A highly regioselective, protecting group controlled, synthesis of bicyclic compounds via Pd-catalysed intramolecular cyclisations

Article abstract of DOI:10.1016/j.tetlet.2013.02.068

Intramolecular Pd-catalysed cyclisation reactions for the preparation of bicyclic compounds have been studied as a model system towards the synthesis of corialstonine and corialstonidine. Significant differences in reactivity have been observed for the cyclic allyl alcohols possessing O-protected and free OH functionalities. Cyclisation via the intramolecular Heck reaction, for both derivatives, proved to be highly regioselective and while the O-protected compound favoured the exo mode of cyclisation, the unprotected alcohol preferred the endo cyclisation pathway. Brief computational studies were carried out in order to obtain further insight into these processes.

Full text of DOI:10.1016/j.tetlet.2013.02.068

Tetrahedron Letters 54 (2013) 2243–2246
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Tetrahedron Letters
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A highly regioselective, protecting group controlled, synthesis of bicyclic
compounds via Pd-catalysed intramolecular cyclisations
Gordana Tasic a, Jelena Randjelovic a, Nikola Vusurovic a, Veselin Maslak b, Suren Husinec c,
Vladimir Savic a,
⇑
a University of Belgrade, Faculty of Pharmacy, Department of Organic Chemistry, Vojvode Stepe 450, 11000 Belgrade, Serbia
b University of Belgrade, Faculty of Chemistry, Department of Organic Chemistry, Studentski trg 16, 11000 Belgrade, Serbia
c Institute for Chemistry, Technology and Metallurgy, Centre for Chemistry, PO Box 815, Njegoseva 12, 11000 Belgrade, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history:
Intramolecular Pd-catalysed cyclisation reactions for the preparation of bicyclic compounds have been
studied as a model system towards the synthesis of corialstonine and corialstonidine. Significant differ-
ences in reactivity have been observed for the cyclic allyl alcohols possessing O-protected and free OH
functionalities. Cyclisation via the intramolecular Heck reaction, for both derivatives, proved to be highly
regioselective and while the O-protected compound favoured the exo mode of cyclisation, the unpro-
tected alcohol preferred the endo cyclisation pathway. Brief computational studies were carried out in
order to obtain further insight into these processes.
Received 30 December 2012
Revised 31 January 2013
Accepted 20 February 2013
Available online 27 February 2013
Keywords:
Bicyclic compounds
Heck reaction
Regioselectivity
Corialstonine
Ó 2013 Elsevier Ltd. All rights reserved.
Corialstonidine
Corialstonine (1) and corialstonidine (2) are quinine-like alka-
loids isolated from Alstonia coriacea, and are shown to have moder-
ate antimalarial activity against Plasmodium falciparum.1 Although
the stereochemistry of these natural products has not been fully
established, their structural properties and biological profiles make
them attractive synthetic targets. Our interest in developing a syn-
thetic route for these alkaloids led us retrosynthetically to struc-
ture 3 which was expected to be accessible via intramolecular
Heck reaction of the cyclopentene derived allyl alcohol 4.
H
H
HN
H
N
COOMe
COOMe
H
HO
O
MeO
MeO
MeO
MeO
N
N
2
1
The success of this approach would rely on the potential to con-
trol the regioselectivity of the cyclisation step.2 Only the C–C bond
formation involving C(3) of cyclopentene derivative 4 (the endo
mode of cyclisation related to the allyl alcohol functionality), upon
b-elimination of palladium hydride and tautomerisation, would
lead to the desired compound. An alternative pathway, via cyclisa-
tion onto C(2), would furnish an unwanted regioisomeric product
(the exo mode of cyclisation related to the allyl alcohol functional-
ity). The Heck reaction of allylic alcohols has been widely explored,
but mainly as an intermolecular variant.3 The reaction of these
unsaturated alcohols usually affords products of C(3)-substitu-
tion.3a However, if the reaction conditions favour the ionic path-
way, the regioselectivity may be reversed to yield the product of
C(3)-substitution.3b Intramolecular processes leading to polycyclic
R
N
HN
I
HO
5
4
3
1
2
O
4
3
Scheme 1.
derivatives may afford products of both exo and endo cyclisations,
which are often controlled by the ring size.4 Some transformations
employing cyclic allyl alcohols, for the synthesis of bicyclic com-
pounds, yielded a mixture of products with the exo cyclisation
pathway as the major one.5 The variability of the results related
to the regioselectivity issue, and lack of information associated
with the synthesis of bicyclic compounds via the general approach
outlined in Scheme 1 prompted the study presented herein.
