Silver-mediated exo-selective tandem desilylative bromination/ oxycyclization of silyl-protected alkynes: Synthesis of 2-bromomethylene- tetrahydrofuran

Article abstract of DOI:10.1002/asia.201100158

Three in one: The exocyclic β-bromo enol ether is a synthetically useful functional moiety in the preparation of functionalized oxygen heterocycles. A new tandem method for the synthesis of this valuable moiety has been achieved by employing silyl-protect

Full text of DOI:10.1002/asia.201100158

DOI: 10.1002/asia.201100158
Silver-Mediated exo-Selective Tandem Desilylative Bromination/
Oxycyclization of Silyl-Protected Alkynes: Synthesis of 2-Bromomethylene-
Tetrahydrofuran
Hyun-Ji Lee,[a] Chaemin Lim,[a] Soonho Hwang,[a] Byeong-Seon Jeong,[b] and
Sanghee Kim*[a]
Dedicated to Professor Eun Lee on the occasion of his retirement and 65th birthday
Tandem reactions are one-pot multi-step processes and
are thus among the most useful synthetic methods.[1] Recent-
ly, the use of transition metals in tandem reactions has re-
ceived considerable attention and has revolutionized the
synthetic strategies in organic chemistry.[2] Among the tran-
sition metal-based tandem reactions, the processes that in-
volve heterocyclization reactions of unsaturated substrates
are of particular interest,[3,4] as they provide a direct method
for the construction of synthetically and biologically valua-
ble heterocycles.
The exocyclic enol ether function, created by the exo-dig
heterocyclization of hydroxyalkynes, is synthetically useful
because it is amenable to further chemical transformations,
such as hetero-Diels–Alder reactions and electrophilic addi-
tions.[8] In the synthesis of functionalized oxygen heterocy-
cles, the exocyclic b-bromo enol ether 1 (Scheme 1) is more
The intramolecular heterocyclization of g or d-hydroxyal-
kynes promises to be a convenient way to access the func-
tionalized tetrahydrofurans and pyrans, which are common
motifs in a variety of natural products. This cyclization can
be driven by transition metal catalysts, and the regioselectiv-
ity of these reactions varies depending on the metal and the
conditions employed.[5] Among the metals thus far reported
for these reactions, silver and gold have been demonstrated
to be the most effective and selective for the formation of
exo-dig products.[6,7] These metals can promote heterocycli-
zation of both internal and terminal alkynes. Even halogen-
ated and silylated alkynes can be converted into the corre-
sponding exo-dig products using these metals.
Scheme 1. Formation of the exocyclic b-bromo enol ether (1).
useful due to the presence of an additional functionality, the
vinyl halide moiety. As shown in Scheme 1, this functionality
can be accessed from the exocyclic enol ether by bromina-
tion.[9] In addition, it can be directly produced by the intra-
molecular heterocyclization of bromoacetylenic alcohols in
the presence of a gold catalyst or base.[5d,10] Mercury-in-
duced heterocyclization of an acetylenic alcohol followed by
interception of the resulting organomercury intermediate
using N-bromosuccinimide (NBS) also leads to the forma-
tion of b-bromo enol ethers.[11]
We envisioned that the exocyclic b-bromo enol ether
functionality could be accessed directly from the silylacety-
lenic alcohol 2 through tandem silver(I)-mediated desilyla-
tive bromination and heterocyclization.[12] In addition to
promoting the heterocyclization of hydroxyalkynes, silver
also has the ability to induce the in situ one-pot desilylative
bromination of silyl-protected acetylenes. The trimethylsilyl
[a] H.-J. Lee,+ C. Lim,+ S. Hwang, Prof. S. Kim
College of Pharmacy
Seoul National University
San 56-1, Shilim, Kwanak, Seoul 151-742 (Korea)
Fax : (+82)2-888-0649
E-mail: pennkim@snu.ac.kr
[b] Prof. B.-S. Jeong
College of Pharmacy
Yeungnam University
214-1 Dae-dong, Gyeongsan, Gyeongsangbukdo 712-749 (Korea)
[+] These two authors have contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100158.
