Metal carbenoid mediated ring opening of furan-2-yloxy derivatives. Stereoselective synthesis of 2,4-hexadienedioic acid and 6-aryl-1-oxo-2,4-hexadienoic acid derivatives

Article abstract of DOI:10.1016/S0040-4020(01)00691-3

2-Methoxy- and 2-trimethylsilyloxyfuran undergo facile ring opening reaction upon treatment with metal carbenoid. Treatment of 2-methoxyfuran with ethyl diazoacetate and aryl-α-diazocarbonyl compounds under metal catalysis afforded (2Z,4E)-hexadienedioate

Full text of DOI:10.1016/S0040-4020(01)00691-3

TETRAHEDRON
Pergamon
Tetrahedron 57 )2001) 7303±7307
Metal carbenoid mediated ring opening of furan-2-yloxy
derivatives. Stereoselective synthesis of 2,4-hexadienedioic acid
and 6-aryl-1-oxo-2,4-hexadienoic acid derivatives
Po Chuen Shieha and Chi Wi Ongb,p
aDepartment of Pharmacy, Tajen Institute of Technology, Ping-Tung, Taiwan, ROC
bDepartment of Chemistry, National Sun Yat Sen University, Koahsiung 804, Taiwan, ROC
Received 21 May 2001; accepted 2 July 2001
AbstractÐ2-Methoxy- and 2-trimethylsilyloxyfuran undergo facile ring opening reaction upon treatment with metal carbenoid. Treatment
of 2-methoxyfuran with ethyl diazoacetate and aryl-a-diazocarbonyl compounds under metal catalysis afforded )2Z,4E)-hexadienedioate and
6-aryl-6-oxo-)2Z,4E)-hexadienoates respectively. When 2-trimethylsilyloxyfuran was used, desilylation occurred to give directly the mono-
protected )Z,E)-muconic acid and 6-aryl-6-oxo-)2Z,4E)-hexadienoic acids. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
array of 2,4-hexadienedioic acid and 6-aryl-6-oxo-
)2Z,4E)-hexadienoic acid derivatives.
Rhodium)II) acetate has been a superior catalyst for the
generation of transient electrophilic metal carbenoid from
a-diazocarbonyl compounds1 and these undergo inter- or
intramolecular carbenoid addition2±4 and insertion
reaction.5±8 In recent years the reactions of this transient
electrophilic carbenoid with furan and 2-alkylfuran have
been recognized as one of the most convenient routes to
1,6-dioxo 2,4-diene derivatives, and are used for the
synthesis of a number of natural products9 and heterocyclic
systems.10 Furthermore, a variation in the products was
encountered by varying the nature of the substituents
attached to the 2-position of furan upon treatment with the
transient electrophilic metal carbenoid generated from
rhodium)II) acetate catalyst. One important ®nding is that
electron-withdrawing substituents at the 2-position of the
furan ring do not inhibit the cyclopropanation but interfere
with the ring opening reaction.11 Although furan and
2-alkylfuran have been found to give a net ring opening
reaction after the initial cycloaddition reaction with the
metal carbenoid,12 far less attention has been given to the
study of a 2-substituted heteroatom electron-donating group
on furan. In this paper we describe the scope of using metal
carbenoid generated from a-diazoester and aryl-a-diazo-
carbonyl compounds with rhodium)II) acetate for the ring
opening reaction of a series of 2-alkoxyfurans. The success-
ful execution of this reaction will represent a new powerful
methodology for the direct construction of a remarkable
At the onset of this work we were uncertain as to whether
the ring opening reaction of furan-2-yloxy derivatives with
ethyl diazoacetate and aryl-a-diazocarbonyl compounds in
the presence of rhodium)II) catalyst can compete favorably
with the cyclopropanation or the insertion reaction
)Scheme 1).
2. Results and discussions
Our initial studies focused on the dirhodium)II) tetraacetate
catalyzed reaction of 2-methoxyfuran 1a with ethyl diazo-
acetate13 in CH2Cl2. In this reaction, the prevalent products
obtained were a mixture of the )2E,4Z)-2a and 1-ethyl 6-
methyl )2E,4E)-hexadienedioate 3a respectively, in a ca.