⇑
Corresponding author. Tel.: +381 113951243; fax: +381 113972840.
E-mail address: vladimir.savic@pharmacy.bg.ac.rs (V. Savic).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.02.068

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G. Tasic et al. / Tetrahedron Letters 54 (2013) 2243–2246
The readily accessible amide 5 was selected as a model com-
TBDMSO
Pd(OAc)2
O
pound to study the cyclisation leading to bicyclic compounds.
Although the results employing 5 would not be directly transfer-
able to the conversion 4?3 due to the formation of different rings
(azabicyclo[4.2.1]nonane from 5 vs azabicyclo[3.2.1]octane from
4), the study was however expected to provide insight into the ele-
ments controlling the regioselectivity. Compound 5 was prepared
starting from crotonaldehyde using adapted literature procedures
(Scheme 2).6 The stereochemistry of the product was established
following the preparation of compound 10, and was elucidated
by analysis of the H–H NOESY data which suggested that 10 was
obtained as a 4.3:1 mixture of cis and trans isomers. Establishing
the exact ratio of the diastereomers for compound 5 was problem-
atic due to 1H NMR complexity caused by restricted rotation
around the amide bond, in addition to the existence of cis/trans iso-
mers. Methylation of compound 10 to produce 5 was necessary
since the attempts to cyclise 10 under various conditions used
for Pd-catalysed processes failed.
Me
N
Me
N
TBDMS
O
I
PPh3
O
base
toluene
110 oC
15 h
5
11
Scheme 3.
Table 1
Variation of the base in the cyclisation reaction of 5a
Entry
Base
Product
Yieldb (%)
a
b
c
Cs2CO3
11
11
11
—
60
51
73
Proton spongec
Et3N
d
d
K2CO3
—
The initial cyclisation experiment, carried out on O-protected
derivative 5, employed Pd(OAc)2/PPh3 as the catalytic system and
Cs2CO3 as the base in refluxing toluene (Scheme 3; Table 1, entry
a).7
a
Reaction conditions: 5 (0.066 mmol), Pd(OAc)2 (10 mol %), PPh3 (20 mol %) base
(0.132 mmol), toluene (6.5 mL), 110 °C, 15 h.
b
Isolated yield.
c
1,8-Bis(dimethylamino)naphthalene.
d
Complex mixture as suggested by analysis of the 1H NMR spectrum.
Product 11 was isolated as a single regioisomer in 60% yield and
was formed via the exo cyclisation mode, involving C(2). Inspection
of the 1H NMR spectrum of the crude reaction mixture did not
show the presence of the regioisomeric product which would have
arisen from the cyclisation reaction involving C(3). We briefly
screened other bases and their effect on the yields (Table 1). In
all cases, the only product isolated was the bicyclic compound
11, while the use of Et3N (Table 1, entry c) proved to be the most
efficient furnishing the product in 73% yield. On the other hand,
K2CO3 afforded a complex mixture which included only a small
quantity of 11, as judged by analysis of the 1H NMR spectrum of
the crude product.
Further studies of these processes were carried out on OH-
derivative 12, which was easily accessible via desilylation of 5 un-
der standard conditions.8 Similarly, as for compound 5, unambigu-
ous determination of the cis/trans ratio of 12 proved difficult due to
the complexity of the 1H NMR spectrum. The initial cyclisation
reaction of compound 12 was carried out using similar conditions
with Et3N as the base (Scheme 4; Table 2, entry a). The reaction
was again highly regioselective furnishing two products via endo
cyclisation, compounds 13 and 14, in 75% yield and in the ratio
1.3:1.9 Interestingly, the observed regioselectivity contrasted with
that represented by the transformation 5?11. Product 13 was
formed via cyclisation at C(3) followed by palladium hydride elim-
ination to generate the enol and then the final product via
tautomerisation.
