Gold-catalyzed tandem cycloisomerization of alkynyloxiranes with nucleophiles: An efficient approach to 2,5-disubstituted furans

Article abstract of DOI:10.1002/adsc.200700319

An efficient approach to 2,5-disubstituted furans has been developed by utilizing gold-catalyzed sequential nucleophilic attack onto metal-complexed alkynes with complete regioselectivity. The reaction proceeds efficiently under mild conditions with comme

Full text of DOI:10.1002/adsc.200700319

FULL PAPERS
DOI: 10.1002/adsc.200700319
Gold-Catalyzed Tandem Cycloisomerization of Alkynyloxiranes
with Nucleophiles: An Efficient Approach to 2,5-Disubstituted
Furans
Xing-Zhong Shu,a Xue-Yuan Liu,a Hui-Quan Xiao,a,c Ke-Gong Ji,a Li-Na Guo,a
Chen-Ze Qi,c and Yong-Min Lianga,b,*
a
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, Peopleꢀs Republic of China
Phone: (+86)-931-891-2593; fax: (+86)-931-891-2582; e-mail: liangym@lzu.edu.cn
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science,
b
Lanzhou 730000, Peopleꢀs Republic of China
Institute of Applied Chemistry, Shaoxing College of Arts and Sciences, Shaoxing 312000, Peopleꢀs Republic of China
c
Received: June 29, 2007; Revised: August 6, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: An efficient approach to 2,5-disubstituted erate to excellent yields (up to 96%) with high diver-
furans has been developed by utilizing gold-catalyzed sity.
sequential nucleophilic attack onto metal-complexed
alkynes with complete regioselectivity. The reaction
proceeds efficiently under mild conditions with com- Keywords: alkynes; alkynyloxiranes; cycloisomeriza-
mercially available catalysts to afford furans in mod- tion; epoxides; furans; gold
Introduction
ed cyclizations are not suitable for the synthesis of
base- or acid-sensitive furans.[5,6] (iii) Mo- and Ru-cat-
Furans are an important class of heterocyclic com- alyzed isomerizations of alkynyloxiranes to furans are
pounds which are extensively used as synthetic build- only suitable for terminal alkynes.[7] (iv) SmI2/Pd
(0 or
G
ing blocks[1] and appear as a subunit in many natural II) systems need stoichiometric amounts of SmI2 with
products which exhibit interesting biological activities a sequential procedure.[8] Furthermore, only the
and substances of relevance for industry.[2] For this minor (Z)-isomers of intermediates II cyclize to the
reason, the efficient synthesis of multiply substituted corresponding furans while the major (E)-isomers
furans continues to attract the interest of synthetic remain unchanged (Scheme 1).[8a]
chemists.[3]
Recently, gold salts have emerged as promising cat-
À
Among many different approaches to furans, the alysts for C O bond formation reactions through acti-
isomerization of alkynyloxiranes to furans looks quite vation of alkynes toward nucleophilic attack.[9] Be-
attractive. However traditional methods of these ap- sides alcohols,[9l,m,q] carbonyl compounds,[9p,t] and car-
proaches have some limitations as follows: (i) The dis- boxylic acids[9s], epoxides also act as good nucleo-
advantages of the Hg catalysis are obvious, which philes. However, only one example of this type of re-
does not meet the contemporary requirement against action has been reported.[9n] Consequently, the use of
hazardous reagents.[4] (ii) Strong base- or acid-promot- gold catalysis in the area of epoxides remains largely
Scheme 1.
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Table 1. Efficiency of transition-metal catalysts for the trans-
formation of 1a into 2a.[a]
Scheme 2.
Entry Catalyst
Temp.
[8C]
Time
[h]
Yield
[%][b]
unexplored. Due to their unique structures, species
formed from alkynes and epoxides might be further
attacked by nucleophiles. This might provide an effi-
1
2
3
4
5
6
Cu
AgSbF6
Pd(OAc)2
PtCl2
AuCl
(OTf)2
r.t.[c]
r.t.
r.t.
r.t.
r.t.
r.t.
