 105.3, 101.3, 66.7, 61.8, 55.7, 47.2, 42.3, 40.5, 32.7,
31.2; IR (thin film): νmax (cm−1) = 3566, 2923, 2865, 1780,
1698, 1605, 1491, 1448, 1388, 1362, 1314, 1224, 1121, 1039,
1021, 955, 921, 747, 668, 615; MS (ESI) 343 ([M + H]+);
HRMS (MALDI) calcd for C19H23N2O4 ([M + H]+): 343.1652.
Found: 343.1649.
3g. White solid, 64% yield, M.p. 145–148 °C. 1H NMR
(400 MHz, CDCl3) δ 6.91 (dd, J = 8.0, 5.6 Hz, 1H), 6.30 (t, J =
10.0 Hz, 1H), 6.05 (dd, J = 10.0, 2.0 Hz, 1H), 5.21 (s, 1H),
4.38–4.28 (m, 2H), 3.97–3.86 (m, 3H), 3.46–3.40 (m, 1H), 2.90
(s, 3H), 2.87–2.82 (m, 2H), 2.39–2.32 (m, 1H), 2.10–1.98
(m, 3H); 13C NMR (100 MHz, CDCl3) δ 172.9, 164.1 (d, J =
240.3 Hz), 153.2, 152.9 (d, J = 11.8 Hz), 126.4 (d, J = 2.0 Hz),
123.9 (d, J = 10.7 Hz), 103.1, 102.6 (d, J = 22.7 Hz), 92.8 (d,
J = 26.9 Hz), 67.0, 61.8, 55.4, 42.5, 40.4, 32.7, 31.0, 30.5; 19F
NMR (376 MHz, CDCl3) δ −114.3 (m); IR (thin film): νmax
(cm−1) = 2925, 2868, 1780, 1698, 1618, 1602, 1500, 1388,
1362, 1324, 1224, 1177, 1118, 1097, 1039, 1016, 964, 824, 759,
705, 618, 540, 490; MS (ESI) 335 ([M + H]+); HRMS (MALDI)
4. To a solution of 3a (158.1 mg, 0.5 mmol) in THF–H2O
(35 mL, 4 : 1) was added H2O2 (30% solution in H2O, 0.57 mL,
5.0 mmol) at 0 °C under argon over 2 min. Then a solution of
LiOH·H2O (126.0 mg, 3.0 mmol) in 2 mL water was added. The
mixture was stirred at 0 °C for 3 h, aqueous solution (5 mL) of
Na2SO3 (756.2 mg, 6.0 mmol) was added. The reaction mixture
was then stirred for 30 min and acidified with 2 N aq. HCl
(5 mL). The mixture was extracted with DCM (3 × 10 mL). The
combined organic phases were dried with Na2SO4 and the
solvent was evaporated under vacuum. The residue was purified
by silica gel column chromatography using PE/EA = 5/2 as the
eluent affording compound 4 as a white solid (127.1 mg 98%
calcd for C17H20FN2O4 ([M
335.1405.
+
H]+): 335.1402. Found:
3h. Viscous colorless oil, 72% yield. 1H NMR (400 MHz,
CDCl3) δ 6.90 (d, J = 8.0 Hz, 1H), 6.59 (d, J = 9.2 Hz, 1H),
6.30 (s, 1H), 5.20 (s, 1H), 4.37–4.28 (m, 2H), 3.94–3.88 (m,
1H), 3.88–3.86 (m, 2H), 3.43–3.38 (m, 1H), 2.89 (s, 3H),
2.86–2.79 (m, 2H), 2.40–2.33 (m, 1H), 2.10–1.97 (m, 3H); 13C
NMR (100 MHz, CDCl3) δ 172.8, 153.2, 152.4, 134.1, 129.5,
124.2, 116.5, 105.0, 102.8, 66.9, 61.8, 55.5, 42.4, 40.3, 32.5,
1
yield), M.p. 109–112 °C. H NMR (300 MHz, CDCl3) δ 10.19
(brs, 1H), 7.03 (t, J = 7.5 Hz, 1H), 6.93 (d, J = 6.9 Hz, 1H),
6.60 (t, J = 7.5 Hz, 1H), 6.29 (d, J = 7.8 Hz, 1H), 5.05 (s, 1H),
3.87 (t, J = 7.2 Hz, 1H), 3.37–3.32 (m, 1H), 2.82 (s, 3H),
2.25–1.92 (m, 6H); 13C NMR (75 MHz, CDCl3) δ 179.0, 151.1,
This journal is © The Royal Society of Chemistry 2012
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130.9, 128.6, 122.9, 117.5, 105.1, 102.4, 67.1, 55.7, 40.2, 32.5,
30.5, 30.2; IR (thin film): νmax (cm−1) = 2926, 1708, 1608,
1494, 1450, 1428, 1389, 1301, 1225, 1157, 1125, 996, 922, 872,
744, 666, 620, 604, 539, 455; MS (ESI) 248 ([M + H]+); HRMS
(MALDI) calcd for C14H17NO3 ([M]+): 247.1208. Found:
247.1214.