1:1 ratio )after crude product separation by preparative
chromatography on silica gel). From the mechanistic point
of view, the stereochemistry of the ring cleavage of
2-methoxyfuran with ethyl diazoacetate under rhodium)II)-
catalysis should exclusively gave the )2E,4Z)-2a. This arises
from the fact that whereas the newly formed double
bond between the 2-methoxyfuran and ethyl diazoacetate
a-carbons could assume a trans con®guration, the double
bond created between the former furan b-carbons main-
tained a cis orientation, re¯ecting the original furan frame-
work.4,5 The formation of the )E,E)-3a thus implicate an
acid induced isomerization of the cis-double bond into the
more stable trans-double bond during silica gel separation
of the crude reaction product.14 The introduction of a
2-methoxy substituent on the furan ring was found to
promote the ring opening reaction. Stereochemical
Keywords: diazo compounds; rhodium acetate; furan-2-yloxy derivatives;
ring opening reaction.
p
Corresponding author. Tel.: 1886-7-5252000; fax: 1886-7-5253908;
e-mail: cong@mail.nsysu.edu.tw
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020)01)00691-3

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P. C. Shieh, C. W. Ong / Tetrahedron 57 .2001) 7303±7307
Scheme 1. Possible products from the reaction of furan-2-yloxy derivative with electrophilic metal carbenoid.
assignments of the products were straightforward upon
analyzing of the H NMR obtained. The )2E,4Z)-isomer
group in muconic diester by generating a highly labile
trimethylsily-ester precursor that undergoes desilylation in
situ.
1
has a typical down®eld chemical shift for the internal
E-double bond proton at approximately d 8.40 )dd, Jˆ11
and 15 Hz), similar to that reported for the )E,Z)-dimethyl-
hexadienedioate.15 On the other hand, the )E,E)-isomer
internal Z-double bond proton has a chemical shift at
approximately d 7.50, similar to that reported for the
)E,E)-dimethylhexadienedioate.16 We have now added
another new repertoire for the preparation of unsymmetrical
muconic diester. Several other reported methods for the
synthesis of muconic diester include: )i) the oxidative
cleavage of catechol17 to the )E,E)-isomer and further
isomerized into the )E,Z) form; )ii) the thermolysis of 1,2-
cyclobutenedicarboxylic anhydride18 to the )E,Z) isomer
and )iii) the Wittig19 or Horner±Emmons20 reaction with
malealdehyde. Each method has its advantages and draw-
backs; thus our method provides another alternative route
)Scheme 2).
These results represent the ®rst example using the reactions
of furan-2-yloxy derivatives with metal carbenoids for the
direct synthesis of unsymmetrical muconic diester and
monoprotected muconic acid.
The success achieved by the rhodium)II)-catalyzed ring
opening offuran-2-yloxy derivatives with ethyl diazoacetate
was extended to a series of aryl diazoketone for the
synthesis of 6-aryl-6-oxo-)2Z,4E)-hexadienoic acid and
derivatives. In fact there are two possible routes to 6-aryl-
6-oxo-2,4-hexadienoic acid by this method: )i) the reaction
of 2-arylfuran with ethyl diazoacetate, and )ii) the reaction
of furan-2-yloxy derivatives with aryl-diazoketone in the
presence of rhodium)II)-catalyst. The preparation of 2-aryl-
furan itself is a cumbersome process and this route was
deemed unsuitable for the preparation of 6-aryl-6-oxo-
)2Z,4E)-hexadienoic acid and derivatives. The fact that
the ring opening reaction of commercially available furan-
2-yloxy derivatives with metal carbenoid provides an
alternative route for the direct synthesis of 6-aryl-6-oxo-
2,4-hexadienoic derivatives is particularly noteworthy.
