Synthesis and photooxygenation of 2-thiofuran derivatives: A mild and direct access to O,S-dimethyl and O-methyl-S-phenyl thiomaleates

Article abstract of DOI:10.1016/S0040-4039(02)02277-3

New 3-bromo-2-thio and 3-bromo-2,5-bis-(thio) furan derivatives have been synthesized and their synthetic potential has been assessed by their conversion into O,S-dimethyl and O-methyl-S-phenyl thiomaleates via methylene blue sensitized photooxygenation.

Full text of DOI:10.1016/S0040-4039(02)02277-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9129–9132
Synthesis and photooxygenation of 2-thiofuran derivatives:
a mild and direct access to O,S-dimethyl and
O-methyl-S-phenyl thiomaleates
Yolanda Arroyo, Justo F. Rodr´ıguez, Ma6 . Ascensio´n Sanz-Tejedor* and Mercedes Santos
Departamento de Qu´ımica Orga´nica, ETS de Ingenieros Industriales, Paseo del Cauce s/n, 47011, Valladolid, Spain
Received 22 July 2002; accepted 8 October 2002
Abstract—New 3-bromo-2-thio and 3-bromo-2,5-bis-(thio) furan derivatives have been synthesized and their synthetic potential
has been assessed by their conversion into O,S-dimethyl and O-methyl-S-phenyl thiomaleates via methylene blue sensitized
photooxygenation. © 2002 Elsevier Science Ltd. All rights reserved.
Substituted furans are key structural units in a wide
range of biologically active natural and synthetic
products1 and they have been frequently employed in
organic synthesis as versatile building blocks.2 One of
the most interesting aspects of furan chemistry is the
oxidative ring cleavage3 to give 1,4-dioxo-2-alkenes
which can be further transformed into complex
molecules.4 In the photooxygenation process, singlet
oxygen, 1O2, is transferred via [4+2] cycloaddition
allowing a selective cis-1,4-dioxygenation.5 In connec-
tion with our studies concerning the photooxygenation
of substituted furans6 we drew our attention toward
2,5-bis-(thio)furan derivatives which could be trans-
formed into maleic acid thioesters by oxidative ring
cleavage of the former compounds. Fumaric acid
thioesters7 and g-oxo-a,b-unsaturated thioesters8 have
been used as useful dienophiles in the construction of
natural products and polycyclic cage compounds and
also as Michael acceptors in reactions with enamines.9
Additionally, they are useful dienophiles in situations
where the corresponding esters lacked the reactivity
needed,7a,c and it is possible to carry out the chemio-
selective reduction of a thioester group without dis-
turbing an ester or a lactone moieties.7b,c
The synthesis of 2,5-bis-(thio)furan derivatives was
planned starting from 3,4-dibromofuran10 1, and 3-bro-
mofuran 2. We select these starting materials because
they can undergo regiospecific metallation with LDA
and subsequent electrophilic reaction at the carbon C2
as well as a second metal-proton exchange reaction at
C5. Additionally, the metal-bromine exchange reaction
at C3 by means of n-BuLi can be carried out.11
2-Thiofurans 3, 4 and 5 were obtained in high yields by
ortho metallation of 1 and 2 with LDA (1 equiv.) and
subsequent treatment with the electrophile reagents
(Me2S2 and Ph2S2; see Scheme 1). When compound 3
was subjected to a new set of metal–proton exchange
reaction and then treatment with Ph2S2 the 2,5-bis-
(phenylthio)furan 6 was obtained in 70% yield (Scheme
In this paper we describe a regiocontrolled approach to
2-thiofuran and 2,5-bis-(thio)furan derivatives and the
study of their sensitized photooxygenation as a direct
and mild access to O,S-dimethyl and O-methyl-S-
phenyl thiomaleates. To the best of our knowledge, the
synthesis of these derivatives has not been described.
Scheme 1. Reagents and conditions: a. (i) LDA (1 equiv.); (ii)
R22S2 (1.1 equiv.); b. (i) LDA (2.2 equiv.); (iii) R23S2 (2.2
equiv.); c. (i) n-BuLi (1 equiv.); (ii) p-Tol2S2 (1.1 equiv.).
