Lewis acid-promoted reaction of β,γ-unsaturated α,α-dimethoxy esters with silyl nucleophiles

Article abstract of DOI:10.1016/j.tetlet.2012.06.078

The Lewis acid-promoted reaction of β,γ-unsaturated α,α-dimethoxy esters, which are easily prepared by the acetalization of β,γ-unsaturated α-keto esters, with silyl nucleophiles is presented. By employing trimethylsilyl enolate and allyltrimethylsilane as nucleophiles, the BF₃-promoted reactions of a series of β,γ-unsaturated α,α-dimethoxy esters bearing aromatic and aliphatic substituents proceeded at the γ-position in an SN2′ manner to furnish γ-substituted α,β-unsaturated α-methoxy esters in good yields with high regioselectivity. In contrast, the reaction using trimethylsilyl cyanide predominantly occurred at the α-position, and the reaction of silyl hydride resulted in a mixture of α- and γ-regioisomers in favor of the γ-substitution products.

Full text of DOI:10.1016/j.tetlet.2012.06.078

Tetrahedron Letters 53 (2012) 4584–4587
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Tetrahedron Letters
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Lewis acid-promoted reaction of b,c-unsaturated a,a-dimethoxy esters with
silyl nucleophiles
⇑
Hideyuki Sugimura , Hiiro Miyazaki, Yui Makita
Department of Chemistry and Biological Science, Aoyama Gakuin University, 5-10-1, Fuchinobe, Chuo-ku, Sagamihara-shi 252-2558, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The Lewis acid-promoted reaction of b,
the acetalization of b,c-unsaturated a
c
-unsaturated
-keto esters, with silyl nucleophiles is presented. By employing tri-
methylsilyl enolate and allyltrimethylsilane as nucleophiles, the BF3-promoted reactions of a series of
b, -unsaturated -dimethoxy esters bearing aromatic and aliphatic substituents proceeded at the
-position in an S -substituted ,b-unsaturated -methoxy esters in good yields
20 manner to furnish
with high regioselectivity. In contrast, the reaction using trimethylsilyl cyanide predominantly occurred
a,a-dimethoxy esters, which are easily prepared by
Received 25 April 2012
Revised 11 June 2012
Accepted 15 June 2012
Available online 20 June 2012
c
a,a
c
N
c
a
a
Keywords:
Mukaiyama aldol reaction
Hosomi–Sakurai allylation
at the
of the
a
c
-position, and the reaction of silyl hydride resulted in a mixture of
-substitution products.
a- and c-regioisomers in favor
Ó 2012 Elsevier Ltd. All rights reserved.
b,
-Substituted
esters
c
-Unsaturated
a-keto esters
,b-unsaturated a-methoxy
c
a
In recent years, there has been considerable interest in the uti-
lization of b, -unsaturated -keto esters as versatile reaction part-
the presence of BF3ꢀOEt2 resulted in poor or moderate yield of
c
a
the Michael adduct even after several attempts.10 At this stage,
ners in organic synthesis because a variety of highly functionalized
structures can be easily constructed using these building blocks.
For instance, they have been utilized as substrates in asymmetric
aldol reactions,1 reactive Michael acceptors,2 electrophiles in Fri-
edel–Crafts type reactions,3 oxo-diene units in hetero-Diels–Alder
reactions,4 and components in other cycloaddition reactions.5
In earlier reports, we described an efficient method for prepar-
we envisaged that if the dimethyl acetal derivatives of b,
rated -keto esters could be activated by Lewis acids, the addition
of silyl nucleophiles might result in -substituted b, -unsaturated
-methoxy esters ( -product) or -substituted ,b-unsaturated
methoxy esters ( -product), as shown in Scheme 1. The
acrylate moieties of the
synthetic products,11 and compounds that possess the
c-unsatu-
a
a
c
a
a
c
a
a-
c
a-alkoxy
c
-products exist in several natural and
a-alkoxy
ing b,
c
-unsaturated
a
-keto esters via the BF3-mediated aldol reac-
acrylate moiety have been employed as key intermediates for the
tion of 2-(trimethylsiloxy)acrylate esters with acetals,6 and we
utilized this procedure for several synthetic processes, including
the chiral auxiliary-induced stereoselective reduction7 and the
synthesis of biologically important compounds.11b,12a,d,f,g,13 Several
general methods are available for the preparation of
rated -methoxy esters and most of them rely either on the
Horner–Wadsworth–Emmons reaction using a-alkoxyphospho-
a,b-unsatu-
a
addition of organometallic compounds to the
ties.8 As a continuation of our efforts in this field, we proposed
examining a Lewis acid-promoted reaction of b, -unsaturated
-keto esters with silyl nucleophiles. Although the Lewis acid-
mediated conjugate additions of silyl nucleophiles to ,b-unsatu-
a-carbonyl moie-
noacetates12 or on the condensation of alkoxyacetates with
aldehydes under basic conditions.11b,13 More recently, the CrCl2-
mediated Reformatsky-type reaction of 2,2-dichloro-2-methoxy-
acetate with aldehydes has been reported by Falck et al. for the
c
a
a
rated carbonyl compounds has been analyzed in previous studies
facile preparation of
we report a novel approach for the preparation of
-methoxy esters via addition of silyl nucleophiles to dimethyl
acetal 2 in the presence of Lewis acid (Scheme 1).
