The synthesis and strecker reaction of 2-chlorotetrafluoroethanesulfinyl ketimines

Article abstract of DOI:10.1021/jo500008e

A series of 2-chlorotetrafluoroethanesulfinyl ketimines were prepared from 2-chlorotetrafluoroethanesulfinamide and ketones in high yields, and their Strecker reactions with TMSCN have been investigated. High yields and excellent diastereoselectivities were achieved in the presence of a catalytic amount of CsF under mild conditions. The six-membered chairlike models were proposed to account for the high stereoselective Strecker reaction.

Full text of DOI:10.1021/jo500008e

Note
pubs.acs.org/joc
The Synthesis and Strecker Reaction of
2‑Chlorotetrafluoroethanesulfinyl Ketimines
Kai Yang, Li-Juan Liu, and Jin-Tao Liu*
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling
Road, Shanghai 200032, P.R. China
S
* Supporting Information
ABSTRACT: A series of 2-chlorotetrafluoroethanesulfinyl
ketimines were prepared from 2-chlorotetrafluoroethanesulfi-
namide and ketones in high yields, and their Strecker reactions
with TMSCN have been investigated. High yields and
excellent diastereoselectivities were achieved in the presence
of a catalytic amount of CsF under mild conditions. The six-membered chairlike models were proposed to account for the high
stereoselective Strecker reaction.
he chiral auxiliary strategy has been developed as an
important technology in asymmetric synthesis.1 Among
chlorotetrafluoroethanesulfinyl ketimines from CTFSA and
ketones and investigated their application in the Strecker
reaction. The results are reported in this contribution.
In our initial experiments, 2-chlorotetrafluoroethanesulfina-
mide (1) was treated with 1.1 equiv of acetophenone (2a) and
2.0 equiv of Ti(OEt)4 in toluene at 90 °C. After 12 h, 19F NMR
spectrum of the reaction mixture showed that 63% of 1
converted and the desired ketimine 3a was formed in 47%
yield. When the reaction was carried out at 80 °C, the
corresponding conversion and yield were reduced to 42% and
38%, respectively. Further decrease of temperature led to much
lower conversion and yield. Taking into account the reaction
efficiency, we chose 80 °C as the reaction temperature in the
following experiments. Screening of solvent revealed that 1,4-
dioxane, acetonitrile, and dimethylsulfoxide (DMSO) could not
afford ketimine 3a, and only trace product was observed in 1,2-
dichloroethane (DCE). When cyclohexane was used as solvent,
the yield of 3a was improved to 64% (Table SI-1 in Supporting
Information (SI)).
Next, the ratio of reagents was screened to find a suitable
protocol for the formation of ketimines. The data revealed that
increasing the amount of 2a and Ti(OEt)4 was beneficial for
the formation of 3a (Table SI-2 in SI). The yield of 3a could be
improved to 92% in the presence of 2.0 equiv of 2a and 4.0
equiv of Ti(OEt)4. Therefore, the optimal reaction conditions
were set to using 2.0 equiv of 2a and 4.0 equiv of Ti(OEt)4 in
cyclohexane at 80 °C.
T
various chiral auxiliaries, N-tert-butanesulfinimines (t-BS-
imines)1a,b,2 and N-p-toluenesulfinimines (p-TS-imines)1c,3 are
two typical ones and have found wide applications as versatile
chiral amine templates. Recently, a novel fluorinated chiral
auxiliary, polyfluoroalkanesulfinamide, was developed in our
laboratory and applied successfully in the Strecker reaction,4a
three-component aza-Diels−Alder reaction,4b and aminoallyla-
tion of aldehydes.4c Due to the strong electron-withdrawing
effect of polyfluoroalkyl groups, imines derived from poly-
fluoroalkanesulfinamides showed high reactivity in the above
reactions, and the reactions usually took place under mild
conditions to achieve good yields and high diastereoselectiv-
ities. Among various polyfluoroalkanesulfinamides investigated
in our previous work,4 2-chlorotetrafluoroethanesulfinamide
(CTFSA) is a promising one due to its easy availability and
high performance.
The Strecker reaction is an efficient approach for the
formation of the C−C bond.5,6 It has been reported that the
Strecker reaction of aldimines could achieve good yields and
stereoselectivities using TBSA7 and p-TSA8 as auxiliaries.
However, the corresponding ketimines9 derived from TBSA
and p-TSA showed much less reactivity than their aldimine
counterparts.10 Therefore, the application of ketimines in the
Strecker reaction still remains a challenge in organic synthesis.11
In 2006, Lu’s group demonstrated that the reaction of
trifluoromethylketimines, prepared from trifluoroacetophenone
and TBSA, with TMSCN could occur in the absence of additive
and gave the adducts with excellent diastereoselectivities.12
Obviously, the reinforced electrophilicity of the ketimines was
attributed to the strong electron-withdrawing effect of
trifluoromethyl group. Recently, Han’s group also succeeded
in developing the asymmetric application of trifluoromethylated
N-tert-butanesulfinylimine.13 Encouraged by their work, we
envisioned that CTFSA4a might serve as a superior auxiliary for
ketimines due to the strong electron-withdrawing effect of the
fluoroalkyl group. Therefore, we studied the synthesis of 2-
It is worth mentioning that ketimine 3a is stable in the
refrigerator, and no decomposition is detected for weeks.
