Thermal intramolecular Alder-Ene cycloisomerization of 1,6-allenynes

Article abstract of DOI:10.1055/s-2008-1032113

Alder-ene cycloisomerizations of 1,6-allenynes occur without catalysts under refluxing conditions. Depending on the substrate, THF, toluene, or xylenes can be used as solvents to provide the corresponding 6-methylene-1- vinylcyclohexene in up to 98% yield

Full text of DOI:10.1055/s-2008-1032113

CLUSTER
751
Thermal Intramolecular Alder–Ene Cycloisomerization of 1,6-Allenynes
Intramolecular
A
ld
l
er–Ene
i
Cyclo
v
isomerizatio
i
of 1, 6
e
-Allenyne
sr Buisine, Vincent Gandon, Louis Fensterbank, Corinne Aubert,* Max Malacria*
Laboratoire de Chimie Organique UMR 7611, Institut de Chimie Moléculaire FR2769, Université Pierre et Marie Curie-Paris 6,
Tour 44-54, 4 Place Jussieu 75252 Paris, France
Fax +33(1)44277360; E-mail: max.malacria@upmc.fr
Received 28 November 2007
external allene carbon. The silylated triene 5b was also
obtained as a single diastereomer in nearly quantitative
yield.3 However, a longer reaction time of four days was
required.
Abstract: Alder–ene cycloisomerizations of 1,6-allenynes occur
without catalysts under refluxing conditions. Depending on the sub-
strate, THF, toluene, or xylenes can be used as solvents to provide
the corresponding 6-methylene-1-vinylcyclohexene in up to 98%
yield.
Key words, allenes, alkynes, cyclizations, ene reactions, trienes
R = H
xylenes, 140 °C
12 h
[2+2]
1,6-Allenynes are versatile substrates for metal-catalyzed
cycloisomerization and cycloaddition reactions. A rich
variety of products can be obtained, as shown on
Scheme 1 (2–6).1 Purely thermal and microwave-assisted
[2+2] cycloadditions leading to compounds of type 6 have
also been described.1j,2 Under metal-free conditions, Al-
der–ene-type products 5 have been obtained as minor
compounds alongside 6.2a In this article, we describe our
findings regarding the selective transformation of 1,6-al-
lenynes into trienes 5 under heating conditions without
catalysts.
MeOH2C
MeOH2C
SiMe3
H
R
•
6a (98%)
SiMe3
MeOH2C
MeOH2C
R = Me
xylenes, 140 °C
1a (R = H)
1b (R = Me)
96 h
Alder–ene
MeOH2C
MeOH2C
SiMe3
5b (98%)
Scheme 2
This first example of highly selective thermal activation
of a 1,6-allenyne toward Alder–ene-type transformation
prompted us to study the scope of this reaction (Table 1).4
In the silylated series, disubstitution at the terminal allene
carbon by methyl groups inhibited any thermal process
(entry 1). On the other hand, when the trimethylsilyl
group was removed (R4 = H), both monosubstituted (1d,
R2 = Me, R3 = H) and disubstituted (1e, R2 = Me,
R3 = Me) allenynes underwent selective Alder–ene reac-
tions, providing trienes 5d and 5e in 98% and 69% respec-
tively (entries 2 and 3). Shorter reaction times of 10 hours
and three days were enough to reach full conversion of the
substrates. Keeping the tert-butyl group at R1, we next
tried a phenyl group at the alkyne (entry 4). As in the case
of 1e, the Alder–ene reaction of the dimethyl-substituted
allenyne 1f furnished the expected triene in 65% isolated
yield after 3 days. We next changed the tert-butyl group
for a phenyl at R1 (entries 5–8). Surprisingly, the thermal
transformation of allenyne 1g took only 10 minutes to
come to completion (entry 5). The resulting triene was
isolated in 95% yield. This unexpectedly short reaction
period led us to monitor the same transformation at a low-
er temperature. We found that allenyne 1g could be trans-
formed into 5g in 98% yield even at room temperature in
about one week (entry 6). The beneficial influence of the
phenyl substituent at R1 on the reaction time of the Alder–
ene transformation was maintained in allenyne 1h, mono-
substituted at the terminal allene carbon (entries 7 and 8).
