Rh-catalyzed intermolecular and enantioselective [4 + 2] cycloaddition of 1,3-dienes with dimethyl acetylenedicarboxylate

Article abstract of DOI:10.1039/b717914f

A Rh-BINAP complex catalyzed an intermolecular and enantioselective [4 + 2] cycloaddition of 1-monosubstituted, 1,1- or 1,2-disubstituted buta-1,3-dienes with dimethyl acetylenedicarboxylate to give chiral cyclohexa-1,4-dienes. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b717914f

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Rh-catalyzed intermolecular and enantioselective [4 + 2] cycloaddition of
1,3-dienes with dimethyl acetylenedicarboxylate
Takanori Shibata,* Daisuke Fujiwara and Kohei Endo
Received 19th November 2007, Accepted 22nd November 2007
First published as an Advance Article on the web 11th December 2007
DOI: 10.1039/b717914f
A Rh-BINAP complex catalyzed an intermolecular and enantioselective [4 + 2] cycloaddition of
1-monosubstituted, 1,1- or 1,2-disubstituted buta-1,3-dienes with dimethyl acetylenedicarboxylate to
give chiral cyclohexa-1,4-dienes.
Table 1 Screening of chiral ligands in Rh-catalyzed intermolecular
Introduction
[4 + 2] cycloadditions
The [4 + 2] cycloaddition of 1,3-dienes with alkenes or alkynes is
widely recognized as a powerful synthetic tool for the construction
of 6-membered ring systems. In particular, the development of
Lewis acid-catalyzed enantioselective Diels–Alder reactions of
dienes with dienophiles, possessing heteroatom(s) as coordination
site(s), has been one of main topics in organic synthesis, and
various efficient chiral Lewis acids have been reported in both in-
Entrya
Ligandb
Yield of 3aa (%)
ee (%)
termolecular and intramolecular reactions.1 On the contrary, tran-
sition metal-catalyzed [4 + 2] cycloaddition is another approach.
For instance, in 1983, iron complex-catalyzed intermolecular [4 +
2] cycloadditions of 1,3-dienes with alkynes were reported.2 Rh
complexes have also shown themselves to be efficient catalysts.3
Since 1989, intramolecular [4 + 2] cycloadditions of dienynes have
been comprehensively studied using Ni4 and Rh catalysts.5 Enan-
tioselective cycloadditions have also been realized using chiral Rh6
and Ir catalysts.7 However, an example of intermolecular and
enantioselective cycloaddition has been reported using a chiral
cobalt catalyst, where a reaction of only a 1-boron functionalized-
3-methylbuta-1,3-diene and trimethylsilylacetylene was disclosed.8
We disclose here the Rh-catalyzed intermolecular and enantios-
elective [4 + 2] cycloaddition of substituted buta-1,3-dienes and
dimethyl acetylenedicarboxylate (DMAD).9
1
2
3
4
5
6
7
(S)-BINAP
64
62
37
72
32
<5
Trace
67
69
72
45
22
9
(S)-Tol-BINAP
(S)-xylyl-BINAP
(S)-BDPP
(S)-NORPHOS
(S)-CHIRAPHOS
(S,S)-Me-DUPHOS
—
a Diene 1a/DMAD = 3 : 1. b Tol-BINAP: 2,2ꢀ-bis(di-p-tolylphosphino)-
1,1ꢀ-binaphthyl, xylyl-BINAP: 2,2ꢀ-bis(di(3,5-xylyl)phosphino)-1,1ꢀ-
binaphthyl, BDPP: 2,4-bis(diphenylphosphino)pentane, NORPHOS:
2,3-bis(diphenylphosphino)bicyclo[2.2.1]hept-5-ene, CHIRAPHOS: 2,3-
bis(diphenylphosphino)butane, Me-DUPHOS: 1,2-bis(2,5-dimethyl-
phospholano)benzene.