The 6-phenyl-6-oxo-)2E,4E)-hexadienoate has recently
been prepared from phenylglyoxal after two consecutive
Wittig reactions with formylmethylenetriphenylphos-
phorane and )methoxycarbonylmethylene)triphenylphos-
phorane.22
It is reasonable to assume that 6-ethyl 1-trimethylsilyl
)2Z,4E)-hexadienedioate can be prepared from the ring
opening reaction of 2-trimethylsilyloxyfuran with ethyl
diazoacetate in the presence of rhodium)II)-catalyst. Selec-
tive deprotection of the trimethylsily-protecting group
should be possible under mild condition to afford mono-
protected muconic acid. Monoprotected muconic acid is
an important intermediate for the synthesis of macrocyclic
trichothecenes.19±21 Interestingly, the reaction of
2-trimethylsilyloxyfuran 1b with ethyl diazoacetate in the
presence of rhodium)II)-catalyst was found to give directly
the )2E,4Z)-2b and )2E,4E)-3b monoprotected muconic
acid in a 1:1 ratio )after chromatography of the crude
product on silica gel). The trimethylsilyl ester group in the
product proved to be extremely labile and undergo selective
deprotection to yield the acid. Earlier report for the prepara-
tion of monoprotected )E,Z)-muconic acid generally
involved a multi-step sequence via hydrolysis of an ester
group in muconic diester that proceed with poor regio-
selectivity.18 Our method circumvents the dif®culty
encounter for the regioselective hydrolysis of an ester
We therefore investigated the reactions of 2-methoxyfuran
1a with a-diazoacetophenone23 4a, a-diazoacetothio-
phene10 4b, 4-methoxy-a-diazoacetophenone23 4c and
a-diazoaceto-2-naphthone24 in the presence of rhodium)II)-
catalyst. The aryl-a-diazocarbonyl compounds 4a±d were
synthesized from the reaction of the corresponding acid
chloride with diazomethane. Treatment of 2-methoxyfuran
with a-diazoacetophenone 4a in the presence of Rh)II)-
catalyst afforded the furan ring opening product, methyl
6-oxo-6-phenylhexa-)2Z,4E)-dienoate 5a, after puri®cation
Scheme 2. Synthesis of 2,4-hexadiendioic acid derivatives from 2-methoxy and 2-trimethylsilyloxyfuran and ethyl diazoacetate under Rh)II)-catalyst.

P. C. Shieh, C. W. Ong / Tetrahedron 57 .2001) 7303±7307
7305
Table 1. Reaction of aryl diazoketone with furan 2-yloxy derivatives
by chromatography. A point worth noting is that the thermo-
dynamically less stable )Z,E)-isomer that correspond to an
overall retention of con®guration of the double bond created
between the former furan b-carbons can be obtained without
undergoing isomerization during chromatography, and this
might be attributed to the increase stability of the cis-double
bond. By changing the aryl diazocarbonyl compound to
the synthesis of 6-aryl-6-oxo-)2Z,4E)-hexadienoic acids
and its esters by the ring opening reaction of
2-trimethylsilyloxyfuran and 2-methoxyfuran with a wide
array of aryl diazoketones in the presence of rhodium)II)
catalyst.
a-diazoacetothiophene
4b,
4-methoxy-a-diazoaceto-
3. Conclusions
phenone 4c and a-diazoaceto-2-naphthone 4d, they were
found to react smoothly with 2-methoxyfuran in the
presence of rhodium)II) catalysis to give the ring opening
product 5b±d respectively, whereby the )Z,E)-stereo-
chemisty was again retained after puri®cation by chromato-
graphy )Table 1). The resistance towards isomerization of
the cis-double bond in 5a±d is particularly worth noting.
In conclusion, this represent the ®rst example of the highly
ef®ciency rhodium)II)-catalyzed ring opening reaction of
2-methoxy- and 2-trimethylsilyloxyfuran with ethyl diazo-
acetate, a-diazoacetophenone, a-diazoacetothiophene,
4-methoxy-a-diazoacetophenone and a-diazoaceto-2-
naphthone for the stereoselective synthesis of 2,4-hexa-
dienedionic acid and 1-aryl-1-oxo-2,4-hexadienoic acid
derivatives which will ®nd wide application in organic
synthesis. The generality of this transformation is also
worth noting.
