Diastereoselective synthesis of 2-aryl-3-vinyl-2,3-dihydrobenzo[b]furans through a Sakurai reaction: A mechanistic proposal

Article abstract of DOI:10.1002/chem.200601017

The condensation of 2,3-dihydrobenzoxasilepins with aromatic aldehydes in the presence of boron trifluoride to form 2,3-dihydrobenzofurans shows a level of diastereoselection which is a function of the electronic nature of the aldehyde and the polarity of

Full text of DOI:10.1002/chem.200601017

FULL PAPER
DOI: 10.1002/chem.200601017
Diastereoselective Synthesis of 2-Aryl-3-vinyl-2,3-dihydrobenzo[b]furans
through a Sakurai Reaction: A Mechanistic Proposal
Leticia JimØnez-Gonzµlez, Sergio García-MuÇoz, Miriam lvarez-Corral,
Manuel MuÇoz-Dorado, and Ignacio Rodríguez-García*[a]
Abstract: The condensation of 2,3-dihydrobenzoxasilepins with aromatic aldehydes
in the presence of boron trifluoride to form 2,3-dihydrobenzofurans shows a level
Keywords: aldehydes · allylation ·
of diastereoselection which is a function of the electronic nature of the aldehyde
diastereoselectivity · heterocycles ·
and the polarity of the solvent. The study of the mechanism of the reaction dem-
silanes
onstrated that it proceeds through a ring-opened allylfluorosilane, which is stable
enough to be isolated and characterized.
Introduction
pendently, by Marsden[25] and Cossy.[26] Our procedure in-
volves the formation of benzoxasilepins by ring-closing
metathesis (RCM) followed by a Sakurai–Hosomi-modified
reaction. We have described the application of this proce-
dure to the synthesis of pterocarpans[24] and neolignans,[27]
although strong differences in the levels of diastereoselectiv-
ity were obtained. Other studies have shown that the dia-
stereoselectivity of this procedure is dependent on the re-
agent.[28–30] Here we present new results which demonstrate
the dependence of the diastereoselectivity obtained on the
electronic density of the aldehyde, therefore enabling us to
control the exclusive formation of cis or trans diastereomers
by adequate selection of the substituents on the aromatic
ring.
The condensation of allylsilanes with carbonyl compounds
under Lewis acid conditions, known as the Sakurai–Hosomi
reaction, has been applied successfully in organic synthesis
[1,2]
À
for the formation of C C bonds.
In addition, allylsilanes
are excellent reagents for the construction of carbocyclic
and heterocyclic compounds.[3,4] The 2,3-dihydrobenzofuran
ring is the core of the skeleton of an important number of
biologically active natural products, such as pterocarpans,[5,6]
neolignans,[7,8] as well as synthetic drugs used in the treat-
ment of pulmonary hypertension and atherosclerotic periph-
eral arterial disease[9] or with protective effects against cen-
tral nervous system trauma and ischemia.[10] Therefore,
much interest has been paid to the development of methods
for the stereoselective synthesis of these compounds, mainly
Lewis acid promoted reactions,[11,12] anionic cyclizations,[13–15]
Results andDiscussion
radical reactions,[16–19] palladium-mediated couplings[20,21] and
[22,23]
À
rhodium catalyzed C H insertions.
Benzoxasilepins: The 2,3-dihidrobenzo[f]oxasilepins re-
quired for this study were prepared by RCM of allylsilyleth-
ers of ortho-allylphenols (Scheme 1). To our knowledge, the
behaviour in solution of seven-membered cyclic allylsilox-
anes has never been reported, although their synthesis has
In a previous communication, we have reported a novel,
two-component methodology, which allows the synthesis of
diastereomerically pure dihydrobenzofurans in an easy
way.[24] A similar procedure, used for the preparation of tri-
substituted tetrahydrofurans, was previously described, inde-
[a] Dr. L. JimØnez-Gonzµlez, Dr. S. García-MuÇoz,
Dr. M. lvarez-Corral, Dr. M. MuÇoz-Dorado,
Dr. I. Rodríguez-García
Dpt. Química Orgµnica, Universidad de Almería
04120 Almería (Spain)
Fax : (+34)950-015481
E-mail: irodrigu@ual.es
Scheme 1. a) Et3N, AllylSi(Me)2Cl, 08C, 4 h, dichloromethane; 90%;
b) Grubbs 2nd generation catalyst, dichloromethane, 30 min, reflux,
91%.
Supporting information for this article is available on the WWW
under http://www.chemistry.org or from the author.
Chem. Eur. J. 2007, 13, 557 – 568
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557

been described in a variety of ways, including radical cycli-
zations,[31] titanocene(II)-promoted cyclizations,[32] rhodium
achieve a better understanding of the factors that control
the diastereoselectivity. Thus, we decided to study the influ-
ence of the nature of the aldehyde on the stereochemical
outcome of the reaction. The benzoxasilepin 3 (without sub-
stituents) was used as a substrate, together with a range of
substituted benzaldehydes 4 (Scheme 3). All experiments
mediated C H insertion[33] and ring-closing metathesis.[34–36]
À
The NMR spectra of 2,3-dihydrobenzo[f]oxasilepin 3
measured in CDCl3 were in agreement with the proposed
1
structure, but when the H NMR spectrum was measured in
CD3OD at room temperature, two sets of signals were ob-
served. This can be understood by considering an equilibri-
um with a zwitterionic, pentacoordinated silicon species (I),
resulting from the nucleophilic addition of the solvent
(Scheme 2).
Scheme 3. Condensation of benzoxasilepin 3 with substituted aldehydes
4.
were carried out under the same reaction conditions by
using BF3·Et2O as a Lewis acid (2equiv) and a slight excess
of aldehyde (1.1 equiv) referred to benzoxasilepin
(Table 1).
3
The relative stereochemistry of both diastereomers was
assigned from their NMR spectroscopic properties. Specially
relevant is the shielding of the signal of H-3 in the trans iso-
mers 6 when compared to the cis isomers 5. This behaviour
can be explained in terms of the influence on this hydrogen
of the anisotropy cone of the aromatic ring, as it has been
previously observed.[27,28] Appropriate crystals for X-ray dif-
fraction were prepared from 5l (Figure 1) and 6b (Figure 2),
thus unambiguously establishing the relative stereochemistry
for these compounds and for all the 5/6 pairs.
Scheme 2. Behaviour of 2,3-dihydrobenzo[f]oxasilepin 3 in CD3OD.
It is commonly accepted that in pentacoordinated silicon
compounds, the most polar atoms prefer the apical positions
of a trigonal-bipyramidal arrangement,[37] although these
systems usually show some degree of fluxionality.[38] There-
fore, we propose structure I for this adduct, which is in
agreement with the observed spectroscopic properties. Thus,
the methyl signals suffer a slight shielding when changing
from 3 to the pentacoordinated species I (from d=0.29 to
0.10 ppm), but only one singlet is observed for both of them
in each substance. The CH2 is somewhat deshielded (from
d=1.56 to 1.85 ppm) and the double-bond protons also
show slight changes in chemical shift and almost the same
multiplicity pattern. The relative ratio of these two species,
1
as deduced from the integrals of the H NMR spectroscopic
signals, is approximately 4:1 for which 3 is the major one.
Variable temperature NMR experiments of this solution en-
abled the construction of a Vanꢁt Hoff plot. The calculated
thermodynamic constants of the system (DH=À4.9 kJmolÀ1
À1
and DS=À2 9 Jmol KÀ1) agree with an enthalpically fav-
oured and entropically disfavoured process. Evaporation of
the methanol affords quantitatively pure 3.
In the same way, when the NMR spectrum was measured
in non-nucleophilic solvents, such as [D3]acetonitrile,
[D6]acetone, [D7]DMF or [D6]bencene, no splitting of the
signals was observed, but in ethanol,[39] again two sets ap-
peared.
Figure 1. Crystal structure of cis-2-(2-pivaloyloxyphenyl)-3-vinyl-2,3-dihy-
drobenzofuran 5l (ORTEP 20%).[40]
Although a Hammett plot could not be successfully con-
structed, it is interesting to note that there is a surprisingly
good correlation between the value of dH (ArCHO) in
1H NMR spectra and the measured cis/trans ratio. Thus,
there is an absolute lack of diastereoselectivity when dH =
10.0 ppm (Table 1, entry 7); moving towards higher dH
values, the major isomer formed has a cis relative stereo-
chemistry (5). On the other hand, when the value is lower
than d=10.0 ppm, the major isomer has trans relative ster-
Diastereoselectivity of the Sakurai reaction as a function of
the nature of the aldehyde: The modified Sakurai–Hosomi
condensation of allylsiloxanes with aldehydes has been re-
ported with high levels of diastereoselectivity by Marsden.[28]
In our previous papers, we have observed that when the re-
action is applied to benzo-fused cyclic allylsiloxanes, com-
plete diastereoselection[24] or a 1:1 mixture is obtained.[27]
More extensive experimentation was obviously needed to
558
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Chem. Eur. J. 2007, 13, 557 – 568

Synthesis of 2-Aryl-3-vinyl-2,3-dihydrobenzo[b]furans
Table 1. Synthesis of isomers cis/trans (5/6) from 3 and 4.[a]
FULL PAPER
given substitution pattern. This
correlation shows that the po-
tentially more reactive alde-
hydes, those with higher elec-
trodeficiency and therefore
higher dH values, offer a greater
ratio of cis isomer. On the
other hand, strong electron-do-
nating groups, such as MeO or
Me2N, lead preferentially to the
formation of trans isomers.
All the aldehydes presented
in Table 1 have oxygenated or
nitrogenated substituents. Halo-
genated substituents were also
assayed, and as in other kinds
of reactions, their behaviour is
somewhat peculiar. Table 2
summarizes the results. meta-
Substituted aldehydes (entries 2
and 5) lead to the exclusive for-
mation of the cis isomers 5,
while ortho- and para-substitut-
ed benzaldehydes (entries 1, 3,
4, and 6) following a common
trend, gave rise to cis/trans mix-
tures in which the cis isomers
predominate. These results can
be explained on the basis of the
two electronic effects of the
halogens: when they are in the
ortho and para positions, the
opposing effects, electron dona-
tion by conjugation and elec-
tron withdrawal by induction,
make the aldehydes less reac-
Entry
Aldehyde[b]
d (CHO) [ppm]
cis/trans[c]
6a)
1
4a
4b
9.720:100 (
2
3
9.80
9.95
0:100 (6b)
4c
4d
4e
17:83 (5c/6c)
4
5
9.95
20:80 (5d/6d)
20:80 (5e/6e)
10.45[d]
6
7
4 f
4g
9.97
33:67 (5 f/6 f)
50:50 (5g/6g)
10.00
8
9
4h
4i
10.07
10.11
75:25 (5h/6h)
83:17 (5i/6i)
10
11
4j
10.01
10.11
100:0 (5j)
100:0 (5k)
4k
12
13
4l
10.14
10.20
100:0 (5l)
4m
100:0 (5m)
[a] In refluxing CH2Cl2 overnight; entries are sorted according to increasing ratios of the cis isomer 5. [b] For
the synthesis of the noncommercial aldehydes see the Supporting Information. [c] Ratio of isomers determined
1
by H NMR spectroscopic analysis of the reaction mixture and confirmed after isolation of the compounds by
column chromatography. [d] dH value is higher than expected due to intermolecular hydrogen bonding.[42]
Figure 2. Crystal structure of trans-2-(4-methoxyphenyl)-3-vinyl-2,3-dihy-
drobenzofuran 6b (ORTEP 50%).[41]
eochemistry (6). This correlation has been represented in
Figure 3 and it can be a useful tool to know what kind of al-
dehyde we can choose in order to obtain the cis or the trans
isomer, or what diastereoselection would be obtained for a
Figure 3. Depiction of the ratio cis/trans towards the 1H NMR chemical
shift of the aldehyde hydrogen in the Sakurai condensation of 2,3-dihy-
dro-2,2-dimethylbenzo[f][1,2]oxasilepin (3) with aromatic aldehydes.
A
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559

I. Rodríguez-García et al.
Table 2. The cis/trans ratio with halogen-substituted benzaldehydes.[a]
Table 4. Influence of the solvent on the ratio of diastereomers.