Z
•
Z
•
•
2
3
Z
Z
Z
Z
1
4
5
6
Scheme 1
In the absence of substituents at the external allene car-
bon, [2+2] cycloaddition between the alkyne and the ter-
minal allene double bond may take place giving
compounds of type 6.2b Following this trend, we found
that allenyne 1a, bearing a silylated group at the alkyne
and a tert-butyl group at the internal allene position, could
be transformed into the bicyclo[4.2.0]octa-1,6-diene 6a
after 12 hours in refluxing xylenes (Scheme 2). In spite of
these harsh reaction conditions, the cycloadduct was iso-
lated in 98% yield after column chromatography. In sharp
contrast, a selective Alder–ene reaction occurred when
starting from allenyne 1b, bearing a methyl group at the
SYNLETT 2008, No. 5, pp 0751–0754
Advanced online publication: 26.02.2008
x
x.
x
x.
2
0
0
8
DOI: 10.1055/s-2008-1032113; Art ID: Y01707ST
© Georg Thieme Verlag Stuttgart · New York

752
O. Buisine et al.
CLUSTER
Table 1 Cycloisomerization of Allenynes 1a–i
R3
R1
R3
R1
R1
R3
R1
R3
R2
R4
R2
•
R2
R
R
+
+
R
R
R
R4
R4
R
R
R
R4
1
(R = CH2OMe)
5
6
7
Entry
1
Solvent
xylenes
xylenes
xylenes
xylenes
xylenes
xylenes
xylenes
THF
Temp (°C) Time (h)
R1
R2
R3
R4
SiMe3
H
Yield (5/6/7, %)a
0:0:0b
1
2
1c
1d
1e
1f
140
140
140
140
140
r.t.
96
10
72
72
t-Bu
t-Bu
t-Bu
t-Bu
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
98:0:0
3
Me
Me
Me
Me
Me
Me
Me
Me
H
69:0:0
4
Ph
Ph
Ph
H
65:0:0
5
1g
1g
1h
1h
1i
0.2
95:0:0
6
172
4
Ph
98:0:0
7
140
66
Ph
83:0:0
8
48
Ph
H
80:0:0
9
xylenes
toluene
140
111
72
Me
Me
Ph
Ph
50:0:21
67:0:0
10
1i
150
a Isolated yield after column chromatography.
b Starting material was recovered.
In refluxing xylenes, triene 5h was isolated in 83% yield In order to gain further insight, the transformation of the
after four hours. More strikingly, this reaction could also model allenynes A into Alder–ene products B were stud-
be carried out in refluxing THF in two days, leading to a ied in silico at the DFT/B3LYP 6-31G(d) level of theory
similar yield of 80%. Interestingly, going back to an alkyl (Scheme 3 and Table 2).5 Two distinct pathways were un-
group at R1 as in 1i (R1 = Me, entries 9 and 10), longer re- covered: a concerted one leading directly to B (path a) and
action times were required again. In refluxing xylenes, the a stepwise mechanism giving B after the intermediacy of
conversion was complete after three days (entry 9). How- the formally zwitterionic intermediate C (see also
ever, a mixture of the expected triene 5i (50%) and of the Figure 1 for the transition states).6
7-methylenebicyclo[3.2.0]hept-5-ene 7i (21%) was ob-
tained. The latter presumably arose from an unprecedent-
ed [2+2] cycloaddition between the alkyne and the
internal allene double bond. It is worthy of note that this
With R1 = Me and R2 = H, the concerted Alder–ene reac-
tion was found faster than the stepwise one (DH298 = 32.1
vs. 35.0 kcal/mol). With R1 = t-Bu and R2 = H, the step-
wise process could even not be reproduced computation-
side reaction could be avoided by lowering the tempera-
ally. In contrast, the presence of a phenyl group at R1
ture. In refluxing toluene, 5i was obtained as a sole prod-
favors the stepwise reaction over the concerted one. With
uct in 67% yield (entry 10). In this case, the reaction
required about 6 days to reach full conversion of the sub-
strate. The reason for the generation of 7i at lower temper-
ature is not clear at this time.