* Corresponding author. Tel.: 0034-983423374; fax: 0034-983423310;
e-mail: atejedor@dali.eis.uva.es
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02277-3

9130
Y. Arroyo et al. / Tetrahedron Letters 43 (2002) 9129–9132
could be removed, as insoluble material, by treatment
with hexane.13 In the same way, the reaction of 7 with
1O2 afforded 3-bromo-O-methyl-S-phenyl thiomaleate
12 as the main product along with a small amount of its
regioisomer 13 in a 94:6 ratio (entry 2). Again, spec-
troscopy data14 of the reaction mixture show the peaks
corresponding to a methoxycarbonyl group (lH=3.89
ppm; lC=164.2 ppm) and a phenylsulfanylcarbonyl
group (lC=193.1 ppm) of the major 12. An experiment
carried out with 7 at −78°C showed that the tempera-
ture of the reaction had no influence on the products
ratio but the starting furan was not consumed after 7 h
of irradiation (entry 3). The use of Rose Bengal as
photosensitizer requires higher temperatures (20°C) and
the regioselectivity decreased slightly (entry 4). We also
performed photooxidation of 7 in the presence of
reduction agents such as Me2S, thiourea and Ph3P
(entries 5–7). We found that the addition of these
compounds had no influence on the nature of the final
products but the regioselectivity decreased and both
higher temperatures and longer irradiation times were
necessary to consume the starting furan 7. The pho-
tooxygenation of 8 afforded 3-bromo-O,S-dimethyl
thiomaleate 14 and S,S-dimethyl bis(thio)maleate 15 in
a 68:32 ratio (entry 8). Spectroscopic data14 of the
reaction mixture show the peaks corresponding to a
methoxycarbonyl group (lH=3.88 ppm; lC=164.1
ppm) and a methylsulfanylcarbonyl group (lH=2.48
ppm; lC=186.9 ppm) for compound 14 while two
methylsulfanylcarbonyl groups are present in the minor
15 (lH=2.47 and 2.39; lC=186.9 and 190.1).
1). In the same way, the syntheses of 7 from 4, and 8
from 5 were achieved in good yields using Ph2S2 and
Me2S2 as electrophile reagents, respectively (Scheme 1).
The unsymmetrical 2,5-bis(thio)furan 9 was similarly
obtained by reaction of 5 with LDA and subsequent
treatment with Ph2S2. On the other hand, when furan 4
was treated with n-BuLi (1 equiv.), it resulted in a
3-lithiofuran intermediate which was quenched with
di-p-tolyldisulfide giving rise to 2,3-bis-(thio)furan 10 in
67% yield (Scheme 1). All thiosubstituted furans have
been isolated as pure compounds and characterized by
their spectroscopy data.12
With the precursors in hand the reactions of thiofuran
derivatives 5-10 with singlet oxygen were then investi-
gated.13 In all experiments performed with 2,5-bis-
(thio)furan derivatives, 6–9, we found that the expected
S,S-bis(thio) maleate derivatives were not formed
except for furan 8. However the corresponding O,S-
dimethyl or O-methyl-S-phenyl thiomaleates were
formed as a result of the loss of a phenylthio or a
methylthio group from the starting furan derivatives.
These results are collected in Table 1.
Thus, when photooxygenation of compound 6 was
1
carried out the H and 13C NMR spectra of the reac-
tion mixture just after irradiation showed the presence
of 2,3-dibromo-O-methyl-S-phenyl thiomaleate 1114
(Table 1; entry 1). The characteristic peak of the
methoxycarbonyl group appeared at lH 3.85 ppm and
at lC 165.1 ppm while the phenylsulfanylcarbonyl
group appeared at lC 186.2 ppm. The reaction was
accompanied by the formation of a by-product result-
ing from the loss of a phenylthio group (only signals
corresponding to aromatic protons were detected) but it
In order to establish the right structure for 12, 13
and 14, the photooxidation of 2-(methylthio)-5-
(phenylthio)-furan 9 was then performed. Thus, after
Table 1. Photooxygenation of 2,5-bis-(thio)furans 6–9
Entry
Dithiofuran
Sensitizer/reduction agent
T (°C)
Irradiation time (h)
Product ratioa (%)
Yield (%)b
1
2
3
4
5
6
7
8
9
6
7
7
7
7
7
7
8
9
MB/none
MB/none
MB/none
RB/none
MB/Me2S
MB/thiourea
MB/Ph3P
MB/none
MB/none
−40
−40
−78
20
20
20
−20
−40
−40
5.5
3.5
7
6
6
7
5
0.75
1
11 (100)
96
93
28c
93
95
15d
91
87
90
12 (94)
12 (94)
12 (86)
12 (88)
12 (86)
12 (87)
14 (68)
14 (93)
13 (6)
13 (6)
13 (14)
13 (12)
13 (14)
13 (13)
15 (32)
13 (7)
a Estimated by 1H NMR.
b Crude yield of major compound.
c 67% of starting material was recovered.
d 78% of starting material was recovered.

