An easy, efficient, and completely stereoselective synthesis of (E)-α,β-unsaturated esters via sequential aldol-type/elimination reactions promoted by samarium diiodide or chromium dichloride

Article abstract of DOI:10.1021/jo050655o

(E)-α,β-Unsaturated esters can be obtained with complete stereoselectivity by reaction of different aldehydes and ethyl clibromoacetate promoted by SmI₂ or CrCl₂. The transformation takes place as two sequential reactions: an aldol-type reaction and a β-elimination reaction.

Full text of DOI:10.1021/jo050655o

SCHEME 1. Synthesis of r,â-Unsaturated Esters 2
An Easy, Efficient, and Completely
Stereoselective Synthesis of
(E)-r,â-Unsaturated Esters via Sequential
Aldol-Type/Elimination Reactions
Promoted by Samarium Diiodide or
Chromium Dichloride
Jose´ M. Concello´n,* Carmen Concello´n, and
Carmen Me´jica
an important number of papers have described their
synthesis.4 However, in most of these methods, complete
control of the stereoselectivity of the carbon-carbon
double-bond formation remained unresolved,5 and in
other cases, tedious multipot procedures or starting
materials that are not readily available are necessary.
Previously, we have described the highly stereoselec-
tive preparation of (E)-R,â-unsaturated esters by treat-
ment of 2-halo-3-hydroxyesters with samarium diiodide6
or chromium dichloride.7 The starting 2-halo-3-hydroxy-
esters were obtained by successive generation of the
corresponding enolate derived from R-haloesters and
condensation with different aldehydes at low tempera-
ture. For this, a synthesis of R,â-unsaturated esters from
commercially available compounds would be necessary.
Our objective in this work is to describe an improved
method of the SmI2-promoted synthesis of (E)-R,â-
unsaturated esters with total stereoselectivity by using
more readily available starting compounds such as com-
mercial ethyl dibromoacetate and a variety of aldehydes.
This transformation takes place as a sequential process,
with the first step being an aldol-type reaction, followed
by a â-elimination reaction as the second step. Moreover,
we have developed another alternative method of this
sequential process by using CrCl2.
Departamento de Quı´mica Orga´nica e Inorga´nica, Facultad
de Quı´mica, Universidad de Oviedo, Julia´n Claverı´a,
8, 33071 Oviedo, Spain
jmcg@fq.uniovi.es
Received April 4, 2005
(E)-R,â-Unsaturated esters can be obtained with complete
stereoselectivity by reaction of different aldehydes and ethyl
dibromoacetate promoted by SmI2 or CrCl2. The transforma-
tion takes place as two sequential reactions: an aldol-type
reaction and a â-elimination reaction.
Sequential reactions constitute very attractive methods
due to their enormous simplicity. In recent years, sa-
marium diiodide has become among the most important
reagents for sequential or cascade reactions performed
under reducing conditions.1 Chromium dichloride has
been also used to perform highly selective sequential
reactions.2 However, to the best of our knowledge, no
cascade reactions from 2,2-dihaloesters to obtain R,â-
unsaturated esters by using SmI2 or CrCl2 have been
described.3
Treatment of a mixture of different aldehydes and
ethyl dibromoacetate with a solution of SmI2
(5 equiv)
in THF8 for 60 min at room temperature afforded, after
hydrolysis, the corresponding (E)-R,â-unsaturated esters
2 with total stereoselectivity and in high yield (Scheme
1).
(4) For recent reviews for the synthesis of R,â-unsaturated esters,
see: (a) Franklin, A. S. J. Chem Soc., Perkin Trans. 1 1998, 2451-
2465. (b) Shen, Y. Acc. Chem. Res. 1998, 31, 584-592. By using the
Wittig reaction: (c) Patil, V. J.; Ma¨vers, U. Tetrahedron Lett. 1996,
37, 1281-1284. (d) Suzuki, K.; Matsukura, H.; Matsuo, G.; Koshino,
H.; Nakata, T. Tetrahedron Lett. 2002, 43, 8653-8655. (e) Beji, F.;
Lebreton, J.; Villieras, J.; Amri, H. Synth. Commun. 2002, 32, 3273-
3278. (f) Huang, Z.; Sun, R. J. Chinese J. Org. Chem. 2002, 20, 1460-
1462. (g) Shen, Y.; Zhang, Y. Heteroatom Chem. 2003, 14, 276-279.
