Sodium tetramethoxyborate: An efficient catalyst for Michael additions of stabilized carbon nucleophiles

Article abstract of DOI:10.1021/jo701354c

(Chemical Equation Presented) Sodium tetramethoxyborate, easily prepared by reaction of inexpensive sodium borohydride with methanol, possesses a suitable combination of a Lewis base and a Lewis acid to catalyze Michael reactions at room temperature under

Full text of DOI:10.1021/jo701354c

Sodium Tetramethoxyborate: An Efficient
Catalyst for Michael Additions of Stabilized
Carbon Nucleophiles
although the simplest and most stable ones (triphenylphospine,
for example) are not reactive enough. The superior chemical
profile of some ruthenium catalysts is due to a combination of
the presence of phospines and the metal center, the latter acting
3
as a Lewis acid. Encouraged by this idea, we thought that other
†
†
Araceli G. Campa n˜ a, Noelia Fuentes,
combinations of a base and a Lewis acid might result in simple,
inexpensive, practical, air-stable catalysts that might work under
practically neutral conditions. Boron derivatives fit these
requirements, their catalytic activity toward the Michael addition
of stabilized carbon nucleophiles having been observed previ-
‡
‡
†
Enrique G o´ mez-Bengoa, Cristina Mateo, J. Enrique Oltra,
,
‡,§
,†
Antonio M. Echavarren,* and Juan M. Cuerva*
Department of Organic Chemistry, Faculty of Sciences, Campus
FuentenueVa s/n, UniVersity of Granada, E-18071 Granada,
Spain, Department of Organic Chemistry, UniVersidad
Aut o´ noma de Madrid, Cantoblanco, 28049 Madrid, Spain, and
Institute of Chemical Research of Catalonia (ICIQ), AV. Pa ¨ı sos
Catalans 16, 43007 Tarragona, Spain
3
7
ously by us and other authors. We describe here our results
in a new and efficient Michael addition reaction catalyzed by
simple NaB(OMe)4 (1).
Boron derivative 1, a stable white solid, was easily prepared
8
aechaVarren@iciq.es; jmcuerVa@ugr.es
ReceiVed June 29, 2007
by reaction of NaBH4 with methanol. Its structure was
confirmed by the singlet signal at 3.02 ppm (CD3OD) observed
in its B NMR spectrum, which is similar to that described for
the closely related reagent LiB(OMe)4 (2.7 ppm, CD3OD).9
1
1
,10
We began to evaluate the catalytic activity of 1 by stirring a
substoichiometric proportion of this catalyst (3 mol %) with a
mixture of malonate 2 (1 mmol) and methyl vinyl ketone (3)
(
1.1 mmol) in acetonitrile at room temperature (eq 1).11 In this
manner, we obtained a 100% yield of the Michael addition
product 4.
Sodium tetramethoxyborate, easily prepared by reaction of
inexpensive sodium borohydride with methanol, possesses
a suitable combination of a Lewis base and a Lewis acid to
catalyze Michael reactions at room temperature under
practically neutral conditions. This reaction provides good
to excellent yields of Michael addition products from a broad
scope of Michael donor and Michael acceptor reagents.
This excellent result prompted us to study the scope of the
reaction using different Michael donors (5-15) and Michael
acceptors (3, 16-18) (Chart 1). The results obtained are
summarized in Table 1.
Michael reaction is one of the cornerstones of organic
synthesis and is widely used in C-C bond-forming reactions.
1
CHART 1
Nevertheless, conventional base-catalyzed processes are usually
affected by undesirable side reactions such as retrograde Michael
1
reactions or polymerizations of the Michael acceptor. Many
catalysts working under neutral conditions have been developed
2
to solve these problems. Transition metals play an important
role in this regard, especially Ru-based catalysts.3,4 It should
be noted that some phosphines are excellent neutral catalysts
3
,5,6
for the Michael addition of stabilized carbon nucleopiles,
†
University of Granada.
Universidad Aut o´ noma de Madrid.
Institute of Chemical Research of Catalonia.
‡
§
(
1) (a) Jung, M. E. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 4, Chapter 1.1 (b) Smith,
M. B.; March, J. March’s AdVanced Organic Chemistry, 6th ed.; Wiley:
New York, 2007; pp 1105-1110.