Purification by flash column chromatography afforded 3a in
81% yield (Table 1, entry 1). Under the optimized conditions,
various ketones 2 were examined and a series of ketimines were
synthesized. As shown in Table 1, the reaction of electron-
deficient ketones proceeded well to afford the corresponding
Received: January 2, 2014
Published: March 5, 2014
© 2014 American Chemical Society
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The Journal of Organic Chemistry
Note
Table 1. Synthesis of Fluoroalkanesulfinyl Ketimines 3
Table 2. Optimization of Reaction Parameters for the
Strecker Reaction of 3a
a
b
entry
2, R
time (h)
3
yield (%)
temp
additive
(equiv)
yield
b
dr (4a/
b
1
2
3
4
5
6
7
8
9
10
2a, C6H5
20
12
24
24
24
12
24
36
48
36
3a
3b
3c
3d
3e
3f
81 (92)
68 (97)
70 (87)
77 (95)
71 (86)
69
a
entry
solvent
DMF
(°C)
(%)
4a′)
2b, p-CF3C6H4
2c, p-FC6H4
1
2
3
rt
rt
rt
−
−
−
NR
0
−
−
33:67
HMPA
2d, p-ClC6H4
DMF/HMPA
(1/1)
36
2e, p-BrC6H4
2f, m-CF3C6H4
2g, p-MeC6H4
4
DCM
rt
Ti(OEt)4
(2.0)
trace
−
3g
3h
3i
63 (82)
69 (86)
41 (55)
75 (99)
56 (86)
2h, p-MeOC6H4
2i, p-O(CH2CH2)2N−C6H4
2j, C6H5CHCH
2k, (CH3)2CHCH2
5
6
7
8
DCM
DCM
DCM
DCM
rt
−25
−25
−25
CsF (2.0)
CsF (2.0)
CsF (0.2)
CsF (0.1)
complex
95
−
97:3
97:3
97:3
3j
94
c
11
12
3k
90
a
b
a
The concentration of 1 was 0.1 M. Yields in parentheses were
Conditions: Ketimine 3a (0.1 mmol), TMSCN (0.2 mmol) and 1 mL
determined by 19F NMR spectroscopy using p-BrC6H4CF3 as internal
b
solvent at room temperature or −25 °C. Determined by 19F NMR
c
standard. The reaction was carried out at 70 °C (oil bath).
using PhCF3 as internal standard.
products 2b−2f in high yields (Table 1, entries 2−6). The
ketones containing electron-donating substituted groups had
lower reactivity and required longer reaction time. For example,
it took 36 h for 4-methoxy acetophenone (2h) to achieve full
conversion (Table 1, entry 8). In the case of ketone 2i, the
reaction time was prolonged to 48 h, and only 41% yield was
obtained (Table 1, entry 9). Ketone 2j containing a styryl group
also reacted smoothly to give the corresponding ketimine 3j in
good yield (Table 1, entry 10). Under similar conditions,
aliphatic ketone 2k reacted with 1 readily at 70 °C to give the
corresponding ketimine 3k, but the isolated yield of 3k was
relatively lower (56%) due to its sensitivity to moisture (Table
1, entry 11).
Having the desired ketimines in hand, we next investigated
their Strecker reactions. When 3a was treated with TMSCN
(2.0 equiv) without additive in dimethylformamide (DMF) at
room temperature, 19F NMR analysis of the reaction mixture
showed that 3a remained unchanged (Table 2, entry 1). When
hexamethylphosphoramide (HMPA) was used as solvent, no
desired product was obtained (Table 2, entry 2). Running the
reaction in a mixed solvent of DMF and HMPA (1:1), the
isomers 4a and 4a′ were obtained in a total yield of 36% with a
ratio of 33:67 (Table 2, entry 3). Attempts to use 2.0 equiv of
Ti(OEt)4 or CsF as additives failed to afford the products in
1,2-dichloromethane (DCM) at room temperature (Table 2,
entries 4−5). However, when the reaction was carried out at
−25 °C in the presence of CsF (2.0 equiv), adducts with syn-
configuration as major isomer, were obtained in a total yield of
95% with 95:5 dr (Table 2, entry 6). Further experiments
indicated that the additive loading could be lowered down to
0.2 equiv without any erosion in yield and stereoselectivity
(Table 2, entries 7−8). Therefore, the optimized conditions of
the Strecker reaction were set as follows: treating 3a with
TMSCN (2.0 equiv) in the presence of catalytic amount of CsF
(0.2 equiv) in DCM at −25 °C.
Table 3. Strecker Reaction of Fluoroalkanesulfinyl Ketimines
time
(h)
yield
b
dr
(4:4′)
a
c
entry
3, R
products
(%)
1
2
3
4
5
6
7
8
3a, C6H5
7
4
4a, 4a′
4b, 4b′
4c, 4c′
4d, 4d′
4e, 4e′
4f, 4f′
4g, 4g′
4h, 4h′
4i, 4i′
93
84
85
87
81
83
89
89
84
97:3
96:4
94:6
96:4
93:7
96:4
97:3
96:4
>97:3
3b, p-CF3C6H4
3c, p-FC6H4
9
3d, p-ClC6H4
3e, p-BrC6H4
3f, m-CF3C6H4
3g, p-MeC6H4
3h, p-MeOC6H4
7
6
7
15
17
27
d
9
3i, p-O(CH2CH2)2N−
C6H4
10
11
3j, C6H5CHCH
17
4
4j, 4j′
4k, 4k′
82
94
97:3
96:4
3k, (CH3)2CHCH2
a
Reaction conditions: 3 (0.2 mmol), TMSCN (0.4 mmol), CsF (0.04
b
c
mmol) in 3 mL DCM at −25 °C for 7 h. Isolated yield. Determined
d
by 19F NMR spectra of the crude reaction mixture. 15% of 3i was
recovered, and the reaction was performed at −5 °C.
substituent on the aromatic ring. It took a much shorter time
for electron-deficient aromatic ketimines 3b−3f and aliphatic
ketimine 3k (Table 3, entries 2−6, 11). The reaction of
ketimine 3i with a morpholine-substituent was carried out at
−5 °C, and the corresponding adducts 4i and 4i′ were obtained
in 82% yield with 15% recovery of ketimine 3i (Table 3, entry
9).
