Efficient highly selective synthesis of methyl 2-(ethynyl)alk-2(E)-enoates and 2-(1′-chlorovinyl)alk-2(Z)-enoates from 2-(methoxycarbonyl)-2,3- allenols

Article abstract of DOI:10.1021/ol9004273

Highly regio- and stereoselective reactions of readily available 2-(methoxycarbonyl)-2,3-allenols 1 with oxalyl chloride in the presence of Et₃N or DMSO afforded methyl 2-(ethynyl)alk-2(E)-enoates(E)-2 and 2-(1′-chlorovinyl)alk-2(Z)-enoates(Z)-

Full text of DOI:10.1021/ol9004273

ORGANIC
LETTERS
2009
Vol. 11, No. 10
2169-2172
Efficient Highly Selective Synthesis of
Methyl 2-(Ethynyl)alk-2(E)-enoates and
2-(1′-Chlorovinyl)alk-2(Z)-enoates from
2-(Methoxycarbonyl)-2,3-allenols
Youqian Deng,† Xin Jin,‡ Chunling Fu,† and Shengming Ma*,†,‡
Laboratory of Molecular Recognition and Synthesis, Department of Chemistry,
Zhejiang UniVersity, Hangzhou 310027, Zhejiang, People’s Republic of China and
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu,
Shanghai 200032, People’s Republic of China
masm@mail.sioc.ac.cn
Received March 2, 2009
ABSTRACT
Highly regio- and stereoselective reactions of readily available 2-(methoxycarbonyl)-2,3-allenols 1 with oxalyl chloride in the presence of Et3N
or DMSO afforded methyl 2-(ethynyl)alk-2(E)-enoates (E)-2 and 2-(1′-chlorovinyl)alk-2(Z)-enoates (Z)-3, respectively, in moderate to good yields.
The efficient stereoselective syntheses of conjugated enynes1
and conjugated 1,3-dienes2 are of current interest since they
are very important intermediates in organic synthesis.3,4
Usually, the stereodefined enynes are prepared by Sono-
gashira-type coupling reactions between stereodefined alk-
enyl halides with terminal alkynes,5cross-metathesis of
alkynes and alkenes with major (Z)-selectivity,6 Wittig-
Horner-Wadsworth-Emmons-type reactions usually with
a poor stereoselectivity,7 the reactions of alkenylborane
derivatives with alkynyllithiums,8 etc.9 On the other hand,
many efficient synthetic methods for the preparation of
conjugated 1,3-dienes have also been developed.2 In addition,
conjugated 1,3-dienes may be synthesized from the reactions
of 2,3-allenols or their derivatives, but always with a lower
stereoselectivity.10 Recently, Lee et al. have developed a
DABCO-catalyzed method for the synthesis of ethyl 2-(ethy-
nyl)alk-2(E)-enoates from 2,3-allenol acetates, prepared from
the acetylation of 2-(ethoxycarbonyl)-2,3-allenols.11 This
prompted us to report here a one-pot highly stereoselective
synthesis of methyl 2-(ethynyl)alk-2(E)-enoates and 2-(1′-
chlorovinyl)alk-2(Z)-enoates from 2-(methoxycarbonyl)-2,3-
allenols12 in moderate to good yields, respectively.
Originally, we were conducting the Swern oxidation13
reaction of 3-(methoxycarbonyl)-1,2-nonadien-4-ol 1a (Scheme
1). Instead of forming the expected oxidation product, i.e.,
1,2-allenic ketone, surprisingly, the reaction afforded methyl
2-(ethynyl)oct-2(E)-enoate (E)-2a and methyl 2-(1′-chlo-
rovinyl)oct-2(Z)-enoate (Z)-3a in 2% and 54% yields,
respectively, with a 16% recovery of 1a; in the absence of
Et3N, the reaction afforded (Z)-3a as the only product in 48%
yield, but also with a 15% recovery of 1a. Interestingly, in
the absence of DMSO, (E)-2a were formed as a major
† Zhejiang University.
‡ Shanghai Institute of Organic Chemistry.