By using the Horner-Emmons reaction: (h) Ando, K. J. Org. Chem.
1999, 64, 6815-6821. (i) Ando, K. J. Org. Chem. 1997, 62, 1934-1939.
(j) Ando, K. Tetrahedron Lett. 1995, 36, 4105-4108. By using Heck
reaction: (k) Moreno-Man˜as, M.; Pe´rez, M.; Pleixants, R. Tetrahedron
Lett. 1996, 37, 7449-7452. (l) Beller, M.; Riermeier, T. H. Tetrahedron
Lett. 1996, 37, 635-638.
(5) One of the great challenges associated with the Wittig reaction
in the preparation of R,â-unsaturated esters relates to stereocontrol:
Maryanoff, B. E.; Reitz, A. B. Chem. Rev. 1989, 89, 863-927.
(6) Concello´n, J. M.; Pe´rez-Andre´s, J. A.; Rodriguez-Solla, H. Angew.
Chem. 2000, 112, 2866-2868; Angew. Chem., Int. Ed. 2000, 39, 2773-
2775.
Moreover, R,â-unsaturated esters are attractive start-
ing materials in organic synthesis, and for this reason,
(1) To see the pioneering paper on synthetic application of SmI2:
(a) Namy, J. L.; Girard, P.; Kagan, H. B. Nouv. J. Chim. 1977, 1, 5-7.
(b) Namy, J. L.; Girard, P.; Kagan, H. B.; Caro, P. E. Nouv. J. Chim.
1981, 5, 479-484. For recent reviews of sequencing reactions with
SmI2, see: (c) Molander, G. A.; Harris, C. R. Tetrahedron 1998, 54,
3321-3354. (d) Molander, G. A.; Harris, C. R. Chem. Rev. 1996, 92,
307-338. For recent reviews of synthetic applications of SmI2, see:
(e) Krief, A.; Laval, A. M. Chem. Rev. 1999, 99, 745-777. (f) Steel, P.
G. J. Chem. Soc., Perkin Trans. 1 2001, 2727-2751. (g) Kagan H. B.
Tetrahedron 2003, 59, 10351-10372. (h) Concello´n J. M.; Rodr´ıguez-
Solla, H. Chem. Soc. Rev. 2005, 33, 599-609. (i) Dahle´n A.; Hilmerson,
G. Eur. J. Inorg. Chem. 2004, 3393-3403.
(2) (a) Takai, K. Org. React. 2004, 64, 253-612. (b) Liu, Y.; Wu, H.;
Zhang, Y. Synth. Commun. 2001, 31, 47-52. (c) Matsubara, S.;
Oshima, K. In Modern Carbonyl Olefination; Takeda, T., Ed.; Wiley:
2004, Chapter 5. (d) Fu¨rstner, A. Chem. Rev. 1999, 99, 991-1045. (e)
Wessjohann, L. A.; Scheid, G. Synthesis 1999, 1-36.
(3) Previously, only one example of the synthesis of the correspond-
ing R,â-unsaturated ester by reaction of 4-tert-butylcyclohexanone with
ethyl dibromoacetate (no mixture of Z/E isomers can be obtained) by
using a mixture of SmI2, Sm, and CrCl3 has been described: Matsub-
ara, S.; Horiuchi, M.; Takai, K.; Utimoto, K. Chem. Lett. 1995, 259-
260.
(7) Concello´n, J. M.; Rodriguez-Solla, H.; Me´jica, C. Tetrahedron
Lett. 2004, 44, 1931-1934.
