New chiral thiophene-salen chromium complexes for the asymmetric Henry reaction

Article abstract of DOI:10.1021/jo802769y

Chiral thiophene-salen chromium complexes were investigated in their monomeric form as soluble catalysts in the enantioselective Henry reaction of several aldehydes. The anodic polymerization of one complex led to an insoluble powder that was successfully used as a heterogeneous catalyst for the transformation of 2-methoxybenzaldehyde with enantiomeric excesses up to 77%. The polymerized catalyst was recovered and also recycled in an original multisubstrate procedure

Full text of DOI:10.1021/jo802769y

the early rare-earth-based asymmetric catalysis developed and
optimized by Shibasaki’s group,5 zinc(II)-based catalysts have
also proved their efficiency in the enantioselective Henry
reaction.6 Copper-based catalysis has been also intensively
studied by different groups, leading to high levels of enantio-
control with a wide variety of ligands.7 More recently, other
metals such as cobalt8 or chromium9 associated with chiral salen
derivatives afforded ꢀ-nitroalcohol in satisfactory yields and
enantioselectivities. By fine-tuning the synthesis of the salen
ligands, up to 94% ee was recently reached with chromium
complexes.9b The Henry reaction has also been successfully
performed by using organocatalysts10 or enzymes.11 The
products of the enantioselective Henry reaction are highly
valuable synthons for the preparation of useful chiral intermedi-
ates in synthetic organic chemistry, and their preparation in high
yield and selectivity following a safe and economic procedure
is still challenging. In such a context and for matching at best
the “principles in green chemistry”,12 some examples have been
reported in which the asymmetric catalytic Henry reaction was
performed under heterogeneous conditions as an attempt to
improve the procedures by the efficient recovery and recycling
of the chiral catalysts. By immobilizing (S)-(-)-binol onto
nanocrystalline magnesium oxide, Choudary, Kantam, and co-
workers were able to carry out efficiently the asymmetric
nitroaldol reaction using five times the same batch of catalyst
without noting any loss in activity nor in enantioselectivity.13
Chiral lanthanum-lithium-binaphthol complexes were also
bonded to silica and MCM-41, and they proved to be as efficient
as their homogeneous counterparts in terms of enantioselectiv-
ity.14 A chiral copper acetate complex tethered to poly(ethylene
glycol) was also used in a recycling procedure, demonstrating
a high activity and stability.15
New Chiral Thiophene-Salen Chromium
Complexes for the Asymmetric Henry Reaction
Ana¨ıs Zulauf, Mohamed Mellah,* and Emmanuelle Schulz*
Equipe de Catalyse Mole´culaire, ICMMO, UMR CNRS 8182,
UniVersite´ Paris-Sud 11, 91405 Orsay cedex, France
emmaschulz@icmo.u-psud.fr
ReceiVed December 19, 2008
Chiral thiophene-salen chromium complexes were inves-
tigated in their monomeric form as soluble catalysts in the
enantioselective Henry reaction of several aldehydes. The
anodic polymerization of one complex led to an insoluble
powder that was successfully used as a heterogeneous
catalyst for the transformation of 2-methoxybenzaldehyde
with enantiomeric excesses up to 77%. The polymerized
catalyst was recovered and also recycled in an original
multisubstrate procedure.
We have recently proposed a method for the polymerization
of new chiral thiophene-containing salen ligands via the
The nitroaldol reaction (or Henry reaction, discovered in
1895)1 is an efficient method for the formation of a C-C bond
between a nucleophile coming from a nitroalkane with a
carbonyl electrophile from an aldehyde or a more challenging
ketone. The first asymmetric catalytic version of this transfor-
mation was reported by Shibasaki et al.,2 nearly one century
later. An optically active lanthanum alkoxide from binaphthol
was used, exhibiting a sufficient basic character for the in situ
generation of a nitronate species and leading to high enantio-
meric excesses for the targeted nitroaldol derivatives. Since this
first issue, numerous successful catalytic asymmetric Henry
reactions have been described in the literature, and these results
have been discussed in recent exhaustive reviews.3,4 Apart from
(6) (a) Trost, B. M.; Yeh, V. S. C. Angew. Chem., Int. Ed. 2002, 41, 861–
863. (b) Trost, B. M.; Yeh, V. S. C.; Ito, H.; Bremeyer, N. Org. Lett. 2002, 4,
2621–2623. (c) Gao, J.; Martell, A. E. Org. Biomol. Chem. 2003, 1, 2801–2806.
