Facile synthesis of oxabicyclic alkenes by ultrasonication-promoted Diels-Alder cycloaddition of furano Dienes

Article abstract of DOI:10.1021/jo049210a

Ultrasonic irradiation effectively promotes the Diels-Alder reaction of substituted furans with reactive dienophiles (i.e., dimethyl acetylenedicarboxylate (DMAD) and dimethyl maleate). Regiospecific furano Diels-Alder cycloaddition of 2-vinylic furans with DMAD furnished functionalized oxabicyclic alkenes in good yield under ultrasonication condition.

Full text of DOI:10.1021/jo049210a

Although ultrasonication (sonochemical method) has
been widely applied in many other synthetic reactions,9
there are only scattered reports10 on the Diels-Alder
reaction affected by ultrasonic (US) irradiation, and very
few cases10i,j where the Diels-Alder reactions of furano
dienes were promoted by US irradiation. In connection
with our recent studies,4c we report here the distinct
beneficial effects of US irradiation on the Diels-Alder
cycloaddition of substituted furano derivatives leading
to a practical and convenient synthesis of oxabicyclic
alkenes.
F a cile Syn th esis of Oxa bicyclic Alk en es by
Ultr a son ica tion -P r om oted Diels-Ald er
Cycloa d d ition of F u r a n o Dien es
Kun Wei, Hai-Tao Gao, and Wei-Dong Z. Li*
State Key Laboratory of Applied Organic Chemistry,
Lanzhou University, Lanzhou 730000, China
liwd@lzu.edu.cn
Received May 10, 2004
Although dimethyl acetylenedicarboxylate (DMAD) has
been a highly reactive dienophile widely used in Diels-
Alder reactions, substituted furano derivatives usually
react sluggishly with DMAD under conventional thermal
conditions or Lewis acid catalysis.11 For example, al-
though 2-methylfuran reacted with DMAD at 40 °C for
5 h to give the corresponding [4 + 2] cycloadduct in ca.
50% yield, the analogous adduct 1 was obtained in a
small yield even in the presence of Lewis acids (i.e.,
BF3‚OEt2, AlCl3, MgBr2, etc.) or in a sealed-tube heated
to 100 °C.12 When a (neat) mixture of 2-substituted or
2,5-disubstituted furan and DMAD was subjected to US
irradiation at ambient temperature for 15 h, cycloadducts
1-3 were isolated as the sole products in good to excellent
yields.
Abstr a ct: Ultrasonic irradiation effectively promotes the
Diels-Alder reaction of substituted furans with reactive
dienophiles (i.e., dimethyl acetylenedicarboxylate (DMAD)
and dimethyl maleate). Regiospecific furano Diels-Alder
cycloaddition of 2-vinylic furans with DMAD furnished
functionalized oxabicyclic alkenes in good yield under ul-
trasonication condition.
Furans are generally much less reactive dienes (or
dienophiles) in Diels-Alder cycloaddition reaction due
to their inherent aromaticity1 and usually facile retro-
Diels-Alder reaction of the resulting adducts.2 In view
of the importance and great potential of Diels-Alder
cycloadducts of furano derivatives as key intermediates
for synthesizing structurally complex targets3 or achiev-
ing highly regio- and stereocontrolled reactions via the
corresponding oxabicyclic adducts,4 a great deal of effort
has been devoted to the development of chemical and
physical means to promote the frequently difficult [4 +
2] cycloaddition of cyclic furano dienes. For example, a
variety of Lewis acids5 were commonly used as catalysts
to promote or accelerate the Diels-Alder reaction of
furans. Some nonthermal forcing conditions, including
high temperature and/or high pressure,6 and microwave
irradiation7 have also been applied to promote cycload-
dition of furans and other hetero-dienes.8
Encouraged by this initial dramatic effect of US
irridiation, we next examined the 2-vinylic furano deriva-
(8) For examples, see: (a) Diaz-Ortiz, A.; De la Hoz, A.; Prieto, P.;
Carrillo, J . R.; Moreno, A.; Neunhoeffer, H. Synlett 2001, 236. (b) Diaz-
Ortiz, A.; Carrillo, J . R.; Gomez-Escalonilla, M. J .; De la Hoz, A.;
Moreno, A.; Prieto, P. Synlett 1998, 1069. (c) Diaz-Ortiz, A.; Carrillo,
J . R.; Diez-Barra, E.; De la Hoz, A.; Gomez-Escalonilla, M. J .; Moreno,
A.; Langa, F. Tetrahedron 1996, 52, 9237. (d) Van der Eycken, E.;
Appukkuttan, P.; De Borggraeve, W.; Dehaen, W.; Dallinger, D.; Kappe,
C. O. J . Org. Chem. 2002, 67, 7904. (e) Hong, B.-C.; Shr, Y.-J .; Liao,
J .-H. Org. Lett. 2002, 4, 663.