The starting dimethyl acetals 2a–g were prepared as E-isomers
by treating (E)-b, -unsaturated -keto esters 1a–g with 2.4 equiv
of trimethyl orthoformate and a catalytic amount of TsOHꢀH2O in
refluxing MeOH (Table 1).15 p-Nitrophenyl-substituted b,
-unsatu-
rated -keto ester 1e showed lower reactivity though an improve-
ment in yield was achieved by prolonging the reaction time. The
reaction of alkyl-substituted b, -unsaturated -keto esters 1f and
a,b-unsaturated
a
-methoxy esters.14 Herein,
,b-unsaturated
as fundamental reactions in organic synthesis,9 to our knowledge,
a
no reports about a similar reaction of b,
have appeared to date. Consequently, we initially examined the
Lewis acid-promoted conjugate addition of silyl nucleophiles to
c
-unsaturated
a
-keto esters
a
b,c
-unsaturated
a
-keto esters. However, the reaction of methyl
c
a
(E)-2-oxo-4-phenyl-3-butenoate (1a) with silyl nucleophiles in
c
a
⇑
Corresponding author. Tel./fax: +81 42 759 6227.
E-mail address: sugimura@chem.aoyama.ac.jp (H. Sugimura).
c
a
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.078

H. Sugimura et al. / Tetrahedron Letters 53 (2012) 4584–4587
4585
Scheme 1. Lewis acid-promoted reaction of b,c-unsaturated a,a-dimethoxy esters with silyl nucleophiles.
1g resulted in moderate yields because of the formation of uniden-
tified by-products.
yield (entry 9). The E/Z ratios of products 4a–g were confirmed on
the basis of the integration of appropriate proton absorptions by 1H
NMR analysis (vide infra).
Table 1
Preparation of acetals 2a–g
We next turned our attention to surveying several other silyl
Entry
R
Time (h)
Product
Yielda (%)
nucleophiles to enhance the scope of this regioselective c-substitu-
1
2
3
4
5
6
7
C6H5
4.5
1
3
1.5
47
5.5
6
2a
2b
2c
2d
2e
2f
91
98
92
95
88
53
59
tion reaction and initially focused on allyltrimethylsilane (5).17 The
results of the reactions using unsaturated acetalderivatives 2a–g
are summarized in Table 3. It is noteworthy that TiCl4 and a cata-
lytic amount of TMSOTf as well as BF3ꢀOEt2 promoted the substitu-
p-MeOC6H4
p-MeC6H4
p-ClC6H4
p-NO2C6H4
PhCH2CH2
c-C6H11
tion reaction smoothly to furnish the
c-products in high yields.
