Efficient one-pot synthesis of biologically interesting diverse furo[2,3-b]pyran-6-ones by rhodium(II)-catalyzed cascade reactions of diazo compound with ethynyl compounds

Article abstract of DOI:10.1016/j.tet.2013.08.054

Rhodium(II)-catalyzed reactions of diazo compound and a variety of ethynyl compounds were carried out. These reactions provide a rapid route for preparing a variety of furo[2,3-b]pyran-6-one derivatives in one-pot via cascade reactions of metal carbenoid reaction/ketene formation/[2+2]cycloaddition/ring expansion.

Full text of DOI:10.1016/j.tet.2013.08.054

Tetrahedron 69 (2013) 9294e9302
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Efficient one-pot synthesis of biologically interesting diverse furo
[2,3-b]pyran-6-ones by rhodium(II)-catalyzed cascade reactions of
diazo compound with ethynyl compounds
,
b
Krishna Bahadur Somai Magar a, Yong Rok Lee a , Sung Hong Kim
*
a School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea
b Analysis Research Division, Daegu Center, Korea Basic Science Institute, Daegu 702-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 May 2013
Received in revised form 14 August 2013
Accepted 16 August 2013
Available online 27 August 2013
Rhodium(II)-catalyzed reactions of diazo compound and a variety of ethynyl compounds were carried
out. These reactions provide a rapid route for preparing a variety of furo[2,3-b]pyran-6-one derivatives in
one-pot via cascade reactions of metal carbenoid reaction/ketene formation/[2þ2]cycloaddition/ring
expansion.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Cascade reaction
Diazo compound
Ethynyl compounds
Furo[2,3-b]pyran-6-ones
Rhodium(II) catalyst
1. Introduction
the rhodium-catalyzed reactions of cyclic diazo compound derived
from Meldrum’s acid with phenylacetylene or arylacetylenes. We
Furopyranone derivatives are important heterocycles that occur
widely in nature.1 These derivatives exhibit a variety of biological
activities, such as, antibacterial,2 antitumor,3 and antiproliferative
effects.4 Because of their importance, a number of methods have
been devised to synthesize furopyranone derivatives.5 Among
these, methods for synthesizing two furo[2,3-c]pyran-4-one com-
pounds using multi-component reactions have been reported.4a,6
Recently, another method of synthesizing furo[2,3-b]pyran-6-one
derivatives using Co(III)-porphyrin catalyzed reactions of diazo
compounds was developed.7 Thus, there is still demand for a more
convenient, efficient synthetic method that can efficiently provide
a variety of furo[2,3-b]pyran-6-one derivatives.
describe herein a general and facile method for an efficient one-pot
synthesis of
(Scheme 1).
a
variety of furo[2,3-b]pyran-6-one derivatives
We have been interested in the rhodium-catalyzed reactions of
cyclic diazodicarbonyl compounds with several substrates. We
developed a novel methodology for the synthesis of dihydrofurans,
dihydrooxepins, oxindoles, oxazoles, cyclopropanes, and versatile
Scheme 1.
2. Results and discussion
b-substituted
a-haloenones starting from cyclic diazodicarbonyl
compounds.8
First, we investigated reactions between diazo compound 1 and
ethynylbenzene in the presence of several metal catalysts (Table 1).
When we tried reactions of 1 with 2.2 equiv of ethynylbenzene in
During our continuing studies on the development of a new
methodology based on cyclic diazo compounds, we investigated
the presence of 10 mol
% of In(OAc)3, Cu(OAc)2, Pd(OAc)2,
Co(PPh3)3Cl, Ru(PPh3)3Cl2, or Au(PPh3)3Cl in toluene at 70 ꢀC for
* Corresponding author. Tel.: þ82 53 810 2529; fax: þ82 53 810 4631; e-mail
address: yrlee@yu.ac.kr (Y.R. Lee).
12 h, no products were produced (entries 1e6). We then examined
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.08.054

K.B. Somai Magar et al. / Tetrahedron 69 (2013) 9294e9302
9295
Table 1
Effect of metal catalysts in the reaction of 1 with ethynylbenzene
Entry
Catalyst
Ethynylbenzene (equiv)
Solvent
Condition
Yield (%)
2
3
4
1
2
3
4
5
6
7
8
In(OAc)3 (10 mol %)
Cu(OAc)2 (10 mol %)
Pd(OAc)2 (10 mol %)
Co(PPh3)3Cl (10 mol %)
Ru(PPh3)3Cl2 (10 mol %)
Au(PPh3)3Cl (10 mol %)
Rh2(OCOCF3)4 (2 mol %)
Rh2(OAc)4 (2 mol %)
Rh2(OPiv)4 (2 mol %)
Rh2(OPiv)4 (2 mol %)
Rh2(OPiv)4 (5 mol %)
Rh2(OPiv)4 (2 mol %)
Rh2(OPiv)4 (2 mol %)
Rh2(OPiv)4 (2 mol %)
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
1.0
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Benzene
PhF
70 ꢀC, 12 h
70 ꢀC, 12 h
70 ꢀC, 12 h
70 ꢀC, 12 h
70 ꢀC, 12 h
70 ꢀC, 12 h
60 ꢀC, 12 h
60 ꢀC, 6 h
60 ꢀC, 0.5 h
80 ꢀC, 0.5 h
60 ꢀC, 0.5 h
60 ꢀC, 0.5 h
60 ꢀC, 0.5 h
60 ꢀC, 0.5 h
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
10
20
60
35
39
41
56
32
9
10
11
12
13
14
Toluene
the catalytic activities of several rhodium complexes in toluene.