(8) A solution of SmI2 in THF was very rapidly generated by reaction
of diiodomethane with samarium powder in the presence of sonic
waves: Concello´n, J. M.; Rodr´ıguez-Solla, H.; Bardales, E.; Huerta,
M. Eur. J. Org. Chem. 2003, 1775-1778.
10.1021/jo050655o CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/25/2005
J. Org. Chem. 2005, 70, 6111-6113
6111

TABLE 1. Synthesis of (E)-r,â-Unsaturated Esters
SCHEME 2. Proposed Mechanism
a
entry
2
MX2
R1
R2
Ta (°C) yield (%)b
1
2
2a SmI2
2a CrCl2
2b SmI2
2b CrCl2
n-C7H15
n-C7H15
Cy
Et
Et
Et
Et
Me
Et
rt
reflux
rt
reflux
rt
reflux
rt
reflux
rt
reflux
rt
reflux
rt
reflux
66
75
76
84
60
79d
69
88
68
74
70
72
81
95
3
4
Cy
5c
6
2c
SmI2
Cy
2d CrCl2
PhCHdCH
7
2e
2e
2f
2f
SmI2
CrCl2
SmI2
CrCl2
CH3CH(Ph) Et
CH3CH(Ph) Et
8
9
Ph2CH
Ph2CH
p-MeOC6H4 Et
p-MeOC6H4 Et
Ph
Ph
Et
Et
10
11
12
13
14
2g SmI2
2g CrCl2
2h SmI2
2h CrCl2
Et
Et
a In all cases, 3 equiv of SmI2 was used. Reactions with CrCl2
were performed with 6 equiv. b Isolated yield after column chro-
matography based on the starting aldehyde 1. c By using methyl
dichloroacetate. d De ) 91%.
The results are compiled in Table 1, showing that the
sequential reaction using aldehydes seems to be gen-
eral: aliphatic (linear, branched, or cyclic) and aromatic
(E)-R,â-unsaturated esters can be obtained. It is nothewor-
thy that when aldehydes with high proclivity to enolize
are used (Table 1, entry 7 and 9), the reaction takes place
in good yield.9
Interestingly, no byproducts were detected from reac-
tions involving aromatic aldehydes known to undergo
pinacol coupling.
Using CrCl2 can also accomplish the same transforma-
tion. Thus, treatment of a mixture of ethyl dibromo-
acetate and various aldehydes with 6 equiv of CrCl2 at
65 °C afforded, after hydrolysis, the corresponding (E)-
R,â-unsaturated esters in high yield and with total
stereoselectivity (Scheme 1 and Table 1). When the
reaction was carried out at room temperature, the
reaction did not achieve completion and starting aldehyde
was recovered after 2 h.
Results in Table 1 show that R,â-unsturated esters can
be prepared from cinnamaldehyde by using CrCl2 (Table
1, entry 6) with very high stereoselectivity (de ) 92%,
GC-MS). The reaction of cinnamaldehyde with SmI2,
afforded a complex mixture of products. In the other
cases, marginally higher yields were obtained by using
CrCl2 instead of SmI2, the stereoselectivity being com-
plete in both cases. Given these facts, the two proposed
methods can be complementary and represent valuable
methods for the preparation R,â-unsaturated esters.
Using SmI2 allows the reaction to be carried out at room
temperature, whereas heating at 65 °C is necessary to
perform this process with CrCl2.
In a manner similar to the SmI2-promoted reaction,
the total stereoselectivity of the sequential reaction using
CrCl2 was established by GC-MS and by 1H and 13C NMR
spectroscopy of crude products, and the relative config-
uration of the CdC bond was assigned on the basis of
the coupling constant between the olefinic protons of
compounds 2.
Formation of products 2 using SmI2 or CrCl2 can be
explained by assuming a sequential process in both cases
(Scheme 2).
The synthesis of R,â-unsaturated esters can be also
carried out by using methyl dichloroacetate. In this case,
the reaction was carried out at reflux and by using a
greater amount of SmI2 (6 equiv) to drive the reaction to
completion, furnishing the R,â-unsaturated esters in
lower yield. For these reasons, it was found to be
advantageous to use dibromo- instead of dichloroacetate.
The presence of a single stereoisomer was shown in
1
the crude reaction products by GC-MS and by H NMR
spectroscopy.
The E stereochemistry in the CdC double bond of R,â-
unsaturated esters 2 was assigned on the basis of the
value of the 1H NMR coupling constant between the
olefinic protons of compounds 210 and, in other cases, by
NOE experiments.
This method constitutes an important improvement on
our previous synthesis of (E)-R,â-unsaturated esters6 due
to increased simplicity, total stereoselectivity, a shorter
reaction time, and commercially available starting com-
pounds. In addition, the yield of the sequential process
is better than the total yield of the two-pot process
(synthesis of starting 2-halo-3-hydroxyesters and SmI2-
promoted elimination reaction).
