(Z)-tamoxifen and tetrasubstituted alkenes and dienes via a regio- and stereospecific three-component magnesium carbometalation palladium(O) cross-coupling strategy

Article abstract of DOI:10.1021/ol0347794

(Matrix presented) The synthesis of various tetrasubstituted alkenes and dienes in a regio- and stereocontrolled manner is described. This three-component coupling strategy involves the addition of Grignard reagents to propargyl alcohols followed by palladium(O)-mediated cross-coupling with aryl or vinyl halides. This protocol has been applied to the synthesis of (Z)-Tamoxifen and related mimics.

Full text of DOI:10.1021/ol0347794

ORGANIC
LETTERS
2003
Vol. 5, No. 17
2989-2992
(Z)-Tamoxifen and Tetrasubstituted
Alkenes and Dienes via a Regio- and
Stereospecific Three-Component
Magnesium Carbometalation
Palladium(0) Cross-Coupling Strategy
Pierre E. Tessier, Andrea J. Penwell,† Fabio E. S. Souza, and Alex G. Fallis*
Centre for Research in Biopharmaceuticals, Department of Chemistry,
UniVersity of Ottawa, 10 Marie Curie, Ottawa, Ontario, Canada K1N 6N5
afallis@science.uottawa.ca
Received May 7, 2003
ABSTRACT
The synthesis of various tetrasubstituted alkenes and dienes in a regio- and stereocontrolled manner is described. This three-component
coupling strategy involves the addition of Grignard reagents to propargyl alcohols followed by palladium(0)-mediated cross-coupling with aryl
or vinyl halides. This protocol has been applied to the synthesis of (Z)-Tamoxifen and related mimics.
We have previously described the versatility of the magne-
sium-mediated carbometalation of propargyl alcohols to
assemble several diverse compounds generated in a single
reaction, for a variety of objectives.1
assembled in a one-pot, four-component coupling is repre-
sentative and illustrates the common theme for this protocol
via the magnesium chelate illustrated in Scheme 1.
These reactions have included the regio- and stereoselec-
tive generation of diene-halides, diene-diols, enediynes,
furans, furanones, and taxoid intermediates.2 The synthesis
of differentially substituted furans in which five bonds are
Scheme 1. Furans by Carbometalation of Propargyl Alcohols
† Present address: AstraZeneca R & D Montreal, AstraZeneca Canada
Inc., 7171 Frederick-Banting, St. Laurent (Montreal), Quebec, H4S 1Z9.
(1) For a review see: (a) Fallis, A. G.; Forgione, P. Tetrahedron 2001,
57, 589. For earlier studies see (b) Eisch, J. J.; Merkley, J. H. J. Organomet.
Chem. 1969, 20, 27. (c) Richey, H. G., Jr.; Von Rein, F. W. J. Organomet.
Chem. 1969, 20, 32. (d) Von Rein, F. W.; Richey, H. G., Jr. Tetrahedron
Lett. 1971, 6, 3777. (e) Jousseaume, B.; Duboudin, J. G. J. Organomet.
Chem. 1975, C1. (f) Jousseaume, B.; Duboudin, J. G. J. Organomet. Chem.
1979, 91, 6903. (g) Duboudin, J. G.; Jousseaume, B. J. Organomet. Chem.
1979, 168, 1. (h) Duboudin, J. G.; Jousseaume, B.; Bonakdar, A. J.
Organomet. Chem. 1979, 168, 227. (i) Negishi, E.; Zhang, Y.; Cederbaum,
F. E.; Webb, M. B. J. Org. Chem. 1986, 51, 4080. (j) Labaundinie`re, L.;
Hanaizi, J.; Normant, J.-F. J. Org. Chem. 1992, 57, 6903.
Multicomponent couplings of this type are of interest in
their own right and also hold potential for diversity-oriented
syntheses,3 especially if the vinyl furans were subjected to
subsequent cycloaddition reactions (R1 or R2 ) vinyl). We
wish to report the extension of these methods to a new,
versatile procedure for the regio- and stereospecific one-pot
palladium(0) cross-coupling of aryl and alkenyl halides from
(2) (a) Wong, T.; Tjepkema, M. W.; Audrain, H.; Wilson, P. D.; Fallis,
A. G. Tetrahedron Lett. 1996, 37, 755. (b) Forgione P.; Fallis, A. G.
Tetrahedron Lett. 2000, 41, 11. (c) Forgione, P.; Wilson, P. D.; Fallis, A.
G. Tetrahedron Lett. 2000, 41, 17. (d) Forgione, P.; Wilson, P. D.; Yap, G.
P. A.; Fallis, A. G. Synthesis 2000, 921.
(3) Kwon, O.; Park, S. B.; Schreiber, S. L. J. Am. Chem. Soc. 2002,
124, 13402.
10.1021/ol0347794 CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/29/2003