(9) For reviews, see: (a) Lindley, J .; Mason, T. J . Chem. Soc. Rev.
1987, 16, 275. (b) Pindur, U.; Lutz, G.; Otto, C. Chem. Rev. 1993, 93,
741. (c) Ho, T.-L. Distinctive Techniques for Organic SynthesissA
Practical Guide; World Scientific Publishing Co. Pte., Ltd.: 1998,
Singapore; pp 90-127. (d) Fringuelli, F.; Taticchi, A. In The Diels-
Alder Reaction-Selected Practical Methods; J ohn Wiley & Sons: 2002,
New York; Chapter 4, pp 143-204. (e) Fillion, H.; Luche, J .-L. In
Synthetic Organic Sonochemistry; Luche, J .-L., Ed.; Plenum Press:
1998, New York; Chapter 3, pp 91-106.
(10) (a) Lee, J .; Snyder, J . K. J . Am. Chem. Soc. 1989, 111, 1522.
(b) Lee, J .; Snyder, J . K. J . Org. Chem. 1990, 55, 4995. (c) Caulier, T.
P.; Reisse, J . J . Org. Chem. 1996, 61, 2547. (d) Raj, C. P.; Dhas, N. A.;
Cherkinski, M.; Gedanken, A.; Braverman, S. Tetrahedron Lett. 1998,
39, 5413. (e) Avalos, M.; Babiano, R.; Cabello, N.; Cintas, P.; Hurst-
house, M. B.; J imenez, J . L.; Light, M. E.; Palacios, J . C. J . Org. Chem.
2003, 68, 7193. (f) Tuulmets, A. Ultrasonics Sonochem. 1997, 4, 189.
(g) Cabello, N.; Cintas, P.; Luche, J .-L. Ultrasonics Sonochem. 2003,
10, 25. (h) Villacampa, M.; Perez, J . M.; Avendano, C.; Menendez, J .
C. Tetrahedron 1994, 50, 10047. For a formal [3 + 4] cycloaddition of
furan, see: (i) J oshi, N. N.; Hoffmann, H. M. R. Tetrahedron Lett. 1986,
27, 687. For an example on dramatic improvement of Diels-Alder
reaction in water under ultrasonication, see: (j) Saksena, A. K.;
Girijavallabhan, V. M.; Chen, Y.-T.; J ao, E.; Pike, R. E.; Desai, J . A.;
Rane, D.; Ganguly, A. K. Heterocycles 1993, 35, 129.
* Current address: Department of Chemistry and Chemical Biology,
Harvard University, 12 Oxford Street, Cambridge, MA 02138.
E-mail: wdli@fas.harvard.edu.
(1) For an account on hetero-aromaticity, see: Balaban, A. T.;
Oniciu, D. C.; Katritzky, A. R. Chem. Rev. 2004, 104, 2777.
(2) For examples, see: (a) Woodward, R. B. J . Am. Chem. Soc. 1940,
62, 1478. (b) Dauben, W. G.; Kessel, C. R.; Takemura, K. H. J . Am.
