Intramolecular Diels-Alder Reaction of Furans with Allenyl Ethers Followed by Sulfur and Silicon Atom-Containing Group Rearrangement

Article abstract of DOI:10.1021/jo980240l

The intramolecular Diels-Alder reactions of a furandiene with an allenyl ether dienophile followed by the alkylthio, alkylsulfinyl, alkylsulfonyl, and trimethylsilyl group rearrangements are reported. Refluxing the propargyl ether 3 with t-BuOK in t-BuOH at 85°C gave the 1,4-rearrangement product 4 exclusively. The alkylthio group 1,4-rearrangement may proceed intramolecularly via the tight ion pairs. Refluxing the 5-(alkylsulfinyl)-2-furfuryl propargyl ethers 9a-e and the 5-(alkylsulfonyl)-2-furfuryl propargyl ethers 19a-e under the same reaction conditions gave the alkylsulfinyl group and the alkylsulfonyl group 1,2-rearrangement products 10a-e and 20a-e, respectively. No detectable amount of the corresponding 1,4-rearrangement products 11a-e and 21a-e was obtained. In the cases of 9b and 19b, which possess two furfurylic hydrogen atoms, no detectable amount of the furan ring-transfer reaction products was obtained. Refluxing the 5-(trimethylsilyl)-2-furfuryl propargyl ethers 25a-d with t-BuOK in t-BuOH at 85°C for 10 h gave the trimethylsilyl group 1,2-rearrangement products 26a-d and Brook rearrangement products 27a-d. The reaction mechanisms for these novel intramolecular Diels-Alder reactions are discussed. In the case of 37, which possesses two furfurylic hydrogen atoms, compound 38 was obtained via an intramolecular Diels-Alder reaction followed by the furan ring-transfer reaction and Brook rearrangement.

Full text of DOI:10.1021/jo980240l

5064
J . Org. Chem. 1998, 63, 5064-5070
In tr a m olecu la r Diels-Ald er Rea ction of F u r a n s w ith Allen yl
Eth er s F ollow ed by Su lfu r a n d Silicon Atom -Con ta in in g Gr ou p
Rea r r a n gem en t
Hsien-J en Wu,* Chia-Hui Yen, and Chi-Te Chuang
Department of Applied Chemistry, National Chiao Tung University, Hinchu, Taiwan, China
Received February 9, 1998
The intramolecular Diels-Alder reactions of a furandiene with an allenyl ether dienophile followed
by the alkylthio, alkylsulfinyl, alkylsulfonyl, and trimethylsilyl group rearrangements are reported.
Refluxing the propargyl ether 3 with t-BuOK in t-BuOH at 85 °C gave the 1,4-rearrangement product
4 exclusively. The alkylthio group 1,4-rearrangement may proceed intramolecularly via the tight
ion pairs. Refluxing the 5-(alkylsulfinyl)-2-furfuryl propargyl ethers 9a -e and the 5-(alkylsulfonyl)-
2-furfuryl propargyl ethers 19a -e under the same reaction conditions gave the alkylsulfinyl group
and the alkylsulfonyl group 1,2-rearrangement products 10a -e and 20a -e, respectively. No
detectable amount of the corresponding 1,4-rearrangement products 11a -e and 21a -e was
obtained. In the cases of 9b and 19b, which possess two furfurylic hydrogen atoms, no detectable
amount of the furan ring-transfer reaction products was obtained. Refluxing the 5-(trimethylsilyl)-
2-furfuryl propargyl ethers 25a -d with t-BuOK in t-BuOH at 85 °C for 10 h gave the trimethylsilyl
group 1,2-rearrangement products 26a -d and Brook rearrangement products 27a -d . The reaction
mechanisms for these novel intramolecular Diels-Alder reactions are discussed. In the case of
37, which possesses two furfurylic hydrogen atoms, compound 38 was obtained via an intramolecular
Diels-Alder reaction followed by the furan ring-transfer reaction and Brook rearrangement.
In tr od u ction
reaction, in all of their cases there was only one carbon
atom and one oxygen atom connection between the furan
diene and the allene dienophile for the cycloaddition. In
these cases, the cycloadducts were not isolated under the
reaction conditions but were further transferred to the
isobenzofuran precursors via ring opening of the bridged
oxygen ring of the cycloadducts.
There is considerable current interest in the intramo-
lecular Diels-Alder reaction, and it has been applied to
a number of synthetic objectives with notable success.1
The vast majority of the work reported in this area has
dealt with reactions utilizing ethylenic and acetylenic
dienophiles. On the other hand, the intramolecular
Diels-Alder reaction of allene has received much less
attention.2 Over a decade ago, Kanematsu et al. dem-
onstrated that the allene unit is a versatile synthon as a
dienophile in the intramolecular cycloaddition due to the
absence of unfavorable nonbonded interactions in the
transition state.3 Afterward, they developed a furan ring-
transfer reaction via the intramolecular Diels-Alder
reaction of a furan diene and an allenyl ether dienophile
and applied this reaction to the synthesis of natural
products.4 For the purpose of the furan ring-transfer
A few years ago, we also investigated this intramo-
lecular cycloaddition. By varying the chain length be-
tween the furan diene and the allenyl ether dienophile,
we found a very high effect of the chain length on the
structure and reactivity of the cycloadducts.5 We also
utilized this intramolecular cycloaddition as a new entry
for the synthesis of indanones and tetralones.6 Later on,
we discovered a novel reaction involving an intramolecu-
lar Diels-Alder reaction of a furan with an allenyl ether
followed by a methylthio group 1,4-rearrangement.7 We
also proposed in general that the intramolecular Diels-
Alder reaction of furfuryl allenyl ethers B, including the
furan ring transfer reaction,4 proceeded via the corre-
sponding zwitterions D as the reaction intermediates8
(Scheme 1). When the X group of A is a hydrogen atom
or an alkyl group, the furan ring transfer reaction takes
place to give the isobenzofuran precursor E. On the other
hand, when the X group of A is a methylthio group, the
(1) For reviews, see: (a) Oppolzer, W. Angew. Chem., Int. Ed. Engl.
1977, 16, 10. (b) Brieger, G.; Bennett, J . N. Chem. Rev. 1980, 80, 63.
