Rhodium(I)-catalyzed [3+2+2]-cyclization of alkenyl Fischer carbene complexes with methyl buta-2,3-dienoate

Article abstract of DOI:10.1016/j.tet.2010.04.130

A variety of alkenyl chromium Fischer carbene complexes react with methyl buta-2,3-dienoate in the presence of a rhodium catalyst to afford substituted cycloheptene derivatives 7, along with minor amounts of cyclopentene derivatives 6. Through this [3+2+2]-cyclization two allene units and one alkenyl carbene ligand are assembled in a selective manner.

Full text of DOI:10.1016/j.tet.2010.04.130

Tetrahedron 66 (2010) 6335e6339
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Rhodium(I)-catalyzed [3þ2þ2]-cyclization of alkenyl Fischer carbene complexes
with methyl buta-2,3-dienoate
*
José Barluenga , Rubén Vicente, Luis A. López, Miguel Tomás
Instituto Universitario de Química Organometálica ‘Enrique Moles’, Unidad Asociada al C.S.I.C., Universidad de Oviedo, Julián Clavería 8, 33071 Oviedo, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A variety of alkenyl chromium Fischer carbene complexes react with methyl buta-2,3-dienoate in the
presence of a rhodium catalyst to afford substituted cycloheptene derivatives 7, along with minor
amounts of cyclopentene derivatives 6. Through this [3þ2þ2]-cyclization two allene units and one
alkenyl carbene ligand are assembled in a selective manner.
Received 12 March 2010
Received in revised form 22 April 2010
Accepted 29 April 2010
Available online 6 May 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Fischer carbenes
Rhodium(I) carbenes
Allenes
Cycloaddition
Carbene transfer
1. Introduction
presence of catalytic amounts of rhodium(I) (Scheme 1). In-
terestingly, we found not only a differential reactivity compared
Fischer carbene complexes of group 6 metals are valuable tools
in stoichiometric transition metal-mediated organic synthesis since
they feature a variety of interesting and unique reaction modes.1 In
a number of cases, complex structural frameworks, hardly accessi-
ble by traditional methodologies, become available in a single step
by using these metal reagents. Despite their synthetic potential,
studies concerning these compounds were restricted mainly to
readily available, fairly stable group 6 metal complexes. Recently,
we and others have reported on new metal carbenes of groups 9e11
by the chromium-metal exchange reaction.2 In particular, we were
able not only to isolate and characterized by X-ray analysis new
rhodium(I) carbenes, but also to find that they feature a divergent
reactivity with respect to that of parent chromium carbenes.3
Doubtless, the reaction with unsaturated carbon-based sub-
strates -cyclopropanation of alkenes and the annulation with
with the uncatalyzed reaction,6 but the reaction course can be
finely modulated depending on the electronic demand of the allene
substrate by appropriate choice of the rhodium(I) catalyst (Scheme
1). Thus, neutral and electron-rich allenes 2 afforded the [3þ2]
cycloadduct 3 upon reaction with carbenes 1 in the presence of
either the cationic complex [Rh(cod)(naphthalene)][SbF6] (Wender
catalyst) or the neutral complex [Rh(CO)2Cl]2 under CO atmo-
sphere.7,8 In a complementary fashion, the use of the neutral
complex [Rh(cod)Cl]2 resulted in the formation of the [3þ2þ2]
cycloadduct 4.9 Finally, initial results with electron-poor allenes 5
revealed that the [3þ2] cycloadduct 6 was formed using the cat-
ionic Wender catalyst under CO atmosphere.8 Interestingly, the
R2
MeO
R3
R2
OMe
€
R2
R3
alkynes (Dotz reaction)- constitutes the basic CeC bond-forming
[Rh(I)]+
ref. 7,8
OMe
[Rh(I)]
ref. 9
+
reactions using group 6 carbene complexes. On the contrary, the
reactivity of these carbenes toward allenes, whose potential for
CeC bond formation has been well established over the last few
years,4 has been much less explored.5
R3
R2
•
R1
R1
(CO)5Cr
R1
R3
4
3
1
2
Recently, we become involved in the study of the reactivity of
Fischer carbene complexes with neutral and activated allenes in the
[Rh(I)]+
CO
ref. 8
OMe
CO2Me
OMe
[Rh(I)]
this work
+
?