Entry
Aldehyde
cis:trans[b]
Entry Substrates Products
cis/trans
[b]
Dichloromethane[a] CD3CN[b] CDCl3
[c]
1
2
3
4
5
3+4a
3+4b
3+4d5d
3+4g
3+4s
5a/6a
5b/6b
/6d
5g/6g
5s/6s
0:100
0:100
20:80
50:50
70:30
0:100
20:80
62:38
63:37
77:23
–
1
72:28 ( 5n/6n)
56:44
71:29
100:0
83:17
2
3
4
100:0 (5o)
[a] Conditions from Table 1. [b] Reaction run in a NMR tube at 408C in
the presence of 2equiv of BF 3·Et2O. [c] The aldehyde precipitates upon
addition of the other components.
72:28 ( 5p/6p)
71:29 (5q/6q)
higher cis/trans ratios. A particularly strong effect is ob-
served for CDCl3. The higher values for the cis isomers in
À
chloroform could arise from a C H···O hydrogen bond be-
5
6
100:0 (5r/6r)
70:30 (5s/6s)
tween chloroform and the carbonyl oxygen, which would
render the carbonyl group more electron-deficient, as has
been previously described for other types of reactions.[44] It
is especially interesting to note that an adequate choice of
solvent could result in a strong change of diastereoselectivi-
ty (entry 4).
[a] Isomers cis/trans (5/6) prepared from 3 and the appropriate halogen-
ated aldehyde by using 2equiv of BF 3·Et2O in refluxing CH2Cl2.
[b] Ratio of isomers determined by H NMR analysis of the reaction mix-
ture and confirmed after isolation of the compounds by column chroma-
tography.
1
Mechanism: Although several mechanisms for this kind of
reaction have been proposed,[25,26,28] they do not provide a
satisfactory explanation for our results, with regards to the
different diastereoselectivity levels as a function of the alde-
hyde substitution or the solvent polarity.
tive than their meta isomers, which have only the inductive
effect. For each position, chloro or bromo derivatives gave
rise to almost identical results.
We have previously described the synthesis of other ben-
zoxasilepins which incorporate methoxyl substituents on the
aromatic ring (7 and 8).[43] It would be interesting to know if
the tendency for diastereomeric behaviour to be affected by
the electronic nature of the aldehyde is extensible to those
substrates. Thus, 7 and 8 were condensed with some of the
previously used aldehydes under the same experimental
conditions. The results are listed in Table 3. Again, higher
ratios of cis isomers were obtained with the more electrode-
ficient aldehydes.
Thus, we decided to follow the reaction through NMR
spectroscopic analysis to detect any possible intermediate
which could help to clarify the mechanism. Thus, when two
equivalents of BF3·Et2O were added to a CDCl3 solution of
benzoxasilepin
3 and the mixture was monitored by
1H NMR spectroscopy, the immediate appearance of a new
species II was observed (Scheme 4). Its relative ratio to-
The influence of the nature of the solvent on the diaster-
eoselectivity of the reaction can be analyzed from the data
in Table 4.
As a general trend, an increase in the polarity of the sol-
vent (changing from dichloromethane to CD3CN) leads to
Table 3. The cis/trans 5/6 ratio starting from different benzoxasilepins.
Entry
Benzoxasilepin
Aldehyde
(d CHO [ppm])
cis:
G
G
trans[a]
4c (9.95)
1
2
3
9:91 (5t/6t)
50:50 (5u/6u)
20:80 (5v/6v)
Scheme 4. Ring opening of 3 leading to 9.
7
4h (10.07)
4c (9.95)
wards 3 (approximately II/3 3:7) remains constant for very
long periods of time or when more Lewis acid is added.
However, when BF3·Et2O was added to a solution of ben-
zoxasilepin 3 in CD3OD, the new species II was formed in-
stantly and quantitatively. The reaction mixture was worked
up, thus enabling the isolation of the fluorinated compound
9. Its HRMS agreed with a molecular formula C11H15FOSi,
4
5
8
8
4h (10.07)
4i (10.11)
91:9 (5w/6w)
100:0 (5x/6x)
1
[a] Ratio of isomers determined by H NMR spectroscopic analysis of the
reaction mixture and confirmed after isolation of the compounds by
column chromatography.
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Synthesis of 2-Aryl-3-vinyl-2,3-dihydrobenzo[b]furans
FULL PAPER
Table 5. Ratio[a] II/3 in different solvents at 258C.
which contains one fluorine and hydrogen atom more than
the initial heterocycle. The 1H NMR spectrum is very similar
to that of the starting material, but for the methyl groups on
silicon, which appear as doublets due to their coupling with
Solvent
e[a]
3[b]
II[b]
[D7]DMF
[D6]acetone
CDCl3
36.71
20.7
4.807
2.274
1
1
1
1
3.45
0.71
0.64
0.50
the 19F (3J
ACHTREUNG
[D6]benzene
[a] For the corresponding undeuterated solvent, at 25 (Æ58C).[46]
[b] Ratio II/3 determined by 1H NMR spectroscopic analysis of the reac-
tion mixture (area of Me3Si signals).
ACHTREUNG
bond can still be established as Z. The presence of the fluo-
rine on the silicon is also revealed by the signals for the
methyl carbons in the 13C NMR spectrum, which appear as
doublets (2J
(C,F)=13.1, 14.9 Hz, respectively). The
However, in methanol the process seems to be irreversi-
ble, as the only product that can be observed at any temper-
ature is 9 (no signals of 3 could be detected even at
À1008C). In CD3OD, a solvent attack on the starting mate-
rial would render the pentavalent silicon species I, which
would then suffer the attack of the fluoride nucleophile, and
eventually, protonation to form 9.
Although the fluorinated species seemed to be a reaction
intermediate, we decided to check whether it was able to
react with the aldehydes. Initially, a solution of 9 and ben-
zaldehyde in dichloromethane was refluxed without Lewis
acid. No reaction products could be detected after 24 h. But
in the presence of one equivalent of BF3·Et2O, the reaction
smoothly gave the condensation products with the same dia-
stereoselection as when the reaction was performed straight
from 3, such as shown in Tables 1 and 2.
19F NMR spectrum shows only one signal at d=
À160.89 ppm (hept t, 3J
ACHTREUNG
bands due to the 19F–29Si coupling (2J
ACHTREUNG
similar spectroscopic behaviour has been reported for other
fluorosilanes.[45]
The existence of pentavalent silicon species is well estab-
lished, and the possibility that a pentavalent intermediate
exists in the nucleophilic substitution of silyl ethers has been
proposed.[38] In CDCl3, this species would be just an inter-
mediate which evolves into the ring-opened species II
(Scheme 4). The position of the equilibrium depends on the
temperature, and therefore it can be studied through varia-
tions on the temperature of acquisition of the NMR spectra.
This phenomenon is better observed in a nonprotic solvent
with a high boiling point, such as [D7]DMF. Figure 4 shows
With all these data, we can propose a mechanism for the
reaction. It seems clear that the first stage of the reaction is
the opening of the benzoxasilepin ring to give a fluorinated
species (through a pentacoordinated silicon intermediate)
which adds to the carbonyl of the aldehyde, previously acti-
vated by the Lewis acid. Two possible diastereomeric transi-
tion states, leading to both diastereomers should be written.
The dependence of the diastereomeric outcome of the reac-
tion on the polarity of the solvent (Table 4) points to a dif-
ference in polarity of those transition states. Bottoni et al.
have investigated the mechanism of the Sakurai reaction at
the DFT computational level.[47] They found four transition
states, quite close in energy, for a model reaction with ace-
taldehyde, allylsilane and BH2F, and predicted two of them
(each one leading to a different diastereomer) to be associ-
ated to the most likely reaction channels in real systems.
These are eight-membered cyclic structures, one resembling
the boat–boat (BB, TS A) conformation of cyclooctane and
the other a type of boat–chair (BC, TS B), in which one of
the fluorine atoms from the BF3 coordinated to the alde-
hyde approaches the silicon atom, while at the same time
the C=C double bond adds to the carbonyl C=O
(Scheme 5). Obviously this hypothesis implies the participa-
tion of a second BF3 molecule in the reaction, as we have
experimentally observed.
Figure 4. Vanꢁt Hoff plot for the equilibrium 3:II in [D7]DMF in the tem-
perature range from 258 to 343 K (values above the line are equilibrium
constants K in mÀ1; values in parentheses are temperatures in K).
a Vanꢁt Hoff plot for the results when using a large excess
of BF3·Et2O (>10 equiv), from which the thermodynamic
constants of the system can be calculated (DH=
À15.94 kJmolÀ1 and DS=À43.1 JmolÀ1 KÀ1). When the tem-
perature is increased, the equilibrium constant drops from
10.0 at 258 K to 1.6 at 343 K. Hence, the reaction with
boron trifluoride seems to be an exothermic process which
is disfavoured by a decrease in entropy, as it should be for a
collision of two molecules into a single one. In other aprotic
solvents the behaviour is basically the same. Table 5 depicts
the position of the equilibrium at 258C, showing that the
more polar solvents favour the displacement of the equilibri-
um towards II.
The differences in polarity between the two transition
states (TS) could explain the diastereoselectivity of the reac-
tion, the most polar being TS B, as it leads to the cis iso-
mers, which are preferred in the more polar solvents. The
differences in polarization of the aldehyde C=O double
bond due to the effect of the substituents on the aromatic
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561

I. Rodríguez-García et al.
and 13C NMR spectra were recorded in deuterated solvent and are refer-
enced to TMS. Carbon substitution degrees were established by DEPT
multipulse sequence, and 13C NMR peak assignments were made with
the aid of 2D NMR spectroscopy (HMBC, HMQC, COSY and NOESY).
HRMS were registered on an Autospec-Q VG Analytical (FISONS)
mass spectrometer. RX data were determined on a Bruker SMART
APEX CCD instrument. All solvents were purified and dried following
standard procedures.
Allyl[2-(1-propenyl)phenoxy]dimethylsilane (2): Commercially available
2-(1-propenyl)phenol (1) (3 g, 22.4 mmol) was dissolved in anhydrous
CH2Cl2 (110 mL) at 08C under
a N2 atmosphere. Anhydrous NEt3
(3.7 mL, 26.8 mmol) and allylchlorodimethylsilane (3.6 mL, 24.6 mmol)
were then added. The reaction mixture was stirred at 08C for 4 h, and
then a saturated solution of NaHCO3/H2O was added. The whole mixture
was extracted with CH2Cl2. The organic layer was dried over anhydrous
Na2SO4 and concentrated in vacuo. The residue was purified by flash
chromatography, yielding 2 (4.55 g, 20 mmol, 90%).
Compound trans-2 (major isomer): Colourless oil; Rf =0.35 (hexane/Et2O
98:2); 1H NMR (300 MHz, CDCl3, 2 58C, TMS): d=7.46 (dd, 3J
ACHTREUNG
Scheme 5. Mechanism for the condensation of II and 4.
3
3
ACHTREUNG
7.7, J
6.97 (brt, 3J
1H; H-6), 6.71 (dq, J
15.9, 6.6 Hz, 1H; H-2’), 5.85 (ddt, J
5.02–4.93 (m, 2H; H-3’’), 1.93 (dd, 3J
(d, 3J(H,H)=8.1 Hz, 2H; H-1’’), 0.30 ppm (s, 6H; SiMe2); 13C NMR
(H,H)=1.5 Hz, 1H; H-3), 7.13 (td, J
A
ACHTREUNG
3
3
ACHTREUNG
A
À
ring or to the C H···O hydrogen bond when the solvent is
3
AHCTREUNG
chloroform, should also correspond with significant differen-
ces in polarization of the transition states, and hence, in
their relative energies.
AHCTREUNG
AHCTREUNG
(75 MHz, CDCl3, 2 58C, TMS): d=151.8 (C, C-1), 133.3 (CH, C-2’’), 129.4
(C, C-2), 127.5 (CH, C-5), 126.3 (CH, C-3), 126.1 (CH, C-2’), 125.9 (CH,
C-1’), 121.7 (CH, C-4), 119.8 (CH, C-6), 114.3 (CH2, C-3’’), 24.7 (CH2, C-
1’’), 18.8 (CH3, C-3’), À1.7 ppm (CH3, SiMe2); IR (film): n˜ =2961, 2912,
It is important to note that we have not detected any epi-
merization processes in these reactions, as has been pro-
posed,[30] and that the reactions performed with other Lewis
acids, which differ from BF3·Et2O, could proceed through
different mechanistic pathways. The main difference be-
tween this hypothesis and those previously published for
nonbenzofused systems[26,28] is the initial formation of the
2881, 2854, 1630, 1596, 1483, 1449, 1253, 1159, 1039, 968, 915, 835 cmÀ1
;
HR-EIMS: m/z: calcd for C12H21OSi: 209.1362 [M+H]+; found:
209.1359.