(2) For selected examples, see: (a) Antonioletti, R.; Bonadies, F.;
Monteagudo, E. S.; Scettri, A. Tetrahedron Lett. 1991, 32, 5373-5374.
b) Ranu, B. C.; Bhar, S.; Sarkar, D. C. Tetrahedron Lett. 1991, 32, 2811-
812. (c) Bonadies, F.; Lattanzi, A.; Orelli, L. R.; Pesci, S.; Scettri, A.
(
2
Tetrahedron Lett. 1993, 34, 7649-7650. (d) Nelson, J. H.; Howells, P. N.;
DeLullo, G. C.; Landen, G. L. J. Org. Chem. 1980, 45, 1246-1249. (e)
Keller, E.; Feringa, B. L. Tetrahedron Lett. 1996, 37, 1879-1882. (f)
Veldurthy, B.; Clacens, J. M.; Figueras, F. AdV. Synth. Catal. 2005, 347,
As we expected, with the use of different combinations of
Michael donors and acceptors, the reaction gave good to
excellent yields of the corresponding Michael addition products.
Moreover, the presence of common functional groups in the
Michael donor such as alkenes, terminal alkynes, nitriles,
7
67-771.
3) G o´ mez-Bengoa, E.; Cuerva, J. M.; Mateo, C.; Echavarren, A. M. J.
Am. Chem. Soc. 1996, 118, 8553-8565.
(
1
0.1021/jo701354c CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/19/2007
J. Org. Chem. 2007, 72, 8127-8130
8127

TABLE 1. NaB(OMe)4-Catalyzed Michael Addition of Activated Carbon Nucleophilesa
a
Unless otherwise stated the reactions were carried out in the presence of 3 mol % of NaB(OMe)4 using 2 equiv of Michael acceptor and 1 equiv of
b
c
d
Michael donor in MeCN at 23 ×bcC. The reaction was carried out using 1 equiv of Michael acceptor. 20b and 21 were obtained as a 2:1 mixture. 31b
and 32 were obtained as a 4:1 mixture. 33a was obtained as a 3:2 mixture of epimers at C-1. 33b was obtained as a 1:1 mixture of epimers at C-1.
e
f
epoxides, or alcohols did not interfere with the catalyst (entries
-9, 14, and 15). Entries 1 and 2 show that either simple or
double additions can be alternatively achieved by choosing the
suitable proportion of the Michael acceptor reagent. Double
addition products, however, can undergo subsequent sodium
tetramethoxyborate-catalyzed intramolecular aldol reactions
yielding cyclic products such as 20, 31, and 33 (entries 2, 12,
and 13) with modest stereoselection. Michael donor 15, which
5
(6) For recent reports, see: (a) Wroblewski, A. E.; Bansal, V.; Kisanga,
P.; Verkade, J. G. Tetrahedron 2003, 59, 561-566. (b) Lumbierres, M.;
Marchi, C.; Moreno-Ma n˜ as, M.; Sebasti a´ n, R. M.; Vallribera, A.; Lago,
E.; Molins, E. Eur. J. Org. Chem. 2001, 2321-2328. (c) Gimbert, C.;
Lumbierres, M.; Marchi, C.; Moreno-Ma n˜ as, M.; Sebasti a´ n, R. M.;
Vallribera, A. Tetrahedron 2005, 61, 8598-8605. (d) Reddy, C.; Reddy,
V.; Verkade, J. G. J. Org. Chem. 2007, 72, 3093-3096.
(
4) (a) Murahashi, S.-I.; Naota, T.; Taki, H.; Mizuno, M.; Takaya, H.;
Komiya, S.; Mizuho, Y.; Oyasato, N.; Hiraoka, M.; Hirano, M.; Fukuoka,
A. J. Am. Chem. Soc. 1995, 117, 12436-12451. (b) Alvarez, S. G.;
Hasegawa, S.; Hirano, M.; Komiya, S. Tetrahedron Lett. 1998, 39, 5209-
5
3
212. (c) Picquet, M.; Bruneau, C.; Dixneuf, P. H. Tetrahedron, 1999, 55,
937-3948. (d) Watanabe, M.; Ikagawa, A.; Wang, H.; Murata, K.; Ikariya,
(7) Abraham, S.; Sundararajan, G. Tetrahedron 2006, 62, 1474-1478.