Similarly, 2-trimethylsiloxyfuran 1b was found to react
favorably with all the aryl diazocarbonyl compounds 4a±
d in the presence of rhodium)II) catalyst to give the ring
opening product, 6-aryl-6-oxo-)2Z,4E)-hexadienoic acid
6a±d respectively, after puri®cation by silica gel chromato-
graphy. The labile trimethylsilyl ester functional group
undergoes hydrolysis readily in situ to give the correspond-
ing acid, and the )Z,E)-double bond was inert towards
isomerization during chromatographic puri®cation. We
have developed a new and highly ef®cient direct entry for
4. Experimental
1H NMR spectra were recorded using 200 MHz spectro-
meter in CDCl3, using tetramethylsilane as an internal
standard. Melting points were measured on a Fisher±John

7306
P. C. Shieh, C. W. Ong / Tetrahedron 57 .2001) 7303±7307
apparatus and uncorrected. Unless otherwise noted, all reac-
tions were performed in oven-dried glassware under an
atmosphere of nitrogen. Separation of the crude reaction
mixture was carried out on preparative silica gel TLC
using ethyl acetate±hexane mixture as the eluent. Ethyl
diazoacetate,13 a-diazoacetophenone,23 a-diazoacetothio-
phene,10 4-methoxy-a-diazoacetophenone23 and a-diazo-
aceto-2-naphthone24 were prepared according to the
previously reported methods. Approximately 5 mg of
catalyst was used to decompose 0.001 mmol of the diazo
compound.
4.1.6. Methyl 6-oxo-6-thienyl-'2Z,4E)-2,4-hexadienoate
5b. Yield: 53% as yellowish solid, mp 95±968C. H NMR
1
)200 MHz, CDCl3) d 3.80 )3H, s), 6.05 )1H, d, Jˆ11 Hz),
6.76 )1H, dd, both Jˆ11 Hz), 7.00 )1H, d, Jˆ15 Hz), 7.10
)1H, dd, Jˆ5, 6 Hz), 7.70 )1H, d, Jˆ6 Hz), 7.81 )1H, d,
Jˆ5 Hz), 8.48 )1H, dd, Jˆ11, 15 Hz); 13C NMR
)500 MHz) d 182.28, 165.75, 164.85, 140.87, 137.63,
134.35, 132.58, 132.38, 128.36, 125.16, 51.74; MS m/z
222. )Found: C, 59.36; H, 4.52%. Calcd for C11H10SO3: C,
59.44; H, 4.53%.)
4.1.7. Methyl 6-oxo-6-'4-methoxyphenyl)-'2Z,4E)-2,4-
hexadienoate 5c. Yield: 84% as yellowish solid, mp 125±
1268C. H NMR )200 MHz, CDCl3) d 3.79 )3H, s), 3.89
)3H, s), 6.03 )1H, d, Jˆ11 Hz), 6.77 )1H, dd, both
Jˆ11 Hz), 6.97 )2H, d, Jˆ8 Hz), 7.10 )1H, d, Jˆ15 Hz),
7.97 )2H, d, Jˆ8 Hz), 8.40 )1H, dd, Jˆ11, 15 Hz); 13C
NMR )500 MHz) d 189.10, 165.86, 163,67, 141.34,
137.61, 133.07, 130.03, 130.46, 124.55, 113.93, 55.52,
51.71; MS m/z 246. )Found: C, 68.19; H, 5.54%. Calcd
for C14H14O4: C, 68.28; H, 5.73%.)
1
4.1. Rhodium'II)-catalyzed ring opening of 2-methoxy
and 2-silyloxyfuran with diazo compounds
To a solution containing diazo compound )0.006 mmol) in
dichloromethane )10 mL) was added dropwise to a solution
containing 2-methoxyfuran or 2-silyloxyfuran )0.005
mmol) and dirhodium)II) tetraacetate )30 mg) in dichloro-
methane )15 mL). The reaction mixture was stirred at room
temperature over overnight. The reaction mixture was
®ltered and concentrated under reduced pressure. The
resulting crude product was puri®ed on preparative TLC.
