Rhodium(II)-catalyzed reaction of iodonium ylides with electron-deficient and conjugated alkynes: Efficient synthesis of substituted furans

Article abstract of DOI:10.1080/00397910600631957

The rhodium(II)-catalyzed reactions of iodonium ylides with electron-deficient and conjugated alkynes have been examined. These reactions provide a rapid route to the preparation of furan derivatives with a variety of substituents on the furan ring. Copyr

Full text of DOI:10.1080/00397910600631957

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Rhodium(II)‐Catalyzed Reaction of Iodonium
Ylides with Electron‐Deficient and
Conjugated Alkynes: Efficient Synthesis of
Substituted Furans
Yong Rok Lee a & Sang Heum Yoon a
a School of Chemical Engineering and Technology , Yeungnam University ,
Gyeongsan, Korea
Published online: 20 Aug 2006.
To cite this article: Yong Rok Lee & Sang Heum Yoon (2006) Rhodium(II)‐Catalyzed Reaction of Iodonium
Ylides with Electron‐Deficient and Conjugated Alkynes: Efficient Synthesis of Substituted Furans, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:14,
1941-1951
To link to this article: http://dx.doi.org/10.1080/00397910600631957
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Synthetic Communicationsw, 36: 1941–1951, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600631957
Rhodium(II)-Catalyzed Reaction of
Iodonium Ylides with Electron-Deficient
and Conjugated Alkynes: Efficient Synthesis
of Substituted Furans
Yong Rok Lee and Sang Heum Yoon
School of Chemical Engineering and Technology, Yeungnam University,
Gyeongsan, Korea
Abstract: The rhodium(II)-catalyzed reactions of iodonium ylides with electron-
deficient and conjugated alkynes have been examined. These reactions provide a
rapid route to the preparation of furan derivatives with a variety of substituents on
the furan ring.
Keywords: Conjugated alkyne, electron-deficient alkyne, iodonoum ylides,
rhodium(II)-catalyzed reaction
INTRODUCTION
Iodonium ylides are useful and important reagents in organic synthesis.[1]
Iodonium ylides are widely used as equivalents of the corresponding diazo
compounds. Intermolecular and intramolecular photochemical and metal-
mediated reactions of iodonium ylides with a number of substrates have
been studied by several groups.[2] The reactions include C-H and N-H
insertions,[3] Wolff rearrangement,[4] cyclopropanation,[5] and cycloaddition.[6]
These reactions usually decompose at lower temperatures than the corres-
ponding diazo compounds, leading to better yields of products. We have
interest in the rhodium(II)-catalyzed reaction of iodonium ylides with
Received in Japan October 24, 2005
Address correspondence to Yong Rok Lee, School of Chemical Engineering and
Technology, Yeungnam University, Gyeongsan 712-749, Korea. Tel: 82-53-810-
2529; Fax: 82-53-810-4631; E-mail: yrlee@yu.ac.kr
1941