Treatment with electron-poor rhodium trifluoroacetate (2 mol %)
as a catalyst in toluene at 60 ꢀC for 12 h produced cycloadduct 4 in
only 10% yield (entry 7), whereas treatment with rhodium acetate
(2 mol %) at 60 ꢀC for 6 h afforded 4 in 20% yield (entry 8). Using
electron-rich rhodium pivalate (2 mol %) in toluene at 60 ꢀC for
0.5 h, compound 4, intractable material, catalyst, acetone, and
remaining ethynylbenzene were shown by the crude 1H NMR
spectrum. After column chromatography, compound 4 was ob-
tained in 60% yield. In this reaction, other possible products of 2 and
3 were not isolated. Elevating temperature to 80 ꢀC and the use of
5 mol % of rhodium pivalate did not provide 4 in increased yield
(entries 10 and 11). Other solvents such as benzene and fluo-
robenzene were less effective (entries 12 and 13). On the other
hand, treatment of 1 with 1 equiv of ethynylbenzene in the pres-
ence of 2 mol % of rhodium pivalate in toluene at 60 ꢀC for 0.5 h
gave compound 4 in 32% yield. The structure of 4 was identified by
analyzing spectral data and by comparing it with reported data.7
1-bromo-4-ethynylbenzene or 4-ethynyl-a,a,a-trifluorotoluene
bearing an electron-withdrawing group on the benzene ring in
toluene at 60 ꢀC for 1e5 h afforded adducts 5ie5l in 43, 40, 41, and
50% yields (entries 9e12), respectively. To confirm the structures of
5ae5l, the structure of 5c was determined by X-ray crystallographic
analysis (Fig. 1).9
Further reactions were then attempted to produce furo[2,3-b]
pyran-6-one derivatives bearing polyconjugated units. When 1 was
treated with 1-ethynyl-4-phenoxybenzene in the presence of
2 mol % of Rh2(OPiv)4 in toluene at 60 ꢀC for 0.5 h, 5m was obtained
in 45% yield (entry 13). Treatment with 1-ethynylnaphthalene at
60 ꢀC for 1 h provided cycloadduct 5n in 54% yield (entry 14),
whereas that with 2-ethynyl-6-methoxynaphthalene at 60 ꢀC for
1 h afforded compound 5o in 35% yield (entry15). Interestingly,
with 9-ethynylphenanthrene, cycloadduct 5p was produced in 40%
yield (entry 16). Reactions between diazo compound 1 and other
ethynyl compounds without the phenylacetylene moiety were also
successful. Treatment with 3-ethynylthiophene and 1-
ethynylcyclohexene afforded cycloadducts 5qe5r in 58 and 49%
yields, respectively (entries 17 and 18). These reactions provide
rapid synthetic routes to a variety of furo[2,3-b]pyran-6-one de-
rivatives bearing polyconjugated units and other substituents on
furopyranone rings. Based on similarities with compounds re-
ported in the literature, the synthesized compounds 5me5p are
expected to exhibit fluorescence properties.10
To further explore the scope and limitation of ethynyl com-
pounds, reactions of non-conjugated terminal alkynes and di-
substituted alkynes were next examined. As non-conjugated
terminal alkynes, treatment of 1 with 1-hexyne in the presence of
2 mol % of rhodium pivalate in toluene at 60 ꢀC for 0.5 h provided
unexpected cyclopropene 6 in 56% yield, without any isolation of
two possible dihydrofuran and furopyranone (Scheme 2). With 3-
phenyl-1-propyne at 60 ꢀC for 0.5 h, 7 was also produced in 62%
yield. The structural assignments of 6 and 7 were identified by
spectroscopic analysis. The 1H NMR of 6 shows one vinylic proton
The 1H NMR spectrum of 4 showed two vinyl protons at
d¼6.87
and 6.21 ppm as two singlets associated with a pyranone and
a furan ring. The structure of 4 was further confirmed by its IR
spectrum, which exhibited the expected carbonyl absorption at
y
¼1724 cmꢁ1 due to an ester group.