1.0
24
24
24
1.5
1.5
0[d]
0
13
11
74
55
U
À
cient approach to the formation of a C O bond and a
remote carbon-nucleophile bond in one pot
(Scheme 2). Herein, we disclose the first results on
the corresponding cycloisomerization of alkynyloxir-
anes with nucleophilies to afford 2,5-disubstituted
furans by utilizing gold-catalyzed nucleophilic domino
attack onto metal-complexed alkynes with complete
regioselectivity. The alkynyloxiranes could be easily
obtained by epoxidizing the corresponding 1-en-4-yn-
3-ols following an acylation process.
2% A
AgBF4
2% A
ACHTREUNG
7
A
r.t.
r.t.
1.5
57
AgSbF6
AuCl3
2% AuCl3/6% AgSbF6 r.t.
8
9
1.5
2
24
20
2
93
53
69
74
92
86
10
11
12
13
14
15
Bu4N
A
r.t.
r.t.
60
HAuCl4·4H2O
HAuCl4·4H2O
HAuCl4·4H2O
10% TFAr.t.
80
2
1.5
0
Results and Discussion
10% ClCH2CO2H
r.t.
1.5
0
[a]
Conditions: 0.3 mmol of 1a with 2 mol% of catalysts in
methanol (1.0 mL).
Isolated yield.
We began our investigation with ester of 1-alkynyl-
2,3-epoxy alcohol 1a[10] (0.3 mmol) in methanol
(1.0 mL) using a variety of transition metal complexes
[b]
[c]
[d]
23–258C.
(2 mol%) (Table 1). In the presence of Cu(OTf)2 or
A
No starting material of 1a remained.
AgSbF6, the formation of furan 2a was not observed
(entries 1 and 2). Treatment of alkynyloxirane 1a with
2 mol% of Pd
(OAc)2 or PtCl2 gave the desired furan
To examine the scope of this cyclization, we first in-
2a in 13% and 11% yields, respectively. However the vestigated a range of esters of 1-alkynyl-2,3-epoxy al-
reaction did not proceed to completion even after cohols with methanol, as depicted in Table 2. Both ar-
24 h at room temperature (entries 3 and 4). Among omatic (entries 1–3) and aliphatic (entry 4) R2 groups
the gold catalysts used, Au(I) and Au(III) all afforded worked well. Electron-rich aryl groups showed better
U
good yields of furan 2a without any 1,3-migration results than those with an electron-withdrawing group
product[11] (entries 5–11). AuCl3 gave the best result in this cyclization (entries 2 vs. 3). These results would
(92%, 1.5 h). Addition of Ag(I) salts to the gold sys- be ascribed to the fact that an intermediate C, having
tems led to a dramatic decrease in the formation of an electron-donating group as the R2 group, would be
2a (entries 6, 7 and 9). On the other hand, on increas- stabilized effectively (vide infra). Less hindered sub-
ing the temperature to 608C, a high yield of up to strates like 1d and 1e gave the corresponding prod-
92% was also obtained by HAuCl4·4H2O (entry 12). ucts in slightly lower yields (entries 4 and 5). Accord-
Considering the acidity of these catalyst systems,[12] ingly, steric hindrance did not appear to be a major
protic acids such as TFA, chloroacetic acid have also
problem. We have also investigated the reactions of
been tested. However, the substrate underwent the alkynyloxiranes containing different R1 groups at the
deacylation process efficiently to afford the starting end of the triple bond with methanol. The reactions
material, 1-alkynyl-2,3-epoxy alcohol, without any de- of alkynes bearing an electron-withdrawing group or
sired furan product (entries 14 and 15). Thus, the use an electron-donating group with methanol all led to
of AuCl3 (2 mol%) at room temperature (conditions good yields of the desired products (entries 6–9). Al-
A) or HAuCl4·4H2O (2 mol%) at 608C (conditions kynes with heteroaromatic groups such as 2-thienyl
B) was found to be the most efficient and was used as proceeded smoothly to afford the corresponding
the standard conditions.[13] In the absence of gold cat- furan, e.g., 2j in 80% yield (entry 10). Alkynyloxirane
alyst, no reaction took place. This result indicates that 1k containing an n-pentyl group also afforded the de-
the gold catalyst is required for the reaction to pro- sired furan 2k in good yield (entry 11). Although
ceed.