4.1.8. Methyl 6-oxo-6-'2-naphthyl)-'2Z,4E)-2,4-hexa-
dienoate 5d. Yield: 60% as yellowish solid, mp 111±
1128C. H NMR )200 MHz, CDCl3) d 3.79 )3H, s), 6.08
1
4.1.1. 1-Ethyl 6-methyl '2E,4Z)-2,4-hexadienedioate 2a.
Yield: 31.5% )total yield of 63% for Z,E- and E,E-isomers)
as clear oil. H NMR )200 MHz, CDCl3) d 1.32 )3H, t,
Jˆ8 Hz), 3.78 )3H, s), 4.25 )2H, q, Jˆ8 Hz), 5.98 )1H, d,
Jˆ11 Hz), 6.12 )1H, d, Jˆ16 Hz), 6.66 )1H, dd, both
Jˆ11 Hz), 8.38 )1H, dd, Jˆ11, 16 Hz); MS m/z 184.
)Found: C, 58.83; H, 6.79%. Calcd for C9H13O4: C, 58.69;
H, 6.57%.)
)1H, d, Jˆ11 Hz), 6.84 )1H, dd, both Jˆ11 Hz), 6.90)1H,
d, Jˆ15 Hz), 7.60 )3H, m) 7.90 )4H, m), 8.33 )1 dd, Jˆ11,
15 Hz); 13C NMR )500 MHz) 190.53, 165.79, 141.17,
138.33, 135.59, 134.85, 132.93, 132.49, 130.34, 129.58,
128.68, 128.59, 127.85, 126.87, 125.01, 124.37, 51.73;
MS m/z 266. )Found: C, 77.01; H, 5.14%. Calcd for
C17H14O4: C, 76.68; H, 5.30%.)
1
4.1.9. 6-Oxo-6-phenyl-'2Z,4E)-2,4-hexadienoic acid 6a.
Yield: 79% as yellowish solid, mp 82±838C. H NMR
1
4.1.2. 1-Ethyl 6-methyl '2E,E)-2,4-hexadienedioate 3a.
Yield: 31.5% as clear oil. H NMR )200 MHz, CDCl3) d
1
)200 MHz, CDCl3) d 6.39 )1H, Jˆ11 Hz), 6.61 )1H, dd,
both Jˆ11 Hz), 7.07 )1H, d, Jˆ15 Hz), 7.55 )3H, m), 7.93
)2H, d, Jˆ9 Hz), 8.25 )1H, dd, Jˆ11, 15 Hz), 11.40 )1H, br.
s); MS m/z 201. )Found: 200.0837. HRMS calcd for
C12H10O3: 200.0841.)
1.32 )3H, t, Jˆ8 Hz), 3.75 )3H, s), 4.24 )2H, q, Jˆ8 Hz),
5.98 )2H, d, Jˆ14 Hz), 7.89 )2H, d, Jˆ14 Hz).
4.1.3. 5-Ethoxycarbonyl-'2Z,4E)-2,4-pentadienoic acid
2b. Yield: 20.5% )total yield of 41% for Z,E- and E,E-
isomers), as colorless solid, mp 89±918C )lit.25 90±928C).
1H NMR )200 MHz, CDCl3) d 1.30 )t, 3H, Jˆ8.0 Hz), 4.22
)2H, q, Jˆ8.0 Hz), 6.07 )1H, d, Jˆ11 Hz), 6.20 )1H, d,
Jˆ15 Hz), 6.73 )1H, dd, both Jˆ11 Hz), 8.20 )1H, dd,
Jˆ11, 15 Hz), 11.04 )1H, br. s), MS m/z 169.
4.1.10. 6-Oxo-6-thienyl-'2Z,4E)-2,4-hexadienoic acid 6b.