General procedure for the metathesis of allyl[2-(1-propenyl)phenoxy] di-
methylsilanes.
[1,2]oxasilepins:
ne)(dichloro
Preparation
of
2,2-dimethyl-2,3-dihydrobenzo[f]-
A
[1,3-Bis-(2,4,6-trimethylphenyl)-(2-imidazolidinylide-
phenylmethylene)(tricyclohexylphosphine)ruthenium]
À
À
C C bond (instead of the C O bond), a step which controls
the stereochemical outcome of the reaction. It should be fol-
lowed by cyclization through nucleophilic attack on the
carbon bearing the OBF2 group with concomitant inversion
of configuration.
(Grubbs 2nd generation catalyst) was added to a stirred solution of
allyl[2-(1-propenyl)phenoxy]dimethylsilanes (0.02m) in anhydrous
CH2Cl2 under a N2 atmosphere. The mixture was stirred under reflux for
30 min, and then the solvent was removed in vacuo. The residue was pu-
rified by flash chromatography.
2,2-Dimethyl-2,3-dihydrobenzo[f][1,2]oxasilepin (3): Reaction of allyl[2-
A
(1-propenyl)phenoxy]dimethylsilane (2) (1.4 g, 6.2mmol) with Grubbs
2nd generation catalyst (26 mg, 0.03 mmol) in anhydrous CH2Cl2
(310 mL) followed by workup, as described in the general procedure,
yielded 3 (1.07 g, 5.6 mmol, 91%). Colourless oil; Rf =0.34 (hexane/Et2O
Conclusions
In this paper we have advanced the knowledge of the mech-
anism of a version of the Sakurai reaction, used to prepare
natural products with a dihydrobenzofuran skeleton, by the
condensation of aromatic aldehydes with benzoxasilepins in
the presence of boron trifluoride. We have isolated and
identified one of the intermediates of the reaction, a ring-
opened allylfluorosilane. The study of the factors that influ-
ence the diastereoselection of the reaction shows that the
electronic features of the aldehyde and the polarity of the
solvent are critical. Two diastereomeric eight-membered
cyclic transition states of different polarity can account for
the observed stereochemical results.
98:2); 1H NMR (300 MHz, CDCl3, 2 58C, TMS): d=7.18 (td, 3J
ACHTREUNG
7.6, 1.3 Hz, 1H; H-7), 7.13 (dd, 3J
3J
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
AHCTREUNG
13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=153.2(C, C-9a), 130.9 (CH,
C-6), 128.5 (C, C-5a), 128.1 (CH, C-4), 127.9 (CH, C-7), 126.6 (CH, C-5),
121.4 (CH, C-8*), 121.2 (CH, C-9), 18.2 (CH2, C-3), À0.5 ppm (CH3,
SiMe2); IR (film): n˜ =3064, 3018, 2958, 2875, 1598, 1565, 1484, 1443 cmÀ1
;
HR-EIMS: m/z: calcd for C11H15OSi: 191.0892[ M+H]+; found:
191.0885; chemical shift and multiplicity of the signals of the mixture 3+
I:3: 1H NMR (300 MHz, CD3OD, 258C, TMS): d=6.34 (d, 3J
ACHTREUNG
10.8 Hz, 1H), 6.06 (dt, 3J
(H,H)=10.9, 3J
ACHTREUNG
AHCTREUNG
A
ACHTREUNG
A
ACHTREUNG
Experimental Section
1H NMR spectra of a solution of 3 in CD3OD were acquired at different
temperatures. The ratio I/3 (K) was determined by measuring the area of
General: IR spectra were recorded in liquid film between NaCl plates on
Si
A
a FTIR Mattson Genesis II spectrometer. NMR spectra were determined
on a Bruker Avance DPX 300 and Bruker Avance-500 spectrometers. H
1
0.193 (313), 0.186 (318), 0.182 (323), 0.178 (328).
562
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 557 – 568

Synthesis of 2-Aryl-3-vinyl-2,3-dihydrobenzo[b]furans
FULL PAPER
2,2-Dimethyl-9-methoxy-2,3-dihydrobenzo[f]
of allyl[2-methoxy-6-(1-propenyl)phenoxy]dimethylsilane
C
(d, 3J
4), 5.99 (ddd, 3J
9.0 Hz, 1H; H-2), 5.26 (ddd, J
(ddd, 3J(H,H)=16.9, 1.5, 0.8 Hz, 1H; H-2’’b), 4.05 (brdd, 3J
G
ACHTREUNG
G
ACHTREUNG
3
ACHTREUNG
3.16 mmol) with Grubbs 2nd generation catalyst (13 mg, 0.01 mmol) in
anhydrous CH2Cl2 (150 mL) followed by workup, as described in the gen-
eral procedure, yielded 7 (650 mg, 2.95 mmol, 93%): White solid; Rf =
0.34 (hexane/Et2O 98:2 );1H NMR (300 MHz, CDCl3, 2 58C, TMS): d=
U
ACHTREUNG
8.4 Hz, 1H; H-3), 3.85 ppm (s, 3H, OCH3); 13C NMR (75 MHz, CDCl3,
258C, TMS): d=159.5 (C, C-4’), 159.2(C, C-7a), 137.0 (CH, C-1 ’’), 132.3
(C, C-1’), 129.4 (C, C-3a), 128.7 (CH, C-6), 127.4 (CH, C-2’, C-6’), 124.7
(CH, C-7), 120.8 (CH, C-5), 117.9 (CH2, C-2’’), 114.0 (CH, C-3’, C-5’),
109.6 (CH, C-4), 89.8 (CH, C-2), 56.1 (CH, C-3), 55.3 ppm (CH3, OCH3);
IR (KBr): n˜ =3070, 2958, 2920, 2835, 1604, 1490, 1469, 1233, 1095, 1029,
969, 801, 750, 668 cmÀ1; HR-FABMS: m/z: calcd for C17H16O2: 252.1150
[M]+; found: 252.1149.
6.93 (dd, 3J
1H; H-8), 6.73 (dd, 3J
10.6 Hz, 1H; H-5), 6.11 (dt, J
A
ACHTREUNG
G
ACHTREUNG
3
AHCTREUNG
OCH3), 1.58 (d, J=10.6 Hz, 2H; H-3), 0.38 ppm (s, 6H; SiMe2);
13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=151.6 (C, C-9), 142.7 (C, C-
9a), 129.6 (C, C-5a), 128.5 (CH, C-4), 126.2 (CH, C-5), 122.3 (CH, C-6),
120.9 (CH, C-7), 109.9 (CH, C-8), 55.9 (OCH3), 18.4 (CH2, C-3),
À0.5 ppm (SiMe2); IR (KBr): n˜ =3014, 2953, 2839, 1572, 1469, 1258,
1079, 918, cmÀ1; HR-FABMS: m/z: calcd for C12H16O2SiNa 243.0817
[M+Na]+; found: 243.0820.
cis-2-(4-Methoxy-2-pivaloyloxyphenyl)-3-vinyl-2,3-dihydrobenzofuran
(5c) and trans-2-(4-methoxy-2-pivaloyloxyphenyl)-3-vinyl-2,3-dihydro-
benzofuran (6c): Reaction of benzoxasilepin (3) (79 mg, 0.41 mmol) with
4-methoxy-2-pivaloyloxybenzaldehyde (4c) (108 mg, 0.45 mmol) and
BF3·Et2O (104 mL, 0.82mmol) in anhydrous CH 2Cl2 (2mL) followed by
workup, as described in the general procedure, yielded 5c/6c 17:83 with
an overall yield of 61%.
2,2-Dimethyl-7-methoxy-2,3-dihydrobenzo[f]
of allyl[4-methoxy-2-(1-propenyl)phenoxy]dimethylsilane
5.7 mmol) with Grubbs 2nd generation catalyst (24 mg, 0.028 mmol) in
anhydrous CH2Cl2 (285 mL) followed by workup, as described in the gen-
eral procedure, yielded 8 (1.13 g, 5.15 mmol, 90%). Colourless oil; Rf =
0.35 (hexane/Et2O 98:2 );1H NMR (300 MHz, CDCl3, 2 58C, TMS): d=
Compound 6c: Colourless oil; Rf =0.28 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.37 (d, 3J
(H,H)=8.7 Hz 1H; H-6’),
G
3
3
ACHTREUNG
7.21 (t, J
6.93 (dt, 3J
H-7), 6.80 (dd, 3J
2.5 Hz, 1H; H-3’), 5.92(ddd, 3J
(d, 3J(H,H)=8.3 Hz, 1H; H-2), 5.17 (dd, 3J
2’’), 4.07 (t, J
9H, OCOC
(CH3)3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=176.6
(C, OCOC(CH3)3), 160.1 (C, C-4’), 159.3 (C, C-7a), 149.5 (C, C-2’), 137.0
A
6.87 (d, 3J
H-8), 6.63 (d, 3J
1H; H-5), 6.09 (dt, J
1.57 (d, 3J(H,H)=7.4 Hz, 2H; H-3), 0.31 ppm (s, 6H; SiMe2); 13C NMR
U
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
G
ACHTREUNG
3
ACHTREUNG
AHCTREUNG
ACHTREUNG
A
ACHTREUNG
3
AHCTREUNG
(75 MHz, CDCl3, 2 58C, TMS): d=153.8 (C, C-7), 147.1 (C, C-9a), 129.0
(C, C-5a), 128.6 (CH, C-4), 126.4 (CH, C-5), 121.8 (CH, C-9), 114.6 (CH,
C-6), 113.9 (CH, C-8), 55.6 (OCH3), 18.2(CH 2, C-3), À0.7 ppm (SiMe2);
IR (film): n˜ =3013, 2950, 2839, 1609, 1490, 1418, 1259, 1220, 1144, 1042,
935, 890, 842, 811 cmÀ1; HR-EIMS: m/z: calcd for C12H16O2Si: 220.0919
[M]+; found: 220.0919.
AHCTREUNG
AHCTREUNG
(CH, C-1’’), 129.0 (C, C-3a), 128.7 (CH, C-6), 128.4 (CH, C-6’), 124.9
(CH, C-4), 124.2 (C, C-1’), 120.9 (CH, C-5), 117.6 (CH2, C-2’’), 112.1
(CH, C-5’), 109.6 (CH, C-7), 108.2(CH, C-3 ’), 84.9 (CH, C-2), 55.5 (CH3,
OCH3), 55.0 (CH, C-3), 39.2(C, CO C
(CH3)3), 27.1 ppm (CH3, COC-
General procedure for the Sakurai reaction: BF3·Et2O (2equiv) was
added to a stirred solution of the corresponding benzoxasilepin in anhy-
drous CH2Cl2 (4 mLmmÀ1ol) under a N2 atmosphere at À458C. After
5 min, a solution of the aldehyde (1.1 equiv) in CH2Cl2 was added drop-
wise. The reaction mixture was stirred at À458C for 2h and was warmed
to room temperature. The mixture was then heated to reflux for 4–10 h,
diluted with CH2Cl2 and washed with brine. The dried (Na2SO4) extract
was concentrated in vacuo and the cis/trans ratio was measured by
1H NMR spectroscopy. The major isomers were purified by chromatogra-
phy over silica gel.
AHCTREUNG
1460, 1258, 1229, 1155, 974 cmÀ1
; HR-FABMS: m/z: calcd for
C22H24O4Na: 375.1572[ M+Na]+; found: 375.1571.
cis-2-(3-Methoxyphenyl)-3-vinyl-2,3-dihydrobenzofuran (5d) and trans-2-
(3-methoxyphenyl)-3-vinyl-2,3-dihydrobenzofuran (6d): Reaction of ben-
zoxasilepin (3) (70 mg, 0.37 mmol) with 3-methoxybenzaldehyde (4d)
(55 mg, 0.4 mmol) and BF3·Et2O (94 mL, 0.74 mmol) in anhydrous CH2Cl2
(2mL) followed by workup, as described in the general procedure, yield-
ed 5d/6d 20:80 with an overall yield of 62%.