CO2Me
•
(CO)5Cr
R1
R1
1
5
6
* Corresponding author. Tel./fax: þ34 985 10 34 50; e-mail address: barluenga@
Scheme 1.
uniovi.es (J. Barluenga).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.04.130

6336
J. Barluenga et al. / Tetrahedron 66 (2010) 6335e6339
chemoselectivity of the cyclization was again complementary to
that observed with electron-rich allenes (the terminal and internal
C]C bond of the allene, respectively were involved).
Taking into consideration the intrinsic interest of the complete
process as well as the significance of the resulting cycloadducts
(cyclopentenone and cycloheptanone precursors) we report herein
our studies of the rhodium-catalyzed reaction of chromium car-
benes 1 and methyl 2,3-butadieoate 5 that establish a protocol for
a new three-component heptannulation reaction.
was formed with higher selectivity (6a/7a¼1:6) albeit in lower
yield (30%). On the other hand, other donor ligand-containing Rh
(I)-catalysts tested (entries 9e11) did not provide satisfactory
results since complex mixtures of unidentified compounds were
obtained.
Accordingly, we decided to use [Rh(CO)2Cl2]2 (10 mol %), as the
optimal catalyst, in dioxane to accomplish the [3þ2þ2] cyclization
with various chromium carbene complexes 1 (Table 2). Thus, when
b-aryl substituted carbene complexes 1aec were reacted with
methyl 2,3-butadienoate cycloheptenes 7aec were obtained in
moderate yields, along with minor amounts of the corresponding
cyclopentenes 6aec. Although the heptannulation reaction works
for aryl substituents of different electronic nature, the presence of
an electron-donating group (OMe) results in increasing the yield of
the [3þ2þ2] cycloadduct. The presence of a second transition-
metal on the carbene 1d (Fc¼ferrocenyl) has no significant effect on
the reaction, affording 7d in comparable yield and selectivity. In-
2. Results and discussion
Previously, it was found that the reaction of carbene complex 1a
(R1¼4-MeO-C6H4) and methyl 2,3-butadienoate 5 using [Rh(cod)
(naphthalene)][SbF6] (10 mol %) under a CO atmosphere leads
exclusively to the cycloadduct 6a.8 We have now observed that
running this reaction in the absence of CO afforded a mixture of
cyclopentene 6a and cycloheptene 7a (48 and 14% yield, respectively,
after separation by column chromatography; Table 1, entry 1).
terestingly, the use of b-alkyl and b,b-dialkyl substituted carbene
complexes led exclusively to the desired cycloheptene derivatives
7eeg. In these cases, however, the E,E diastereoisomer was formed
along with minor amounts of the E,Z diastereoisomer (from 3:1 to
4:1 ratio).10 Unfortunately, this protocol seems restricted to mon-
osubstiuted allenes like 5. Thus, further attempts with 2,3-buta-
dienoate methyl esters having an alkyl/aryl substituent at either
C-2 or C-4 were unsuccessful.
Table 1
Optimization for Rh(I)-catalyzed [3þ2þ2]-cyclization of alkenyl Fischer carbene
complex 1a with methyl 2,3-butadienoate 5a
CO2Me
MeO
+
OMe
CO2Me
OMe
[Rh] cat.
+
CO2Me
solvent
•
PMP
(CO)5Cr
25ºC
PMP
PMP
CO2Me
Table 2
1a
5
6a
7a
Rh(I)-catalyzed [3þ2þ2]-cyclization of alkenyl Fischer carbene complexes 1a-g with
methyl 2,3-butadienoate 5a,b
Entry
[Rh]
Solvent
6ab (%)
7ab (%)
1
2
3
4
5
6
7
8
[Rh(cod)(C10H8][SbF6]
[Rh(CO)2Cl]2
[Rh(CO)2Cl]2
[Rh(CO)2Cl]2
[Rh(CO)2Cl]2
[Rh(CO)2Cl]2
[Rh(CO)2Cl]2
[Rh(cod)Cl]2
Rh(PPh3)3Cl
CH2Cl2
CH2Cl2
DMF
MeCN
THF
48
42
dc
40
27
40
15
5
14
28
dc
d
27
20
53
30
ec
ec
ec
DME
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
9
10
11
ec
ec
ec
Rh(PPh3)2(CO)Cl
Rh(IMes)(cod)Cl
a
Reaction conditions: 1a (0.5 mmol),
5 (1.5 mmol), [Rh] (10 mol %), solvent
(10 mL), 25 ꢁC, 4e24 h.