To produce a variety of furo[2,3-b]pyran-6-one derivatives with
different substituents on the benzene ring, additional reactions
between diazo compounds and several ethynyl compounds were
conducted in the presence of 2 mol % rhodium pivalate in toluene at
60 ꢀC for 0.5e5 h. Results are summarized in Table 2. To investigate
the influence of substituents on reactivity, a variety of ethynyl
compounds bearing electron-donating and -withdrawing groups
on the benzene ring were used. Reactions of several ethynyl com-
pounds possessing an electron-donating group on the benzene ring
were first attempted. Reaction of 1 with 3-ethynyltoluene, 4-
ethynyltoluene, 1-ethynyl-2,4,5-trimethylbenzene, 1-ethynyl-4-n-
propylbenzene, 1-tert-butyl-4-ethynylbenzene, 1-ethynyl-4-n-
pentylbenzene, or 1-ethynyl-4-methoxy-2-methylbenzene in the
presence of 2 mol % of Rh2(OPiv)4 in toluene at 60 ꢀC for 0.5 h gave
compounds 5ae5h in 46e65% yield (entries 1e8, Table 2). Treat-
ment with 1-fluoro-3-ethynylbenzene,1-chloro-3-ethynylbenzene,
on the cyclopropene ring at
d
¼6.16 ppm as a singlet and the gem-
dimethyl peaks at
d
¼1.78 and 1.77 ppm as two singlets. The 1H NMR
of 7 exhibits one vinylic proton at
d¼6.24 ppm as a singlet and one
methylene proton at
d¼3.80 ppm as a singlet. Further confirmation

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K.B. Somai Magar et al. / Tetrahedron 69 (2013) 9294e9302
Table 2
Rhodium(II)-catalyzed reactions of 1 with ethynyl compounds to give a variety of furo[2,3-b]pyran-6-one derivatives
Entry
Arylacetylene
Temperature (ꢀC)
Time (h)
Product
Yielda (%)
1
60
0.5
54
2
60
0.5
55
3
60
0.5
47
4
60
0.5
62
5
60
0.5
46
6
60
0.5
65
7
60
0.5
57

K.B. Somai Magar et al. / Tetrahedron 69 (2013) 9294e9302
9297
Table 2 (continued )
Entry
Arylacetylene
Temperature (ꢀC)
Time (h)
Product
Yielda (%)
8
60
0.5
52
9
60
1
43
10
60
1
40
11
60
2
41
12
60
5
50
13
60
0.5
45
14
60
1
54
(continued on next page)

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K.B. Somai Magar et al. / Tetrahedron 69 (2013) 9294e9302
Table 2 (continued )
Entry
Arylacetylene
Temperature (ꢀC)
Time (h)
Product
Yielda (%)
15
60
1
35
16
60
1
40
17
60
0.5
58
18
60
0.5
49
a
Isolated yield.
for the structural assignment of 6 and 7 was obtained from spectral
data of their 13C NMR by comparison with those of reported com-
pound 8 (Fig. 2).11 The 13C NMR of 6 clearly shows two vinylic
evidence of this mechanism, the cyclopropenes 6 and 7 were iso-
lated from reactions of diazo compound 1 and non-conjugated
terminal alkynes. The formation of cyclopropenes followed by di-
rect ring opening and cyclization to give corresponding furans from
diazodicarbonyl compounds has been also reported.13 Conversion
of intermediate 3 to final product 4 is explained by ketene forma-
tion followed by [2þ2]cycloaddition and subsequent ring expan-
sion.14 Ketene formation from the 2,2-dimethyl-4H-1,3-dioxin-4-
one moiety in 3 and [2þ2]cycloaddition via the trapping of ole-
fins or alkynes have already described.15 Importantly, in the present
study, the expected cycloadduct 4a formed by [4þ2]cycloaddition
was not isolated.
carbons at
¼30.1 ppm on the cyclopropene ring, whereas 7 exhibits two vi-
nylic carbons at
¼107.2 and 89.9 ppm and a quaternary carbon at
¼30.5 ppm.
However, efforts to prepare the desired products from di-
d¼108.6 and 87.6 ppm and a quaternary carbon at
d
d
d
substituted alkynes were unsuccessful. Reaction of 1 with diphe-
nylacetylene or 1-phenyl-1-propyne gave intractable decomposed
mixtures without any isolation of pure products.
The Co(III)-complex catalyzed formation of compound 4 from
diazo compound 1 was recently reported to occur via double and
iterative radical cyclization processes.7 However, the Rh(II)-
catalyzed formation of 4 may be explained as shown in Scheme
3. We propose that the diazo compound 1 first gives a carbenoid
9 by displacement of nitrogen by Rh2(OPiv)4. The carbenoid 9 is
then trapped by the triple bond of ethynylbenzene to give in-
termediate 10, which may give intermediate 3 via cyclopropenation
intermediate 2 followed by a ring cleavage and a cyclization.12 As an
3. Conclusion
In conclusion, the rhodium(II)-catalyzed reactions of the cyclic
diazo compound 1 with a variety of ethynyl compounds are in-
vestigated. These reactions provide a rapid approach for synthe-
sizing a variety of biologically interesting furo[2,3-b]pyran-6-one
derivatives in one-pot cascade reactions of metal carbenoid

K.B. Somai Magar et al. / Tetrahedron 69 (2013) 9294e9302
9299
reaction/ketene formation/[2þ2]cycloaddition/subsequent ring
expansion. This methodology has the advantages of mild reaction
conditions, efficient catalytic ability, and simple experimentation
requirements.
4. Experimental
4.1. General
All experiments were carried out under nitrogen atmosphere.
Merck pre-coated silica gel plates (Art. 5554) with a fluorescent
indicator were used for analytical TLC. Flash column chromatog-
raphy was performed using silica gel 9385 (Merck). Melting points
were determined with micro-cover glasses on a Fisher-Johns ap-
paratus and are uncorrected. 1H NMR spectra were recorded on
a Varian-VNS (300 MHz) spectrometer in CDCl3 using 7.24 ppm as
the solvent chemical shift. 13C NMR spectra were recorded on
a Varian-VNS (75 MHz) spectrometer in CDCl3 using 77.0 ppm as
the solvent chemical shift. IR spectra were recorded on a JASCO FTIR
5300 spectrophotometer. High-resolution mass spectra were ob-
tained with a JEOL JMS-700 spectrometer at the Korea Basic Science
Institute.