(c) Fallis, A. G. Can. J . Chem. 1984, 62, 183. (d) Ciganek, E. Org. React.
1984, 32, 1. (e) Weinreb, S. M.; Staib, R. R. Tetrahedron 1982, 38, 3087.
(f) Weinreb, S. M. Acc. Chem. Res. 1985, 18, 16. (g) Craig, D. Chem.
Soc. Rev. 1987, 16, 187.
(2) For some examples of the intramolecular Diels-Alder reactions
of allenic dienophiles, see: (a) Bartlett, A. J .; Laird, T.; Ollis, W. D. J .
Chem. Soc., Perkin Trans. 1 1975, 1315. (b) Himbert, G.; Diehl, K.;
Maas, G. J . Chem. Soc., Chem. Commun. 1984, 900. (c) Harrison, R.
M.; Hobson, J . D.; Midgley, A. W. J . Chem. Soc., Perkin Trans. 1 1973,
1960. (d) Saxton, H. M.; Sutherland, J . K.; Whaley, C. J . Chem. Soc.,
Chem. Commun. 1987, 1449. (e) Hayakawa, K.; Yasukouchi, T.;
Kanematus, K. Tetrahedron Lett. 1986, 27, 1837. (f) Hayakawa, K.;
Nagatsugi, F.; Kanematsu, K. J . Org. Chem. 1988, 53, 860. (g)
Hayakawa, K.; Yasukouchi, T.; Kanematsu, K. Tertahedron Lett. 1987,
28, 5895. (h) Yasukouchi, T.; Kanematsu, K. Tetrahedron Lett. 1989,
30, 6559. (i) Yoshida, M.; Hiromatsu, M.; Kanematsu, K. Heterocycles
1986, 24, 881. (j) Hayakawa, K.; Ohsuki, S.; Kanematsu, K. Tetrahe-
dron Lett. 1986, 27, 947.
(4) (a) Hayakawa, K.; Yamaguchi, Y.; Kanematsu, K. Tetrahedron
Lett. 1985, 26, 2689. (b) Yamaguchi, Y.; Hayakawa, K.; Kanematsu,
K. J . Chem. Soc., Chem. Commun. 1987, 515. (c) Yamaguchi, Y.;
Yamada, H.; Hayakawa, K.; Kanematsu, K. J . Org. Chem. 1987, 52,
2040. (d) Yamaguchi, Y.; Tatsuta, N.; Hayakawa, K.; Kanematsu, K.
J . Chem. Soc., Chem. Commun. 1989, 470. (e) Kanematsu, K.; Soejima,
S. Heterocycles 1991, 32, 1483.
(5) Wu, H. J .; Lin, S. H.; Lin, C. C. Heterocycles 1994, 38, 1507.
(6) Lin, C. C.; Chen, L. H.; Wu, H. J . J . Chin. Chem. Soc. 1991, 38,
613.
(7) Wu, H. J .; Shao, W. D.; Ying, F. H. Tetrahedron Lett. 1994, 35,
729.
(8) Wu, H. J .; Ying, F. H.; Shao, W. D. J . Org. Chem. 1995, 60, 6168.
(3) Hayakawa, K.; Yodo, M.; Ohsuki, S.; Kanematsu, K. J . Am.
Chem. Soc. 1984, 106, 6735.
S0022-3263(98)00240-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/27/1998

Diels-Alder Reaction of Furans with Allenyl Ethers
J . Org. Chem., Vol. 63, No. 15, 1998 5065
Sch em e 1
Sch em e 2
methylthio group 1,4-rearrangement takes place to give
the product G via F .
Recently, we have accomplished a series of reactions
involving the intramolecular Diels-Alder reaction of a
furan diene with an allenyl ether dienophile followed by
the trimethylsilyl group,9 alkylsulfinyl group, and alkyl-
sulfonyl group rearrangements. In this paper, we wish
to report the full details of our finding.
Resu lts a n d Discu ssion
First of all, to understand in a more detail about the
nature of the methylthio group 1,4-rearrangement as
shown in Scheme 1, the furfuryl propargyl ether 3 was
prepared. Metalation of furan with n-BuLi in dry tet-
rahydrofuran (THF) at 25 °C followed by addition of
diethyl disulfide gave 2-ethylthiofuran (1) in 85% yield.
Treatment of 1 with n-BuLi in dry THF followed by
addition of 3-pentanone gave the alcohol 2 in 95% yield.
Reaction of 2 with propargyl bromide in aqueous NaOH
solution in the presence of n-Bu4NBr as a phase-transfer
catalyst at 25 °C gave the furfuryl propargyl ether 3 in
95% yield. Refluxing the propargyl ether 3 with t-BuOK
in t-BuOH at 85 °C for 6 h gave the 1,4-rearrangement
product 4 in 90% yield (Scheme 2). Refluxing the mixture
of the propargyl ethers 5a 8 and 3 with t-BuOK in t-BuOH
at 85 °C for 6 h gave the rearrangement products 6 and
4. No detectable amount of the cross-rearrangement
products 7 or 8 was obtained. Thus, this reaction may
involve an intramolecular Diels-Alder reaction of furan
with allenyl ether followed by an intramolecular alkylthio
group 1,4-rearrangement via the zwitterion H and the
tight ion pair I.
75-85% yields, respectively. Refluxing 9a -e with t-
BuOK in t-BuOH at 85 °C for 6 h gave the sulfinyl group
1,2-rearrangement products 10a -e in 80-90% yields
(Scheme 3). No detectable amount of the sulfinyl group
1,4-rearrangement products 11a -e was obtained. In the
case of 9b, which possesses two hydrogen atoms at the
furfurylic position, no detectable amount of the furan ring
transfer products 12 or 13 was obtained.
The IR spectra of 10a -e showed strong absorptions
at 3500-3200 cm-1 for the phenolic hydroxy group. The
1H NMR spectrum of 10a revealed two singlets at δ 6.84
and 6.76 for the two protons on the benzene ring and a
singlet at δ 4.99 for the methylene protons. The 13C NMR
spectrum of 10a displayed four singlets at δ 157.68,
144.39, 138.97, and 123.65 for the four quaternary
benzene ring carbons, two peaks at δ 116.80 and 111.64
for the other two benzene ring carbons, and one peak at
δ 70.33 for the methylene carbon. The structure of 10a
was also proved by X-ray analysis.10
A mechanism is proposed for this cycloaddition reation.