Chem. Soc. 1980, 102, 6893. (c) Kappe, C. O.; Murphree, S. S.; Padwa,
A. Tetrehedron 1997, 53, 14179.
(3) For examples, see: (a) Hudlicky, T.; Entwistle, D. A.; Pitzer, K.
K.; Thorpe, A. J . Chem. Rev. 1996, 96, 1195. (b) Grieco, P. A.; Zelle, R.
E.; Lis, R.; Finn, J . J . Am. Chem. Soc. 1983, 105, 1403. (c) Schmidt, R.
R.; Beitzke, C.; Forrest, A. K. J . Chem. Soc., Chem. Commun. 1982,
909.
(4) For an excellent review (Using Ring-Opening Reactions of
Oxabicyclic Compounds as a Strategy in Organic Synthesis), see: (a)
Chiu, P.; Lautens, M. Top. Curr. Chem. 1997, 190, 1-85. For recent
examples from this laboratory: (b) Zhang, Z.; Li, W.-D. Z.; Li, Y. Org.
Lett. 2001, 3, 2555. (c) Li, W.-D. Z.; Wei, K. Org. Lett. 2004, 6, 1333.
(5) For examples, see: (a) Brion, F. Tetrehedron Lett. 1982, 23, 5299.
(b) Grieco, P. A.; Nunes, J . J .; Gaul, M. D. J . Am. Chem. Soc. 1990,
112, 4595. (c) Hayashi, Y.; Nakamura, M.; Nakao, S.; Inoue, T.; Shoji,
M. Angew. Chem., Int. Ed. 2002, 41, 4079 and references therein.
(6) For examples, see: (a) J enner, G.; Salem, R. B. Tetrehedron 1997,
53, 4637. (b) Dauben, W. G.; Lam, J . Y. L.; Guo, Z. R. J . Org. Chem.
1996, 61, 4816. (c) Dauben, W. G.; Krabbenhoft, H. O. J . Am. Chem.
Soc. 1976, 98, 1992.
(7) For an example, see: De la Hoz, A.; Diaz-Ortiz, A.; Fraile, J .
M.; Gomez, M. V.; Mayoral, J . A.; Moreno, A.; Saiz, A.; Vazquez, E.
Synlett 2001, 753.
10.1021/jo049210a CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/14/2004
J . Org. Chem. 2004, 69, 5763-5765
5763

SCHEME 1
SCHEME 2
tives as dienes in the Diels-Alder reaction with DMAD,
for there are two types of diene systems present, namely,
the cyclic furano diene and exo-cyclic diene. Although a
small yield (ca. 5%) of 6 (R ) H) was obtained from the
reaction of 2-vinyl furan 4 (R ) H) and DMAD at room
temperature,13 along with small yield (ca. 5%) of the
aromatized benzofuran 5 (R ) H), as previously docu-
mented by Elix and co-workers,14 the reaction of substi-
tuted 2-vinylic furan 4 (R * H) with DMAD mainly led
to polymerized materials, without any detectable furano
diene cycloadducts under thermal or Lewis acid-catalysis
(vide supra) conditions. Remarkably, the analogous reac-
tion under US irradiation conditions furnished solely the
cyclic furano diene cycloadducts 6a -c from substituted
2-vinylic furan 4 in good isolated yields (Scheme 1). Thus,
a highly regioselective Diels-Alder cycloadditions of
2-vinylic furans with DMADC can be achieved under US
irradiation. We reason that this dramatic effect15 may
be attributed to the sonochemical (kinetic) activation16
of the furan nuclei (oxa-cyclic diene) by polarization-
induced electronic excitation or polarization-facilitated
(single) electron transfer to the dienophile (a redox
process).17
Since dimethyl maleate (DMM) is much less reactive
dienophile than DMAD, the corresponding Diels-Alder
reactions with furano derivatives are very slow and
sluggish under thermal (sealed tube) conditions or Lewis
acid catalysis, giving only a small yield of the correspond-
ing oxabicyclic adducts, due to the facile retro-Diels-
Alder process.4c In contrast, the cycloaddition of DMM
with 2-substituted or 2,5-disubstituted furans under US
irradiation (neat, rt, 30 h) produced solely the corre-
sponding [4 + 2] cycloadducts 7-10 in 20∼26% yields.18
Although the conversion and isolated yields are relatively
low, the exo/ endo selectivities are generally very high
(>50:1). It is apparent that US irradiation promotes the
furano [4 + 2] cycloaddition leading to the thermody-
namically favorable exo-isomers, due presumably to the
accelerating effect of US irradiation on the retro-Diels-
Alder reaction of the kinetically favorable endo-isomer.