2g
Interestingly, depending on the promoter used, the E/Z-ratios for
product 6a varied from approximately 1:1 (BF3ꢀOEt2) to 1:4 (TiCl4)
and 4:1 (TMSOTf). Again, substrates 2b–d with p-substituted phe-
a
Isolated yields.
nyl groups or alkyl groups at the
in the reaction with allylsilane 5 in the presence of BF3ꢀOEt2 to give
-products 6b–g in moderate to excellent yields (Table 3, entries
5–10).
c-position were good substrates
We initiated our experiment by screening suitable Lewis acids
c
for the reaction of methyl (E)-2,2-dimethoxy-4-phenyl-3-buteno-
ate (2a) with silyl enolate 3. Several representative results are
summarized in Table 2, entries 1–4. Each reaction afforded only
c-product 4a and no
a-regioisomer was observed. Among the Le-
wis acids examined, BF3ꢀOEt2 was identified as the most effective,
providing c-product 4a in 91% yield as a mixture of E/Z isomers
(Table 2, entry 1).16 In contrast, TiCl4 and trimethylsilyl trifluoro-
methanesulfonate (TMSOTf) were not sufficiently reactive and
were ineffective in promoting the addition reaction. With the
optimal reaction conditions using BF3ꢀOEt2, we then tested the sub-
The reaction was also conducted using silyl cyanide and silyl
hydride as nucleophiles. The substitution reaction employing
TMSCN, however, occurred predominately at the -position to
form (E)-b, -unsaturated ester 8 in 86% yield along with 9% yield
of (E)- -methoxy- ,b-unsaturated ester 7. The reaction of
Et3SiH also afforded a significant amount of (E)- -substitution
product 10, though -substitution product 9 was the major
product. The use of more bulky Ph3SiH improved selectivity for
strate scope of the reaction by varying the
c-substituent of the
unsaturated acetals. -Aryl-substituted b,
c
c
-unsaturated -dime-
a
,
a
a
thoxy esters 2b–e having both electron-donating and electron-
withdrawing substituents were effective substrates in this
c
a
a
transformation (entries 4–8).
c
-Alkyl-substituted b,
c
-unsaturated
a
a,a
-dimethoxy esters 2f and 2g were also good substrates for the
c
reaction, but the primary-alkyl substituent resulted in a moderate
Table 2
Reactions of acetals 2a–g with trimethylsilyl enolate 3
Entry
Acetal
Lewis acid
Conditions
Product
Yielda (%)
E/Z Ratiob
1
2
3
4
5
6
7
8
9
2a
2a
2a
2a
2b
2c
2d
2e
2f
BF3ꢀOEt2
TiCl4
ꢁ78–0 °C for 2 h, then 0 °C, 2 h
ꢁ78–0 °C for 2 h, then 0 °C, 20 h
0 °C, 3 h
4a
4a
4a
4a
4b
4c
4d
4e
4f
91
5
56:44
100:0
—
65:35
64:36
85:15
68:32
80:20
89:11
96:4
TMSOTfc
TMSOTf
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
11
48
98
92
81
92
37e
85
d
0 °C, 3 h
ꢁ78–0 °C for 2 h, then 0 °C, 5 h
ꢁ78–0 °C for 2 h, then 0 °C, 4 h
ꢁ78–0 °C for 2 h, then 0 °C, 4 h
ꢁ78–0 °C for 2 h, then 0 °C, 4 h
ꢁ78–0 °C for 2 h, then 0 °C, 20 h
ꢁ78–0 °C for 2 h, then 0 °C, 4 h
10
2g
4g
a
b
c
Isolated yields.
Determined by 1H NMR.
The reaction was carried out using 0.1 equiv of TMSOTf.
This ratio was not determined.
The corresponding a-product was also obtained in 11% yield.
d
e

4586
H. Sugimura et al. / Tetrahedron Letters 53 (2012) 4584–4587
Table 3
Reactions of acetals 2a–g with allyltrimethylsilane 5
Entry
Acetal
Lewis acid
Conditions
Product
Yielda (%)
E/Z Ratiob
1
2
3
4
5
6
7
8
9
2a
2a
2a
2a
2b
2c
2d
2e
2f
BF3ꢀOEt2
TiCl4
ꢁ78–0 °C for 2 h, then 0 °C, 2 h
ꢁ78 °C, 25 min
6a
6a
6a
6a
6b
6c
6d
6e
6f
93
90
92
94
95
84
92
82
69
86
46:54
23:77
80:20
67:33
59:41
38:62
52:48
83:17
94:6
TMSOTfc
TMSOTfc
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
BF3ꢀOEt2
ꢁ78 °C 3 h, then 0 °C, 2 h
0 °C, 2 h
ꢁ78–0 °C for 2 h, then 0 °C, 1 h
ꢁ78–0 °C for 2 h, then 0 °C, 3 h
ꢁ78–0 °C for 2 h, then 0 °C, 1 h
ꢁ78–0 °C for 2 h, then 0 °C, 6 h
ꢁ78–0 °C for 2 h, then 0 °C, 20 h
ꢁ78–0 °C for 2 h, then 0 °C, 3 h
10
2g
6g
86:14
a
b
c
Isolated yields.