(1) For recent reviews, see: (a) Shirtcliff, L. D.; McClintock, S. P.; Haley,
M. M. Chem. Soc. ReV. 2008, 37, 343. (b) Negishi, E.; Huang, Z.; Wang,
G.; Mohan, S.; Wang, C.; Hattori, H. Acc. Chem. Res. 2008, 41, 1474. (c)
Doucet, H.; Hierso, J.-C. Angew. Chem., Int. Ed. 2007, 46, 834. (d)
Chinchilla, R.; Na´jera, C. Chem. ReV. 2007, 107, 874. (e) Zeni, G.; Braga,
A. L.; Stefani, H. A. Acc. Chem. Res. 2003, 36, 731.
10.1021/ol9004273 CCC: $40.75
Published on Web 04/21/2009
 2009 American Chemical Society

Scheme 1
Table 1. Temperature and Base Effects of the Reaction of 1a
T1
T2
(E)-2aa (Z)-3aa
entry
(°C)/t1(h)
base
Et3N
Et3N
Et3N
Et3N
EtN(Pr-i)2
pyridine
(°C)/t2(h)
(%)
(%)
1
2
3
4
5
6
-78 °C-rt/4
rt/2
reflux/2
reflux/2
reflux/2
reflux/2
rt/1
rt/3
reflux/3
rt/5
rt/3
68
66
64
87
71
0
7
5
6
<0.5
8
25
a NMR yield. b 16% of 1a was recovered. c 15% of 1a was recovered.
rt/21
a NMR yield using 1,3,5-trimethylbenzene as the internal standard.
product in 68% yield together with 7% yield of (Z)-3a
(Scheme 1). In order to further optimize the reaction
conditions for the selective formation of (E)-2a or (Z)-3a,
temperature and base effects were tested. Some of the most
typical results are summarized in Table 1. 3-(Methoxycar-
bonyl)-1,2-nonadien-4-ol 1a was first treated with oxalyl
chloride under reflux, which was followed by the elimination
at rt using Et3N as the base to afford the product (E)-2a in
the highest yield with the best selectivity (compare entries
1-4, Table 1). Among the bases screened, EtN(Pr-i)2
demonstrated a poor selectivity affording (E)-2a and (Z)-3a
in 71% and 8% yields, respectively (entry 5, Table 1); when
(2) For reports on the synthesis of 1,3-dienes, see: (a) Trost, B. M.;
Fortunak, J. M. J. Am. Chem. Soc. 1980, 102, 2841. (b) Tsuboi, S.; Masuda,
T.; Takeda, A. J. Org. Chem. 1982, 47, 4478. (c) Mandai, T.; Yanagi, T.;
Araki, K.; Morisaki, Y.; Kawada, M.; Otera, J. J. Am. Chem. Soc. 1984,
106, 3670. (d) Trost, B. M.; Brandi, A. J. Org. Chem. 1984, 49, 4811. (e)
Cacchi, S.; Morera, E.; Ortar, G. Tetrahedron Lett. 1984, 25, 2271. (f)
Mandai, T.; Moriyama, T.; Tsujimoto, K.; Kawada, M.; Otera, J. Tetrahe-
dron Lett. 1986, 27, 603. (g) Trost, B. M.; Schmidt, T. J. Am. Chem. Soc.
1988, 110, 2301. (h) Hettrick, C. M.; Kling, J. K.; Scott, W. J. J. Org.
Chem. 1991, 56, 1489. (i) Mitsudo, T.-A.; Zhang, S.-W.; Nagao, M.;
Watanabe, Y. J. Chem. Soc., Chem. Commun. 1991, 598. (j) Guo, C.; Lu,
X. Tetrahedron Lett. 1992, 33, 3659. (k) Trost, B. M.; Kazmaier, U. J. Am.
Chem. Soc. 1992, 114, 7933. (l) Guo, C.; Lu, X. J. Chem. Soc., Perkin
Trans. 1 1993, 1921. (m) Rychnovsky, S. D.; Kim, J. J. Org. Chem. 1994,
59, 2659. (n) Kitahara, T.; Matsuoka, T.; Kiyota, H.; Warita, Y.; Kurata,
H.; Horiguchi, A.; Mori, K. Synthesis 1994, 692. (o) Bernabeu, M. C.;
Chinchilla, R.; Na´jera, C. Tetrahedron Lett. 1995, 36, 3901. (p) Kinoshita,
A.; Sakakibara, N.; Mori, M. J. Am. Chem. Soc. 1997, 119, 12388. (q) Qi,
X.; Montgomery, J. J. Org. Chem. 1999, 64, 9310. (r) Yao, Q. Org. Lett.