Analysis of mass spectral data for the by-product 14 suggested
loss of oxygen (m/z: observed M+ 199). Further examination, spe-
cifically of the H–H COSY and H–H NOESY spectra, supported the
proposed structure. Particularly notable was the absence of any
correlation between NCH and ArCH and correlation of both of these
with the CH2 and different olefinic protons. This product was likely
formed via initial addition of ArPdI onto the double bond followed
by b-elimination of the [Pd]OH species to afford 14.4d,10 Since com-
TBDMSCl
OTBDMS
NaCN
OH
CN
CHO
imidazole
AcOH
MeOH
0 oC to r.t.
CN
7
DMF
6
0 oC to r.t.
a) allylMgBr
68% (over two steps)
Et2O
b) NaBH4
MeOH
- 70 oC to r.t
O
H
TBDMSO
N
COCl
I
I
50%
OTBDMS
Et3N
NH2
9
CH2Cl2
8
0 oC to r.t.
83%
toluene
60 oC
98%
Grubbs II (3%)
O
Me
N
O
H
TBDMS
O
I
MeI
TBDMS
O
N
I
NaH
THF
0 oC to r.t.
87%
10
4.3:1 cis:trans
5
Scheme 2.

G. Tasic et al. / Tetrahedron Letters 54 (2013) 2243–2246
2245
O
Me
N
TBDMSO
I
5
Me
N
OTBDMS
O
TBAF
THF
Me
N
O
91%
r.t.
O
1h
Pd(OAc)2
PPh3
O
13
Me
N
+
HO
I
Me
N
11
base
toluene
110 °C
15h
TS1
O
12
14
Scheme 4.
Me
N
TBDMSO
O
O
O
Table 2
Variation of the base in the cyclisation reaction of 12a
Entry
Base
Product
Yieldb (%)
17
a
b
c
Et3N
13/14
13
—
75 (1.3:1)
38
TS2
Cs2CO3
Proton spongec
—
d
a
Reaction conditions: 12 (0.303 mmol), Pd(OAc)2 (10 mol %), PPh3 (20 mol %),
base (0.606 mmol), toluene (30 mL), 110 °C, 15 h.
b
Isolated yield.
c
1,8-Bis(dimethylamino)naphthalene.
d
Complex mixture as suggested by analysis of the 1H NMR spectrum.
Me
N
O
pound 12 was used as a mixture of cis and trans isomers it is likely
that the by-product 14 was produced via the reaction involving cis-
12 (Scheme 5). Upon the syn addition of Ar[Pd]I to form the C(3)-Ar
bond, intermediate 15 was generated which then furnished 14 via
elimination of the syn positioned [Pd] and OH groups.4d,10 The
same transformation for trans-12 produced the target compound
13 via intermediate 16, in which [Pd] and H were syn positioned
(Scheme 5). Although the mechanistic explanation seems reason-
able, the ratio of 13/14 does not reflect the cis/trans ratio of 10. This
may suggest that the reaction conditions used for the sequence
10?5?12 altered the cis/trans ratio or that other processes inter-
fere with the expected syn [Pd]X (X = OH or H) elimination, and
this remains to be investigated further.11 The cyclisation reaction
was also carried out with two other bases (Table 2, entries b and
c). When Cs2CO3 was used under the described conditions the ex-
pected product 13 was obtained in 38% yield, while only traces of
13
TS3
OH
Me
N
18
TS4
Figure 1. Transition states of the cyclisation reactions of 5 and 12.
OH
H
OH
H
Me
N
[Pd]
NMe
14 were observed. The use of proton sponge afforded a mixture of
products in very low yields and with only partial conversion.
To gain a better insight into the origin of the highly regioselec-
tive cyclisation with O-protected 5 and the free alcohol 12, we
studied transition states leading to products 11 and 13 and their
regioisomeric equivalents 17 and 18 by computational methods.
All initial calculations were performed on derivatives cis-5 and
cis-12. The transition state searches were carried out using DFT
with the B3LYP hybrid functional and def2-SVP basis set for all
atoms, in vacuum, and the single point energies at the located
transition states were recalculated using the same functional and
def2-TZVP basis set for all atoms in vacuum.12 The transition states
were characterised by frequency analysis.13 Although the aryl io-
dide initiated the oxidative addition in the studied reactions, the
NMe
O
H
H
O
O
I
- [Pd]OH
14
15
OH
12-cis
OH
H
OH
H
H
Me
N
[Pd]
NMe
O
NMe
O
O
H
H
13
I
- [Pd]H
16
Scheme 5.