Thus, reaction of 2 equiv of SmI2 or CrCl2 with ethyl
dibromoacetate generates a samarium or chromium
enolate 3, which reacts with the carbonyl compound to
afford the corresponding 2-bromoester 4. A metalation
of 4 with 2 equiv of samarium diiodide or chromium
dichloride gives the enolate intermediate 5. Chelation of
the SmIII or CrIII centers with the oxygen atom of the
alcohol group produces a six-membered ring. Tentatively,
we propose a half-chair transition state model I11 with
the bulkier group R1 in the equatorial orientation (to
avoid 1,3-diaxial interactions). Elimination from I affords
(E)-R,â-unsaturated esters.
To the best of our knowledge, this sequential C-C bond
formation/elimination reaction constitutes the first method
described in the literature to perform the olefination of
aldehydes using SmI2 with total stereoselectivity.
(9) When the Wittig reactions are carried out with enolizable
carbonyl compounds, alkenes are obtained in very low yield (see ref
5).
(10) Coupling constants between the olefinic protons of compounds
2 ranging between J ) 15.7 and 16.3 Hz were in accordance with the
average literature values. Silverstein, R. M.; Bassler, G. C.; Morrill,
T. C. Spectrometric Identification of Organic Compounds; John Wiley
and Sons: New York, 1991; Chapter 4, Appendix F, p 221.
6112 J. Org. Chem., Vol. 70, No. 15, 2005

eV) calcd for C11H18O2 [M+] 182.1307, found 182.1319; IR (neat)
2926, 1723 cm-1; Rf ) 0.5 (hexane/EtOAc 3:1). Anal. Calcd for
C11H18O2: C, 72.49; H, 9.95. Found: C, 72.71; H, 10.07.
Methyl (E)-3-Cyclohexylprop-2-enoate (2c): orange oil; 1H
NMR (300 MHz, CDCl3) δ 6.91 (dd, J ) 6.8, 15.9 Hz, 1 H), 5.76
(dd, J ) 2.8, 15.9 Hz, 1 H), 3.72 (s, 3 H), 2.15-0.87 (m, 11 H);
13C NMR (75 MHz, CDCl3) δ 167.2, 154.3, 118.2, 51.1, 40.1, 31.4,
25.6, 25.4; MS (70 eV, EI) m/z (%) 168 [M+, 58], 136 (50), 95
(50), 87 (96), 59 (17); IR (neat) 2926, 1726, 1653, 1275 cm-1; Rf
) 0.54 (hexane/EtOAc 3:1). Anal. Calcd for C10H16O2: C, 71.39;
H, 9.59. Found: C,70.99; H, 9.51.
Ethyl (2E,4E)-5-Phenyl-pent-2,4-dienoate (2d): yellowish
oil; 1H NMR (300 MHz, CDCl3) δ 7.38-7.30 (m, 7 H), 6.87 (d, J
) 4.8 Hz, 1 H), 5.99 (d, J ) 15.2 Hz, 1 H), 4.23 (q, J ) 7.1 Hz,
2 H), 1.32 (t, J ) 7.1 Hz, 3 H); 13C NMR (75 MHz, CDCl3) δ
166.9, 144.4, 140.2, 135.9, 128.9, 128.7, 127.1, 126.1, 121.2, 60.2,
14.2; MS (70 eV, EI) m/z (%) 202 [M+, 20], 157 (22), 129 (100),
77 (12); IR (neat) 2982, 1708, 1626 cm-1; Rf ) 0.41 (hexane/
EtOAc 5:1). Anal. Calcd for C13H14O2: C, 77.20; H, 6.98. Found:
C, 76.91; H, 6.86.
In conclusion, in this paper we have presented two
easy, rapid, straightforward, general methods for the
preparation of R,â-unsaturated esters in high yield and
with total stereoselectivity starting from dibromoacetate
and aldehydes (both commercially available), promoted
by samarium diiodide or chromium dichloride. This
transformation takes place through a sequential pro-
cess: an aldol-type reaction, as the first step, followed
by a totally E-stereoselective â-elimination reaction, as
the second step. A mechanism to explain the high
stereoselectivity has been proposed on the basis of the
chelation of the SmIII or CrIII centers with both oxygen
atoms.
Experimental Section
Synthesis of R,â-Unsaturated Esters (2) Using SmI2.
Under nitrogen, a solution of SmI2 (3 mmol) was added to a
stirred suspension of ethyl dibromoacetate (0.5 mmol) and the
corresponding aldehyde (0.5 mmol) in THF. After stirring at
room temperature for 2 h, the reaction was quenched with
aqueous 0.1 M HCl. The organic layer was then extracted with
dichloromethane. The combined extracts were dried over Na2-
SO4, and the solvent was removed in vacuo. Purification by flash
chromatography on silica gel (hexane/EtOAc 10:1) afforded
products 2.
