Synthesis of 2(5H)-furanones via oxidative ring expansion of 4-hydroxy-2-cyclobutenones

Article abstract of DOI:10.1055/s-2005-864805

A new method to reduce cyclobutenediones towards 4-hydroxy-2-cyclobutenones was established. The latter compounds gave rise to new 5-halo-2(5H)-furanones by reaction with halogenating agents via a cationic ring-opening-ring-closure mechanism.

Full text of DOI:10.1055/s-2005-864805

LETTER
947
Synthesis of 2(5H)-Furanones via Oxidative Ring Expansion of 4-Hydroxy-2-
cyclobutenones
S
ynthesis of 2(5
H
)
u
-Furanones ido Verniest, Norbert De Kimpe*
Department of Organic Chemistry, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000 Gent, Belgium
Fax +32(9)2646243; E-mail: norbert.dekimpe@UGent.be
Received 26 January 2005
by treating (semi)squaric acid derivatives with Grignard
Abstract: A new method to reduce cyclobutenediones towards 4-
reagents or organolithium species.1–8 When 4-alkyl-4-hy-
droxy-2-cyclobutenones are used as starting material for
hydroxy-2-cyclobutenones was established. The latter compounds
gave rise to new 5-halo-2(5H)-furanones by reaction with haloge-
the reaction with iodine, Pb(OAc)4 or PhI(OAc)2, the re-
sulting 5-iodo- or 5-acetoxy-5-alkyl-2(5H)-furanones of-
ten give rise to 5-alkylidene-2(5H)-furanones as a side
product, due to the easy elimination of HI or HOAc.5a,6,7
nating agents via a cationic ring-opening–ring-closure mechanism.
Key words: cyclobutenones, 2(5H)-furanones, ring expansion, re-
arrangements, oxidations
4-Hydroxy-2-cyclobutenones are known as versatile
building blocks in organic synthesis due to the facile ring-
opening of these small compounds, relieving their ring
strain. Consequently, 4-hydroxy-2-cyclobutenones have
been used in a variety of ring expansion reactions towards
five- and six-membered carbo- and heterocycles such as
phenols and quinones,1 2-pyrones,2 2-pyridinones,3 and
cyclopentenones.4 Only a limited number of publications
describe the synthesis of furanones starting from substi-
tuted 4-hydroxy-2-cyclobutenones.1b,c,3b,5–8 The thermal
or photolytic rearrangement of 4-hydroxy-2-cyclobuten-
ones by an initial electrocyclic ring-opening resulting in
vinylketenes, yielded substituted furanones after ring-clo-
sure.1b,c,3b,5 Another synthetic pathway leading to fura-
nones using hydroxycyclobutenones proceeds via a
radical mechanism by reaction of the latter compounds
with lead(IV) tetraacetate or by photolysis and thermoly-
sis of hypoiodides synthesized from hydroxycyclobuten-
ones with iodine in the presence of mercury(I) oxide.5a,6
Besides these radical ring transformations of 4-hydroxy-
cyclobutenones an alternative cationic mechanism was
discovered when treating 4-alkyl- or 4-aryl-2,3-diethoxy-
4-hydroxy-2-cyclobutenones with PhI(OAc)2.5a,7,8
O
O
O
O
O
O
OMe
OMe
RO
O
R
OR
O
2
3
1
Cl
O
Cl
O
O
O
O
O
O
O
H
MeO
O
O
HO
Cl
losigamone (4)
spirodiclofen (5)
spiromesifen (6)
®
)
(Envidor
Figure 1
In our approach, a new methodology was developed to
synthesize 4-hydroxycyclobutenones without additional
substituents at the 4-position. These compounds proved to
be good precursors for the synthesis of new 5-halo-2(5H)-
furanones bearing only a halogen substituent at the 5-po-
sition and which consequently do not suffer from further
dehalogenation reactions towards 5-alkylidene-2(5H)-
furanones.