(8) (a) Davis R. E.; Gottbrath, J. A. J. Am. Chem. Soc. 1962, 84, 895-
898. (b) Golden, J. H.; Schreier, C.; Singaram, B.; Williamson, S. M. Inorg.
Chem. 1992, 31, 1533-1535.
T. J. Am. Chem. Soc. 2004, 126, 11148-11149. (e) Guo, R.; Morris, R.
H.; Song, D. J. Am. Chem. Soc. 2005, 127, 516-517.
(
5) For a recent review, see: Methot, J. L.; Roush, W. R. AdV. Synth.
Catal. 2004, 346, 1035-1050.
(9) Hermanek, S. Chem. ReV. 1992, 92, 325-362.
8
128 J. Org. Chem., Vol. 72, No. 21, 2007

giving 2.1 g of 1 (100% yield): white solid; 11B NMR (128.32
MHz, CD OD) δ 3.02 (s).
Synthesis of Michael Donor 14. A sample of (Z)-BrCH
has two nucleophilic positions, only gave Michael adduct 35.
This chemoselectivity can be related to differences in acidity
3
12
between the 1,3-diester (pKa ) 16, DMSO) and alcohol (pKa
2
CHd
CHCH OCO Et (500 mg, 2.2 mmol) was added to a mixture of
20
29, DMSO)12 functionalities, the Michael reaction being
)
2
2
NaH (106 mg, 4.4 mmol) and dimethyl malonate (871 mg, 6.6
mmol) in DMF (40 mL). This solution was stirred at room
effective from the most acidic one. Nevertheless, a direct
correlation between the acidity of the Michael donor reagent
and the reaction rate is not always to be found. In fact bis-
2
temperature for 4 h and then diluted with Et O, washed with 2 N
HCl, and dried with anhydrous Na SO , and the solvent was
2
4
1
2
(
phenylsulphonyl)methane 11 (pKa ) 12.25, DMSO), which
removed. The residue was submitted to flash chromatography (7:3
1
2
1
is more acidic than dimethyl malonate 5 (pKa ) 16, DMSO),
hexane/EtOAc) to give 14 (442 mg, 72%) as a colorless oil; H
required a much longer reaction time. Control experiments
showed that the reaction between malonate 5 (1.0 mmol) and
methyl vinyl ketone 3 (2.0 mmol) was complete in 30 min (20-
NMR (500 MHz, CDCl
3
) δ 5.60-5.54 (m, 1H), 5.52-5.47 (m,
H), 4.59 (d, J ) 5.5 Hz, 2H), 4.08 (q, J ) 7.1 Hz, 2H), 3.64 (s,
H), 3.34 (t, J ) 7.4 Hz, 1H), 2.61 (t, J ) 6.8 Hz, 2H), 1.19 (t, J
7.1 Hz, 3H); 13C NMR (125 MHz, CDCl
; DEPT) δ 169.2 (C),
55.3 (C), 130.3 (CH), 126.5 (CH), 64.2 (CH ), 63.3 (CH ), 52.8
); FABHRMS calcd for
Na m/z 297.0950, found m/z 297.0949.
-Catalyzed Michael
1
6
)
1
3
2
1, 100%), whereas a 30-min reaction between disulfone 11
2
2
(1.0 mmol) and the same Michael acceptor (3, 2.0 mmol) only
(CH
3
), 51.5 (CH), 27.2 (CH
2
), 14.46 (CH
3
gave traces of the Michael adduct 30. This intriguing behavior
suggests that NaB(OMe)4-catalyzed Michael reactions are not
C
12
H O
18 7
Model Procedure for the NaB(OMe)
4
simple base-catalyzed reactions. It is possible that a boron
Reaction. The Michael acceptor (1.0 or 2.0 mmol, see Table 1)
was added to a mixture of Michael donor (1.0 mmol) and NaB-
derivative, probably B(OMe)3,1
3,14
is acting as a Lewis acid
activating not only the Michael acceptor but also the Michael
4
(OMe) (0.03 mmol, 3 mol %) in MeCN (3 mL) at room
donor toward the base,15 thus facilitating an efficient Michael
temperature. The resulting solution was stirred at room temperature
for 3-24 h (see Table 1). The solvent was removed, and the residue
was chromatographed (hexane/EtOAc mixtures) to give adducts 4,
addition reaction even with less acidic substrates as diesters.
