Cu(I)-catalyzed domino reaction of 3-cyclopropylideneprop-2-en-1-ones

Article abstract of DOI:10.1021/jo302312y

CuCl-catalyzed cyclization-dimerization reactions of 3- cyclopropylideneprop-2-en-1-ones provide an interesting route to benzofuran-7(3aH)-one derivatives with one highly strained three-membered ring and one four-membered ring via intramolecular cycloisom

Full text of DOI:10.1021/jo302312y

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Note
Cu(I)-Catalyzed Domino Reaction of 3-Cyclopropylideneprop-2-en-1-ones
Maozhong Miao, Jian Cao, Jingjing Zhang, Xian Huang, and Luling Wu
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo302312y • Publication Date (Web): 15 Feb 2013
Downloaded from http://pubs.acs.org on February 16, 2013
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Cu(I)-Catalyzed Domino Reaction of 3-Cyclopropylideneprop-2-en-1-ones
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Maozhong Miao†, Jian Cao†, Jingjing Zhang†, Xian Huang†,‡,§ and Luling Wu*,†
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† Department of Chemistry, Zhejiang University, Hangzhou 310028, P. R. China
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‡ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, Shanghai 200032, P. R. China
wululing@zju.edu.cn
CuCl-catalyzed cyclization-dimerization reactions of 3-cyclopropylideneprop-2-en-1-ones provide an
interesting route to benzofuran-7(3aH)-one derivatives with one highly strained three-membered ring
and one four-membered ring via intramolecular cycloisomerization, sequential bimolecular [4+2]
cycloaddition, opening of the oxa-bridge and ring-contraction. Furthermore, the reaction was monitored
by NMR experiments to unveil some key intermediates.
Furans are important five-membered heterocycles in non-natural and natural organic compounds.1 They
are also important intermediates in synthetic transformations.2 In addition, benzofuran-7(3aH)-one and
its analogs are present in bioactive natural products,3 pharmaceuticals4 and intermediates for synthesis
of natural products5 (Figure 1). Some methods have been well developed for the synthesis of the
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benzofuran-7(3aH)-one core;6 however, the development of conceptually different synthetic approaches
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is still of great interest.
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Figure 1. Bioactive compounds containing a common motif that consists of a benzofuran-7(3aH)-
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one core
In recent years, increasing attention has been paid to metal-catalyzed cyclization/1,2-migration
domino methodology which provides rapid access to complex molecular frameworks.7 Specifically,
cyclization of allenyl ketones via 1,2-migration of various groups is an efficient approach for the
assembly of the furan ring.8 Marshall and Hashmi have shown an efficient approach for the assembly of
the furan ring via a formal 1,2-hydrogen shift of allenyl ketones (eq 1, Scheme 1).8b-d Gevorgyan has
reported the metal-catalyzed cyclization of allenyl ketones with 1,2-alkyl migration as a key step in the
formation of highly substituted furans (eq 1, Scheme 1).8e In this regard, we have reported a PdCl2-
catalyzed oxidative cycloisomerization of 3-cyclopropylideneprop-2-en-1-ones which can be prepared
from substituted ethynylcyclopropane according to the known procedure (Scheme 2), providing a facile
synthesis of highly strained functionalized 2-alkylidenecyclobutanones (eq 2, Scheme 1).9 As a
continuing exploration of the synthetic utility of 3-cyclopropylideneprop-2-en-1-ones, herein we wish to
disclose our unexpected observation of a copper-catalyzed cyclization-dimerization reaction of 3-
cyclopropylideneprop-2-en-1-ones, which resulted in a complex molecular structure of benzofuran-
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7(3aH)-ones with one highly strained three-membered ring and one four-membered ring (eq 3, Scheme
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1).
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Scheme 1. Transition metal catalyzed cycloisomerization of 1,2-allenylketones and 3-
cyclopropylideneprop-2-en-1-ones 1
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Scheme 2. The procedures for the synthesis of starting materials
In a preliminary experiment, we were successfully able to convert 3-cyclopropylideneprop-2-en-1-one
1a into the 2-alkylidenecyclobutanone upon treatment with 10% PdCl2 as the catalyst and 2.5 equiv
Dess-Martin periodinane (DMP) as the oxidant in the open air.9 Next, we observed that the reaction of
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1a in the presence of 10 mol % CuI in THF at 50 C in a N2 atmosphere gave a structurally very
different complex product. Finally, careful examination of the single crystal X-ray diffraction study of
product 3c10 (Table 2, entry 3) and 3i11 (Table 2, entry 9) revealed that the structurally very different
complex product is the benzofuran-7(3aH)-one with a highly strained spiro-three-membered ring at the
6-position and a highly strained four-membered ring on the bridge (Figure 2). Both three-membered
ring and four-membered ring are useful functional groups because of the inherent ring strains.
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Figure 2. ORTEP representation of 3c (top) and 3i (bottom)
With these encouraging results, a further study on optimizing the reaction conditions for the selective
formation of 3a was immediately undertaken. We first investigated the effect of metal salts on the
reaction. The best result was obtained when 10 mol % CuCl was used as the catalyst, and 3a could be
obtained in 87% yield (Table 1, entry 3). Examination of the solvent effects indicated that THF is most
suitable (Table 1, entry 3). Further experiments showed that the temperature had a dramatic effect on the
reaction (Table 1, entries 9-10). Finally, we were able to define the best conditions for this
transformation: the reaction in THF at 50 oC using 10 mol % CuCl as the catalyst (Table 1, entry 3).