Reduction of the Diels-Alder adduct 10 by LiAlH4 in
THF afforded (98%) the oxabicyclic diol 11, which then
cyclized upon treatment with trifluoromethanesulfonic
acid (Scheme 2) to give (63%) the 1,4-epoxy ring-opened
cyclization products 12a and 12b (3:1), analogous to our
recent report.4c
In summary, the above experimental results have
demonstrated the significant sonochemical effects on
Diels-Alder reaction of furano dienes with reactive
dienophiles in terms of the reaction yield and regio- and
stereoselectivities. This convenient and practical labora-
tory technique provides a simple physical means for the
activation of the relatively inert furano diene system.
Although the fundamental mechanism of US activation
on Diels-Alder reaction is not well understood currently
and needs to be further elucidated,10c-g the simple
technique described herein could be generally useful in
the cycloaddition of furano derivatives.
(11) For an example, see: Alcaide, B.; Biurrum, C.; Plumet, J .
Heterocycles 1993, 36, 1795.
(12) Cycloaddition product was severely contaminated with aroma-
tized and other polymeric byproducts under these forcing conditions.
(13) Thermal conditions (at 80 °C) did not result in a better yield of
cycloaddition product
presumably due to the faster retro-Diels-Alder process of 6; cf. ref
14a.
6 but instead led to more side reactions,
Exp er im en ta l Section 19
Typ ica l Exp er im en ta l P r oced u r e for th e Ultr a son ica -
tion -P r om oted Diels-Ald er Cycloa d d ition of F u r a n . A
degassed mixture of 2-[â-(m-methoxyphenyl)ethyl]furan (202 mg,
1.0 mmol) and DMAD (170 mg, 1.2 mmol) in a test tube was
(14) (a) Davidson, W. J .; Elix, J . A. Aust. J . Chem. 1973, 26, 1059.
(b) Elix, J . A.; Tronson, D. Aust. J . Chem. 1973, 26, 1093. For an earlier
study, see also: (c) Herz, W. J . Am. Chem. Soc. 1946, 68, 2732.
(15) Cis substituent of 2-vinylic furan 4 may further (sterically)
prevent the access of exo-cyclic diene towards dienophile.
(16) Cf.: (a) Suslick, K. S.; J ohnson, R. E. J . Am. Chem. Soc. 1984,
106, 6856. (b) Diaba, F.; Lewis, I.; Grignon-Dubois, M.; Navarre, S. J .
Org. Chem. 1996, 61, 4830. See also ref 10d.
sonicated while submerged in
a water bath of a common
(18) Unreacted furano derivatives can be easily recovered in good
yield.
(19) For General Experimental Procedures, cf. Supporting Informa-
tion of ref 4c.
(17) For relevant accounts, see: (a) Woodward, R. B. J . Am. Chem.
Soc. 1942, 64, 3058. (b) Woodward, R. B.; Baer, H. J . Am. Chem. Soc.
1944, 66, 645.