Compound 5d: Colourless oil; Rf =0.30 (hexane/Et2O 95:5); 1H NMR
trans-2-(4-Dimethylaminophenyl)-3-vinyl-2,3-dihydrobenzofuran
(6a):
(300 MHz, CDCl3, 2 58C, TMS): d=7.24 (m, 1H; H-5’), 7.22 (brd, 3J-
Reaction of benzoxasilepin (3) (90 mg, 0.47 mmol) with dimethylamino-
benzaldehyde (4a) (77 mg, 0.52mmol) and BF 3·Et2O (120 mL,
0.94 mmol) in anhydrous CH2Cl2 (2mL) followed by workup, as descri-
bed in the general procedure, yielded 6a (48%). Colourless oil; Rf =0.30
(hexane/Et2O 95:5); 1H NMR (300 MHz, CDCl3, 2 58C, TMS): d=7.32
A
ACHTREUNG
ACHTREUNG
AHCTREUNG
A
ACHTREUNG
3
3
ACHTREUNG
(d, J
H-6), 7.12(d, 3J
H-5), 6.87 (d, 3J(H,H)=8.4 Hz, 1H; H-7), 6.75 (d, 3J
H-3’, H-5’), 5.96 (ddd, 3J(H,H)=16.9, 9.7, 8.9 Hz, 1H; H-1’’), 5.32(d, 3J-
(H,H)=8.9 Hz, 1H; H-2), 5.21 (d, 3J
(H,H)=9.7 Hz, 1H; H-2’’a), 5.19 (d,
3J
(H,H)=16.9 Hz, 1H; H-2’’b), 4.01 (dd, J
2.98 ppm (s, 6H; N
(CH3)2); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=
159.3 (C, C-7a), 150.6 (C, C-4’), 137.2(CH, C-1 ’’), 129.6 (C, C-1’)*, 128.6
(CH, C-6), 128. 0 (C, C-3a)*, 127.3 (CH, C-2’, C-6’), 124.7 (CH, C-4),
1120.5 (CH, C-5), 117.5 (CH2, C-2’’), 112.4 (CH, C-3’, C-5’), 109.5 (CH,
A
AHCTREUNG
3H, OCH3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=159.5* (C, C-
3’), 159.5* (C, C-7a), 139.5 (C, C-1’), 136.0 (CH, C-1’’), 129.2 (CH, C-5’),
129.0 (C, C-3a), 128.7 (CH, C-6), 125.6 (CH, C-4), 120.9 (CH, C-5), 118.8
(CH, C-6’), 116.7 (CH2, C-2’’), 113.0 (CH, C-4’)#, 112.1 (CH, C-2’)#, 109.5
(CH, C-7), 87.1 (CH, C-2), 55.2 (CH3, OCH3), 51.6 ppm (CH, C-3); * and
A
ACHTREUNG
G
ACHTREUNG
AHCTREUNG
G
ACHTREUNG
3
E
(H,H)=8.9, 8.5 Hz, 1H; H-3),
#
ACHTREUNG
may be interchanged; HR-EIMS: m/z: calcd for C17H16O2: 252.1150
[M]+; found: 252.1148.
Compound 6d: Colourless oil; Rf =0.29 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.31 (t, 3J
ACHTREUNG
7.22 (t, 3J
A
ACHTREUNG
C-7), 90.3 (CH, C-2), 55.5 (CH, C-3), 40.5 ppm (CH3,
N(CH3)2);
C
AHCTREUNG
trans-2-(4-Methoxyphenyl)-3-vinyl-2,3-dihydrobenzofuran (6b): Reaction
of benzoxasilepin (3) (104 mg, 0.55 mmol) with 4-methoxybenzaldehyde
(4b) (82mg, 0.60 mmol) and BF 3·Et2O (140 mL, 1.12mmol) in anhydrous
CH2Cl2 (2.5 mL) followed by workup, as described in the general proce-
dure, yielded 6b (60%). White solid; Rf =0.28 (hexane/Et2O 95:5); m.p.
A
ACHTREUNG
G
ACHTREUNG
3
3
ACHTREUNG
E
A
ACHTREUNG
8.9, 8.5 Hz, 1H; H-3), 3.83 ppm (s, 3H, OCH3); 13C NMR (75 MHz,
CDCl3, 2 58C, TMS): d=159.8 (C, C-3’)*, 159.2(C, C-7a)*, 142.1 (C, C-
1’), 137.1 (CH, C-1’’), 129.6 (CH, C-5’), 129.1 (C, C-3a), 128.7 (CH, C-6),
124.8 (CH, C-4), 120.9 (CH, C-5), 118.0 (CH, C-6’), 117.9 (CH2, C-2’’),
1
3
ACHTREUNG
54–568C; H NMR (300 MHz, CDCl3, 2 58C, TMS): d=7.40 (d, J(H,H)=
3
ACHTREUNG
3
8.8 Hz, 2H; H-2’, H-6’), 7.25 (t, J
(H,H)=7.4 Hz, 1H; H-7), 6.96 (dt, 3J
A
ACHTREUNG
Chem. Eur. J. 2007, 13, 557 – 568
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
563

I. Rodríguez-García et al.
113.6 (CH, C-4’)#, 111.2(CH, C-2 ’)#, 109.6 (CH, C-7), 89.6 (CH, C-2),
1H; H-7), 5.99 (ddd, 3J
(H,H)=9.0 Hz, 1H; H-2), 5.27 (dd, 3J
5.21 (ddd, 3J(H,H)=16.9, 1.6, 0.8 Hz, 1H; H-2’’b), 4.02(brt, 3J
8.9 Hz, 1H; H-3), 1.39 ppm (s, 9H; OCOC
(CH3)3); 13C NMR (75 MHz,
CDCl3, 2 58C, TMS): d=177.0 (C, OCOC(CH3)3), 159.1 (C, C-7a), 150.8
(C, C-4’), 137.8 (C, C-1’), 136.8 (CH, C-1’’), 129.1 (C, C-3a), 128.8 (CH,
C-6), 126.8 (CH, C-2’, C-6’), 124.8 (CH, C-4), 121.6 (CH, C-3’, C-5’),
121.0 (CH, C-5), 118.1 (CH2, C-2’’), 109.6 (CH, C-7), 89.2(CH, C-2), 56.3
ACHTREUNG
#
56.2(CH, C-3), 55.2ppm (CH 3, OCH3); * and may be interchanged;
G
ACHTREUNG
HR-EIMS: m/z: calcd for C17H16O2: 252.1150 [M]+; found: 252.1146.
N
ACHTREUNG
AHCTREUNG
cis-2-(2-Methoxyphenyl)-3-vinyl-2,3-dihydrobenzofuran (5e) and trans-2-
(2-methoxyphenyl)-3-vinyl-2,3-dihydrobenzofuran (6e): Reaction of ben-
zoxasilepin (3) (102mg, 0.53 mmol) with 2-methoxybenzaldehyde ( 4e)
(80 mg, 0.59 mmol) and BF3·Et2O (134 mL, 1.06 mmol) in anhydrous
CH2Cl2 (2.5 mL) followed by workup, as described in the general proce-
dure, yielded 5e/6e 20:80 with an overall yield of 63%.
AHCTREUNG
(CH, C-3), 39.1 (C, COC
N
N
(film): n˜ =3078, 3050, 2974, 2932, 2873, 1750, 1595, 1507, 1477, 1459,
1396, 1365, 1277, 1229, 1199, 1164, 1116, 970, 750 cmÀ1; HR-FABMS: m/
z: calcd for C21H22O3Na: 345.1466 [M+Na]+; found: 345.1464.
Compound 5e: White solid; Rf =0.27 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.47 (dd, 3J
ACHTREUNG
H-6’), 7.28 (dt, 3J
A
ACHTREUNG
cis-2-Phenyl-3-vinyl-2,3-dihydrobenzofuran (5g) and trans-2-phenyl-3-
vinyl-2,3-dihydrobenzofuran (6g): Reaction of benzoxasilepin (3) (85 mg,
0.45 mmol) with benzaldehyde (4g) (52mg, 0.49 mmol) and BF 3·Et2O
(114 mL, 0.90 mmol) in anhydrous CH2Cl2 (2.5 mL) followed by workup,
as described in the general procedure, yielded 5g/6g 50:50 with an over-
all yield of 58%.
AHCTREUNG
AHCTREUNG
A
ACHTREUNG
AHCTREUNG
ACHTREUNG
13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=159.4 (C, C-7a), 155.6 (C, C-
2’), 136.5 (CH, C-1’’), 129.6 (C, C-3a), 128.4 (CH, C-6), 128.4 (CH, C-4’),
126.9 (C, C-1’), 126.3 (CH, C-6’), 125.7 (CH, C-4), 120.6 (CH, C-5), 120.3
(CH, C-5’), 115.4 (CH2, C-2’’), 109.7 (CH, C-3’), 109.5 (CH, C-7), 83.2
(CH, C-2), 55.1 (CH3, OCH3), 50.2ppm (CH, C-3); IR (KBr): n˜ =3078,
2962, 2928, 2837, 1604, 1493, 1477, 1461, 1233, 1098, 1029, 969, 801, 750,
668 cmÀ1; HR-FABMS m/z: calcd for C17H16O2: 252.1150 [M]+; found:
252.1151.
Compound 5g: Colourless oil; Rf =0.30 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.50–7.20 (m, 6H; H-2’, 3’, 4’, 5’, 6’,
6); 7.14 (brd, 3J
5.91 (d, 3J
8.8 Hz, 1H; H-1’’); 5.04 (dd, 3J
(dd, 3J
ACHTREUNG
G
ACHTREUNG
AHCTREUNG
(H,H)=10.0, 1.9 Hz, 1H; H-2’’b), 4.34 ppm (dd, 3J
ACHTREUNG
8.8 Hz, 1H; H-3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=159.6 (C,
C-7a); 137.9 (C, C-1’); 136.1 (CH, C-1’’); 129.0 (C, C-3a); 128.7 (CH, C-
6)*; 128.1 (CH, C-3’, C-5’); 127.6 (CH, C-4’)*; 126.4 (CH, C-2’, C-6’);
Compound 6e: Colourless oil; Rf =0.26 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.43 (dd, 3J
ACHTREUNG
125.6 (CH, C-4); 120.9 (CH, C-5)#; 116.8 (CH2, C-2’’); 109.5 (CH, C-7)#;
87.2(CH, C-2); 51.6 ppm (CH, C-3); * and
H-6’), 7.31 (ddd, 3J
3J
ACHTREUNG
#
may be interchanged; IR
G
ACHTREUNG
(film): n˜ =3065, 3030, 2967, 2924, 1730, 1595, 1470, 1304, 1260, 1227,
1089, 1013, 974, 921, 747, 698 cmÀ1; HR-EIMS: m/z: calcd for C16H15O:
223.1123 [M+H]+; found: 223.1121.
Compound 6g: Colourless oil; Rf =0.29 (hexane/Et2O 95:5); 1H NMR
AHCTREUNG
AHCTREUNG
AHCTREUNG
3H, OCH3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=159.5 (C, C-
7a), 156.6 (C, C-2’), 138.6 (CH, C-1’’), 129.3 (C, C-1’), 129.0 (C, C-3a),
128.9 (CH, C-4’), 128.5 (CH, C-6), 126.4 (CH, C-6’), 125.2 (CH, C-4),
120.6 (CH, C-5), 120.6 (CH, C-5’), 115.8 (CH2, C-2’’), 110.6 (CH, C-3’),
109.5 (CH, C-7), 85.1 (CH, C-2), 55.3 (CH3, OCH3), 54.8 ppm (CH, C-3);
IR (film): n˜ =3077, 2934, 2835, 1596, 1477, 1460, 1282, 1244, 1234, 1027,
(300 MHz, CDCl3, 2 58C, TMS): d=7.47–7.33 (m, 5H; H-2’, H-3’, H-4’,
H-5’, H-6’), 7.23 (dd, 3J
C
ACHTREUNG
(H,H)=7.3 Hz, 1H; H-5), 6.92(d, 3J
ACHTREUNG
7.9 Hz, 1H; H-7), 6.00 (ddd, J
3
(d, J
A
974, 750 cmÀ1
; HR-FABMS: m/z: calcd for C17H16O2Na: 275.1048
5.20 (brd, 3J
A
[M+Na]+; found: 275.1042.