the intermediate chelate 2 to give a variety of tetrasubstituted
alkenes and dienes (Scheme 2). This protocol has also been
Table 1. Three-Component Coupling for Alkenes and Dienesa
Scheme 2. General Synthesis of Tetrasubstituted Alkenes
extended to a direct synthesis of (Z)-Tamoxifen.
An additional motivation for this study is our interest in
developing an efficient route to compounds that will be
recognized by the estrogen receptors such as Tamoxifen itself
and related mimics. These compounds have proven utility
for adjuvant therapy in women who have suffered from breast
cancer.4 In addition, recent studies suggest Tamoxifen may
have cancer preventative properties5 and effectiveness in the
preservation of bone mineral density related to osteoporosis.6
The related unsaturated acid 4 and raloxifene show similar
promise7 and thus these tetrasubstituted olefins have potential
for broader medicinal applications.8
Previous investigations in our laboratory have demon-
strated that the solvent plays an important role in these
carbometalation reactions. Nonpolar solvents such as cyclo-
hexane were essential for the direct condensation of the
magnesium chelate intermediate with aldehydes and nitriles.2b,c
However, toluene/THF mixtures (1:1) provided a significant
improvement. This is a consequence of the improved
solubility of the magnesium salts which afforded higher
yields especially when vinyl Grignard reagents were em-
ployed.
a Reagents and conditions: (a) R1MgX (3.2 equiv), toluene/THF, 1:1,
reflux, 14 h; (b) [Pd(PPh3)4], (0.05 equiv), R2X (3 equiv), reflux, 24 h.
Table 1 with a variety of combinations. These include alkyl-,
aryl-, allyl-, and trimethylsilyl-substituted propargyl alcohols.
The aryl, vinyl, and allyl Grignard reagents were exposed
to iodo or bromo coupling partners in the presence of
palladium(0).
The fully substituted alkenes in entries 1a and 1b displayed
no significant difference in yield upon variation of the halide.
Skipped dienes were generated with allyl substituents (entries
3 and 4). In these cases the allyl functionality may be
introduced as either the magnesium or palladium component.
Dienes were prepared efficiently as demonstrated by entries
5, 6, and 7. In theses examples also, depending upon the
synthetic objective, the second vinyl component can be
introduced at the coupling stage, the Grignard addition step,
or via an oxidation of the primary alcohol followed by a
Wittig reaction (Scheme 3). Unfortunately, alkyl Grignard
reagents such as methylmagnesium chloride are problematic.
This is partly attributed to the beneficial influence of
Figure 1. Tamoxifen and related drugs that recognize the estrogen
receptors.
The application of this carbometalation-palladium cou-
pling protocol is outlined in Scheme 2 and summarized in
(7) Willison, T. M.; Henke, B. R.; Momtahen, T. M.; Charifson, P. S.;
Batchelor, K. W.; Lubahn, D. B.; Moore, L. B.; Oliver, B. B.; Sauls, H.
R.; Baer, P. G. J. Med. Chem. 1994, 37, 1550.
(4) Furr, B. J. A.; Jordan, V. C. Pharmacol. Ther. 1984, 25, 127.
(5) Jordan, V. C. J. Steroid Biochem. Mol. Biol. 2000, 74, 269.
(6) Grey, A. B.; Stapleton, M. C.; Evans, M. C.; Tatnell, R. W.; Ames,
B.; Reid, I. R. Am. J. Med. 1995, 99, 636.
(8) Grese, T. A.; Dodge, J. A. Estrogen Recptor Modulators: Effects In
Non-Tradition Target Tissues. In Annual Reports in Medicinal Chemistry;
Bristol, J. A. Ed., Academic Press: San Diego, CA, 1996; Vol. 31, pp 181-
90.
2990
Org. Lett., Vol. 5, No. 17, 2003

