Valine-derived phosphinothiourea as organocatalyst in enantioselective Morita-Baylis-Hillman reactions of acrylates with aromatic aldehydes

Article abstract of DOI:10.1016/j.tetasy.2009.07.047

A new type of chiral bifunctional phosphinothiourea derived from l-valine is synthesized and used as an organocatalyst in the enantioselective Morita-Baylis-Hillman reaction of aromatic aldehydes with acrylates. The desired products were obtained in good

Full text of DOI:10.1016/j.tetasy.2009.07.047

Tetrahedron: Asymmetry 20 (2009) 2117–2120
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Tetrahedron: Asymmetry
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Valine-derived phosphinothiourea as organocatalyst in enantioselective
Morita–Baylis–Hillman reactions of acrylates with aromatic aldehydes
*
Jing-Jing Gong, Kui Yuan, Xin-Yan Wu
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new type of chiral bifunctional phosphinothiourea derived from L-valine is synthesized and used as an
Received 22 June 2009
Accepted 30 July 2009
Available online 3 October 2009
organocatalyst in the enantioselective Morita–Baylis–Hillman reaction of aromatic aldehydes with acry-
lates. The desired products were obtained in good enantioselectivities (up to 83% ee) and in excellent
yields (up to 96%) under mild reaction conditions.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Herein we report the MBH reaction of aldehydes and acrylates cat-
alyzed by the bifunctional phosphinothioureas.
The Morita–Baylis–Hillman (MBH) reaction is an important car-
bon–carbon formation process1 and much research effort has been
concentrated on improving its efficiency and selectivity over the last
decade.2 Various chiral organocatalysts including (S)-proline,3 quin-
idine derivatives,4 BINOL derivatives,5 bis(thio)ureas,6 bifunctional
aminothiourea,7 bifunctional phosphinothiourea,8 and chiral amino
alcohol-derived thiourea9 have been developed for the asymmetric
MBH reaction to achieve high enantioselectivity. Although the MBH
reaction of enone with aldehydes and aza-MBH reactions have
achieved good results,2a–d the asymmetric MBH reaction of alde-
hydes with acrylates is still a challenge. A few effective chiral organ-
ocatalysts have been tested for this process.2e,4a,c,10 For example,
Hatakeyama reported the excellent enantioselective MBH reaction
of 1,1,1,3,3,3-hexafluoroisopropyl acrylate (HFIPA) with aldehydes
(up to 99% ee),4a,c and Shi used a quinidine-derived chiral amine as
2. Results and discussion
The organocatalysts 2a–f are easily prepared by condensation of
amino-phosphine 1 with 1.1 equiv of the corresponding iso(thio)
cyanate under mild conditions (Scheme 1).
Initially the asymmetric Morita–Baylis–Hillman reaction of 4-
nitrobenzaldehyde with methyl acrylate was performed with
10 mol % of catalysts 2a–f at 25 °C in THF under an N2 atmosphere.
The results summarized in Table 1 indicate that the thiourea moi-
ety obviously affected the MBH reaction in terms of both yields and
enantioselectivities. The organocatalysts containing aromatic thio-
urea unit led to higher yields and enantioselectivities than those
bearing aliphatic or alicyclic thiourea moiety (entries 1–4 vs en-
tries 5 and 6). There was not an obvious diversity of the enantio
selectivities catalyzed by the aromatic thiourea 2a–d, but the yield
was lower when 2c was used as a catalyst. Comparatively, 2a was
the best catalyst for this transformation and furnished the desired
organocatalyst for the MBH reaction of aldehydes with a-naphthyl
acrylate to obtain as high as 92% ee but poor yield (only 17%).10a
To the best of our knowledge, the best result of the organocata-
lyzed-MBH reaction of aldehydes with unactivated alkyl acrylates
(such as methyl, ethyl, or butyl acrylate) was achieved by using a ter-
tiary amine as a catalyst to give the corresponding adducts with
moderate ee.2e
product in 93% yield and 81% ee (entry 1). The L-valine-derived
phosphinothioureas led to the MBH reaction products with an
(R)-configuration; the absolute configuration was assigned by
comparing the specific rotation value with those reported in the
literatures.4a,10c,12 For comparison, phosphinothiourea 2g and
2h8b derived from (R,R)-1-amino-2-(diphenylphosphino)cyclohex-
ane were examined (Scheme 2). As expected, organocatalysts 2g
and 2h provided (S)-configuration MBH products. It is noteworthy
that organocatalyst 2g was more reactive than other screened
organocatalysts, and the MBH reaction was completed in 8 h.