12-trans

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G. Tasic et al. / Tetrahedron Letters 54 (2013) 2243–2246
2. Various concepts of regioselective transformations have been reviewed, see: (a)
Mahatthananchai, J.; Dumas, A. M.; Bode, J. W. Angew. Chem., Int. Ed. 2012, 51,
10954–10990; (b) Rousseau, G.; Breit, B. Angew. Chem., Int. Ed. 2011, 50, 2450–
2494.
calculations were performed on transition states having acetate as
the Pd-ligand rather than iodide, since Pd(OAc)2 was used as a
source of Pd(0).14 Compound cis-5 afforded product 11 via transi-
tion state TS1 (Fig. 1) positioning the phenyl substituent syn to
the OTBDMS moiety and leading to the exo cyclisation mode. Alter-
natively, the endo cyclisation is favoured by TS2 leading to the
unobserved compound 17. The calculated energy difference be-
tween TS1 and TS2 is 3.4 kcal/mol in favour of TS1, corroborating
the observed results. Inspection of the molecular models did not
reveal any obvious steric effects accountable for the high level of
the experimentally observed regioselectivity. It is possible that
the phenyl moiety, due to its planarity, is less involved in steric
interactions with the large TBDMS group (TS1) than the acetate
3. For reviews, see: (a) Muzart, J. Tetrahedron 2005, 61, 4179–4212; (b) Ruan, J.;
Xiao, J. Acc. Chem. Res. 2011, 44, 614–626; For some recent examples, see: (c)
Brandt, D.; Bellosta, V.; Cossy, J.; Panther, J. Org. Lett. 2012, 14, 5594–5597; (d)
Rohrich, A.; Müller, T. J. J. ARKIVOC 2012, iii, 297–311; (e) Colbon, P.; Ruan, J.;
Purdie, M.; Mulholland, K.; Xiao, J. Org. Lett. 2011, 13, 5456–5459; (f) Stone, M.
T. Org. Lett. 2011, 13, 2326–2329; (g) Schmidt, B.; Holter, F.; Kelling, A.; Schilde,
U. J. Org. Chem. 2011, 76, 3357–3365; (h) Liu, J.; Zhu, J.; Jiang, H.; Wang, W.; Li, J.
Chem. Commun. 2010, 46, 415–417; (i) Alacid, E.; Najera, C. Adv. Synth. Catal.
2007, 349, 2572–2584; (j) Calo, V.; Nacci, A.; Monopoli, A.; Ferola, V. J. Org.
Chem. 2007, 72, 2596–2601; (k) Mo, J.; Xu, L.; Ruan, J.; Liu, S.; Xiao, J. Chem.
Commun. 2006, 3591–3593.
4. (a) Gaudin, J.-M. Tetrahedron Lett. 1991, 32, 6113–6116; (b) Shi, L.; Narula, C. K.;
Mak, K. T.; Kao, L.; Xu, Y.; Heck, R. F. J. Org. Chem. 1983, 48, 3894–3900; (c)
Kondo, K.; Sodeoka, M.; Mori, M.; Shibasaki, M. Tetrahedron Lett. 1993, 34,
4219–4222; (d) Kondo, K.; Sodeoka, M.; Mori, M.; Shibasaki, M. Synthesis 1993,
920–930.
(TS2) possessing an sp3 carbon. Additionally, the
p–p interactions
of the phenyls from the phosphine ligand and the aryl group in-
volved in C–C bond formation may provide support in directing
the reacting phenyl to minimise steric clashes in TS1. Compound
cis-12 showed completely opposite results. It favoured the endo
mode of cyclisation furnishing exclusively product 13 via transi-
tion state TS3, while the regioisomeric product 18, obtained via
TS4, was not detected. The energy difference between TS3 leading
to 13 and TS4 furnishing 18, 13.6 kcal/mol, was higher than that
calculated for TS1/TS2. This was somewhat surprising since the re-
moval of TBDMS was expected to reduce steric interactions. The
observed results were attributed to the additional stabilisation of
TS3 due to potential H-bonding interactions between the syn posi-
tioned acetate and OH functionalities.