Determined by 1H NMR.
The reaction was carried out using 0.1 equiv of TMSOTf.
the
c
-substitution product (entry 2 in Table 4). This result
In summary, we have successfully developed the Lewis acid-
promoted -substitution reaction of b, -unsaturated -dime-
thoxy esters with silyl nucleophiles. The reactions using silyl
enolate and allylsilane proceed smoothly to provide -substituted
,b-unsaturated -methoxy esters in excellent yields with various
E/Z-ratios. In contrast, the reactions using silyl cyanide and silyl
hydride resulted in mixtures of - and -substitution products.
implies that the regioselectivity of the substitution of the silyl
nucleophiles is dependent on the steric effects of the nucleo-
philes used.
c
c
a,a
c
a
a
a
c
The regioselectivity seems to be dependent on the steric bulk of
the silyl nucleophiles. Further, attempts to extend this reaction
to other silyl nucleophiles and to render the reaction sequence
asymmetric are currently being pursued in our laboratory.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.06.078.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
Table 4
Reactions of acetal 2a with silyl hydride
References and notes
Entry R3SiH
Conditions
ꢁ78–0 °C, 6 h 69
Ph3SiH ꢁ78–0 °C, 1 h 86
Yield of 9a E/Z Ratiob Yield of 10a (%)
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1
2
Et3SiH
86:14
86:14
21
12
a
Isolated yields.
Determined by 1H NMR.
b
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c
-Substituted a,b-unsaturated a-methoxy esters were obtained
as E/Z-mixtures with variable ratios. The assignment of the E- and
Z- isomers was established on the basis of the chemical shift (d)
value of the vinyl proton in the 1H NMR spectra and NOE experi-
ments. The representative d value for 4a is depicted in Figure 1.
Generally, the d values of the vinyl protons in the Z-isomers of
a
-alkoxy-2-alkenoates are larger than those of the vinyl protons
in the corresponding E-isomers by approximately 1 ppm.18 In
addition, NOE measurements on the E-isomer of 4a revealed a clear
enhancement between the vinyl proton and the neighboring
methoxy group.
3. For selected examples, see: (a) Jensen, K. B.; Thorhauge, J.; Hazell, R. G.;
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Figure 1. Assignment of the E- and Z-isomers for compound 4a.

H. Sugimura et al. / Tetrahedron Letters 53 (2012) 4584–4587
4587
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15. A representative procedure for acetalization of 1a: To a solution of 1a (3.17 g,
16.7 mmol) in dry MeOH (4.4 mL) and HC(OMe)3 (4.4 mL, 40 mmol) was added
p-TsOHꢀH2O (0.31 g, 1.7 mmol), and the mixture was refluxed for 5 h. The
solution was cooled to rt, neutralized with satd NaHCO3 aq, and extracted with
Et2O (2 ꢂ 30 mL). Combined organic layers were dried and then concentrated
under reduced pressure to give crude product, which was purified by silica gel
column chromatography. Elution of the column with hexane/AcOEt (4:1)
mixture gave 2a (3.74 g, 95%) as a white solid. 1H NMR (500 MHz, CDCl3) d:
3.35 (s, 6H), 3.81 (s, 3H), 6.11 (d, J = 16 Hz, 1H), 6.97 (d, J = 16 Hz, 1H), 7.30–
7.36 (m, 3H), 7.40–7.44 (m, 2H); 13C NMR (125 MHz, CDCl3) d: 50.4, 52.9, 100.6,
124.4, 126.9, 128.6, 128.6, 135.3, 135.3, 169.0.
16.
A representative procedure for the substitution reaction of 2a with 1-
(trimethylsiloxy)styrene 3: To a solution of 2a (318 mg, 1.34 mmol) and 3
(0.402 g, 2.09 mmol) in dry CH2Cl2 (13 mL) was added BF3ꢀOEt2 (0.200 mL,
1.47 mmol) at ꢁ78 °C under Ar. The mixture was gradually warmed to 0 °C and
continued to stir at 0 °C for 2 h. After completion of the reaction (TLC), it was
quenched with satd NaHCO3 aq and extracted with CH2Cl2 (2 ꢂ 10 mL).