1942
Y. R. Lee and S. H. Yoon
several substrates. Recently, we reported that the rhodium(II)-catalyzed
reaction of iodonium ylides with halides and olefins gave b-substituted
a-halo compounds and dihydrofurans in good yields.[7] While continuing
our work based on iodonium ylides, we have expanded this work to the
synthesis of biologically interesting furan derivatives. Although the
rhodium-catalyzed reactions of iodonium ylides with electron-rich substrates
have been reported by another group,[8] there is little information available on
the rhodium(II)-catalyzed reactions of iodonium ylides with electron-deficient
and conjugated alkynes. We describe herein very efficient and convenient
synthesis of substituted furans using rhodium(II)-catalyzed reactions of
iodonium ylides with electron-deficient and conjugated alkynes.
RESULTS AND DISCUSSION
Iodonium ylides 1–4 were prepared by the reaction of the corresponding
1,3-dicarbonyl compounds with iodobenzene diacetate according to Koser’s
method (Fig. 1).[9] To check the reactivity of electron-deficient alkynes,
reactions with ethyl propiolate were first examined utilizing several
catalysts (Table 1). When iodonium ylide 2 was treated with ethyl propiolate
as a solvent and a reactant at 608C for 24 h in the presence of 10 mol% of
copper acetate and palladium acetate, no products were produced. Both
Rh2(OCOCF3)4 (1 mol%, rt, 12 h) and Rh2(OAc)4 (1 mol%, rt, 12 h)
catalysts afforded a small fraction of the product 5 in 10 and 24% yields,
respectively, whereas the Rh2(OPiv)4 (1 mol%, rt, 5 h) catalyst provided the
two products 5 and 6 in 42 and 19% yields, respectively. We found that
the more electron-rich Rh2(OPiv)4 was a much superior catalyst for this
cycloaddition than Rh2(OCOCF3)4 or Rh2(OAc)4. When fluorobenzene was
used as solvent, cycloadducts 5 (32%) and 6 (10%) were produced in lower
yields. The two regioisomers 5 and 6 were easily separated by column
chromatography and assigned by spectroscopic analysis. The 1H NMR
spectrum of 5 showed one vinyl proton on the furan ring at d 7.36 ppm as a
singlet, whereas the vinyl proton of 6 was observed at d 7.88 ppm as a singlet.
To extend the utility of these cycloadditions, additional reactions of
iodonium ylides with propiolates, 3-butyn-2-one, and 2-methyl-1-buten-3-
yne were next investigated under optimized reaction conditions. The results
Figure 1. Iodonium ylides.

Efficient Synthesis of Substituted Furans
1943
Table 1. Effect of catalysts in the reaction of 2 and ethyl propiolate
Product (%)
Catalyst
Condition
5
6
Cu(OAc)2
Pd(OAc)2
10 mol%, 608C, 24 h
10 mol%, 608C, 24 h
1 mol%, rt, 12 h
1 mol%, rt, 12 h
1 mol%, rt, 5 h
0
0
0
10
24
42
0
0
Rh2(OCOCF3)4
Rh2(OAc)4
Rh2(OPiv)4
0
19
are summarized in Table 2. Reaction of 1 with methyl propiolate as a solvent
and a reactant in the presence of 1 mol% of Rh2(OPiv)4 at room temperature
for 5 h afforded the two products 7 and 8 in 42 and 15% yields (entry 1)
respectively. Similarly, treating of 2 with methyl propiolate gave cycloadducts
9 and 10 in 46 and 12% yields (entry 2). Reaction of 1 with ethyl propiolate
gave two adducts 11 and 12 in 41 and 10% yields (entry 3), whereas reaction
of 3 gave adduct 13 as a single compound in 34% yield (entry 4).
With 3-butyn-2-one, the reactions were also successful. When iodonium
ylide 1 was treated with 3-butyn-2-one in the presence of 1 mol% of
Rh2(OPiv)4 at room temperature for 5 h, furan 14 was obtained in 43%
yield (entry 5). Similarly, the reaction of 2 and 3 with 3-butyn-2-one
afforded adducts 15 and 16 in 45 and 40% yields, respectively (entries 6, 7).
On the other hand, treatment of 4 with 3-butyn-2-one afforded adduct 17
(37%) and rearranged product 18 (12%) (entry 8). The assignment of 18
was confirmed by spectral analysis and by comparison with the earlier
reported data.[7a]
With 2-methyl-1-buten-3-yne, the expected cycloadducts were also
produced. Reaction of 1 and 2 with 2-methyl-1-buten-3-yne in the presence
of 1 mol% of Rh2(OPiv)4 at room temperature for 5 h gave 19 and 20 in 53
and 47% yields, respectively (entries 9, 10).
Interestingly, in view of our results, reactions of iodonium ylides with less
electron-deficient alkynes such as 3-butyn-2-one and 2-methyl-1-buten-3-yne
than propiolates gave products as a single compound.
Although the exact mechanism of the reaction is still not clear, it is best
described as shown in Scheme 1. All the previously mentioned reactions could
be explained by a carbene or carbenoid intermediate, as proposed by
Hadjiarapoglou.[10] The iodonium ylide 2 first gives a carbene 21 (or
carbenoid) by displacement of iodobenzene by Rh2(OPiv)4, which reacts