However, with 2g and 2h as catalysts, the enantioselectivity was
lower than the corresponding organocatalysts 2a and 2d,
respectively (entry 7 vs 1, entry 8 vs 4).
The L
-valine-derived amino-phosphine 111a is a useful chiral
precursor for constructing N, P-ligands, which are efficient in var-
ious metal-catalyzed asymmetric transformations, such as allylic
alkylation,11b–f transfer hydrogenation,11g and diethylzinc addition
to enones.11h Recently, we developed a new class of phos-
phinothiourea organocatalysts 2a–f derived from the amino-phos-
phine 1, and evaluated them in the enantioselective MBH reaction.
* Corresponding author. Tel.: +86 21 64252011; fax: +86 21 64252758.
E-mail address: xinyanwu@ecust.edu.cn (X.-Y. Wu).
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.07.047

2118
J.-J. Gong et al. / Tetrahedron: Asymmetry 20 (2009) 2117–2120
S
2a: R = C6H5
2b: R = 4-MeOC6H4
2c: R = 4-ClC6H4
2d: R = 3,5-(CF3)2C6H3
2e: R = c-C6H11
RNCS
NH2
PPh2
HN
NHR
L-Valine
PPh2
2f: R = n-C8H17
1
2a-f
Scheme 1. Synthesis of the phosphinothiourea catalysts.
Table 1
Table 2
Screening of the catalysts for the reaction of methyl acrylate and 4-
The optimization of the reaction conditionsa
nitrobenzaldehydea
CHO
O
O
OH
CHO
O
O
OH
10 mol% 2a
MeO
+
MeO
10 mol% 2a-h
Solvent, 25oC
MeO
MeO
+
THF, 25oC, 48h
NO2
NO2
NO2
NO2
3
4
Entry
Catalyst
Yieldb (%)
eec (%)
Entry
Solvent
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7e
8
2a
2b
2c
2d
2e
2f
93
89
79
94
80
76
96
82
81d
78
80
79
70
76
72f
61f
1
2
3
4
5
6
7
8
9
10
11d
12e
13f
THF
Ether
1,4-Dioxane
Toluene
EtOH
48
26
48
48
20
48
40
40
48
48
96
72
48
93
77
54
44
53
64
77
67
62
32
81
55
91
81
74
75
63
58
32
30
28
28
4
DMF
2g
2h
DMSO
CH2Cl2
CHCl3
CH3CN
THF
a
The reactions were conducted with 10 mol % of organocatalyst and 5 equiv of
methyl acrylate in THF (0.2 M) under N2 at 25 °C.
80
71
77
b
Isolated yields.
THF
THF
c
Determined by chiral HPLC using Chiralcel OD-H column.
½ ꢀ ¼ ꢁ38:8 (c 0.4, MeOH).
a 2D0
d
e
f
a
The reaction time is 8 h.
Unless stated otherwise, the reactions were conducted with 10 mol % of
organocatalyst and 5 equiv of methyl acrylate in solvent (0.2 M) under N2 at 25 °C.
The absolute configuration of the MBH reaction product is (S).
b
Isolated yields.
Determined by chiral HPLC using Chiralcel OD-H column.
The reaction was conducted with 3 equiv of methyl acrylate.
