Practical, modular, and general synthesis of benzofurans through extended pummerer annulation/cross-coupling strategy

Article abstract of DOI:10.1002/anie.201403288

Operationally simple, efficient, and widely applicable Pummerer annulations of simple phenols with ketene dithioacetal monoxides, with the aid of trifluoroacetic anhydride, have been shown to provide a variety of benzofurans having a methylthio group at the 2-position. Subsequent and newly developed nickel-catalyzed arylation at the methylthio group culminates in diversity-oriented synthesis of multisubstituted benzofurans. Our extended Pummerer annulation/cross-coupling sequence is powerful enough to synthesize biologically active natural products as well as highly fluorescent benzofuran derivatives.

Full text of DOI:10.1002/anie.201403288

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Angewandte
Communications
DOI: 10.1002/anie.201403288
Synthetic Methods
Practical, Modular, and General Synthesis of Benzofurans through
Extended Pummerer Annulation/Cross-Coupling Strategy**
Kei Murakami, Hideki Yorimitsu,* and Atsuhiro Osuka
Abstract: Operationally simple, efficient, and widely applica-
ble Pummerer annulations of simple phenols with ketene
dithioacetal monoxides, with the aid of trifluoroacetic anhy-
dride, have been shown to provide a variety of benzofurans
having a methylthio group at the 2-position. Subsequent and
newly developed nickel-catalyzed arylation at the methylthio
group culminates in diversity-oriented synthesis of multisub-
stituted benzofurans. Our extended Pummerer annulation/
Scheme 1. Modular synthesis of substituted benzofurans. EDG=elec-
tron-donating group, EWG=electron-withdrawing group, Tf=trifluoro-
methanesulfonyl, TFAA=trifluoroacetic anhydride.
cross-coupling sequence is powerful enough to synthesize
biologically active natural products as well as highly fluores-
cent benzofuran derivatives.
B
enzofuran is an ubiquitous core found in various natural
a wide range of KDMs,[12] would provide a tailor-made[13]
multisubstituted benzofurans.
A proposed mechanism of the extended Pummerer
annulation with the acid anhydride A2O is shown in
Scheme 2. First, the KDM 1 is activated with A2O to give
the monocationic intermediate A. Unless an appropriate
combination of A2O and the R group is employed, A readily
products,[1] biologically active molecules,[2] and organic func-
tional molecules.[3] Development of straightforward and
diversity-oriented reactions to construct benzofurans[4]
would therefore have a big impact for pharmaceutical
chemists as well as materials chemists who need libraries for
novel molecules. While recent active investigations on direct
two-component annulations of simple phenols describe an
ideal route,[5] it has still been difficult to achieve modular and
precise syntheses of substituted benzofurans.
We recently reported the synthesis of 3-trifluoromethyl-
benzofurans,[6] which have a convertible alkylthio group[7] at
the 2-position, from simple phenols and perfluoroalkyl-
substituted ketene dithioacetal monoxide (KDM) through
an extended Pummerer annulation (Scheme 1).[8–10] Our
diversity-oriented synthesis of substituted benzofurans was
accomplished through an extended Pummerer annulation/
cross-coupling[11] sequence. Although the annulation is metal-
free, rapid, efficient, and regioselective, only perfluoroalkyl-
substituted KDM is applicable and the scope is thus very
limited. Naturally, we envisioned that the development of
new extended Pummerer annulations, which proceed with
Scheme 2. Proposed mechanism.
[*] Prof. Dr. K. Murakami, Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University
decomposes through the dicationic intermediate A’.[9d] The
intermediate A reacts with a phenol to provide B. Charge-
accelerated [3,3]-sigmatropic rearrangement of B[14] gives C,
which smoothly cyclizes into D. Methanethiol is eliminated
from D to afford the corresponding benzofuran E. Since the
choice of acid anhydride directly affects the stability and
reactivity of A, we first screened acid anhydrides. After the
screening, we found trifluoroacetic anhydride (TFAA) to be
the best activator (see the Supporting Information).
