4-exo-dig cyclocarbopalladation: A straightforward synthesis of cyclobutanediols from acyclic γ-bromopropargylic diols under microwave irradiation conditions

Article abstract of DOI:10.1002/ejoc.200500813

Treatment of acyclic γ-bromopropargylic diols with tributyl-stannylated alkynes under palladium catalysis and microwave irradiation conditions gives high yields of the bis(alkylidene)cyclobutanediol derivatives and cyclobutenediols through an efficient 4-exo-dig cyclocarbopalladation. The cyclization is general with a wide variety of alkyne derivatives and gives access to new cyclobutane ring systems bearing one exocyclic double bond and one eneyne substituent as well as bicyclic dienes sharing a common double bond that may be of interest for further elaborations of complex molecules. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejoc.200500813

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200500813
4
-exo-dig Cyclocarbopalladation: A Straightforward Synthesis of
Cyclobutanediols from Acyclic γ-Bromopropargylic Diols under Microwave
Irradiation Conditions
[a]
[a]
Christophe Bour and Jean Suffert*
In memory of our friend and colleague Dr Philippe Klotz
Keywords: Cyclocarbopalladation / 4-exo-dig cyclization / Microwaves / Bis(alkylidene)cyclobutane
Treatment of acyclic γ-bromopropargylic diols with tributyl-
stannylated alkynes under palladium catalysis and micro-
wave irradiation conditions gives high yields of the bis(alkyl-
idene)cyclobutanediol derivatives and cyclobutenediols
through an efficient 4-exo-dig cyclocarbopalladation. The
cyclization is general with a wide variety of alkyne deriva-
tives and gives access to new cyclobutane ring systems bear-
ing one exocyclic double bond and one eneyne substituent
as well as bicyclic dienes sharing a common double bond
that may be of interest for further elaborations of complex
molecules.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
vinylic bromides of type 10 (Scheme 2) and secondly to ex-
plore new reaction conditions involving the use of micro-
wave irradiation as a source of molecular activation of the
catalytic process. Here we present the synthesis of highly
substituted cyclobutanedienynes 11, in high yields ranging
from 71–86%. Bis(alkylidene)cyclobutane derivatives have
The discovery of new reactions that allow the construc-
tion of new, complex or exotic molecules, particularly
through the use of transition metal catalysis, has been
[
1]
[2]
widely explored in recent years. Notably, palladium,
[
3]
[4]
[5]
platinum, gold and rhodium have recently been ap-
plied in several ring-expansion, tandem, cascade or domino
processes for the construction of new polycyclic systems
and the formation of new C–C or C–X bonds (X = O, N,
S) in single operations. In this context we have recently dis-
covered a new cyclization reaction based on a 4-exo-dig cy-
clocarbopalladation[ that gives strained polycyclic deriva-
tives (4–9, Scheme 1), in some cases possessing anti-Bredt
double bonds shared by three cycles, as shown in com-
pounds 4 and 5 (Scheme 1).
[
8]
been known for some time and have recently been ob-
tained in good yields through organometallic-mediated syn-
[
9]
thesis, with use of copper or palladium activation.
In the presence of heteroaromatic tin derivatives, it is also
possible to obtain new heteroaromatic substrates of type 12.
If a vinylic stannylated reagent is used in place of an alkyn-
ylic one, bicyclic systems of type 13 can be obtained in good
to moderate yields after a 6π-electrocylization reaction
6]
(Scheme 2). We have already previously reported several ex-
amples of this kind of reaction affording exotic polycy-
Eventually, access to the tricyclic core structures 8 and 9
of the ophiobolin and aleurodiscal natural products by this
methodology was shown to be possible,[ the reaction origi-
nally being carried out with compounds of type 1–3 with
Pd(PPh ) catalysis in benzene or toluene at 85–90 °C in
[
6b]
cles. The starting γ-bromopropargylic diol 16 (Scheme 3)
was easily accessible from the trans-cinnamaldehyde (14) in
a two-step sequence.
7]
Compound 14 was first subjected to a bromination/de-
3
4
[
10]
bromination step by standard procedures.
The product
the presence of a vinyl- or alkynylstannane reagent. The
potential synthetic utility of this reaction prompted us to
study its scope and limitation.