The reaction was conducted with 5 mol % of organocatalyst.
c
d
e
CF3
f
The reaction was conducted with 20 mol % of organocatalyst.
S
S
N
H
N
H
CF3
NH
NH
Table 3
The MBH reactions of different acrylates with 4-nitrobenzaldehyde catalysed by 2aa
PPh2
PPh2
CHO
O
2g
O
OH
2h
R1O
10 mol% 2a
Scheme 2. Structure of the phosphinothiourea 2g and 2h.
R1O
+
THF, 25oC
NO2
NO2
Next, we investigated the solvent effects on this process with 2a
as the organocatalyst (Table 2). Among the solvents screened, THF
was the optimal one for both yield and ee (entry 1). With ether and
1,4-dioxane as solvents, the product was obtained in moderate ee,
and the chemical yields decreased remarkably (entries 2 and 3 vs
entry 1). The reaction proceeded quickly in EtOH, but a by-product
was formed, and the desired product was provided in 53% yield. In
non-protonic polar solvents, such as DMF and DMSO, the MBH
product was obtained in moderate yields with low ee (entries 6
and 7). In the case of CH3CN, the product was provided in very poor
ee and in low yield. Furthermore, other reaction conditions includ-
ing the ratio of acrylate to aldehyde 3/4 and the loading of the cat-
alyst were investigated. The results indicated that when lowering
the ratio of 3/4 from 5/1 to 3/1, the chemical yield was reduced
(entry 1 vs entry 11). The catalyst loading affected both the chem-
ical yields and enantioselectivities of the product (entry 1 vs en-
tries 12 and 13). When the catalyst loading was reduced to
5 mol %, the reaction became slower, and only 55% chemical yield
was obtained after 72 h, while the enantioselectivity decreased
(entry 12). An improvement in the enantioselectivity or yield
was not observed when using 20 mol % organocatalyst. Thus
10 mol % of organocatalyst 2a was selected for further study.
Entry
R1
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
Me
Et
n-Bu
Bn
t-Bu
Ph
1-Naphthyl
48
48
48
42
48
46
46
93
92
94
92
72
24
36
81
80
83
76
74
4
2
a
The reactions were conducted with 10 mol % of organocatalyst and 5 equiv of
acrylate in THF (0.2 M) under N2 at 25 °C.
b
Isolated yields.
c
Determined by chiral HPLC using Chiralcel OD-H, Chiralpak AS-H, or Chiralpak
AD-H column.
Under the established optimal reaction conditions (10 mol % 2a
as a catalyst, 5 equiv of acrylate, THF as the solvent at 25 °C under
N2), the MBH reactions of different acrylates with 4-nitrobenzalde-
hyde were surveyed using 2a as a catalyst. The results are summa-
rized in Table 3. There was not obvious change in the
enantioselectivities or chemical yields when un-branched alkyl
acrylates (R1 = n-Bu, Me and Et) and benzyl acrylate were used as
Michael donor, while t-butyl acrylate provided lower yield and

J.-J. Gong et al. / Tetrahedron: Asymmetry 20 (2009) 2117–2120
2119
enantioselectivity (entries 1–4 vs entry 5). On the other hand,
phenyl acrylate and 1-naphthyl acrylate gave the product in very
low yields with poor ee and dioxanone was observed as a by-prod-
uct (entries 6 and 7).4a The results indicated that the structure of
acrylate is critical for the MBH reaction, and the bulky-hindrance
group of the acrylate had a negative effect on the enantioselectivity
and chemical yield (entries 1–5 vs entries 6 and 7).
phinoyl associated alkoxy enolate attacks the activated carbonyl
from the si-face to give the product in an (R)-configuration.