Fig. 1. X-ray structure of compound 5c.
Scheme 2.
4.2. General procedure for the synthesis of 6H-furo[3,2-b]
pyran-6-ones (4 and 5ae5r)
To a solution of diazo compound 1 (0.5 mmol) and alkynes
(1.1 mmol) in toluene (2 mL) was added rhodium(II) pivalate
(0.01 mmol). The reaction mixture was stirred at 60 ꢀC for 0.5e5 h.
The solvent was evaporated in rotary evaporator under reduced
pressure to give the residue. The residue was purified by flash
column chromatography on silica gel to give the product.
Fig. 2. Comparison of 13C NMR chemical shifts of synthesized compounds 6 and 7 with
reported compound 8.
Scheme 3.

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K.B. Somai Magar et al. / Tetrahedron 69 (2013) 9294e9302
4.2.1. 2,4-Diphenyl-6H-furo[2,3-b]pyran-6-one (4). Reaction of
1
1602, 1514, 1375, 1273, 1013, 831, 550 cmꢁ1; HRMS m/z (Mþ) calcd
(85 mg, 0.5 mmol) and phenylacetylene (112 mg, 1.1 mmol) under
for C27H28O3: 400.2038. Found: 400.2038.
Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded 4 (86 mg, 60%) as a solid:
mp 88e90 ꢀC; 1H NMR (300 MHz, CDCl3)
d
7.64e7.62 (4H, m),
7.55e7.52 (3H, m), 7.39 (2H, t, J¼7.5 Hz), 7.30 (1H, d, J¼7.5 Hz),
6.87 (1H, s), 6.21 (1H, s); 13C NMR (75 MHz, CDCl3)
159.6, 159.1,
4.2.7. 2,4-Bis(4-pentylphenyl)-6H-furo[2,3-b]pyran-6-one
(5f).
Reaction of 1 (85 mg, 0.5 mmol) and 1-ethynyl-4-pentylbenzene
(189 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded
5f (143 mg, 65%) as a solid: mp 118e120 ꢀC; 1H NMR (300 MHz,
d
154.0, 148.7, 135.3, 131.0, 129.4, 129.1, 128.5, 127.7, 123.7, 105.0,
101.0, 99.9; IR (KBr) 1724, 1607, 1526, 1379, 1295, 1195, 826, 761,
687 cmꢁ1; HRMS m/z (Mþ) calcd for C19H12O3: 288.0786. Found:
288.0790.
CDCl3)
d
7.48e7.44 (4H, m), 7.25 (2H, d, J¼7.8 Hz), 7.11 (2H, d,
J¼7.8 Hz), 6.74 (1H, s), 6.09 (1H, s), 2.60 (2H, t, J¼7.5 Hz), 2.51 (2H, t,
J¼7.5 Hz), 1.58e1.50 (4H, m), 1.27e1.23 (8H, m), 0.83e0.78 (6H, m);
13C NMR (75 MHz, CDCl3)
d 159.5, 158.8, 153.7, 148.7, 146.3, 143.4,
4.2.2. 2,4-Di-m-tolyl-6H-furo[2,3-b]pyran-6-one (5a). Reaction of 1
(85 mg, 0.5 mmol) and 3-ethynyltoluene (127 mg, 1.1 mmol)
under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded 5a (85 mg, 54%) as
132.5, 129.2, 128.8, 127.5, 126.3, 123.5, 104.0, 100.1, 99.6, 35.7, 35.6,
31.4, 31.4, 30.9, 30.8, 22.4,13.9; IR (KBr) 2932,1713,1602,1513,1375,
1190, 1014, 821, 747 cmꢁ1; HRMS m/z (Mþ) calcd for C29H32O3:
428.2351. Found: 428.2349.
a solid: mp 98e100 ꢀC; 1H NMR (300 MHz, CDCl3)
d 7.46e7.41
(5H, m), 7.36e7.33 (1H, m), 7.30e7.25 (1H, m), 7.12 (1H, d,
J¼7.2 Hz), 6.84 (1H, s), 6.19 (1H, s), 2.45 (3H, s), 2.38 (3H, s); 13C
4.2.8. 2,4-Bis(4-methoxyphenyl)-6H-furo[2,3-b]pyran-6-one (5g).