To further demonstrate the application of 2-chlorotetra-
fluoroethanesulfinyl ketimines in asymmetric synthesis, (S)-3a
with 97.8% ee was prepared from (S)-2-chlorotetrafluoroetha-
nesulfinamide (99% ee) and allowed to react with TMSCN
under standard conditions to make optically pure 4a (Scheme
1). HPLC analysis confirmed the excellent de value (94.0%)
and ee value (97.5%) of the reaction. It was reported that the
asymmetric strecker reaction of tert-butanesulfinyl and p-
toluenesulfinyl ketimines derived from acetophenone gave the
Under the optimized conditions, the scope of Strecker
reaction was then examined. The data in Table 3 showed that
the reaction of various ketimines 3 worked well to afford the
corresponding products in good yields with excellent
diastereoselectivities. It is worth mentioning that the Strecker
reaction was very sensitive to the electronic property of the
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Note
Procedure for the Synthesis of Racemic 1. To a flask
containing HMDS (0.075 mol) was added 2-chlorotetrafluoroethane-
sulfinyl chloride (0.075 mol) dropwise at 0 °C. After addition, stirring
was continued for 2 h at room temperature. The mixture was
concentrated under reduced pressure, and the residue was treated with
50 mL of saturated NH4Cl (aq) for 2 h at room temperature. The
resulting mixture was extracted with Et2O (30 mL × 4) and dried over
anhydrous Na2SO4. After concentration, the residue was purified by
flash column chromatography on silica gel (EtOAc/petroleum ether
1:5).
Scheme 1. Preparation of (S)-5a from (S)-3a
1
Compound 1. H NMR (300 MHz, CDCl3): δ 4.96 (s, 2H) ppm;
19F NMR (282 MHz, CDCl3): δ −67.27 to −67.32 (m, 2F), −119.74
(d, JFF = 243.6 Hz, 1F), −122.47 (d, JFF = 243.6 Hz, 1F) ppm; 13C
NMR (100 MHz, CDCl3): δ 122.4 (tt, J = 298.4, 33.4 Hz), 116.3 (ddt,
J = 306.0, 302.0, 35.1 Hz) ppm; HRMS (EI) calcd for C2H2ClF4NOS
(M+) requires 198.9482, found 198.9480.
Procedure for the Synthesis of (S)-1. n-BuLi (2.64 mL, 2.5 M in
hexane) was added dropwise to the solution of (S)-4-phenyl-
oxazolidin-2-one (6 mmol) in THF (30 mL) at −78 °C under the
protection of N2. After addition, the mixture was stirred for 0.5 h.
Then 2-chlorotetrafluoroethanesulfinyl chloride (19.8 mmol) in 10 mL
of THF was added. After being stirred for 3 h, the mixture was allowed
to warm to room temperature. Solvent was removed, and the residue
was purified by flash column chromatography on silica gel (EtOAc/
petroleum ether, 1:7). (Rs, Sc)-4-Phenyl-N-(2-chlorotetrafluoroethane-
sulfinyl)oxazolidin-2-one was obtained as a white solid.
corresponding adducts with 84% de11a and 78% de,11b
respectively. Furthermore, the X-ray crystal structure of 4a
was obtained, indicating that the absolute configuration of 4a
was (Ss,Sc) (Figure SI-1 in SI).14 No racemization was observed
during the deprotection of (Ss,Sc)-4a and the formation of (S)-
5a.
On the basis of the above experimental results, the six-
membered chairlike models15 T1 and T2 were proposed to
account for the high stereoselectivity of the Strecker reaction.
As shown in Figure 1, transition state T2 was an unfavored one
A solution of (Rs,Sc)-4-phenyl-N-(2-chlorotetrafluoroethane-
sulfinyl)oxazolidin −2-one (0.5 mmol) in CH2Cl2 (7 mL) was
added slowly to a solution of LiHMDS (1.0 mmol) in CH2Cl2 (3 mL)
in 3 h at −78 °C under the protection of N2. After addition, saturated
NH4Cl (aq) (5 mL) was added. The resulting mixture was extracted
with CH2Cl2 (10 mL × 3), and the organic solution was dried over
anhydrous Na2SO4. After concentration, the residue was purified by
flash column chromatography on silica gel (EtOAc/petroleum ether,
1:5).
Compound (S)-1: yield 72%; white solid; mp 42−43 °C; [α]20
−15.5 (c = 1.0, CHCl3, 99% ee).
=
D
Typical Procedure for the Preparation of Ketimines 3. To a
solution of 2-chlorotetrafluoroethanesulfinamide (0.2 mmol) and
titanium(IV) ethoxide (0.8 mmol) in cyclohexane (2.0 mL) was
added ketone 2 (0.4 mmol). The mixture was stirred at 80 °C and
monitored by TLC. After the reaction was completed, brine (5 mL)
was added, and the resulting mixture was filtered through Celite. The
filter cake was washed with EtOAc. The aqueous phase was extracted
with EtOAc (5.0 mL × 3), and the combined organic phase was dried
over anhydrous Na2SO4. After concentration, the residue was purified
by flash column chromatography on silica gel to give ketimine 3
(EtOAc/petroleum ether, 1:20).
Figure 1. Proposed transition states for the Strecker reaction.
because of the predominant steric hindrance raised by the
phenyl group. Therefore, the transition state T1 predominated
in this reaction and afforded the syn-adduct as a major product.
In summary, 2-chlorotetrafluoroethanesulfinyl ketimines 3
have been synthesized successfully from 2-chlorotetrafluoroe-
thanesulfinamide, and their Strecker reaction with TMSCN was
achieved in the presence of a catalytic amount of CsF, affording
the corresponding adducts in good yields with excellent
stereoselectivities. Compared with tert-butanesulfinyl and p-
toluenesulfinyl ketimines, these fluoroalkanesulfinyl ketimines
showed much higher diastereoselectivity in these Strecker
reactions.