2001, 3, 2069. (s) Su, M.; Kang, Y.; Yu, W.; Hua, Z.; Jin, Z. Org. Lett.
2002, 4, 691. (t) Wang, Y.; West, F. G. Synthesis 2002, 99. (u) Paih, J. L.;
Monnier, F.; De´rien, S.; Dixneuf, P. H.; Clot, E.; Eisenstein, O. J. Am.
Chem. Soc. 2003, 125, 11964. (v) Fu, C.; Ma, S. Org. Lett. 2005, 7, 1707.
(w) Cella, R.; Orfa˜o, A. T. G.; Stefani, H. A. Tetrahedron Lett. 2006, 47,
5075. (x) Tayama, E.; Sugai, S. Tetrahedron Lett. 2007, 48, 6163. (y) Clark,
D. A.; Basile, B. S.; Karnofel, W. S.; Diver, S. T. Org. Lett. 2008, 10,
4927. (z) Li, G.; Zhang, G.; Zhang, L. J. Am. Chem. Soc. 2008, 130, 3740.
(aa) Mundal, D. A.; Lutz, K. E.; Thomson, R. J. Org. Lett. 2009, 11, 465.
(bb) Dudnik, A. S.; Schwier, T.; Gevorgyan, V. J. Organomet. Chem. 2009,
694, 482.
(5) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
16, 4467. (b) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508. (c)
Alami, M.; Ferri, F.; Linstrumelle, G. Tetrahedron Lett. 1993, 34, 6403.
(d) Lemay, A. B.; Vulic, K. S.; Ogilvie, W. W. J. Org. Chem. 2006, 71,
3615. (e) Hatakeyama, T.; Yoshimoto, Y.; Gabriel, T.; Nakamura, M. Org.
Lett. 2008, 10, 5341.
(6) (a) Kang, B.; Kim, D.; Do, Y.; Chang, S. Org. Lett. 2003, 5, 3041.
(b) Kang, B.; Lee, J. M.; Kwak, J.; Lee, Y. S.; Chang, S. J. Org. Chem.
2004, 69, 7661. (c) Hansen, E. C.; Lee, D. Org. Lett. 2004, 6, 2035.
(7) Gibson, A. W.; Humphrey, G. R.; Kennedy, D. J.; Wright, S. H. B.
Synthesis 1991, 414.
(8) Negishi, E.; Yoshida, T.; Abramovitch, A.; Lew, G.; Williams, R. M.
Tetrahedron 1991, 47, 343.
(9) (a) Rao, W.; Zhang, X.; Sze, E. M. L.; Chan, P. W. H. J. Org. Chem.
2009, 74, 1740. (b) Sim, S. H.; Park, H.-J.; Lee, S. I.; Chung, Y. K. Org.
Lett. 2008, 10, 433. (c) Yamauchi, Y.; Onodera, G.; Sakata, K.; Yuki, M.;
Miyake, Y.; Uemura, S.; Nishibayashi, Y. J. Am. Chem. Soc. 2007, 129,
5175. (d) Ikeda, S.; Sato, Y. J. Am. Chem. Soc. 1994, 116, 5975.
(10) For reports on the synthesis of 1,3-dienes from 2,3-allenols or their
derivatives, see: (a) Pornet, J. Tetrahedron Lett. 1981, 22, 453. (b) Kleijn,
H.; Wertmijze, H.; Meijer, J.; Vermeer, P. Recl. TraV. Chim. Pays-Bas 1983,
102, 378. (c) Wang, K. K.; Nikam, S. S.; Marcano, M. M. Tetrahedron
Lett. 1986, 27, 1123. (d) Djahanbini, D.; Cazes, B.; Gore, J. Tetrahedron
1987, 43, 3441. (e) Nokami, J.; Maihara, A.; Tsuji, J. Tetrahedron Lett.
1990, 31, 5629. (f) Friesen, R. W.; Kolaczewska, A. E.; Khazanovich, N.
Tetrahedron Lett. 1992, 33, 6715. (g) Imada, Y.; Vasapollo, G.; Alper, H.