Table 1. Reaction conditions optimization for the formation of 3aa
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Entry
MXn (10 mol %)
CuI
Temp (°C)
Solvent
THF
Yield of 3a (%)b
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83
87
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66
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86
CuBr
THF
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CuCl
50
THF
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AgOTf
PdCl2
50
THF
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50
THF
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AgNO3
CuCl
50
THF
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50
toluene
CH3CN
THF
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CuCl
50
9
CuCl
rt
10
CuCl
reflux
THF
a Unless otherwise specified, the reaction was carried out using 1a (0.15 mmol) in 3 mL of solvent in a
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N2 atmosphere. b Isolated yields.
Inspired by these results, derivatives of 1 in which R1 or R2 are varied as shown in Table 2. The nature
and position of substituents on the aromatic R1 or R2 have a limited effect on this reaction. Interestingly,
o
when we examined the reaction of 1m by treating with 10 mol % CuCl at 50 C, furan-fused
cyclobutene 2m can be obtained in 91% yield by flash chromatography on neutral Al2O3 (Table 2, entry
13). However, no dimer was isolated from the reaction of 1m in the presence of CuCl even after 10
hours heating at 50 oC.
Table 2. The scope of the reaction for the formation of 3a
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1
Yield of 3
(%)b
R1
4-MeC6H4
Ph
R2
Ph (1a)
Entry
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9
87 (3a)
83 (3b)
61 (3c)
91 (3d)
64 (3e)
86 (3f)
63 (3g)
78 (3h)
93 (3i)
86 (3j)c
85 (3k)
78 (3l)
91 (2m)d
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2
Ph (1b)
3
4-FC6H4
4-MeOC6H4
3,4,5-(MeO)3C6H2
2-furyl
2-thienyl
4-MeC6H4
Ph
Ph (1c)
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Ph (1d)
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Ph (1e)
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Ph (1f)
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Ph (1g)
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4-MeC6H4 (1h)
3-MeC6H4 (1i)
4-MeOC6H4 (1j)
4-FC6H4 (1k)
2-naphthyl (1l)
Ph (1m)
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Ph
Ph
Ph
i-Bu
a Unless otherwise specified, the reaction was carried out using 1 (0.15 mmol) in presence of 10 mol %
CuCl in 3 mL of THF at 50 oC in a N2 atmosphere. b Isolated yields. c When the reaction was carried out
using 1j in 1 mmol, 276 mg scale catalyzed by 10% CuCl, the product 3j was obtained in 89% yield
d
(246 mg). The reaction was carried out for 10 min and the structure of 2m is 3-isobutyl-4-phenyl-2-
oxabicyclo[3.2.0]hepta-1(5),3-diene.
The reaction was slow at room temperature when we used the CuCl as the catalyst. In order to
elucidate the mechanism, we conducted the reaction of 1a in the presence of 10 mol % AgNO3 instead
of 10 mol % CuCl as the catalyst in CDCl3 under N2 atmosphere. This reaction was then monitored by
NMR experiments to detect the formation of any intermediates (Scheme 3). Initially, the reaction gave
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rise to two sets of CH2 signals Nos. 6 and 5 at 3.39 (t, J = 2.8 Hz, 2H), 3.01 (t, J = 2.8 Hz, 2H) together
with one methyl resonance signal No. 4 at 2.33 ppm (3H, s) in the high field region (Figure S1 b, c), and
these two sets of characteristic CH2 shifts 6, 5 are similar to the furan-fused cyclobutene 2m (for the 1H
NMR spectra of 2m see Supporting Information). This result indicates that 1a was converted to furan-
fused cyclobutene 2a at first. Subsequent transformation of 2a at room temperature gave the [4+2]
intermediate 4a, which was identified by the four sets of characteristic CH2 signals in the two four-
membered rings Nos. 9, 10, 11 and 12 at 3.32-3.45 (m, 1H), 3.07-3.21 (m, 1H), 2.84-3.06 (m, 2H), 2.55-
2.75 (m, 2H), 2.36-2.51 (m, 1H), 2.04-2.21 ppm (m, 1H) and two methyl signals Nos. 7 and 8 at 2.27
(3H, s), 2.24 ppm (3H, s) in the high field region (Figure S1 d-g). Meanwhile, the structure of [4+2]
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intermediate 4a was further supported by C NMR, DEPT 135 (Figure S1 h, i) and ESI-MS spectra
(see Supporting Information). Finally, the transformation of bimolecular 4a to 3a at 50 oC (Figure S1 j,
k) indicated that the [4+2] product 4a was the key intermediate for this reaction.
Scheme 3. Controlled experiment in the NMR tube.