Typical Procedure for the Strecker Reaction. To a solution of
2-chlorotetrafluoroethanesulfinyl ketimine 3 (0.1 mmol) and CsF
(0.02 mmol) in anhydrous DCM (1.0 mL) was added TMSCN (0.2
mmol) dropwise via a syringe at −25 °C. The resulting mixture was
stirred at −25 °C and monitored by 19F NMR. After the reaction was
completed, the solvent was evaporated, and the residue was purified by
column chromatography on silica gel (EtOAc/petroleum ether, 1:10)
to give the corresponding products 4a and 4a′.
Compound 3a: 48.8 mg, 81%; yellow oil; IR (film): 3067, 1610,
1
1594, 1573, 1285, 1174, 1014, 800, 688 cm−1; H NMR (300 MHz,
CDCl3): δ 7.94−7.91 (m, 2H), 7.58−7.55 (m, 1H), 7.50−7.45 (m,
2H), 2.85 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ −65.94 (s, 2
F), −115.48 (d, JFF = 231.7 Hz, 1F), −116.91 (d, JFF = 231.7 Hz, 1F)
ppm; 13C NMR (100 MHz, CDCl3): δ 179.9, 137.2, 133.0, 128.7,
127.8, 122.3 (tt, JC−F = 248.8, 25.1 Hz), 117.0 (tt, JC−F = 253.5, 25.1
Hz), 20.3 ppm; ESI-MS (m/z, %): 302.0 (100) [M + H]+; HRMS
(ESI) Calcd for C10H8ClF4NNaOS [M + Na]+ requires 323.9843,
found 323.9828.
EXPERIMENTAL SECTION
■
General Information. Unless otherwise mentioned, solvents and
reagents were purchased from commercial sources and used as
received. CH2Cl2 was freshly distilled by standard procedure prior to
use. Melting points were measured on a Melt-Temp apparatus and
uncorrected. IR spectra were recorded with an infrared spectrometer.
1H NMR and 19F NMR were recorded on 1H at 300 Hz (or 400 MHz)
and 19F at 282 MHz (or 376 MHz), respectively. 13C NMR spectra
were recorded on 13C at 100 MHz. The mass analyzer type was
FTICR-MS used for the HRMS measurements. Enantiomeric excesses
were determined by chiral HPLC.
Compound (S)-3a: 48.4 mg, 80%; yellow oil; [α]20 = −79.6 (c =
D
1.0, CHCl3, 97.8% ee).
Compound 3b: 50.0 mg, 68%; yellow oil; IR (film): 3085, 2925,
1
1608, 1570, 1408, 1328, 1177, 1014, 847, 795, 606 cm−1; H NMR
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Note
(300 MHz, CDCl3): δ 8.01 (d, J = 8.2 Hz, 2H), 7.73 (d, J = 8.2 Hz,
3H), 2.90 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ −62.60 (s,
3F), −65.44 (s, 2F), −114.32 (d, JFF = 230.2 Hz, 1F), −115.64 (d, JFF
= 230.2 Hz, 1F) ppm; 13C NMR (100 MHz, CDCl3): δ 178.3, 140.4,
134.4 (q, JC−F = 32.7 Hz), 128.1, 125.7 (q, JC−F = 3.5 Hz), 123.5 (q,
ppm; 19F NMR (282 MHz, CDCl3): δ −66.36 (s, 2F), −117.16 (d, JFF
= 234.3 Hz, 1F), −118.10 (d, JFF = 234.3 Hz, 1F) ppm; 13C NMR
(100 MHz, CDCl3): δ 178.9, 154.4, 130.2, 127.2, 113.3, 66.5, 47.3,
19.9 ppm; ESI-MS (m/z, %): 387.0 (100) [M + H]+; HRMS (ESI)
Calcd for C14H15ClF4N2NaO2S [M + Na]+ requires 409.0371, found
409.0381.
JC−F = 270.6 Hz), 122.3 (tt, JC−F = 298.5, 35.9 Hz), 117.3 (tt, JC−F
=
308.1, 35.8 Hz), 20.4 ppm; ESI-MS (m/z, %): 370.0 (100) [M + H]+;
HRMS (ESI) Calcd for C11H8ClF7NOS [M + H]+ requires 369.9898,
found 369.9893.
Compound 3j: 49.2 mg, 75%; yellow oil; IR (film): 3060, 1668,
1609, 1257, 975, 749, 690, 559 cm−1; 1H NMR (300 MHz, CDCl3): δ
7.53−7.48 (m, 3H), 7.40 (d, J = 3.0 Hz, 3H), 6.70 (d, J = 16.2 Hz,
1H), 2.38 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ −65.95 (d, J
= 24.6 Hz, 2F), −115.64 to −116.70 (m, 2F) ppm; 13C NMR (100
MHz, CDCl3): δ 198.5, 143.5, 134.4, 130.5, 129.0, 128.3, 127.1, 27.4
ppm; ESI-MS (m/z, %): 327.9 (100) [M + H]+; HRMS (ESI) Calcd
for C12H11ClF4NOS [M + H]+ requires 328.0181, found 328.0176.