to attach a cleavable tether tied to a breast cancer drug.
Therefore the mimic should act as a site recognition agent,
bind to an estrogen receptor, and subsequently release the
drug by hydrolysis.
Scheme 3. Synthesis of Tamoxifen and Related Olefinsa
Tamoxifen’s anti-estrogen biological activity resides en-
tirely in the (Z) isomer. There are a limited number of
stereoselective syntheses of (Z)-Tamoxifen,9 although the
nickel-catalyzed carbozincation sequence employing the
addition of diphenylzinc to 1-phenyl-1-butyne followed by
an iodine quench and palladium coupling is very direct
(Figure 2).10
Figure 2. Comparison of (Z)-Tamoxifen syntheses.
Our synthesis commenced with a Sonogashira cross-
coupling of the aryl halide 15 with propargyl alcohol 16 to
give 17 (83%). This substituted alkynol was then subjected
to the standard carbometalation protocol with phenylmag-
nesium chloride followed by the addition of Pd(PPh3)4 and
phenyl iodide as the cross-coupling partner to give the
Tamoxifen mimic 18 in 72% yield (Scheme 3). In a parallel
carbometalation-palladium cross-coupling sequence, the
methoxy ether 19 was prepared and may be converted, after
demethylation, to the Tamoxifen metabolite 22 (X ) Me)
or provide a second hydroxyl group for substituent attach-
ment.
Alkenol 18 was oxidized with Dess-Martin periodinane
in CH2Cl2 to give the corresponding unsaturated aldehyde
20 (96%), which was transformed directly with methyl
triphenylphosphonium bromide to give diene 21. Selective
hydrogenation of the less hindered double bond (10% Pd/C,
H2) afforded (Z)-Tamoxifen in 69% yield for the combined
steps.
a Reagents and conditions: (a) PdCl2(PPh3)2 (0.1 equiv), CuI (0.1
equiv), Et3N, THF, rt, (22 °C), 18 h, 83%. (b) (1) PhMgCl (3.2
equiv), MePh, reflux, 16 h; (2) Pd(PPh3)4 (0.05 equiv), PhI, 72%.
(c) (1) PhMgCl (3.2 equiv), MePh, reflux, 16 h; (2) Pd(PPh3)4 (0.05
equiv), p-MeOPhI (3 equiv), reflux, 24 h, 60%. (d) DMP (3 equiv),
CH2Cl2, 22 °C, 12 h, 96%. (e) KOt-Bu (1.2 equiv), PPh3CH2Br
(1.1 equiv), THF, reflux, 16 h, 81%. (f) H2/Pd/C, EtOAc, 22 °C, 2
h. 85%.
π-orbitals in the addition component. Thus the poor result
with the methylmagnesium chloride (entry 8) is due to the
inefficiency of the carbometalation and not to the subsequent
cross-coupling. The yield of the Grignard additions to
propargyl alcohols can be enhanced, in some cases, by the
addition of copper iodide.1g In our experience this frequently
reduces the yield in condensations with carbonyl electro-
philes. However, a reviewer recommended we examine the
potential influence of catalytic copper iodide on one of these
combined reactions due to its established beneficial influence
in palladium couplings as well as conjugate additions. The
effect was significant as the yield of 7 in entry 1a increased
to 82% in the presence of 7.5 mol % of cuprous iodide, a
yield improvement of 12%. The parallel reaction of meth-
ylmagnesium chloride in entry 8 was even more dramatic.
The yield of 14 increased to 30%. Clearly, the best procedure
for synthetic applications is to include catalytic CuI (7-10
mol %) for improved yields. We intend to investigate this
beneficial effect in more detail.
A comparison of the various (Z)-Tamoxifen syntheses is
summarized in Figure 2, although a Friedel-Crafts reaction
is required to generate the starting material 24. However,
only the syntheses commencing with 25 and 17 afford the
pure Z isomer directly, although variation of the substitution
pattern in 17 or the aryl Grignard allows greater versatility
for analogue synthesis.
The net effect of these carbometalations is the regiocon-
trolled anti addition of the Grignard component relative to
In view of Tamoxifen’s beneficial medicinal properties
discussed above, we are interested in developing a short route
to Tamoxifen mimics, as well as Tamoxifen itself. We
desired a family of related mimics with alcohol substitution
(9) (a) Millar, R. B.; Al-Hassan, M. I. J. Org. Chem. 1985, 50, 2121.
(b) Potter, G. A.; McCague, R. J. Org. Chem. 1990, 55, 6187.
(10) Studemann, T.; Knochel, P. Angew. Chem., Int. Ed. Engl. 1997,
36, 93.
Org. Lett., Vol. 5, No. 17, 2003
2991