Kitashirakawa, Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: yori@kuchem.kyoto-u.ac.jp
Prof. Dr. K. Murakami
The Hakubi Center for Advanced Research, Kyoto University (Japan)
Prof. Dr. H. Yorimitsu
ACT-C (Japan) Science and Technology Agency (Japan)
[**] This work was supported by Grants-in-Aid from MEXT (Nos.:
24106721 “Reaction Integration” and 25107002 “Science of Atomic
Layers”) and JSPS [Nos.: 25220802 (Scientific Research (S)),
24685007 (Young Scientists (A)), 23655037 (Exploratory Research)].
K.M. acknowledges JSPS Postdoctoral Fellowship.
In contrast to the previous report[6,9a–d] where only CF3-
subtituted KDM could be applied, a wide variety of KDMs
have become applicable with the aid of TFAA (Table 1).
Treatment of 4-tert-butylphenol with 1a in the presence of
TFAA afforded the corresponding benzofuran 2a in 87%
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403288.
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Table 1: Scope with regard to KDM and substituted phenol.
Table 2: Reaction of various hydroxyaromatics.
Entry
R
1: R’
2 or 3
Yield [%]
1
4-tBu
4-tBu
4-tBu
4-tBu
4-tBu
4-tBu
4-tBu
4-tBu
4-tBu
4-tBu
4-tBu
4-Br
4-Bpin
4-CF3
4-MeO
4-EtO2C
4-CHO
4-TIPSO
3-MeO
2-Ph
1a: Ph
1b: H
1c: Me
1d: Cyclohexyl
1e: 4-CF3C6H4
1 f: 4-MeOC6H4
1g: 4-ClC6H4
1h: 1-naphthyl
1i: 3-thienyl
1j: C6F5
2a
2b
2c
2d
2e
2 f
2g
2h
2i
87
67
78
63
95
51
96
78
85
54
61
87
76
60
72
77
60
65
63
70
2[a]
3
4
5
6
7
8
9
10[a,b]
11[a,b]
12
13
14
15[d]
16
17[e]
18[a]
19
20
2j
[c]
1k: CF3
2k
3a
3b
3c
3d
3e
3 f
3g
3h[f]
3i[g]
1a: Ph
1a: Ph
1a: Ph
1a: Ph
1a: Ph
1a: Ph
1a: Ph
1a: Ph
1a: Ph
[a] 1 (1 equiv), TFAA (2 equiv), phenol (2 equiv). [b] 408C, 1.5 h. [c] The
E/Z ratio of 1k was 4:1. [d] 1a (1 equiv), TFAA (1.2 equiv), phenol
(1.2 equiv). [e] 1a (1 equiv), TFAA (2 equiv), phenol (2 equiv), K2CO3
(2 equiv), 1 h. [f] 6-Methoxy-2-methylthio-3-phenylbenzo[b]furan. [g] 2-
Methylthio-3,7-diphenylbenzo[b]furan. TIPS=triisopropylsilyl.
[a] Alcohol (2 equiv), 1 (1 equiv) TFAA (2 equiv). [b] Alcohol (1 equiv),
1 (2 equiv), TFAA (2 equiv). [c] Vanillin (2 equiv), 1a (1 equiv), TFAA
(2 equiv), K2CO3 (2 equiv). [d] Diol (1 equiv), 1 (3 equiv), TFAA (3 equiv).