The intramolecular Diels-Alder reactions of 9a -e gave
compounds 10a -e as the sole product respectively,
presumably via the corresponding allenyl ethers 14a -e
and the cycloadducts 15a -e. We proposed that the
cycloadducts are highly strained and under the reaction
conditions they easily undergo ring opening of the bridged
To understand the effects of various substituent groups
at the fifth position of the furan ring on the intramolecu-
lar Diels-Alder reaction and on the intramolecular
rearrangement, the following experiments were per-
formed. Oxidation of compounds 5a -d 8 and 3 with 1
equiv of m-chloroperoxybenzoic acid (m-CPBA) in dichlo-
romethane at 25 °C gave the sulfoxides 9a -d and 9e in
(10) The author has deposited atomic coordinates for 10a with the
Cambridge Crystallographic Data Center. The coordinates can be
obtained, on request, from the Director, Cambridge Crystallogriphic
Data Center, 12 Union Road, Cambridge, CB2 1EZ, UK.
(9) Wu, H. J .; Yen, C. H.; Chuang, C. T. Tetrahedron Lett. 1996, 37,
7395.

5066 J . Org. Chem., Vol. 63, No. 15, 1998
Wu et al.
Sch em e 3
Sch em e 6
of methylthio group 1,2-rearrangement. Thus, we can
perform the alkylthio group 1,4-rearrangement, such as
the conversion of 3 to 4, or the alkylthio group 1,2-
migration via the sulfinyl group 1,2-rearrangement, such
as the conversion of 5a to 18 via 9a and 10a .
Sch em e 4
Oxidation of compounds 9a -e with excess m-CPBA in
dichloromethane at 25 °C gave the sulfones 19a -e.
Refluxing 19a -e with t-BuOK in t-BuOH at 85 °C for 5
h gave the sulfonyl group 1,2-rearrangement products
20a -e in 75-85% yields (Scheme 6). No detectable
amount of the sulfonyl group 1,4-rearrangement products
21a -e was obtained. Also, no detectable amount of the
furan ring transfer products 22 or 13 was obtained for
the case of 19b, which possesses two hydrogen atoms at
the furfurylic position. A mechanism similar to the
sulfinyl group 1,2-rearrangement, such as Scheme 4, can
be used to account for the sulfonyl group 1,2-rearrange-
ment.
Metalation of furan with 1 equiv of n-BuLi in dry THF
at 25 °C followed by addition of trimethylchlorosilane
gave 2-(trimethylsilyl)furan (23). Metalation of 23 with
n-BuLi in dry THF at 25 °C followed by addition of
acetaldehyde, propioaldehyde, acetone, and 3-pentanone
gave the alcohols 24a -d in 75-85% yields, respectively.
Treatment of 24a -d with propargyl bromide in saturated
aqueous NaOH solution in the presence of n-Bu4NBr as
a phase-transfer catalyst at 25 °C gave the furfuryl
propargyl ethers 25a -d in 75-95% yields. Refluxing the
propargyl ethers 25a -d with t-BuOK in t-BuOH at 85
°C for 10 h gave the trimethylsilyl group 1,2-rearrange-
ment products 26a -d (40-55%) and the trimethylsilyl
group Brook rearrangement11 products 27a -d (45-35%)
(Scheme 7). No detectable amount of the trimethylsilyl
group 1,4-rearrangement products 28a -d or compounds
29a -d was obtained.
Sch em e 5
oxygen atom to form the zwitterions 16a -e as the
reaction intermediates (Scheme 4). Elimination of the
alkylsulfinyl group by the alkoxide ion of 16 followed by
nucleophilic attack of the alkylsulfinyl group on the
partially positively charged neighboring carbon C6 gave
the 1,2-rearrangement intermediates 17a -e, which un-
derwent aromatization to give the final products 10a -
e. It must be determined if the neighboring alkoxide ion
of the zwitterion intermediates, such as 16, may play a
controlling role on the alkylsulfinyl group 1,2-rearrange-
ment and the alkylthio group 1,4-rearrangement.
Treatment of compound 26 with potassium tert-butox-
ide in refluxing tert-butyl alcohol (85 °C) for 6 h resulted
in unchanged starting compound 26. No conversion of
26 to 27 was obtained. Thus, compounds 26 and 27 were
Treatment of the sulfoxide 10a with P4S10 in dichlo-
romethane at 25 °C gave the reduced product 18 in a
quantitative yield (Scheme 5). The overall transforma-
tion from 5 to 18 via 9 and 10 portrays the equivalence
(11) Brook, A. G. Acc. Chem. Res. 1974, 7, 77.

Diels-Alder Reaction of Furans with Allenyl Ethers
J . Org. Chem., Vol. 63, No. 15, 1998 5067
Sch em e 7
Sch em e 9
Sch em e 8
trimethylsilyl group of the zwitterions 32a -d (route b)
gave the rearranged intermediates 34a -d , which un-
derwent aromatization to give the products 35a -d .
Hydrolysis of the trimethylsilyl group of 35a -d by the
base or solvent afforded the phenols 27a -d . The ratio
of 26:27 (1:1) may imply the proximate reaction rates for
the Brook rearrangement and the 1,2-rearrangement.
Again, the problem of whether the neighboring alkoxide
ion of the zwitterions 32 may play a controlling role on
the trimethylsilyl group 1,2-rearrangement and Brook
rearrangement needs to be proven by further experimen-
tal studies.
Metalation of furfuryl alcohol with 2.5 equiv of n-BuLi
in dry THF at 25 °C followed by addition of 3 equiv of
trimethylchlorosilane gave the disilylated product 36 in
90% yield. Reaction of 36 with propargyl bromide in
saturated aqueous NaOH solution in the presence of
n-Bu4NBr as a phase transfer catalyst gave the propargyl
ether 37 in 95% yield. Refluxing 37 with t-BuOK in
t-BuOH at 85 °C for 8 h gave compound 38 in 90% yield
(Scheme 9). Compound 38 was obtained from 37 via an
intramolecular Diels-Alder reaction followed by the
furan ring transfer reaction, which proceeded via proton
abstraction of the proton Ha by the base and by the Brook
rearrangement of the zwitterion intermediate 39. Hy-
drolysis of 40 gave the final product 38. The amount of
the 1,2-rearrangement product 41 and the Brook rear-
rangement product 42 without furan ring-transfer reac-
tion was too small to be isolated.
obtained from 25 via different reaction pathways.