1H; H-3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=159.2(C, C-7a),
140.4 (C, C-1’), 137.0 (CH, C-1’’), 129.2 (C, C-3a), 128.7 (CH, C-6), 128.5
(CH, C-3’, C-5’), 128.1 (CH, C-4’), 125.8 (CH, C-2’, C-6’), 124.8 (CH, C-
4), 120.9 (CH, C-5), 117.9 (CH2, C-2’’), 109.6 (CH, C-7), 89.7 (CH, C-2),
56.2ppm (CH, C-3); IR (film): n˜ =3064, 3031, 2959, 2922, 2885, 1638,
1596, 1476, 1262, 1228, 1099, 1014, 982, 922, 865, 750, 697 cmÀ1; HR-
EIMS: m/z: calcd for C16H15O: 223.1123 [M+H]+; found: 223.1128.
cis-2-(4-Pivaloyloxyphenyl)-3-vinyl-2,3-dihydrobenzofuran (5f) and trans-
2-(4-pivaloyloxyphenyl)-3-vinyl-2,3-dihydrobenzofuran (6f): Reaction of
benzoxasilepin (3) (110 mg, 0.58 mmol) with 4-pivaloyloxybenzaldehyde
(4 f) (130 mg, 0.63 mmol) and BF3·Et2O (150 mL, 1.16 mmol) in anhy-
drous CH2Cl2 (3.0 mL) followed by workup, as described in the general
procedure, yielded 5 f/6 f 33:67 with an overall yield of 60%.
Compound 5 f: White solid; Rf =0.27 (hexane/Et2O 95:5); 1H NMR
cis-2-(2,4-Dipivaloyloxyphenyl)-3-vinyl-2,3-dihydrobenzofuran (5h) and
(300 MHz, CDCl3, 2 58C, TMS): d=7.29 (d, 3J
ACHTREUNG
trans-2-(2,4-dipivaloyloxyphenyl)-3-vinyl-2,3-dihydrobenzofuran
(6h):
H-6’), 7.23 (brt, 3J
A
ACHTREUNG
Reaction of benzoxasilepin (3) (120 mg, 0.63 mmol) with 2,4-dipivaloy-
loxybenzaldehyde (4h) (212 mg, 0.69 mmol) and BF3·Et2O (160 mL,
1.26 mmol) in anhydrous CH2Cl2 (3.0 mL) followed by workup, as descri-
bed in the general procedure, yielded 5h/6h 75:25 with an overall yield
of 57%.
A
ACHTREUNG
AHCTREUNG
A
ACHTREUNG
ACHTREUNG
Compound 5h: White solid; Rf =0.29 (hexane/Et2O 95:5); 1H NMR
A
ACHTREUNG
(300 MHz, CDCl3, 2 58C, TMS): d=7.50 (brd, 3J
A
AHCTREUNG
3
3
ACHTREUNG
AHCTREUNG
6’), 7.23 (dt, J
(H,H)=7.3, 1.4 Hz, 1H; H-6), 7.14 (brd, J
1H; H-4), 6.95 (m, 4H; H-5, H-7, H-3’, H-5’), 5.94 (d, 3J
1H; H-2), 5.33 (ddd, 3J
3J
(CH, C-1’’),135.2(C, C-1 ’), 128.9 (C, C-3a), 128.7 (CH, C-6), 127.4 (CH,
C-2’, C-6’), 125.6 (CH, C-4), 121.2 (CH, C-3’, C-5’), 121.0 (CH, C-5),
117.0 (CH2, C-2’’), 109.5 (CH, C-7), 86.7 (CH, C-2), 51.6 (CH, C-3), 39.0
AHCTREUNG
A
ACHTREUNG
(C, COC
E
G
A
ACHTREUNG
2974, 1752, 1596, 1476, 1459, 1275, 1228, 1199, 1164, 1116, 1015, 984, 930,
AHCTREUNG
866, 750 cmÀ1
; HR-FABMS: m/z: calcd for C21H22O3Na: 345.1467
N
ACHTREUNG
[M+Na]+; found: 345.1463.
7a), 150.6 (C, C-4’), 147.6 (C, C-2’), 135.5 (CH, C-1’’), 128.8 (CH, C-6),
128.6 (C, C-3a), 127.5 (CH, C-6’), 127.4 (C, C-1’), 125.8 (CH, C-4), 121.1
(CH, C-5), 118.6 (CH, C-5’), 116.8 (CH2, C-2’’), 115.4 (CH, C-3’), 109.7
(CH, C-7), 82.3 (CH, C-2), 50.6 (CH, C-3), 39.2 (C, COC
(C, COC(CH3)3), 27.1 (CH3, COC(CH3)3), 27.1 ppm (CH3, COC
IR (KBr): n˜ =2974, 2933, 2873, 1755, 1595, 1478, 1460, 1230, 1107, 1028,
Compound 6 f: Colourless oil; Rf =0.26 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.46 (d, 3J
ACHTREUNG
H-6’), 7.23 (ddd, 3J
A
ACHTREUNG
3J
C
G
H
H
ACHTREUNG
ACHTREUNG
564
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 557 – 568

Synthesis of 2-Aryl-3-vinyl-2,3-dihydrobenzo[b]furans
FULL PAPER
978, 901, 751 cmÀ1; HR-FABMS: m/z: calcd for C26H30O5Na: 445.1991
[M+Na]+; found: 445.1990.
64–668C; 1H NMR (300 MHz, CDCl3, 2 85C, TMS): d=7.52(dd,
3J
A
ACHTREUNG
5’), 7.24 (m, 2H; H-4’, H-6), 7.15 (d, 3J
ACHTREUNG
cis-2-(5-Methoxy-2-pivaloyloxyphenyl)-3-vinyl-2,3-dihydrobenzofuran
(5i) and trans-2-(5-methoxy-2-pivaloyloxyphenyl)-3-vinyl-2,3-dihydroben-
zofuran (6i): Reaction of benzoxasilepin (3) (112mg, 0.59 mmol) with 5-
methoxy-2-pivaloyloxybenzaldehyde (4i) (153 mg, 0.65 mmol) and
BF3·Et2O (150 mL, 1.18 mmol) in anhydrous CH2Cl2 (3.0 mL) followed by
workup, as described in the general procedure, yielded 5i/6i 83:17 with
an overall yield of 63%.
3
3
ACHTREUNG
(dd, J
C
C
ACHTREUNG
AHCTREUNG
A
ACHTREUNG
AHCTREUNG
AHCTREUNG
AHCTREUNG
Compound 5i: Colourless oil; Rf =0.30 (hexane/Et2O 95:5); 1H NMR
1’), 128.8 (CH, C-6), 128.7 (C, C-3a), 128.4 (CH, C-5’), 127.2 (CH, C-3’),
125.8 (CH, C-4), 125.5 (CH, C-4’), 121.7 (CH, C-6’), 121.0 (CH, C-5),
116.6 (CH2, C-2’’), 109.7 (CH, C-7), 82.5 (CH, C-2), 50.6 (CH, C-3), 39.2
(300 MHz, CDCl3, 2 58C, TMS): d=7.23 (dd, 3J
ACHTREUNG
H-6), 7.15 (d, 3J
A
ACHTREUNG
3
3
ACHTREUNG
H-6’), 7.01 (d, J
N
(C, COC
N
N
H-7), 6.95 (dd, 3J
ACHTREUNG
2974, 1752, 1596, 1476, 1459, 1275, 1228, 1199, 1164, 1116, 1015, 984, 930,
AHCTREUNG
866, 750 cmÀ1
; HR-FABMS: m/z: calcd for C21H22O3Na: 345.1467
3
ACHTREUNG
A
[M+Na]+; found: 345.1466.
A
ACHTREUNG
cis-2-(4-Nitrophenyl)-3-vinyl-2,3-dihydrobenzofuran (5m): Reaction of
benzoxasilepin (3) (107 mg, 0.56 mmol) with 4-nitrobenzaldehyde (4m)
(94 mg, 0.62mmol) and BF 3·Et2O (140 mL, 1.12mmol) in anhydrous
CH2Cl2 (3 mL) followed by workup, as described in the general proce-
dure, yielded 5m (30%). Yellow oil; Rf =0.29 (hexane/Et2O 95:5);
ACHTREUNG
ACHTREUNG
1’’), 131.1 (C, C-1’), 128.7 (CH, C-6), 128.7 (C, C-3a), 125.8 (CH, C-4),
122.6 (CH, C-3’), 121.0 (CH, C-5), 116.5 (CH2, C-2’’), 113.7 (CH, C-4’),
112.1 (CH, C-6’), 109.7 (CH, C-7), 82.6 (CH, C-2), 55.6 (CH3, OCH3),
1H NMR (300 MHz, CDCl3, 2 58C, TMS): d=8.23 (d, 3J
2H; H-3’, H-5’), 7.48 (d, 3J
(H,H)=8.9 Hz, 2H; H-2’, H-6’), 7.26 (m, 1H;
H-6), 7.14 (d, 3J
(H,H)=7.1 Hz, 1H; H-4), 6.99 (m, 2H; H-5, H-7), 5.97
(d, 3J(H,H)=9.5 Hz, 1H; H-2), 5.17 (ddd, 3J
(H,H)=16.9, 9.4, 8.8 Hz,
1H; H-1’’), 5.06 (dd, 3J
(H,H)=16.9, 2.3 Hz, 1H; H-2’’b), 4.95 (dd,
3J(H,H)=9.4, 2.3, Hz, 1H; H-2’’a), 4.40 ppm (dd, 3J
(H,H)=9.5, 8.9 Hz,
(H,H)=8.9 Hz,
AHCTREUNG
50.6 (CH, C-3), 39.1 (C, OCOC
G
G
AHCTREUNG
IR (film): n˜ =2973, 2934, 2873, 2836, 1751, 1596, 1496, 1478, 1462, 1275,
1232, 1183, 1118, 1040, 980, 752 cmÀ1
; HR-FABMS: m/z: calcd for
A
ACHTREUNG
C22H24O4Na [M+Na]+ 375.1572; found: 375.1573.
AHCTREUNG
A
ACHTREUNG
cis-2-(3-Pivaloyiloxyphenyl)-3-vinyl-2,3-dihydrobenzofuran (5j): Reac-
tion of benzoxasilepin (3) (66 mg, 0.35 mmol) with m-pivaloyloxybenzal-
dehyde (4j) (79 mg, 0.38 mmol) and BF3·Et2O (88 mL, 0.70 mmol) in an-
hydrous CH2Cl2 (2mL) followed by workup, as described in the general
procedure, yielded 5j (58%). Colourless oil; Rf =0.28 (hexane/Et2O
1H; H-3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=159.0 (C, C-7a),
147.4 (C, C-4’), 145.4 (C, C-1’), 135.3 (CH, C-1’’), 129.1 (CH, C-6), 128.3
(C, C-3a), 127.3 (CH, C-2’, C-6’), 125.7 (CH, C-4), 123.4 (CH, C-3’, C-5’),
121.5 (CH, C-5), 117.7 (CH2, C-2’’), 109.7 (CH, C-7), 86.0 (CH, C-2),
51.7 ppm (CH, C-3); HR-EIMS: m/z: calcd for C16H13NO3: 267.0895
[M]+; found: 267.0897.
95:5); 1H NMR (300 MHz, CDCl3, 2 58C, TMS): d=7.36 (t, 3J
7.7 Hz, 1H; H-5’), 7.24 (dt, 3J(H,H)=7.3, 1.2Hz, 1H; H-6), 7.14 (d, 3J-
(H,H)=7.7 Hz, 2H; H-4’, H-6’), 7.03–6.92(m, 4H; H-5, H-7, H-2 ’, H-4),
5.89 (d, 3J(H,H)=9.3 Hz, 1H; H-2), 5.32 (ddd, 3J
(H,H)=16.9, 10.1,
9.0 Hz, 1H; H-1’’), 5.02(dd, 3J
(H,H)=16.9, 1.2Hz, 1H; H-2 ’’b), 4.95 (dd,
3J(H,H)=10.1, 1.6 Hz, 1H; H-2’’a), 4.31 (t, 3J
(H,H)=9.3 Hz, 1H; H-3),
1.37 ppm (s, 9H; OCO
(CH3)3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS):
d=176.8 (COCOC(CH3)3), 159.3 (C, C-7a), 151.1 (C, C-3’), 139.6 (C, C-
N
AHCTREUNG
cis-2-(2-Bromophenyl)-3-vinyl-2,3-dihydrobenzofuran (5n) and trans-2-
(2-bromophenyl)-3-vinyl-2,3-dihydrobenzofuran (6n): Reaction of ben-
zoxasilepin (3) (107 mg, 0.56 mmol) with o-bromobenzaldehyde (4n)
(65 mg, 0.35 mmol) and BF3·Et2O (81 mL, 0.64 mmol) in anhydrous
CH2Cl2 (3 mL) followed by workup, as described in the general proce-
dure, yielded 5n/6n (72:28) with an overall yield of 58%; Rf =0.27
(hexane/Et2O 95:5).