Table 2. Reaction of iodonium ylides with electron-deficient and conjugated alkynes
Entry
Iodonium ylide
Alkyne
Product (yield)
1
2
3
4
5

Efficient Synthesis of Substituted Furans
1945

1946
Y. R. Lee and S. H. Yoon
Scheme 1.
further with ethyl propiolate to cyclopropene 22.The intermediate 22
undergoes bond cleavage and ring closure to give 5 (path a) and 6 (path b).
In conclusion, the rhodium(II)-catalyzed reaction of iodonium ylides to
electron-deficient and conjugated alkynes is described. These reactions
provide a rapid entry into the synthesis of a variety of furan derivatives.
EXPERIMENTAL
All experiments were carried out under a nitrogen atmosphere. Merck
precoated silica-gel plates (Art. 5554) with fluorescent indicator were used
for analytical TLC. Flash-column chromatography was performed using
silica gel 9385 (Merck). Melting points were determined in capillary tubes
on a Fisher-Johns apparatus and are uncorrected. 1H NMR spectra were
recorded on a Bruker Model ARX (300 MHz) spectrometer in CDCl3 using
7.26 ppm as the solvent chemical shift. 13C NMR spectra were recorded on
a Bruker Model ARX (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 (HRMS) spectra were carried out
on JEOL JMS-700 spectrometer by the Korea Basic Science Institute.
General Procedure for the Reactions of Iodonium Ylides
with Alkynes
To a solution of iodonium ylides (1.0 mmol) in alkyne (2 mL) was added
Rh2(OPiv)4 (0.01 mmol) at room temperature under an N2 atmosphere. The
reaction mixture was stirred at room temperature for 5 h. The reaction
mixture was evaporated under reduced pressure to give the residue. The
residue was purified by flash-column chromatography on silica gel to give
the product.