Synthesis of SmI2 by Sonication. Samarium powder (3
mmol) was placed in a Schlenk tube at room temperature, and
dry THF (30 mL) was added. The Schlenk tube was then
partially submerged to the solvent level in a conventional cleaner
sonicator (ultrasound laboratory cleaner 230 V, 150 W, 50 Hz),
and diidodomethane (3 mmol) was added. After 5 min, a deep
blue solution of SmI2 was obtained.
Synthesis of R,â-Unsaturated Esters (2) Using CrCl2. To
a stirred suspension of CrCl2 (6 mmol) in THF (10 mL) was
added a solution of ethyl dibromoacetate (1 mmol) and the
corresponding aldehyde (1 mmol), at 65 °C. After stirring at this
temperature for 2 h, the reaction was quenched with 0.1 M HCl
and extracted with hexane (3 × 10 mL). The combined extracts
were dried over Na2SO4 and concentrated. The organic layer was
then filtered through a pad of Celite, and the solvents were
removed in vacuo. Purification by column chromatography on
silica gel (hexane/EtOAc 10:1) afforded the R,â-unsaturated
esters 2.
Ethyl (E)-4-Phenylpent-2-enoate (2e): colorless oil; 1H
NMR (300 MHz, CDCl3) δ 7.37-7.21 (m, 5 H), 7.14 (dd, J ) 6.9,
16.0 Hz, 1 H), 5.83 (dd, J ) 1.35, 15.6 Hz, 1 H), 4.20 (q, J ) 7.1
Hz, 2 H), 3.64 (q, J ) 6.7 Hz, 1 H), 1.46 (d, J ) 6.9 Hz, 3 H),
1.30 (t, J ) 7.0 Hz, 3 H); 13C NMR (75 MHz, CDCl3) δ 166.5,
152.4, 143.0, 128.8, 127.1, 126.5, 119.9, 60.0, 41.8, 19.9, 14.0;
MS (70 eV, EI) m/z (%)204 [M+, 28], 159 (38), 131 (100), 115
(76), 91 (54), 77 (29); IR (neat) 2977, 1717, 1650 cm-1; Rf ) 0.4
(hexane/EtOAc 3:1). Anal. Calcd for C13H16O2: C, 76.44; H, 7.90.
Found: C, 76.68; H, 7.78.
Ethyl (E)-4,4-Diphenylbut-2-enoate (2f): pale orange oil;
1H NMR (300 MHz, CDCl3) δ 7.45-7.20 (m, 11 H), 5.78 (d, J )
15.6 Hz, 1 H), 4.90 (d, J ) 7.3 Hz, 1 H), 4.22 (q, J ) 7.1 Hz, 2
H), 1.30 (d, J ) 7.1 Hz, 3 H); 13C NMR (75 MHz, CDCl3) δ )
166.3, 149.7, 141.4, 129.0, 128.9, 128.5, 128.4, 126.8, 122.8, 60.3,
53.3, 14.1; MS (70 eV, EI) m/z (%) 266 [M+, 12], 237 (10), 221
(20), 193 (100), 165 (34), 115 (94), 91 (23), 77 (10); IR (neat) 2982,
1720 cm-1; Rf ) 0.5 (hexane/EtOAc 5:1). Anal. Calcd for
C18H18O2: C, 81.17; H, 6.81. Found: C, 81.26; H, 6.75.
Ethyl (E)-3-(4-Methoxyphenyl)prop-2-enoate (2g): color-
less oil; 1H NMR (200 MHz, CDCl3) δ 7.63 (d, J ) 15.9 Hz, 1 H),
7.48-7.43 (m, 2 H), 6.91-6.85 (m, 2 H), 6.29 (d, J ) 15.9 Hz, 1
H), 4.24 (q, J ) 7.2 Hz, 2 H), 3.80 (s, 3 H), 1.32 (t, J ) 7.2 Hz,
3 H); 13C NMR (50 MHz, CDCl3) δ 167.3, 161.2, 144.2, 129.6,
126.9, 115.6, 114.2, 60.3, 55.3, 14.3; MS (70 eV, EI) m/z (%) 206
[M+, <62], 161 (100), 89 (10); HRMS (70 eV) calcd for C12H14O3
[M+] 206.0943, found 206.0927; IR (neat) 2980, 1708, 1605, 1171
cm-1; Rf ) 0.2 (hexane/EtOAc 3:1). Anal. Calcd for C12H14O3:
C, 69.88; H, 6.84. Found: C, 69.90; H, 6.90.