On the basis of the above results, we proposed a plausible pathway for the formation of product 3a as
shown in Scheme 4. At first, CuCl may activate the relatively electron-rich C3-C4 double bond and
trigger the nucleophilic attack of the carbonyl oxygen at the C4 atom of the allenone moiety to form the
spirocyclic oxonium salt X. The latter intermediate would evolve into the Cu(I) carbenoid Y, and
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subsequent bond cleavage followed by elimination of the metal would provide the intermediate
compound 2a. The next dimerization of intermediate 2a via [4+2] cycloaddition may afford the
bimolecular intermediate 4a. Subsequent cleavage of the oxa-bridge catalyzed by CuCl gives the allylic
cationic mesomeric forms H and I. The ring-contraction reaction of the cyclobutyloxyl anion I furnishes
the product 3a and regenerates the catalyst CuCl.
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Scheme 4. Plausible mechanism for the formation of 6,7a-dihydrobenzofuran-7(3aH)-one
derivatives 3a
Both three-membered ring and four-membered ring have shown interesting reactivity in organic
synthesis. Considering the readily available bifuctional (three-membered ring and four-membered ring)
benzofuran-7(3aH)-one derivatives 3 with our protocol, thus, we observed an interesting route to
substituted 2,3-dihydrobenzofuro[7,6-b]furan 5j from 3j in 80% yield at elevated temperature (Scheme
5). The overall sequence of reactions can be described as proceeding by an initial ethylene elimination
and a subsequent Cloke-Wilson cyclopropyl ketone rearrangement.12 The facility of the process might
be related to the aromaticity gained in the final step.
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Scheme 5. Synthesis of tricyclic compound 5j
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In conclusion, a new copper (I) catalyzed dimerization pathway of 3-cyclopropylideneprop-2-en-1-
ones into spirocyclic-oxa-[4,3,2]propellanes containing benzofuran-7(3aH)-one core has been revealed.
In this process, the intermediate furan-fused cyclobutenes demonstrate a new type of reactivity; it
involves the unusual [4+2] cycloaddition which can present as diene and dienophile in one reaction. The
reaction is accompanied by the formation of bridged four-menbered ring and spiro-three-membered ring.
Also, the propellane scaffolding can be easily transformed to substituted 2,3-dihydrobenzofuro[7,6-
b]furan in high yield.
1. Experimental Procedures
1.1 General Methods
1
Melting points are uncorrected. H and 13C NMR spectra were recorded at 400 and 100 MHz
respectively using tetramethylsilane as the internal standard. Chemical shifts are expressed in ppm and J
values are given in Hz. Organic solvents used were dried by standard methods when necessary. The
THF and toluene were distilled from sodium-benzophenone; The DCM and CH3CN were distilled from
CaH2. Commercially obtained available reagents were used without further purification. Petroleum ether
refers to the fraction with boiling point in the range 60~90 °C. All reactions were monitored by TLC
with GF 254 silica gel coated plates. Flash column chromatography was carried out using 300-400 mesh
silica gel or 200-300 mesh Al2O3 at increased pressure.
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1.2 Procedure for Synthesis of 3a-3l
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(1) 2,4-Bis(4-methylphenyl)-3a,7a-ethylene-3,5-diphenyl-3aH-spiro[benzofuran-6,1'-cyclopropan]-
7(7aH)-one (3a)
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Typical procedure: Under an atmosphere of dry nitrogen, CuCl (1.5 mg, 0.015 mmol, 10 mol %) was
added to a solution of 1a (39 mg, 0.150 mmol) in 3 mL of anhydrous THF at 50 oC. After being stirred
for 10-12 hours (monitored by TLC), the mixture was quenched with 5 mL of water and extracted with
EtOAc (3 × 10 mL). The combined organic layer was dried over anhydrous MgSO4. After filtration and
removal of the solvent in vacuo, the residue was purified with flash silica gel chromatography
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(petroleum ether/ethyl acetate 15:1 v/v) to afford 3a (34 mg, 87%) as a white solid: M.p. 179-180 C
1
(petroleum ether/ethyl acetate); H NMR (400 MHz, CDCl3): δ = 7.20 (d, J = 8.0 Hz, 2H), 7.11 (t, J =
7.2 Hz, 1H), 6.89-7.06 (m, 6H), 6.82 (t, J = 8.0 Hz, 2H), 6.72-6.76 (m, 2H), 6.69 (d, J = 7.2 Hz, 1H),
6.29-6.35 (m, 4H), 3.04-3.15 (m, 1H), 2.85-2.99 (m, 2H), 2.73-2.83 (m, 1H), 2.24 (s, 3H), 1.96 (s, 3H),
1.74-1.82 (m, 1H), 1.45-1.53 (m, 1H), 1.08-1.16 (m, 1H), 0.88-0.96 (m, 1H); 13C NMR (100 MHz,
CDCl3): δ = 17.8, 18.9, 20.8, 21.2, 30.5, 30.8, 34.4, 61.8, 83.8, 117.0, 125.5, 126.3, 126.9, 127.2, 127.5,
127.6, 128.2, 128.4, 129.5, 130.3, 130.6, 130.8, 134.7, 135.0, 135.7, 137.1, 138.2, 138.2, 151.4, 205.4;
IR (neat): 1698, 1626, 1510, 1084, 900, 825, 764, 701 cm-1; MS (70 eV, EI) m/z: 520 (M+), 492 (100);
TOF HRMS (EI): calcd for C38H32O2 (M+): 520.2402. Found, 520.2407.