A
mechanism is proposed for this reaction (Scheme 8). The
intramolecular Diels-Alder reactions of 25a -d gave
26a -d and 27a -d , respectively, presumably via the
corresponding allenyl ethers 30a -d and the cycloadducts
31a -d . Under the reaction conditions, the cycloadducts
31a -d easily underwent ring opening of the bridged
oxygen atom to form the zwitterions 32a -d as the
reaction intermediates. Elimination of the trimethylsilyl
group by the alkoxide ion followed by 1,2-shift of the
trimethylsilyl group (route a) gave the rearranged inter-
mediate 33a -d , which underwent aromatization to give
26a -d . On the other hand, Brook rearrangement of the
Con clu sion
We have discovered novel intramolecular Diels-Alder
reactions of a furandiene with an allenyl ether dienophile
followed by the alkylsulfinyl group, alkylsulfonyl group,

5068 J . Org. Chem., Vol. 63, No. 15, 1998
Wu et al.
P r epar ation of r,r-Dieth yl-5-(eth ylth io)-2-fu r fu r yl P r o-
p a r gyl Eth er (3). To a mixture of compound 2 (1.0 g, 4.8
mmol) and saturated NaOH solution (20 mL) were added
propargyl bromide (0.60 g, 5.0 mmol) and a catalytic amount
(0.020 g) of n-Bu4NBr at 25 °C. The reaction mixture was
stirred at 25 °C for 8 h. After addition of saturated NH4Cl
(50 mL) and extraction with ether, the organic layer was
washed with brine, dried over MgSO4, and evaporated, and
the residue was purified by column chromatography to give 3
(5.6 g, 95%): pale yellow oil; IR (CHCl3) 3320, 2980, 2150, 1500
and trimethylsilyl group rearrangements. In the cases
of 5-(alkylthio)-2-furfuryl propargyl ethers, the alkylthio
group 1,4-rearrangement products were obtained exclu-
sively. Also, the alkylthio group 1,4-rearrangement may
proceed intramolecularly via the tight ion pairs. In the
cases of the 5-(alkylsulfinyl)-2-furfuryl propargyl ethers
9a-e and the 5-(alkylsulfonyl)-2-furfuryl propargyl ethers
19a -e, the alkylsulfinyl group and the alkylsulfonyl
group 1,2-rearrangement products 10a -e and 20a -e
were obtained, respectively. In the cases of the 5-(tri-
methylsilyl)-2-furfuryl propargyl ethers 25a -d , the tri-
methylsilyl group 1,2-rearrangement products 26a -d
and Brook rearrangement products 27a -d were ob-
tained. In the case of the conversion of 37 to 38, a
mechanism via an intramolecular Diels-Alder reaction
followed by the furan ring-transfer reaction and Brook
rearrangement is proposed.
1
cm-1; H NMR (300 MHz, CDCl3) δ 6.44 (d, J ) 3.0 Hz, 1H),
6.30 (d, J ) 3.0 Hz, 1H), 3.85 (d, J ) 2.4 Hz, 2H), 2.77 (q, J )
6.6 Hz, 2H), 2.34 (t, J ) 2.4 Hz, 1H), 1.90 (q, J ) 6.6 Hz, 4H),
1.25 (t, J ) 6.6 Hz, 3H), 0.81 (t, J ) 6.6 Hz, 6H); 13C NMR (75
MHz, CDCl3, DEPT) δ 158.35 (C), 145.27 (C), 116.98 (CH),
110.48 (CH), 80.61 (CH), 79.83 (C), 72.98 (C), 50.81 (CH2),
29.97 (CH2), 25.63 (2CH2), 14.94 (CH3), 7.36 (2CH3); LRMS
m/z (rel int) 252 (M+, 5), 149 (100); HRMS (EI) calcd for
C14H20O2S 252.1184, found 252.1176. Anal. Calcd for
C
14H20O2S: C, 66.64; H, 7.99. Found: C, 66.52; H, 7.95.
In tr a m olecu la r Diels-Ald er Rea ction of 3. Compound
Exp er im en ta l Section
3 (2.0 g, 7.9 mmol) was dissolved in 2-methyl-2-propanol (100
mL) in a round-bottomed flask. Potassium tert-butoxide (1.8
g, 17 mmol) was added to the solution, and the reaction
mixture was refluxed at 85 °C for 6 h. After cooling, saturated
NH4Cl (50 mL) was added, and the reaction mixture was
extracted with ether. The organic layer was washed with
brine, dried over MgSO4, and evaporated, and the residue was
purified by column chromatography to give the 1,4-rearrange-
ment product 4 (1.7 g, 85%): pale yellow oil; IR (CHCl3) 3500-
3200, 1610, 1100 cm-1; 1H NMR (300 MHz, CDCl3) δ 7.06 (brs,
1H), 6.88-6.83 (m, 3H), 6.45 (s, 1H), 2.78-2.65 (m, 2H), 1.90-
1.74 (m, 4H), 1.29 (t, J ) 6.9 Hz, 3H), 0.90 (t, J ) 6.9 Hz, 3H),
0.69 (t, J ) 6.9 Hz, 3H); 13C NMR (75 MHz, CDCl3, DEPT) δ
155.38 (C), 140.52 (C), 134.60 (C), 121.38 (CH), 115.72 (CH),
108.70 (CH), 92.79 (C), 87.64 (CH), 32.65 (CH2), 24.81 (CH2),
24.64 (CH2), 14.62 (CH3), 8.18 (CH3), 7.51 (CH3); LRMS m/z
(rel int) 252 (M+, 8), 223 (20), 191 (100); HRMS (EI) calcd for
Gen er a l Meth od s. Melting points were determined in
capillary tubes with a Laboratory Devices melting point
apparatus and left uncorrected. Infrared spectra were re-
corded in CHCl3 solutions or on neat thin films between NaCl
disks. 1H NMR spectra were determined at 300 MHz, and 13C
NMR spectra were determined at 75 MHz, on Fourier trans-
form spectrometers. Chemical shifts are reported in ppm
relative to TMS in the solvents specified. The multiplicities
of 13C signals were determined by DEPT techniques. High-
resolution mass values were obtained with a high-resolution
mass spectrometer at the Department of Chemistry, National
Tsing Hua University. Elemental analyses were performed
at the microanalysis laboratory of this department. X-ray
analyses were carried out on a diffractometer at the Depart-
ment of Chemistry, National Tsing Hua University. For thin-
layer chromatography (TLC) analysis, precoated TLC plates
(Kieselgel 60 F254) were used, and column chromatography was
done by using Kieselgel 60 (70-230 mesh) as the stationary
phase. THF was distilled immediately prior to use from
sodium benzophenone ketyl under nitrogen. CH2Cl2 was
distilled from CaH2 under nitrogen.