2(5H)-Furanones are of importance as physiologically
active compounds, which resulted already in numerous
patents. For instance, 3-phenylfuranones 1 and 2 were
patented as herbicides, plant protection agents and pesti-
cides,9 whereas 5-alkoxyfuranones 3 in general were pat-
ented for their potential in the treatment of osteoporosis
(Figure 1).10 Losigamone (4) is a well-studied furanone
and is used as an antiepilepticum.11 Spirodiclofen (5,
Envidor®) and spiromesifen (6) are furanones with pro-
nounced insecticidal and acaricidal activities.12
In the present literature only very few methods are known
to synthesize 2-hydroxycyclobutenones via reduction of
(semi)squarates or (semi)squaramides. In one publication,
sodium borohydride was used to reduce 4-phenylsemi-
squaramides,5c while other publications make use of
DIBALH or LiAlH(t-BuO)3.13 In an evaluation of differ-
ent reducing agents to synthesize hydroxycyclobutenones
from 4-arylsemisquaric esters 714 (Scheme 1), it was
found that an efficient reduction could be established us-
ing zinc in acetic acid at room temperature. This proce-
dure provides a new, easy to perform and cheap method to
synthesize 4-hydroxy-2-cyclobuten-1-ones 8 from 7. In
Above mentioned methodologies to synthesize furanones
make use of hydroxycyclobutenones which are obtained
SYNLETT 2005, No. 6, pp 0947–0950
0
6.
0
4.
2
0
0
5
Advanced online publication: 23.03.2005
DOI: 10.1055/s-2005-864805; Art ID: G01605ST
© Georg Thieme Verlag Stuttgart · New York

948
G. Verniest, N. De Kimpe
LETTER
that way, various new hydroxycyclobutenones (8a–d) The mechanism of this ring expansion is rationalized by a
were synthesized. New O-acyl derivatives 9 were pre- cationic ring-opening of the intermediate hypobromite 10
pared in good yield by treatment of hydroxycyclobuten- and subsequent attack of the expelled bromide to the gen-
ones 8 with suitable acid chlorides (Scheme 1).
erated positive charge (Scheme 2). A radical mechanism
is not likely because the reaction proceeds at low temper-
ature and the same results are obtained when performing
the reaction in the dark. Keeping in mind the necessity of
thermolysis or photolysis of analogous hypoiodides to in-
duce a radical triggered ring expansion,5a,6 the current re-
action pathway probably proceeds via a cationic
mechanism. It is not surprising that next to this formal
acyl shift, also dehydrobromination of the intermediate
hypobromite 10 occurs towards the oxidized compound
7a (R1 = H, R2 = CH3). Because the reaction of hydroxy-
cyclobutenone with bromine produced only small
amounts of ring expanded product 12, also reactions with
other halogenating agents were evaluated (see Table 1).
Treatment of 8a with NBS gave rise to an increased
amount of 2(5H)-furanone 12, which was accompanied by
an equal amount of oxidation product. Better results were
obtained with the use of t-butyl hypochlorite in dichlo-
romethane. After reaction at room temperature for 15
hours, a ratio 7a/13a of 23:77 was obtained (calculated
O
OH
O
O
5 equiv Zn
HOAc, r.t., 4 h
OR2
OR2
R1
R1
7
8a R1 = H, R2 = Me (84%)
8b R1 = H, R2 = Et (61%)
8c R1 = H, R2 = i-Pr (54%)
8d R1 = Cl, R2 = Me (70%)
O
O
1 equiv. R3COCl
1 equiv. Et3N
O
R3
CH2Cl2, 0 °C, 2 h
OR2
R1
9a R1 = H, R2 = Me, R3 = Me (75%)
9b R1 = H, R2 = Me, R3 = Ph (71%)
9c R1 = H, R2 = Me, R3 = 2,4-Cl2C6H3 (80%)
9d R1 = H, R2 = Me, R3 = Bn (89%)
9e R1 = Cl, R2 = Me, R3 = Me (84%)
9f R1 = H, R2 = Et, R3 = Me (87%)
1
from H NMR spectra). Performing the same reaction at
reflux temperatures shifted this ratio towards 16:84
(Table 1). It is clear that the ring-opening of intermediates
yielding furanones is preferred over a single dehydrohalo-
genation towards cyclobutenediones when higher temper-
atures are applied. The best results were obtained by
reaction of hydroxycyclobutenone 8a with 1.05 equiva-
lents of NCS in CCl4 at reflux temperature for 2 hours.