Efficient Synthesis of Substituted Furans
1947
6,6-Dimethyl-4-oxo-4,5,6,7-tetrahydrobenzofuran-2-carboxylic Acid
Ethyl Ester (5) and 6,6-Dimethyl-4-oxo-4,5,6,7-tetrahydrobenzofuran-
3-carboxylic Acid Ethyl Ester (6)
Reaction of 2 (342 mg, 1.0 mmol) and ethyl propiolate (2 mL) under
Rh2(OPiv)4 (6 mg) at room temperature for 5 h afforded 5 (99 mg, 42%) and
1
6 (45 mg, 19%) as oils. 5: H NMR (300 MHz, CDCl3) d 7.36 (1H, s), 4.35
(2H, q, J ¼ 7.1 Hz), 2.80 (2H, s), 2.39 (2H, s), 1.36 (3H, t, J ¼ 7.1 Hz),
1.13 (6H, s); 13C NMR (75 MHz, CDCl3) d 193.6, 169.2, 158.8, 145.4,
121.7, 114.1, 61.8, 52.4, 37.9, 35.6, 28.9, 14.7; IR (neat) 3136, 2961, 2874,
1726, 1690, 1593, 1543, 1451, 1372, 1290, 1215, 1152, 1103, 1032,
762 cm21; HRMS: m/z (Mþ) calcd. for C13H16O4: 236.1049. Found:
236.1046. 6: 1H NMR (300 MHz, CDCl3) d 7.88 (1H, s), 4.31 (2H, q,
J ¼ 7.1 Hz), 2.74 (2H, s), 2.41 (2H, s), 1.34 (3H, t, J ¼ 7.1 Hz), 1.12
(6H, s); 13C NMR (75 MHz, CDCl3) d 192.1, 168.0, 162.3, 148.8, 118.0,
117.7, 61.4, 53.5, 37.9, 35.3, 28.8, 14.6; IR (neat) 2961, 2928, 2868, 1742,
1691, 1555, 1443, 1298, 1279, 1109, 1034, 760 cm2 1;HRMS: m/z (Mþ)
calcd. for C13H16O4: 236.1049. Found: 236.1050.
4-Oxo-4,5,6,7-tetrahydrobenzofuran-2-carboxylic Acid Methyl Ester
(7) and 4-Oxo-4,5,6,7-tetrahydrobenzofuran-3-carboxylic Acid Methyl
Ester (8)
Reaction of 1 (314 mg, 1.0 mmol) and methyl propiolate (2 mL) under
Rh2(OPiv)4 (6 mg) at room temperature for 5 h afforded 7 (82 mg, 42%) and
8 (29 mg, 15%). 7: mp 67–688C; 1H NMR (300 MHz, CDCl3)d 7.37
(1H, s), 3.89 (3H, s), 2.95 (2H, t, J ¼ 6.3 Hz), 2.53 (2H, t, J ¼ 5.9 Hz),
2.24–2.16 (2H, m); 13C NMR (75 MHz, CDCl3) d 194.1, 170.1, 159.3,
144.7, 122.9, 114.4, 52.7, 38.0, 24.0, 22.6; IR (KBr) 3113, 2955, 1718,
1678, 1591, 1547, 1460, 1439, 1358, 1319, 1298, 1233, 1190, 1146, 1005,
924, 895 cm21; HRMS: m/z (Mþ) calcd. for C10H10O4: 194.0579. Found:
1
194.0577. 8: oil; H NMR (300 MHz, CDCl3) d 7.86 (1H, s), 3.85 (3H, s),
2.89 (2H, t, J ¼ 6.2 Hz), 2.54 (2H, t, J ¼ 5.8 Hz), 2.20–2.12 (2H, m); 13C
NMR (75 MHz, CDCl3) d 192.6, 168.9, 163.6, 118.5, 52.5, 39.1, 30.1, 22.6;
IR (neat) 2926, 2854, 1739, 1695, 1555, 1456, 1296, 1134, 1111, 1030,
1003, 802 cm21; HRMS: m/z (Mþ) calcd. for C10H10O4: 194.0579. Found:
194.0581.
6,6-Dimethyl-4-oxo-4,5,6,7-tetrahydrobenzofuran-2-carboxylic Acid
Methyl Ester (9) and 6,6-Dimethyl-4-oxo-4,5,6,7-
tetrahydrobenzofuran-3-carboxylic Acid Methyl Ester (10)
Reaction of 2 (342 mg, 1.0 mmol) and methyl propiolate (2 mL) under
Rh2(OPiv)4 (6 mg) at room temperature for 5 h afforded 9 (102 mg, 46%)
1
and 10 (27 mg, 12%) as oils. 9: H NMR (300 MHz, CDCl3) d 7.37 (1H, s),