(d, J = 7.8 Hz, 1H), 5.04 (s, 1H), 3.88–3.82 (m, 1H), 3.45 (t, J =
6.6 Hz, 2H), 3.37–3.31 (m, 1H), 2.82 (s, 3H), 2.06–1.88 (m,
2H), 1.80–1.64 (m, 3H), 1.51–1.343 (m, 1H), 1.32–1.24 (m,
1H); 13C NMR (75 MHz, CDCl3) δ 151.1, 132.3, 128.1, 122.8,
117.1, 104.8, 102.9, 67.0, 62.8, 56.1, 40.4, 34.3, 30.6, 28.6; IR
(thin film): νmax (cm−1) = 3405, 3051, 2938, 2871, 1607, 1494,
1449, 1428, 1388, 1357, 1300, 1222, 1156, 1125, 1058, 1020,
942, 911, 742, 675, 606, 539, 516, 456; MS (ESI) 234
([M + H]+); HRMS (MALDI) calcd for C14H19NO2 ([M]+):
233.1416. Found: 233.1424.
5. To a solution of 3a (94.8 mg, 0.3 mmol) in CH3OH
(10 mL) at 0 °C was added CH3MgBr (1 M in THF, 0.45 mL)
with a syringe. After stirring at the same temperature for 1 h, the
reaction was complete (monitored by TLC). Then the reaction
mixture was loaded directly on silica gel and purified by
silica gel column chromatography (PE/EA = 5/1) to afford the
desired product 5 as a white solid (78.9 mg, 99% yield),
M.p. 59–62 °C. 1H NMR (300 MHz, CDCl3) δ 7.03 (t, J =
7.5 Hz, 1H), 6.94 (d, J = 7.2 Hz, 1H), 6.60 (t, J = 7.5 Hz, 1H),
6.29 (d, J = 7.8 Hz, 1H), 5.03 (s, 1H), 3.87 (t, J = 7.5 Hz, 1H),
3.53 (s, 3H), 3.38–3.29 m, 1H), 2.83 (s, 3H), 2.20–1.93 m, 6H);
13C NMR (75 MHz, CDCl3) δ 173.7, 151.2, 131.1, 128.5,
122.9, 117.3, 105.0, 102.5, 67.1, 55.8, 51.5, 40.3, 32.8, 30.6,
30.3; IR (thin film): νmax (cm−1) = 3051, 2926, 2869, 1736,
1607, 1493, 1436, 1376, 1301, 1260, 1197, 1165, 1125, 1013,
923, 872, 799, 744, 666, 605, 456; MS (ESI) 262 ([M + H]+);
HRMS (MALDI) calcd for C14H17NO3 ([M]+): 261.1365.
Found: 261.1371.
Acknowledgements
We thank the National Basic Research Program of China (973
Program 2009CB825300), the National Natural Science Foun-
dation of China (21025209, 20932008, 21102160, 21121062),
and the Chinese Academy of Sciences for generous financial
support.
References
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7.5 Hz, 1H), 6.93 (d, J = 7.2 Hz, 1H), 6.62 (t, J = 7.5 Hz, 1H),
6.31 (d, J = 7.8 Hz, 1H), 5.02 (s, 1H), 3.88 (t, J = 8.1 Hz, 1H),
3.36–3.32 (m, 1H), 2.84 (s, 3H), 2.40–1.91 (m, 6H); 13C NMR
(75 MHz, CDCl3) δ 201.6, 151.2, 131.1, 128.9, 123.0, 117.5,
105.0, 102.4, 67.2, 55.6, 40.4, 39.9, 30.6, 30.1; IR (thin film):
νmax (cm−1) = 3420, 3051, 2924, 2869, 2723, 1756, 1722, 1607,
1493, 1448, 1389, 1362, 1302, 1260, 1223, 1125, 1020, 940,
913, 873, 801, 745, 661, 606, 539, 508, 456; MS (ESI) 232
([M + H]+); HRMS (MALDI) calcd for C14H17NO2 ([M]+):
231.1259. Found: 231.1267.
7. To a stirred solution of 3a (94.8 mg, 0.3 mmol) in MeOH
(10 mL) at 0 °C was added NaBH4 portionwise (34.2 mg,
0.9 mmol). The reaction mixture was allowed to stir for 1 h at
same temperature and then quenched with saturated aqueous
NH4Cl. The solvent was removed under reduced pressure and
the resulting residue was diluted with water and extracted with
EtOAc (3 × 10 mL). The combined organic layers were dried
with Na2SO4 and concentrated under reduced pressure. The
crude product was purified by silica gel column chromatography
(PE/EA = 5/3) to afford the product 7 (65.1 mg, 93%) as viscous
1
colorless oil. H NMR (300 MHz, CDCl3) δ 7.02 (t, J = 8.1 Hz,
1H), 6.93 (d, J = 7.2 Hz, 1H), 6.58 (t, J = 7.2 Hz, 1H), 6.28
7182 | Org. Biomol. Chem., 2012, 10, 7177–7183
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This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 7177–7183 | 7183