(d) Palomo, C.; Oiarbide, M.; Laso, A. Angew. Chem., Int. Ed. 2005, 44, 3881–
3884. (e) Liu, S.; Wolf, C. Org. Lett. 2008, 10, 1831–1834.
(7) (a) Christensen, C.; Juhl, K.; Hazell, R. G.; Jørgensen, K. A. J. Org.
Chem. 2002, 67, 4875–4881. (b) Xiong, Y.; Wang, F.; Huang, X.; Wen, Y.;
Feng, X. Chem.sEur. J. 2007, 13, 829–833. (c) Bandini, M.; Piccinelli, F.;
Tommasi, S.; Umani-Ronchi, A.; Ventrici, C. Chem. Commun. 2007, 616–618.
(d) Arai, T.; Takashita, R.; Endo, Y.; Watanabe, M.; Yanagisawa, A. J. Org.
Chem. 2008, 73, 4903–4906. (e) Ma, K.; You, J. Chem.sEur. J. 2007, 13, 1863–
1871.
(8) Kogami, Y.; Nakajima, T.; Ikeno, T.; Yamada, T. Synthesis 2004, 1947–
1950.
(9) (a) Kowalczyk, R.; Sidorowicz, Ł.; Skarz˙ewski, J. Tetrahedron: Asym-
metry 2007, 18, 2581–2586. (b) Kowalczyk, R.; Kwiatkowski, P.; Skarz˙ewski,
J.; Jurczak, J. J. Org. Chem. 2009, 74, 753–756.
(10) (a) Chinchilla, R.; Na´jera, C.; Sa´nchez-Agullo´, P. Tetrahedron: Asym-
metry 1994, 5, 1393–1402. (b) Sohtome, Y.; Hashimoto, Y.; Nagasawa, K. Eur.
J. Org. Chem. 2006, 2894–2897. (c) Marcelli, T.; van der Haas, R. N. S.; van
Maarseveen, J. H.; Hiemstra, H. Angew. Chem., Int. Ed. 2006, 45, 929–931.
(11) Gruber-Khadjawi, M.; Purkarthofer, T.; Skrane, W.; Griengl, H. AdV.
Synth. Catal. 2007, 349, 1445–1450.
(12) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice;
Oxford University Press: Oxford, 1988; p 30.
(13) Choudary, B. M.; Ranganath, K. V. S.; Pal, U.; Kantam, M. L.; Sreedhar,
B. J. Am. Chem. Soc. 2005, 127, 13167–13171.
(14) Bhatt, A. P.; Pathak, K.; Jasra, R. V.; Kureshy, R. I.; Khan, N.-u. H.;
Abdi, S. H. R. J. Mol. Catal. A: Chem. 2006, 244, 110–117.
(15) Bandini, M.; Benaglia, M.; Sinisi, R.; Tommasi, S.; Umani-Ronchi, A.
Org. Lett. 2007, 9, 2151–2153.
(1) Henry, L. C. R. Hebd. Seances Acad. Sci. 1895, 120, 1265–1267.
(2) Sasai, H.; Suzuki, T.; Arai, S.; Arai, T.; Shibasaki, M. J. Am. Chem.
Soc. 1992, 114, 4418–4420.
(3) Boruwa, J.; Gogoi, N.; Saikia, P. P.; Barua, N. C. Tetrahedron: Asymmetry
2006, 17, 3315–3326.
(4) Palomo, C.; Oiarbide, M.; Laso, A. Eur. J. Org. Chem. 2007, 2561–274.
(5) (a) Shibasaki, M.; Sasai, H.; Arai, T. Angew. Chem., Int. Ed. Engl. 1997,
36, 1236–1256, and literature cited therein. See also: (b) Sohtome, Y.; Kato,
Y.; Handa, S.; Aoyama, N.; Nagawa, K.; Matsunaga, S.; Shibasaki, M. Org.