Combined organic layers were dried and then concentrated under reduced
pressure to give crude product, which was purified by silica gel column
chromatography. Elution of the column with hexane/AcOEt (95:5) mixture
gave (E)-4a (224 mg, 51%) and (Z)-4a (176 mg, 40%), respectively. (E)-isomer:
1H NMR (500 MHz, CDCl3) d: 3.35 (dd J = 7.3, 16.0 Hz, 1H), 3.51 (dd, J = 6.4,
16.0 Hz, 1H), 3.54 (s, 3H), 3.75 (s, 3H), 5.06 (ddd, J = 6.4, 7.3, 10.1 Hz, 1H), 5.34
(d, J = 10.1 Hz, 1H), 7.19 (t, J = 7.3 Hz, 1H), 7.29 (t, J = 7.8 Hz, 2H), 7.3 (d,
J = 7.3 Hz, 2H), 7.41 (t, J = 7.3 Hz, 2H), 7.51 (t, J = 7.3 Hz, 1H), 7.91 (d, J = 7.3 Hz,
2H); 13C NMR (125 MHz, CDCl3) d: 38.0, 45.8, 51.9, 55.3, 115.7, 126.4, 127.1,
127.9, 128.4, 128.5, 132.8, 136.7, 143.7, 145.3, 163.3, 197.8; (Z)-isomer: 1H
NMR (500 MHz, CDCl3) d: 3.43 (dd, J = 7.8, 17.0 Hz, 1H), 3.46 (dd, J = 6.4,
17.0 Hz, 1H), 3.64 (s, 3H), 3.70 (s, 3H), 4.61 (ddd, J = 6.4, 7.8, 10.1 Hz, 1H), 6.40
(d, J = 10.1 Hz, 1H), 7.16–7.32 (m, 5H), 7.38–7.42 (m, 2H), 7.48–7.53 (m, 1H),
7.90–7.94 (m, 2H); 13C NMR (125 MHz, CDCl3) d: 37.3, 44.3, 51.6, 59.6, 126.6,
127.2, 127.8, 128.4, 128.5, 129.8, 132.9, 136.5, 142.4, 145.1, 163.8, 197.2.
6. Sugimura, H.; Yoshida, K. Bull. Chem. Soc. Jpn. 1992, 65, 3209–3211.
7. Sugimura, H.; Yoshida, K. J. Org. Chem. 1993, 58, 4484–4486.
8. Sugimura, H.; Watanabe, T. Synlett 1994, 175–177.
9. For selected examples, see: (a) Evans, D. A.; Scheidt, K. A.; Johnston, J. N.; Willis,
M. C. J. Am. Chem. Soc. 2001, 123, 4480–4491; (b) Brown, S. P.; Goodwin, N. C.;
MacMillan, D. W. C. J. Am. Chem. Soc. 2003, 125, 1192–1194; (c) Gnaneshwar,
R.; Wadgonkar, P. P.; Sivaram, S. Tetrahedron Lett. 2003, 44, 6047–6049; (d)
Wang, W.; Li, H.; Wang, J. Org. Lett. 2005, 7, 1637–1639; (e) Sarabèr, F. C. E.;
Dratch, S.; Bosselaar, G.; Jansen, B. J. M.; de Groot, A. Tetrahedron 2006, 62,
1717–1725; (f) Xu, L.-W.; Yang, M.-S.; Qiu, H.-Y.; Lai, G.-Q.; Jiang, J.-X. Synth.
Commun. 2008, 38, 1011–1019; (g) Tamagaki, H.; Nawate, Y.; Nagase, R.;
Tanabe, Y. Chem. Commun. 2010, 46, 5930–5932; (h) Kemppainen, E. K.; Sahoo,
G.; Valkonen, A.; Pihko, P. M. Org. Lett. 2012, 14, 1086–1089.