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1943

COMMUNICATION
Table 1. AgNO3-mediated bromination/oxycyclization of TMS-protected
acetylene 3a and terminal acetylene 9.
(TMS)-protected acetylenes can be converted into haloace-
tylenes through the use of a catalytic amount of AgNO3 and
a
halogen source, such as NBS or N-iodosuccinimide
(NIS).[13] Acetylenes with a bulky silyl group, such as tert-bu-
tyldimethylsilyl (TBDMS) and triisopropylsilyl (TIPS), can
also be transformed into bromoacetylenes in the presence of
AgF and NBS.[14] Thus, the employment of silver would
allow the desired tandem transformation to produce the syn-
thetically useful exocyclic b-halo enol ether functionality
from the readily obtainable silyl-protected acetylenic alco-
hols. Herein, we report our studies on this subject and de-
scribe the total synthesis of pachastrissamine from the
tandem product to demonstrate the potential of the devel-
oped synthetic method.
Entry Substrate
AgNO3
[equiv]
Solvent
t
Combined yield 7:8[b]
[h]
[%][a]
1
2
3
4
5
3a
3a
3a
9
0.1
1.2
0.1
0.1
1.2
Acetone
Acetone
MeCN
Acetone
Acetone
7
2
48
3
70
72
49:51
52:48
–
67:33
71:29
NR[c]
72
9
1
73
[a] Yields determined by GC analysis. [b] Product ratio was determined
by GC and 1H NMR spectroscopic analyses. [c] Starting material 3a was
primarily recovered.
We chose to study substrate 3 (Scheme 2) as a model,
which contains an oxygen atom at the propargylic position.
À
The interaction between the propargylic C O bond and the
an almost equal ratio of the b-bromo enol ether 7 and the b-
dibromo enol ether 8 in 70% combined yield. The obtained
enol ether 7 has the Z stereochemistry, which was deter-
mined by NOESY analysis. An increase in the amount of
AgNO3 beyond one equivalent did not significantly alter the
ratio of the two products (entry 2). The same reaction in
acetonitrile did not proceed well (entry 3). When the termi-
nal acetylene 9 was employed as a substrate, the tandem re-
action also afforded a mixture of 7 and 8 with low selectivity
(entries 4–5).
After initial feasibility studies with AgNO3, we then pro-
ceeded to AgF as a silver ion source. Exposure of the TMS-
protected acetylene 3a to AgF (1.2 equiv) in the presence of
a slight excess of NBS (1.1 equiv) in acetonitrile resulted in
selective formation of the mono-bromo product 7 as the
major product (Table 2, entry 1). Only a small amount of
the bis-bromo product 8 was detected. When this system
was applied to the terminal acetylene 9, the selectivity was
also very high (entry 2). Under these reaction conditions,
even the acetylene 3b with a bulky TIPS group could under-
go bromination/oxycyclization to produce the mono-bromo
product 7 along with a minor amount of bis-bromo product
Scheme 2. Synthesis of the silyl-protected acetylenic alcohol 3.
adjacent triple bond has been reported to alter the electron
density of the p system and affect the metal–alkyne coordi-
nation and eventually the heterocyclization step.[15] As the
electronic influence of the propargylic substituent can be ex-
amined by changing the benzoate protecting group of 3 to
the electron-donating protecting group (see below), sub-
strate 3 is a suitable initial model for testing the feasibility
of the silver(I)-mediated tandem reaction and evaluating
the scope of these reactions. The silyl-protected acetylenic
alcohol 3 was prepared from the known aldehyde 4[16] by the
addition of lithium acetylide, protection of the resulting
propargylic alcohol, and deprotection of the primary alcohol
(Scheme 2).