alkynyl diester, the reaction gave a complex mixture.10 Product 3ab
could not be completely purified but its ee was comparable with
that of DMAD (Table 2, entry 2). In contrast, in the case of 1,4-
diphenyl-2-butyne-1,4-dione (2c), the corresponding cycloadduct
3ac was obtained in good yield but the ee was very poor (Table 2,
entry 3). [4 + 2] Cycloadducts could not be detected in the
reaction with N,N,Nꢀ,Nꢀ-tetramethyl acetylenedicarboxamide (2d)
under the same reaction conditions (Table 2, entry 4). Alkynyl
monoesters did not give the corresponding [4 + 2] cycloadducts at
all (Table 2, entries 5 and 6). These results imply that the oxygen
atom of the carbonyl moiety is important for the induction of high
enantioselectivity, and that two electron-withdrawing groups are
indispensable for the cross-coupling of 1,3-dienes and alkynes.11
Various 1-substituted buta-1,3-dienes were submitted to the
reaction with DMAD (Table 3). (E)- and (Z)-penta-1,3-diene
clearly showed different reactivities (Table 3, entries 1 and 2). That
is, it took only 4 h for the complete consumption of DMAD, and
the highest ee of 94% was achieved by the opposite enantiomer 3ba.
It is noteworthy that both enantiomers were selectively obtained
from the choice of diene geometry using the same chiral catalyst.
When a phenyl group was introduced, the ee decreased (Table 3,
Results and discussion
We chose the reaction of 1-alkyl-substituted (E)-buta-1,3-diene
1a and DMAD as a model reaction, and examined several chiral
phosphorus ligands in cationic Rh complexes, which are already
known as efficient catalysts in [4 + 2] cycloadditions (Table 1).3
BINAP derivatives were appropriate ligands, and cyclohexa-1,4-
diene 3aa was obtained in moderate ee (Table 1, entries 1–3). A
Rh-BDPP complex gave the product in good yield, yet with lower
ee (Table 1, entry 4), and CHIRAPHOS and Me-DUPHOS were
unsuitable ligands (Table 1, entries 6 and 7).
When an isolated Rh-BINAP complex was used as the chiral cat-
alyst, the enantioselectivity exceeded 80% (Table 2, entry 1). Under
the same reaction conditions, we next examined several alkynes
as coupling partners (Table 2, entries 2–6). When 3-hexyne-2,5-
dione (2b), namely an alkynyl diketone, was used in place of an
Department of Chemistry and Biochemistry, School of Advanced Science and
Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo, 169-8555,
Japan. E-mail: tshibata@waseda.jp; Fax: +81 352868098
464 | Org. Biomol. Chem., 2008, 6, 464–467
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Table 2 Screening of alkynes in Rh-catalyzed intermolecular [4 + 2]
Scheme 2 shows a proposed mechanism for our intermolecular
[4 + 2] cycloadditions. p-Complexation of a diene and an alkyne to
the chiral rhodium complex forms the beginning of the reaction.
The 3,4-position of a 1-substituted-1,3-diene is an exo olefin and
less sterically hindered than the 1,2-position. Therefore, oxidative
coupling proceeds between the 3,4-position of the diene and
DMAD. Metallacyclopentene A would therefore be preferentially
obtained because of steric repulsion between the methoxycarbonyl
and alkenyl groups in Aꢀ. A following 1,3-allylic rearrangement
and reductive elimination then give the cyclohexa-1,4-diene. When
a (Z)-1,3-diene (Rꢀ is not hydrogen) is used, the 1,3-allylic
rearrangement would probably proceed readily because of steric
repulsion between Rꢀ and the ligand at the metal center. The
enantioselectivity is determined at the oxidative coupling step,
and the steric discrimination of a large alkenyl group and a small
hydrogen would induce high enantioselectivity. In practice, when
hydrogen is replaced by a methyl group at the 3-position, the ee
drastically decreases (Scheme 3).
cycloadditions
Entrya
R
Rꢀ
Alkyne
Yield (%)
ee (%)
1
2
3
4
5
6
CO2Me
C(O)Me
C(O)Ph
C(O)NMe2
CO2Me
CO2Me
CO2Me
C(O)Me
C(O)Ph
C(O)NMe2
Me
2a
2b
2c
2d
2e
2f
65 (3aa)
ca. 30 (3ab)
69 (3ac)
—
—
—
82
78
7
—
—
—
Ph
a Diene 1a/alkyne = 3 : 1.