J. Org. Chem. 1996, 61, 7982. (h) Horva´th, A.; Ba¨ckvall, J.-E. J. Org.
Chem. 2001, 66, 8120. (i) Shen, Q.; Hammond, G. B. Org. Lett. 2001, 3,
2213. (j) Ma, S.; Wang, G. Tetrahedron Lett. 2002, 43, 5723. (k) Cho,
Y. S.; Jun, B. K.; Pae, A. N.; Cha, J. H.; Koh, H. Y.; Chang, M. H.; Han,
S.-Y. Synthesis 2004, 2620. (l) Alcaide, B.; Almendros, P.; Aragoncillo,
M.; Redondo, M. C. Eur. J. Org. Chem. 2005, 98. (m) Ma, S.; Gu, Z. J. Am.
Chem. Soc. 2005, 127, 6182. (n) Alcaide, B.; Almendros, P.; Mart´ınez del
Campo, T. Angew. Chem., Int. Ed. 2006, 45, 4501. (o) Deng, Y.; Li, J.;
Ma, S. Chem.sEur. J. 2008, 14, 4263. (p) Deng, Y.; Yu, Y.; Ma, S. J.
Org. Chem. 2008, 73, 585. (q) Alcaide, B.; Almendros, P.; Mart´ınez del
Campo, T.; Carrascosa, R. Chem. Asian. J. 2008, 3, 1140.
(3) For some selected recent references on the reactions of conjugated
enynes, see: (a) Yu, X.; Ren, H.; Xiao, Y.; Zhang, J. Chem.sEur. J. 2008,
14, 8481. (b) Dunetz, J. R.; Danheiser, R. L. J. Am. Chem. Soc. 2005, 127,
5776. (c) Hayes, M. E.; Shinokubo, H.; Danheiser, R. L. Org. Lett. 2005,
7, 3917. (d) Nakao, Y.; Hirata, Y.; Ishihara, S.; Oda, S.; Yukawa, T.;
Shirakawa, E.; Hiyama, T. J. Am. Chem. Soc. 2004, 126, 15650. (e) Wender,
P. A.; Gamber, G. G.; Scanio, M. J. C. Angew. Chem., Int. Ed. 2001, 40,
3895. (f) Saito, S.; Tanaka, T.; Koizumi, T.; Tsuboya, N.; Itagaki, H.;
Kawasaki, T.; Endo, S.; Yamamoto, Y. J. Am. Chem. Soc. 2000, 122, 1810.
(g) Saito, S.; Ohmori, O.; Yamamoto, Y. Org. Lett. 2000, 2, 3853.
(4) (a) Wasserman, A. Diels-Alder Reactions; Elsevier: Amsterdam,
1965. (b) Wallweben, H. Diels-Alder Reaction; Thieme: Stuttgart, 1972.
(c) Fringuelli, F.; Taticchi, A. Dienes in the Diels-Alder Reaction; John
Wiley: New York, 1990. (d) Oppolzer, W. In ComprehensiVe Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991;
Vol. 5, Chapter 4.1, pp 315-400. (e) Roush, W. R. In ComprehensiVe
Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford,
1991; Vol. 5, Chapter 4.4, pp 513-550. (f) Mehta, G.; Rao, S. P. In The
Chemistry of Dienes and Polyenes; Rappoport, Z., Ed.; John Wiley:
Chichester, 1997; Chapter 9, pp 361-467. (g) Fringuelli, F.; Taticchi, A.
The Diels-Alder Reaction: Selected Practical Methods; John Wiley:
Chichester, 2002.
(11) Choe, Y.; Lee, P. H. Org. Lett. 2009, 11, 1445.
(12) (a) Deng, Y.; Jin, X.; Ma, S. J. Org. Chem. 2007, 72, 5901. (b)
Winkler, J. D.; Quinn, K. J.; MacKinnon, C. H.; Hiscock, S. D.; McLaughlin,
E. C. Org. Lett. 2003, 5, 1805.
(13) (a) Omura, K.; Swern, D. Tetrahedron 1978, 34, 1651. For the
oxidation of 2,3-allenols to corresponding 1,2-allenic ketones, see: (b) Ma,
S.; Yu, S.; Yin, S. J. Org. Chem. 2003, 68, 8996.