HAuCl4·4H2O is a suitable catalyst for many reac-
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An Efficient Approach to 2,5-Disubstituted Furans
FULL PAPERS
Table 2. Au
C
Entry
1[c]
R1
R2
Conditions
2
Yields [%][d]
1
2
3
4
5
6
7
8
1a (7:5)
1b (10:7)
1c (2:1)
1d (2:1)
1e (9:7)
1f (2:1)
1g (2:1)
1h (7:5)
1i (9:7)
1j (2:1)
1k (5:3)
Ph
Ph
Ph
Ph
Ph
A, 1.5 h; B, 2 h
A , 1 h
A , 2 h; B, 2 h
B, 4 h
A, 2 h; B, 2.5 h
B, 4 h
B, 1 h
B, 2 h
A, 1 h; B, 2 h
A, 1 h
2a
2b
2c
2d
2e
2f
2g
2h
2i
93; 92
83
67; 70
62
78; 75
63
72
74
96; 55
80
p-CH3-C6H4
p-Cl-C6H4
CH3
H
Ph
Ph
Ph
Ph
Ph
H
Ph
p-Cl-C6H4
m-Cl-C6H4
m-CH3-C6H4
p-CH3O-C6H4
2-thienyl
n-C5H11
9
10
2j
2k
11[b]
A, 0.5 h
57
[a]
Unless noted, all reactions were carried out using 1 (0.3 mmol) with 2 mol% of catalysts in MeOH (1.0 mL). Conditions
A: AuCl 3 at room temperature. Conditions B: HAuCl4·4H2O at 608C.
[b]
[c]
[d]
Reaction run with 5 mol% catalyst loading.
syn/anti mixtures of the substrate were used; syn:anti ratio determined by H NMR.
1
Isolated yield.
tions, AuCl3 acts as a more active catalyst in some Table 3. Synthesis of 2,5-disubstituted furans 2 using differ-
ent nucleophiles.[a]
cases. For example, the cyclization of 1i afforded
prouct 2i in 96% yield by AuCl3 catalyst (entry 9).
However, only a moderate yield of 2i was obtained
when HAuCl4·4H2O was used (entry 9).
Then we investigated the scope of nucleophiles in
this cyclization (Table 3). It was found that, in addi-
tion to methanol, a variety of alcohols could be used
Entry[b]
ROH
Time
2
Yields (%)[c]
as effective nucleophiles for this reaction. Treatment
of 1e with ethanol resulted in the formation of 2l with
an ethoxy group in 70% yield (entry 2). When isopro-
pyl alcohol was employed, the corresponding furan
2m was formed smoothly in 62% yield (entry 3). The
use of sterically more hindered alcohols such as t-
BuOH also proceeded to afford the cyclized product
2n in 60% yield (entry 4). In addition, allylic alcohol
and benzylic alcohol were also readily reacted (en-
tries 5 and 6).
On the basis of the above observations, a possible
reaction mechanism is proposed in Scheme 3, which
may involve the following steps. (i) Coordination of
the alkynyl moiety of A to Au catalyst gives the com-
plex B. (ii) The subsequent domino nucleophilic
attack/anti-endo-dig cyclization affords organogold in-
termediate E. (iii) Protonation of E yields 2,3-dihy-
1
2
3
4
MeOH
EtOH
i-PrOH
t-BuOH
2 h
6 h
6 h
6 h
6 h
6 h
2e
2l
2m
2n
2o
2p
(78)[d]
(70)
(62)
(60)
(60)
(61)
5
6
[a]
Unless noted, all reactions were carried out using 1e
(0.3 mmol) with 2 mol% of AuCl3 in alcohols (1.0 mL) at
room temperature (conditions A).
syn/anti mixtures of 1e were used in the reaction.
Isolated yield.
75% yield of 2e was isolated when the reaction was pro-
ceeded under 2 mol% of HAuCl4·4H2O at 608C for
2.5 h.
[b]
[c]
[d]
drofuran F and regenerates the catalyst Au. (iv) F un- is formed by the nucleophilic attack of epoxide
dergoes direct reductive elimination of acetic acid, oxygen to the gold-coordinated alkynes.[17] The oxoni-
presumably via the corresponding oxonium ion, to um ion C undergoes the subsequent reaction with al-
afford the 2,5-disubstituted furan G.[15] Alternatively, cohols followed by protonation to regenerate the Au
the reaction may involve an oxonium ion C,[16] which catalyst and produce 2,3-dihydrofuran F.
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Xing-Zhong Shu et al.
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Scheme 3. Proposed reaction mechanism for Au-catalyzed cyclization.