Lett. 2008, 10, 2231–2234. (c) Tosaki, S. Y.; Hara, K.; Gnanadesikan, V.;
Morimoto, H.; Harada, S.; Sugita, M.; Yamagiwa, N.; Matsunaga, S.; Shibasaki,
M. J. Am. Chem. Soc. 2006, 128, 11776–11777. (d) Saa, J. M.; Tur, F.; Gonzalez,
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2242 J. Org. Chem. 2009, 74, 2242–2245
10.1021/jo802769y CCC: $40.75 2009 American Chemical Society
Published on Web 02/11/2009

TABLE 1. Asymmetric Henry Reaction Catalyzed by Complexes
1-4
iPr2EtN
time
(h)
T
(°C)
yield 10
ee 10
entry
cat
(equiv)a
(%)b
(%)c
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
2
2
2
3
4
1
1
1
24
24
24
48
48
72
48
24
48
48
24
24
24
48
20
20
20
74
83
61
77
92
74
79
70
91
75
86
74
88
82
34
47
69
74
85
79
82
54
81
81
49
32
74
73
0.5
0.05
1
1
1
0.5
1
1
0.5
1
1
-20
-40
-78
-40
20
-40
-40
20
FIGURE 1. Structures of thiophene salen complexes 1-4.
SCHEME 1. Synthetic Pathway for the Preparation of
Chromium Complex 3
9
10
11
12
13d
14d
20
20
-40
1
1
a The amount of base is calculated according to the quantity of
aldehyde. b Isolated yield. c Enantiomeric excesses are determined by
chiral HPLC analyses (see Supporting Information). d Reaction run in
MTBE.
cyclohexane-1,2-diamine yielded the new chiral ligand 8 that
was readily transformed in the corresponding chromium com-
plex 3, following a procedure described by Jacobsen.21
All chromium complexes were then evaluated in their capacity
to promote the enantioselective nitroaldol reaction. Complex 1
was first tested for the transformation of 2-methoxybenzaldehyde
9 with nitromethane into 1-(2-methoxyphenyl)-2-nitroethanol
10 in dichloromethane (DCM). Catalyst 1 afforded the desired
compound 10 with 34% ee in 74% isolated yield when the
reaction was performed at room temperature in the presence of
1 equiv of diisopropylethylamine (Table 1, entry 1). A similar
result was obtained when triethylamine was used as the base,
whereas K3PO4 led to a much less efficient catalytic system
(19% yield and 9% ee). The decrease of the amount of base
allowed interestingly an enhancement in the reaction selectivity
(Table 1, entries 2 and 3), albeit accompanied by a loss of
activity if 5 mol % of base was used. The reaction temperature
had an important effect on both the activity and enantioselec-
tivity of the transformation (Table 1, entries 4-7). The best
compromise was obtained by working at -40 °C using 1 equiv
of base. Product 10 was indeed obtained with a very high
isolated yield and 85% ee after 48 h reaction time (Table 1,
entry 5). A further decrease in the temperature (Table 1, entry
6) or using a lower amount of base (entry 7) did not improve
the results.
Complex 2, bearing sterically hindered alkyl-donating groups
on the third positions of the thiophene ring proved to be a more
enantioselective catalyst for the preparation of compound 10 at
ambient temperature than complex 1 (Table 1, compare entries
1 and 8). Unfortunately, these results were not improved by
lowering the temperature (Table 1, compare entries 5 and 7 to
entries 9 and 10). Complexes 3 and 4 promoted also efficiently
the Henry reaction of 2-methoxybenzaldehyde with nitromethane,
albeit without leading to enhanced enantioselectivities. Different
solvents were then investigated for performing this transforma-
tion. Similar reactions were run with complex 1 in methyl tert-
electrochemical oxidation of their metal complexes (Co, Cu,
Cr, and Ni).16 Various chromium-containing polymers were thus
prepared and used to promote the asymmetric Diels-Alder
reaction between Danishefsky’s diene and different aldehydes.17
A procedure for the reuse of these heterogeneous catalysts was
optimized, and they gratifyingly exhibited a very high stability,
allowing their recycling in an original multisubstrate process.18
We report here the efficient use of monomeric chiral thiophene-
salen chromium complexes as soluble catalysts in the Henry
reaction under homogeneous conditions. After anodic polym-
erization, one complex was engaged as insoluble powder to
catalyze the asymmetric nitroaldol reaction under heterogeneous
conditions. The reuse of this catalyst is also described.
Thiophene-salen chromium complexes 1, 2, and 4 were
synthesized as we previously reported.18 Another new complex
was prepared, compound 3, in which the alkyl groups on the
thiophene rings are placed in the fourth position for an
evaluation of steric effects on the enantioselectivity of the
catalytic transformation compared to complex 2 (Figure 1).