5764 J . Org. Chem., Vol. 69, No. 17, 2004

laboratory ultrasonic cleaner (40 kHz, 160 W) for 15 h. The bath
temperature was maintained between 25∼35 °C by occasional
cooling. The resulting orange gum was then purified by column
chromatography on silica gel eluting with EtOAc-petroleum
ether (1:8, v/v) to yield 336 mg (95%) of oxabicyclic diene 2 as a
colorless oil. Rf (EtOAc-petroleum ether, 1:4) ) 0.27. IR (film):
Preparation of oxabicyclic diol 11 and its cyclization.
Lithium aluminum hydride reduction4c of 10 in THF gave 11
in 98% yield as a colorless oil. Rf (EtOAc-petroleum ether, 1:2)
) 0.34. IR (film): υmax 3349, 2931, 1601, 1489, 1454, 1256, 1152,
1033 cm-1 1H NMR (200 MHz, CDCl3): δ 1.59(s, 3H), 2.04(m,
.
2H), 2.51(m, 2H), 3.26(t, J ) 11.0 Hz, 2H), 3.65(d, J ) 11.0 Hz,
2H), 3.75(m, 2H), 3.80(s, 3H), 6.08(s, 2H), 6.72(m, 3H), 7.15(t, J
) 7.6 Hz, 1H) ppm. 13C NMR (50 MHz, CDCl3): δ 18.0, 31.0,
33.9, 51.3, 52.8, 55.0, 60.8, 60.9, 86.3, 89.4, 111.2, 114.0, 120.6,
129.3, 136.9, 138.2, 143.6, 159.6 ppm. LRMS (EI) m/z: 304 (M+,
0.01%), 286 (0.01), 216 (8), 121 (4), 95 (100).
υ
max 3011, 2951, 1769, 1731, 1586, 1490, 1438, 1369, 1268, 1190
1
cm-1. H NMR (200 MHz, CDCl3): δ 2.43(m, 2H), 2.69(m, 2H),
3.79(s, 3H), 3.80(s, 3H), 3.83(s, 3H), 5.68(d, J ) 2.0 Hz, 1H),
6.73(m, 3H), 6.98(d, J ) 5.2 Hz, 1H), 7.20(dd, J 1 ) 1.8 Hz, J 2
)
5.0 Hz, 1H), 7.21(m, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ
30.5, 30.8, 51.8, 51.9, 54.7, 83.0, 96.8, 111.0, 113.8, 120.3, 129.1,
142.6, 144.3, 144.5, 151.5, 155.4, 162.4, 164.6 ppm. LRMS (EI)
m/z: 344 (M+, 0.1%), 312 (4), 283 (7), 202 (6), 121 (100), 81 (68).
HRMS (SIMS) m/z: [M + H]+ 345.1331 (C19H20O6 requires
345.1333).
To a solution of compound 11 (157 mg, 0.5 mmol) in 5 mL of
CH2Cl2 was added CF3SO3H (0.013 mL, 0.15 mmol) at 0 °C. The
resulting mixture was stirred for 0.5 h at room temperature,
quenched with saturated aqueous NaHCO3 (5 mL), and ex-
tracted (3 × 50 mL) with AcOEt. The organic layer was washed
with H2O and brine, dried (MgSO4), and concentrated, and the
crude product was purified by silica gel chromatography eluting
with EtOAc-petroleum ether (1:2) to yield 65 mg (48%) of 12a
and 21 mg (15%) of 12b as colorless oils: Rf of 12a (EtOAc-
petroleum ether, 2:1) ) 0.36; Rf of 12b (EtOAc-petroleum ether,
2:1) ) 0.38. 12a IR (film): υmax 3415, 2930, 2871, 1609, 1500,
Oxabicyclic diene 3, colorless oil. Rf (EtOAc-petroleum ether,
1:4) ) 0.40. IR (film): υmax 2998, 2838, 1716, 1635, 1603, 1490,
1437, 1307, 1253, 1139, 1036 cm-1. 1H NMR (200 MHz, CDCl3):
δ 1.83(s, 3H), 2.46(m, 2H), 2.70(m, 2H), 3.76(s, 6H), 3.79(s, 3H),
6.71(m, 3H), 6.95(d, J ) 1.6 Hz, 1H), 7.15(t, J ) 7.8 Hz, 1H)
ppm. 13C NMR (75 MHz, CDCl3): δ 15.1, 30.9, 31.0, 51.8, 51.8,
54.7, 91.8, 95.0, 111.0, 113.8, 120.4, 129.1, 142.9, 142.9, 145.9,
147.0, 153.7, 155.2, 159.3, 164.0 ppm. LRMS (EI) m/z: 358 (M+,
0.2%), 326 (5), 163 (94), 202 (6), 121 (58), 95 (100). HRMS (SIMS)
m/z: [M + H]+ 359.1494 (C20H22O6 requires 359.1489).