Conclusions
In summary, a novel catalytic approach to 2,5-disub-
stituted furans has been developed through the cycli-
zation of esters of 1-alkynyl-2,3-epoxy alcohols with
various nucleophiles in moderate to excellent yields
under gold catalysts. The reaction proceeds efficiently
under mild conditions with commercially available
catalysts without any additive. Further studies on the
mechanism and the scope of this reaction are in prog-
ress in our laboratory.
by addition of saturated aqueous ammonium chloride
(40 mL) and extracted with ethyl ether (2”40 mL). The
combined organic layers were washed with brine, dried over
Na2SO4, and concentrated under reduced pressure. The
crude material was purified by flash column chromatogra-
phy to obtain the pure propargylic alcohol in quantitative
yield.
Experimental Section
The propargylic alcohol in dichloromethane (0.35M) was
cooled to 08C. To this cooled solution was added m-CPBA
(1.2 equivs., 77% w/v in water) and the mixture was stirred
for 10 h at room temperature, then the mixture was poured
into a mixture of saturated aqueous solutions of NaHSO3
and NaHCO3 (40 mL, 1:1). The aqueous layer was extracted
with ethyl ether (2”30 mL). The combined organic layer
was dried (Na2SO4), concentrated under reduced pressure,
and purified by flash column chromatography on silica gel
to obtain the 1-alkynyl-2,3-epoxy alcohol as a mixture of
diastereoisomers.
To a mixture of the epoxy alcohol in dichloromethane
(0.35M) was added triethylamine (3 equivs.), acetic anhy-
dride (1.2 equivs.), and DMAP (10 mol%) under a nitrogen
atmosphere at room temperature. The resultant mixture was
stirred for 3 h then washed with water, brine, dried over
Na2SO4 and the solvent was removed under reduced pres-
sure. The concentrate was purified by flash column chroma-
tography to obtain the desired esters of 1-alkynyl-2,3-epoxy
alcohols as a mixture of diastereoisomers.
General Remarks
Column chromatography was carried out on silica gel.
1H NMR spectra were recorded on 300 MHz in CDCl3 and
13C NMR spectra were recorded on 75 MHz in CDCl3. IR
spectra were recorded on a FT-IR spectrometer and only
major peaks are reported in cmÀ1. Melting points were de-
termined on a microscopic apparatus and were uncorrected.
All compounds were further characterized by elemental
analysis; copies of their H NMR and 13C NMR spectra are
1
provided in the Supporting Information. Room temperature
is 23–258C. Commercially available reagents and solvents
were used without further purification. THF was distilled
immediately before use from Na/benzophenone.
Representative Procedure for the Synthesis of
Alkynyloxiranes 1a–1d, 1f, 1i
To a stirred solution of the appropriate terminal alkyne
(1.2 equivs.) in THF (1.0M) was added ethylmagnesium bro-
mide (1.0M in THF, 1.1 equivs.) at room temperature. The
resulting solution was stirred for 1 h at 508C. Then a,b-unsa-
turated aldehyde (1.0 equiv.) in THF (0.35M) was added
slowly by syringe to the resulting solution at room tempera-
ture and stirred for 3 h. The reaction mixture was quenched
Representative Procedure for the Synthesis of
Alkynyloxiranes 1e, 1k
The propargylic alcohol was prepared according to the
above method. The propargylic alcohol in benzene (0.2M)
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An Efficient Approach to 2,5-Disubstituted Furans
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Epoxidation and acylation were performed as described
in the representative procedure.
General Procedure for the Cycloisomerization of
Esters of 1-Alkynyl-2,3-epoxy Alcohols to Substituted
Furans
Method A: To a solution of esters of 1-alkynyl-2,3-epoxy al-
cohols
1 (0.30 mmol) in alcohols (1.0 mL) was added
1.81 mg (0.006 mmol, 2 mol%) of AuCl3 under air at room
temperature. When the reaction was considered complete as
determined by TLC analysis, the reaction mixture was dilut-
ed with ethyl ether (40 mL), washed with water, saturated
brine, dried over Na2SO4 and evaporated under reduced
pressure. The residue was purified by chromatography on
silica gel to afford corresponding furans.