Table 3 Intermolecular [4 + 2] cycloaddition of various 1,3-dienes with
DMAD
Entrya
Rꢀꢀ
Dieneb
Time/h
Yield (%)
ee (%)
1
2
3
4
5
6
Me
Me
Ph
1b-E
1b-Zc
1c
24
4
24
24
24
24
51 (3ba)
51 (3ba)
65 (3ca)
54 (3da)
62 (3ea)
66 (3fa)
78
−94
65
89
84
BnO(CH2)2
TBSO(CH2)2
MeTsN(CH2)2
1d
1e
1f
89
a Diene 1/alkyne = 3 : 1. b (E)-1,3-Dienes were used, except entry 2. c (Z)-
1,3-Diene was used.
entry 3). Functionalized 1,3-dienes 1d–1f, with an oxygen or
nitrogen atom, were also tolerable, and the corresponding products
were obtained in relatively high ee (Table 3, entries 4–6).
We further examined 1,1-disubstituted buta-1,3-diene 1g under
the same reaction conditions and obtained cyclohexa-1,4-diene
3ga, with a quaternary carbon stereocenter, in good ee (Scheme 1).
1,2-Disubstituted buta-1,3-diene 1h was also a good substrate, and
bicyclic compound 3ha was obtained.
Scheme 2
Scheme 3
Conclusion
We have developed an intermolecular and enantioselective [4 +
2] cycloaddition. The chiral Rh complex-catalyzed reaction of
1-monosubstituted, and 1,1- and 1,2-substituted 1,3-dienes with
DMAD gave cyclohexa-1,4-dienes in good to high ee.
Scheme 1
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27.0, 32.4, 52.1, 52.2, 121.3, 128.9, 129.4, 140.2, 167.7 and 169.2;
HRMS (FAB, positive): [M + Na]+ found m/z 233.0791, calc. for
C11H14O4Na 233.0790; [a]D24 = 26.27 (c 0.87 in CHCl3, 78% ee from
(E)-penta-1,3-diene). The ee was determined by HPLC analysis
using Daicel Chiralpak AS-H (eluent: 1% 2-propanol in hexane,
flow rate: 1.0 mL min−1, retention time: 10 min for the minor
isomer and 12 min for the major isomer).
Experimental
General
Anhydrous 1,2-dichloroethane (DCE) is commercially available.
It was dried over molecular sieves 4A (MS 4A) and de-gassed by
argon bubbling before use. All reactions were conducted under an
argon atmosphere. IR spectra were recorded with a Horiba FT730
spectrophotometer. NMR spectra were measured with JEOL
AL-400 and Lambda500 spectrometers using tetramethylsilane
as an internal standard and CDCl3 as a solvent. Mass spectra
were measured with a JEOL JMS-SX102A instrument. Optical
rotations were measured with a Jasco DIP-1000 polarimeter.
Dimethyl 3-phenylcyclohexa-1,4-diene-1,2-dicarboxylate (3ca)
Colorless oil. IR (CH2Cl2): 2951, 1724 and 1259 cm−1; 1H NMR:
d 3.02–3.05 (m, 1H), 3.20–3.23 (m, 1H), 3.53 (s, 3H), 3.77 (s, 3H),
4.37–4.39 (m, 1H), 5.74–5.82 (m, 2H) and 7.18–7.36 (m, 5H); 13
C
NMR: d 27.4, 44.1, 51.9, 52.2, 121.2, 127.1, 127.4, 128.4, 128.6,
131.0, 136.9, 141.4, 168.0 and 168.1; HRMS (FAB, positive): [M +
H]+ found m/z 273.1147, calc. for C16H17O4 273.1127; [a]2D2 = 75.45
(c 1.00 in CHCl3, 65% ee). The ee was determined by HPLC
analysis using Daicel Chiralpak AS-H (eluent: 10% 2-propanol
in hexane, flow rate: 0.5 mL min−1, retention time: 16 min for the
minor isomer and 18 min for the major isomer).
Typical experimental procedure (Table 3)
[Rh(cod)(binap)]BF4 (9.2 mg, 0.010 mmol) was stirred in CH2Cl2
(1.0 mL) at room temperature under an atmosphere of argon. The
flask was purged with hydrogen gas and the solution stirred for a
further 30 min. After the solvent and hydrogen had been excluded
under reduced pressure, argon gas was introduced. DCE (0.1 mL)
was added to the flask, followed by a DCE solution (0.4 mL) of
1,3-diene 1 (0.30 mmol) an◦d DMAD (14.2 mg, 0.1 mmol), and the
mixture was stirred at 40 C for 24 h. The solvent was removed
under reduced pressure and the resulting crude products were
purified by thin-layer chromatography to give pure cycloadduct 3.