1948
Y. R. Lee and S. H. Yoon
3.89 (3H, s), 2.80 (2H, s), 2.40 (2H, s), 1.13 (6H, s); 13C NMR (75 MHz,
CDCl3) d 193.6, 169.3, 159.2, 145.1, 121.7, 114.3, 52.7, 52.4, 37.9, 35.6,
28.9; IR (neat) 3136, 2959, 2874, 1732, 1688, 1593, 1545, 1453, 1290,
1217, 1152, 1105, 1032, 981 cm21; HRMS: m/z (Mþ) calcd. for C12H14O4:
1
222.0892. Found: 222.0893. 10: H NMR (300 MHz, CDCl3) d 7.89 (1H, s),
3.85 (3H, s), 2.75 (2H, s), 2.42 (2H, s), 1.12 (6H, s); IR (neat) 2957, 2926,
2855, 1742, 1694, 1555, 1441, 1298, 1281, 1113, 1032, 802 cm21; HRMS:
m/z (Mþ) calcd. for C12H14O4: 222.0892. Found: 222.0894.
4-Oxo-4,5,6,7-tetrahydrobenzofuran-2-carboxylic Acid Ethyl Ester (11)
and 4-Oxo-4,5,6,7-tetrahydrobenzofuran-3-carboxylic
Acid Ethyl Ester (12)
Reaction of 1 (314 mg, 1.0 mmol) and ethyl propiolate (2 mL) under
Rh2(OPiv)4 (6 mg) at room temperature for 5 h afforded 11 (85 mg, 41%)
1
and 12 (21 mg, 10%). 11: mp 548C; H NMR (300 MHz, CDCl3) d 7.36
(1H, s), 4.35 (2H, q, J ¼ 7.1 Hz), 2.95 (2H, t, J ¼ 6.3 Hz), 2.52 (2H, t,
J ¼ 5.9 Hz), 2.24–2.15 (2H, m), 1.36 (3H, t, J ¼ 7.1 Hz); 13C NMR
(75 MHz, CDCl3) d 194.2, 170.0, 158.9, 145.1, 122.8, 114.2, 61.8, 38.0,
24.0, 22.6, 14.7; IR (KBr) 3003, 2978, 2938, 1713, 1689, 1592, 1544, 1441,
1372, 1306, 1208, 1187, 1148, 1018, 1001, 892 cm21; HRMS: m/z (Mþ)
calcd. for C11H12O4: 208.0736. Found: 208.0738. 12: oil; 1H NMR
(300 MHz, CDCl3) d 7.86 (1H, s), 4.31 (2H, q, J ¼ 7.1 Hz), 2.88 (2H, t,
J ¼ 6.3 Hz), 2.53 (2H, t, J ¼ 6.3 Hz), 2.19–2.11 (2H, m), 1.36 (3H, t,
J ¼ 7.1 Hz); IR (neat) 2955, 1740, 1690, 1551, 1431, 1369, 1293, 1227,
1108, 1034, 1003, 834 cm21; HRMS: m/z (Mþ) calcd. for C11H12O4:
208.0736. Found: 208.0734.
4-Oxo-6-phenyl-4,5,6,7-tetrahydrobenzofuran-2-carboxylic Acid Ethyl
Ester (13)
Reaction of 3 (390 mg, 1.0 mmol) and ethyl propiolate (2 mL) under
Rh2(OPiv)4 (6 mg) at room temperature for 5 h afforded 13 (97 mg, 34%).
1
Mp 72–738C; H NMR (300 MHz, CDCl3) d 7.38–7.24 (5H, m), 4.35 (2H,
q, J ¼ 7.1 Hz), 3.61–3.51 (1H, m), 3.29–3.05 (2H, m), 2.88–2.71 (2H, m),
1.36 (3H, t, J ¼ 7.1 Hz); 13C NMR (75 MHz, CDCl3) d 192.6, 169.0, 158.8,
145.6, 142.2, 131.3, 129.3, 127.9, 127.1, 122.8, 114.1, 61.8, 45.4, 41.3,
30.7, 14.6; IR (KBr) 2995, 2955, 1730, 1674, 1543, 1451, 1291, 1211,
1021, 849 cm21; HRMS: m/z (Mþ) calcd. for C17H16O4: 284.1049. Found:
284.1050.
2-Acetyl-4,5,6,7-Tetrahydrobenzofuran-4-one (14)
Reaction of 1 (314 mg, 1.0 mmol) and 3-butyn-2-one (2 mL) under
Rh2(OPiv)4 (6 mg) at room temperature for 5 h afforded 14 (77 mg, 43%).