Table 2. AgF-mediated bromination/oxycyclization of compounds 3a, 3b,
and 9.
The tandem desilylative bromination/oxycyclization was
initially conducted with the TMS-protected acetylene 3a.
Our first attempts to accomplish the silver-mediated tandem
reactions of 3a involved employing the NBS/AgNO3 condi-
tions.[13] Treatment of 3a with the conventional catalytic
amount of AgNO3 (0.1 equiv) and a slight excess of NBS
(1.1 equiv) in acetone (Table 1, entry 1) led to the desired
tandem bromination/oxycyclization. The reaction was com-
pleted in seven hours and yielded only the exo-dig products.
However, these reaction conditions produced a mixture of
Entry Substrate NBS [equiv] Solvent t [h] Yield [%][a]
7:8[b]
1
2
3
4
5
6
3a
9
3b
3b
3a
3b
1.1
1.1
1.1
1.1
2.5
2.5
MeCN
MeCN
MeCN
DMF
MeCN
MeCN
9
8
11
24
18
20
95 (7+8)
83 (7+8)
82 (7+8)
72 (7+8)
94:6
91:9
91:9
69:31
71 (only 8)[c] 0:100
73 (only 8)[c] 0:100
[a] Yields determined by GC analysis. [b] Product ratio was determined
1
by GC and H NMR spectroscopic analyses. [c] Yield of isolated product.
1944
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Chem. Asian J. 2011, 6, 1943 – 1947

Tandem Desilylative Bromination/Oxycyclization
8 in a ratio of 91:9 (entry 3). The reaction with TIPS-acety-
lene 3b in N,N-dimethylformamide (DMF) required a
longer reaction time (entry 4), and this resulted in a lower
product yield and selectivity than those obtained using ace-
tonitrile as a solvent. When the reaction was conducted in
acetone, tetrahydrofuran (THF), dimethyl sulfoxide
(DMSO), and dichloromethane, the starting material was
primarily recovered. These failures could be due to the low
solubility of AgF in these solvents.
Notably, the use of excess NBS (2.5 equiv) in the reaction
of 3 with AgF in acetonitrile resulted in the formation of
bis-bromo product 8 as the only detectable product (en-
tries 5 and 6). In the absence of NBS, TIPS-protected acety-
lenic alcohol 3b underwent smooth desilylative oxycycliza-
tion to give the corresponding nonhalogenated exocyclic
enol ether in high yield (93%).
the b-bromo enol ether 7. Alternatively, the bromolysis of D
with NBS present in the reaction mixture would lead to the
formation of b-dibromo enol ether 8. The involvement of
silver-vinylidene D during this sequence is supported by the
observation that the isolated mono-bromo compound 7 was
not transformed into the bis-bromo compound 8 upon treat-
ment with NBS, regardless of the presence or absence of
AgF.
Although TMS has been used as a classical protecting
group for terminal acetylenes, the bulkier TIPS protecting
group is also very useful and attractive because of its stabili-
ty under a wider range of reaction conditions. Thus, despite
the observation that TMS-protected 3a gave better yield
and selectivity than TIPS-protected 3b (Table 2, entry 1 vs.
entry 3), TIPS-protected acetylenic alcohol was chosen to
examine the scope of this tandem desilylative bromination/
oxycyclization. We prepared the TIPS-protected acetylenic
alcohols 11, 12 (Scheme 4), and 13 (Scheme 5) and subjected
them to the above-mentioned conditions. These reaction
The formation of b-bromo enol ether 7 from 3 appears to
occur via the intermediate bromoacetylene 10 (Scheme 3).