ACHTREUNG
N
ACHTREUNG
AHCTREUNG
G
ACHTREUNG
ACHTREUNG
ACHTREUNG
1’), 135.9 (CH, C-1’’), 129.0 (CH, C-5’), 128.9 (C, C-3a), 128.7 (CH, C-6),
125.7 (CH, C-4), 123.6 (CH, C-6’), 121.0 (CH, C-5), 120.6* (CH, C-4’),
119.6* (CH, C-2’), 116.9 (CH2, C-2’’), 109.6 (CH, C-7), 86.6 (CH, C-2),
Compound 5n: Colourless oil; 1H NMR (300 MHz, CDCl3, 2 58C, TMS):
d=7.56 (m, 2H; H-3’, H-5’), 7.36–7.15 (m, 4H; H-4, H-6, H-4’, H-6’),
6.96 (m, 2H; H-5, H-7)), 6.12 (d, 1H; 3J
ACHTREUNG
51.5 (CH, C-3), 39.0 (C, OCOC
G
HR-EIMS: m/z: calcd for C21H22O3: 322.1569 [M]+; found: 322.1568
3J
G
ACHTREUNG
AHCTREUNG
cis-2-(3-Nitrophenyl)-3-vinyl-2,3-dihydrobenzofuran (5k): Reaction of
benzoxasilepin (3) (69 mg, 0.36 mmol) with m-nitrobenzaldehyde (4k)
(60 mg, 0.40 mmol) and BF3·Et2O (91 mL, 0.72mmol) in anhydrous
CH2Cl2 (2mL) followed by workup, as described in the general proce-
dure, yielded 5k (41%). Yellow oil; Rf =0.30 (hexane/Et2O 95:5);
1H NMR (300 MHz, CDCl3, 2 58C, TMS): d=8.17 (m, 2H; H-2’, H-4’),
AHCTREUNG
258C, TMS): d=159.1 (C, C-7a), 137.8 (C, C-1’), 135.5 (CH, C-1’’), 132.3
(CH, C-3’), 129.0* (CH, C-6’), 128.8 (C, C-3a), 128.7 (CH, C-6), 127.9*
(CH, C-5’), 127.2* (CH, C-4’), 125.9 (CH, C-4), 121.5 (C, C-2’), 121.1
(CH, C-5), 116.4 (CH2, C-2’’), 109.6 (CH, C-7), 86.3 (CH, C-2), 49.6 ppm
3
3
ACHTREUNG
(CH, C-3);
* may be interchanged; HR-EIMS: m/z: calcd for
7.64 (d, 1H; J
7.26 (m, 1H; H-6), 7.15 (d, 3J
5, H-7), 5.97 (d, 3J(H,H)=9.1 Hz, 1H; H-2), 5.21 (ddd, 3J
9.5, 9.1 Hz, 1H; H-1’’), 5.06 (dd, J
(dd, 3J(H,H)=9.5, 2.0 Hz, 1H; H-2’’a), 4.39 ppm (t, 3J
C
(H,H)=7.7 Hz, H-5’),
C16H1379BrO: 300.0150 [M]+; found: 300.0151.
AHCTREUNG
G
ACHTREUNG
cis-2-(3-Bromophenyl)-3-vinyl-2,3-dihydrobenzofuran (5o): Reaction of
benzoxasilepin (3) (69 mg, 0.36 mmol) with m-bromobenzaldehyde (4o)
(74 mg, 0.40 mmol) and BF3·Et2O (91 mL, 0.72mmol) in anhydrous
CH2Cl2 (2mL) followed by workup, as described in the general proce-
dure, yielded 5o (58%). Colourless oil; Rf =0.29 (hexane/Et2O 95:5);
1H NMR (300 MHz, CDCl3, 2 58C, TMS): d=7.44 (m, 2H; H-2’, H-4’),
3
AHCTREUNG
G
ACHTREUNG
7.21 (m, 3H; H-6, H-5’, H-6’), 7.14 (1H, dd, 3J
6.97 (m, 2H; H-5, H-7), 5.85 (d, 3J
(H,H)=8.9 Hz, 1H; H-2), 5.27 (ddd,
3J(H,H)=17.0, 9.8, 8.8 Hz, 1H; H-1’’), 5.04 (dd, 3J
(H,H)=17.0, 1.9 Hz,
1H; H-2’’b), 4.97 (dd, 3J
(H,H)=9.8, 1.9 Hz, 1H; H-2’’a), 4.32ppm (t,
3J(H,H)=8.9 Hz, 1H; H-3). 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=
159.2(C, C-7a), 140.3 (C, C-1 ’), 135.7 (CH, C-1’’), 130.7* (CH, C-5’),
129.7* (CH, C-4’), 129.4* (CH, C-2’), 128.8 (CH, C-6), 128.7 (C, C-3a),
125.6 (CH, C-4)#, 125.1 (CH, C-6’)#, 122.4 (C, C-3’), 121.1 (CH, C-5),
117.2(CH 2, C-2’’), 109.6 (CH, C-7), 86.2(CH, C-2), 51.6 ppm (CH, C-3);
N
#
109.0 (CH, C-7), 85.0 (CH, C-2), 50.8 ppm (CH, C-3); * and may be in-
AHCTREUNG
terchanged; HR-EIMS: m/z: calcd for C16H13NO3: 267.0895 [M]+; found:
A
267.0893.
AHCTREUNG
cis-2-(2-Pivaloyloxyphenyl)-3-vinyl-2,3-dihydrobenzofuran (5l): Reaction
of benzoxasilepin (3) (115 mg, 0.6 mmol) with 2-pivaloyloxybenzaldehyde
(4l) (137 mg, 0.66 mmol) and BF3·Et2O (150 mL, 1.2mmol) in anhydrous
CH2Cl2 (3 mL) followed by workup, as described in the general proce-
dure, yielded 5l (61%). White solid; Rf =0.28 (hexane/Et2O 95:5); m.p.
ACHTREUNG
Chem. Eur. J. 2007, 13, 557 – 568
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
565

I. Rodríguez-García et al.
* and may be interchanged; HR-EIMS: m/z: calcd for C16H1379BrO:
3J
A
G
#
300.0150 [M]+; found: 300.0148.
H-3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=159.3 (C, C-7a), 136.5
(C, C-1’), 135.8 (CH, C-1’’), 133.3 (C, C-4’), 128.8 (CH, C-6), 128.7 (C, C-
3a), 128.3 (2CH, C-3’, C-5’), 127.8 (2CH, C-2’, C-6’), 125.6 (CH, C-4),
121.1 (CH, C-5), 117.1 (CH2, C-2’’), 109.5 (CH, C-7), 86.5 (CH, C-2),
51.5 ppm (CH, C-3); HR-EIMS: m/z: calcd for C16H13ClO: 256.0655
[M]+; found: 256.0653.
cis-2-(4-Bromophenyl)-3-vinyl-2,3-dihydrobenzofuran (5p) and trans-2-
(4-bromophenyl)-3-vinyl-2,3-dihydrobenzofuran (6p): Reaction of ben-
zoxasilepin (3) (84 mg, 0.44 mmol) with p-bromobenzaldehyde (4p)
(90 mg, 0.48 mmol) and BF3·Et2O (110 mL, 0.88 mmol) in anhydrous
CH2Cl2 (2mL) followed by workup, as described in the general proce-
dure, yielded 5p/6p (72:28) with an overall yield of 56%.
cis-7-Methoxy-2-(4-methoxy-2-pivaloyloxyphenyl)-3-vinyl-2,3-dihydro-
benzofuran (5t) and trans-7-methoxy-2-(4-methoxy-2-pivaloyloxyphen-
yl)-3-vinyl-2,3-dihydrobenzofuran (6t): Reaction of 9-methoxybenzoxasi-
lepin (7) (116 mg, 0.53 mmol) with 4-methoxy-2-pivaloyloxybenzaldehyde
(4c) (149 mg, 0.63 mmol) and BF3·Et2O (130 mL, 1.06 mmol) in anhy-
drous CH2Cl2 (2.5 mL) followed by workup, as described in the general
procedure, yielded 5t/6t 9:91 with an overall yield of 57%.
Compound 5p: Colourless oil; Rf =0.30 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.48 (d, 3J
A
3
3
ACHTREUNG
H-5’), 7.24 (dt, J
A
2H; H-2’, H-6’), 7.13 (brd, 3J
5, H-7), 5.85 (d, 3J
9.7, 8.8 Hz, 1H; H-1’’), 5.04 (ddd, J
4.96 (dd, 3J
ACHTREUNG
A
ACHTREUNG
3
AHCTREUNG
Compound 6t: Colourless oil; Rf =0.25 (hexane/Et2O 95:5); 1H NMR
A
ACHTREUNG
9.1 Hz, 1H; H-3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=159.3 (C,
C-7a), 137.0 (C, C-4’), 135.8 (CH, C-1’’), 131.3 (CH, C-3’, C-5’), 128.8
(CH, C-6), 128.7 (C, C-1’), 128.1 (CH, C-2’, C-6’), 125.6 (CH, C-4), 121.5
(C, C-3a), 121.1 (CH, C-5), 117.1 (CH2, C-2’’), 109.5 (CH, C-7), 86.5 (CH,
C-2), 51.5 ppm (CH, C-3); HR-EIMS: m/z: calcd for C16H1379BrO:
300.0150 [M]+; found: 300.0146.
(300 MHz, CDCl3, 2 58C, TMS): d=7.39 (d, 3J
ACHTREUNG
6.90 (dd, 3J
G
ACHTREUNG
N
ACHTREUNG
AHCTREUNG
G
ACHTREUNG
AHCTREUNG
AHCTREUNG
cis-2-(2-Chlorophenyl)-3-vinyl-2,3-dihydrobenzofuran (5q) and trans-2-
(2-chlorophenyl)-3-vinyl-2,3-dihydrobenzofuran (6q): Reaction of ben-
zoxasilepin (3) (74 mg, 0.39 mmol) with o-chlorobenzaldehyde (4q)
(60 mg, 0.42mmol) and BF 3·Et2O (100 mL, 0.78 mmol) in anhydrous
CH2Cl2 (2mL) followed by workup, as described in the general proce-
dure, yielded 5q/6q 71:29 with an overall yield of 59%.
À
(75 MHz, CDCl3, 2 58C, TMS): d=176.6 (C, COO ), 160.0 (C, C-4’),
149.3 (C, C-2’), 147.6 (C, C-7a), 144.4 (C, C-7), 136.8 (CH, C-1’’), 130.1
(C, C-3a), 128.5 (CH, C-6’), 124.1 (C, C-1’), 121.5 (C, C-5), 117.6 (CH2,
C-2’’), 117.0 (CH, C-4), 112.2 (CH, C-5’), 111.9 (CH, C-6), 107.9 (CH, C-
3’), 85.3 (CH, C-2), 56.0 (OCH3), 55.7 (CH, C-3), 55.4 (OCH3), 39.2(C,
Compound 5q: Colourless oil; Rf =0.27 (hexane/Et2O 95:5); 1H NMR
C
A
E
(300 MHz, CDCl3, 2 58C, TMS): d=7.57 (dd, 3J
ACHTREUNG
H-3’), 7.38 (dd, 3J
ACHTREUNG
cis-2-(2,4-Dipivaloyloxyphenyl)-7-methoxy-3-vinyl-2,3-dihydrobenzofuran
(5u) and trans-2-(2,4-dipivaloyloxyphenyl)-7-methoxy-3-vinyl-2,3-dihy-
drobenzofuran (6u): Reaction of 9-methoxybenzoxasilepin (7) (157 mg,
0.72mmol) with 2,4-dipivaloyloxybenzaldehyde ( 4h) (149 mg, 0.63 mmol)
and BF3·Et2O (130 mL, 1.06 mmol) in anhydrous CH2Cl2 (3.5 mL) fol-
lowed by workup, as described in the general procedure, yielded 5u/6u
50:50 with an overall yield of 60%.