Y. Arroyo et al. / Tetrahedron Letters 43 (2002) 9129–9132
9131
irradiation 3-bromo-O,S-dimethyl thiomaleate 14 was
formed as the main product together with 2-bromo-O-
methyl-S-phenyl thiomaleate 13 in a 93:7 ratio (entry
9). Compound 14 comes from the loss of the phenylthio
group at C5 and spectroscopy data are coincident with
those of the major compound obtained from 8. The
minor 13 results from the loss of the methylthio group
at C2 and spectroscopic data are coincident with the
minor compound obtained from 7. These results show
that during the photooxygenation process of 2,5-
bis(thio)furans the loss of the thiophenyl or thiomethyl
group at C5 is preferred.
produces cis-2-butene-1,4-diones.5 For furans unsubsti-
tuted at C5, dihydrofurans such as B% are formed giving
rise to alkenals, which is in accord with the result
obtained for 5. In photooxidations performed with
2,5-bis(thio)furans we found that a reduction agent is
not necessary in order to obtain the final products and
moreover the addition of such a reduction agent during
the irradiation had no influence on the reaction course.
These results suggest that a ready transformation of
dihydrofurans B into compounds 11–14 takes place as a
consequence of the higher ability of phenylthio and
methylthio groups to act as leaving groups in compari-
son with the methoxy one. The fact that compound 15
was formed in photooxidation of 8 could be explained
taking into account that a methylthio group is a poorer
leaving group than a phenylthio group. It should be
noted that in order to remove the by-products resulting
from the reduction process, the reaction mixture was
treated with hexane or water to give a clean reaction
mixture where only the photooxygenation products
were detected.13
In the same way we studied the photooxygenation of
furans unsubstituted at C5 5 and 10. Reaction of 5 with
1O2 over 25 minutes, after treatment with Me2S, lead to
the expected methyl (E)-2-bromo-4-oxo-2-butenethioate
16 as the only product in 90% yield. This (E)-alkenal
isomerized spontaneously to the corresponding Z iso-
mer after seven days at room temperature (Scheme 2).
In contrast with the results obtained in the photooxy-
genation of 3-bromofurans (compounds 5–9), which
afforded open chain a,b-unsaturated thioesters (11–16),
the reaction performed with 2,3-bis-(thio)furan 10 lead,
after treatment with Me2S, to the g-butyrolactone 1715
as the sole product in nearly quantitatively yield (97%
crude yield) (Scheme 3).
In summary the results describe above demonstrate that
3-bromo-2-thiofurans 5–9 are pivotal compounds in the
synthesis of O,S-dimethyl, and O-methyl-S-phenyl
thiomaleates (compounds 11, 12 and 14) as well as the
4-oxo-2-butenethioate 16 by methylene blue sensitized
photooxygenation using methanol as solvent. We also
report that the 2,3-bis-(thio)furan 10 bearing a tolyl-
thiogroup at C3 lead only to the g-butyrolactone 17.
Studies devoted to the use of compounds 11–16 as
dienophiles and as Michael acceptors are currently in
progress.
As it is well documented5 photooxygenation of furan
derivatives gives unstable endoperoxides A which, in
alcoholic solutions, usually are regioselectively trans-
formed into alkoxy-hydroperoxydihydrofurans (B or
B%; Scheme 4) the regioselectivity being determined by
the stabilization of the developing positive charge.5a
The subsequent reduction of dihydrofurans, B and B%,
References
1. For a review, see: Keay, B. A.; Dibble, P. W.; In Compre-
hensive Heterocyclic Chemistry II; Katrizky, A. R.; Rees,
C. W.; Scriven, E. F. V.; Bird, C. W., Eds.; Oxford:
Pergamon, 1996; Vol. 2, pp. 395–436.