Table 2 Enthalpies (kcal/mol) for the Concerted (path a) and Step-
wise (path b) Alder–Ene Cycloisomerization of Allenyne A
A → B
DH298
A → C
DH298
C → B
DH298
R1
R1
DH298
DH298
DH298
•
path a
R1 = Me, –45.6
R2 = H
32.1
35.3
35.0
–80.9
2.3
H
R2
R1 = t-Bu, –36.8
34.3
32.6
–
–
–
–
a
B
R2
A
R2 = H
R1
R1
•
R1 = Ph,
R2 = H
–42.6
–
29.3
21.1
30.1
30.5
–71.9
–57.6
2.9
2.4
path b
B
R2
H
R1 = Ph,
R2 = Ph
–
a
R2
C
A
Scheme 3
a Could not be determined.
Synlett 2008, No. 5, 751–754 © Thieme Stuttgart · New York

CLUSTER
Intramolecular Alder–Ene Cycloisomerization of 1,6-Allenynes
753
R2 = H, the enthalpy of activation to give C is 2.5 kcal/ by the substitution pattern at the allene moiety. Although
mol lower (DH298 = 30.1 vs. 32.6 kcal/mol). However, the quite long in most cases, a low period of 10 minutes has
first step is markedly endothermic by 29.3 kcal/mol, thus, also been recorded. On the basis of DFT calculations, we
even with a low enthalpy of activation of 2.9 kcal/mol for believe that the reaction times can be rationalized in terms
the 1,5-proton shift, the overall process culminates at 32.2 of electronic effects. Even when several days are required,
kcal/mol (29.3 + 2.9). This value is quite close to the 32.6 the products can be isolated in high yields. Besides, even
kcal/mol of enthalpy of activation required for the con- if the thermal reaction takes longer than a catalyzed one,
certed cycloisomerization, therefore both mechanisms we encountered no problem of isomerization of the double
may coincide. Interestingly, with R2 = Ph, only the step- bonds of the triene.1f Moreover, the thermal reaction is
wise mechanism could be reproduced. The first step re- compatible with a silylated triple bond, which can be
quires 30.5 kcal/mol of enthalpy of activation and is less poorly reactive under catalytic conditions.7
endothermic (21.1 kcal/mol). From C, only 2.4 kcal/mol
are required to reach the transition state to B. Consequent-
ly, the first step is the rate-determining one.
All manipulations were carried out under argon.
It makes sense that the presence of aryl groups at R1 and
General Procedure for the Cycloisomerization of Allenynes
The allenyne was dissolved into xylenes or toluene (distilled from
R2 favors the stepwise mechanism by stabilizing the de-
CaH2). The mixture was heated under reflux. After completion (see
veloping positive and negative charges in the transition
Table 1), the solution was cooled to r.t. and concentrated in vacuo.
state. In agreement with the experimental results, the step-
The residue was submitted to flash chromatography on silica gel
wise mechanism requires less energy than the concerted
one. It explains why with one phenyl group at the allene
(Table 1, entries 9 and 10), only 4 hours are required for
the conversion of 1h in xylenes and THF can also be used
as solvent. The hypothesis of a parallel mechanism is also
corroborated with two phenyl groups (entries 7 and 8), the
reaction of 1g being over in 10 minutes in refluxing xy-
lenes. With this set of results, one can predict that the pres-
ence of a strong electron-releasing group at R1 and of a
strong electron-withdrawing group at R2 would make the
thermal Alder–ene cycloisomerization of 1,6-allenynes
very facile (push–pull effect) or even compromise the iso-
lation of the starting compound.
(PE–Et2O = 9:1).
{2-tert-Butyl-4,4-bis(methoxymethyl)bicyclo[4.2.0]octa-1,6-
dien-7-yl}trimethylsilane (6a)
According to the general procedure, 6a (98.0 mg, 98%) was pre-
1
pared from 1a (100 mg, 0.31 mmol) using 10 mL of xylenes. H
NMR (400 MHz, CDCl3): d = 0.10 (s, 9 H), 1.05 (s, 9 H), 2.03 (s, 2
H), 2.13 (t, J = 2.6 Hz, 2 H), 3.12 (t, J = 2.6 Hz, 2 H), 3.18–3.26
(ABm, 4 H), 3.32 (s, 6 H). 13C NMR (50 MHz, CDCl3): d = –1.1,
28.7 (2 C), 29.6, 35.7, 39.7, 41.9, 59.4, 76.3, 127.5, 132.4, 142.1,
157.8. IR (film): 1570, 1470, 1450, 1350, 1250, 1240, 1190, 1100,
860, 840, 750 cm–1.