In summary, NaB(OMe)4 (1), easily prepared by reaction of
inexpensive sodium borohydride with methanol, possesses an
ideal combination of a Lewis base and a Lewis acid to catalyze
Michael additions of stabilized carbon nucleophiles. The reaction
takes place at room temperature under practically neutral
conditions and affords good to excellent yields of Michael
addition products with a broad scope of Michael donors and
acceptors. This procedure gives access to a variety of function-
alized substrates of the type commonly used in studies about
new cyclization reactions. Moreover, the inclusion of chiral
ligands in the boron-based catalyst might lead to enantio-
selective Michael reactions.16 We are currently working toward
this goal.
1
9-35 at the yields indicated in eq 1 and Table 1.
Compound 28: colorless oil; H NMR (500 MHz, CDCl
.74 (s, 3H), 3.72 (s, 3H), 2.71 (dd, J ) 7.4, 4.2 Hz, 1H), 2.56 (dt,
1
3
) δ
3
J ) 17.8, 7.8 Hz, 1H), 2.40 (dt, J ) 17.8, 7.6 Hz, 1H), 2.26 (t, J
) 7.5 Hz, 2H), 2.22 (dd, J ) 14.9, 4.2 Hz, 1H), 2.13 (s, 3H), 1.98
1
3
(dd, J ) 14.9, 7.4 Hz, 1H), 1.27 (s, 3H), 1.24 (s, 3H); C NMR
(
(
(
125 MHz, CDCl
CH), 58.1 (C), 56.0 (C), 52.9 (CH
CH ), 30.1 (CH ), 27.5 (CH ), 24.9 (CH
Na m/z 309.1314, found m/z 309.1308.
3
; DEPT) δ 207.2 (C), 171.6 (C), 171.5 (C), 60.0
), 52.8 (CH ), 38.9 (CH ), 33.4
), 18.9 (CH ). FABHRMS
3
3
2
2
3
2
3
3
22 6
calcd for C14H O
Compound 31b. Minor diastereomer 31b was obtained as a 4:1
mixture with the known polyketone 32 and showed the following
NMR data: 1H NMR (300 MHz, CDCl
3
) δ 2.70 (dd, J ) 12.9, 3.7
Hz, 1H), 2.60 (ddd, J ) 13.9, 3.1, 2.7 Hz, 1H), 2.47-2.38 (m,
1
H), 2.31 (s, 3H), 2.16 (s, 3H), 2.14 (s, 3H), 1.85-1.65 (m, 2H),
.46 (td, J ) 14.0 , 4.1 Hz, 1H), 1.18 (s, 3H), 1.26-1.10 (m, 1H);
1
Experimental Section
1
3
C NMR (100 MHz, CDCl
C), 71.7 (C), 67.9 (C), 55.4 (CH), 38.5 (CH
CH ), 27.1 (CH ), 26.4 (CH ), 25.9 (CH ), 21.7 (CH
Na m/z 263.1259, found m/z 263.1258.
3
; DEPT) δ 211.5 (C), 206.2 (C), 205.1
), 31.7 (CH ), 29.2
); FABHRMS
(
(
2
3
General. Dry MeCN was obtained by distillation under Ar from
. Substances 917 and 15 were prepared according to known
18
2
2
3
3
3
CaH
2
20 4
calcd for C13H O
procedures. The following known compounds were isolated as pure
samples and showed NMR spectra identical to those in our
previously reported data: 4, 19, 20a-b, 21, 22, 23, 24, 25, 26, 29,
Compound 33b. Minor diastereomer 33b was obtained as a 1:1
1
mixture of epimers at C-1; colorless oil; one diastereomer: H NMR
3
(500 MHz, CDCl ) δ 3.77 (s, 3H), 2.68 (dd, J ) 9.0, 3.8 Hz, 1H),
3
3
0, 31a, 32, and 33a. The known compound 27 was isolated as a
19
pure sample and showed NMR spectra identical to those reported.
11
(16) For Lewis acid catalysis in Michael additions, see: (a) Bonadies,
F.; Lattanzi, A.; Orelli, L. R.; Pesci, S.; Scettri, A. Tetrahedron Lett. 1993,
B NMR was obtained at 128.32 MHz, and the chemical shifts
are in δ units relative to Et O-BF (0.0 ppm in CDCl ).