Compound 3k: 31.5 mg, 56%; yellow oil; IR (film): 2963, 1625,
1370,1174, 1127, 1015, 910, 801 cm−1; 1H NMR (400 MHz, CDCl3):
δ 2.37−2.29 (m, 5H), 2.16−2.07 (m, 1H), 0.94 (dd, J = 12.3, 3.0 Hz,
6H) ppm; 19F NMR (376 MHz, CDCl3): δ −65.69 (s, 2F), −116.58
(d, JFF = 230.7 Hz, 1F), −117.08 (d, JFF = 230.7 Hz, 1F) ppm; 13C
NMR (100 MHz, CDCl3): δ 190.2, 120.6 (tt, JC−F = 299.9, 33.6 Hz),
117.5 (tt, JC−F = 251.3, 33.3 Hz), 52.7, 29.6, 25.8, 22.2 ppm; ESI-MS
(m/z, %): 282.0 (100) [M + H]+; HRMS (ESI) Calcd for
C8H13ClF4NOS [M + H]+ requires 282.0337, found 282.0327.
Compound 4a: 30.5 mg, 93%; yellow solid; mp: 87−88 °C; IR
Compound 3c: 44.7 mg, 70%; yellow oil; IR (film): 3079, 2930,
1
1600, 1507, 1300, 1135, 1012, 841, 577 cm−1; H NMR (400 MHz,
CDCl3): δ 7.92 (dd, J = 8.6, 5.6 Hz, 2H), 7.73 (t, J = 8.6 Hz, 2H), 2.84
(s, 3H) ppm; 19F NMR (376 MHz, CDCl3): δ −65.76 (s, 2F),
−104.69 (d, J = 3.4 Hz, 1F), −115.38 (d, JFF = 229.9 Hz, 1F), −116.51
(d, JFF = 229.9 Hz, 1F) ppm; 13C NMR (100 MHz, CDCl3): δ 178.3,
165.8 (d, JC−F = 255.8 Hz), 133.5 (d, JC−F = 3.6 Hz), 130.4 (d, J = 8.9
Hz), 115.9 (d, J = 21.8 Hz), 20.3 ppm; ESI-MS (m/z, %): 320.0 (100)
[M + H]+; HRMS (ESI) Calcd for C10H7ClF5NNaOS [M + Na]+
requires 341.9749, found 341.9735.
Compound 3d: 51.7 mg, 77%; yellow oil; IR (film): 3104, 1682,
1
1587, 1488, 1398, 1265, 1176, 1013, 802 cm−1; H NMR (400 MHz,
CDCl3): δ 7.88 (d, J = 8.0 Hz, 2H), 7.46 (d, J = 8.0 Hz, 2H), 2.85 (s,
3H) ppm; 19F NMR (376 MHz, CDCl3): δ −65.71 (d, J = 2.5 Hz, 2F),
−115.10 (d, JFF = 231.0 Hz, 1F), −116.29 (d, JFF = 231.0 Hz, 1F)
ppm; 13C NMR (100 MHz, CDCl3): δ 178.4, 139.6, 135.6, 129.2,
129.1, 20.2 ppm; ESI-MS (m/z, %): 336.0 (100) [M + H]+; HRMS
(ESI) Calcd for C10H7Cl2F4NNaOS [M + Na]+ requires 357.9453,
found 357.9448.
1
(KBr): 3433, 3171, 1624, 1400, 1175, 1130, 1018, 803, 699 cm−1; H
NMR (300 MHz, CDCl3): δ 7.63−7.60 (m, 2H), 7.49−7.46 (m, 3H),
5.33 (s, 1H), 2.06 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ
−65.55 to −66.95 (m, 2F), −116.08 (d, JFF = 232.1 Hz, 1F), −118.00
(d, JFF = 232.1 Hz, 1F) ppm; 13C NMR (100 MHz, CDCl3): δ 136.5,
130.2, 129.5, 126.0, 119.6, 57.7, 30.3 ppm; ESI-MS (m/z, %): 346.0
(100) [M + NH4]+; HRMS (ESI) Calcd for C11H9ClF4N2NaOS [M +
Na]+ requires 350.9952, found 350.9965. (Ss,Sc)-4a: yield 93%; 97.5%
Compound 3e: 54.0 mg, 71%; yellow oil; FT-IR (film): 3085, 2927,
1
2856, 1609, 1557, 1302, 1136, 1009, 829, 688, 576 cm−1; H NMR
(400 MHz, CDCl3): δ 7.78 (d, J = 8.0 Hz, 2H), 7.60 (d, J = 8.0 Hz,
2H), 2.83 (s, 3H) ppm; 19F NMR (376 MHz, CDCl3): δ −65.42 (s,
2F), −114.89 (d, JFF = 231.0 Hz, 1F), −116.05 (d, JFF = 231.0 Hz, 1F)
ppm; 13C NMR (100 MHz, CDCl3): δ 178.6, 136.1, 132.1, 129.3,
128.3, 20.2 ppm; ESI-MS (m/z, %): 379.9 (100) [M + H]+; HRMS
(ESI) Calcd for C10H7BrClF4NNaOS [M + Na]+ requires 401.8949,
found 401.8949.
Compound 3f: 50.0 mg, 69%; yellow oil; IR (film): 3074, 2930,
2359, 1614, 1435, 1337, 1133, 905, 797, 693, 565 cm−1; 1H NMR (400
MHz, CDCl3): δ 8.15−8.09 (m, 2H), 7.82 (d, J = 8.1 Hz, 1H), 7.60 (t,
J = 8.1 Hz, 1H), 2.89 (s, 3H) ppm; 19F NMR (376 MHz, CDCl3): δ
−62.37 to −62.47 (m, 3F), −65.61 (s, 2F), −114.89 (d, JFF = 231.0
Hz, 1F), −116.05 (d, JFF = 231.0 Hz, 1F) ppm; 13C NMR (100 MHz,
CDCl3): δ 178.6, 138.1, 131.5 (q, JC−F = 33.1 Hz), 130.8, 129.5, 129.4
(q, JC−F = 3.6 Hz), 124.6 (q, JC−F = 4.6 Hz), 123.6 (q, JC−F = 271.3
Hz), 20.2 ppm; ESI-MS (m/z, %): 370.0 (100) [M + H]+; HRMS
(ESI) Calcd for C11H8ClF7NOS [M + H]+ requires 369.9893, found
369.9892.