To this end, and to broaden the usefulness of the 4-exo-
dig cyclocarbopalladation, we firstly decided to use acyclic
15 was obtained in 84% yield as a mixture of trans and
cis stereoisomers in a 1:1 ratio and this mixture was easily
separated by chromatography on silica gel. For the first ex-
periments, diol 15trans was selected as the starting material
in order to minimize the stereoelectronic interactions be-
[
a] Université Louis Pasteur de Strasbourg, Faculté de Pharmacie, tween the phenyl group on the double bond and the new
(UMR 7175-LC1 CNRS/ULP)
incorporated triple bond or double bond that might – in
7
4, route du Rhin, B.P. 24, 67401 Illkirch Cedex, France
15cis, for example – prevent the efficiency of the final coup-
Fax: +33-3-90244310
E-mail: jean.suffert@pharma.u-strasbg.fr
ling and eventually the 6π-electrocyclization process. The
1390
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4-exo-dig Cyclocarbopalladation
SHORT COMMUNICATION
Scheme 1. 4-exo-dig cyclocarbopalladation – routes to polycyclic structures.
Scheme 2. Reactivity of acyclic γ-bromopropargylic diols.
anti diol 16 and the syn 17 diol were isolated in 84% total (PPh ) ] in benzene afforded the cyclocarbopalladation
3
4
yield and in a 3:2 ratio through the addition of the lithium products 23a–d, which immediately cyclized through a 6π-
salt of a suitably protected propargylic alcohol to the bro- electrocyclization process to give bicyclic products 24a–d. It
moaldehyde 15trans, followed by deprotection and chroma- is important to note that under these conditions the protec-
tographic separation.[
11]
tion of the diol as a dioxolane gave 24b in a very high yield
The stereochemistries of the two diols were unambigu- (91%) as a mixture of two diastereomers in a 1:1 ratio,
ously determined by NMR 2D-NOESY experiments on the whilst if unprotected diol 16 was used, 24a (49%) was iso-
two dioxolane derivatives 18 and 19. In 18, strong corre- lated as a single diastereomer, the phenyl group anti to the
3
4
lations were observed between the protons H and H and two OH groups. The sensitive role of the protection is not
just one methyl, but in 19 each methyl correlated with only yet well understood and will be further studied.
3
4
one proton (H or H ; Scheme 3). From our previous re-
In a second set of experiments, diol 16 was treated under
sults obtained with 4-exo-dig cyclocarbopalladations of cy- “classical conditions” with 1.3 equiv. of (tributylstannyl)-
clic propargylic diols[ we first intended to study only the phenylacetylene
6b]
[14]
(22) and Pd(PPh ) (10 mol-%) in ben-
3 4
reactivity of isomer 16, which was expected to give higher zene at 85 °C for 15 h. The stable cyclobutanediol 25 was
yields of the cyclized products. In order to avoid decompo- isolated after silica gel chromatography in 56% yield. When
sition of these highly sensitive compounds we turned our 16 was then subjected to the same reaction conditions but
attention to the use of microwave irradiation, which would under microwave irradiation for 20 min, 25 was isolated in
be expected to give better results in a shorter reaction time. 85% yield (Scheme 4). Similarly, treatment of 16 in the pres-
Many examples describing the enhancement of yields, im- ence of the aromatic heterocycles derived from thiophene
provements in the efficiency in the course of the reaction, and furan produced 26 and 27 in 63 and 46% yields, respec-
the absence of side products and short reaction times are tively (Scheme 4). These optimized conditions were selected
available in the literature.[ In a first set of experiments, for the formation of other products containing the dienyne
12]
[
13]
treatment of 16, 18 and 20 with vinyltributylstannane 21
moiety; the investigation of additional substrates is summa-
in the presence of palladium catalysis [10 mol-% Pd- rized in Table 1. In all cases the reaction was run at 130 °C
Eur. J. Org. Chem. 2006, 1390–1395
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1391

C. Bour, J. Suffert
SHORT COMMUNICATION
Scheme 3. The preparation of acyclic γ-bromopropargylic diols.
Scheme 4. First results in the 4-exo-dig cyclocarbopalladation.
over 20 min under initial microwave irradiation at 300 W, in electrocyclization was not observed when the tributylvinyl-
a degassed solution of benzene in the presence of 1.3 equiv. stannane was coupled, 45 being isolated as the sole product
[
15]
of the stannylated reagent and Pd(PPh ) (10 mol-%).
in a modest yield of 40% (Scheme 5).