[e] Diol (2 equiv), 1a (1 equiv), TFAA (1.5 equiv).
yield. The unsubstituted KDM 1b or KDM having other alkyl
groups such as methyl and bulky cyclohexyl groups partici-
pated to give 2b–d in good yields (entries 2–4). The reactions
with various substituents on the aryl groups of the KDMs,
groups such as electron-withdrawing and electron-donating
moieties and halogens, were successful (entries 5–7). Bulky 1-
naphthyl- and heteroaryl-substituted 1h and 1i reacted to
provide 2h and 2i, respectively, in high yields (entries 8 and
9). KDMs having perfluorinated substituents also reacted
similarly (entries 10 and 11). The scope of substituted phenols
was also studied and showed high functional-group compat-
ibility (entries 12–20). Phenols having halogen, boron, elec-
tron-withdrawing, or electron-donating groups smoothly
reacted (entries 12–15). While the reaction of 4-ethoxycarbo-
nylphenol provided 3e with the ester group intact (entry 16),
a formyl group did not tolerate the original reaction
conditions because the resulting trifluoroacetic acid catalyzed
dithioacetalization of the carbonyl moiety with methanethiol.
An addition of K2CO3 suppressed this side reaction to afford
3 f (entry 17). A bulky triisopropylsiloxy group remained
untouched (entry 18). To our delight, the reaction of 3-
methoxyphenol proceeded at the 6-position, the less crowded
ortho position, regioselectively to yield 6-methoxy-2-methyl-
thio-3-phenylbenzo[b]furan (3h) (entry 19). Bulky ortho-
substituted 2-phenylphenol reacted smoothly (entry 20).
The reactions were applied to various hydroxy-substituted
aromatics (Table 2). The reactions of 2-naphthols regioselec-
tively proceeded to give 4a–c in high yield.[15] In the presence
of K2CO3, vanillin participated to furnish the corresponding
benzofuran 4d in 45% yield with concomitant formation of
4d’ in 22% yield. The reaction of 4-[(E)-1-propenyl]phenol
with 1c smoothly proceeded in the presence of TFAA to give
4e in 79% yield. Gratifyingly, the reactions of dihydroxylated
arenes such as resolcinol and naphthalenediols afforded the
corresponding difurans 4 f–h directly in one pot. Since the
solubility of 2,6-naphthodiol in dichloromethane is poor,
employment of an excess amount of 2,6-naphthodiol provided
the corresponding difuran 4h in moderate yield. To demon-
strate the robustness of the extended Pummerer annulation,
the reaction of 2,6-naphthalenediol was performed on
a 5 mmol scale, thus affording 1.4 grams of 4g after simple
filtration though a pad of silica gel and recrystallization from
CH2Cl2/MeOH (1:2).
We applied the Pummerer annulation/coupling strategy
toward the diversity-oriented syntheses of Eupomatenoids
(Scheme 3),[16] which were first isolated from Eupomatia
laurina and have interesting biological activities such as
anticancer[17a] or anti-Trypanosoma cruzi activity.[17b] In the
recent report by Bach and Bartels,[18] the linear syntheses of
eupomatenoids started from the arylation of the bromo group
at the 2-position of 2,3,5-tribromobenzo[b]furan. Since the
eupomatenoid family has variety of aryl groups, installation of
an aryl group at the 2-position should be ideally done at the
late stage of a synthesis. In contrast, our convergent strategy
centers on the synthesis of the core benzofuran (i.e., 4e),
which is then transformed into eupomatenoids. To realize the
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Table 3: Tailor-made synthesis of difurans.[a]
Scheme 3. Syntheses of Eupomatenoids. IPr=1,3-bis(2,6-diisopropyl-
phenyl)-1,3-dihydro-2 H-imidazol-2-ylidene, PEPPSI=pyridine-
enhanced precatalyst preparation stabilization and initiation,
TBDMS=tert-butyldimethylsilyl.
strategy, we developed new Kumada–Tamao–Corriu condi-
tions for bulky thioethers[7,8,19] catalyzed by [IPrNiCl2-
(PPh3)].[20] Treatment of 4e with a Grignard reagent in
toluene at 608C for 12 h in the presence of the [IPrNiCl2-
(PPh3)] catalyst furnished Eupomatenoid-3 (E3) in 79%
yield. Since methoxy moieties are not tolerated by the highly
active Ni/IPr catalyst,[20c] the synthesis of Eupomatenoid-5
(E5) was accomplished under our Negishi conditions.[6] The
reaction of 4e with the 3-methoxy-4-siloxyphenylzinc
reagent[21] provided TBS-protected E5(TBS) in 57% yield,
and was smoothly deprotected with TBAF to give E5 in 96%
yield.