Oxabicyclic triene 6b, colorless oil. Rf (EtOAc-petroleum
ether, 1:4) ) 0.25. IR (film): υmax 2984, 2955, 1729, 1644, 1437,
1312, 1266, 1201, 1157, 1113 cm-1. 1H NMR (200 MHz, CDCl3):
δ 1.20(t, J ) 7.2 Hz, 3H), 2.34(m, 2H), 2.48(m, 2H), 3.77(s, 3H),
3.82(s, 3H), 4.06(q, J ) 7.2 Hz, 2H), 5.68(d, J ) 2.0 Hz, 1H),
1463, 1440, 1255, 1236, 1067, 1038 cm-1 1H NMR (200 MHz,
.
CDCl3): δ 1.81(s, 3H), 1.96(m, 3H), 2.59(m, 1H), 2.87(m, 2H),
3.32(bs, 1H), 3.51(t, J ) 10.2 Hz, 1H), 3.76(s, 3H), 3.83(m, 2H),
4.05(dd, J 1 ) 4.0 Hz, J 2 ) 7.2 Hz, 1H), 5.73(br, 1H), 6.61(d, J )
2.6 Hz, 1H), 6.71(dd, J 1 ) 2.6 Hz, J 2 ) 8.4 Hz, 1H), 7.16(d, J )
8.2 Hz, 1H) ppm. 13C NMR (50 MHz, CDCl3): δ 22.4, 28.9, 29.4,
43.2, 45.5, 49.2, 55.1, 60.4, 73.9, 82.7, 112.2, 112.9, 120.4, 127.6,
131.2, 136.2, 140.7, 157.4 ppm. LRMS (EI) m/z: 286 (M+, 10%),
268 (1), 256 (7), 95 (100). HRMS (ESI) m/z: [M + H]+ 287.1640
(C18H23O3 requires 287.1643). 12b IR (film): υmax 3402, 2930,
5.83(m, 2H), 7.11(d, J ) 5.0 Hz, 1H), 7.18(dd, J 1 ) 2.0 Hz, J 2
)
5.0 Hz, 1H) ppm. 13C NMR (75 MHz, CDCl3): δ 14.1, 24.2, 33.7,
52.2, 52.2, 60.2, 83.5, 94.2, 121.9, 137.3, 143.6, 146.2, 149.4,
158.0, 162.4, 164.8, 172.7 ppm. LRMS (EI) m/z: 336 (M+, 0.6%),
310 (1), 259 (5), 194 (32), 120 (100). HRMS (SIMS) m/z: [M +
H]+ 337.1285 (C17H20O7 requires 337.1282).
2872, 1580, 1464, 1438, 1248, 1060, 1022 cm-1 1H NMR (200
.