3
4
Usually the solution turned dark brown after irradiation
In summary, we have developed a new application of the
and the product was isolated by standard procedures and 4-exo-dig cyclocarbopalladation of acyclic γ-bromopropar-
chromatographic purification on silica gel. The 4-exo-dig gylic diols that allows an efficient preparation of new deriv-
cyclocarbopalladation is effective with a large variety of atives of cyclobutanediols in good yields. The efficiency of
functionalities on the triple bond, and yields of the corre- the reaction is greatly improved by the use of brief micro-
sponding dienynecyclobutanediols were between 71 and wave irradiation at 130 °C. If the reaction was conducted in
86% (Entries 1–14, 25, 28–40). If 42 was used in place of the presence of a vinyltributylstannane reagent a further 6π-
16 as a starting diol, the 4-exo-dig reaction was still effective electrocyclization occurred, providing new bicyclic systems.
(Scheme 3), with the incorporation of triple bonds giving All the new products constitute new building blocks for the
43 in 76% yield and 44 in 71% yield (Scheme 5). The 6π- elaboration of substituted polycyclic molecules. Further
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4-exo-dig Cyclocarbopalladation
SHORT COMMUNICATION
Table 1. 4-exo-dig Cyclocarbopalladation of acyclic propargylic diols with some alkynestannanes.
Scheme 5. Reactivity of the cis-γ-bromopropargylic diol.
studies will focus on the use of this methodology in this
direction.
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1393

C. Bour, J. Suffert
SHORT COMMUNICATION
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[15] General Procedure. 4-exo-dig Cyclocarbopalladation under Mi-
crowave Irradiation (Biotage System Initiator): The reaction
was carried out in a special BIOTAGE flask, under argon at-
mosphere. To a solution of the diol (1 equiv.) in dried benzene
2
004, 6, 1509–1511; m) A. Fürstner, P. W. Davies, T. Gress, J.
Am. Chem. Soc. 2005, 127, 8244–8245; n) S. Anjum, J. Marco-
Contelles, Tetrahedron 2005, 61, 4793–4803.
3 4
(2 mL), was added Pd(PPh ) (0.1 equiv.) and followed by the
stannylated reagent (1.3 equiv.). The mixture was purged 20
min with argon and heated at 130 °C under microwave irradia-
tion. The reaction was followed by TLC. After the reaction
[
4] a) C. Nieto-Oberhuber, S. Loópez, A. M. Echavarren, J. Am.
Chem. Soc. 2005, 127, 6178–6179; b) L. Zhang, S. A. Kozmin,
J. Am. Chem. Soc. 2004, 126, 11806–11807; c) J. P. Markham,
S. T. Staben, F. D. Toste, J. Am. Chem. Soc. 2005, 127, 9708–
2
time, the mixture was cooled, diluted with Et O (10 mL),
treated with active charcoal, filtrated over celite, and concen-
trated to dryness. The reaction mixture was purified by flash
chromatography (in the majority of the cases two chromatog-
raphy are necessary).
(1S,2R,3Z,4Z)-3-Benzylidene-4-[3-phenyl-1-(trimethylsilyl)prop-
2-ynylidene]cyclobutane-1,2-diol (25): This compound was pre-
pared according to the General Procedure on 0.295 mmol scale
and with a reaction time of 20 min. Purification by flash
9
709; d) J. J. Kennedy-Smith, S. T. Staben, F. D. Toste, J. Am.
Chem. Soc. 2004, 126, 4526–4527; e) S. Antoniotti, E. Genin,
V. Michelet, J.-P. Genêt, J. Am. Chem. Soc. 2005, 127, 9976–
9
977; f) V. Mamane, T. Gress, H. Krause, A. Furstner, J. Am.
Chem. Soc. 2004, 126, 8654–8655; g) A. S. K. Hashmi, L.
Schwartz, J.-H. Choi, T. M. Frost, Angew. Chem. Int. Ed. 2000,
3
9, 2285–2288; h) G. Dyker, D. Hildebrandt, J. Liu, K. Merz,
Angew. Chem. Int. Ed. 2003, 42, 4399–4402; i) N. Asao, K.
Takahashi, S. Lee, T. Kasahara, Y. Yamamoto, J. Am. Chem.