Computational studies were also carried out on trans-5 and
trans-12 as well, and the results showed the same trend as de-
scribed for related cis-5 and cis-12 (these results will be discussed
in a full account of this work).
In conclusion, our brief study of the intramolecular Heck reac-
tion on cyclic allyl alcohols has revealed the ability of the proximal
OH-group to influence the regioselectivity of the cyclisation. Fur-
ther study of these processes and their application in the synthesis
of corialstonine and corialstonidine is currently underway.
5. Kelly, S. A.; Foricher, Y.; Mann, J.; Bentley, J. M. Org. Biomol. Chem. 2003, 1,
2865–2876.
6. van den Nieuwendijk, A. M. C. H.; Ghisaidoobe, A. B. T.; Overkleeft, H. S.;
Brussee, J.; van der Gen, A. Tetrahedron 2004, 60, 10385–10396.
7. General procedure for the preparation of compound 11:
A
mixture of compound
5
(30 mg, 0.066 mmol), Pd(OAc)2 (1.5 mg,
0.0066 mmol), PPh3 (3.5 mg, 0.0131 mmol), base (2 equiv, see Table 1) in
toluene (6.5 mL) was refluxed under a nitrogen atmosphere for 15 h. The
solvent was evaporated under reduced pressure and the residue was dissolved
in CH2Cl2 (20 mL), washed with H2O (2 Â 5 mL), dried (Na2SO4) and filtered.
The solvent was then evaporated under reduced pressure and the residue
purified by flash chromatography (SiO2, 8:2 v/v petroleum ether/Et2O) to afford
the product 11 as a light yellow oil (see Table 1 for yields).
IR mmax 2927, 2856, 1615, 1390, 776, 740. 1H NMR (500 MHz, CDCl3): d À0.12,
0.03 (2s, 6H, SitBuMe2), 0.63 (s, 9H, SitBuMe2), 3.25 (s, 3H, N-CH3), 3.61 (dd, 1H,
J = 6.5 Hz, J = 3.5 Hz, CH-C6H4), 3.97 (dd, 1H, J = 6.5 Hz, J = 3 Hz, NCH-
CHOTBDMS), 4.55 (t, 1H, J = 6.5 Hz, CHOTBDMS), 5.83 (dd, 1H, J = 6 Hz,
J = 3 Hz, CH(C6H4)CH@CH), 6.06 (dd, 1H, J = 6 Hz, J = 3.5 Hz, CH(C6H4)CH@CH),
7.10 (dd, 1H, J = 7.0 Hz, J = 1.5 Hz, ArH), 7.26–7.34 (m, 2H, ArH), 8.57 (dd, 1H,
J = 8.0 Hz, J = 1.5 Hz, ArH). 13C NMR (125 MHz, CDCl3): d À5.0, À4.96, 17.6, 25.3,
40.0, 56.0, 66.5, 69.6, 126.5, 127.4, 129.21, 130.8, 133.9, 134.7,138.3, 139.5,
167.8. m/z (EI): 329.1 (M+), 314.1, 272.1, 215.0, 198.1, 156. HRMS (ESI): calcd
for C19H28NO2Si (M+H)+ 330.18838, found 330.18711
8. Corey, E. J.; Venkateswarlu, A. J. Am. Chem. Soc. 1972, 94, 6190–6191.
9. General procedure for the preparation of compounds 13/14: Cyclisation 12?13/
14 was carried out according to the general procedure described for compound
11 (see Table 2 for yields).7
Compound 13
IR mmax 1745, 1615, 1387, 1264, 1241, 703. 1H NMR (500 MHz, CDCl3): d 2.34
(dd, 1H, J = 14.5 Hz, J = 3 Hz, CH2CH-N), 2.52 (dd, 1H, J = 19 Hz, J = 3 Hz,
CH2C@O), 2.58–2.64 (m, 1H, CH2CH-N), 2.78 (dd, 1H, J = 19 Hz, J = 8.5 Hz,
CH2C@O), 3.36 (s, 3H, N-CH3), 3.76 (t, 1H, J = 7.5 Hz, CH-C6H4), 3.81 (d, 1H,
J = 8 Hz, CH-N), 7.22 (dd, 1H, J = 7.5 Hz, J = 1.5 Hz, ArH), 7.24–7.42 (m, 2H, ArH),
8.57 (dd, 1H, J = 8.0 Hz, J = 1.5 Hz, ArH). 13C NMR (125 MHz, CDCl3): d 33.8, 40.2,
42.4, 48.2, 63.3, 127.1, 129.0, 130.5, 131.7, 135.0, 143.9, 165.8, 212.0. m/z (EI):
215.1 (M+), 197.1, 172.1, 158.1, 144.1, 131.1. HRMS (ESI): calcd for C13H14NO2
(M+H)+ 216.10191, found 216.10168.