(E)-{[3-tert-Butyl-5,5-bis(methoxymethyl)-2-vinylcyclohex-2-
enylidene]methyl}trimethylsilane (5b)
According to the general procedure, 5b (98.0 mg, 98%) was pre-
1
pared from 1b (100 mg, 0.30 mmol) using 10 mL of xylenes. H
NMR (200 MHz, CDCl3): d = 0.03 (s, 9 H), 1.09 (s, 9 H), 2.07 (s, 2
H), 2.22 (s, 2 H), 3.25 (s, 6 H), 3.26 (s, 4 H), 4.81 (dd, J1 = 17.9 Hz,
J2 = 2.5 Hz, 1 H), 4.98 (dd, J1 = 17.9 Hz, J2 = 11.0 Hz, 1 H), 5.21
(dd, J1 = 11.0 Hz, J2 = 2.5 Hz, 1 H), 5.49 (br s, 1 H). 13C NMR (100
MHz, CDCl3): d = 0.3, 27.9, 30.9, 35.8, 37.4, 38.5, 59.5, 74.5,
118.0, 125.8, 133.8, 138.5, 142.9, 151.4.
1-tert-Butyl-5,5-bis(methoxymethyl)-3-methylene-2-vinylcyclo-
hex-1-ene (5d)
According to the general procedure, 5d (98.0 mg, 98%) was pre-
1
pared from 1d (100 mg, 0.37 mmol) using 10 mL of xylenes. H
NMR (200 MHz, CDCl3): d = 1.18 (s, 9 H), 2.24 (s, 2 H), 2.27 (s, 2
H), 3.21 (s, 6 H), 3.31–3.35 (ABm, 4 H), 4.77 (br s, 1 H), 4.94 (dd,
J1 = 17.9 Hz, J2 = 2.3 Hz, 1 H), 5.00 (br s, 1 H), 5.27 (dd, J1 = 11.1
Hz, J2 = 2.3 Hz, 1 H), 6.45 (dd, J1 = 17.9 Hz, J2 = 11.1 Hz, 1 H). 13
C
NMR (100 MHz, CDCl3): d = 30.8, 33.2 (2 C), 37.1, 41.5, 59.5,
75.9, 112.2, 117.8, 132.2, 137.6, 142.3, 143.3. IR (CH2Cl2): 1530,
1450, 1360, 1240, 1100, 850 cm–1.
Figure 1 Transition states corresponding to A → B (left) and A →
C (right), with distances in Å
1-tert-Butyl-5,5-bis(methoxymethyl)-3-methylene-2-(prop-1-
en-2-yl)cyclohex-1-ene (5e)
According to the general procedure, 5e (69.0 mg, 69%) was pre-
In summary, we have shown that, in addition to previous-
ly described [2+2] cycloadditions, selective Alder–ene
transformations of 1,6-allenynes are also possible. We
have prepared several trienes in good to excellent yields,
without catalyst, in boiling solvents (xylenes, toluene,
THF). The reaction time seems to be strongly influenced
1
pared from 1e (100 mg, 0.35 mmol) using 10 mL of xylenes. H
NMR (200 MHz, CDCl3): d = 1.12 (s, 9 H), 1.59 (s, 3 H), 1.62 (d,
J = 2.8 Hz, 3 H), 2.20 (s, 2 H), 3.23–3.26 (ABm, 4 H), 3.26 (s, 6 H),
4.64 (br s, 1 H), 5.00 (br s, 1 H), 6.18 (t, J = 2.8 Hz, 1 H). 13C NMR
(50 MHz, CDCl3): d = 20.7, 27.9, 29.3, 34.9, 41.7, 42.4, 59.2, 74.0,
116.4, 118.8, 126.7, 135.6, 138.2, 143.5.