Starting from octylthiophene 5, 4,4,5,5-tetramethyl-2-(4-
octylthiophen-2-yl)-[1,3,2]dioxaborolane 6 was almost selec-
tively prepared in the presence of LDA as base19 with
2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane at low
temperature (Scheme 1). Compound 6 was isolated with 60%
yield and then treated in a Suzuki-Miyaura coupling20 with
5-bromo-3-tert-butyl-2-hydroxybenzaldehyde to yield 7 in a high
yield after purification. A condensation reaction with (S,S)-
(16) Voituriez, A.; Mellah, M.; Schulz, E. Synth. Met. 2006, 156, 166–175.
(17) Mellah, M.; Ansel, B.; Patureau, F.; Voituriez, A.; Schulz, E. J. Mol.
Catal. A: Chem. 2007, 272, 20–25.
(18) Zulauf, A.; Mellah, M.; Guillot, R.; Schulz, E. Eur. J. Org. Chem. 2008,
2118–2129.
(19) Jayakannan, M.; Van Hal, P. A.; Janssen, R. A. J. J. Polym. Sci. Part
A: Polym. Chem. 2002, 40, 251–261.
(20) Mohanakrishnan, A. K.; Hucke, A.; Lyon, M. A.; Lakshmikantham,
M. V.; Cava, M. P. Tetrahedron 1999, 55, 11745–11754.
(21) Schaus, S. E.; Branålt, J.; Jacobsen, E. N. J. Org. Chem. 1998, 63, 403–
405.
J. Org. Chem. Vol. 74, No. 5, 2009 2243

TABLE 2. Asymmetric Henry Reaction with Different Aldehydes
Aliphatic aldehydes proved to be more reactive substrates, and
the corresponding nitroalcohols were isolated in up to 92% yield
(Table 2, entry 14). In each case, higher enantioselectivities were
observed when the reaction was performed in MTBE instead
of DCM at room temperature, indicating probably a faster
competitive racemic reaction in this latter solvent. However,
running the reaction in DCM at -40 °C allowed the preparation
of the targeted products with the highest enantioselectivities (up
to 83% for the preparation of 1-hexyl-2-nitroethanol 12f) as
the best conditions for this homogeneous reaction. Substrate
11c was an exception since better results were obtained in
MTBE (Table 2, entry 10).
Catalyzed by Thiophene-Salen Complex 1
time
T
(°C)
yield 12 ee 12
entry
aldehyde R ) solvent (h)
(%)a
(%)b
1
2
3
DCM
MTBE 24 12a
DCM
DCM
24
20
20
-40
39
37
62
25
34
62
11a Ph
48
24
4
5
6
20
20
-40
42
62
43
25
36
64
11b p-Cl-C6H4
11c p-NO2-C6H4
MTBE 24 12b
DCM
48
Taking into account these results, we attempted to perform
the Henry reaction under heterogeneous conditions, in the
presence of a polymerized catalyst, poly-1.22 This insoluble
complex was prepared by electrochemical oxidative polymer-
ization of monomer 1 in an undivided cell in the presence of
nBu4NBF4 as supporting electrolyte at a fixed intensity. We have
already reported this procedure in a previous paper.18 Poly-1
was recovered as an insoluble powder from the electrode surface
and used as catalyst to promote the Henry reaction of 2-meth-
oxybenzaldehyde 9 with nitromethane under heterogeneous
conditions. Reactions were performed either in dichloromethane
at -40 °C or in MTBE at room temperature. After complete
conversion of the substrate, the solution was removed from the
Schlenk tube through a filtration syringe, and the remaining
powdered catalyst was washed successively with DCM or
MTBE, water and then dried. New substrates were then added
to the same set of catalyst to evaluate its recycling ability. The
polymerized catalyst promoted satisfyingly the Henry reaction
in both solvents, and the expected products were recovered with
yields similar to those observed under homogeneous conditions.
It is noticeable that the polymerized catalyst does not show any
loss in activity compared to its homogeneous counterpart, as is
often the case for heterogeneous catalysts. Following experi-
ments were then performed to confirm that the transformation
was promoted by the catalyst under heterogeneous conditions.