Yield: 55% as yellowish solid, mp 102±1038C. H NMR
1
)500 MHz, CDCl3) d 6.08 )1H, d, Jˆ11 Hz), 6.86 )1H,
dd, both Jˆ11 Hz), 7.05 )1H, d, Jˆ15 Hz), 7.20 )1H, dd,
Jˆ5, 6 Hz), 7.76 )1H, d, Jˆ6 Hz), 7.98 )1H, d, Jˆ5 Hz),
8.50 )1H, dd, Jˆ11, 15 Hz), 11.30 )1H, br. s); MS m/z 208.
)Found: 208.0194. HRMS calcd for C10H8O3S: 208.0194.)
4.1.4. 5-Ethoxycarbonyl-'2E,4E)-2,4-pentadienoic acid
3b. Yield: 20.5% as colorless solid, mp 143±1458C )lit.26
145±1468C). H NMR )200 MHz, CDCl3) d 1.31 )3H, t,
1
4.1.11. 6-Oxo-6-'4-methoxyphenyl)-'2Z,4E)-2,4-hexa-
dienoic acid 6c. Yield: 57% as yellowish solid, mp 126±
Jˆ8 Hz), 4.20 )2H, q, Jˆ8 Hz), 6.00 )1H, d, Jˆ14 Hz),
7.85 )1H, d, Jˆ14 Hz), 7.94 )1H, dd, both Jˆ14 Hz), 8.01
)1H, dd, both Jˆ14 Hz), 11.24 )1H, br. s), MS m/z 169.
1
1278C. H NMR )200 MHz, CDCl3) d 3.89 )3H, s), 6.05
)1H, d, Jˆ11 Hz), 6.84 )dd, 1H, both Jˆ11 Hz), 6.90 )2H,
d, Jˆ9 Hz), 7.15 )1H, d, Jˆ15 Hz), 7.92 )1H, d, Jˆ9 Hz),
8.42 )1H, dd, Jˆ11, 15 Hz), 11.34 )1H, br. s); MS m/z
232. Found: 232.0735. HRMS calcd for C13H12O4:
2320745.)
4.1.5. Methyl 6-oxo-6-phenyl-'2Z,4E)-2,4-hexadienoate
5a. Yield: 79% as yellowish solid, mp 90±928C. H NMR
1
)200 MHz, CDCl3) d 3.79 )3H, s), 6.05 )1H, d, Jˆ11 Hz),
6.78 )1H, dd, both Jˆ11 Hz), 7.09 )1H, d, Jˆ15 Hz), 7.53
)3H, m), 7.95 )2H, d, Jˆ8 Hz), 8.41 )1H, dd, Jˆ11, 15 Hz);
13C NMR )500 MHz) d 190.969, 165.78, 141.16, 138.44,
137.50, 133.11, 133.03, 128.69, 128.67, 125.01, 51.74; MS
m/z 216. )Found: C, 71.94; H, 5.67%. Calcd for C13H12O3:
C, 72.21; H, 5.59%.)
4.1.12. 6-Oxo-6-'2-naphthyl)-'2Z,4E)-2,4-hexadienoic acid
6d. Yield: 55% as yellowish solid, mp 87±888C. H NMR
1
)200 MHz, CDCl3) d 6.07 )1H, d, Jˆ11 Hz), 6.78 )1H, dd,
both Jˆ11 Hz), 7.13 )1H, d, Jˆ15 Hz), 7.65 )3H, m), 7.90
)4H, m), 8.43 )1H, dd, Jˆ11, 15 Hz), 11.64 )1H, br. s); MS

P. C. Shieh, C. W. Ong / Tetrahedron 57 .2001) 7303±7307
7307
3135. Ong, C. W.; Chen, C. M.; Wang, L. F.; Jan, J. J.; Shieh,
P. C. J. Org. Chem. 1998, 63, 9131. Ong, C. W.; Chen, C. M.;
Wang, L. F.; Shieh, P. C. Tetrahedron Lett. 1998, 39, 9191.
11. Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic
Methods for Organic Synthesis with Diazo Compounds;
Wiley-Interscience: New York, 1998 see p 336.
m/z 252. Found: 252.0786. HRMS calcd for C16H12O3:
252.0787.
Acknowledgements
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