C
14H20O2S 252.1184, found 252.1189. Anal. Calcd for
C14H20O2S: C, 66.64; H, 7.99. Found: C, 66.58; H, 7.93.
In tr a m olecu la r Diels-Ald er Rea ction of th e Mixtu r e
of Com p ou n d s 3 a n d 5a . Compounds 3 (1.0 g, 4.0 mmol)
and 5a 8 (0.84 g, 4.0 mmol) were dissolved in 2-methyl-2-
propanol (100 mL) in a round-bottomed flask. Potassium tert-
butoxide (2.2 g, 20 mmol) was added to the solution, and the
reaction mixture was refluxed at 85 °C for 6 h. After cooling,
saturated NH4Cl (60 mL) was added, and the reaction mixture
was extracted with ether. The organic layer was washed with
brine, dried over MgSO4, and evaporated, and the residue was
purified by column chromatography to give compounds 4 and
6 in 85% yields. No detectable amount of the cross-rearrange-
ment products 7 or 8 was obtained. Compound 6 is a known
compound.8
P r ep a r a tion of 2-(Eth ylth io)fu r a n (1). To a solution of
furan (5.0 g, 73 mmol) in dry THF (100 mL) was added n-BuLi
(46 mL, 75 mmol) at 0 °C. The reaction mixture was stirred
at room temperature for 4 h. To this solution was added
diethyl disulfide (9.8 g, 81 mmol) at 0 °C, and the reaction
mixture was stirred at 25 °C for 4 h. After addition of
saturated NH4Cl (80 mL) and extraction with ether, the
organic layer was washed with brine, dried over MgSO4, and
evaporated, and the residue was purified by distillation to give
1 (9.2 g, 85%): bp 138-140 °C (760 mmHg); IR (CHCl3) 1500,
750 cm-1 1H NMR (300 MHz, CDCl3) δ 7.50-7.49 (m, 1H),
;
Gen er a l P r oced u r e for th e Oxid a tion of Com p ou n d s
5a -d a n d 3 w ith 1 Equ iv of m -CP BA. To a solution of 5a
(1.0 g, 4.8 mmol) in dichloromethane (50 mL) was added 1
equiv of m-CPBA (0.82 g, 4.8 mmol) at 0 °C. The reaction
mixture was stirred at 25 °C for 1 h. The solvent was
evaporated, and sodium bicarbonate solution (1 N, 15 mL) was
added. The mixture was extracted with ether. The organic
layer was washed with brine, dried over MgSO4, and evapo-
rated, and the residue was purified by column chromatography
to give compound 9a (0.86 g, 80%).
6.49-6.47 (m, 1H), 6.38-6.36 (m, 1H), 2.77 (q, J ) 6.3 Hz,
2H), 1.26 (t, J ) 6.3 Hz, 3H); 13C NMR (75 MHz, CDCl3, DEPT)
δ 145.68 (C), 145.24 (CH), 116.77 (CH), 111.32 (CH), 29.97
(CH2), 15.03 (CH3); LRMS m/z (rel int) 128 (M+, 10), 99 (100).
P r ep a r a tion of r,r-Dieth yl-5-(eth ylth io)-2-fu r fu r yl Al-
coh ol (2). To a solution of 2-(ethylthio)furan (5.0 g, 39 mmol)
in dry THF (100 mL) was added n-BuLi (17 mL, 43 mmol) at
0 °C. The reaction mixture was stirred at 25 °C for 4 h. To
this solution was added 3-pentanone (3.5 g, 41 mmol) at 0 °C,
and the reaction mixture was stirred at 25 °C for 1 h. After
addition of saturated NH4Cl (90 mL) and extraction with ether,
the organic layer was washed with brine, dried over MgSO4,
and evaporated, and the residue was purified by column
chromatography to give 2 (7.9 g, 95%): pale yellow oil; IR
(CHCl3) 3500-3200, 1500 cm-1; 1H NMR (300 MHz, CDCl3) δ
6.45 (d, J ) 1.8 Hz, 1H), 6.21 (d, J ) 1.8 Hz, 1H), 2.74 (q, J )
6.6 Hz, 2H), 2.08 (brs, 1H), 1.90-1.80 (m, 4H), 1.23 (t, J ) 6.6
Hz, 3H), 0.82 (t, J ) 6.6 Hz, 6H); 13C NMR (75 MHz, CDCl3,
DEPT) δ 161.61 (C), 144.10 (C), 117.82 (CH), 107.30 (CH),
75.11 (C), 31.93 (2CH2), 30.24 (CH2), 14.91 (CH3), 7.74 (2CH3);
LRMS m/z (rel int) 214 (M+, 22), 196 (95), 185 (100).
r,r-Dim eth yl-5-(m eth ylsu lfin yl)-2-fu r fu r yl p r op a r gyl
eth er (9a ): pale yellow oil; IR (CHCl3) 2150, 1380, 1100, 1050
1
cm-1; H NMR (300 MHz, CDCl3) δ 6.90 (d, J ) 3.6 Hz, 1H),
6.40 (d, J ) 3.6 Hz, 1H), 3.92 (d, J ) 2.4 Hz, 2H), 2.97 (s, 3H),
2.32 (t, J ) 2.4 Hz, 1H), 1.59 (s, 3H), 1.58 (s, 3H); 13C NMR
(75 MHz, CDCl3, DEPT) δ 161.44 (C), 152.87 (C), 115.69 (CH),
108.70 (CH), 80.26 (CH), 73.97 (C), 73.45 (C), 51.74 (CH2),
38.34 (CH3), 25.40 (CH3), 25.69 (CH3); LRMS m/z (rel int) 226
(M+, 10), 211 (20), 155 (100); HRMS (EI) calcd for C11H14O3S
226.0663, found 226.0669. Anal. Calcd for C11H14O3S: C,
58.39; H, 6.24. Found: C, 58.25; H, 6.30.