The fact that reactions using t-BuOCl yield more cy-
clobutenedione 7a as compared to NCS could be ex-
plained by the presence of t-butoxide in the reaction
mixture in the former case, which acts as a base and pro-
motes the dehydrochlorination. These reaction conditions
directed the transformation predominantly to 2(5H)-fura-
none 13a, while only a slight amount of 2-methoxy-3-
phenylcyclobutenedione 7a (4%) was produced.
Scheme 1
In a preliminary experiment to evaluate the reactivity of 4-
hydroxycyclobutenones 8 towards halogenating agents,
4-hydroxy-3-methoxy-2-phenyl-2-cyclobutenone
(8a)
was treated with 1 equivalent of bromine in dichlo-
romethane at 0 °C for 3 hours. Two major compounds
were determined in the 1H NMR spectrum, namely start-
ing material 8a (37%) and the oxidized product 7a (31%,
Scheme 2). Besides these compounds, also a third minor
compound was detected. After chromatography, this com-
pound could be isolated in 5% yield and proved to be
2(5H)-furanone 12.15
Using the optimized reaction conditions, a simple and ef-
ficient route was established towards various new fura-
nones 13 bearing a halogen at the 5-position (Scheme 3).
It should be emphasized that this synthetic pathway is per-
formed only with low cost reagents (Zn in HOAc, NCS,
etc.) and standard organic synthesis procedures.
O
OH
OMe
8a
Br
H
O
O
O
O
1 equiv Br2
CH2Cl2, 0 °C
3 h
The presence of a halogen at the 5-position of 13 makes
these compounds of particular interest for further organic
transformations to furanones bearing specific substitution
patterns. The reaction of 4-alkoxy-3-aryl-5-chloro-2(5H)-
furanones 13 with sodium alkoxides under mild condi-
tions, resulted in 4,5-dialkoxyfuranones 15 in good yields.
The reaction of furanone 13a with an excess of isopro-
panol gave only a substitution of the chloro atom, afford-
ing furanone 16 in 72% yield. In addition, heating of
dimethoxyfuranone 13a in refluxing isopropylamine re-
sulted in 4-isopropylamino-5-methoxy-3-phenyl-2(5H)-
furanone (17).
OMe
OMe
10
7a (31%)
Br
O
O
O
O
–Br
Br
O
O
OMe
OMe
OMe
12 (5%)
10
11
Scheme 2
Synlett 2005, No. 6, 947–950 © Thieme Stuttgart · New York

LETTER
Synthesis of 2(5H)-Furanones
949
Table 1 Optimization of the Rearrangement of 4-Hydroxy-2-cyclobutenone (8a) to 2(5H)-Furanones 12 and 13a
Entry
Reaction conditions
Yield of 8a (%)a
Yield of 7a (%)a
Yield of 12, 13a (%)a
1
2
3
4
5
1.1 Equiv NBS, CCl4, D, 1 h
0
13
0
50
50
23
16
4
50
37
77
84
96
1.1 Equiv t-BuOCl, CH2Cl2, 0 °C, 1 h
1.1 Equiv t-BuOCl, CH2Cl2, r.t., 15 h
1.1 Equiv t-BuOCl, CH2Cl2, D, 4 h
1.05 Equiv NCS, CCl4, D, 2 h
0
0
a Ratios calculated from 1H NMR spectra.
Due to the specific substitution pattern of furanones 13, In conclusion, it can be stated that an efficient pathway
bearing a chlorine at the 5-position, these compounds are towards interesting furanones with a remarkable sub-
of interest as precursors for 5-metalated furanones which stitution pattern was developed, which renders these com-
could be trapped with electrophiles. Indeed, the reaction pounds suitable for conversion to polysubstituted
of chlorinated furanones 13 with activated zinc yielded 5- physiologically interesting 2(5H)-furanones.
dechlorinated furanones 14, resulting from an in situ pro-
tonation of the intermediate organo zinc compounds.