The following compounds 3b-3l were prepared similarly.
(2) 3a,7a-Ethylene-2,3,4,5-tetraphenyl-3aH-spiro[benzofuran-6,1'-cyclo- propan]-7(7aH)-one (3b)
The reaction of 1b (40 mg, 0.163 mmol) and CuCl (1.5 mg, 0.015 mmol, 10 mol %) at 50 oC afforded
3b (33 mg, 83%) as white solid: M.p. 177-179 oC (petroleum ether/ethyl acetate); 1H NMR (400 MHz,
CDCl3): δ = 7.32 (d, J = 7.6 Hz, 2H), 7.06-7.21 (m, 4H), 6.86-7.06 (m, 4H), 6.83 (t, J = 7.2 Hz, 2H),
6.77 (d, J = 8.0 Hz, 2H), 6.58-6.70 (m, 2H), 6.55 (t, J = 7.2 Hz, 2H), 6.46 (d, J = 7.6 Hz, 2H), 3.08-3.20
(m, 1H), 2.90-3.02 (m, 2H), 2.75-2.88 (m, 1H), 1.75-1.84 (m, 1H), 1.47-1.55 (m, 1H), 1.10-1.20 (m,
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1H), 0.90-1.00 (m, 1H); 13C NMR (100 MHz, CDCl3): δ = 18.0, 19.0, 30.6, 30.7, 34.4, 61.6, 83.9,
117.7, 125.4, 126.0, 126.4, 126.5, 126.9, 127.6, 127.7, 127.7, 127.8, 128.4, 129.7, 130.2, 130.5, 130.7,
131.1, 134.6, 135.6, 136.8, 138.1, 138.8, 151.4, 205.2; IR (neat): 1687, 1638, 1491, 1442, 1072, 764,
694 cm-1; MS (70 eV, EI) m/z: 492 (M+), 464 (100); TOF HRMS (EI): calcd for C36H28O2 (M+):
492.2089. Found, 492.2086.
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(3) 2,4-Bis(4-fluorophenyl)-3a,7a-ethylene-3,5-diphenyl-3aH-spiro[benzofuran-6,1'-cyclopropan]-
7(7aH)-one (3c)
The reaction of 1c (41 mg, 0.155 mmol) and CuCl (1.5 mg, 0.015 mmol, 10 mol %) at 50 oC afforded
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1
3c (25 mg, 61%) as white solid: M.p. 208-209 C (petroleum ether/ethyl acetate); H NMR (400 MHz,
CDCl3): δ = 7.30 (dd, J = 8.4 Hz, 5.6 Hz, 2H), 7.12 (t, J = 7.6 Hz, 1H), 6.93-7.05 (m, 4H), 6.79-6.91 (m,
4H), 6.75 (d, J = 7.2 Hz, 2H), 6.65 (d, J = 7.6 Hz, 1H), 6.41 (dd, J = 8.4 Hz, 6.0 Hz, 2H), 6.23 (t, J = 8.8
Hz, 2H), 3.09-3.21 (m, 1H), 2.87-3.02 (m, 2H), 2.75-2.88 (m, 1H), 1.77-1.86 (m, 1H), 1.46-1.55 (m,
13
1H), 1.10-1.19 (m, 1H), 0.92-1.01 (m, 1H); C NMR (100 MHz, CDCl3): δ = 17.9, 19.5, 30.7, 34.7,
2
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61.5, 83.9, 113.4 (d, JC-F = 21 Hz), 114.8 (d, JC-F = 21 Hz), 117.1, 126.3, 126.6, 127.1, 127.1, 127.8,
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Hz), 136.2, 136.6, 137.0, 150.6, 160.6 (d, JC-F = 243 Hz), 162.5 (d, JC-F = 247 Hz), 204.9; IR (neat):
1702, 1638, 1506, 1226, 1081, 1063, 902, 844, 700 cm-1; MS (70 eV, EI) m/z: 528 (M+), 500 (100);
TOF HRMS (EI): calcd for C36H26O2F2 (M+): 528.1901. Found, 528.1899.
(4) 2,4-Bis(4-methoxyphenyl)-3a,7a-ethylene-3,5-diphenyl-3aH-spiro[benzofuran-6,1'-
cyclopropan]-7(7aH)-one (3d)
The reaction of 1d (43 mg, 0.156 mmol) and CuCl (1.5 mg, 0.015 mmol, 10 mol %) at 50 oC afforded
3d (39 mg, 91%) as white solid: M.p. 178-179 oC (petroleum ether/ethyl acetate); 1H NMR (400 MHz,
CDCl3): δ = 7.22-7.28 (m, 2H), 7.12 (t, J = 6.8 Hz, 1H), 6.90-7.06 (m, 4H), 6.87 (t, J = 7.2 Hz, 2H),
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6.76 (d, J = 8.0 Hz, 2H), 6.63-6.71 (m, 3H), 6.35 (d, J = 8.0 Hz, 2H), 6.09 (d, J = 8.0 Hz, 2H), 3.72 (s,
3H), 3.53 (s, 3H), 3.04-3.16 (m, 1H), 2.84-2.99 (m, 2H), 2.72-2.83 (m, 1H), 1.73-1.84 (m, 1H), 1.44-
1.54 (m, 1H), 1.07-1.17 (m, 1H), 0.89-0.97 (m, 1H); 13C NMR (100 MHz, CDCl3): δ = 17.9, 19.1, 30.6,
30.8, 34.4, 54.9, 55.1, 61.7, 83.8, 112.1, 113.1, 116.0, 123.7, 125.8, 126.4, 126.9, 127.6, 129.1, 130.3,
130.6, 130.7, 130.8, 131.3, 135.1, 135.2, 137.1, 137.8, 151.1, 157.1, 159.5, 205.5; IR (neat): 1685,
1605, 1509, 1246, 1178, 1094, 1028, 909, 833, 734, 716 cm-1; MS (70 eV, EI) m/z: 552 (M+), 524
(100); TOF HRMS (EI): calcd for C38H32O4 (M+): 552.2301. Found, 552.2296.