3. Conclusions
In conclusion, we have developed a new kind of chiral bifunc-
tional phosphinothiourea derived from
L-valine. These organocata-
Finally, various aromatic aldehydes were investigated under the
optimal reaction conditions (10 mol % 2a as a catalyst, 5 equiv of
acrylate, THF as the solvent at 25 °C under N2). As indicated in
Table 4, the strong electron-deficient benzaldehydes were trans-
formed to the desired products in excellent yields and moderate-
to-good enantioselectivities (entries 1–12). The results indicated
that the position of the substituent on benzaldehyde was very
important for the enantioselectivity, ortho substitution such as 2-
nitrobenzaldehyde had a deleterious effect on the enantioselectiv-
ities (entries 1–3 and 7–9 vs entries 4–6). The 4-trifluoromethyl-
benzaldehyde reacted with methyl acrylate providing higher ee
than other acrylates, but the yield was lower (entries 10–12). The
reaction with weakly electron-deficient benzaldehydes and non-
substituted benzaldehydes gave relatively poor yields and moder-
ate enantioselectivities (entries 13 and 14).
lysts were efficient for the asymmetric MBH reaction of acrylates
with aldehydes. In the presence of 10 mol % 2a, the MBH product
was obtained in up to 83% ee and moderate-to-excellent yields
(up to 96%). The advantage of this catalyst is that the chiral starting
material is facile and inexpensive. The further refinement of the
catalyst structure and extension of the utility to other MBH reac-
tions are under active investigation.
4. Experimental
4.1. General
Melting points are taken without correction. Optical rotations
were measured on a WZZ-2A digital polarimeter at the wavelength
of the sodium D-line (598 nm). 1H NMR spectra were recorded on
Bruker 500 (500 MHz) spectrometer, and chemical shifts were re-
corded in parts per million (ppm, d) relative to tetramethylsilane
(d, 0.00) with the solvent resonance as an internal standard (CDCl3:
7.24 ppm) and coupling constants (Hz). 13C NMR spectra were re-
corded on Bruker 500 (125 MHz) or 400 (100 MHz) instrument
with complete proton decoupling, and chemical shifts were re-
ported in parts per million (ppm) from tetramethylsilane with
the solvent as the internal standard (CDCl3: 77.0 ppm). High Reso-
lution Mass spectra (HRMS) were recorded on Micromass GCT
spectrometer with Electron Ionization (EI) resource. HPLC analysis
was performed on Waters 510 with 2487 detector using Daicel
Chiralpak AS-H, Chiralcel OD-H, Chiralcel OJ, or Chiralpak AD-H
column.
Table 4
The MBH reactions between various aromatic aldehydes with different acrylates
catalyzed by 2aa
CHO
O
O
OH
R1O
10 mol% 2a
R1O
R2
+
R2
THF, 25ºC
Entry
R1
R2
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Me
Et
n-Bu
Me
Et
n-Bu
Me
Et
n-Bu
Me
Et
4-NO2
4-NO2
4-NO2
2-NO2
2-NO2
2-NO2
3-NO2
3-NO2
3-NO2
4-CF3
4-CF3
4-CF3
4-Br
48
48
48
55
55
47
55
48
47
55
55
50
65
72
93
92
94
86
91
94
86
87
96
61
82
81
28
11
81
80
83
55
50
50
79
74
78
83
67
66
59
56
4.2. Synthesis of phosphine-thiourea catalysts 2a–f
To a solution of (S)-2-amino-1-diphenylphoshino-3-methylbu-
tane 111a (271 mg, 1.0 mmol) in 2.0 mL CH2Cl2 was added isothio-
cyanate (1.1 mmol) at room temperature, and the corresponding
mixture was stirred at this temperature until the completion of
the reaction (monitored by TLC). The solvent was removed under
reduced pressure and the residue was purified by column chroma-
tography (petroleum ether/ethyl acetate = 4/1 or 3/1) to afford the
chiral phosphine-thiourea compounds 2a–f.
n-Bu
n-Bu
n-Bu
H
a
The reactions were conducted with 10 mol % of organocatalyst and 5 equiv of
acrylate in THF (0.2 M) under N2 at 25 °C.
b
Isolated yields.
c
Determined by chiral HPLC using Chiralpak AS-H, Chiralcel OD-H, Chiralcel OJ,
or Chiralpak AD-H column.