2. For reviews, see: (a) Heaney, H.; Anh, J. S.; In Compre-
hensive Heterocyclic Chemistry II; Katrizky, A. R.; Rees,
C. W.; Scriven, E. F. V.; Bird, C. W., Eds.; Oxford:
Pergamon, 1996; Vol. 2, pp. 297–350; (b) Shipman, M.;
Contemp. Org. Synth. 1995, 2, 1-17; (c) Lipshutz, B. H.
Chem. Rev. 1986, 86, 795–819.
Scheme 2. Reagents and conditions: (a) O2, MB, hw, CH3OH,
−40°C, 25 min; (b) Me2S.
3. For a review, see: Piancatelli, G.; D’Onofrio, F. Synthesis
1994, 867–889.
4. (a) Norley, M. C.; Kocie´nsky, P. J.; Faller, A. Synlett
1994, 77–78; (b) Cottier, L.; Descotes, G.; Eymard, L.;
Rapp, K. Synthesis 1995, 303–306; (c) Shiraki, R.;
Sumino, A.; Tadano, K.-J.; Ogawa, S. Tetrahedron Lett.
1995, 36, 5551–5554; (d) Maezaki, N.; Gijsen, J. M.; Sun,
Li-Q; Paquette, L. A. J. Org. Chem. 1996, 61, 6685–6692.
5. For reviews, see: (a) Feringa, B. L.; Rec. Tr. Chim. Pays
Bas 1987, 469–488; (b) Clenan, E. L. Tetrahedron 1991,
47, 1343–1382; (c) Adam, W.; Griesbeck, A. In Methoden
Organic Chemistry (Houben-Weyl); Hoffmann, R. W.,
Ed., Vol. E21e, Stuttgart: Georg Thieme Verlag, 1995;
4928–4946. For more recent references, see: (d) Iesce, M.
R.; Cermola, F.; Scarpati, R.; Graziano, M. L. Synthesis
1994, 944–948; (e) Iesce, M. R.; Cermola, F.; Piazza, A.;
Scheme 3. Reagents and conditions: (a) O2, MB, hw, CH3OH,
−40°C, 30 min; (b) Me2S.
Scheme 4.

9132
Y. Arroyo et al. / Tetrahedron Letters 43 (2002) 9129–9132
1
Scarpati, R.; Graziano, M. L. Synthesis 1995, 439–443;
(f) Onitsuka, S.; Nishino, H.; Kurosawa, K. Tetrahedron
2001, 57, 6003–6009.
m/z (%) 302/300 (44/46, M+). Compound 10: H NMR:
2.30 (s, 3H, CH3ꢀC6H4ꢀ), 6.37 (d, 1H, J 2.1 Hz, H-4),
7.25–7.28 (m, 9H, H-arom); 7.52 (d, 1H, H-5); 13C NMR:
21.0 (CH3–C6H4–), 115.3 (C-4), 125.6 (C-3), 126.7, 128.5,
129.1, 129.8, 130.1 (CH-arom), 131.4, 135.1, 136.9 (C-
arom), 144.6, 146.1 (C-2, C-5); MS: m/z (%) 298 (100,
M+).
6. (a) Ba´n˜ez, J. M.; Galisteo, D.; Lo´pez Sastre, J. A.;
Rodr´ıguez Amo, J. F.; Romero, C.; Santos, M.; Sanz
Tejedor, M. A. Carbohydr. Res. 1996, 179–188; (b)
Arroyo, Y.; Carren˜o, M. C.; Garc´ıa-Ruano, J. L.;
Rodr´ıguez, J. F.; Santos, M.; Sanz-Tejedor, M. A. Tetra-
hedron: Asymmetry 2000, 11, 1183–1191.
7. (a) Chen, Ch.-Y.; Hart, D. J. J. Org. Chem. 1993, 58,
3840–3849; (b) Hart, D. J.; Wu, W.-L.; Kozikowski, A.
P. J. Org. Chem. 1997, 62, 5023–5033; (c) Hart, D. J.; Li,
J.; Wu, W.-L. J. Am. Chem. Soc. 1995, 117, 9369–9371;
(d) Byeon, Ch.-H.; Chen, Ch.-Y; Ellis, D. A.; Hart, D. J.;
Li, J. Synlett 1998, 596–598.