Diels-Alder Reaction of Furans with Allenyl Ethers
J . Org. Chem., Vol. 63, No. 15, 1998 5069
Gen er a l P r oced u r e for th e In tr a m olecu la r Diels-
Ald er Rea ction of Com p ou n d s 9a -e. The same reaction
conditions and procedure as for the intramolecular Diels-
Alder reaction of 3 were applied for the intramolccular Diels-
Alder reaction of 9a -e to give the alkylsulfinyl group 1,2-
rearrangement products 10a -e in 80-90% yields.
Sp ectr a l d a ta for 10a : viscous oil; 85% yield; IR (CHCl3)
3500-3200, 1610, 1440, 1300, 1050 cm-1; 1H NMR (300 MHz,
CDCl3) δ 6.84 (s, 1H), 6.76 (s, 1H), 4.99 (s, 2H), 2.95 (s, 3H),
1.44 (s, 6H); 13C NMR (75 MHz, CDCl3, DEPT) δ 157.68 (C),
144.39 (C), 138.97 (C), 123.65 (C), 116.80 (CH), 111.64 (CH),
85.39 (C), 70.33 (CH2), 41.60 (CH3), 28.40 (2CH3); LRMS m/z
(rel int) 226 (M+, 20), 211 (100), 193 (70); HRMS (EI) calcd for
C11H14O3S 226.0663, found 226.0660. Anal. Calcd for
1H), 6.39 (dd, J ) 3.0, 2.4 Hz, 1H), 0.29 (s, 9H); 13C NMR (75
MHz, CDCl3, DEPT) δ 160.39 (C), 146.49 (CH), 119.42 (CH),
109.28 (CH), -1.65 (3CH3); LRMS m/z (rel int) 140 (M+, 12),
71 (100).
Gen er a l P r oced u r e for th e P r ep a r a tion of th e F u r fu -
r yl Alcoh ols 24a -d . To a solution of 2-(trimethylsilyl)furan
(2.80 g, 20.0 mmol) in dry THF (80 mL) was added n-BuLi (10
mL, 25 mmol) at 0 °C. The reaction mixture was stirred at
room temperature for 4 h. To this solution was added
acetaldehyde (1.32 g, 30.0 mmol) at 0 °C, and the reaction
mixture was stirred at 25 °C for 2 h. After addition of
saturated NH4Cl (60 mL) and extraction with ether, the
organic layer was washed with brine, dried over MgSO4, and
evaporated, and the residue was purified by column chroma-
tography to give R-methyl-5-(trimethylsilyl)-2-furfuryl alcohol
24a (3.1 g, 85%): pale yellow oil; IR (CHCl3) 3500-3200, 1260,
1100, 850 cm-1; 1H NMR (300 MHz, CDCl3) δ 6.53 (d, J ) 3.0
Hz, 1H), 6.19 (d, J ) 3.0 Hz, 1H), 4.88 (q, J ) 6.9 Hz, 1H),
2.31 (brs, 1H), 1.52 (d, J ) 6.9 Hz, 3H), 0.11 (s, 9H); 13C NMR
(75 MHz, CDCl3, DEPT) δ 161.96 (C), 159.43 (C), 120.09 (CH),
104.86 (CH), 63.57 (CH), 21.26 (CH3), -1.73 (3CH3); LRMS
m/z (rel int) 184 (M+, 7), 169 (100).
C
11H14O3S: C, 58.39; H, 6.24. Found: C, 58.52; H, 6.34.
Red u ction of Com p ou n d 10a w ith P 4S10. To a solution
of compound 10a (0.50 g, 2.2 mmol) in dichloromethane (50
mL) was added P4S10 (1.47 g, 3.32 mmol) at 25 °C. The
reaction mixture was stirred at 25 °C for 6 h. The solvent
was evaporated, and the crude product was purified by column
chromatography to give 18 (0.46 g, 98%).
Sp ectr a l d a ta for 18: pale yellow oil; IR (CHCl3) 3500-
1
3200, 1610, 1050 cm-1; H NMR (300 MHz, CDCl3) δ 7.21 (s,
Gen er a l P r oced u r e for th e P r ep a r a tion of th e 5-(Tr i-
m eth ylsilyl)-2-fu r fu r yl P r op a r gyl Eth er s 25a -d . The
same reaction conditions and procedure as for the preparation
of the propargyl ether 3 were applied for the preparation of
the 5-(trimethylsilyl)-2-furfuryl propargyl ethers 25a -d .
r-Meth yl-5-(tr im eth ylsilyl)-2-fu r fu r yl p r op a r gyl eth er
25a : pale yellow oil; 92% yield; IR (CHCl3) 3300, 2100, 1100,
1H), 6.80 (s, 1H), 5.00 (s, 2H), 2.34 (s, 3H), 1.48 (s, 6H); 13C
NMR (75 MHz, CDCl3, DEPT) δ 155.55 (C), 141.30 (C), 139.35
(C), 125.89 (CH), 120.09 (C), 107.16 (CH), 85.39 (C), 70.15
(CH2), 28.37 (2CH3), 19.48 (CH3); LRMS m/z (rel int) 210 (M+,
20), 119 (62), 85 (95), 71 (100); HRMS (EI) calcd for C11H14O2S
210.0715, found 210.0710.