Efficient Synthesis of Substituted Furans
1949
Mp 54–558C; 1H NMR (300 MHz, CDCl3) d 7.35 (1H, s), 2.94 (2H, t,
J ¼ 6.3 Hz), 2.52 (2H, t, J ¼ 6.3 Hz), 2.43 (3H, s), 2.23–2.15 (2H, m); 13C
NMR (75 MHz, CDCl3) d 194.1, 186.7, 170.5, 152.4, 123.0, 114.1, 38.0,
26.3, 24.0, 22.5; IR (KBr) 3093, 2958, 1674, 1582, 1531, 1454, 1415, 1355,
1300, 1219, 1138, 1085, 1003, 882 cm21; HRMS: m/z (Mþ) calcd. for
C10H10O3: 178.0630. Found: 178.0633.
2-Acetyl-6,6-dimethyl-4,5,6,7-tetrahydrobenzofuran-4-one (15)
Reaction of 2 (342mg, 1.0 mmol) and 3-butyn-2-one (2mL) under Rh2(OPiv)4
(6 mg) at room temperature for 5 h afforded 15 (93 mg, 45%). Mp 80–818C; 1H
NMR (300MHz, CDCl3) d 7.33 (1H, s), 2.78 (2H, s), 2.42 (3H, s), 2.35 (2H, s),
1.11 (6H, s); 13C NMR (75 MHz, CDCl3) d 193.5, 186.6, 169.7, 152.8, 121.9,
114.1, 52.4, 37.9, 35.5, 28.9, 28.7, 26.3; IR (KBr) 3110, 2956, 1673, 1581,
1525, 1451, 1281, 1222, 1134, 1085, 1038, 964 cm21; HRMS: m/z (Mþ)
calcd. for C12H14O3: 206.0943. Found: 206.0941.
2-Acetyl-6-phenyl-4,5,6,7-tetrahydrobenzofuran-4-one (16)
Reaction of 3 (390 mg, 1.0 mmol) and 3-butyn-2-one (2 mL) under
Rh2(OPiv)4 (6 mg) at room temperature for 5 h afforded 16 (102 mg, 40%).
1
Mp 134–1358C; H NMR (300 MHz, CDCl3) d 7.40 (1H, s), 7.38–7.24
(5H, m), 3.63–3.49 (1H, m), 3.30–3.07 (2H, m), 2.84–2.73 (2H, m), 2.46
(3H, s); 13C NMR (75 MHz, CDCl3) d 192.9, 186.8, 169.7, 152.7, 142.1,
129.5, 128.0, 127.1, 122.9, 114.2, 53.9, 45.4, 41.2, 31.8, 26.5; IR (KBr)
3097, 2962, 2921, 1674, 1531, 1452, 1280, 1220, 1145, 1034, 969,
933 cm21; HRMS: m/z (Mþ) calcd. for C16H14O3: 254.0943. Found:
254.0943.
9-Acetylphenaleno[1,2-b] furan-7-one (17) and 2-Iodo-3-
phenoxyphenalen-1-one (18)
Reaction of 4 (398 mg, 1.0 mmol) and 3-butyn-2-one (2 mL) under
Rh2(OPiv)4 (6 mg) at room temperature for 5 h afforded 17 (97 mg, 37%)
1
and 18 (48 mg, 12%). 17: mp 179–1808C; H NMR (300 MHz, CDCl3) d
8.71 (1H, d, J ¼ 7.4 Hz), 8.36 (1H, d, J ¼ 7.3 Hz), 8.22 (1H, d, J ¼ 8.0 Hz),
8.10 (1H, d, J ¼ 8.0 Hz), 7.79 (1H, dd, J ¼ 8.0, 7.4 Hz), 7.74 (1H, s), 7.69
(1H, dd, J ¼ 8.0, 7.3 Hz), 2.51 (3H, s); 13C NMR (75 MHz, CDCl3) d
187.0, 180.5, 161.3, 152.9, 136.1, 133.3, 132.9, 131.7, 129.8, 127.5, 126.9,
125.8, 123.0, 120.1, 115.6, 26.8; IR (KBr) 3103, 2925, 2854, 1675, 1655,
1586, 1523, 1377, 1292, 1260, 1204, 884, 776 cm21; HRMS: m/z (Mþ)
1
calcd. for C17H10O3: 262.0630. Found: 262.0630. 18: mp 183–1848C; H
NMR (300 MHz, CDCl3) d 8.76 (1H, d, J ¼ 7.4 Hz), 8.25 (1H, d,
J ¼ 8.0 Hz), 8.10 (1H, d, J ¼ 8.0 Hz), 7.99 (1H, d, J ¼ 7.4 Hz), 7.80 (1H,
dd, J ¼ 8.0, 7.4 Hz), 7.53 (1H, dd, J ¼ 8.0, 7.4 Hz), 7.31 (2H, dd, J ¼ 8.6,