2170
Org. Lett., Vol. 11, No. 10, 2009

Scheme 2
pyridine was used as the base, the reaction afforded (Z)-3a
as the only product in 25% yield (entry 6, Table 1). In
conclusion, the reaction afforded methyl 2-ethynyloct-2(E)-
enoate (E)-2a in 87% yield when conducted with 3 equiv
each of oxalyl chloride and Et3N (entry 4, Table 1). 1H NMR
analysis of the crude product indicated the exclusive forma-
tion of the (E)-stereoisomer. The stereochemistry was
established by the NOESY analysis of (E)-4a, which was
prepared by the reduction of (E)-2a with DIBAL-H in toluene
in 82% yield (Scheme 2).
Figure 1. ORTEP representation of (E)-2e and (Z)-3i (right).
typical results are summarized in Table 3. Alkyl groups
(entries 1-4, Table 3), aryl groups (entries 5-6, Table 3),
and alkynyl group-substituted secondary alcohols (entry 7,
Table 3) all afforded the conjugated 1,3-diene products (Z)-3
in good yields.
Table 3. Synthesis of Methyl 2-(1′-Chlorovinyl)alk-2(Z)-enoates
(Z)-3
Table 2. Synthesis of Methyl 2-(Ethynyl)alk-2(E)-enoates (E)-2a
entry
R
t1 (h) t2 (h) yield of (E)-2 (%)
entry
R
time (h)
yield of (Z)-3 (%)
1
2
3
4
n-C5H11 (1a)
C2H5 (1b)
n-C3H7 (1c)
i-C3H7 (1d)
Ph (1e)
2
2.5
2
2
7
7
4
7
9
5
4.5
4
3.5
10.5
15
7
10.5
12
83 ((E)-2a)
62 ((E)-2b)
65 ((E)-2c)
67 ((E)-2d)
83 ((E)-2e)
85 ((E)-2e)
69 ((E)-2f)
83 ((E)-2g)
56 ((E)-2h)
1
2
3
4
5
6
7
n-C5H11 (1a)
C2H5 (1b)
n-C3H7 (1c)
i-C3H7 (1d)
Ph (1e)
5
5
5
5
5
6
6
83 ((Z)-3a)
76 ((Z)-3b)
85 ((Z)-3c)
76 ((Z)-3d)
78 ((Z)-3e)
80 ((Z)-3g)
64 ((Z)-3h)
5
6b
7
Ph (1e)
p-CH3C6H4 (1f)
p-BrC6H4 (1g)
n-C4H9Ct C (1h)
p-BrC6H4 (1g)
n-C4H9Ct C (1h)
8c
9c, d
a Allenol 1 (0.4 mmol) was used. b Allenol 1e (4 mmol) was used. c
5
equiv each of oxalyl chloride and Et3N were used. d (Z)-3h was also afforded
When R is cinnamyl group, the reaction of 4-(methoxy-
carbonyl)-1(E)-phenyl-1,4,5-hexatrien-3-ol E-1i and oxalyl
chloride occurred directly to afford methyl 2-(1′-chlorovinyl)-
5-phenylpenta-(2Z,4E)-dienoate (Z,E)-3i in 66% yield, which
was also established by the X-ray diffraction studies (Figure
1)15 (Scheme 3).
in 12% NMR yield.
With the optimized conditions in hand, the scope of this
reaction was explored. Some of the typical results are
summarized in Table 2. It is concluded that R may be a
primary (entries 1-3, Table 2) or secondary (entry 4, Table
2) alkyl group and a phenyl- or methyl/bromo-substituted
phenyl group (entries 5-8, Table 2); in addition, even an
alkynyl group may be introduced to this position (entry 9,
Table 2). The structure of the product (E)-2 was further
established by the X-ray diffraction studies of (E)-2e (Figure
1).14
On the other hand, we further optimized the reaction
conditions for the highly selective formation of (Z)-3a from
the reaction of 3-(methoxycarbonyl)-1,2-nonadien-4-ol 1a
with oxalyl chloride in the presence of DMSO (Scheme 1).