1
Gen er a l P r oced u r e for t h e Oxid a t ion of 9a -e w it h
m -CP BA. To a solution of 9a (1.13 g, 5.0 mmol) in dichlo-
romethane (50 mL) was added an excess of m-CPBA (1.3 g,
7.4 mmol) at 0 °C. The reaction mixture was stirred at 25 °C
for 2 h. The solvent was evaporated, and sodium bicarbonate
solution (1 N, 20 mL) was added. The mixture was extracted
with ether. The organic layer was washed with brine, dried
over MgSO4, and evaporated, and the residue was purified by
column chromatography to give the sulfone 19a (0.98 g, 84%).
r,r-Dim eth yl-5-(m eth ylsu lfon yl)-2-fu r fu r yl p r op a r gyl
850 cm-1; H NMR (300 MHz, CDCl3) δ 6.55 (d, J ) 3.0 Hz,
1H), 6.28 (d, J ) 3.0 Hz, 1H), 4.74 (q, J ) 6.6 Hz, 1H), 4.14,
4.01 (doublet of ABq, J ) 15.9, 2.4 Hz, 2H), 2.41 (t, J ) 2.4
Hz, 1H), 1.55 (d, J ) 6.6 Hz, 3H), 0.26 (s, 9H); 13C NMR (75
MHz, CDCl3, DEPT) δ 160.39 (C), 158.89 (C), 119.92 (CH),
107.48 (CH), 79.86 (CH), 73.93 (C), 69.31 (CH), 55.41 (CH2),
19.57 (CH3), -1.69 (3CH3); LRMS m/z (rel int) 222 (M+, 4), 57
(100); HRMS (EI) calcd for C12H18O2Si 222.1076, found 222.1073.
Anal. Calcd for C12H18O2Si: C, 64.83; H, 8.17. Found: C,
64.94; H, 8.25.
eth er (19a ): pale yellow oil; IR (CHCl3) 2150, 1325, 1140 cm-1
;
Gen er a l P r oced u r e for th e In tr a m olecu la r Diels-
Ald er Rea ction of 5-(Tr im eth ylsilyl)-2-fu r fu r yl P r op a r -
gyl Eth er s 25a -d . Compound 25a (2.2 g, 10 mmol) was
dissolved in tert-butyl alcohol (100 mL) in a round-bottomed
flask. Potassium tert-butoxide (2.2 g, 20 mmol) was added to
the solution, and the reaction mixture was refluxed at 85 °C
for 10 h. After cooling, saturated NH4Cl (80 mL) was added,
and the reaction mixture was extracted with ether. The
organic layer was washed with brine, dried over MgSO4, and
evaporated, and the residue was purified by column chroma-
tography to give the trimethylsilyl group 1,2-rearrangement
product 26a (1.1 g, 50%) and the Brook rearrangement product
27a (0.59 g, 40%).
Sp ectr a l d a ta for 26a : viscous oil; IR (CHCl3) 3500-3200,
1100 cm-1; 1H NMR (300 MHz, CDCl3) δ 7.11 (s, 1H), 6.54 (s,
1H), 6.08 (brs, 1H), 5.29 (q, J ) 6.6 Hz, 1H), 5.05, 4.97 (ABq,
J ) 12.6 Hz, 2H), 1.49 (d, J ) 6.6 Hz, 3H), 0.31 (s, 9H); 13C
NMR (75 MHz, CDCl3, DEPT) δ 160.71 (C), 142.06 (C), 134.78
(C), 127.14 (CH), 124.87 (C), 106.87 (CH), 79.97 (CH), 71.84
(CH2), 21.93 (CH3), -1.00 (3CH3); LRMS m/z (rel int) 222 (M+,
5), 207 (100), 191 (93); HRMS (EI) calcd for C12H18O2Si
222.1076, found 222.1072. Anal. Calcd for C12H18O2Si: C,
64.83; H, 8.17. Found: C, 64.96; H, 8.27.
1H NMR (300 MHz, CDCl3) δ 7.12 (d, J ) 3.3 Hz, 1H), 6.46 (d,
J ) 3.3 Hz, 1H), 3.99 (d, J ) 2.4 Hz, 2H), 3.17 (s, 3H), 2.36 (t,
J ) 2.4 Hz, 1H), 1.63 (s, 6H); 13C NMR (75 MHz, CDCl3, DEPT)
δ 162.40 (C), 148.59 (C), 117.70 (CH), 108.61 (CH), 80.06 (CH),
73.82 (C), 73.59 (C), 51.74 (CH2), 43.06 (CH3), 25.40 (2CH3);
LRMS m/z (rel int) 242 (M+, 62), 143 (100); HRMS (EI) calcd
for C11H14O4S 242.0612, found 242.0616. Anal. Calcd for
C
11H14O4S: C, 54.53; H, 5.83. Found: C, 54.65; H, 5.90.
Gen er a l P r oced u r e for th e In tr a m olecu la r Diels-
Ald er Rea ction of th e Su lfon es 19a -e. The same reaction
conditions and procedure as for the cycloaddition of 3 were
applied for the intramolecular Diels-Alder reaction of com-
pounds 19a -e to give the alkylsulfonyl group 1,2-rearrange-
ment products 20a -e in 75-85% yields.
Sp ectr a l d a ta for 20a : viscous oil; 82% yield; IR (CHCl3)
3500-3200, 1440, 1305, 1120 cm-1; 1H NMR (300 MHz, CDCl3)
δ 7.41 (s, 1H), 6.87 (s, 1H), 4.98 (s, 2H), 3.11 (s, 3H), 1.55 (s,
3H); 13C NMR (75 MHz, CDCl3, DEPT) δ 155.61 (C), 149.23
(C), 138.13 (C), 121.29 (C), 120.33 (CH), 111.30 (CH), 85.39
(C), 70.15 (CH2), 44.89 (CH3), 28.40 (CH3); LRMS m/z (rel int)
228 (M+, 5), 213 (100); HRMS (EI) calcd for C11H14O4S
242.0612, found 242.0608. Anal. Calcd for C11H14O4S: C,
54.53; H, 5.83. Found: C, 54.68; H, 5.93.