1950
Y. R. Lee and S. H. Yoon
7.5 Hz), 7.07 (1H, dd, J ¼ 8.6, 7.5 Hz), 7.07 (1H, t, J ¼ 7.5 Hz), 6.99 (2H, d,
J ¼ 8.6 Hz); 13C NMR (75 MHz, CDCl3) d 181.5, 164.9, 157.3, 150.2, 136.1,
133.7, 132.9, 132.6, 130.4, 129.1, 127.9, 127.5, 127.2, 127.1, 125.3, 123.3,
116.1, 101.4; IR (KBr) 3065, 1638, 1572, 1553, 1489, 1402, 1312, 1225,
1209, 1165, 1152, 1074, 1024, 947 cm21; HRMS: m/z (Mþ) calcd. for
C19H11IO2: 397.9804. Found: 397.9802.
2-Isopropenyl-4,5,6,7-tetrahydrobenzofuran-4-one (19)
Reaction of 1 (314 mg, 1.0 mmol) and 2-methyl-1-buten-3-yne (2 mL) under
Rh2(OPiv)4 (6 mg) at room temperature for 5 h afforded 19 (93 mg, 53%) as
an oil. 1H NMR (300 MHz, CDCl3) d 6.50 (1H, s), 5.50 (1H, s), 5.00 (1H, s),
2.84 (2H, t, J ¼ 6.3 Hz), 2.45 (2H, t, J ¼ 6.3 Hz), 2.20–2.09 (2H, m), 1.95
(3H, s); 13C NMR (75 MHz, CDCl3) d 195.1, 167.3, 155.6, 132.5, 122.9,
111.6, 102.6, 37.9, 23.8, 22.9, 19.6; IR (neat) 3115, 2947, 1672, 1592, 1546,
1415, 1367, 1239, 1214, 1183, 1132, 1103, 1003, 953, 889, 818 cm21
HRMS: m/z (Mþ) calcd. for C11H12O2: 176.0837. Found: 176.0839.
;
2-Isopropenyl-6,6-dimethyl-4,5,6,7-tetrahydrobenzofuran-4-one (20)
Reaction of 2 (342mg, 1.0 mmol) and 2-methyl-1-buten-3-yne (2 mL) under
Rh2(OPiv)4 (6mg) at room temperature for 5 h afforded 20 (96 mg, 47%) as an
oil. 1H NMR (300MHz, CDCl3) d 6.50 (1H, s), 5.50 (1H, s), 5.01 (1H, s), 2.73
(2H, s), 2.35 (2H, s), 1.98 (3H, s), 1.12 (6H, s); 13C NMR (75 MHz, CDCl3)
d 194.5, 166.5, 155.9, 132.6, 124.8, 111.5, 102.6, 52.3, 37.8, 35.7, 29.0, 27.8,
19.6; IR (neat) 2959, 1679, 1585, 1451, 1415, 1372, 1221, 1120, 1038,
843 cm21; HRMS: m/z (Mþ) calcd. for C13H16O2: 204.1150. Found: 204.1149.
ACKNOWLEDGMENT
This work was supported by the Korea Research Foundation Grant R05-2004-
000-10224-0, funded by the Korean government. We thank Ronald Tepper for
discussion of this work.
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