During the reaction, we could observe bromoacetylene 10,
Scheme 4. AgF-mediated bromination/oxycyclization of compounds 11
and 12.
conditions were also found to be effective in the preparation
of the functionalized pyran. The substrate 11 underwent an
equally facile, AgF-mediated exo-selective tandem reaction
to give pyran 14a in 79% yield without notable formation
of the bis-bromo product 14b. Substrate 12, bearing an elec-
tron-donating benzyl group, was also converted into the
exo-dig tandem product 15. The mono-bromide 15a was the
major product of the reaction; however, selectivity for the
formation of mono-bromide 15a and bis-bromide 15b was
lower than that for the benzoate-containing substrate 3.
Unlike 7, isolated 15a was somewhat unstable and decom-
posed gradually at room temperature. Substrate 13, derived
from (S)-Garnerꢁs aldehyde (16) over three steps
(Scheme 5), readily cyclized to again yield the Z stereoiso-
mer 19 along with a minor amount of bis-bromo product in
a ratio of 81:19. These results suggested that the presented
tandem reaction would be applicable to substrates with vari-
ous functionalities although the product selectivity (mono-
vs. bis-bromide) varies depending on the substrate type.
The exocyclic b-halo enol ether 19 was envisioned to be a
useful starting point for the total synthesis of pachastrissa-
mine[17,18] (22; Scheme 5) through cross-coupling reactions.
Scheme 3. Proposed mechanism for the AgF-mediated tandem reaction.
and isolate this intermediate. When isolated 10 was treated
with AgF, this compound readily cyclized, again resulting in
bromo enol ether 7 in high yield (96%). These results to-
gether with the observed Z stereochemistry suggest a mech-
anism for the AgF-mediated tandem reaction (Scheme 3).
The reaction could be initiated by the coordination of the
silver ion to the silylated alkyne to form the p-alkynyl com-
plex A. This coordination would favor nucleophilic displace-
ment of the silyl group by silicophilic fluoride, thereby lead-
ing to the formation of the alkynyl silver B. The resulting al-
kynyl silver reacts with NBS to give the intermediate bro-
moacetylene 10. When produced, the latter would be acti-
vated by silver ions again to form the p-alkynyl complex C.
A subsequent anti-intramolecular addition of the alcohol
group to the p-complex C would lead to the s-complex D.
The protonolysis of the carbon–silver bond in D with reten-
tion of configuration at the carbon atom would then liberate
Chem. Asian J. 2011, 6, 1943 – 1947
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1945

S. Kim et al.
COMMUNICATION
large excess of NBS afforded the bis-bromo product exclu-
sively. As the exocyclic b-halo enol ether functionality could
serve as a handle for further chemical transformations, this
method is a valuable and efficient tool for the synthesis of
natural or biologically active products. Further studies to
expand the scope of this method as well as to develop its
synthetic applications are now underway.
Experimental Section
General Procedure for Silver-Mediated Bromination/Oxycyclization
(Table 2, entries 1–3)
NBS (1.1 equiv) and AgF (1.2 equiv) were added to a solution of acety-
lenic alcohol (0.2 mmol) in acetonitrile (4 mL, 0.05m) in the dark. The re-
action mixture was stirred at room temperature for 8–11 h, quenched
with water, and extracted twice with EtOAc (15 mL). The combined or-
ganic layers were washed with brine, dried over MgSO4, and then filtered
through a Celite pad. For analytical purposes, a small amount of the or-
ganic filtrate was subjected to GC analysis with dodecane as an internal
standard. The crude product was purified using silica-gel column chroma-
tography (EtOAc/hexane, 1:10) to afford the corresponding pure prod-
ucts 7 and 8.