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(5) 3a,7a-Ethylene-3,5-diphenyl-2,4-bis(3,4,5-trimethoxy- phenyl)-3aH-spiro[benzofuran-6,1'-
cyclopropan]-7(7aH)-one (3e)
The reaction of 1e (50 mg, 0.149 mmol) and CuCl (1.5 mg, 0.015 mmol, 10 mol %) at 50 oC afforded
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3e (32 mg, 64%) as white solid: M.p. 177-178 C (petroleum ether/ethyl acetate); H NMR (400 MHz,
CDCl3): δ = 7.16-7.24 (m, 2H), 7.07 (t, J = 7.2 Hz, 1H), 6.96-7.04 (m, 4H), 6.86-6.91 (m, 2H), 6.66 (d,
J = 7.2 Hz, 1H), 6.54 (s, 2H), 5.76 (s, 2H), 3.77 (s, 3H), 3.60 (s, 3H), 3.51 (s, 6H), 3.38 (s, 6H), 3.11-
3.22 (m, 1H), 2.97-3.10 (m, 1H), 2.85-2.96 (m, 1H), 2.72-2.84 (m, 1H), 1.82-1.92 (m, 1H), 1.45-1.55
(m, 1H), 1.20-1.25 (m, 1H), 0.94-1.03 (m, 1H); 13C NMR (100 MHz, CDCl3): δ = 17.9, 20.0, 30.9, 31.0,
33.9, 55.5, 55.6, 60.4, 60.7, 61.5, 83.5, 104.6, 107.5, 117.1, 126.0, 126.5, 126.7, 126.8, 127.6, 128.2,
130.5, 130.6, 134.1, 135.0, 135.5, 135.7, 137.0, 137.5, 137.9, 150.6, 151.4, 152.3, 205.0; IR (neat):
1708, 1575, 1504, 1411, 1350, 1241, 1121, 1001, 833, 700 cm-1; MS (70 eV, EI) m/z: 672 (M+), 644
(100); TOF HRMS (EI): calcd for C42H40O8 (M+): 672.2723. Found, 672.2726.
(6) 2,4-Di(furan-2-yl)-3a,7a-ethylene-3,5-diphenyl-3aH-spiro[benzofuran-6,1'-cyclopropan]-
7(7aH)-one (3f)
The reaction of 1f (35 mg, 0.148 mmol) and CuCl (1.5 mg, 0.015 mmol, 10 mol %) at 50 oC afforded
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CDCl3): δ = 7.24-7.42 (m, 4H), 7.06-7.17 (m, 4H), 6.90-7.05 (m, 3H), 6.41 (s, 1H), 6.23 (t, J = 1.6 Hz,
1H), 5.98 (d, J = 7.2 Hz, 1H), 5.63 (t, J = 1.6 Hz, 1H), 4.65 (d, J = 7.2 Hz, 1H), 3.01-3.12 (m, 1H), 2.78-
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2.98 (m, 2H), 2.62-2.75 (m, 1H), 1.55-1.64 (m, 2H), 1.15-1.23 (m, 1H), 0.89-0.95 (m, 1H); C NMR
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(100 MHz, CDCl3): δ = 16.2, 20.6, 29.3, 30.9, 33.6, 61.1, 84.2, 109.7, 110.1, 110.2, 110.9, 118.2, 126.4,
126.5, 127.6, 127.7, 128.1, 129.0, 129.4, 129.8, 134.0, 134.6, 137.5, 139.6, 142.5, 144.1, 145.7, 151.0,
204.0; IR (neat): 1698, 1163, 1090, 1007, 977, 867, 745, 702 cm-1; MS (70 eV, EI) m/z: 472 (M+), 444
(100); TOF HRMS (EI): calcd for C32H24O4 (M+): 472.1675. Found, 472.1677.