After being stirred at -78 °C for about 18 min and then rt
for another 5 h, the reaction afforded (Z)-3a in 83% yield
without the recovery of 1a (entry 1, Table 3). Some of the
However, the reaction of the substrate with an alkyl
substituent at the 4-position, i.e., 1-(4′-chlorophenyl)-2-
(14) Crystal data for (E)-2e: C12H10O2, MW ) 186.20, triclinic, space
group P-1, final R indices [I > 2σ(I)], R1 ) 0.0731, wR2 ) 0.1959, R
indices (all data) R1 ) 0.0829, wR2 ) 0.2063, a ) 9.4588(14) Å, b )
9.6985(14) Å, c ) 11.9322(17) Å, R ) 74.013(2)°, ꢀ ) 88.980(3)°, γ )
70.029(2)°, V ) 985.5(2) Å3, T ) 293(2) K, Z ) 4, reflections collected/
unique: 5435/3807 (Rint ) 0.1539), number of observations [>2σ(I)] 2929,
parameters: 271. Supplementary crystallographic data have been deposited
at the Cambridge Crystallographic Data Centre: CCDC 722253.
(15) Crystal data for (Z)-3i: C14H13ClO2, MW ) 248.69, monoclinic,
space group P2(1)/c, final R indices [I > 2σ(I)], R1 ) 0.0425, wR2 )
0.1163, R indices (all data) R1 ) 0.0477, wR2 ) 0.1240, a ) 7.9734(5)
Å, b ) 16.9107(10) Å, c ) 9.8279(6) Å, R ) 90.00°, ꢀ ) 99.353(2)°, γ
) 90.00°, V ) 1307.54(14) Å3, T ) 296(2) K, Z ) 4, reflections collected/
unique: 14705/2303 (Rint ) 0.0222), number of observations [>2σ(I)] 2005,
parameters: 163. Supplementary crystallographic data have been deposited
at the Cambridge Crystallographic Data Centre: CCDC 722252.
Org. Lett., Vol. 11, No. 10, 2009
2171

Scheme 3
Scheme 5
(ethoxycarbonyl)deca-2,3-dien-1-ol 1j,16 failed to afford the
corresponding products 2j or 3j under the standard conditions
(Scheme 4).
Scheme 4
In conclusion, we have developed a one-pot highly
selective synthesis of methyl 2-(ethynyl)alk-2(E)-enoates
(E)-2 and 2-(1′-chlorovinyl)alk-2(Z)-enoates (Z)-3, which are
useful building blocks in organic synthesis, from the readily
available 2-(methoxycarbonyl)-2,3-allenols12 in moderate to
good yields, respectively. Due to the easy availability of the
starting materials and the potentials of these two types of
products, this method will be useful in organic synthesis.
Further studies including the scope and synthetic application
of this type of reaction are being carried out in our laboratory.
A possible mechanism for these two types of reactions is
shown in Scheme 5. First, the reaction of 1 with oxalyl
chloride yields the intermediate M1; deprotonation of allenyl
proton with Et3N17 followed by the elimination reaction
would afford (E)-2 directly. Similarly, the reaction of 1 with
M2 generated in situ from the reaction of oxalyl chloride
with DMSO13a affords the intermediate M3. Subsequent
SN2′-type reaction of M3 with chloride anion would afford
(Z)-3.
Acknowledgment. Financial support from the National
Natural Science Foundation of China (20732005) and
Major State Basic Research and Development Program
(2006CB806105) is greatly appreciated. S.M. is a Qiu Shi
Adjunct Professor at Zhejiang University. We thank Mr.
Wangqing Kong in this group for reproducing the results
presented in entries 2 and 9 in Table 2 and entries 2 and
7 in Table 3.
(16) The substrate 1j was prepared according to the literature procedure:
Xu, B.; Hammond, G. B. Angew. Chem., Int. Ed. 2008, 47, 689.
(17) (a) Hoff, S.; Steenstra, B. H.; Brandsma, L.; Arens, J. F. Recl. TraV.
Chim. Pays-Bas 1969, 88, 1284. (b) Bridges, A. J.; Fischer, J. W. J. Chem.
Soc., Chem. Commun. 1982, 665. (c) Back, T. G.; Lai, E. K. Y.;
Muralidharan, K. R. J. Org. Chem. 1990, 55, 4595. (d) Padwa, A.; Yeske,
P. E. J. Org. Chem. 1991, 56, 6386. (e) Kitagaki, S.; Teramoto, S.; Mukai,
C. Org. Lett. 2007, 9, 2549.
Supporting Information Available: Experimental pro-
1
cedures and copies of H and 13C NMR spectra of all
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL9004273
2172
Org. Lett., Vol. 11, No. 10, 2009