4-Hydroxy-3-methoxy-2-phenyl-2-cyclobuten-1-one (8a)
To a solution of 1.00 g (5.32 mmol) 3-methoxy-4-phenyl-3-cy-
O
OH
O
1.05 equiv
NCS, CCl4
clobuten-1,2-dione (7a) in 25 mL of AcOH at r.t. was added 1.74 g
(26.60 mmol, 5 equiv) zinc powder in portions during 10 min. Cool-
ing with an ice bath was applied when the reaction temperature ex-
ceeded 30 °C. After stirring for 4 h, the heterogeneous mixture was
filtered over Celite®, the solids were washed with CH2Cl2 (3 ꢀ 20
mL) and the filtrate was diluted with 50 mL H2O. The organic layer
was separated and the aqueous layer was treated with NaHCO3 until
the mixture was only slightly acidic (ca. pH 6–7) and extracted
again with CH2Cl2 (3 ꢀ 50 mL). After drying over MgSO4 and cold
evaporation (<25 °C) of the solvent, the resulting oil was further
evaporated at high vacuum (<25 °C) to remove residual AcOH. Af-
ter evaporation, a white solid was obtained, which could be purified
by washing with cold Et2O or by flash chromatography. When
during the procedure, including workup, the temperature exceeded
30 °C a significant amount (ca. 10%) of O-acetylated product 9a
was obtained. Purification by washing with cold Et2O; yield 84%,
mp 122 °C (decomposed at 135 °C). 1H NMR (CDCl3): d = 4.31 (3
H, s, OCH3), 5.43 (1 H, s, CHOH), 7.28–7.39 (3 H, m, 3 ꢀ CHar),
7.71–7.74 (2 H, m, 2 ꢀ CHar). 13C NMR (CDCl3): d = 60.6 (OCH3),
82.2 (CHOH), 124.2 (Cquat), 2 ꢀ 127.0 (2 ꢀ CHar), 2 ꢀ 128.5
(2 ꢀ CHar), 2 ꢀ 128.3 (CHar and Cquat), 180.1 (C=COCH3), 188.1
(C=O). IR (KBr): nmax = 3303, 1735, 1589 cm–1. MS: m/z (%) = 190
(100) [M+], 162 (31), 131 (29), 130 (37), 1239 (35), 119 (51), 102
(67), 91 (51), 77 (20). Anal. Calcd for C11H10O3: C, 69.46; H, 5.30.
Found: C, 69.17; H, 5.41.
Cl
O
OR2
OR2
∆, 2 h
R1
R1
8
13a R1 = H, R2 = Me (81%)
13b R1 = H, R2 = Et (62%)
13c R1 = H, R2 = i-Pr (65%)
13d R1 = Cl, R2 = Me (78%)
1 equiv NaOR2
R2OH, r.t., 15 h
1.5 equiv Zn/Cu
HOAc, r.t., 2 h
O
O
OR2
OR2
O
O
OR2
R1
R1
14a R1 = H, R2 = Me (89%)
14b R1 = H, R2 = Et (80%)
14c R1 = H, R2 = i-Pr (73%)
14d R1 = Cl, R2 = Me (81%)
15a R1 = H, R2 = Me (87%)
15b R1 = H, R2 = Et (80%)
15c R1 = H, R2 = i-Pr (51%)
15d R1 = Cl, R2 = Me (86%)
O
O
Oi-Pr
O
Cl
O
excess
i-PrOH,
5-Chloro-4-methoxy-3-phenyl-2(5H)-furanone (13a)
To a solution of 1.50 g (7.89 mmol) 4-hydroxy-3-methoxy-2-phe-
nyl-2-cyclobuten-1-one (8a) in 50 mL of CCl4 was added 1.11 g
(8.29 mmol, 1.05 equiv) NCS. The mixture was refluxed for 2 h and
after cooling to 0 °C the supernatant succinimide was filtered. The
filtrate was cooled again to –20 °C (2 h) and subsequently filtered.