The ee was determined by HPLC analysis using a chiral column.
Dimethyl 3-(2-(benzyloxy)ethyl)cyclohexa-1,4-diene-1,
2-dicarboxylate (3da)
1
Colorless oil. IR (CH2Cl2): 2949, 2862, 1722 and 1257 cm−1; H
NMR: d 1.71–1.80 (m, 1H), 1.93–2.01 (m, 1H), 2.86–3.04 (m,
2H), 3.33–3.42 (m, 1H), 3.51 (d, J = 6.8 Hz, 1H), 3.53 (d, J =
6.8 Hz, 1H), 3.73 (s, 3H), 3.77 (s, 3H), 4.44 (d, J = 11.8 Hz, 1H),
4.50 (d, J = 11.8 Hz, 1H), 5.71–5.75 (m, 2H) and 7.24–7.37 (m,
5H); 13C NMR: d 27.2, 33.9, 34.9, 52.2, 67.2, 72.9, 122.6, 126.9,
127.5, 127.6, 128.3, 130.8, 138.4, 138.7, 167.7 and 168.9 (a signal
in the aliphatic region was overlapped); HRMS (FAB, positive):
Dimethyl 3-heptylcyclohexa-1,4-diene-1,2-dicarboxylate (3aa)
Colorless oil. IR (CH2Cl2): 2927, 1730 and 1257 cm−1; 1H NMR:
d 0.87 (t, J = 6.8 Hz, 3H), 1.21–1.33 (m, 10H), 1.41–1.58 (m, 2H),
2.85–3.06 (m, 2H), 3.19–3.28 (m, 1H), 3.76 (s, 3H), 3.79 (s, 3H),
5.64–5.70 (m, 1H) and 5.72–5.78 (m, 1H); 13C NMR: d 14.1, 22.6,
25.2, 27.1, 29.1, 29.6, 31.8, 33.9, 37.5, 52.1, 52.1, 122.5, 127.2,
129.9, 139.8, 167.6 and 169.4; HRMS (FAB, positive): [M + H]+
found m/z 295.1911, calc. for C17H27O4 295.1909; [a]2D2 = 62.04 (c
0.21 in CHCl3, 82% ee). The ee was determined by HPLC analysis
using Daicel Chiralpak AS-H (eluent 1% 2-propanol in hexane,
flow rate: 0.5 mL min−1, retention time: 12 min for the major
isomer and 14 min for the minor isomer).
[M + H]+ found m/z 331.1581, calc. for C19H23O5 331.1545; [a]2D2
=
70.29 (c 0.90 in CHCl3, 89% ee). The ee was determined by HPLC
analysis using Daicel Chiralcel OJ-H (eluent: 10% 2-propanol in
hexane, flow rate: 1.0 mL min−1, retention time: 20 min for the
major isomer and 24 min for the minor isomer).
Dimethyl 3-(2-(tert-butyldimethylsiloxy)ethyl)cyclohexa-1,
4-diene-1,2-dicarboxylate (3ea)
Colorless oil. IR (CH2Cl2): 2952, 1728 and 1257 cm−1; 1H NMR:
d 0.04 (s, 6H), 0.88 (s, 9H), 1.57–1.66 (m, 2H), 1.83–1.91 (m, 1H),
2.87–3.05 (m, 2H), 3.32–3.35 (m 1H), 3.64–3.76 (m, 1H), 3.79
(s, 3H), 3.80 (s, 3H) and 5.75 (s, 2H); 13C NMR: d −5.4, −5.3,
18.2, 25.9, 27.2, 34.7, 37.1, 52.1, 60.0, 122.4, 126.9, 130.3, 139.3,
167.6 and 169.0; HRMS (FAB, positive): [M + H]+ found m/z
355.1933, calc. for C18H31O5Si 355.1941; [a]2D5 = 44.63 (c 1.20 in
CHCl3, 84% ee). The ee was determined by HPLC analysis using
Daicel Chiralpak AD-H (eluent: 1% 2-propanol in hexane, flow
rate: 0.5 mL min−1, retention time: 8 min for the minor isomer and
9 min for the major isomer).
