A convenient base-mediated synthesis of 3-aryol-4-methyl (or benzyl)-2-methylthio furans from α-oxo ketene dithioacetals and propargyl alcohols via domino coupling/annulations

Article abstract of DOI:10.1039/c4ob01300j

A convenient base-mediated strategy to synthesize 3-aryol-4-methyl (or benzyl)-2-methylthio furans 2 (trisubstituted furans) has been developed through the domino coupling/annulations between available α-oxo ketene dithioacetals 1 and propargyl alcohols.

Full text of DOI:10.1039/c4ob01300j

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PAPER
A convenient base-mediated synthesis of 3-aryol-
4-methyl (or benzyl)-2-methylthio furans from
α-oxo ketene dithioacetals and propargyl alcohols
via domino coupling/annulations†
Cite this: DOI: 10.1039/c4ob01300j
Xiaobing Yang,a,b Fangzhong Hu,*a,b Hongjing Di,a,b Xinxin Cheng,a,b Dan Li,a,b
Xiaoli Kan,a,b Xiaomao Zoua,b and Qichun Zhang*c
A convenient base-mediated strategy to synthesize 3-aryol-4-methyl (or benzyl)-2-methylthio furans 2
(trisubstituted furans) has been developed through the domino coupling/annulations between available
α-oxo ketene dithioacetals 1 and propargyl alcohols. In this strategy, these types of bases play an impor-
tant role in driving the domino coupling reaction of propargyl alcohols and further intramolecular annula-
tions to realize the target compounds. The possible mechanism for the formation of the various products
is believed to involve the generation of allenes 7, followed by intramolecular annulations.
Received 24th June 2014,
Accepted 8th September 2014
DOI: 10.1039/c4ob01300j
www.rsc.org/obc
provide an efficient linkage between ketene dithioacetal func-
Introduction
tionality and a variety of other functional groups that have
shown promising properties for applications.1a,b
α-Oxo ketene dithioacetals (β,β-bis(alkylthio)-α,β-enones) are
highly functionalized α,β-unsaturated carbonyl intermediates,
which can undergo a variety of transformations.1 Generally, the
reaction centers in α-oxo ketene dithioacetals could be the car-
bonyl group, the double bond, or sulfur atoms, and deprotona-
tion can occur at several sites, which really depend upon the
structure of the α-oxo ketene dithioacetals.1c The presence of
two β-alkylthio substituents in α-oxo ketene dithioacetals affords
a higher level of oxidation in manipulation of functional groups
and in many cases, generates a product containing an S-functio-
nalized group, which can be further employed in additional syn-
thetic transformations.1a Numerous one-pot transformations,
involving a cascade of 1,2- and 1,4-nucleophilic addition reac-
tions to α-oxo ketone dithioacetals, have been widely employed
to synthesize a variety of heterocyclic compounds, suggesting
that α-oxo ketene dithioacetal compounds can act as an extre-
mely versatile three-carbon synthon for the manipulation of
functional groups and the construction of C–C bonds.1b Elabor-
ations on an α-oxo ketene dithioacetal skeleton, particularly on
those involving the C–C bond formation with various carboelec-
trophiles (aldehydes, simple ketones, enones, alcohols), would
Highly substituted furans play an important role as struc-
tural elements in many biologically active natural molecules,
pharmaceutics (e.g., ranitidine or zantac), and functional
macromolecules.2 Moreover, they are useful intermediates in
synthetic organic chemistry.3 So developing novel synthetic
routes toward the formation of polysubstituted furan rings is
very important.4 Among the reported approaches to multiply
substituted furans,5 cycloisomerizations of alkynyl ketones
catalyzed by transition metals are particularly attractive.6
This method could be further developed using allenyl
ketones as starting materials and Rh(I),6a,b Ag(I),6c,d Au(III),6e,f,g
or Pd-(0/II)6h,i,j complexes as catalysts for 5-endotrig cycliza-
tions. Although transition-metal-catalyzed cyclization is very
convenient to prepare trisubstituted furans,7 there are some
drawbacks in this type of cyclization process: (a) transition-
metal reagents are normally expensive, poisonous, and envir-
onmentally unfriendly; (b) the starting materials are generally
difficult to obtain so that transition-metal-catalyzed cyclization
has limited practical applications. Recently, transition-metal-
free processes for the formation of C–C, C–N, C–O and C–S
bonds have become an important field.8 Considering the
environmental friendliness and atom-economical purpose,
developing a new system for the synthesis of functionalized
and polysubstituted furans should be a promising and attrac-
tive route. In this context, we will present a convenient base-
mediated protocol for the intramolecular cyclization of α-oxo
ketene dithioacetals and propargyl alcohols to generate 2,3,4-
trisubstituted furans at room temperature.