Samples of poly-1 were stirred for 16 h in DCM and in MTBE
in the presence of the base and nitromethane. The solutions were
then removed from the insoluble powder by filtration. 2-Meth-
oxybenzaldehyde was added to these solutions that were further
stirred for an additional 24 h at room temperature. After classical
workup, only traces of alcohol 10 were obtained from the MTBE
solution. Accordingly, 10 was isolated in 33% yield as racemic
compound from the DCM solution. These results indicate that
no chiral complex was leaching from the polymer into the
solution as asymmetric catalyst for the transformation. The first
use of poly-1 in both solvents afforded thus the expected
products with high enantioselectivity values, showing however
a slight decrease compared to the results obtained under
homogeneous conditions. These polymers were reused four
times, but each recycling was accompanied with a diminution
of the yield and of the enantioselectivity (see Scheme 2).
7
8
9
10
DCM
24
20
20
-40
-40
66
77
87
81
<5
36
15
40
MTBE 24 12c
DCM 48
MTBE 48
11
DCM
24
20
20
-40
13
34
29
18
48
65
12 11d p-MeO-C6H4 MTBE 24 12d
13
DCM
DCM
MTBE 24 12e
DCM
48
24
14
20
20
-40
92
79
59
64
69
80
15 11e cyclohexyl
16
48
17
DCM
MTBE 24 12f
DCM 48
24
20
20
-40
63
33
72
59
69
83
18 11f C6H13
19
a Isolated yield. b Enantiomeric excesses are determined by chiral
HPLC analyses (see Supporting Information).
butyl ether (MTBE), and a highly improved enantioselectivity,
at least at room temperature (up to 74% ee), was obtained,
compared to the reaction in DCM (Table 1, compare entries 1
and 13). In this case, however, the temperature showed no
influence on the enantioselectivity (Table 1, compare entries
13 and 14). We further demonstrated that the uncatalyzed
nitroaldol reaction between nitromethane and 9 occurred at room
temperature. Racemic product 10 was indeed isolated with 37%
yield when the reaction was run in DCM, whereas less than
5% was produced in MTBE. These control experiments are thus
in total accordance with the results of the catalyzed transforma-
tions and can explain the low enantioselectivity observed in
DCM. Indeed, the competitive noncatalyzed racemic reaction
is probably favored in this more polar solvent. The reaction
was also conducted in ethanol, and the targeted product 10 was
isolated in poor yield (33%) and enantioselectivity (23%)
together with the important formation of the corresponding
acetal 1-(1,1-diethoxyethyl)-2-methoxybenzene.
Complex 1 was thus chosen as the best catalyst of the series,
considering its easy synthesis and its efficiency and further
involved in the transformation of various substrates for widening
the scope of its application. The transformation of various
aldehydes with nitromethane in the presence of 2 mol % of
catalyst 1 was investigated, and the reactivity and enantiose-
lectivity of the catalyst were compared at different temperatures
in DCM or in MTBE. In each case, the catalyst proved to be
active for promoting the transformations, which led in general
to better enantioselectivities at room temperature in MTBE
compared to those obtained in DCM.
The versatility of poly-1 as catalyst was demonstrated in a
multisubstrate procedure. New structurally different aldehydes
were introduced at each recycling of the polymer catalyst (see
Table 3) since the efficiency of the corresponding monomer
complex 1 was previously evaluated with different substrates
in homogeneous conditions.
Aromatic aldehydes were fairly transformed, but the corre-
sponding products were isolated in only modest yields, except
for 2-nitro-1-(4-nitrophenyl)ethanol (Table 2, entries 7 and 10).
(22) For a recent review on heterogeneous asymmetric catalysis promoted
by salen complexes, see: Baleiza˜o, C.; Garcia, H. Chem. ReV. 2006, 106, 3987–
4043.
2244 J. Org. Chem. Vol. 74, No. 5, 2009

SCHEME 2. Asymmetric Henry Reaction with
2-Methoxybenzaldehyde Catalyzed by poly-1 under
Heterogeneous ConditionssRecycling of the Catalyst
Experimental Section
Representative Procedure for Henry Reactions. Homoge-
neous Conditions. A Schlenk tube was charged with the catalyst
(2 mol %) and maintained under an argon atmosphere by three
successive vacuo-argon cycles. DCM or MTBE (4 mL), the
aldehyde (1 mmol), and nitromethane (2 mL, 37.5 mmol) were
introduced. In the case of reactions performed at low temperature,
the mixture was first cooled to the desired temperature, then
followed by the addition of a solution of diisopropylethylamine (1
or 0.5 equiv) in DCM or MTBE (4 mL). The resulting solution
was stirred for the specified amount of time. The solvents were
then removed under reduced pressure, and the residue was purified
by flash chromatography on silica gel for the determination of the
yield of the reaction and the enantiomeric excess of the product.