the involvement of a unimolecular one-electron-transfer
mechanism, although a concerted [π2s + σ2a] process has
also been suggested.13 The increased yield observed with
methylmagnesium chloride in the presence of CuI suggests
additional mechanisms involving metal complexation must
also be operative.
In summary, we have developed a versatile direct synthesis
of tetrasubstituted alkenes by a one-pot magnesium-mediated
carbometalation of propargyl alcohols followed by pal-
ladium(0) cross-coupling with vinyl or aryl halides. This
method has been applied to the regio- and stereoselective
synthesis of (Z)-Tamoxifen and potential estrogen mimics
with therapeutic potential.
Figure 3. Rational for trans addition of vinyl Grignard.
the second carbon-carbon bond. We have discussed previ-
ously the preference for sp2 and sp centers for these Grignard
additions.2b We believe the preferred pathway involves an
intramolecular delivery of the vinyl group in which the π-π
interactions are beneficial and the bonding between a-b and
c-d occurs in a synchronous manner as illustrated by 25. A
close analogy is provided by the anti-hydroalumination of
propargyl alcohols with LiAlH4 or Red-Al,11 which are
considered to involve intermediates of type 26. Thus the
initial addition of the vinylmagnesium chloride to the
complexed species related to 25 is followed by collapse to
2. No evidence was detected in earlier investigations12 for
Acknowledgment. We are grateful to the Natural Sci-
ences and Engineering Research Council of Canada (NSERC)
and Torcan Chemical Ltd. for financial support of this
research. We appreciate the constructive comments of the
reviewers. A.J.P. thanks NSERC for a Scholarship and both
A.J.P. and P.E.T. thank the University of Ottawa for Graduate
Scholarships.
Supporting Information Available: Experimental details
and characterization for compounds 7-21. This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) Marshall, J. A.; DeHoff, B. S. J. Org. Chem. 1986, 51, 863.
(12) Eisch, J. J.; Merkley, J. H. J. Am. Chem. Soc. 1979, 101, 1148.
(13) Negishi. E.; Choueiry, D. Reaction of Alkynes with Organometallic
Reagents. In Preparation of Alkenes; A Practical Approach; Williams, J.
M. J., Ed.; Oxford University Press: New York, 1996; Chapter 7, p 152.
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