Ethyl (E)-Dec-2-enoate (2a): orange oil; 1H NMR (300 MHz,
CDCl3) δ 6.97 (dt, J ) 7.5, 15.9 Hz, 1 H), 5.81 (d, J ) 15.9 Hz,
1 H), 4.18 (q, J ) 6.9 Hz, 2 H), 2.20 (apparent q, J ) 7.0 Hz, 2
H), 1.70-0.89 (m, 13 H), 1.29 (t, J ) 6.9 Hz, 3 H); 13C NMR (75
MHz, CDCl3) δ 166.6, 149.3, 121.1, 59.9, 32.1, 31.6, 29.0, 28.8,
27.9, 22.5, 14.1, 13.9; MS (70 eV, EI) m/z (%) 198 [M+, 4], 169
(10), 141 (23), 123 (100); HRMS (70 eV) calcd for C12H22O2 [M+]
198.1620, found 198.1605; IR (neat) 2927, 1724, 1466 cm-1; Rf
) 0.4 (hexane/EtOAc 10:1). Anal. Calcd for C12H22O2: C, 72.68;
H, 11.18. Found: C, 72.91; H, 11.07.
Ethyl (E)-3-Phenylprop-2-enoate (2h): yellow oil; 1H NMR
(200 MHz, CDCl3) δ 7.69 (d, J ) 16.0 Hz, 1 H), 7.53-7.35 (m, 5
H), 6.44 (d, J ) 16.0, 1 H), 4.26 (q, J ) 7.1 Hz, 2 H), 1.33 (t, J
) 7.1 Hz, 3 H); 13C NMR (50 MHz, CDCl3) δ 166.7, 144.3, 134.1,
129.9, 128.1, 127.7, 117.9, 60.1, 14.0; MS (70 eV, EI) m/z (%)
176 [M+, <30], 147 (17), 131 (100), 103 (56), 77 (45); IR (neat)
2980, 1712, 1637 cm-1; Rf ) 0.5 (hexane/EtOAc 3:1). Anal. Calcd
for C11H12O2: C, 74.98; H, 6.86. Found: C, 74.69; H 6.74.
Ethyl (E)-3-Cyclohexylprop-2-enoate (2b): white solid; 1H
NMR (300 MHz, CDCl3) δ 6.89 (dd, J ) 6.7, 15.8 Hz, 1 H), 5.74
(d, J ) 15.8 Hz, 1 H), 4.16 (q, J ) 6.9 Hz, 2 H), 2.12-0.81 (m,
11 H), 1.26 (t, J ) 6.9 Hz, 3 H); 13C NMR (50 MHz, CDCl3) δ
166.9, 154.0, 118.6, 59.9, 40.2, 31.5, 25.7, 25.5, 14.0; MS (70 eV,
EI) m/z (%) 182 [M+, 34], 153 (3), 137 (65), 109 (22); HRMS (70
Acknowledgment. We thank Ministerio de Educa-
cio´n y Cultura (CTQ2004-01191) for financial support
and Euan C. Goddard for his revision of the English.
C.C. thanks Ministerio de Ciencia y Tecnolog´ıa, and
C.M. thanks Ministerio de Educacio´n y Cultura for a
predoctoral fellowship. J.M.C. thanks Carmen Ferna´n-
dez-Flo´rez for her time.
Supporting Information Available: 13C NMR spectra of
compounds 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
(11) Similar transition state model involving a chair transition state
was proposed to explain the selectivity in other reactions of SmI2: (a)
Molander, G. A.; Etter, J. B.; Zinke, P. W. J. Am. Chem. Soc. 1987,
109, 453-463. (b) Urban, D.; Skrydstrup, T.; Beau, J. M. J. Org. Chem.
1998, 63, 2507-2516. (c) Enemrke, R. J.; Larsen, J.; Hjollund, G. H.;
Skrydstrup, T.; Daasbjerg, K. Organometallics 2005, 24, 1252-1262.
(d) Davis, T. A.; Chopade, P. R.; Hilmersson, G.; Flowers, R. A. Org.
Lett. 2005, 7, 119-122.
JO050655O
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