A
ACHTREUNG
AHCTREUNG
G
ACHTREUNG
ACHTREUNG
G
ACHTREUNG
1H; H-3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=159.1 (C, C-7a),
137.8 (C, C-1’)*, 136.3 (C, C-2’)*, 135.5 (CH, C-1’’), 129.0 (CH, C-5’)#,
128.9 (C, C-3a), 128.8 (CH, C-6, C-3’#), 127.4 (CH, C-6’)#, 126.7 (CH, C-
4’)#, 125.9 (CH, C-4), 121.1 (CH, C-5), 116.4 (CH2, C-2’’), 109.6 (CH, C-
Compound 5u: Colourless oil; Rf =0.29 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.56 (d, 3J
(H,H)=8.1 Hz, 1H; H-6’),
R
#
3
6.97 (dd, 3J
(H,H)=7.1 Hz, 1H; H-5), 6.85* (dd, 3J
ACHTREUNG
7), 84.5 (CH, C-2), 49.8 ppm (CH, C-3); * and may be interchanged;
HR-EIMS: m/z: calcd for C16H13ClO: 256.0655 [M]+; found: 256.0654.
A
ACHTREUNG
3
3
ACHTREUNG
cis-2-(3-Chlorophenyl)-3-vinyl-2,3-dihydrobenzofuran (5r): Reaction of
benzoxasilepin (3) (78 mg, 0.41 mmol) with m-chlorobenzaldehyde (4r)
(63 mg, 0.45 mmol) and BF3·Et2O (105 mL, 0.82mmol) in anhydrous
CH2Cl2 (2mL) followed by workup, as described in the general proce-
dure, yielded 5r (59%). Colourless oil; 1H NMR (300 MHz, CDCl3,
6.78* (brd, J
2), 5.35 (ddd, 3J
3J(H,H)=16.9 Hz; H-2’’b), 4.95 (brd, 3J
4.24 (dd, 3J
(H,H)=8.9, 8.5 Hz, 1H; H-3), 3.95 (s, 3H; OCH3), 1.38 (s,
9H; C(CH3)3), 1.35 ppm (s, 9H; C(CH3)3); * may be interchanged;
G
(H,H)=8.9 Hz, 1H; H-
AHCTREUNG
R
ACHTREUNG
AHCTREUNG
A
ACHTREUNG
13
258C, TMS): d=7.29 (m, 3H; H-2’, H-4’, H-6’), 7.23 (m, 1H; H-6), 7.18
À
C NMR (75 MHz, CDCl3, 2 58C, TMS): d=176.6 (C, COO ), 175.8 (C,
3
(m, 1H; H-5’), 7.13 (d, J
G
À
COO ), 150.6 (C, C-2’), 147.6 (C, C-7a), 147.4 (C, C-4), 144.6 (C, C-7),
135.4 (CH, C-1’’), 129.9 (C, C-3a), 127.9 (CH, C-6’), 127.0 (C, C-1’), 121.7
(C, C-5), 118.5 (CH, C-5’), 117.9 (CH, C-4), 116.9 (CH2, C-2’’), 115.3
(CH, C-3’), 111.9 (CH, C-6), 83.0 (CH, C-2), 56.1 (OCH3), 50.9 (CH, C-
7), 5.86 (d, 3J
9.3 Hz, 1H; H-1’’), 5.05 (dd, J
3J
(H,H)=9.3 Hz, 1H; H-2), 5.28 (ddd, 3J
ACHTREUNG
3
AHCTREUNG
R
ACHTREUNG
3); 13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=159.2(C, C-7a), 140.1
(C, C-1’), 135.7 (CH, C-1’’), 134.2(C, C-3 ’), 129.4 (CH, C-5’), 128.8 (CH,
C-6), 128.7 (C, C-3a), 127.8* (CH, C-4’), 126.5* (CH, C-2’), 125.6 (CH,
C-4), 124.7 (CH, C-6’), 121.1 (CH, C-5), 117.2 (CH2, C-2’’), 109.6 (CH, C-
7), 86.3 (CH, C-2), 51.6 ppm (CH, C-3); * may be interchanged; HR-
EIMS: m/z: calcd for C16H13ClO: 256.0655 [M]+; found: 256.0657.
3), 39.2(C,
C
C
G
A
EIMS: m/z: calcd for C27H32O6: 452.2199 [M]+; found: 452.2196.
Compound 6u: White solid; Rf =0.28 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.49 (d, 3J
G
3
3
ACHTREUNG
6.97 (dd, J
A
1H; H-5), 6.86 (m, 2H; H-3’,6), 6.73 (brd, 3J
5.91 (ddd, 3J
cis-2-(4-Chlorophenyl)-3-vinyl-2,3-dihydrobenzofuran (5s) and trans-2-
(4-chlorophenyl)-3-vinyl-2,3-dihydrobenzofuran (6s): Reaction of ben-
zoxasilepin (3) (61 mg, 0.32mmol) with p-chlorobenzaldehyde (4s)
(49 mg, 0.35 mmol) and BF3·Et2O (82 mL, 0.64 mmol) in anhydrous
CH2Cl2 (2mL) followed by workup, as described in the general proce-
dure, yielded 5s/6s 70:30 with an overall yield of 58%.
H
AHCTREUNG
AHCTREUNG
13
À
A
175.3 (C, COO ), 151.0 (C, C-2’), 148.6 (C, C-4’), 147.5 (C, C-7a), 144.4
(C, C-7), 136.7 (CH, C-1’’), 129.9 (C, C-3a), 129.4 (C, C-1’), 128.1 (CH,
C-6’), 121.7 (CH, C-5), 119.2 (CH, C-5’), 117.8 (CH2, C-2’’), 117.0 (CH,
C-4), 116.0 (CH, C-3’), 111.9 (CH, C-6), 85.0 (CH, C-2), 56.0 (OCH3),
Compound 5s: Colourless oil; 1H NMR (300 MHz, CDCl3, 2 58C, TMS):
3
d=7.33 (d, J
ACHTREUNG
6’), 7.13 (brd, 3J
ACHTREUNG
(d, 3J
(H,H)=9.4 Hz, 1H; H-2), 5.26 (ddd, 3J
G
55.9 (CH, C-3), 39.2(C,
C
G
N
1H; H-1’’), 5.04 (ddd, 3J
G
ACHTREUNG
566
www.chemeurj.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 557 – 568

Synthesis of 2-Aryl-3-vinyl-2,3-dihydrobenzo[b]furans
FULL PAPER
1106, 956, 919, 756, 727 cmÀ1;HR-EIMS: m/z: calcd for C27H32O6:
452.2199 [M]+; found: 452.2196.
6.97 (dd, 3J
H-3’), 6.81 (d, 3J
2.7 Hz, 1H; H-6), 6.68 (d, 3J
(H,H)=16.9, 10.8, 8.6 Hz, 1H; H-1’’), 5.52(d, 3J
2), 5.19 (brd, 3J(H,H)=10.8 Hz, 1H; H-2’’a), 5.18 (brd, 3J
16.9 Hz, 1H; H-2’’b), 4.01 (t, 3J
(H,H)=8.1 Hz, 1H; H-3), 3.78 (s, 3H;
OCH3), 1.36* (s, 9H; COC(CH3)3), 1.31* ppm (s, 9H; COC(CH3)3);
13C NMR (75 MHz, CDCl3, 2 58C, TMS): d=176.5* (C, COC
(CH3)3),
(CH3)3), 154.6 (C, C-5), 153.3 (C, C-7a), 151.0 (C, C-4’),
C
ACHTREUNG
G
ACHTREUNG
AHCTREUNG
cis-5-Methoxy-2-(4-methoxy-2-pivaloyloxyphenyl)-3-vinyl-2,3-dihydro-
benzofuran (5v) and trans-5-methoxy-2-(4-methoxy-2-pivaloyloxyphen-
yl)-3-vinyl-2,3-dihydrobenzofuran (6v): Reaction of 7-methoxybenzoxasi-
lepin (8) (231 mg, 1.05 mmol) with 4-methoxy-2-pivaloyloxybenzaldehyde
(4c) (322 mg, 1.36 mmol) and BF3·Et2O (260 mL, 2.1 mmol) in anhydrous
CH2Cl2 (5.0 mL) followed by workup, as described in the general proce-
dure, yielded 5v/6v 20:80 with an overall yield of 60%.
E
ACHTREUNG
G
ACHTREUNG
AHCTREUNG
N
ACHTREUNG
ACHTREUNG
ACHTREUNG
Compound 5v: Colourless oil; Rf =0.28 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.38 (d, 3J
A
3
3
ACHTREUNG
6.85 (d, J
A
H-6), 6.76* (dd, 3J
2.1 Hz, 1H; H-4), 6.64 (d, J
A
(OCH3), 55.6 (CH, C-3), 39.2(C, CO C
N
N
#
3
(CH3, COC(CH3)3), 27.0 ppm (CH3, COC(CH3)3); * and may be inter-
N
N
(H,H)=2.5 Hz, 1H; H-3’), 5.87 (d, J
3
ACHTREUNG
changed; IR (film): n˜ =2972, 2933, 2874, 1757, 1611, 1486, 1432, 1397,
1367, 1248, 1203, 1144, 1106, 1030, 806 cmÀ1; HR-FABMS: m/z: calcd for
C27H32O6Na: 475.2100 [M+Na]+; found: 475.2100.
8.9 Hz, 1H; H-2), 5.35 (ddd, J
(d, 3J
A
ACHTREUNG
3H; OCH3), 1.39 ppm (s, 9H; C
(CH3)3); 13C NMR (75 MHz, CDCl3,
N
cis-5-Methoxy-2-(5-methoxy-2-pivaloyloxyphenyl)-3-vinyl-2,3-dihydro-
benzofuran (5x): Reaction of 7-methoxybenzoxasilepin (8) (243 mg,
1.1 mmol) with 5-methoxy-2-pivaloyloxybenzaldehyde (4i) (285 mg,
1.21 mmol) and BF3·Et2O (280 mL, 2.2 mmol) in anhydrous CH2Cl2
(5.5 mL) followed by workup, as described in the general procedure,
above yielded 5x (262 mg, 0.68 mmol, 62%). Colourless oil; Rf =0.28
À
258C, TMS): d=176.1 (C, COO ), 159.6 (C, C-4’), 154.4 (C, C-5), 153.3
(C, C-7a), 148.2(C, C-2 ’), 135.7 (CH, C-1’’), 129.7, (C, C-3a), 127.9 (CH,
C-6’), 122.2 (C, C-1’), 116.7 (CH2, C-2’’), 113.8 (CH, C-6), 111.6# (CH, C-
4), 111.5#, (CH, C-5’), 109.6 (CH, C-7), 107.4 (CH, C-3’), 82.6 (CH, C-2),
55.9 (OCH3), 55.4 (OCH3), 51.2(CH, C-3), 39.2(C,
C(CH3)3), 27.2 ppm
A
#
(CH3, C(CH3)3); * and may be interchanged; IR (film): n˜ =2927, 1753,
G
(hexane/Et2O 95:5); 1H NMR (300 MHz, CDCl3, 2 58C, TMS): d=7.06
1486, 1210, 1113, 913, 745 cmÀ1; HR-EIMS: m/z: calcd for C23H26O5:
3
(d, J
G
ACHTREUNG
382.1780 [M]+; found: 382.1785.