b
Yield of isolated products after chromatographic separation (SiO2, hexane/ethyl
acetate, 3:1 v/v).
c
A complex mixture of unidentified compounds was obtained.
Despite that a 6a/7a mixture was indeed formed it is relevant
that the formation of the [3þ2þ2] cycloadduct 7a occurs in
a chemo-, regio- and stereoselective manner. Therefore, further
efforts were made in order to uncover this particular heptannula-
tion reaction in a more efficient way. To achieve this goal several
rhodium(I) catalysts and solvents were screened (see Table 1, en-
tries 2e11). Firstly, the neutral catalyst [Rh(CO)2Cl2]2 (10 mol %,
CH2Cl2) afforded a mixture of 6a/7a in moderate yield (7a, 28%) and
selectivity (6a:7a¼1.5:1) (Table 1, entry 2). Although this result
seems rather modest, the enhancement of the selectivity toward 7a
proved the feasibility to control the reaction toward the cyclo-
heptene derivatives. Thus, we tested a variety of solvents (Table 1,
entries 2e7). Running the reaction in highly polar solvents either
led to a complex mixture of unidentified products (DMF, entry 3) or
gave exclusively 6a (MeCN, entry 4). When ethereal solvents (THF,
DME, dioxane) were used more promising results were reached in
terms of yield and selectivity (entries 5e7), particularly in the case
of dioxane (7a, 53%, 6a/7a¼1:3.5, entry 7). At this point, we
observed a dual effect when the CO ligands of the catalyst were
replaced with 1,5-cyclooctadiene (entry 8). Thus, using the catalyst
[Rh(cod)Cl2]2, under the same reaction conditions, compound 7a
a
Reaction conditions: 1aeg (0.5 mmol), 5 (1.5 mmol), [Rh(CO)2Cl]2 (10 mol %),
dioxane (10 mL), 25 ꢁC, 16e24 h.
b
Yields of isolated products.
c Obtained as a mixture of stereoisomers (estimated by 1H NMR). Fc¼Ferrocenyl.
The 1D- and 2D-NMR spectroscopic data of compounds 7aeg
are in agreement with their structure. Moreover, the structure of 7b
was unambiguously determined by a single-crystal X-ray diffrac-
tion analysis (Fig. 1).11 Analysis of the E,E-dienyl moiety reveals
a slightly deviation from the coplanar conformation in the solid-
state (C15eC5eC6eC18 dihedral angle¼ꢀ28.3ꢁ).12
A mechanistic proposal for the formation of compounds 7 is
illustrated in Scheme 2. The process would initiate by Cr(0)-to-Rh
(I) carbene ligand transfer leading to the Rh(I) carbene complex
I.2,3,7e9,13 Then, a chemo- and regioselective metalla-[4þ2]-cy-
cloaddition involving the terminal C]C bond of the allene would

J. Barluenga et al. / Tetrahedron 66 (2010) 6335e6339
6337
carbon (II and III; regioselectivity), (ii) the insertion of the allene
unit occurs into the bond between Rh and the methoxy-substituted
carbon (III; regioselectivity), (iii) the E,E-diastereoisomer is exclu-
sively or preferentially formed (III, stereoselectivity).