10. For example, the BF3-mediated reaction of methyl (E)-2-oxo-4-phenyl-3-
butenoate (1a) with allyltrimethylsilane in CH2Cl2 at 0 °C for 6 h afforded the
corresponding Michael adduct in only 8% yield along with 70% yield of
recovered 1a. As another example using TMS enolate as a silyl nucleophile,
treatment of 1a with 1-(trimethylsiloxy)styrene in CH2Cl2 at 0 °C for 4 h in the
presence of BF3ꢀOEt2 furnished a mixture of 1,4- and 1,2-adducts in 60% and 8%
yields, respectively.
11. For example, see: (a) Bosch, J.; Salas, M.; Amat, M.; Alvarez, M.; Morgó, I.;
Adrover, B. Tetrahedron 1991, 47, 5269–5276; (b) Hanessian, S.; Ma, J.; Wang,
W. J. Am. Chem. Soc. 2001, 123, 10200–10206.
12. (a) Ireland, R. E.; Müller, R. H.; Willard, A. K. J. Org. Chem. 1976, 41, 986–996; (b)
Ireland, R. E.; Müller, R. H.; Willard, A. K. J. Am. Chem. Soc. 1976, 98, 2868–2877;
(c) Bottin-Strzalko, T.; Corset, J.; Froment, F.; Pouet, M.-J.; Seyden-Penne, J.;
Simonnin, M.-P. J. Org. Chem. 1980, 45, 1270–1276; (d) Paterson, I.; McLeod, M.
D. Tetrahedron Lett. 1997, 38, 4183–4186; (e) Seneci, P.; Leger, I.; Souchet, M.;
Nadler, G. Tetrahedron 1997, 53, 17097–17114; (f) Scheidt, K. A.; Bannister, T.
D.; Tasaka, A.; Wendt, M. D.; Savall, B. M.; Fegley, G. J.; Roush, W. R. J. Am. Chem.
17.
A
representative procedure for the substitution reaction of 2a with
allyltrimethylsilane 5: To solution of 2a (124 mg, 0.52 mmol) and
a
5
(0.10 mL, 0.60 mmol) in dry CH2Cl2 (5 mL) was added BF3ꢀOEt2 (0.070 mL,
0.55 mmol) at ꢁ78 °C under Ar. The mixture was gradually warmed to 0 °C and
continued to stir at 0 °C for 2 h. After completion of the reaction (TLC), it was
quenched with satd NaHCO3 aq and extracted with CH2Cl2 (2 ꢂ 10 mL).
Combined organic layers were dried and then concentrated under reduced
pressure to give crude product, which was purified by silica gel column
chromatography. Elution of the column with hexane/AcOEt (95:5) mixture
gave (E)-6a (55 mg, 43%) and (Z)-6a (65 mg, 50%), respectively. (E)-isomer: 1H
NMR (500 MHz, CDCl3) d: 2.40–2.47 (m, 1H), 2.48–2.55 (m, 1H), 3.51 (s, 3H),
3.72 (s, 3H), 4.48 (ddd, J = 5.0, 7.8, 10.1 Hz, 1H), 4.93 (d, J = 10.1 Hz, 1H), 5.01 (d,
J = 16.0 Hz, 1H), 5.26 (d, J = 10.1 Hz, 1H), 5.67 (ddt, J = 6.8, 10.1, 16.0 Hz, 1H),
7.12–7.30 (m, 5H); 13C NMR (125 MHz, CDCl3) d: 41.3, 41.3, 51.5, 55.0, 116.1,
116.9, 125.9, 127.0, 128.1, 135.7, 144.1, 144.8, 163.3; (Z)-isomer: 1H NMR
(500 MHz, CDCl3) d:2.42–2.48 (m, 1H), 2.50–2.56 (m, 1H), 3.61 (s, 3H), 3.74 (s,
3H), 3.96 (ddd, J = 5.0, 8.2, 10.1 Hz, 1H), 4.99 (dd, J = 0.9, 10.1 Hz, 1H), 5.04 (dd,
J = 1.3, 17.0 Hz, 1H), 5.70 (ddt, J = 7.3, 10.1, 17.0 Hz, 1H), 6.38 (d, J = 10.1 Hz,
1H), 7.17–7.32 (m, 5H); 13C NMR (125 MHz, CDCl3) d: 40.1, 41.7, 51.8, 59.9,
116.6, 126.5, 127.3, 128.5, 131.0, 135.6, 142.8, 145.1, 164.0.
18. Fuchibe, K.; Iwasawa, N. Tetrahedron 2000, 56, 4907–4915.