(d, 3J
ACHTREUNG
Compound 6v: Colourless oil; Rf =0.27 (hexane/Et2O 95:5); 1H NMR
A
ACHTREUNG
(300 MHz, CDCl3, 2 58C, TMS): d=7.37 (d, 3J
N
3
ACHTREUNG
ACHTREUNG
3
6.81–6.69 (m, 3H; H-4, 6, 5’, 7), 6.59 (d, J
ACHTREUNG
AHCTREUNG
(ddd, 3J
A
A
ACHTREUNG
1H; H-2), 5.18 (d, 3J
C
ACHTREUNG
AHCTREUNG
ACHTREUNG
AHCTREUNG
157.0 (C, C-5’), 154.5 (C, C-5), 153.2(C, C-7a), 141.0 (C, C-2 ’), 135.5
(CH, C-1’’), 131.2(C, C-1 ’), 129.7 (C, C-3a), 122.6 (CH, C-3’), 116.7
(CH2, C-2’’), 114.0 (CH, C-6), 113.6 (CH, C-4’), 112.1 (CH, C-6’), 111.6
(CH, C-4), 109.7 (CH, C-7), 82.8 (CH, C-2), 56.0* (CH3, OCH3), 55.5*
À
(75 MHz, CDCl3, 2 58C, TMS): d=176.62(C, COO ), 160.1 (C, C-4’),
154.4* (C, C-7a), 153.3* (C, C-5), 149.4 (C, C-2’), 136.7 (CH, C-1’’), 130.0,
(C, C-3a), 128.4 (CH, C-6’), 124.1 (C, C-1’), 117.8 (CH2, C-2’’), 113.8#
(CH, C-4), 110.8# (C, C-6), 109.6#, (CH, C-7), 112.1 (CH, C-5’), 108.1
(CH3, OCH3), 51.1 (CH, C-3), 39.1 (C, OCOC
A
(CH, C-3’), 85.1 (CH, C-2), 56.0 (CH, C-3), 55.5 (OCH3), 55.4 (OCH3),
OCOC(CH3)3); * may be interchanged; IR (film): n˜ =3078, 2972, 2936,
AHCTREUNG
#
39.1 (C, C
(CH3)3), 27.1 ppm (CH3, C
(CH3)3); * and may be inter-
2910, 2834, 1750, 1607, 1487, 1429, 1364, 1273, 1210, 1118, 1034, 982,
800 cmÀ1; HR-FABMS: m/z: calcd for C23H26O5Na: 405.1678 [M+Na]+;
found: 405.1676.
changed; IR (film): n˜ =2967, 1753, 1618, 1508, 1486, 1273, 1210, 1112,
1033 cmÀ1; HR-EIMS: m/z: calcd for C23H26O5: 382.1780 [M]+; found:
382.1776.
(Z)-2-(3-Fluorodimethylsilyl-1-propenyl)phenol (9): To a solution of 2,2-
cis-2-(2,4-Dipivaloyloxyphenyl)-5-methoxy-3-vinyl-2,3-dihydrobenzofuran
(5w) and trans-2-(2,4-dipivaloyloxyphenyl)-5-methoxy-3-vinyl-2,3-dihy-
drobenzofuran (6w): Reaction of 7-methoxybenzoxasilepin (8) (168 mg,
0.76 mmol) with 2,4-dipivaloyloxybenzaldehyde (4h) (280 mg, 0.92 mmol)
and BF3·Et2O (190 mL, 1.52mmol) in anhydrous CH 2Cl2 (3.5 mL) fol-
lowed by workup, as described in the general procedure, yielded 5w/6w
91:9 with an overall yield of 60%.
dimethyl-2,3-dihydrobenzo[f][1,2]oxasilepin (3) (50 mg, 0.26 mmol) in
N
methanol (0.5 mL) was added BF3·Et2O (66 mL, 0.52mmol). The mixture
was stirred at room temperature for 5 min and then diluted with CH2Cl2
(10 mL) and washed with brine (5 mL). The organic phase was dried
over MgSO4 and evaporated under vacuo to give compound 9 (43 mg,
1
0.24 mmol, 90% yield). Colourless oil; H NMR (300 MHz, CDCl3, 2 58C,
TMS): d=7.20 (dd, 3J
C
ACHTREUNG
Compound 5w: Colourless oil; Rf =0.28 (hexane/Et2O 95:5); 1H NMR
(300 MHz, CDCl3, 2 58C, TMS): d=7.50 (d, 3J
ACHTREUNG
6.97 (dd, 3J
ACHTREUNG
A
3J
A
ACHTREUNG
A
19F NMR (289 MHz, CDCl3, 2 58C): À161.74 ppm (hept t, 3J
A
ACHTREUNG
3J(F,H)=5.8 Hz, 1F; SiMe2F); 13C NMR (75 MHz, CDCl3, 2 58C, TMS):
A
3
3
ACHTREUNG
(ddd, J
1H; H-2’’b), 4.93 (brd, J
8.9, 8.5 Hz, 1H; H-3), 3.78 (s, 3H; OCH3), 1.39 (s, 9H; C
N
3
3
ACHTREUNG
d=152.4 (C, C-1), 130.5* (CH, C-3), 129.6* (CH, C-5), 128.6 (CH, C-2’),
123.3 (C, C-2), 122.8 (CH, C-1’), 120.3 (CH, C-4), 115.1 (CH, C-6), 20.0
(H,H)=9.8 Hz, 1H; H-2’’a), 4.22 (dd, J
AHCTREUNG
2
2
ACHTREUNG
(CH2, d, J
A
(C,F)=14.9 Hz, C-
1.36 ppm (s, 9H; C
ACHTREUNG
À
À
4’); * may be interchanged; IR (film): n˜ =3567, 3528, 3423, 2958, 2926,
2870, 1601, 1579, 1484, 1449, 1381, 1337, 1256, 1212, 1158, 1090, 874, 843,
798 cmÀ1; HR-EIMS: m/z: calcd for C11H15OsiF: 210.0876 [M+Na]+;
found: 210.0873; mixture 3/II: 1H NMR (300 MHz, CDCl3, 2 58C, TMS);
common signals: d=7.19 (m, 2H), 7.13 (m, 2H), 6.95 (m, 4H), 6.39 ppm
176.5 (C, COO ), 175.8 (C, COO ), 154.5 (C, C-5), 153.2(C, C-7a),
150.6 (C, C-4’), 147.6 (C, C-2’), 135.4 (CH, C-1’’), 129.6, (C, C-3a), 127.6
(CH, C-6’), 127.5 (C, C-1’), 118.6 (CH, C-5’), 117.0 (CH2, C-2’’), 115.4
(CH, C-3’), 114.0 (CH, C-6), 111.6 (CH, C-4), 109.7 (CH, C-7), 82.1 (CH,
C-2), 56.0 (OCH3), 51.2(CH, C-3), 39.2( C, C
N
G
(d, 2H, J=11.4 Hz; H-1’); signals due to 3: d=6.10 (dt, 3J
3J(H,H)=7.5 Hz, 1H; H-4), 1.58 (d, 3J
(H,H)=7.4 Hz, 2H; H-3),
(H,H)=11.1,
(H,F)=5.8 Hz, 2H; H-3’);
G
27.1 (CH3, C
A
ACHTREUNG
G
ACHTREUNG
C27H32O6: 452.2199 [M]+; found: 452.2197.
0.33 ppm (SiMe2; 6H); signals due to II: d=6.02(dt, 3J
8.7 Hz, 1H; H-2’), 1.78 (dd, 3J
(H,H)=7.8, 3J
ACHTREUNG
Compound 6w: Colourless oil; Rf =0.27 (hexane/Et2O 95:5); 1H NMR
3
(300 MHz, CDCl3, 2 58C, TMS): d=7.47 (d, 3J
U
0.23 ppm (d, J
A
Chem. Eur. J. 2007, 13, 557 – 568
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
567

I. Rodríguez-García et al.
General procedure used to obtained data in Table 4: BF3·Et2O
(0.32mmol) was added to solution of benzoxasilepin (30 mg,
[15] M. Plotkin, S. Y. Chen, P. G. Spoors, Tetrahedron Lett. 2000, 41,
2269–2273.
a
0.16 mmol) and the aldehyde (0.16 mmol) in CDCl3 (0.5 mL) in a NMR
tube. The mixture was heated to 408C in an oil bath and, after 6 h, the
1H NMR spectrum was acquired. The ratio cis/trans was determined by
measuring the area of H-2and H-3 signals for both diasteromers.
[16] G. Boisvert, R. Giasson, Tetrahedron Lett. 1992, 33, 6587–6590.
[17] D. L. J. Clive, N. Etkin, T. Joseph, J. W. Lown, J. Org. Chem. 1993,
58, 2442–2445.
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6270–6274; Angew. Chem. Int. Ed. 2004, 43, 6144–6148.
[22] H. M. L. Davies, M. V. A. Grazini, E. Aouad, Org. Lett. 2001, 3,
1475–1477.
General procedure used to obtained data of Figure 4: BF3·Et2O
(1.6 mmol) was added to a solution of benzoxasilepin (30 mg, 0.16 mmol)
in [D7]DMF (0.5 mL) in a NMR tube. The mixture was heated to the fol-
lowing temperatures and the 1H NMR spectrum was acquired. The ratio
II/3 (K) was determined by measuring the area of Si(CH3)3 signals for
A
each compound. K (T [K]): 10.0 (258), 5.5 (278), 3.5 (298), 2.6 (308), 2.3
(323), 1.6 (343).
[23] H. Saito, H. Oishi, S. Kitagaki, S. Nakamura, M. Anada, S. Hashi-
moto, Org. Lett. 2002, 4, 3887–3890.
X-ray crystallography of 5l and6b
[24] L. Jimenez-Gonzalez, M. Alvarez-Corral, M. MuÇoz-Dorado, I. Ro-
driguez-Garcia, Chem. Commun. 2005, 2689–2691.
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[26] C. Meyer, J. Cossy, Tetrahedron Lett. 1997, 38, 7861–7864.
[27] S. Garcia-MuÇoz, L. Jimenez-Gonzalez, M. Alvarez-Corral, M.
MuÇoz-Dorado, I. Rodriguez-Garcia, Synlett 2005, 3011–3013.
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Chem. 2004, 69, 6874–6882.
Crystal data for 5l: C21H16O3, M=316.34, orthorhombic, space group
Pbca, a=7.3789(6), b=19.3297(16), c=25.302(2) Š, a=90, b=90, g=
908; V=3608.8(5) Š3, T=273 K, Z=8, m(MoKa)=0.077 mmÀ1. A total of
G
16407 reflections were collected, 2579 unique reflections (Rint =0.0430)
which were used in all calculations. The final wR (F2) was 0.2107 (all
data).
Crystal data for 6b: C17H16O2, M=252.30, orthorhombic, space group
P2(1)2(1)2(1), a=6.0770(14), b=7.5441(16), c=29.082(7) Š, a=90, b=
[29] T. Akindele, S. P. Marsden, J. G. Cumming, Tetrahedron Lett. 2005,
46, 7235–7238.
90, g=908, V=1333.3(5) Š3, T=273 K, Z=4, m(MoKa)=0.081 mmÀ1. A
E
total of 6113 reflections were collected, 1909 unique reflections (Rint
=
[30] T. Akindele, S. P. Marsden, J. G. Cumming, Org. Lett. 2005, 7, 3685–
3688.
0.0838) which were used in all calculations. The final wR (F2) was 0.1397
(all data).
[31] S. Bogen, L. Fensterbank, M. Malacria, C. R. Acad. Sci. Ser. IIc
2001, 4, 423–426.
[32] T. Fujiwara, K. Yanai, K. Shimane, M. Takamori, T. Takeda, Eur. J.
Org. Chem. 2001, 155–161.
[33] P. Chiu, X. M. Zhang, R. Y. Y. Ko, Tetrahedron Lett. 2004, 45, 1531–
1534.
CCDC-613456 and -613457 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[34] S. B. Chang, R. H. Grubbs, Tetrahedron Lett. 1997, 38, 4757–4760.
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Acknowledgements
We wish to acknowledge the Spanish Ministerio de Educación y Ciencia
for financial support (Projects PB98-1006 and BQU2002-03254) and for
scholarships to L. JimØnez-Gonzµlez and S. García MuÇoz.
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[40] Selected bond lengths [Š] and bond angles [8]: O1 C9 1.462(3),
À
À
À
À
C7 C9 1.561(4), C7 C121.490(4), C12 C19 1.293(5), C4 C9
À
À
1.513(4), C7 H7 0.980, C9 H9 0.980; C6-O1-C9 107.4(2), C7-C12-
C19 124.1(3), O1-C9-C7 105.6(2), O1-C9-C4 109.8(2), C5-C7-C9
101.2(2), H7-C7-C9-H9 À22.2.
À
[41] Selected bond lengths [Š] and bond angles [8]: O1 C8 1.463(5),
À
À
À
À
C7 C8 1.557(5), C8 C9 1.502(5), C7 C15 1.485(6), C15 C16
À
À
À
À
1.312(6), C8 H8 0.980, C7 H7 0.980, C6 C7 1.532(6), C1 O1
1.375(5); C23-C22-H22 106.3, C1-O1-C8 106.4(3), C7-C15-C16
121.6(4), O1-C8-C9 109.0(3), C6-C7-C8 98.4(3), C15-C7-C6 114.5(4),
C1-C6-C2108.9(4), H7-C7-C8-H8 157.4.
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Received: July 17, 2006
Published online: September 29, 2006
568
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