3. Experimental section
All reactions were carried out under N2 using standard Schlenck
techniques. THF, Et2O, and dioxane were destilled from
ꢀ
sodiumebenzophenone, DMF were dried using activated MS 4A,
and CH2Cl2 and MeCN were destilled from CaH2. TLC was per-
formed on aluminum-backed plates coated with silica gel 60 with
F254 indicator. Flash column chromatography was carried out on
silica gel 60 (230e240 mesh). 1H NMR (300, 400 MHz) and 13C NMR
(75, 100 MHz) spectra were measured in CDCl3 at ambient tem-
perature on a Bruker AV-300, DPX-300, AV-400 and AMV-400
instruments, respectively, with tetramethylsilane (
d
¼0.0 ppm, 1H
NMR), CDCl3
(d
¼77.0, 13C NMR) as internal standard. Carbon mul-
tiplicities were assigned by DEPT techniques. IR spectra were car-
ried out in a Mattson 3000 FTIR instrument using CCl4. Elemental
analyses were carried out on PerkineElmer 2400 and Carlo Erba
1108 microanalyzers. Fischer carbene complexes 1aeg were
prepared from the corresponding alkenyllithium compounds fol-
lowing the general procedure described by Fischer.16 Methyl 2,3-
butadienoate 5 was prepared according to a literature procedure.17
Rhodium catalysts, except [Rh(cod)(naphthalene)][SbF6],18 were
commercially available and were used without further purification.
Figure 1. X-ray structure of compound 7b (ellipsoids at 30% probability level).
lead to the metallacyclohexene species II.8 The participation of
intermediate II seems reasonable to account for the formation of
both cyclopentene 6 and cycloheptene 7 derivatives. Thus, when
using a cationic Rh(I)-catalyst, particularly in the presence of the
p
-acceptor ligand CO, the reductive elimination is favored yielding
the cycloadduct 6.7,8 Conversely, more electron rich neutral Rh(I)-
catalysts retard the reductive elimination allowing for the in-
sertion of a second allene unit. Interestingly, the insertion step
takes place regioselectively involving the bond between rhodium
and the more nucleophilic sp2-carbon giving intermediate III,14
which in turn evolves into the cycloadduct 7 by reductive elimi-
nation. Probably minimizing the steric repulsion between the ester
groups and rhodium ligands (intermediate III) is responsible for
the observed E,E-stereochemistry of 7.
3.1. Representative procedure for Rh(I)-catalyzed [3D2D2]-
cyclization of alkenyl Fischer carbene complexes with methyl
2,3-butadienoate 5 (Table 1, entry 7)
To a suspension of alkenyl Fischer carbene complex 1a (184 mg,
0.50 mmol) and [Rh(CO)2Cl]2 (19 mg, 0.05 mmol) in dioxane
(10 mL), methyl 2,3-butadienoate 5 (147 mg, 1.5 mmol) was added.
The resulting mixture was stirred for 16 h at ambient temperature.
The solvent was removed under vacuum and the residue was trit-
urated with n-pentane/Et2O (1:1), filtered through a short plug of
CeliteÒ and the resulting solution was concentrated under vacuum.
The resulting residue was purified by flash column chromatography
(SiO2, hexane/EtOAc, 3:1 v/v) to yield 7a (99 mg, 53%) as a pale
yellow oil.
CO2Me
OMe
•
OMe
R
(CO)5Cr
[Rh]
MeO
CO2Me
[Rh]
5
1
[4+2]
+
R
I
R
II
[Rh(CO)2Cl]2
3.1.1. (5E,7E)-1-Methoxy-5,7-(di(methoxycarbonyl)methylen)-3-(4-
methoxyphenyl)cyclohept-1-ene (7a). 1H NMR (CDCl3): 2.85 (dd,
J¼13.5, 11.4 Hz, 1H), 3.48 (s, 3H), 3.52e3.84 (m, 12H), 4.29 (d,
J¼17.7 Hz, 1H), 4.62 (d, J¼4.4 Hz, 1H), 6.00 (br s, 1H), 6.04 (s, 1H),
6.86 (d, J¼8.5 Hz, 2H), 7.26 (d, J¼8.5 Hz, 2H). 13C NMR (CDCl3): 32.8
(CH2), 37.2 (CH2), 42.2 (CH), 51.3 (CH3), 51.5 (CH3), 54.7 (CH3), 55.3
(CH3),100.2 (CH),113.8 (CH),115.6 (CH),116.4 (CH),128.4 (CH),137.8
(C), 153.6 (C), 157.0 (C), 158.2 (C), 159.0 (C), 166.2 (C), 166.4 (C). IR
(NaCl): 3055, 2983, 1712, 1648, 1419, 1263, 736 cmꢀ1. Elemental
analysis calcd for C21H24O6: C, 67.73; H, 6.50; found C, 67.91; H, 6.43.