8. (a) Lin, Ch.; Huang, F.-J.; Lin, J.; Wu, H.-J. J. Chinese
Chem. Soc. 1996, 43, 177–186; (b) Tsai, S.-H.; Chung,
W.-S.; Wu, H.-J. J. Chinese Chem. Soc. 1996, 43, 281–
288; (c) Wu, H.-J.; Tsai, S.-H.; Chung, W.-S. Tetrahedron
Lett. 1996, 37, 8209–8212.
13. Sensitized photooxygenation reactions were carried out in
anhydrous methanol as solvent, at −40°C, with bubbling
oxygen and in the presence of methylene blue (MB) as
photosensitizer (4×10−3 mmol), under irradiation with a
500 W tungsten lamp. When each reaction was complete,
the solution was degassed at rt by bubbling with Argon.
For furans 5 and 10, Me2S (2 equiv.) in CCl4 was added
to each solution just after irradiation. The solvent was
removed on a rotatory evaporator and each residue was
washed with hexane (for 6, 7, and 9) or water (for 5, 8
and 10) and extracted with Cl2CH2.
14. Compounds 11–17 were characterized from the crude
reaction mixture on the basis of their 1H NMR (l in
ppm, CDCl3, 300 MHz), 13C NMR (l in ppm, CDCl3, 75
MHz), IR and MS spectral data.
9. Byeon, Ch.-H.; Hart, D. J.; Lai, Ch.-S.; Unch, J. Synlett
2000, 119–121.
10. The synthesis of 1 was achieved from 2,3-dibromo-2-
buten-1,4-diol by the one-pot procedure previously
described by Rewicky et al. Gorzynsky, M.; Rewicki, D.
Liebigs Ann Chem. 1986, 625–637.
11. (a) Davies, G. M.; Davies, P. S. Tetrahedron Lett. 1972,
3507–3508; (b) Danso-Danquah, R. E.; Scott, A. I. Tet-
rahedron 1993, 49, 8195–8210; (c) Alva´rez-Ibarra, C.;
Quiroga, M. L.; Toledano, E. Tetrahedron 1996, 52,
4065–4078.
1
Compound 11: H NMR: 3.85 (s, 3H, CH3O), 7.13–7.32
(m, 5H, H-arom); 13C NMR: 53.9 (CH3O), 127.1, 127.4,
129.0 (CH-arom), 131.1, 133.0 (C-2, C-3), 137.0 (C-
arom), 165.1 (COOMe), 186.2 (COSPh); IR (KBr) w
(cm−1): 1741, 1730; MS: m/z (%) 382/380/378 (1/5/1, M+).
Compound 12: 1H NMR: 3.89 (s, 3H, CH3O), 6.53 (s,
1H, H-2), 7.17–7.47 (m, 5H, H-arom); 13C NMR: 53.8
(CH3O), 126.0, 129.8 (C-2, C-3), 127.2, 127.5, 129.1 (CH-
arom), 137.0 (C-arom), 164.2 (COOMe), 193.1 (COSPh);
IR (KBr) w (cm−1): 1719, 1700; MS: m/z (%) 302/300 (3/1,
12. All new 2-thiofurans were characterized on the basis of
1
their H NMR (l in ppm, CDCl3, 300 MHz), 13C NMR
M+). Compound 13: H NMR: 3.81 (s, 3H, CH3O), 6.66
1
(l in ppm, CDCl3, 75 MHz) and MS spectral data.
1
Compound 3: mp 66–67°C (hexane); H NMR: 7.24–7.28
(s, 1H, H-3), 7.17–7.47 (m, 5H, H-arom). Compound 14:
1H NMR: 2.48 (s, 3H, SCH3), 3.88 (s, 3H, CH3O), 6.51
(s, 1H, H-2); 13C NMR: 12.6 (SCH3), 53.6 (CH3O), 125.1,
129.6 (C-2, C-3), 164.1 (COOMe), 186.9 (COSMe); IR
(KBr) n (cm−1): 1727, 1668; MS: m/z (%) 209/207 (45/43,
M+–OMe). Compound 15: 1H NMR: 2.47, 2.39 (two s,
3H, SCH3), 6.67 (s, 1H, H-3); 13C NMR: 12.0, 12.4
(SCH3), 125.8, 129.5 (C-2, C-3); 186.9, 190.1 (COSMe).