Reaction of 1 (85 mg, 0.5 mmol) and 4-ethynylanisole (145 mg,
1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded 5g (99 mg,
NMR (75 MHz, CDCl3)
d 159.5, 158.9, 154.0, 148.7, 139.1, 138.6,
135.2, 131.5, 129.2, 129.1, 128.8, 128.8, 128.1, 124.7, 124.2, 120.8,
104.6, 100.7, 99.76, 21.5, 21.4; IR (KBr) 2933, 1729, 1609, 1528,
1294, 1182, 786 cmꢁ1; HRMS m/z (Mþ) calcd for C21H16O3:
316.1099. Found: 316.1099.
57%) as a solid: mp 203e205 ꢀC; 1H NMR (300 MHz, CDCl3)
d 7.61
(2H, d, J¼8.7 Hz), 7.57 (2H, d, J¼8.7 Hz), 7.03 (2H, d, J¼8.7 Hz), 6.92
(2H, d, J¼8.7 Hz), 6.74 (1H, s), 6.14 (1H, s), 3.88 (3H, s), 3.82 (3H, s);
13C NMR (75 MHz, CDCl3)
d 161.8, 159.7, 158.7, 153.3, 148.6, 129.2,
4.2.3. 2,4-Di-p-tolyl-6H-furo[2,3-b]pyran-6-one (5b). Reaction of 1
(85 mg, 0.5 mmol) and 4-ethynyltoluene (127 mg, 1.1 mmol) under
Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded 5b (87 mg, 55%) as a solid:
127.5, 125.1, 121.8, 114.6, 114.3, 103.2, 99.6, 99.2, 55.5, 55.3; IR (KBr)
2924, 1722, 1602, 1513, 1249, 1173, 1021, 821, 632 cmꢁ1; HRMS m/z
(Mþ) calcd for C21H16O5: 348.0998. Found: 348.0998.
mp 70e72 ꢀC; 1H NMR (300 MHz, CDCl3)
d
7.55e7.51 (4H, m), 7.33
(2H, d, J¼7.8 Hz), 7.11 (2H, d, J¼7.8 Hz), 6.81 (1H, s), 6.18 (1H, s), 2.43
(3H, s), 2.35 (3H, s); 13C NMR (75 MHz, CDCl3)
159.5, 158.5, 154.0,
4.2.9. 2,4-Bis(4-methoxy-2-methylphenyl)-6H-furo[2,3-b]pyran-6-
one (5h). Reaction of 1 (85 mg, 0.5 mmol) and 1-ethynyl-4-
methoxy-2-methylbenzene (160 mg, 1.1 mmol) under Rh2(OPiv)4
(6 mg, 0.01 mmol) afforded 5h (98 mg, 52%) as a solid: mp
d
149.0, 141.5, 138.6, 132.6, 130.1, 129.8, 127.7, 126.4, 123.8, 104.3,
100.4, 100.0, 21.6, 21.5; IR (KBr) 2938, 1755, 1615, 1427, 1199, 1018,
817, 520 cmꢁ1; HRMS m/z (Mþ) calcd for C21H16O3: 316.1099.
Found: 316.1099.
123e125 ꢀC; 1H NMR (300 MHz, CDCl3)
d
7.46 (1H, d, J¼8.1 Hz), 7.18
(1H, d, J¼7.8 Hz), 6.79e6.69 (4H, m), 6.22 (1H, s), 5.93 (1H, s), 3.78
(3H, s), 3.74 (3H, s), 2.33 (3H, s), 2.27 (3H, s); 13C NMR (75 MHz,
4.2.4. 2,4-Bis(2,4,5-trimethylphenyl)-6H-furo[2,3-b]pyran-6-one
(5c). Reaction of 1 (85 mg, 0.5 mmol) and 1-ethynyl-2,4,5-
trimethylbenzene (158 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg,
0.01 mmol) afforded 5c (87 mg, 47%) as a solid: mp 168e170 ꢀC; 1H
CDCl3) d 160.4, 159.6, 159.4, 157.9, 154.9, 148.3, 137.0, 136.7, 129.7,
128.6, 127.5, 116.6, 116.5, 111.5, 111.5, 106.4, 103.3, 101.1, 55.3, 55.2,
21.9, 21.5; IR (KBr) 2943, 1727, 1604, 1513, 1291, 1247, 1162, 1109,
1048, 855, 797 cmꢁ1; HRMS m/z (Mþ) calcd for C23H20O5: 376.1311.
Found: 376.1309.
NMR (300 MHz, CDCl3)
(1H, s), 6.36 (1H, s), 6.02 (1H, s), 2.38 (3H, s), 2.31e2.29 (9H, m), 2.26
(6H, s); 13C NMR (75 MHz, CDCl3)
159.4, 158.0, 155.3, 148.6, 138.2,
d 7.40 (1H, s), 7.12 (1H, s), 7.09 (1H, s), 7.00
d
4.2.10. 2,4-Bis(3-fluorophenyl)-6H-furo[2,3-b]pyran-6-one
(5i).
137.0, 134.4, 134.2, 132.6, 132.5, 132.3, 132.2, 132.0, 129.2, 128.0,
125.5, 106.5, 103.9, 100.9, 21.0, 19.4, 19.4, 19.3, 19.1; IR (KBr) 2930,
1727, 1611, 1516, 1452, 1376, 1174, 1020, 851, 745 cmꢁ1; HRMS m/z
(Mþ) calcd for C25H24O3: 372.1725. Found: 372.1723.