Synlett 2008, No. 5, 751–754 © Thieme Stuttgart · New York

754
O. Buisine et al.
CLUSTER
(E)-{[3-tert-Butyl-5,5-bis(methoxymethyl)-2-(prop-1-en-2-
yl)cyclohex-2-enylidene]methyl}benzene (5f)
References and Notes
(1) For gold-, platinum-, and gallium-catalyzed
According to the general procedure, 5f (65.0 mg, 65%) was pre-
pared from 1f (100 mg, 0.28 mmol) using 5 mL of xylenes. 1H NMR
(400 MHz, CDCl3): d = 1.23 (s, 9 H), 1.94 (s, 4 H), 2.18 (s, 3 H),
3.20–3.26 (ABm, 4 H), 3.32 (s, 6 H), 4.83 (1 H), 5.18 (s, 1 H), 6.60
(s, 1 H), 7.00–7.50 (m, 5 H). 13C NMR (50 MHz, CDCl3): d = 25.2,
30.4, 32.0, 32.9, 37.8, 38.5, 59.4, 73.8, 117.0, 125.8, 126.4, 128.0,
129.3, 136.2, 139.1, 140.5, 145.5. IR (CDCl3): 1630, 1600, 1570,
1450, 1190, 1100, 800, 690 cm–1.
cycloisomerization of 1,6-allenynes, see: (a) Lin, G.-Y.;
Yang, C.-Y.; Liu, R.-S. J. Org. Chem. 2007, 72, 6753.
(b) Matsuda, T.; Kadowaki, S.; Goya, T.; Murakami, M.
Synlett 2006, 575. (c) Matsuda, T.; Kadowaki, S.;
Murakami, M. Helv. Chim. Acta 2006, 89, 1672. (d) Lee, S.
I.; Sim, S. H.; Kim, S. M.; Kim, K.; Chung, Y. K. J. Org.
Chem. 2006, 71, 7120. (e) Cadran, N.; Cariou, K.; Hervé,
G.; Aubert, C.; Fensterbank, L.; Malacria, M.; Marco-
Contelles, J. J. Am. Chem. Soc. 2004, 126, 3408.
(f) Lemière, G.; Gandon, V.; Agenet, N.; Goddard, J.-P.; de
Kozak, A.; Aubert, C.; Fensterbank, L.; Malacria, M.
Angew. Chem. Int. Ed. 2006, 45, 7596. For rhodium-
catalyzed cycloisomerizations of 1,6-allenynes, see:
(g) Brummond, K. M.; Painter, T. O.; Probst, D. A.; Mitasev,
B. Org. Lett. 2007, 9, 347. (h) Brummond, K. M.; Chen, H.;
Sill, P.; You, L. J. Am. Chem. Soc. 2002, 124, 15186. For
cobalt-mediated Alder–ene reactions of allenynes, see:
(i) Llerena, D.; Aubert, C.; Malacria, M. Tetrahedron Lett.
1996, 37, 7027. For metal-catalyzed [2+2] cycloadditions of
1,6-allenynes, see: (j) Oh, C. H.; Gupta, A. K.; Park, D. I.;
Kim, N. Chem. Commun. 2005, 5670. (k) Shen, Q.;
Hammond, G. B. J. Am. Chem. Soc. 2002, 126, 6534.
(l) Brummond, K. M.; Kerekes, A. D.; Wan, H. J. Org.
Chem. 2002, 67, 5156.
(E)-[3-Benzylidene-5,5-bis(methoxymethyl)-2-(prop-1-en-2-
yl)cyclohex-1-enyl]benzene (5g)
According to the general procedure, 5g (95.0 mg, 95%) was pre-
1
pared from 1g (100 mg, 0.26 mmol) using 10 mL of xylenes. H
NMR (400 MHz, CDCl3): d = 1.76 (s, 3 H), 2.49 (s, 2 H), 2.69 (s, 2
H), 3.34 (br s, 10 H), 4.73 (s, 1 H), 5.07 (d, J = 1.4 Hz, 1 H), 6.63
(d, J = 1.4 Hz, 1 H), 7.21–7.37 (m, 10 H). 13C NMR (50 MHz,
CDCl3): d = 24.3, 31.0, 37.2, 39.0, 59.5, 76.0, 118.0, 126.2, 126.3,
127.7, 128.0, 128.1, 128.2, 129.3, 134.5, 135.1, 138.5, 138.8, 143.0,
144.2. IR (film): 3050, 2100, 1950, 1875, 1800, 1650, 1100, 900,
795, 750, 700 cm–1. Anal. Calcd for C26H30O2: C, 83.39; H, 8.07.