[Rh(I)]
[Rh(I)]+
5
MeO2C
MeO
CO2Me
6
MeO
R
[Rh]
CO2Me
CO2Me
R
7
III
Scheme 2.
In summary, we have discovered a new three-component
[3þ2þ2]-cyclization mode of readily available alkenyl Fischer car-
bene complexes by reaction with methyl 2,3-butadienoate in the
presence of Rh(I) as catalyst. Again, a qualitative modification of the
3.1.2. (5E,7E)-3-Phenyl-1-methoxy-5,7-(di(methoxycarbonyl)
methylen)cyclohept-1-ene (7b). The representative procedure was
followed using 1b (168 mg, 0.50 mmol). After 24 h, purification by
column chromatography (SiO2, hexane/EtOAc, 3:1) yielded 7b
(70 mg, 41%) as a pale yellow oil. A slow diffusion of Et2O in a so-
lution of 7b in CH2Cl2 at -20 ꢁC afforded appropriate crystals for X-
ray diffraction studies. 1H NMR (CDCl3): 2.88 (dd, J¼13.8, 11.6 Hz,
1H), 3.48 (s, 3H), 3.61 (dd, J¼13.8, 4.8 Hz, 1H), 3.68 (br s, 1H), 3.73 (s,
3H), 3.75 (s, 3H), 3.82e3.88 (m, 1H), 4.31 (d, J¼17.7 Hz, 1H), 4.64 (d,
J¼4.6 Hz, 1H), 6.01 (d, J¼1.9 Hz, 1H), 6.05 (s, 1H), 7.20e7.32 (m, 5H).
13C NMR (CDCl3): 32.7 (CH2), 37.0 (CH2), 43.0 (CH), 51.4 (CH3), 51.5
(CH3), 54.7 (CH3), 99.8 (CH), 115.6 (CH), 116.4 (CH), 126.5 (CH), 127.4
reactivity of group
6 metal carbenes is achieved by metal
exchange.15 In spite of the moderate yield and selectivity, this
process allows access to highly functionalized bis-1,2-alkylidene
cycloheptenes in an operationally simple process. According to our
proposal, the whole process comprises some notable features in
terms of selectivity, (i) the [4þ2] cycloaddition and Rh/C insertion
reactions involve the unactivated allene C]C bond (II and III;
chemoselectivity), rhodium being linked to the central allene

6338
J. Barluenga et al. / Tetrahedron 66 (2010) 6335e6339
(CH), 128.5 (CH), 145.7 (C), 153.7 (C), 156.9 (C), 158.9 (C), 166.2 (C),
166.3 (C). IR (NaCl): 3053, 2987, 1719, 1644, 1421, 1265, 908,
736 cmꢀ1. Elemental analysis calcd for C20H22O5: C, 70.16; H, 6.48;
found: C, 70.28; H, 6.53.
assignable signals are listed): 1H NMR (CDCl3): 1.04 (s, 9H), 2.91 (dd,
J¼17.0, 11.9 Hz, 1H), 3.09e3.17 (m, 1H), 3.10e3.16 (m, 1H), 3.59 (s,
3H), 3.69 (s, 3H), 4.60e4.66 (m, 1H), 5.20 (d, J¼5.1 Hz, 1H), 5.90 (br
s, 1H), 6.28 (d, J¼2.0 Hz, 1H). 13C NMR (CDCl3): 27.8 (CH3), 33.9
(CH2), 38.6 (CH2), 44.1 (CH), 51.1 (CH3), 55.6 (CH3), 110.3 (CH), 116.0
(CH), 117.8 (CH). IR (NaCl): 3053, 2966, 2901, 1713, 1633, 1434, 1265,
1167, 895, 740 cmꢀ1. Elemental analysis calcd for C18H26O5 (mixture
of stereoisomers): C, 67.06; H, 8.13; found C, 67.21; H, 8.19.