aState Key Laboratory and Institute of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071, China
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300071, China
cSchool of Materials Science Engineering, Nanyang Technological University,
50 Nanyang Avenue, Singapore. E-mail: fzhu@nankai.edu.cn, qczhang@ntu.edu.sg
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4ob01300j
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Table 1 Optimized reaction conditions for synthesis of 2aa
Results and discussion
Initially, the starting α-oxo ketene dithioacetals were prepared
via the reaction of the corresponding aryl or heteroaryl ethyl
ketones with carbon disulphide in the presence of potassium
tert-butoxide, followed by methylation with dimethylsulfate
(Scheme 1).9
b
Temp Time Yieldb
Next, 3,3-bis (methylthio)-1-phenyl-2-propen-1-one 1a and
propargyl alcohol b were chosen as model substrates for the
initially-attempted new cascade processes in the presence of
t-BuOK in THF. It was found that the combination of 1 equiv.
of 1a, 1.2 equiv. of b and 2.2 equiv. of t-BuOK afforded only
traces of the furan product 2a after stirring at room tempera-
ture (RT) and 60 °C for 10–24 h (Table 1, entries 1 and 2). Most
of the starting material could be recovered. When the amount
of t-BuOK was increased to 1.2 equiv., the desired furan 2a was
obtained in 13–15% yield (Table 1, entries 3 and 4). This result
encouraged us to screen suitable reaction conditions for this 12
type of reaction. It is noteworthy to point that both 2.2 equiv.
of b and t-BuOK furnished the furan product 2a in 65% yield
after stirring at RT for 4 h (Table 1, entry 6). Continuously 16
increasing the amount of both b and t-BuOK resulted in lower
yield (Table 1, entry 8). The yields also became lower when the
reaction temperature was increased from room temperature to 20
60 °C (Table 1, entries 5, 6, 9 and 10). Interestingly, the reac-
tion could proceed at even lower temperature (0 °C), however,
a long time (normally, several days) is required in order to
achieve a complete conversion (Table 1, entry 7). Other alkali
metal bases such as t-BuONa and CH3ONa have poor perform-
ance in this type of reaction (Table 1, entries 11 and 12). Soluble
amine bases such as DBU and Et3N have also been tried,
however, none of them could kick this type of reaction going
(Table 1, entry 13). When the base t-BuOK was replaced by NaH,
the yield decreased to 30% (Table 1, entry 10). Changing the
solvents to dioxane or toluene, slightly decreased yields were
obtained (Table 1, entries 14 and 15). However, if other solvents
such as CH2Cl2, DMF, Et2O, CH3CN and DMSO were employed,
a poor reaction yield was observed (Table 1, entries 16–20).
Entry (equiv.) Base (equiv.)
Solvent
(°C)
(h)
(%)
1
2
3
4
5
6
7
8
1.2
1.2
1.2
1.2
2.2
2.2
2.2
3.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
t-BuOK (2.2)
t-BuOK (2.2)
t-BuOK (1.2)
t-BuOK (1.2)
t-BuOK (2.2)
t-BuOK (2.2)
t-BuOK (2.2)
t-BuOK (3.2)
NaH (2.2)
NaH (2.2)
t-BuONa (2.2) THF
CH3ONa (2.2) THF
DBU (2.2)
t-BuOK (2.2)
t-BuOK (2.2)
t-BuOK (2.2)
t-BuOK (2.2)
t-BuOK (2.2)
t-BuOK (2.2)
t-BuOK (2.2)
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
60
rt
60
rt
60
rt
0
rt
60
rt
rt
rt
10
24
10
15
2
4
60
3
2
20
2
20
24
2
Traces
Traces
15
13
38
65
50
40
10
9
10
11
30
50
12
13
14
15
THF
Dioxane rt
Toluene rt
CH2Cl2
DMF
Et2O
rt
s.m.c
50
6
45
rt
rt
rt
rt
rt
10
24
6
2
2
12
17
18
19
s.m.c
20
CH3CN
DMSO
Traces
25
a Reaction conditions: b and base were stirred in 10 mL of solvent at rt
under N2 for 15 min, followed by addition of 1a (0.5 mmol, 1.0 equiv.).