Reaction of 1 (85 mg, 0.5 mmol) and 1-ethynyl-3-fluorobenzene
(132 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded
5i (69 mg, 43%) as a solid: mp 128e130 ꢀC; 1H NMR (300 MHz,
CDCl3)
6.99e6.96 (1H, m), 6.84 (1H, s), 6.18 (1H, s); 13C NMR (75 MHz,
CDCl3) 164.6, 161.4, 159.0, 158.9, 152.3, 147.4, 137.0, 136.9, 131.1,
d 7.39e7.37 (3H, m), 7.34e7.27 (3H, m), 7.20e7.18 (1H, m),
4.2.5. 2,4-Bis(4-propylphenyl)-6H-furo[2,3-b]pyran-6-one
(5d).
d
Reaction of 1 (85 mg, 0.5 mmol) and 1-ethynyl-4-propylbenzene
(158 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded
5d (115 mg, 62%) as a solid: mp 160e162 ꢀC; 1H NMR (300 MHz,
130.7, 123.3, 119.3, 117.9, 115.4, 114.7, 110.7, 105.6, 101.6, 99.4; IR
(KBr) 2928, 1761, 1715, 1588, 1437, 1240, 1181, 1076, 926, 784,
692 cmꢁ1; HRMS m/z (Mþ) calcd for C19H10F2O3: 324.0598. Found:
324.0599.
CDCl3)
d
7.57e7.52 (4H, m), 7.33 (2H, d, J¼8.1 Hz), 7.19 (2H, d,
J¼8.1 Hz), 6.82 (1H, s), 6.17 (1H, s), 2.66 (2H, t, J¼7.8 Hz), 2.58 (2H, t,
J¼7.8 Hz), 1.73e1.60 (4H, m), 1.00e0.91 (6H, m); 13C NMR (75 MHz,
4.2.11. 2,4-Bis(3-chlorophenyl)-6H-furo[2,3-b]pyran-6-one
(5j).
CDCl3)
d
159.5, 158.8, 153.7, 148.7, 146.0, 143.1, 132.5, 129.3, 128.9,
Reaction of 1 (85 mg, 0.5 mmol) and 3-chloro-1-ethynylbenzene
(149 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded
5j (71 mg, 40%) as a solid: mp 198e200 ꢀC; 1H NMR (300 MHz,
127.5, 126.4, 123.5, 104.0, 100.1, 99.6, 37.8, 37.7, 24.3, 13.7, 13.7; IR
(KBr) 2943, 1713, 1600, 1509, 1291, 1188, 1015, 809 cmꢁ1; HRMS m/z
(Mþ) calcd for C25H24O3: 372.1725. Found: 372.1725.
CDCl3)
d
7.57e7.56 (2H, m), 7.49e7.45 (4H, m), 7.31e7.19 (2H, m),
159.1, 158.8,
6.82 (1H, s), 6.16 (1H, s); 13C NMR (75 MHz, CDCl3)
d
4.2.6. 2,4-Bis(4-tert-butylphenyl)-6H-furo[2,3-b]pyran-6-one
(5e). Reaction of 1 (85 mg, 0.5 mmol) and 4-tert-butylphenylace-
tylene (173 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol)
afforded 5e (92 mg, 46%) as a solid: mp 220e222 ꢀC; 1H NMR
152.2, 147.3, 136.7, 135.4, 135.1, 130.9, 130.6, 130.4, 130.2, 128.4,
127.5,125.6,123.6,121.7,105.7,101.6, 99.4; IR (KBr) 3084,1748,1606,
1526, 1261, 1180, 1094, 785 cmꢁ1; HRMS m/z (Mþ) calcd for
C19H10Cl2O3: 356.0007. Found: 356.0007.
(300 MHz, CDCl3)
(1H, s), 6.18 (1H, s), 1.39 (9H, s), 1.33 (9H, s); 13C NMR (75 MHz,
CDCl3) 159.4, 158.8, 154.4, 153.6, 151.5, 148.6, 132.2, 127.3, 126.1,
125.7, 123.4, 104.0, 100.3, 99.6, 34.9, 34.6, 31.1; IR (KBr) 2955, 1724,
d
7.61e7.55 (6H, m), 7.42 (2H, d, J¼8.1 Hz), 6.86
4.2.12. 2,4-Bis(4-bromophenyl)-6H-furo[2,3-b]pyran-6-one
Reaction of 1 (85 mg, 0.5 mmol) and 1-bromo-4-ethynylbenzene
(199 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol)
(5k).
d

K.B. Somai Magar et al. / Tetrahedron 69 (2013) 9294e9302
9301
afforded 5k (91 mg, 41%) as a solid: mp 228e230 ꢀC; 1H NMR
(300 MHz, CDCl3)
7.68 (2H, d, J¼8.4 Hz), 7.55e7.49 (6H, m),
6.84 (1H, s), 6.20 (1H, s); 13C NMR (75 MHz, CDCl3)
159.1,
1533, 1411, 1266, 1166, 1032, 962, 826, 750 cmꢁ1; HRMS m/z (Mþ)
calcd for C35H20O3: 488.1412. Found: 488.1411.
d
d
158.9, 152.5, 147.7, 133.9, 132.6, 132.1, 129.0, 127.7, 125.5, 125.1,
105.2, 101.1, 99.4; IR (KBr) 2925, 1714, 1597, 1520, 1296, 1194,
4.2.18. 2,4-Di(thiophen-3-yl)-6H-furo[2,3-b]pyran-6-one
(5q).