Acknowledgments
Financial support from the Serbian Ministry of Education, Sci-
ence and Technological Development (Grant no. 172009) is greatly
appreciated. We thank the Faculties of Pharmacy and Chemistry,
Belgrade University for their assistance. The work incorporates re-
sults produced by the High-Performance Computing Infrastructure
for South East Europe’s Research Communities (HP-SEE), a project
co-funded by the European Commission (under Contract Number
261499) through the Seventh Framework Programme HP-SEE
(http://www.hp-see.eu/). We would also like to thank Dr A. E. A.
Porter for fruitful discussions. J.R. would like to thank the Serbian
Ministry of Education, Science and Technological Development
for a PhD scholarship.
Compound 14
IR mmax 1610, 1593,1389, 1251, 761, 739. 1H NMR (500 MHz, CDCl3): d 2.1 (d,
1H, J = 12.5 Hz, CH2), 2.46 (dt, 1H, J = 12.5 Hz, CH2), 3.27 (s, 3H, N-CH3), 3.75
(dd, 1H, J = 6.5 Hz, J = 3 Hz, CH-C6H4), 4.31 (dd, 1H, J = 7.5 Hz, J = 2.5 Hz, CH-N),
5.88 (dd, 1H, J = 5.5 Hz, J = 2.5 Hz, @CH–CH–N), 6.05 (dd, 1H, J = 5.5 Hz, J = 3 Hz,
CH@CH–CH–N), 7.22 (dd, 1H, J = 7.5 Hz, J = 1.5 Hz, ArH), 7.26–7.30 (m,1H, ArH),
7.32–7.35 (m, 1H, ArH), 8.6 (dd, 1H, J = 8.0 Hz, J = 1.5 Hz, ArH). 13C NMR
(125 MHz, CDCl3): d 37.1, 39.0, 51.4, 65.3, 126.5, 127.9, 128.11, 131.02, 132.0,
135.6, 138.13, 144.44, 166.4. m/z (EI): 199.1 (M+), 184.0, 170.0, 141.0, 128.1,
115.0. HRMS (ESI): calcd for C13H14NO (M+H)+ 200.10699, found 200.10611.
10. Zhao, H.; Ariafard, A.; Lin, Z. Organometallics 2006, 25, 812–819.
11. For some examples of anti-hydride elimination in Pd-catalysed reactions, see:
(a) Lautens, M.; Fang, Y.-Q. Org. Lett. 2003, 5, 3679–3682; (b) Maeda, K.;
Farrington, E. J.; Galardon, E.; John, B. D.; Brown, J. M. Adv. Synth. Catal. 2002,
344, 104–109.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
02.068.
12. (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652; (b) Lee, C.; Yang, W.; Parr, R.
G. Phys. Rev. B 1988, 37, 785–789; (c) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J.
Phys. 1980, 58, 1200–1211; (d) Weigend, F.; Alhrichs, R. Phys. Chem. Chem. Phys.
2005, 7, 3297–3305.
13. McIver, J. W.; Komornicki, A. K. J. Am. Chem. Soc. 1972, 94, 2625–2633.
14. (a) Amatore, C.; Came, E.; Jutand, A.; M’Barki, M. A.; Meyer, G. Organometallics
1995, 14, 5605–5614; (b) Jutand, A. In The Mizoroki–Heck Reaction; Oestreich,
M., Ed.; Wiley: Chichester, 2009; pp 1–50.
References and notes
1. (a) Cherif, A.; Massiot, G.; Le Men-Olivier, L. Heterocycles 1987, 26, 3055–3058;
(b) Wright, C. W.; Alle, D.; Phillipson, J. D.; Kirby, G. C.; Warhurst, D. C.; Massiot,
G.; Le Men-Olivier, L. J. Ethnopharmacol. 1993, 40, 41–45.