Synthesis of NaB(OMe) . A solution of NaBH (500 mg) in
MeOH (25 mL) was refluxed for 30 min. The solvent was removed,
2
3
3
3
4, 7649-7650. (b) Itoh, K.; Kanemasa, S. J. Am. Chem. Soc. 2002, 124,
4
4
13394-13395. (c) Nakajima, M.; Yamaguchi, Y.; Hashimoto, S. Chem.
Commun. 2001, 1596-1597. (d) Annamalai, V.; DiMauro, E. F.; Carroll,
P. J.; Kozlowski, M. C. J. Org. Chem. 2003, 68, 1973-1981. (e) Itoh, K.;
Oderaotoshi, Y.; Kanemasa, S. Tetrahedron: Asymmetry 2003, 14, 635-
639. (f) Nakajima, M.; Yamamoto, S.; Yamaguchi, Y.; Nakamura, S.;
Hashimoto, S. Tetrahedron 2003, 59, 7307-7313. (g) de Rosa, M.; Palombi,
L.; Acocella, M. R.; Fruilo, M.; Villano, R.; Soriente, A.; Scettri, A. Chirality
2003, 15, 579-583. (h) Watanabe, M.; Ikagawa, A.; Wang, H.; Murata,
K.; Ikariya, T. J. Am. Chem. Soc. 2004, 126, 11148-11149. (i) Inokuma,
T.; Hoashi, Y.; Takemoto, Y. J. Am. Chem. Soc. 2006, 128, 9413-9419.
(j) Liu, T.-Y.; Li, R.; Chai, Q.; Long, J.; Li, B.-J.; Wu, Y.; Ding, L.-S.;
Chen, Y.-C. Chem. Eur. J. 2007, 13, 319-327.
(
(
10) NaBH4 shows a quintuplet signal at -35.6 ppm (DMSO-d6).
11) We also carried out the same reaction using other solvents, but the
yields were lower. See Supporting Information.
12) Although significant differences in absolute values between pKa
(
values measured in DMSO or acetonitile are expected, the relative acidities
do not differ greatly. For that discussion and pKa values of organic
compounds in DMSO at 25 °C, see: Bordwell, F. G. Acc. Chem. Res. 1988,
2
1, 456-463.
-
(
13) A log K ) 5.62 was determined for the equilibrium between MeO
(17) Dang H-S.; Roberts, B. P. J. Chem. Soc., Perkin Trans. I 1993,
891-897.
-
+
B(OMe)3 and [B(OMe)4] in MeOH: (a) Gut, R. HelV. Chim. Acta 1964,
4
1
7, 2262-2278. (b) Hutton, W. C.; Crowell, T. I. J. Am. Chem. Soc. 1978,
00, 6904-6907.
(18) Michael donor 15 was obtained as 9:1 mixture of E:Z diastereo-
mers: Fern a´ ndez-Rivas, C.; M e´ ndez, M.; Nieto-Oberhuber, C.; Echavarren,
A. M. J. Org. Chem. 2002, 67, 5197-5201.
(14) NaBF4 is unable to promote the Michael reaction, ruling out the
role of sodium cation as Lewis acid.
15) Sulfones have been described as poor Lewis bases: Carr, R. V.;
Paquette, L. A. J. Am. Chem. Soc. 1980, 102, 853-855.
(19) Chande, M. S.; Khanwelkar, R. R. Tetrahedron Lett. 2005, 46,
7787-7792.
(
(20) Oppolzer, W.; F u¨ rstner, A. HelV. Chim. Acta 1993, 76, 2369-2337.