Finally, we performed modular and tailor-made syntheses
of multisubstituted difurans because substituents on the furan
cores significantly alter their photophysical properties.[3,6] Our
iterative Pummerer annulation/cross-coupling strategy leads
to the construction multisubstituted difurans at will (Table 3).
The details of the strategy are as follows. The siloxy-
substituted 2-methylthiofuran 3 or 4 is subjected to our
[IPrNiCl2(PPh3)]-catalyzed Kumada–Tamao–Corriu coupling
with an organomagnesium reagent. The siloxy group on furan
5 is then removed with TBAF to yield 6. The second furan
structure is constructed under the extended Pummerer
annulation conditions with the KDM 1. The resulting difuran
7 is then converted, by the Kumada–Tamao–Corriu or Negishi
reaction, to afford the tetrasubstituted difuran 8. By using this
strategy, we synthesized various multisubstituted difurans
which have four different substituents at the designated
positions. The structure of 8a was confirmed by X-ray
crystallographic analysis (Figure S7 in the Supporting Infor-
mation). Gratifyingly, the resulting multisubstituted furans 8
showed a bright blue fluorescence with high quantum yields
[quantum yield FF = 0.71 (8a), 0.81 (8b), 0.71 (8c), 0.62 (8d);
see the Supporting Information]. Overall, the results clearly
demonstrate the iterative annulation/coupling strategy is
promising for accessing a library of multisubstituted difurans
for materials science.
[a] Reaction conditions: a) [IPrNiCl2(PPh3)] (10 mol%), R1 MgBr/THF
(1m, 2.5 equiv), toluene (0.1m), 1008C, 12 h; b) TBAF/THF (1m,
3 equiv), THF (0.1m), 258C, 3 h; c) 1 (2 equiv), TFAA (2 equiv), CH2Cl2,
258C, 1 h; d1) [IPrNiCl2(PPh3)] (10 mol%), R3 MgBr/THF (1m,
2.5 equiv), toluene (0.1m), 1008C, 12 h; d2) [Pd/PEPPSI-IPr] (10 mol%),
R3ZnI·LiCl/THF[21] (1m, 2.5 equiv), toluene/MeCN (1:1, 0.05m), 608C,
12 h. [b] E/Z ratio of 1d used in this reaction was 3:2. [c] CH2Cl2/MeCN
(5:1, 0.08m) was used as a solvent. TBAF=tert-n-butylammonium
fluoride.
lyzed Kumada–Tamao–Corriu and palladium-catalyzed
Negishi conditions were broadly applicable to the bulky
thioethers. The newly developed strategy was powerful
enough to synthesize highly fluorescent benzofuran mole-
cules and Eupomatenoid natural products.
Experimental Section
Preparation of 2a is representative (Table 1). A dichloromethane
(2 mL) solution of 4-tert-butylphenol (30 mg, 0.20 mmol) and 1a
(51 mg, 0.24 mmol) was placed in a flask. Trifluoroacetic anhydride
(33 mL, 0.24 mmol) was added at 258C, and the resulting mixture was
stirred for 30 min. The mixture was filtered through a pad of alumina
and concentrated in vacuo. Purification by chromatography on silica
gel (n-hexane/CH2Cl2 = 5:1) provided 5-tert-butyl-2-methylthio-3-
phenylbenzo[b]furan (2a, 51 mg, 0.17 mmol, 87%).
In summary, we invented the extended Pummerer annu-
lation/coupling strategy toward diversity-oriented syntheses
of multisubstituted benzofurans. Our improved nickel-cata-
Received: March 13, 2014
Published online: June 12, 2014
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