(Z)-2-(Bromomethylene)-tetrahydrofuran-3-yl benzoate (7)
White solid: m.p. 80.1–82.58C; IR (CH2Cl2): n˜max =2903, 1719, 1667, 1451,
1253 cmÀ1 1H NMR (C6D6, 400 MHz): d=8.01 (d, J=7.6 Hz, 2H), 7.14
;
(t, J=7.4 Hz, 1H), 7.05 (t, J=7.6 Hz, 2H), 5.53 (t, J=4.1 Hz, 1H), 5.45
(s, 1H), 3.81 (td, J=8.2, 8.0 Hz, 1H), 3.67–3.62 (m, 1H), 1.58–1.53 ppm
(m, 2H); 13C NMR (C6D6, 100 MHz): d=165.5, 157.2, 133.2, 130.2, 130.0
(2C), 128.6 (2C), 79.1, 73.1, 69.4, 32.5 ppm; MS (FAB): m/z: 285 (30), 283
([M+H]+, 32), 203 (34), 154 (100); HRMS (FAB): calcd for C12H12BrO3:
282.9892 ([M+H]+); found: 282.9970.
Scheme 5. Synthesis of substrate 13 and total synthesis of pachastrissa-
mine from b-halo enol ether 19. brsm=based on recovered starting mate-
rial. HMPA=hexamethylphosphoramide; LiHMDS=lithium hexame-
thyldisilazane.
2-(Dibromomethylene)-tetrahydrofuran-3-yl benzoate (8)
White solid: m.p. 90.6–94.58C; IR (CH2Cl2): n˜max =2903, 1721, 1601, 1451,
For this purpose, the obtained bromo-methylene 19 was sub-
jected to Sonogashira conditions with tridec-1-yne.[19] The
reaction occurred readily in acetonitrile[20] and gave the de-
sired coupling product 20 in 64% yield (73% based on re-
covery of starting material) with complete retention of con-
figuration. Full reduction of the enyne moiety in 20 with
palladium/carbon under hydrogen furnished 21 with the de-
sired configuration at C-1 as the sole diastereomer (90%).
Finally, standard hydrolysis of 21 yielded the desired pachas-
trissamine (22) in 72% yield, the spectral data of which
were in good agreement with those reported in the litera-
ture.[18b] Our total synthetic route for pachastrissamine from
Garnerꢁs aldehyde is short and efficient owing to the
tandem desilylative bromination/oxycyclization, and this
route is also applicable to the preparation of pachastrissa-
mine derivatives because various side chains may be intro-
duced through cross-coupling reactions with the b-halo enol
ether moiety in 19.
1276 cmÀ1 1H NMR (C6D6, 400 MHz): d=8.09 (d, J=7.2 Hz, 2H), 7.14
;
(t, J=7.4 Hz, 1H), 7.05 (t, J=7.6 Hz, 2H), 5.95 (t, J=3.9 Hz, 1H), 3.79–
3.73 (m, 1H), 3.62–3.57 (m, 1H), 1.62–1.57 ppm (m, 2H); 13C NMR
(C6D6, 100 MHz): d=165.3, 156.1, 133.3, 130.1 (2C), 130.0, 128.7 (2C),
73.7, 71.1, 67.0, 33.2 ppm; MS (FAB): m/z: 364 (35), 362 (68), 360 ([M]+,
35), 241 (100); HRMS (FAB): calcd for C12H10Br2O3: 359.8997 ([M]+);
found: 359.8997.
Acknowledgements
This work was supported by the SRC/ERC program (R11-2007-107-
02001-0) and the WCU program (R32-2008-000-10098-0) through
KOSEF funded by MEST.
Keywords: alkynes · oxygen heterocycles · silver · synthetic
methods · tandem reactions
In summary, we have developed a tandem method for the
synthesis of exocyclic b-bromo enol ethers starting from
easily accessible silyl-protected acetylenic alcohols. The
AgF/NBS system is used to promote a tandem desilylative
bromination/oxycyclization reaction at room temperature to
give functionalized oxolane and oxane functionalities. This
reaction preferentially provided a mono-bromo enol ether
product in the presence of a slight excess of NBS, and a
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Tandem Desilylative Bromination/Oxycyclization
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[20] In other solvents such as DMF and THF, the reaction gave a compli-
cated product mixture.
Received: February 15, 2011
Published online: April 27, 2011
Chem. Asian J. 2011, 6, 1943 – 1947
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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