Heterogeneous Conditions. A Schlenk tube was charged with
the catalyst poly-1 (27.4 mg, 4 mol %), and the procedure is
analogous to that described above. After the resulting suspension
was stirred for the specified amount of time, it was then filtered
with a filtering syringe and the precipitate was thoroughly washed
twice with DCM or MTBE. The solvents of the combined filtrates
were removed under reduced pressure, and the residue was purified
by flash chromatography on silica gel for the determination of the
yield of the reaction and the enantiomeric excess of the product.
In the Schlenk tube, the powdered catalyst was washed with water
and THF or MTBE then dried under vacuum, and new substrates
and solvents were added for its recycling.
TABLE 3. Asymmetric Heterogeneous Henry Reaction with
Different Aldehydes Catalyzed by poly-1sMultisubstrate Procedurea
product cycle yield (%)b ee (%)c cycle yield (%)b ee (%)c
10
12e
12f
1st
2nd
3rd
92
66
13
54
51
38
4th
5th
6th
77
27
11
62
52
47
Preparation of (R)-1-(2-Methoxyphenyl)-2-nitroethanol 10.
Solvent for flash chromatography: pentane/diethyl ether 4/1. Yel-
lowish oil, [R]20D -36.2 (c 1.01, CHCl3) for 74% ee, lit9 [R]D +42.3
(c 1.1, CH2Cl2) for 73% ee material. The ee was determined by
a Reaction run in MTBE, 4 mol % of poly-1, 20 °C, 24 h. b Isolated
yield. c Enantiomeric excesses are determined by chiral HPLC analyses.
HPLC analysis using an IB column (flow rate ) 1.0 mL ·min-1
;
The recycling procedure under these conditions afforded all
expected products with satisfying yields (except for compound
12f as volatile species) that were interestingly easily isolated
without contamination with any product traces from the preced-
ing run at each cycle. The enantioselectivity values were similar
to that obtained under homogeneous conditions and remained
remarkably stable by comparing each entry and the correspond-
ing run of the preceding sequence.
New chiral chromium Schiff base complexes substituted by
thiophene units at the 5,5′-positions of the phenolic rings were
thus efficiently used as enantioselective catalysts for the Henry
reaction between nitromethane and various aldehydes. The
desired nitroaldols were produced with enantiomeric excesses
up to 83% ee. These results compare well in terms of
enantioselectivity with those obtained by the best complexes
described in the literature for this transformation. A correspond-
ing electrogenerated polymer, poly-1, promoted also successfully
the heterogeneous Henry reaction, and this catalyst was recycled
four times, albeit with a slight loss in efficiency. The same
catalyst batch was engaged in an original multisubstrate
procedure and used also many times by adding a structurally
different aldehyde at each recycling. As far as we know, this
report is the first use of a chiral heterogenized salen complex
active in the Henry reaction.
90% hexane, 10% isopropanol, λ ) 254 nm), which resolved both
enantiomers (tR ) 9.3 min, tS ) 10.2 min). The absolute stereo-
chemistry was assigned as R based on comparison of the measured
1
rotation with the literature value: H NMR (300 MHz, CDCl3) δ
7.44 (dd, J ) 1.5, 7.5 Hz, 1H), 7.32 (ddd, J ) 1.5, 7.9, 9.4 Hz,
1H), 7.00 (dd, J ) 0.9, 7.5 Hz, 1H), 6.91 (dd, J ) 0.9, 8.1 Hz,
1H), 5.62 (dd, J ) 3.2, 9.2 Hz, 1H), 4.63 (dd, J ) 3.2, 13.0 Hz,
1H), 4.55 (dd, J ) 9.2, 13.0 Hz, 1H), 3.88 (s, 3H), 3.47 (br s, 1H);
13C NMR (90 MHz, CDCl3) δ 155.8, 129.6, 127.0, 126.0, 120.9,
110.4, 79.7, 67.5, 55.2; MS (CI) 215 [MNH4+] (100), 197 (14),
137 (14).
Acknowledgment. The CNRS, the Ministe`re de l’Enseigne-
ment Supe´rieur et de la Recherche, and the program “Chimie
et De´veloppement Durables” du CNRS are acknowledged for
financial support. X. Hong is sincerely thanked for technical
assistance.
Supporting Information Available: Complete experimental
procedures, characterization, and ee determinations. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO802769Y
J. Org. Chem. Vol. 74, No. 5, 2009 2245