4.2.1. Phosphine-thiourea catalyst 2a
White solid, 60% yield, mp: 54.6–55.7 °C; ½a D25
¼ þ32:1 (c 0.7,
ꢀ
A possible mechanism of this asymmetric MBH reaction can be
explained as a Michael addition and an aldol reaction on the basis
of the generally accepted reaction mechanism, and the proposed
transition state is illustrated in Scheme 3. The thiourea moiety
forms a hydrogen-bond with the aldehyde carbonyl, and the phos-
CHCl3); IR (KBr, cmꢁ1):
m 3229, 3056, 2957, 1598, 1540, 1494,
1043, 565; 1H NMR (CDCl3, 500 MHz): d 7.8 (s, 1H), 7.49–7.42
(m, 4H), 7.40–7.35 (t, 2H, J = 7.7 Hz), 7.34–7.23 (m, 7H), 7.07 (d,
2H, J = 7.7 Hz), 5.97 (d, 1H), 4.59 (br, 1H), 2.44–2.38 (m, 1H),
2.32–2.24 (m, 1H), 2.17–2.09 (m, 1H), 0.87 (d, 3H, J = 6.8 Hz),
0.83 (d, 3H, J = 6.8 Hz); 13C NMR (CDCl3, 100 MHz): d 180.08,
137.99, 136.19, 132.96 (d, J = 5.2 Hz), 132.77 (d, J = 5.2 Hz),
129.99, 128.86, 128.60, 128.57, 128.53, 128.49, 126.94, 125.09,
58.36 (d, J = 14.2 Hz), 31.77 (d, J = 8.6 Hz), 31.23 (d, J = 13.5 Hz),
18.88, 18.03; HRMS (EI) calcd for C24H27N2PS ([M]+) 406.1633, obsd
406.1635.
S
N
N
H
O
OH
H
R1O
Ar
O
P+
si-attack
Ar
Ph
Ph
4.2.2. Phosphine-thiourea catalyst 2b
-O
H
Colorless oil, 73% yield, ½a D25
¼ þ51:0 (c 0.5, CHCl3); IR (KBr,
ꢀ
OR1
cmꢁ1):
m
3378, 2943, 1535, 1509, 1240, 1159, 1031, 693; 1H NMR
(CDCl3, 500 MHz): d 7.59 (s, 1H), 7.53–7.40 (m, 4H), 7.38–7.30
Scheme 3. Proposed transition state of 2a-catalyzed MBH reaction.

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J.-J. Gong et al. / Tetrahedron: Asymmetry 20 (2009) 2117–2120
(m, 6H), 7.00 (d, 2H, J = 8.4 Hz), 6.88 (d, 2H, J = 8.80 Hz), 5.77 (d, 1H,
J = 8.4 Hz), 4.57 (br, 1H), 3.83 (s, 3H), 2.46–2.38 (m, 1H), 2.31–2.22
(m, 1H), 2.19–2.06 (m, 1H), 0.90–0.84 (d, 3H, J = 6.8 Hz), 0.84–0.78
(d, 3H, J = 6.8 Hz); 13C NMR (CDCl3, 100 MHz): d 180.84, 158.59,
132.99 (d, J = 5.2 Hz), 132.81 (d, J = 5.2 Hz), 130.65, 128.72,
128.70, 128.65, 128.63, 127.52, 114.99, 57.92 (d, J = 10.6 Hz),
55.48, 31.87 (d, J = 8.6 Hz), 30.84, 18.83, 18.05; HRMS (EI) calcd
for C25H29N2OPS ([M]+) 436.1738, obsd 436.1740.