3-Heptyl-1,2-bis(phenylcarbonyl)cyclohexa-1,4-diene (3ac)
Colorless oil. IR (CH2Cl2): 2927, 1658 and 1265 cm−1; 1H NMR:
d 0.85 (t, J = 7.0 Hz, 3H), 1.12–1.57 (m, 12H), 2.89–2.97 (m, 1H),
3.32–3.39 (m, 1H), 3.58 (s, 1H), 5.88–5.92 (m, 2H) and 7.25–7.52
(m, 10H); 13C NMR: d 14.1, 22.6, 26.0, 29.0, 29.1, 29.5, 31.7, 34.7,
38.2, 122.7, 128.2, 128.3, 128.5, 129.1, 129.2, 133.0, 133.1, 136.8,
137.6, 138.2, 142.0, 198.4 and 198.5; HRMS (FAB, positive): [M +
H]+ found m/z 387.2362, calc. for C27H31O2 387.2324; [a]2D1 = 36.28
(c 1.18 in CHCl3, 7% ee). The ee was determined by HPLC analysis
using Daicel Chiralpak AS-H (eluent: 10% 2-propanol in hexane,
flow rate: 0.5 mL min−1, retention time: 8 min for the minor isomer
and 9 min for the major isomer).
Dimethyl 3-(2-(N-methyl-N-tosylamino)ethyl)cyclohexa-1,
4-diene-1,2-dicarboxylate (3fa)
Dimethyl 3-methylcyclohexa-1,4-diene-1,2-dicarboxylate (3ba)
Colorless oil. IR (CH2Cl2): 2951, 1726 and 1265 cm−1; 1H NMR:
d 1.62–1.69 (m, 1H), 1.84–1.94 (m, 1H), 2.43 (s, 3H), 2.69 (s, 3H),
2.74–2.80 (m, 1H), 2.95–2.98 (m 2H), 3.26–3.33 (m, 2H), 3.76 (s,
3H), 3.79 (s, 3H), 5.80 (s, 2H), 7.31 (d, J = 8.0 Hz, 2H) and 7.64
Colorless oil. IR (CH2Cl2): 2952, 1720 and 1255 cm−1; 1H NMR:
d 1.15 (d, J = 3.6 Hz, 3H), 2.84–3.09 (m, 2H), 3.16–3.27 (m, 1H),
3.76 (s, 3H), 3.80 (s, 3H) and 5.60–5.73 (m, 2H); 13C NMR: d 20.5,
466 | Org. Biomol. Chem., 2008, 6, 464–467
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(d, J = 8.0 Hz, 2H); 13C NMR: d 21.4, 27.2, 31.6, 34.6, 34.7,
46.7, 52.2, 123.3, 126.1, 127.3, 129.6, 131.4, 134.5, 137.8, 143.2,
167.4 and 168.6 (a signal in the aliphatic region was overlapped);
HRMS (FAB, positive): [M + H]+ found m/z 408.1506, calc. for
C20H26O6NS 408.1481; [a]2D4 = 40.67 (c 0.70 in CHCl3, 89% ee).
The ee was determined by HPLC analysis using Daicel Chiralpak
AD-H (eluent: 10% 2-propanol in hexane, flow rate: 1.0 mL min−1,
retention time: 14 min for the major isomer and 19 min for the
minor isomer).
retention time: 8 min for the major isomer and 12 min for the
minor isomer).
Acknowledgements
We thank Prof. P. A. Evans (University of Liverpool, UK) for
his helpful advice on the synthesis of N,N,Nꢀ,Nꢀ-tetramethyl
acetylenedicarboxamide. We also thank Ai Kawachi (Waseda
University, Japan) for her experimental assistance. This research
was supported by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan.
Dimethyl 3-((benzyloxy)methyl)-3-methylcyclohexa-1,4-diene-1,2-
dicarboxylate (3ga)
Colorless oil. IR (CH2Cl2): 2952, 1726 and 1261 cm−1; 1H NMR:
d 1.19 (s, 3H), 2.99 (s, 2H), 3.30–3.39 (m, 1H), 3.44–3.52 (m, 1H),
3.74 (s, 6H), 4.44–4.58 (m, 2H), 5.50–5.57 (m, 1H), 5.74–5.84 (m,
1H) and 7.20–7.35 (m, 5H); 13C NMR: d 15.1, 23.7, 27.0, 41.0,
51.9, 52.1, 73.4, 122.4, 127.4, 127.4, 127.5, 128.2, 128.7, 131.3,
138.5, 166.7 and 169.0; HRMS (FAB, positive): [M + H]+ found
m/z 331.1544, calc. for C19H23O5 331.1546. The ee was determined
by HPLC analysis using Daicel Chiralpak AS-H (eluent: 5% 2-
propanol in hexane, flow rate: 0.5 mL min−1, retention time: 11 min
for the major isomer and 12 min for the minor isomer).