(7) 3a,7a-Ethylene-3,5-diphenyl-2,4-di(thiophen-2-yl)-3aH-spiro[benzofuran-6,1'-cyclopropan]-
7(7aH)-one (3g)
The reaction of 1g (38 mg, 0.151 mmol) and CuCl (1.5 mg, 0.015 mmol, 10 mol %) at 50 oC afforded
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3g (24 mg, 63%) as white solid: M.p. 176-177 C (petroleum ether/ethyl acetate); H NMR (400 MHz,
CDCl3): δ = 7.04-7.32 (m, 8H), 6.96-7.04 (m, 3H), 6.86 (dd, J = 8.4 Hz, 4.4 Hz, 2H), 6.63 (d, J = 4.8
Hz, 1H), 6.17 (t, J = 4.4 Hz, 1H), 5.98 (d, J = 3.6 Hz, 1H), 2.99-3.08 (m, 1H), 2.88-2.98 (m, 1H), 2.70-
2.88 (m, 2H), 1.55-1.70 (m, 2H), 1.19-1.29 (m, 1H), 0.84-0.91 (m, 1H); 13C NMR (100 MHz, CDCl3): δ
= 17.1, 20.3, 29.2, 31.3, 33.4, 62.4, 84.1, 117.1, 124.6, 125.0, 126.5, 126.6, 126.8, 127.0, 127.4, 127.5,
128.1, 128.2, 128.5, 130.2, 130.5, 130.8, 132.8, 134.0, 136.3, 136.9, 139.5, 147.5, 204.3; IR (neat):
1701, 1638, 1248, 1197, 1082, 694 cm-1; MS (70 eV, EI) m/z: 504 (M+), 476 (100); TOF HRMS (EI):
calcd for C32H24O2S2 (M+): 504.1218. Found, 504.1223.
(8) 2,3,4,5-Tetrakis(4-methylphenyl)-3a,7a-ethylene-3aH-spiro[benzofuran-6,1'-cyclopropan]-
7(7aH)-one (3h)
The reaction of 1h (41 mg, 0.150 mmol) and CuCl (1.5 mg, 0.015 mmol, 10 mol %) at 50 oC afforded
3h (32 mg, 78%) as white solid: M.p. 172-173 oC (petroleum ether/ethyl acetate); 1H NMR (400 MHz,
CDCl3): δ = 7.23 (d, J = 8.0 Hz, 2H), 6.86-6.97 (m, 4H), 6.76 (d, J = 7.6 Hz, 1H), 6.53-6.67 (m, 5H),
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6.33 (q, J = 8.0 Hz, 4H), 3.01-3.12 (m, 1H), 2.82-2.98 (m, 2H), 2.70-2.80 (m, 1H), 2.24 (s, 3H), 2.19 (s,
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1H); C NMR (100 MHz, CDCl3): δ = 17.8, 18.9, 20.8, 21.0, 21.1, 21.2, 30.5, 30.9, 34.4, 61.8, 83.7,
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117.0, 127.2, 127.6, 127.6, 128.3, 128.4, 128.4, 128.5, 130.0, 130.4, 130.6, 132.0, 134.1, 134.6, 134.8,
135.2, 135.8, 136.0, 138.1, 138.1, 150.9, 205.6; IR (neat): 1705, 1509, 1339, 1071, 1036, 893, 812, 721
cm-1; MS (70 eV, EI) m/z: 548 (M+), 520 (100); TOF HRMS (EI): calcd for C40H36O2 (M+): 548.2715.
Found, 548.2712.
(9) 3,5-Bis(3-methylphenyl)-3a,7a-ethylene-2,4-diphenyl-3aH-spiro[benzofuran-6,1'-cyclopropan]-
7(7aH)-one (3i)
The reaction of 1i (40 mg, 0.154 mmol) and CuCl (1.5 mg, 0.015 mmol, 10 mol %) at 50 oC afforded
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3i (37 mg, 93%) as white solid: M.p. 170-172 C (petroleum ether/ethyl acetate); H NMR (400 MHz,
CDCl3): δ = 7.34 (d, J = 7.2 Hz, 2H), 6.42-7.20 (m, 16H), 3.08-3.20 (m, 1H), 2.88-3.01 (m, 2H), 2.74-
2.86 (m, 1H), 2.13 (d, J = 71.2 Hz, 3H), 1.96 (s, 3H), 1.73-1.84 (m, 1H), 1.45-1.55 (m, 1H), 1.11-1.20
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(m, 1H), 0.91-1.00 (m, 1H); C NMR (100 MHz, CDCl3): δ = 18.1, 19.1, 19.3, 21.1, 21.3, 30.5, 30.6,
30.7, 34.5, 61.5, 83.8, 117.8, 125.4, 126.2, 126.7, 126.9, 127.1, 127.1, 127.3, 127.4, 127.5, 127.6, 127.7,
127.7, 127.8, 128.3, 129.6, 130.9, 131.2, 131.2, 131.4, 134.4, 135.5, 135.6, 136.3, 136.7, 137.0, 137.8,
137.8, 138.9, 151.0, 205.4; IR (neat): 1701, 1494, 1447, 1242, 1090, 900, 768, 696, 667 cm-1; MS (70
eV, EI) m/z: 520 (M+), 492 (100); TOF HRMS (EI): calcd for C38H32O2 (M+): 520.2402. Found,
520.2404.