3.1.3. (5E,7E)-3-(4-Chlorophenyl)-1-methoxy-5,7-(di(methox-
ycarbonyl)methylen)cyclohept-1-ene (7c). The representative pro-
cedure was followed using 1c (185 mg, 0.50 mmol). After 24 h,
purification by column chromatography (SiO2, hexane/EtOAc, 3:1)
yielded 7c (75 mg, 40%) as a pale yellow oil. 1H NMR (CDCl3): 2.92
(dd, J¼13.7, 11.2 Hz, 1H), 3.52 (s, 3H), 3.61 (dd, J¼13.7, 5.0 Hz, 1H),
3.74e3.92 (2sþm, 8H), 4.31 (d, J¼17.6 Hz, 1H), 4.62 (d, J¼4.6 Hz,
1H), 6.05 (d, J¼1.9 Hz, 1H), 6.09 (s, 1H), 7.29e7.32 (m, 4H). 13C NMR
(CDCl3): 32.7 (CH2), 36.7 (CH2), 42.4 (CH), 51.4 (CH3), 51.5 (CH3),
54.8 (CH3), 99.3 (CH), 115.9 (CH), 116.5 (CH), 128.6 (CH), 128.8 (CH),
132.1 (C), 144.0 (C), 154.2 (C), 156.5 (C), 158.5 (C), 166.1 (C), 166.3
(C). IR (NaCl): 3053, 2986, 1704, 1605, 1442, 1167, 895 cmꢀ1. Ele-
mental analysis calcd for C20H21ClO5: C, 63.75; H, 5.62; found: C,
63.59; H, 5.66.
3.1.7. 3,3-Dimethyl-1-methoxy-5,7-(di(methoxycarbonyl)methylen)
cyclohept-1-ene (7g). The representative procedure was followed
using 1g (145 mg, 0.50 mmol). After 24 h, purification by column
chromatography (SiO2, hexane/EtOAc, 3:1) yielded 7g (45 mg, 31%,
4:1 mixture of stereoisomers) as a colorless oil. Major isomer: 1H
NMR (CDCl3): 1.03 (s, 6H), 2.11 (s, 2H), 2.79 (s, 2H), 3.70 (s, 3H), 3.72
(br s, 6H), 5.55 (s, 1H), 6.13 (s, 1H), 6.84 (s, 1H). 13C NMR (CDCl3):
29.4 (CH3), 38.0 (C), 40.6 (CH2), 44.5 (CH2), 51.1 (CH3), 51.3 (CH3),
55.4 (CH3), 96.1 (CH), 109.8 (CH), 116.9 (CH), 158.3 (C), 159.1 (C),
166.7 (C), 167.2 (C), 167.3 (C). Minor isomer (only clearly assignable
signals are listed): 1H NMR (CDCl3): 1.09 (s, 6H), 2.17 (s, 2H), 2.97 (s,
2H), 3.55 (s, 3H), 3.60 (s, 3H), 5.30 (s, 1H), 5.52 (s, 1H), 5.79 (s, 1H).
13C NMR (CDCl3): 30.1 (CH3), 42.1 (CH2), 45.5 (CH2), 55.6 (CH3), 96.4
(CH). IR (NaCl): 2961, 1710, 1621, 1432, 1139, 895, 737 cmꢀ1. Ele-
mental analysis calcd for C16H22O5 (mixture of stereoisomers): C,
65.29; H, 7.53; found C, 65.15; H, 7.66.