Compound 16: (E)-isomer: 1H NMR: 2.43 (s, 3H, SCH3),
6.65 (d, 1H, J 6.5, H-3), 9.94 (d, 1H, CHO); 13C NMR:
13.8 (SCH3), 137.2, 137.6 (C-2, C-3), 188.6, 189.3 (CHO,
(m, 5H, H-arom), 7.44 (s, 1H, H-5); 13C-NMR: 104.0,
114.0 (C-3, C-4), 127.6, 129.3, 129.5 (CH-arom), 132.9
(C-arom), 141.6, 147.1 (C-5, C-2); MS: m/z (%) 336/334/
332 (48/100/51, M+). Compound 4: mp 25°C (hexane); 1H
NMR: 6.55 (d, 1H, J 2.1 Hz, H-4), 7.18–7.29 (m, 5H,
H-arom), 7.5 (d, 1H, H-5); 13C NMR: 110.6 (C-3), 115.5
(C-4), 126.8, 128.2, 129.2 (CH-arom), 134.5 (C-arom)
142.9, 146.4 (C-5, C-2); MS: m/z (%) 256/254 (74/76,
M+). Compound 5: 1H NMR: 2.37 (s, 3H, SCH3), 6.44
(d, 1H, J 2.2 Hz, H-4), 7.41 (d, 1H, H-5); 13C NMR: 17.9
(SCH3), 106.2 (C-3), 115.0 (C-4), 144.9, 145.6 (C-5, C-2);
MS: m/z (%) 194/192 (48/51 M+). Compound 6: mp
77–78°C (hexane); 1H NMR: 7.26–7.28 (m, 10H, H-
arom); 13C NMR: 113.0 (C-3, C-4), 127.6, 129.3, 129.5
(CH-arom), 132.9 (C-arom), 147.0 (C-2, C-5). MS: m/z
(%) 444/442/440 (4/9/3 M+). Compound 7: mp 77–78°C
(hexane); 1H NMR: 6.73 (s, 1H, H-4), 7.19–7.25 (m, 10H,
H-arom); 13C NMR: 110.9 (C-3), 122.2 (C-4), 127.1,
127.3, 129.2 (CH-arom), 133.4 (C-arom), 146.4, 148.5
(C-2, C-5); MS: m/z (%) 364/362 (35/32, M+). Compound
1
COSMe). (Z) Isomer: H NMR: 2.42 (s, 3H, SCH3), 7.19
(d, 1H, J 6.5, H-3), 10.00 (d, 1H, CHO); 13C NMR: 13.8
(SCH3), 130.8, 136.5 (C-2, C-3), 188.7 and 192.2 (CHO,
COSMe); IR (KBr) n (cm−1): 1726, 1685; MS: m/z (%)
210/208 (17/17, M+). Compound 17: 1H NMR: 2.31 (s,
3H, CH3–C6H4–), 5.34 (d, 1H, J 3.5 Hz, H-4), 6.42 (d,
1H, H-5), 7.08–7.53 (m, 9H, H-arom); 13C NMR: 21.2
(CH3-C6H4–), 111.9 (C-5), 126.9 (C-4), 127.2, 128.9,
129.7, 136.5, 136.6 (CH-arom), 134.2 (C-3), 135.4, 140.6,
141.9 (C-arom), 164.9 (C-2); IR (KBr) w (cm−1): 1790,
1767; MS: m/z (%) 314 (4%, M+).
1
8: H NMR: 2.42, 2.44 (two s, each 3H, SCH3), 6.44 (s,
1H, H-4); 13C NMR: 18.1 (SCH3), 106.8 (C-3), 117.6
(C-4), 147.5, 150.6 (C-2, C-5); MS: m/z (%) 240/238 (3/1,
M+). Compound 9: 1H NMR: 2.42 (s, 3H, SCH3), 6.72 (s,
1H, H-4), 7.22–7.32 (m, 5H, H-arom); 13C NMR: 17.6
(SCH3), 105.8 (C-3), 122.7 (C-4), 127.5, 128.6, 129.2
(CH-arom), 133.8, 134.6 (C-arom, C-2), 146.3 (C-5); MS:
15. There are several reports in which endoperoxides rear-
ranged to give butyrolactones. See Ref. 4a, 5a and (a)
Waldemar, A.; Schuhmann, R. M. Liebigs Ann. 1996,
635–640; (b) Freeman-Cook, K. D.; Halcomb, R. L.
Tetrahedron Lett. 1998, 39, 8567–8570.