(10) 3,5-Bis(4-methoxyphenyl)-3a,7a-ethylene-2,4-diphenyl-3aH-spiro[benzofuran-6,1'-
cyclopropan]-7(7aH)-one (3j)
The reaction of 1j (42 mg, 0.152 mmol) and CuCl (1.5 mg, 0.015 mmol, 10 mol %) at 50 oC to afford
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3j (36 mg, 86%) as white solid: M.p. 159-160 C (petroleum ether/ethyl acetate); H NMR (400 MHz,
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CDCl3): δ = 7.35 (d, J = 6.8 Hz, 2H), 7.10-7.21 (m, 3H), 6.93 (d, J = 8.4 Hz, 1H), 6.53-6.71 (m, 7H),
6.43-6.51 (m, 3H), 6.38 (d, J = 8.4 Hz, 2H), 3.67 (s, 3H), 3.64 (s, 3H), 3.03-3.15 (m, 1H), 2.86-3.00 (m,
2H), 2.72-2.83 (m, 1H), 1.73-1.83 (m, 1H), 1.45-1.55 (m, 1H), 1.09-1.19 (m, 1H), 0.89-0.98 (m, 1H);
13C NMR (100 MHz, CDCl3): δ = 17.9, 19.1, 30.5, 31.0, 34.3, 54.9, 55.1, 61.4, 83.6, 112.4, 112.9,
113.3, 117.3, 125.2, 126.6, 126.9, 127.6, 127.7, 128.2, 129.1, 129.7, 131.1, 131.7, 131.7, 135.1, 138.3,
139.1, 150.8, 157.8, 157.8, 205.5; IR (neat): 1702, 1604, 1509, 1284, 1242, 1175, 1031, 907, 728, 696
cm-1; MS (70 eV, EI) m/z: 552 (M+), 524 (100); TOF HRMS (EI): calcd for C38H32O4 (M+): 552.2301.
Found, 552.2297.
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(11) 3,5-Bis(4-fluorophenyl)-3a,7a-ethylene-2,4-diphenyl-3aH-spiro[benzofuran-6,1'-
cyclopropan]-7(7aH)-one (3k)
The reaction of 1k (40 mg, 0.152 mmol) and CuCl (1.5 mg, 0.015 mmol, 10 mol %) at 50 oC afforded
3k (34 mg, 85%) as white solid: M.p. 163-164 oC (petroleum ether/ethyl acetate); 1H NMR (400 MHz,
CDCl3): δ = 7.31 (d, J = 6.8 Hz, 2H), 7.11-7.23 (m, 3H), 6.97-7.05 (m, 1H), 6.83 (t, J = 8.4 Hz, 1H),
6.71 (dt, J = 8.6 Hz, 5.2 Hz, 3H), 6.62 (t, J = 7.6 Hz, 4H), 6.51 (t, J = 8.8 Hz, 2H), 6.45 (d, J = 6.8 Hz,
2H), 3.05-3.16 (m, 1H), 2.90-3.03 (m, 2H), 2.74-2.87 (m, 1H), 1.78-1.88 (m, 1H), 1.47-1.56 (m, 1H),
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127.8, 128.6, 129.6, 130.5 (d, 4JC-F = 3 Hz), 130.8, 131.6 (d, 3JC-F = 7 Hz), 132.2 (d, 3JC-F = 8 Hz), 132.6
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(d, JC-F = 3 Hz), 134.6, 138.6, 138.7, 151.7, 161.3 (d, JC-F = 245 Hz), 204.8; IR (neat): 1705, 1599,
1507, 1225, 1069, 902, 834, 766, 694 cm-1; MS (70 eV, EI) m/z: 528 (M+), 500 (100); TOF HRMS (EI):
calcd for C36H26O2F2 (M+): 528.1901. Found, 528.1897.
(12) 3a,7a-Ethylene-3,5-di(naphthalen-2-yl)-2,4-diphenyl-3aH-spiro[benzofuran-6,1'-
cyclopropan]-7(7aH)-one (3l)
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3l (35 mg, 78%) as white solid: M.p. 141-142 C (petroleum ether/ethyl acetate); H NMR (400 MHz,
CDCl3): δ = 7.53-7.74 (m, 3H), 6.76-7.51 (m, 16H), 6.49 (d, J = 7.2 Hz, 2H), 6.10-6.26 (m, 3H), 3.20-
3.40 (m, 1H), 2.99-3.17 (m, 2H), 2.80-2.96 (m, 1H), 1.75-1.91 (m, 1H), 1.45-1.57 (m, 1H), 1.06-1.35
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(m, 1H), 0.80-1.16 (m, 1H); C NMR (100 MHz, CDCl3): δ = 17.2, 18.8, 19.0, 19.8, 30.4, 30.9, 31.0,
34.4, 35.0, 61.6, 61.8, 83.7, 84.2, 117.4, 117.6, 125.2, 125.3, 125.7, 125.7, 125.8, 126.3, 126.4, 126.5,
127.1, 127.2, 127.4, 127.6, 127.7, 127.7, 127.8, 127.9, 128.4, 128.5, 128.6, 128.9, 129.0, 129.3, 129.6,
131.0, 131.0, 131.7, 131.8, 132.1, 132.2, 132.6, 133.0, 134.2, 134.4, 135.2, 135.4, 138.2, 138.5, 138.6,
138.7, 151.7, 151.8, 205.1, 205.2; IR (neat): 1705, 1495, 1339, 1069, 1020, 894, 855, 819, 744, 695 cm-
1; MS (70 eV, EI) m/z: 592 (M+), 564 (100); TOF HRMS (EI): calcd for C44H32O2 (M+): 592.2402.