b Isolated yield after silica gel column chromatography. c s.m.
starting material.
=
With the optimized reaction conditions in hand, we exam-
ined the substrate scope of this strategy for the synthesis of
2,3,4-trisubstituted furans using a variety of α-oxoketene
dithioacetals and propargyl alcohol, and the results are shown
in Table 2. We first investigated the electronic effects of substi-
tuents in different positions on the aryl ring. It was found that
para-substituents of the aromatic ring with stronger electron-
withdrawing groups such as fluoro and chloro delivered rela-
tively lower yields compared to those with weak electron-with-
drawing groups such as bromo, iodo, phenyl and hydrogen
(2a–2e and 2h). Electron-withdrawing groups in the meta-posi-
tion (1m–1o) gave the corresponding furans in moderate yields
of 45–41%. These results indicated that electron-withdrawing
groups on the benzene ring to some extent were slightly un-
favorable for the reaction. In contrast, substrates with various
electron-donating groups in the para-substituted position,
such as methyl, tert-butyl, methoxy and phenoxy (1f, 1g, 1i and
1j) provided products in relatively high yields. ortho-Substi-
tuted substrates with a methyl or a methoxy group produced
2k or 2l in only 42% or 40% yield. Substrate 1p bearing the
methyl group at the meta-position furnished 2p in 51% yield.
Compound 1q, in which the meta-position is carried with a
methoxy group, was smoothly cyclized producing 2q in good
yield. The Ar of the starting materials could also be heteroaryl
groups, such as 2-furyl (1r), 2-(5-mefuryl) (1s), and 2-thienyl
(1t), furnishing 2r, 2s, and 2t in 47, 50 and 54% yields, respect-
Scheme 1 Starting materials of 1a–1y. 1a: Ar = C6H5; 1b: Ar = 4-FC6H4;
1c: Ar = 4-ClC6H4; 1d: Ar = 4-BrC6H4; 1e: Ar = 4-IC6H4; 1f: Ar =
4-MeC6H4; 1g: Ar = 4-tert-BuC6H4; 1h: Ar = 4-phenyl–phenyl; 1i: Ar =
4-MeOC6H4; 1j: Ar
= 4-PhOC6H4; 1k: Ar = 2-MeC6H4; 1l: Ar =
2-MeOC6H4; 1m: Ar = 3-FC6H4; 1n: Ar = 3-CF3C6H4; 1o: Ar = 3-acet-
ylC6H4; 1p: Ar = 3-MeC6H4; 1q: Ar = 3-MeOC6H4; 1r: Ar = 2-furyl; 1s:
Ar = 2-(5-mefuryl); 1t: Ar = 2-thienyl; 1u: Ar = 1-naphthyl; 1v: Ar = 3,4-
Cl2C6H3; 1w: Ar = benzo[d][1,3]dioxol-3-yl; 1x: Ar = 3,4-Me2C6H3.
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Table 2 Synthesis of 3-substituted-4-methyl-2-methylthio-furan derivativesa,b
a Reaction conditions: the mixture of b (1.1 mmol, 2.2 equiv.) and t-BuOK (1.1 mmol, 2.2 equiv.) was stirred in 10 mL of THF at rt under N2 for
15 min, followed by addition of 1 (0.5 mmol, 1.0 equiv.) at room temperature. b Isolated yield after silica gel column chromatography.
c b (2.2 mmol, 4.4 equiv.) and t-BuOK (2.2 mmol, 4.4 equiv.) were used.
ively. Naphthyl-substituted substrate 1u gave a good result. In Interestingly, the O,S-acetal intermediates 4 can afford 5 in
addition, substrates with various polysubstituents (1v–1x) were yields ranging from 40 to 67% in the presence of 1.1 equiv. of
also tested, and in most cases, satisfactory yields of the corres- t-BuOK (Scheme 3). However, no desired product was obtained
ponding cyclized products were achieved. Interestingly, the if 2-butyn-1-ol or 3-butyn-2-ol was employed to react with 1a in
difuran product 2y can also be obtained from the treatment of the presence of t-BuOK in THF.