Soc. 2002, 124, 12650–12651.
chromatography with elution with Et
2
O/heptanes (20:80) af-
= 0.36
2 3
(Et O/heptanes, 30:70). H NMR (300 MHz, CDCl ): δ = 7.81
forded the product (90 mg, 85%) as a yellow solid. R
f
1
[
5] a) S. R. Gilbertson, B. DeBoef, J. Am. Chem. Soc. 2002, 124,
(d, J = 1.5 Hz, 1 H, ArH), 7.63 (d, J = 7.1 Hz, 2 H, ArH), 7.48
(d, J = 1.5 Hz, 1 H, ArH), 7.45 (d, J = 1.5 Hz, 1 H, ArH),
7.38–7.31 (m, 6 H, ArH, Ph–CH), 4.99 (t, J = 5.6 Hz, 1 H,
HO–CH–CH–OH), 4.81 (t, J = 6 Hz, 1 H, HO–CH–CH–OH),
3.74 (br.s, 1 H, OH), 3.61 (br.s, 1 H, OH), 0.32 (s, 9 H,
8
784–8785; b) T. Matsuda, M. Makino, M. Murakami, Angew.
Chem. Int. Ed. 2005, 44, 4608–4611; c) C. H. Oh, H. R. Sung,
S. H. Jung, Y. M. Lim, Tetrahedron Lett. 2001, 42, 5493–5495;
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L. Zhang, J. Am. Chem. Soc. 2005, 127, 2836–2837; f) H. A.
Wegner, A. de Meijere, P. A. Wender, J. Am. Chem. Soc. 2005,
1
3
SiMe
135.8, 131.2, 129.7, 129.5, 128.6, 128.5, 128.3, 128.1, 124.2,
121.0, 103.4, 91.9, 72.4, 71.3, –1.0 (SiMe ) ppm. IR (FITR,
film): ν˜ = 3400 (br), 2118, 1646, 1485, 1249, 1047, 976, 844,
3 3
) ppm. C NMR (75 MHz, CDCl ): δ = 158.8, 142.8,
3
1
27, 6530–6531; g) P. A. Wender, T. M. Pedersen, M. J. C.
–
1
Scanio, J. Am. Chem. Soc. 2002, 124, 15154–15155; h) M.-H.
Baik, E. W. Baum, M. C. Burland, P. A. Evans, J. Am. Chem.
Soc. 2005, 127, 1602–1603; P. A. Evans, E. W. Baum, J. Am.
Chem. Soc. 2004, 126, 11150–11151.
756 cm . HRMS (ESI, positive ion 180 eV): calcd. for
+
(C23
24 2
H O SiNa) : 383.1441; found 383.1502.
(7R,8S)-5-Phenyl-2-(trimethylsilyl)bicyclo[4.2.0]octa-1(6),2-diene-
7,8-diol (24a): This compound was prepared by the general pro-
cedure on 0.295 mmol scale, with use of 0.383 mmol of stan-
nane and a reaction time of 30 min. Purification by flash
chromatography with elution with AcOEt/heptanes (20:80) af-
[
6] a) B. Salem, P. Klotz, J. Suffert, Synthesis 2004, 2, 298–308; b)
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1
2107–12108.
7] B. Salem, J. Suffert, Angew. Chem. Int. Ed. 2004, 43, 2826–
830.
forded the product (65 mg, 49%) as a white solid. R
f
= 0.36
): δ =
1
[
(Et
2
O/heptanes, 30:70). H NMR (200 MHz, CDCl
3
2
7.34–7.21 (m, 3 H, ArH), 7.17–7.12 (m, 2 H, ArH, Ph–CH),
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4-exo-dig Cyclocarbopalladation
SHORT COMMUNICATION
6
.10 (t, J = 4.9 Hz, 1 H, ArH), 4.85 (d, J = 3 Hz, 1 H, HO–
CH–CH–OH), 4.73 (d, J = 3 Hz, 1 H, HO–CH–CH–OH), 3.70
dd, J = 11 Hz, J = 5 Hz, 1 H, CH–CH ), 2.94–2.45 (m, 4 H,
(br), 2924, 2845, 1652, 1490, 1448, 1250, 1107, 1062, 893,
–
1
749 cm . HRMS (ESI, positive ion 180 eV): calcd. for
+
(
2
1
(C17
H
22
O
2
SiNa) : 309.1287; found 309.1253.
Received: October 17, 2005
Published Online: February 3, 2006
3
CH
CDCl
2.9, 36.1, 35.4, –1.5 (SiMe
2
, OH), 0.18 (s, 9 H, SiMe
) δ = 149.2, 147.2, 137.7, 132.1, 128.7, 127.2, 126.6, 73.0,
) ppm. IR (FITR, film): ν˜ = 3410
3
) ppm. C NMR (75 MHz,
3
7
3
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