Found: C, 83.00; H, 8.17.
(E)-{[5,5-Bis(methoxymethyl)-3-methyl-2-(prop-1-en-2-yl)cy-
clohex-2-enylidene]methyl}benzene (5i)
According to the general procedure, 5i (209.0 mg, 67%) was pre-
1
(2) (a) Brummond, K. M.; Chen, D. Org. Lett. 2005, 124, 3473.
(b) Ohno, H.; Mizutani, T.; Kadoh, Y.; Miyamura, K.;
Tanaka, T. Angew. Chem. Int. Ed. 2005, 44, 5113.
(3) The configuration of 5b, as well as 5c,f,g,i (see Table 1), was
ascertained by NOE experiment between the vinyl proton at
R4 and the R3 substituent.
pared from 1i (312 mg, 1.00 mmol) using 20 mL of toluene. H
NMR (400 MHz, CDCl3): d = 1.81 (s, 3 H), 1.89 (t, J = 1.0 Hz, 3 H),
2.16 (s, 2 H), 2.46 (s, 2 H), 3.31 (s, 4 H), 3.36 (s, 6 H), 4.77 (q,
J = 1.0 Hz, 1 H), 5.24 (q, J = 1.0 Hz, 1 H), 6.51 (s, 1 H), 7.20–7.36
(m, 5 H). 13C NMR (100 MHz, CDCl3): d = 21.5, 23.4, 31.0, 36.1,
38.9, 59.5, 76.8, 115.6, 128.1, 128.6, 129.2, 130.0, 134.5, 137.0,
138.8, 144.1. Anal. Calcd for C21H28O2: C, 80.73; H, 9.02. Found:
C, 80.58; H, 9.10.
(4) We have also observed a few thermal Alder–ene
cycloisomerizations of hydroxylated 1,5-allenynes silylated
at the triple bond, see: Zriba, R.; Gandon, V.; Aubert, C.;
Fensterbank, L.; Malacria, M. Chem. Eur. J. 2008, 14, 1482.
(5) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A. Jr.;
Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.;
Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi,
M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.;
Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V.
G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.;
Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
Revision C.02; Gaussian Inc.: Wallingford (CT), 2004.
(6) The vinyl carbanionic fragment in compounds of type C can
be cis or trans. The isomer depicted in Scheme 3 was found
to be formed preferably in all cases by ca 2 kcal/mol of
enthalpy of activation.
3,3-Bis(methoxymethyl)-1-methyl-6-phenyl-7-(propan-2-
ylidene)bicyclo[3.2.0]hept-5-ene (7i)
According to the general procedure, 7i (66.0 mg, 21%) was pre-
1
pared from 1i (312 mg, 1.00 mmol) using 20 mL of xylenes. H
NMR (400 MHz, CDCl3): d = 1.54 (s, 6 H), 1.74 (s, 3 H), 2.07 (A
of AB, 2 H), 2.56 (B of AB, 2 H), 3.11–3.22 (ABm, 2 H), 3.27–3.32
(ABm, 2 H), 3.31 (s, 6 H), 7.20–7.36 (m, 5 H). 13C NMR (100 MHz,
CDCl3): d = 18.0, 24.2, 27.2, 34.9, 41.6, 50.8, 59.5, 76.3, 125.9,
126.0, 126.1, 128.1, 135.3, 138.1, 141.6, 144.1. IR (film): 3050,
2075, 1950, 1875, 1800, 1725, 1600, 1100, 750, 700 cm–1.
Acknowledgment
This work was supported by CNRS, MRES, IUF, and ANR BLAN
‘allènes’ 06-2_159258. We used extensively the computing faci-
lities of CRIHAN, Plan Interrégional du Bassin Parisien (project
2006-013).
(7) See, for example, ref. 1f and: (a) Harrison, T. J.; Dake, G. R.
Org. Lett. 2004, 6, 5023. (b) Cariou, K.; Mainetti, E.;
Fensterbank, L.; Malacria, M. Tetrahedron 2004, 60, 9745.
Synlett 2008, No. 5, 751–754 © Thieme Stuttgart · New York

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