4.3. General procedure for the organocatalytic Morita–Baylis–
Hillman reaction
To a solution of the phosphine-thiourea (0.02 mmol) in THF
(1.0 mL) was added the acrylate (1.0 mmol) at 25 °C under N2. After
stirring at this temperature for 10 min, the aromatic aldehyde
(0.2 mmol) was added. And the reaction mixture was stirred at
25 °C until the completion of the reaction (monitored by TLC).
The solvent was removed under reduced pressure and the residue
was purified by a flash column chromatography to afford the Bay-
lis–Hillman adduct and the ee value was determined by HPLC anal-
ysis on a chiral stationary phase.
4.2.3. Phosphine-thiourea catalyst 2c
White solid, 70% yield, mp: 121.2–122.2 °C; ½a D25
¼ þ59:0 (c 0.5,
ꢀ
CHCl3); IR (KBr, cmꢁ1):
m 3225, 3037, 2960, 1528, 1494, 1341, 1299,
1085, 685; 1H NMR (CDCl3, 500 MHz): d 7.52–7.40 (m, 5H), 7.38–
7.29 (m, 7H), 7.00 (d, 2H, J = 5.79 Hz), 5.89 (br, 1H), 4.59 (br, 1H),
2.50–2.42 (m, 1H), 2.32–2.22 (m, 1H), 2.18–2.09 (m, 1H), 0.92–
0.82 (m, 6H); 13C NMR (CDCl3, 100 MHz): d 180.08, 137.88,
134.86, 132.92 (d, J = 10.0 Hz), 132.73 (d, J = 10.0 Hz), 132.30,
130.01, 128.97, 128.91, 128.65, 128.61, 128.58, 128.54, 126.25,
58.45 (d, J = 14.0 Hz), 31.91 (d, J = 8.5 Hz), 31.48 (d, J = 13.1 Hz),
18.74, 18.27; HRMS (EI) calcd for C24H26ClN2PS ([M]+) 440.1243,
obsd 440.1276.
Acknowledgments
We are grateful for the financial support from National Natural
Science Foundation of China (20772029) and the Program for New
Century Excellent Talents in University (NCET-07-0286).
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1781–1789; (f) Liu, Y.-H.; Chen, L.-H.; Shi, M. Adv. Synth. Catal. 2006, 348, 973–
979; (g) Jiang, Y.-Q.; Shi, Y.-L.; Shi, M. J. Am. Chem. Soc. 2008, 130, 7202–7203.
6. (a) Sohtome, Y.; Tanatani, A.; Hashimoto, Y.; Nagasawa, K. Tetrahedron Lett.
2004, 45, 5589–5592; (b) Berkessel, A.; Roland, K.; Neudörfl, J. M. Org. Lett.
2006, 8, 4195–4198; (c) Shi, M.; Liu, X.-G. Org. Lett. 2008, 10, 1043–1046.
7. Wang, J.; Yu, X.; Zu, L.; Wang, W. Org. Lett. 2005, 7, 4293–4296.
8. (a) Shi, Y.-L.; Shi, M. Adv. Synth. Catal. 2007, 349, 2129–2135; (b) Yuan, K.;
Zhang, L.; Song, H.-L.; Hu, Y.-J.; Wu, X.-Y. Tetrahedron Lett. 2008, 49, 6262–
6264.
4.2.5. Phosphine-thiourea catalyst 2e
Colorless oil, 65% yield, ½a D25
¼ ꢁ81:7 (c 0.3, CHCl3); IR (KBr,
ꢀ
cmꢁ1):
m 3279, 3049–2925, 2852–1547, 1538, 1532, 1023, 557;
1H NMR (CDCl3, 500 MHz): d 7.52–7.41 (m, 4H), 7.41–7.31 (m,
6H), 5.44 (br, 1H), 2.52–2.41 (m, 1H), 2.39–2.26 (m, 1H), 2.22–
2.16 (m, 1H), 1.95–1.81 (m, 2H), 1.76–1.64 (m, 2H), 1.40–1.20
(m, 4H), 1.20–1.00 (m, 3H), 0.98–0.93 (m, 6H); 13C NMR (CDCl3,
100 MHz):
d 180.08, 138.01, 133.01, 132.90, 132.82, 132.71,
128.94, 128.64, 128.91, 128.68, 128.65, 128.62, 128.58, 57.72,
52.47, 32.74, 32.20 (d, J = 8.3 Hz), 31.42, 25.38, 24.62, 24.58,
18.97, 18.06; HRMS (EI) calcd for C24H33N2PS ([M]+) 412.2102, obsd
412.2105.