3.1.4. (5E,7E)-3-Ferrocenyl-1-methoxy-5,7-(di(methoxycarbonyl)
methylen)cyclohept-1-ene (7d). The representative procedure was
followed using 1d (223 mg, 0.50 mmol). After 16 h, purification by
column chromatography (SiO2, hexane/EtOAc, 3:1) yielded 7d
(126 mg, 56%) as a pale red viscous oil. 1H NMR (CDCl3): 2.95 (dd,
J¼15.6, 12.5 Hz, 1H), 3.50e3.62 (sþm, 6H), 3.76 (s, 3H), 3.77 (s, 3H),
4.11e4.19 (sþm, 9H), 4.26 (d, J¼18.0,1H), 4.85 (d, J¼4.3 Hz,1H), 5.98
(br s, 2H). 13C NMR (CDCl3): 33.4 (CH2), 35.5 (CH), 37.2 (CH2), 51.3
(CH3), 51.4 (CH3), 54.6 (CH3), 66.7 (CH), 66.8 (CH), 67.2 (CH), 67.5
(CH), 68.5 (CH), 93.9 (C),100.9 (CH),115.9 (CH),116.4 (CH),153.0 (C),
157.2 (C), 159.5 (C), 166.2 (C), 166.3 (C). IR (NaCl): 3053, 2986, 1713,
1633, 1422, 1265, 1166, 895, 738 cmꢀ1. Elemental analysis calcd for
C24H26FeO5: C, 64.01; H, 5.82; found C, 63.95; H, 5.86.
Acknowledgements
We are grateful to the Ministerio de Ciencia e Innovación
(MICINN) of Spain (Grant CTQ-2007-61048) and the Principado de
Asturias (Grant IB 08-088). R.V. thanks the MICINN for a Juan de la
Cierva contract. We are also grateful to Dr. César J. Pastor (SIdI,
Universidad Autónoma de Madrid) for his assistance in the collec-
tion of the X-ray data.
3.1.5. 3-n-Butyl-1-methoxy-5,7-(di(methoxycarbonyl)methylen) cy-
clohept-1-ene (7e). The representative procedure was followed
using 1e (159 mg, 0.50 mmol). After 24 h, purification by column
chromatography (SiO2, hexane/EtOAc, 3:1) yielded 7e (63 mg, 39%,
3:1 mixture of stereoisomers) as a colorless oil. Major isomer: 1H
NMR (CDCl3): 0.88e0.94 (m, 3H), 1.32e1.63 (m, 6H), 2.43e2.46 (m,
1H), 2.76e2.84 (m, 1H), 3.06e3.25 (m, 2H), 3.46 (s, 3H), 3.71 (s, 3H),
3.72 (s, 3H), 4.15 (d, J¼17.8 Hz, 1H), 4.46 (d, J¼5.2 Hz, 1H), 5.91 (s,
2H). 13C NMR (CDCl3): 14.1 (CH3), 22.8 (CH2), 29.2 (CH2), 33.8 (CH2),
35.0 (CH), 35.7 (CH2), 36.9 (CH2), 51.2 (CH3), 51.4 (CH3), 54.5 (CH3),
101.2 (CH), 115.7 (CH), 116.2 (CH), 153.3 (C), 157.9 (C), 161.1 (C), 166.2
(C), 166.5 (C). Minor isomer (only clearly assignable signals are lis-
ted): 1H NMR (CDCl3): 2.62e2.66 (m,1H), 3.06e3.10 (m,1H), 3.56 (s,
3H), 3.67 (s, 3H), 4.61 (d, J¼14.3 Hz, 1H), 5.02 (d, J¼4.1 Hz, 1H), 5.94
(br s, 1H), 6.27 (s, 1H). 13C NMR (CDCl3): 13.9 (CH3), 29.4 (CH2), 33.7
(CH), 37.0 (CH2), 37.6 (CH2), 38.6 (CH2), 50.8 (CH3), 55.2 (CH3), 113.6
(CH), 115.4 (CH), 117.0 (CH), 149.7 (C), 153.2 (C), 159.6 (C), 166.9 (C),
167.4 (C). IR (NaCl): 2953, 2930, 2872, 1716, 1634, 1456, 1378, 1195,
1171, 906, 729, 650 cmꢀ1. Elemental analysis calcd for C18H26O5
(mixture of stereoisomers): C, 67.06; H, 8.13; found C, 67.01; H, 8.25.
Supplementary data
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.tet.2010.04.130.
References and notes
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cyclohept-1-ene (7f). The representative procedure was followed
using 1f (159 mg, 0.50 mmol). After 24 h, purification by column
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free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
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