1y and propargyl alcohol in 35% yield.
The suggested reaction mechanism is shown in Scheme 4.
It is interesting to note that the stepwise process leading to The cyclization between 1a and propargyl alcohol b was
the formation of 2,3,4-trisubstituted furans was found to be far carried out in the presence of t-BuOK in THF with 2.2 equiv. of
less efficient than the one-pot process (Scheme 2). In a separ- t-BuOD. After completion of the reaction, the mixture was
ate reaction, propargyl alcohol adduct 310 was separated via quenched with H2O, followed by the usual workup and purifi-
dimethylsulfonium perchlorate salt11 and subjected to 1.1 cation. Product 2aa-d was obtained with 62% deuterium incor-
equiv. of t-BuOK. Furan 2a was obtained in 31% overall yield porated at the –CH3 group of the furan ring. A similar level of
from α-oxo ketene dithioacetal 1a, while the same product was deuterium incorporation product 2ab-d was observed when the
obtained in 65% yield from 1a, highlighting the distinct reaction was performed employing t-BuOK and t-BuOD in
advantage of the one-pot domino process.
THF-d8 followed by quenching with D2O. These results
Under the standard conditions for the cyclization, the reac- suggested that the reaction might undergo the intermediate 8
tion between 1a and 3-phenylpropargyl alcohol c only pro- containing anions generated from the deprotonation of 1a by
duced 10% yield of the corresponding cyclized product 5a. t-BuOK and the hydrogen of the CvC double bond in 1a is the
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Scheme 2 An alternative method for the synthesis of 2a.
source of protonation at the –CH3 group at the 4th position of
the furan ring (2a).
On the basis of the deuteration experiments, the mechan-
ism for the reactions between α-oxo ketene dithioacetals and
propargyl alcohols is proposed in Scheme 5. Firstly, deprotona-
tion of propargyl alcohol b by t-BuOK gave the propargyloxya-
nion, which undergoes the Michael addition reaction with 1 to
form the O,S-acetal intermediate 6, followed by generation of
allene 7 in the presence of excess t-BuOK rather than the invol-
vement of the Claisen-rearrangement reaction.10b After an
intramolecular nucleophilic attack on the intermediate allene
7, the cyclization product 8 is formed, whose proton at the C-3
position is transferred intramolecularly to furnish 9. The final
product 2 was obtained through the isomerization of 9.
In conclusion, a convenient base-mediated strategy has
been developed for the synthesis of trisubstituted furans
through the reaction between α-oxo ketene dithioacetals and
propargyl alcohol via domino coupling/annulations. The
cyclization reactions are promoted by t-BuOK and could have
occurred in THF at room temperature. Many functional groups
have been tried to produce a wide range of synthetically rele-
vant furans. The easy accessibility of the starting materials,
simplicity of execution and mild reaction conditions should
make this new strategy more convenient for research and
application.
Scheme 3 Synthesis of 5 from the intermediates 4. Ar = C6H5 (5a,
yield: 55%); Ar = 4-ClC6H4 (5b, yield: 67%); Ar = 4-MeC6H4 (5c, yield:
60%); Ar = 2-furyl (5d, yield: 40%); Ar = 1-naphthyl (5e, yield: 63%).
Scheme 4 Deuteration experiments.
Scheme 5 Proposed mechanism of the domino coupling/annulations.
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Z. Yang, K.-S. Yeung and H. N. C. Wong, Prog. Heterocycl.
Chem., 2008, 19, 176.
Acknowledgements
We gratefully acknowledge financial support from the National
“Twelfth Five-Year” Plan for Science & Technology of China
(no. 2011BAE06B03-12).
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