20
ee; [α]D = −20.7 (c = 1.0 in CHCl3)
Compound 4b: 33.3 mg, 84%; yellow oil; IR (film): 3465, 3161,
1621, 1416, 1329, 1176, 1127, 1015, 801 cm−1 ; 1H NMR (300 MHz,
CDCl3): δ 7.76 (s, 4H), 5.73 (s, 1H), 2.07 (s, 3H) ppm; 19F NMR
(282 MHz, CDCl3): δ −62.50 (s, 3F), −66.14 to −66.20 (m, 2F),
−115.06 (ddd, JFF = 232.1, 5.4, 2.5 Hz, 1F), −118.89 (dd, JFF = 232.1,
2.0 Hz, 1F) ppm; 13C NMR (100 MHz, CDCl3): δ 141.2, 132.5 (q,
JC−F = 33.3 Hz), 126.7 (q, JC−F = 3.5 Hz), 126.2, 123.4 (q, JC−F = 270.8
Hz), 119.0, 57.4, 29.7 ppm; ESI-MS (m/z, %): 414.0 (100) [M +
NH4]+; HRMS (ESI) Calcd for C12H8ClF7N2NaOS [M + Na]+
requires 418.9826, found 418.9838.
Compound 4c: 29.5 mg, 85%; yellow oil; IR (film): 3203, 2824,
1
2359, 1605, 1511, 1239, 1167, 1129, 1016, 838, 801 cm−1; H NMR
(300 MHz, CDCl3): δ 7.63−7.58 (m, 2H), 7.19−7.13 (m, 2H), 5.58
(s, 1H), 2.04 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ-66.37 to
−66.43 (m, 2F), −109.99 to −110.09 (m, 1F), −115.62 (dt, J = 231.8,
3.9 Hz, 1F), −119.03 (dt, J = 231.8, 3.1 Hz, 1F) ppm; 13C NMR (100
MHz, CDCl3): δ 163.5 (d, JC−F = 249.1 Hz), 133.0 (d, JC−F = 3.7 Hz),
127.9 (d, JC−F = 8.6 Hz), 119.4, 116.6 (d, JC−F = 21.3 Hz), 57.1, 30.0
ppm; ESI-MS (m/z, %): 364.0 (100) [M + NH4]+; HRMS (ESI)
Calcd for C11H8ClF5N2NaOS [M + Na]+ requires 368.9858, found
368.9864.
Compound 4d: 31.6 mg, 87%; white solid; mp: 77−79 °C; IR
(KBr): 3442, 3187, 1493, 1402, 1179, 1132, 1013, 803 cm−1; 1H NMR
(300 MHz, CDCl3): δ 7.55 (d, J = 8.4 Hz, 2H), 7.45 (d, J = 8.7 Hz,
2H), 5.71 (s, 1H), 2.02 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ
−65.63 to −66.97 (m, 2F), −116.22 (d, JFF = 231.8 Hz, 1F), −118.09
(d, JFF = 231.8 Hz, 1F) ppm. 13C NMR (100 MHz, CDCl3): δ 136.3,
135.7, 129.8, 127.1, 119.3, 57.2, 29.9 ppm; ESI-MS (m/z, %): 380.0
(100) [M + NH4]+; HRMS (ESI) Calcd for C11H8Cl2F4N2NaOS [M
+ Na]+ requires 384.9562, found 384.9574.
Compound 3g: 39.8 mg, 63%; yellow oil; IR (film): 3037, 2924,
1
1596, 1439, 1284, 1174, 1011, 817, 579 cm−1; H NMR (300 MHz,
CDCl3): δ 7.83 (d, J = 8.1 Hz, 2H), 7.26 (d, J = 8.1 Hz, 2H), 2.83 (s,
3H), 2,42 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ −66.38 (s,
2F), −116.32 (d, JFF = 233.7 Hz, 1F), −117.48 (d, JFF = 233.7 Hz, 1F)
ppm; 13C NMR (100 MHz, CDCl3): δ 179.7, 144.2, 134.6, 129.5,
128.0, 21.6, 20.3 ppm; ESI-MS (m/z, %): 316.0 (100) [M + H]+;
HRMS (ESI) Calcd for C11H11ClF4NOS [M + H]+ requires 316.0176,
found 316.0181.
Compound 3h: 45.8 mg, 69%; yellow oil; IR (film): 2936, 2842,
1
1589, 1560, 1420, 1290, 1261, 1177, 1027, 836 cm−1; H NMR (300
MHz, CDCl3): δ 7.92 (d, J = 9 Hz, 2H), 6.94 (d, J = 9 Hz, 2H), 3.90
(s, 3H), 2.82 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ −65.43 (s,
2 F), −115.70 (d, JFF = 232.4 Hz, 1F), −116.80 (d, JFF = 232.4 Hz, 1F)
ppm; 13C NMR (100 MHz, CDCl3): δ 179.0, 163.8, 130.2, 129.7,
113.9, 55.5, 20.1 ppm; ESI-MS (m/z, %): 332.0 (100) [M + H]+;
HRMS (ESI) Calcd for C11H11ClF4NO2S [M + H]+ requires
332.0129, found 332.0121.