Method B: To a solution of esters of 1-alkynyl-2,3-epoxy
alcohols 1 (0.30 mmol) in alcohols (1.0 mL) was added
2.47 mg (0.006 mmol, 2 mol%) of HAuCl4·4H2O under air
at 608C. When the reaction was considered complete as de-
termined by TLC analysis, the reaction mixture was allowed
to cool to room temperature and diluted with ethyl ether
(40 mL). The mixture was washed with water, saturated
brine, dried over Na2SO4 and evaporated under reduced
pressure. The residue was purified by chromatography on
silica gel to afford corresponding furans.
was added VO(acac)2 (10 mol%) at room temperature.
N
After stirring for 5 mim, a solution of t-BuOOH (2 equivs.,
70% w/v in water) was added and the mixture allowed to
refux for 3 h. The reaction mixture was cooled to 08C and
an aqueous solution of saturated Na2S2O3 was added
(10 mL). The mixture was stirred for 20 min at room tem-
perature and then extracted with ether (2”40 mL). The
combined organic layer was dried over Na2SO4 and concen-
trated under reduced pressure to yield the crude product,
which was purified by flash column chromatography to
obtain the pure epoxy alcohol as a mixture of diastereoiso-
mers.
Acylation were performed as described in the above pro-
cedure.
Representative Procedure for the Synthesis of
Alkynyloxiranes 1g, 1h, 1j
Supporting Information Available
To
a stirred solution of ethynylmagnesium bromide
Characterization data for the compounds prepared and
(3 equivs., 0.35M in THF) was added cinnamaldehyde
1
copies of H and 13C NMR spectra for 2a–f are given in the
Supporting Information file.
Acknowledgements
We thank the NSF (NSF-20621091, NSF-20672049) and the
“Hundred Scientist Program” from the Chinese Academy of
Sciences for financial support.
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[10] syn/anti mixtures of the substrate esters of 1-alkynyl-
2,3-epoxy alcohols were used. The reaction products
were not found to be affected by the stereochemical
configuration of the substrate. For a full explanation
and definition of the terms syn and anti as used herein
see: C. M. Marson, D. W. M. Benzies, A. D. Hobson,
Tetrahedron 1991, 47, 5491–5506.
[11] The substrate bearing a propargyl acetate functionality
might undergo a 1,3-migration to form the correspond-
ing products, a common reaction pathway of propargyl
carboxylates. See: N. Marion, S. P. Nolan, Angew.
Chem. Int. Ed. 2007, 46, 2750–2752, and references
cited therein.
[12] W. Robb, Inorg. Chem. 1967, 6, 382–386.
[13] Based on simpler and inexpensive gold species,
HAuCl4·4H2O was employed as the catalyst for many
transformations.
[14] Recently we have reported that Bu4N[AuCl4] is an effi-
R
cient gold catalyst in the synthesis of highly substituted
furans. See: X. Liu, Z. Pan, X. Shu, X. Duan, Y. Liang,
Synlett 2006, 1962–1964.
[15] Similar processes have been described by Marson, see:
refs.[4f,h]
[16] Similar intermediate have been reported by Sarpong,
see: a) B. G. Pujanauski, B. A. Bhanu Prasad, R. Sar-
pong, J. Am. Chem. Soc. 2006, 128, 6786–6787. For the
transition metal-catalyzed oxonium ion formation, see:
b) N. Asao, K. Takahashi, S. Lee, T. Kasahara, Y. Ya-
mamoto, J. Am. Chem. Soc. 2002, 124, 12650–12651;
c) N. Asao, T. Nogami, S. Lee, Y. Yamamoto, J. Am.
Chem. Soc. 2003, 125, 10921–10925; d) N. Asao, H.
Aikawa, Y. Yamamoto, J. Am. Chem. Soc. 2004, 126,
7458–7459; e) J. Zhu, A. R. Germain, J. A. J. Porco,
Angew. Chem. 2004, 116, 1259–1263; Angew. Chem.
Int. Ed. 2004, 43, 1239–1243; f) K. Sato, S. S. Yudha, N.
Asao, Y. Yamamoto, Synthesis 2004, 9, 1409–1412;
g) H. Kusama, H. Funami, M. Shido, Y. Hara, J.
Takaya, N. Iwasawa, J. Am. Chem. Soc. 2005, 127,
2709–2716.
[17] Asimilar mechanism has been reported by Marson,
see: C. M. Marson, J. Campbell, Tetrahedron Lett. 1997,
38, 7785–7788.
2498
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Adv. Synth. Catal. 2007, 349, 2493 – 2498