9. Lattanzi, A. Synlett 2007, 2106–2110.
10. (a) Shi, M.; Jiang, J.-K. Tetrahedron: Asymmetry 2002, 13, 1941–1947; (b) Xu,
J.-Y.; Guan, Y.-Y.; Yang, S.-H.; Ng, Y. R.; Peh, G. R.; Tan, C.-H. Chem. Asian J. 2006,
1, 724–729; (c) Krishna, P. R.; Kannan, V.; Reddy, P. V. N. Adv. Synth. Catal. 2004,
346, 603–606.
4.2.6. Phosphine-thiourea catalyst 2f
Colorless oil, 65% yield, ½a D25
¼ ꢁ11:0 (c 0.5, CHCl3); IR (KBr,
ꢀ
cmꢁ1):
m
3270, 3064, 2921, 2853, 2127, 1543, 1351, 741, 690; 1H
11. (a) Ito, M.; Osaku, A.; Kobayashi, C.; Shiibashi, A.; Ikariya, T. Organometallics 2009,
28, 390–393; (b) Saitoh, A.; Morimoto, T.; Achiwa, K. Tetrahedron: Asymmetry
1997, 8, 3567–3570; (c) Saitoh, A.; Misawa, M.; Morimoto, T. Tetrahedron:
Asymmetry 1999, 10, 1025–1028; (d) Saitoh, A.; Uda, T.; Morimoto, T.
Tetrahedron: Asymmetry 1999, 10, 4501–4511; (e) Anderson, J. C.; Cubbon, R. J.;
Harling, J. D. Tetrahedron: Asymmetry 2001, 12, 923–935; (f) Laurent, R.;
Caminade, A.-M.; Majoral, J.-P. Tetrahedron Lett. 2005, 46, 6503–6506; (g)
Quirmbach, M.; Holz, J.; Tararov, V. I.; Börner, A. Tetrahedron 2000, 56, 775–780;
(h) Kawamura, K.; Fukuzawa, H.; Hayashi, M. Org. Lett. 2008, 10, 3509–3512.
12. (a) Radha Krishna, P.; Kannan, V.; Ilangovan, A.; Sharma, G. V. M. Tetrahedron:
Asymmetry 2001, 12, 829–837; (b) Bhuniya, D.; Narayanan, S.; Lamba, T. S.;
Reddy, K. V. S. R. K. Synth. Commun. 2003, 33, 3717–3726.
NMR (CDCl3, 500 MHz): d 7.53–7.42 (m, 4H), 7.47–7.29 (m, 6H),
5.46 (br, 1H), 3.20–2.70 (br, 1H), 2.51–2.42 (m, 1H), 2.45–2.36
(m, 1H), 2.19–2.06 (m, 1H), 1.47–1.39 (m, 2H), 1.37–1.20 (m,
12H), 0.98–0.91 (m, 6H), 0.91–0.88 (t, 3H, J = 6.8 Hz); 13C NMR
(CDCl3, 100 MHz): d 181.18, 137.32, 132.98 (d, J = 6.6 Hz), 132.79
(d, J = 6.8 Hz), 129.08, 128.71, 128.68, 128.65, 128.62, 57.70,
43.53, 32.19 (d, J = 8.2 Hz), 31.75, 31.18, 29.21, 29.14, 28.83,
26.88, 22.61, 18.84, 18.14, 14.07; HRMS (EI) calcd for C26H39N2PS
([M]+) 442.2572, obsd 442.2573.