Compound 3i: 37.1 mg, 41%; yellow solid; mp: 103−104 °C; IR
(KBr): 3432, 3136, 1607, 1526, 1399, 1181, 1109, 788 cm−1; 1H NMR
(300 MHz, CDCl3): δ 7.90 (d, J = 8.7 Hz, 2H), 6.86 (d, J = 8.7 Hz,
2H), 3.86 (t, J = 5.4 Hz, 2H), 3.34 (t, J = 4.8 Hz, 2H), 2,76 (s, 3H)
Compound 4e: 33.0 mg, 81%; colorless oil; IR (film): 3200, 2920,
1
2361, 1590, 1490, 1398, 1264, 1180, 1009, 801, 520 cm−1; H NMR
(300 MHz, CDCl3): δ 7.63−7.59 (m, 2H), 7.50−7.46 (m, 2H), 5.51
(s, 1H), 2.02 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ −66.21 to
−66.24 (m, 2F), −116.09 (d, JFF = 232.0 Hz, 1F), −119.80 (d, JFF
=
232.0 Hz, 1F) ppm; 13C NMR (100 MHz, CDCl3): δ 136.2, 132.8,
127.3, 124.5, 119.2, 57.3, 29.9 ppm; ESI-MS (m/z, %): 423.9 (100)
3218
dx.doi.org/10.1021/jo500008e | J. Org. Chem. 2014, 79, 3215−3220

The Journal of Organic Chemistry
Note
[M + NH4]+; HRMS (ESI) Calcd for C11H8BrClF4N2NaOS [M +
Na]+ requires 428.9057, found 428.9067.
After cooling to room temperature, the precipitate was removed by
filtration. The filtrate was concentrated, and the residue was purified
by flash column chromatography on silica gel (AcOEt/hexane, 1/6) to
give the Boc-protected amino acid (S)-5a (11.4 mg, 84% from (Ss,Sc)-
4a, 94.5% ee) as a white solid.
Compound 4f: 32.9 mg, 83%; yellow oil; IR (film): 3202, 2926,
1
2853, 1492, 1437, 1331, 1177, 803, 701 cm−1; H NMR (300 MHz,
CDCl3): δ 7.86−7.83 (m, 2H), 7.75 (d, J = 7.8 Hz, 1H), 7.65 (t, J =
7.8 Hz, 1H), 5.36 (s, 1H), 2.09 (s, 3H) ppm; 19F NMR (282 MHz,
CDCl3): δ −62.50 (s, 3F), −66.21 to −66.24 (m, 2F), −115.83 (d, JFF
= 233.3 Hz, 1F), −118.43 (d, JFF = 233.3 Hz, 1F) ppm. 13C NMR
(100 MHz, CDCl3): δ 138.6, 132.2 (q, JC−F = 32.6 Hz), 130.3, 129.0,
Compound 5a: This is a known compound.15 94.5% ee, colorless
1
oil; H NMR (300 MHz, CDCl3): δ 7.46−7.28 (m, 5H), 5.82 (s, br,
1H), 3.69 (s, 3H), 1.99 (s, 3H), 1.37 (s, 9H) ppm; [α]D20 = −17.9 (c =
1.00 in CHCl3).
127.1 (q, JC−F = 3.1 Hz), 123.4 (q, JC−F = 271.3 Hz), 122.5 (q, JC−F
=
13.6 Hz), 118.9, 57.3, 29.8 ppm; ESI-MS (m/z, %): 413.9 (100) [M +
NH4]+; HRMS (ESI) Calcd for C12H8ClF7N2NaOS [M + Na]+
requires 418.9826, found 418.9838.
ASSOCIATED CONTENT
■
S
* Supporting Information
The crystal data of (Ss, Sc)-4a; H, 19F, and 13C NMR spectra
1
Compound 4g: 30.5 mg, 89%; colorless oil; IR (film): 3212, 2926,
for compounds 3 and 4. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
1513, 1451, 1377, 1264, 1176, 1018, 801 cm−1; H NMR (300 MHz,
CDCl3): δ 7.50 (d, J = 8.7 Hz, 2H), 7.27 (d, J = 8.7 Hz, 2H), 5.57 (s,
1H), 2.38 (s, 3H), 2.03 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ
−66.90 to −67.06 (m, 2F), −116.44 (dd, JFF = 228.7, 5.4 Hz, 1F),
−119.78 (dd, JFF = 228.7, 2.5 Hz, 1F) ppm; 13C NMR (100 MHz,
CDCl3): δ 140.4, 134.0, 130.2, 125.6, 119.7, 57.5, 29.9, 21.1 ppm; ESI-
MS (m/z, %): 360.0 (100) [M + NH4]+; HRMS (ESI) Calcd for
C12H11ClF4N2NaOS [M + Na]+ requires 365.0109, found 365.0114.
Compound 4h: 31.9 mg, 89%; colorless oil; IR (film): 3213, 2937,
AUTHOR INFORMATION
Corresponding Author
*E-mail: jtliu@sioc.ac.cn
Notes
■
The authors declare no competing financial interest.
1
1609, 1514, 1180, 1017, 801 cm−1; H NMR (300 MHz, CDCl3): δ
ACKNOWLEDGMENTS
7.53 (d, J = 9.0 Hz, 2H), 6.97 (d, J = 9.0 Hz, 2H), 5.56 (s, 1H), 3.84
(s, 1H), 2.04 (s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ −65.64 to
−67.00 (m, 2F), −115.92 (d, J = 284.1 Hz, 1F), −118.83 (d, J = 284.1
Hz, 1F) ppm; 13C NMR (100 MHz, CDCl3): δ 160.8, 128.6, 127.3,
119.8, 114.8, 57.3, 55.4, 29.9 ppm; ESI-MS (m/z, %): 376.0 (100) [M
+ NH4]+; HRMS (ESI) Calcd for C12H11ClF4N2NaO2S [M + Na]+
requires 381.0058, found 381.0061.
■
Financial support from the National Natural Science
Foundation of China (No. 21172243) is gratefully acknowl-
edged.
REFERENCES
■
(1) (a) Robak, M. T.; Herbage, M. A.; Ellman, J. A. Chem. Rev. 2010,
110, 3600. (b) Ferreira, F.; Botuha, C.; Chemla, F.; Perez-Luna, A.