Found, 592.2405.
1.3 Syntheis of furan-fused cyclobutenes 2m from 1m in the presence of 10 mol % CuCl
Under an atmosphere of dry nitrogen, CuCl (1.5 mg, 0.015 mmol, 10 mol %) was added to a solution
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of 1m (34 mg, 0.15 mmol) in 3 mL of dry THF at 50 C. After being stirred for 10 min (monitored by
TLC), filtration and removal of the solvent in vacuo, flash chromatography on Al2O3 (petroleum ether)
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afforded 2m in 91% yield as yellow oil: H NMR (400 MHz, DMSO-d6): δ = 7.35-7.42 (m, 4H), 7.20-
7.30 (m, 1H), 3.29-3.34 (m, 2H), 2.95-2.30 (m, 2H), 2.71 (d, J = 2.8 Hz, 2H), 1.92-2.05 (m, 1H), 0.88 (d,
J = 6.8 Hz, 6H); 13C NMR (100 MHz, DMSO-d6): δ = 22.2, 24.7, 28.1, 34.0, 36.3, 120.1, 122.9, 126.3,
127.1, 128.7, 133.1, 149.6, 153.6. Known compound see Rf. 9.
1.4 Syntheis of 4,6-bis(4-methoxyphenyl)-5,7-diphenyl-2,3-dihydrobenzofuro[7,6-b]furan 5j
Under an atmosphere of dry nitrogen, a solution of 3j (35 mg, 0.06 mmol) in 2 mL of DMF was
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stirred at 160 C for 24 hour (monitored by TLC). The mixture was quenched with 5 mL of water and
extracted with EtOAc (3 × 10 mL). The combined organic layer was dried over anhydrous MgSO4.
After filtration and removal of the solvent in vacuo, the residue was purified with flash silica gel
chromatography (petroleum ether/ethyl acetate 15:1 v/v) to afford 5j (28 mg, 80%) as a white solid:
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127.8, 128.1, 129.7, 130.8, 131.09, 131.12, 131.3, 131.9, 133.2, 136.9, 137.7, 142.1, 151.0, 157.7, 158.2
; IR (neat): 2957, 2835, 1607, 1512, 1454, 1413, 1381, 1337, 1285, 1243, 1179, 1148, 1098, 1033 cm-1;
MS (70 eV, EI) m/z: 524 (M+), 524 (100); TOF HRMS (EI): calcd for C36H28O4 (M+): 524.1988.
Found, 524.1986.
Notes
§ Professor Huang passed away on March 6, 2010. He was fully in charge of this project. Professor
Luling Wu is helping to finish all the projects with assistance from Professor Shengming Ma.
ACKNOWLEDGMENT:
We are grateful to the National Natural Science Foundation of China (Project Nos. 20872127, 20732005,
and J0830431) and National Basic Research Program of China (973 Program, 2009CB825300) and
CAS Academician Foundation of Zhejiang Province and the Fundamental Research Funds for the
Central Universities for financial support.
Supporting Information.
Copies of 1H and 13C NMR spectra for 3, 2m, X-ray crystallographic data (CIF file) for 3c and 3i. This
material is available free of charge via the Internet at http://pubs.acs.org.
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(10) X-ray crystal data for 3c: C36H26F2O2; M=528.57; crystal system: triclinic; space group: P-1; final
R indices [I > 2σ(I)] R1=0.0558, wR2=0.1508, R indices (all data) R1=0.0659, wR2=0.1600;
a=7.8825(8) Å, b=10.8228(11) Å, c=15.9897(17) Å; α=87.010(2), β=88.663(2), γ=76.653(2),
V=1325.4(2) Å3, T=293(2) K, Z=2; F(000) 552; reflections collected/unique: 7036/4871 [R(int)=0.1084];
number of observations [I > 2σ(I)]: 3909; parameters: 362. Supplementary crystallographic data have
been deposited at the Cambridge Crystallographic Data Centre, CCDC 807078.
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(11) X-ray crystal data for 3i: C38H32O2; M=520.64; crystal system: triclinic; space group: P-1; final R
indices [I > 2σ(I)] R1=0.0756, wR2=0.2222, R indices (all data) R1=0.1499, wR2=0.2521; a=8.0059(5)
Å, b=13.2066(8) Å, c=14.6206(11) Å; α=79.046(6), β=75.191(6), γ=81.987(5), V=1460.53(17) Å3,
T=293(2) K, Z=2; F(000) 552; reflections collected/unique: 12425/5337 [R(int)=0.1084]; number of
observations [I > 2σ(I)]: 2232; parameters: 358. Supplementary crystallographic data have been
deposited at the Cambridge Crystallographic Data Centre, CCDC 807081.
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(12) (a) Curtis, E. A.; Sandanayaka, V. P.; Padwa, A. Tetrahedron Lett. 1995, 36, 1989-1992. (b) Gould,
N. P.; Lee, T.-J. J. Org. Chem. 1980, 45, 4530-4532.
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