Notes and references
1 Recent selected reviews: (a) D. Carmona, M. P. Lamata and L. A. Oro,
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Weinheim, 2001, pp. 5; (c) E. J. Corey, Angew. Chem., Int. Ed., 2002,
41, 1650.
2 H. T. Dieck and R. Diercks, Angew. Chem., Int. Ed. Engl., 1983, 22,
778.
3 (a) I. Matsuda, M. Shibata, S. Sato and Y. Izumi, Tetrahedron Lett.,
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4 (a) P. A. Wender and T. E. Jenkins, J. Am. Chem. Soc., 1989, 111, 6432;
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Dimethyl bicyclo[4.4.0]deca-1,4-diene-4,5-dicarboxylate (3ha)
Colorless oil. IR (CH2Cl2): 2931, 1728 and 1263 cm−1; 1H NMR:
d 1.16–1.52 (m, 3H), 1.82–2.00 (m, 4H), 2.26–2.29 (m, 1H), 2.85–
3.03 (m, 3H), 3.75 (s, 3H), 3.79 (s, 3H) and 5.36 (bs, 1H); 13C NMR:
d 26.5, 27.4, 28.0, 33.2, 35.2, 40.1, 52.1, 52.1, 113.9, 128.7, 137.0,
139.6, 167.7 and 169.5; HRMS (FAB, positive): [M + H]+ found
m/z 251.1302, calc. for C14H19O4 251.1283; [a]2D1 = 12.36 (c 1.68 in
CHCl3, 78% ee). The ee was determined by HPLC analysis using
Daicel Chiralpak AS-H (eluent: 10% 2-propanol in hexane, flow
rate: 0.5 mL min−1, retention time: 12 min for the minor isomer
and 16 min for the major isomer).
Dimethyl 3-heptyl-5-methylcyclohexa-1,4-diene-1,2-dicarboxylate
(3ia)
7 T. Shibata, K. Takasaku, Y. Takesue, N. Hirata and K. Takagi, Synlett,
2002, 1681.
Colorless oil. IR (CH2Cl2): 2927, 1728 and 1259 cm−1; 1H NMR:
d 0.81–0.93 (m, 3H), 1.16–1.64 (m, 12H), 1.74 (s, 3H), 2.71–2.99
(m, 2H), 3.16–3.29 (m, 1H), 3.76 (s, 3H), 3.79 (s, 3H) and 5.37
(s, 1H); 13C NMR: d 14.1, 22.6, 22.6, 25.3, 29.2, 29.7, 31.8, 31.8,
34.1, 38.7, 52.0, 52.1, 121.6, 129.7, 129.9, 140.2, 167.5 and 169.5;
HRMS (FAB, positive): [M + H]+ found m/z 309.2066, calc. for
C18H29O4 309.2066; [a]2D2 = −4,71 (c 0.90 in CHCl3, 10% ee). The
ee was determined by HPLC analysis using Daicel Chiralpak AS-
H (eluent: 5% 2-propanol in hexane, flow rate: 0.5 mL min−1,
8 G. Hilt, W. Hess and K. Harms, Org. Lett., 2006, 8, 3287.
9 Fairly recently, an enantioselective [4 + 2] cycloaddition using a Rh-
chiral diene complex was reported that includes two examples of
intermolecular reactions: R. Shintani, Y. Sannohe, T. Tsuji and T.
Hayashi, Angew. Chem., Int. Ed., 2007, 46, 7277.
10 The only identified by-product was a [2 + 2 + 2] cycloadduct of two
alkynes and diene 1a.
11 Actually, the reaction of diene 1a with ethyl 4,4,4-trifluorobut-2-
ynoate gave the [4 + 2] cycloadducts, but as an inseparable mixture
of regioisomers.
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The Royal Society of Chemistry 2008
Org. Biomol. Chem., 2008, 6, 464–467 | 467
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