Reaction of 1 (85 mg, 0.5 mmol) and 3-ethynylthiophene (118 mg,
1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded 5q (87 mg,
1007, 816 cmꢁ1
;
HRMS m/z (Mþ) calcd for C19H10Br2O3:
443.8997. Found: 443.9001.
58%) as a solid: mp 189e191 ꢀC; 1H NMR (300 MHz, CDCl3)
d
7.81
(1H, s), 7.51e7.49 (2H, m), 7.42e7.36 (2H, m), 7.30e7.28 (1H, m),
6.76 (1H, s), 6.23 (1H, s); 13C NMR (75 MHz, CDCl3)
159.6, 158.7,
4.2.13. 2,4-Bis(4-(trifluoromethyl)phenyl)-6H-furo[2,3-b]pyran-6-
d
one (5l). Reaction of 1 (85 mg, 0.5 mmol) and 4-ethynyl-
a
,
a
,
a
-tri-
147.4,145.7,136.4,130.3,127.6,127.1,127.0,126.2,124.0,120.3,103.4,
100.4, 98.9; IR (KBr) 3122, 1710, 1522, 1409, 1279, 786 cmꢁ1; HRMS
m/z (Mþ) calcd for C15H8O3S2: 299.9915. Found: 299.9912.
fluorotoluene (187 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg,
0.01 mmol) afforded 5l (106 mg, 50%) as a solid: mp 158e160 ꢀC; 1H
NMR (300 MHz, CDCl3)
d
7.82 (2H, d, J¼8.1 Hz), 7.76e7.73 (4H, m),
7.65 (2H, d, J¼8.1 Hz), 6.94 (1H, s), 6.26 (1H, s); 13C NMR (75 MHz,
4.2.19. 2,4-Dicyclohexenyl-6H-furo[2,3-b]pyran-6-one
(5r).
CDCl3)
d
158.6, 152.1, 147.3, 138.4, 131.9, 128.2, 127.9, 126.4, 126.3,
Reaction of 1 (85 mg, 0.5 mmol) and 1-ethynylcyclohexene (116 mg,
1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded 5r (72 mg,
126.3, 126.1, 126.0, 125.1, 123.7, 106.4, 102.4, 99.5; IR (KBr) 3125,
1728, 1616, 1528, 1325, 1127, 837, 672 cmꢁ1; HRMS m/z (Mþ) calcd
for C21H10F6O3: 424.0534. Found: 424.0532.
49%) as a solid: mp 142e144 ꢀC; 1H NMR (300 MHz, CDCl3)
d
6.50
(1H, s), 6.29 (2H, s), 5.89 (1H, s), 2.30e2.24 (4H, m), 2.22e2.17 (4H,
m), 1.77e1.62 (8H, m); 13C NMR (75 MHz, CDCl3)
160.3, 158.2,
d
4.2.14. 2,4-Bis(4-phenoxyphenyl)-6H-furo[2,3-b]pyran-6-one (5m).
Reaction of 1 (85 mg, 0.5 mmol) and 1-ethynyl-4-phenoxybenzene
(213 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded
5m (106 mg, 45%) as a solid: mp 162e164 ꢀC; 1H NMR (300 MHz,
154.7, 149.5, 134.7, 133.7, 125.5, 124.3, 101.5, 99.8, 98.1, 26.4, 26.0,
25.1, 24.3, 22.3, 22.0, 21.9, 21.5; IR (KBr) 2930, 1719, 1511, 1278, 1128,
817, 557 cmꢁ1; HRMS m/z (Mþ) calcd for C19H20O3: 296.1412.
Found: 296.1414.
CDCl3)
d
7.64e7.54 (4H, m), 7.42e7.33 (4H, m), 7.21e6.98 (10H, m),
160.2, 159.5,
6.79 (1H, s), 6.18 (1H, s); 13C NMR (75 MHz, CDCl3)
d
4.2.20. 1-Butyl-6,6-dimethyl-5,7-dioxaspiro[2.5]oct-1-ene-4,8-dione
(6). Reaction of 1 (85 mg, 0.5 mmol) and 1-hexyne (90 mg,
1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded 6 (63 mg,
157.7, 155.7, 148.3, 130.0, 129.8, 129.4, 129.3, 126.5, 125.3, 124.5,
123.9, 123.8, 119.9, 119.5, 119.3, 118.9, 118.4, 118.3, 103.9, 99.9, 99.6;
IR (KBr) 1730, 1587, 1489, 1237, 1171, 1015, 840, 754, 689 cmꢁ1
;
56%) as a liquid; 1H NMR (300 MHz, CDCl3)
d 6.16 (1H, s), 2.55e2.50
HRMS m/z (Mþ) calcd for C31H20O5: 472.1311. Found: 472.1310.
(2H, m), 1.78 (3H, s), 1.77 (3H, s), 1.64e1.54 (2H, m), 1.45e1.33 (2H,
m), 0.88 (3H, t, J¼7.5 Hz); 13C NMR (75 MHz, CDCl3)
d 169.7, 108.6,
4.2.15. 2,4-Di(naphthalen-1-yl)-6H-furo[2,3-b]pyran-6-one (5n).