J. Org. Chem, Vol. 72, No. 21, 2007 8129

2
1
1
.49 (dd, J ) 3.8, 2.5 Hz, 1H), 2.45 (t, J ) 2.9 Hz, 1H), 2.29 (m,
4.06 (d, J ) 5.8 Hz, 2H), 3.70 (s, 6H), 2.62 (d, J ) 7.2 Hz, 2H),
2.44 (t, J ) 8.0 Hz, 2H), 2.12 (s, 3H), 2.11 (t, J ) 7.5 Hz, 2H);
H), 2.25 (s, 3H), 2.14 (s, 3H), 1.75 (t, J ) 13.5 Hz, 1H), 1.69-
13
13
.64 (m, 1H), 1.44-1.38 (m, 1H), 1.16 (s, 3H); C NMR (125
C NMR (125 MHz, CDCl ; DEPT) δ 207.4 (C), 171.5 (C), 134.2
3
MHz, CDCl
1.4 (C), 55.9 (CH), 53.0 (CH
CH ), 27.9 (CH ), 26.3 (CH ), 21.7 (CH
H NMR (500 MHz, CDCl ) δ 3.70 (s, 3H), 2.69 (dd, J ) 8.5, 4.0
3
; DEPT) δ 211.8 (C), 204.6 (C), 172.0 (C), 71.8 (C),
), 38.8 (CH ), 31.6 (CH ), 30.2
). Another diastereomer:
(
3
CH), 125.7 (CH), 63.3 (CH
6.8 (CH ), 30.1 (CH ), 26.8 (CH
Na m/z 295.1157, found m/z 295.1159.
2
), 57.1 (C), 52.6 (CH
3 2
), 38.7 (CH ),
6
3
2
3
2
3
2
); FABHRMS calcd for C13H O -
20 6
(
2
2
3
3
1
3
Hz, 1H), 2.52 (dd, J ) 4.0, 2.5 Hz, 1H), 2.42 (t, J ) 2.9 Hz, 1H),
2
1
Acknowledgment. This work was supported by the Spanish
Ministry of Science and Technology (Projects CTQ2005-
08402/BQU and CTQ2004-02869, Consolider Ingenio 2010,
Grant CSD2006-0003), the “Junta de Andaluc ´ı a” (PAI
group FQM339), and the ICIQ Foundation. We thank Dr. Ali
.32 (m, 1H), 2.24 (s, 3H), 2.17 (s, 3H), 1.82 (t, J ) 13.4 Hz, 1H),
.71-1.68 (m, 1H), 1.61-1.53 (m, 1H), 1.18 (s, 3H); 13C NMR
(
(
(
125 MHz, CDCl
C), 59.7 (C), 55.4 (CH), 53.0 (CH
CH
3
; DEPT) δ 211.7 (C), 204.2 (C), 172.3 (C), 71.6
), 37.9 (CH ), 30.2 (CH ), 29.7
), 22.1 (CH ); FABHRMS calcd for
Na m/z 279.1208, found m/z 279.1208.
3
2
3
2
), 27.8 (CH
2
), 25.9 (CH
3
3
C
13
H O
20 5
11
Haidour for his collaboration in B NMR experiments and
1
3
Compound 34: colorless oil; H NMR (500 MHz, CDCl ) δ
their English colleague Dr. J. Trout for revising the English
text. A.G.C. thanks the Spanish Ministry of Science and
Technology for a predoctoral grant enabling her to pursue these
studies.
5
4
2
1
2
.62-5.56 (m, 1H), 5.44-5.39 (m, 1H), 4.55 (d, J ) 6.8 Hz, 2H),
.08 (q, J ) 7.1 Hz, 2H), 3.61 (s, 6H), 2.60 (d, J ) 7.5 Hz, 2H),
.36 (t, J ) 8.2 Hz, 2H), 2.04 (t, J ) 8.2 Hz, 2H), 2.02 (s, 3H),
.19 (t, J ) 7.1 Hz, 3H); 13C NMR (125 MHz, CDCl
; DEPT) δ
07.1 (C), 171.3 (C), 155.2 (C), 128.5 (CH), 127.2 (CH), 64.2
), 63.1 (CH ), 56.8 (C), 52.7 (CH ), 38.7 (CH ), 31.9 (CH ),
0.0 (CH ), 26.9 (CH ), 14.4 (CH ); FABHRMS calcd for C16
3
(CH
2
2
3
2
2
Supporting Information Available: Additional experiments
and characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
3
3
2
3
24 8
H O -
Na m/z 367.1368, found m/z 367.1363.
1
Compound 35: colorless oil; H NMR (500 MHz, CDCl
3
) δ
5
.70 (dt, J ) 15.0, 5.7 Hz, 1H), 5.52 (dt, J ) 15.0, 5.7 Hz, 1H),
JO701354C
8
130 J. Org. Chem., Vol. 72, No. 21, 2007