Sp ectr a l d a ta for 27a : viscous oil; IR (CHCl3) 3500-3200,
1
P r ep a r a tion of (Tr im eth ylsilyl)fu r a n (23). To a solution
of furan (2.04 g, 30.0 mmol) in dry THF (100 mL) was added
n-BuLi (13.2 mL, 33.0 mmol) at 0 °C. The reaction mixture
was stirred at room temperature for 4 h. To this solution was
added trimethylchlorosilane (3.58 g, 33.0 mmol) at 0 °C, and
the reaction mixture was stirred at 25 °C for 5 h. After
addition of saturated NH4Cl (60 mL) and extraction with ether,
the organic layer was washed with brine, dried over MgSO4,
and evaporated, and the residue was purified by distillation
to give (trimethylsilyl)furan (23) (3.61 g, 86%): bp 108-109
1100 cm-1; H NMR (300 MHz, CDCl3) δ 6.99 (d, J ) 8.7 Hz,
1H), 6.75 (dd, J ) 8.7, 2.1 Hz, 1H), 6.68 (d, J ) 2.1 Hz, 1H),
6.60 (brs, 1H), 5.30 (q, J ) 6.6 Hz, 1H), 5.08, 5.00 (ABq, J )
12.6 Hz, 2H), 1.47 (d, J ) 6.6 Hz, 3H); 13C NMR (75 MHz,
CDCl3, DEPT) δ 155.76 (C), 140.52 (C), 134.92 (C), 121.70
(CH), 114.62 (CH), 107.83 (CH), 79.91 (CH), 71.93 (CH2), 21.87
(CH3); LRMS m/z (rel int) 150 (M+, 18), 135 (100); HRMS (EI)
calcd for C9H10O2 150.0681, found 150.0689. Anal. Calcd for
C9H10O2: C, 71.97; H, 6.72. Found: C, 71.83; H, 6.60.
P r ep a r a tion of 5-(Tr im eth ylsilyl)-2-fu r fu r yl Tr im eth -
ylsilyl Eth er (36). To a solution of furfuryl alcohol (0.50 g,
5.0 mmol) in dry THF (30 mL) was added n-BuLi (5.0 mL,
°C (760 mmHg); IR (CHCl3) 1280, 850 cm-1
;
1H NMR (300
MHz, CDCl3) δ 7.66 (d, J ) 2.4 Hz, 1H), 6.65 (d, J ) 3.0 Hz,

5070 J . Org. Chem., Vol. 63, No. 15, 1998
Wu et al.
12.5 mmol) at 0 °C. The reaction mixture was stirred at room
temperature for 6 h. To this solution was added trimethyl-
chlorosilane (1.6 g, 15 mmol) at 0 °C, and the reaction mixture
was stirred at 25 °C for 4 h. After addition of saturated
NaHCO3 (40 mL) and extraction with ether, the organic layer
was washed with brine, dried over MgSO4, and evaporated,
and the residue was purified by column chromatography to
give 5-(trimethylsilyl)-2-furfuryl trimethylsilyl ether 36 (1.1
g, 90%): pale yellow oil; IR (CHCl3) 1280, 850 cm-1; 1H NMR
(300 MHz, CDCl3) δ 6.53 (d, J ) 3.0 Hz, 1H), 6.22 (d, J ) 3.0
Hz, 1H), 4.64 (s, 2H), 0.26 (s, 9H), 0.10 (s, 9H); 13C NMR (75
MHz, CDCl3, DEPT) δ 160.13 (C), 157.88 (C), 120.30 (CH),
107.80 (CH), 57.60 (CH2), -0.45 (3CH3), -1.61 (3CH3); LRMS
m/z (rel int) 242 (M+, 18), 73 (100).
intramolecular Diels-Alder reaction of compound 3 were
applied for the reaction of 37 to give the product 38 in 90%
yield: viscous oil; IR (CHCl3) 3500-3200, 1100 cm-1; 1H NMR
(300 MHz, CDCl3) δ 7.32 (s, 1H), 7.20 (s, 1H), 6.51 (d, J ) 9.6
Hz, 1H), 5.92 (dd, J ) 9.6, 3.6 Hz, 1H), 4.52-4.48 (m, 1H),
2.88, 2.78 (doublet of ABq, J ) 16.8, 6.6 Hz, 2H), 2.03 (brs,
1H); 13C NMR (75 MHz, CDCl3, DEPT) δ 138.19 (CH), 137.14
(CH), 129.62 (CH), 120.33 (C), 119.74 (CH), 117.27 (C), 65.46
(CH), 27.82 (CH2); LRMS m/z (rel int) 136 (M+, 8), 119 (100);
HRMS (EI) calcd for C8H8O2 136.0524, found 136.0530.
Ack n ow led gm en t. We thank the National Science
Council of the Republic of China for financial support
(Grant No. NSC84-2113-M009-008). We also thank Dr.
Sue-Lein Wang and Ms. Fen-Ling Liao at the Depart-
ment of Chemistry, National Tsing Hua University, for
their help in carrying out the X-ray crystallographic
analysis.
P r ep a r a tion of 5-(Tr im eth ylsilyl)-2-fu r fu r yl P r op a r gl
Eth er (37). The same reaction conditions and prodecure as
for the preparation of the propargyl ether 3 were applied for
the preparation of compound 37 in 95% yield: pale yellow oil;
1
IR (CHCl3) 3300, 2140, 1100, 850 cm-1; H NMR (300 MHz,
CDCl3) δ 6.56 (d, J ) 3.0 Hz, 1H), 6.35 (d, J ) 3.0 Hz, 1H),
4.59 (s, 2H), 4.17 (d, J ) 2.4 Hz, 2H), 2.47 (t, J ) 2.4 Hz, 1H),
0.26 (s, 9H); 13C NMR (75 MHz, CDCl3, DEPT) δ 161.18 (C),
154.85 (C), 120.24 (CH), 109.95 (CH), 79.39 (CH), 74.67 (C),
63.19 (CH2), 56.69 (CH2), -1.67 (3CH3); LRMS m/z (rel int)
208 (M+, 26), 153 (80), 73 (100); HRMS (EI) calcd for C11H16O2-
Si 208.0920, found 208.0927. Anal. Calcd for C11H16O2Si: C,
63.43; H, 7.75. Found: C, 63.51; H, 7.80
Su p p or tin g In for m a tion Ava ila ble: Spectral data for
compounds 9b -e, 10b -e, 19b -e, 20b -e, 24b -d , 25b -d ,
26b-d , and 27b-d and X-ray crystallographic data for
compound 10a (15 pages). This material is contained in
libraries on microfiche, immediately follows this article in the
microfilm version of the journal, and can be ordered from the
ACS; see any current masthead page for ordering information.
In tr a m olecu la r Diels-Ald er Rea ction of Com p ou n d
37. The same reaction conditions and procedure as for the
J O980240L