104.3, 87.6, 30.1, 28.3, 27.7, 27.7, 22.9, 22.0, 13.4; IR (neat) 3143,
2955, 2871, 1742, 1461, 1388, 1290, 1204, 1128, 1016, 922, 832,
740 cmꢁ1; HRMS m/z (Mþ) calcd for C12H16O4: 224.1049. Found:
224.1047.
Reaction of 1 (85 mg, 0.5 mmol) and 1-ethynylnaphthalene
(167 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded
5n (105 mg, 54%) as a solid: mp 88e90 ꢀC; 1H NMR (300 MHz,
CDCl3)
J¼7.8 Hz), 7.68 (1H, d, J¼6.9 Hz), 7.58e7.47 (7H, m), 6.48 (1H, s),
6.32 (1H, s); 13C NMR (75 MHz, CDCl3)
159.4, 158.5, 154.1, 147.8,
133.7, 133.7, 133.0, 130.5, 129.9, 129.8, 129.5, 128.6, 127.0, 126.6,
126.5, 126.4, 126.1, 125.3, 125.1, 125.0, 124.6, 107.7, 105.8, 101.4; IR
(KBr) 3055, 1743, 1620, 1521, 1397, 1238, 1175, 961, 910, 781,
730 cmꢁ1; HRMS m/z (Mþ) calcd for C27H16O3: 388.1099. Found:
388.1099.
d 8.23e8.21 (1H, m), 8.00e7.95 (3H, m), 7.84 (2H, d,
4.2.21. 1-Benzyl-6,6-dimethyl-5,7-dioxaspiro[2.5]oct-1-ene-4,8-
dione (7). Reaction of 1 (85 mg, 0.5 mmol) and 3-phenyl-1-propyne
(128 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded 7
d
(81 mg, 62%) as a liquid; 1H NMR (300 MHz, CDCl3)
d
7.21e7.14 (5H,
m), 6.24 (1H, s), 3.80 (2H, s), 1.65 (3H, s), 1.44 (3H, s); 13C NMR
(75 MHz, CDCl3) 169.4, 133.8, 128.8, 127.4, 107.2, 104.4, 89.9, 30.5,
d
29.4, 27.6, 27.3; IR (neat) 3146, 2999, 1739, 1628, 1386, 1289, 1202,
1127, 1018, 922, 837, 739 cmꢁ1; HRMS (FAB) m/z (Mþ) calcd for
4.2.16. 2,4-Bis(6-methoxynaphthalen-2-yl)-6H-furo[2,3-b]pyran-6-
one (5o). Reaction of 1 (85 mg, 0.5 mmol) and 2-ethynyl-6-
methoxynaphthalene (200 mg, 1.1 mmol) under Rh2(OPiv)4
(6 mg, 0.01 mmol) afforded 5o (78 mg, 35%) as a solid: mp
C15H14O4: 258.0892. Found: 258.0896.
Acknowledgements
188e190 ꢀC; 1H NMR (300 MHz, CDCl3)
d
8.11e8.03 (2H, m),
7.90e7.85 (1H, m), 7.78e7.70 (4H, m), 7.60e7.58 (1H, m), 7.20e7.12
(4H, m), 7.01 (1H, s), 6.32 (1H, s); 13C NMR (75 MHz, CDCl3)
159.5,
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology
(2012R1A1A4A01009620).
d
158.9,158.0,153.7,148.7,135.5,134.1,131.8,130.0,129.9,129.5,129.1,
128.3, 127.7, 127.4, 127.3, 124.7, 122.2, 121.8, 119.8, 119.4, 105.7, 105.5,
104.2, 100.4, 99.8, 96.4, 55.2, 55.1; IR (KBr) 2941, 1709, 1614, 1484,
1393, 1266, 1199, 1027, 851, 731 cmꢁ1; HRMS m/z (Mþ) calcd for
Supplementary data
1H NMR, 13C NMR spectra and X-ray data of synthesized com-
pounds. Supplementary data associated with this article can be
found in the online version, at http://dx.doi.org/10.1016/
j.tet.2013.08.054.
C
29H20O5: 448.1311. Found: 448.1313.
4.2.17. 2,4-Di(phenanthren-9-yl)-6H-furo[2,3-b]pyran-6-one (5p).
Reaction of 1 (85 mg, 0.5 mmol) and 9-ethynylphenanthrene
(222 mg, 1.1 mmol) under Rh2(OPiv)4 (6 mg, 0.01 mmol) afforded
5p (98 mg, 40%) as a solid: mp 156e158 ꢀC; 1H NMR (300 MHz,
CDCl3)
m), 7.78e7.53 (8H, m), 6.52 (1H, s), 6.42 (1H, s); 13C NMR (75 MHz,
CDCl3) 159.3, 158.4, 154.2, 147.8, 131.8, 130.7, 130.6, 130.5, 130.5,
130.2, 129.1, 129.0, 128.8, 128.5, 128.4, 128.0, 127.9, 127.5, 127.4,
127.2, 127.1, 126.9, 126.9, 126.8, 126.7, 125.9, 125.3, 124.8, 123.1,
122.9, 122.5, 122.3, 107.5, 106.0, 101.3; IR (KBr) 2923, 1714, 1646,
References and notes
d
8.81e8.62 (4H, m), 8.23 (1H, d, J¼7.5 Hz), 8.04e7.85 (5H,
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