After evaporation of the solvent, furanone 13a was obtained in al-
most quantitative yield. To remove minor impurities the product
was purified by flash chromatography. Flash chromatography (hex-
ane–EtOAc, 4:1; Rf = 0.27); yield 81%. 1H NMR (CDCl3): d = 4.04
(3 H, s, OCH3), 6.54 (1 H, s, CHCl), 7.34–7.44 (3 H, m, 3 ꢀ CHar),
7.71–7.75 (2 H, m, 2 ꢀ CHar). 13C NMR (CDCl3): d = 59.1 (OCH3),
80.8 (CHCl), 103.3 (Cquat-C=O), 127.7 (Cquat), 2 ꢀ 128.0
(2 ꢀ CHar), 2 ꢀ 128.1 (2 ꢀ CHar), 128.2 (CHar), 168.6 (Cquat-OCH3),
170.6 (C=O). IR (NaCl): nmax = 1782, 1655, 1372, 1065 cm–1. MS:
OMe
OMe
∆, 15 h
16 (72%)
13a
O
OMe
O
O
OMe
O
excess
i-PrNH2
OMe
NHi-Pr
∆, 12 h
17 (69%)
15a
Scheme 3
Synlett 2005, No. 6, 947–950 © Thieme Stuttgart · New York

950
G. Verniest, N. De Kimpe
LETTER
(4) (a) Liebeskind, L. S.; Chidambaram, R.; Mitchell, D.;
m/z (%) = 224/26 (100) [M+], 195/97 (93), 189 (97), 118 (63), 89
(70). Anal. Calcd for C11H9ClO3: C, 58.81; H, 4.04. Found: C,
59.03; H, 4.19.
Foster, B. S. Pure Appl. Chem. 1988, 60, 27.
(b) Liebeskind, L. S.; Bombrun, A. J. Org. Chem. 1994, 59,
1149. (c) Yamamoto, Y.; Ohno, M.; Eguchi, S. Tetrahedron
Lett. 1995, 36, 5539. (d) Liebeskind, L. S.; Mitchell, D.;
Foster, B. S. J. Am. Chem. Soc. 1987, 109, 7908.
4,5-Dimethoxy-3-phenyl-2(5H)-furanone (15a)
To a solution of 0.50 g (2.23 mmol) 5-chloro-4-methoxy-3-phenyl-
2(5H)-furanone (13a) in 5 mL of MeOH was added 0.61 mL (2.45
mmol, 1.1 equiv) of 4 M NaOMe in MeOH at r.t. After stirring over-
night (15 h) the mixture was poured in H2O (20 mL) and extracted
with CH2Cl2 (3 ꢀ 20 mL). Drying (MgSO4) and removal of the sol-
vents in vacuo yielded dimethoxyfuranone 15a. When the same
procedure was followed in the reaction of 5-chloro-4-methoxy-3-
phenyl-2(5H)-furanone with 2.2 equiv of NaOEt in EtOH, di-
ethoxyfuranone 15b was obtained. When 5-chloro-4-methoxy-3-
phenyl-2(5H)-furanone was heated in i-PrOH only substitution of
the chloro atom occurred towards 16.
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1994, 59, 4707. (b) Yamamoto, Y.; Ohno, M.; Eguchi, S. J.
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M.; Eguchi, S. J. Org. Chem. 1996, 61, 9264.
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Chem. 1999, 64, 707.
(9) (a) Akiyoshi, Y.; Tsutsumiuchi, K.; Narita, I.; Okada, T.;
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Compound 15a: flash chromatography (hexane–EtOAc, 65:35;
Rf = 0.24); yield 87%. 1H NMR (CDCl3): d = 3.59 (3 H, s, OCH3),
4.04 (3 H, s, OCH3), 5.82 (OCHO), 7.30–7.42 (3 H, m, 3 ꢀ CHar),
7.78–7.82 (2 H, m, 2 ꢀ CHar). 13C NMR (CDCl3): d = 55.4 (OCH3),
58.6 (OCH3), 96.4 (CH), 104.4 (Cquat-C=O), 127.7 (CHar), 2 ꢀ 127.8
(2 ꢀ CHar), 2 ꢀ 127.9 (2 ꢀ CHar), 128.4 (Cquat), 168.3 (Cquat-OCH3),
169.7 (C=O). IR (NaCl): nmax = 1767, 1755, 1659, 1386 cm–1. MS:
m/z (%) = 220 (100) [M+], 205 (10), 189 (24), 161 (59), 145 (42),
118 (79). Anal. Calcd for C12H12O4: C, 65.45; H, 5.49. Found: C,
65.33; H, 5.28.
Acknowledgment
The authors are indebted to the IWT (Flemish Institute for the
Promotion of Scientific-Technological Research in Industry), the
FWO-Flanders and Ghent University (GOA) for financial support.
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recently described procedure starting from 2,2-dichloro-
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details, see: Verniest, G.; Colpaert, J.; Törnroos, K. W.; De
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Synlett 2005, No. 6, 947–950 © Thieme Stuttgart · New York