MHz, CDCl3): δ 1.77(s, 3H), 1.88(m, 2H), 2.05(m, 1H), 2.58(m,
1H), 2.78(m, 2H), 3.59(m, 2H), 3.83(s, 3H), 3.92(d, J ) 7.2 Hz,
2H), 4.08(dd, J 1 ) 4.4 Hz, J 2 ) 7.4 Hz, 1H), 6.10(br, 1H),
6.68(d, J ) 8.0 Hz, 2H), 7.04(t, J ) 8.2 Hz, 1H) ppm. 13C NMR
(50 MHz, CDCl3): δ 22.7, 29.2, 29.3, 41.9, 46.6, 49.2, 55.0, 60.5,
74.3, 83.2, 108.3, 120.7, 121.1, 126.3, 127.3, 137.1, 141.3, 157.7
ppm. LRMS (EI) m/z: 286 (M+, 12%), 268 (1), 256 (10), 225 (100),
84 (45). HRMS (ESI) m/z: [M + H]+ 287.1643 (C18H23O3 requires
287.1643).
Oxabicyclic alkene 9, colorless oil. Rf (EtOAc-petroleum ether,
1:4) ) 0.22. IR (film): υmax 3086, 2951, 2838, 1743, 1601, 1202,
1165, 1043 cm-1 1H NMR (400 MHz, CDCl3): δ 2.13(m, 1H),
.
2.41(m, 1H), 2.67(m, 2H), 3.14(d, J ) 10.0 Hz, 1H), 3.60(dd, J 1
) 4.4 Hz, J 2 ) 10.1 Hz, 1H), 3.64(s, 3H), 3.65(s, 3H), 3.80(s,
3H), 5.13(dd, J 1 ) 1.5 Hz, J 2 ) 4.2 Hz, 1H), 6.47(d, J ) 5.7 Hz,
1H), 6.54(dd, J 1 ) 1.5 Hz, J 2 ) 5.5 Hz, 1H), 6.74(m, 2H), 6.54(d,
J ) 7.5 Hz, 1H), 7.20(t, J ) 7.6 Hz, 1H) ppm. 13C NMR (100
MHz, CDCl3): δ 30.9, 33.5, 50.5, 51.4, 51.7, 54.9, 79.3, 91.2,
111.2, 113.8, 120.5, 129.2, 135.1, 136.7, 143.1, 159.6, 170.5, 170.9
ppm. LRMS (EI) m/z: 346 (M+, 0.1%), 328 (0.05), 315 (0.2), 202
(18), 121 (44), 81(100). HRMS (SIMS) m/z: [M + H]+ 347.1496
(C19H22O6 requires 347.1489).
Ack n ow led gm en t. We thank the National Natural
Science Foundation of China (Distinguished Youth Fund
29925204, Funds 20021001 and 20172022) and Ministry
of National Education of China (Funds 99114 and 2000-
66) for financial support. The Cheung Kong Scholars
program is gratefully acknowledged.
Oxabicyclic alkene 10, colorless oil. Rf (EtOAc-petroleum
ether, 1:4) ) 0.30. IR (film): υmax 2951, 2840, 1742, 1601, 1582,
1490, 1454, 1436, 1197, 1158, 1038 cm-1 1H NMR (200 MHz,
.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
CDCl3): δ 1.69(s, 3H), 2.34(m, 2H), 2.68(m, 2H), 3.61(s, 3H),
3.63(s, 3H), 3.80(s, 3H), 6.36(m, 2H), 6.72(m, 3H), 7.15(t, J )
7.8 Hz, 1H) ppm. 13C NMR (50 MHz, CDCl3): δ 18.0, 31.0, 33.8,
51.6, 52.1, 54.6, 55.0, 55.9, 87.5, 90.6, 111.2, 113.9, 120.6, 129.3,
136.7, 138.2, 143.3, 159.6, 170.9 ppm. LRMS (EI) m/z: 360 (M+,
0.03%), 342 (0.02), 216 (6), 113 (40), 121 (4), 95 (100). HRMS
(SIMS) m/z [M + H]+ 361.1656 (C20H24O6 requires 361.1646).
cedures of preparation of substituted furans, spectral data of
1
compounds 1, 6a , 6c, 7, and 8, and copies of spectra (IR, H
and 13C NMR) of compounds 1, 6b, 9, 11, 12a , and 12b. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O049210A
J . Org. Chem, Vol. 69, No. 17, 2004 5765