Chem. Soc. Rev. 2009, 38, 1162. (c) Davis, F. A. J. Org. Chem. 2006, 71,
8993.
Compound 4i: 34.8 mg, 84%; colorless solid; mp: 133−135 °C; IR
(KBr): 3447, 3125, 1610, 1517, 1384, 1227, 1170, 1117, 1015, 802,
1
535 cm−1; H NMR (300 MHz, CDCl3): δ 7.48 (d, J = 8.7 Hz, 2H),
6.93 (d, J = 8.7 Hz, 2H), 5.46 (s, 1H), 3.85 (s, 4H), 3.20 (s, 4H), 2.03
(s, 3H) ppm; 19F NMR (282 MHz, CDCl3): δ −66.95 (d, J = 23.4 Hz,
2F), −116.08 (d, J = 232.6 Hz, 1F), −119.38 (d, J = 232.6 Hz, 1F)
ppm; 13C NMR (100 MHz, CDCl3): δ 152.2, 126.9, 126.7, 119.9,
115.3, 66.6, 57.2, 48.2, 29.8 ppm; ESI-MS (m/z, %): 414.0 (100) [M +
H]+; HRMS (ESI) Calcd for C15H17ClF4N3O2S [M + H]+ requires
414.0660, found 414.0669.
Compound 4j: 29.1 mg, 82%; colorless oil; IR (film): 3419, 3202,
2921, 1401, 1172, 1138, 960, 805 cm−1; 1H NMR (300 MHz, CDCl3):
δ 7.46−7.36 (m, 5H), 7.07 (d, J = 15.9 Hz, 1H), 6.31 (d, J = 15.9 Hz,
1H), 5.62−5.59 (m, 1H), 1.91 (s, 3H) ppm; 19F NMR (282 MHz,
CDCl3): δ −65.55 to −66.90 (m, 2F), −115.75 (d, J = 232.4 Hz, 1F),
−118.69 (ddd, J = 232.4, 5.6, 3.7 Hz, 1F) ppm. 13C NMR (100 MHz,
CDCl3): δ 135.3, 134.1, 129.4, 128.9, 127.3, 125.2, 118.6, 55.8, 28.0
ppm; ESI-MS (m/z, %): 372.0 (100) [M + NH4]+; HRMS (ESI)
Calcd for C13H11ClF4N2NaOS [M + Na]+ requires 377.0106, found
377.0113.
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W.; Harbottle, G.; Stockman, R. A. Org. Lett. 2013, 15, 2030. (g) Guo,
T.; Song, R.; Yuan, B.-H.; Chen, X.-Y.; Sun, X.-W.; Lin, G.-Q. Chem.
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Compound 4k: 29.0 mg, 94%; yellow oil; IR (film): 3208, 2965,
1
2877, 1471, 1382, 1264, 1127, 1016, 910, 799, 605 cm−1; H NMR
Chem. 2012, 77, 525. (f) Vyas, D. J.; Frohlich, R.; Oestreich, M. Org.
Lett. 2011, 13, 2094.
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̈
(300 MHz, CDCl3): δ 5.35 (s, 1H), 1.95−1.74 (m, 3H), 1.70 (s, 3H),
1.07−1.04 (m, 6H) ppm; 19F NMR (282 MHz, CDCl3): δ −67.02 to
−67.06 (m, 2F), −116.91 (dd, J = 236.0, 2.8 Hz, 1F), −119.95 (d, J =
236.0 Hz, 1F) ppm. 13C NMR (100 MHz, CDCl3): δ 120.4, 53.6, 49.6,
26.9, 24.6, 23.6, 23.5 ppm; ESI-MS (m/z, %): 326.0 (100) [M +
NH4]+; HRMS (ESI) Calcd for C9H13ClF4N2NaOS [M + Na]+
requires 331.0265, found 331.0281.
Procedure for the Preparation of (S)-5a from (Ss, Sc)-4a. A
suspension of (Ss, Sc)-4a (0.0482 mmol) in the mixture of conc. HCl
(1 mL) and MeOH (0.1 mL) was stirred at 80 °C for 8 h. After
cooling to room temperature, the solvent was removed under reduced
pressure, and the residue was dissolved in dry MeOH (0.5 mL). To
this mixture, SOCl2 (50 μL) was added dropwise at room temperature,
and the resulting mixture was refluxed for 9 h. After cooling to room
temperature, the solvent was removed in vacuo, and the residue was
dissolved in CH3CN (2 mL). NaHCO3 (3.6 mmol) and Boc2O (0.206
mmol) were added, and the mixture was stirred at 70 °C for 12 h.
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(14) The crystal data of (Ss,Sc)-4a have been deposited at the
Cambridge Crystallographic Data Center. CCDC: 965936. Empirical
Formula: C11H9ClF4N2OS; Formula Weight: 328.71; Crystal Color,
Habit: yellow, prismatic; Crystal Dimensions: 0.342 × 0.213 × 0.116
mm3; Crystal System: Trigonal; Lattice Type: Primitive; Lattice
Parameters: a = 10.278(5)Å, b = 10.278(5)Å, c = 11.406(4)Å, α = 90°,
β = 90°, γ = 120°, V = 1043.5(9)Å3; Space group: P3(1)/n; Z = 3;
Dcalc = 1.569 g/cm3; F000 = 498; Residuals: R; Rw: 0.0466, 0.0503.
(15) Hose, D. R. J.; Mahon, M. F.; Molloy, K. C.; Raynham, T.; Wills,
M. J. Chem. Soc., Perkin Trans. 1 1996, 691.
3220
dx.doi.org/10.1021/jo500008e | J. Org. Chem. 2014, 79, 3215−3220