discussion addendum for: convenient preparation of 3...
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Org. Synth. 2019, 96, 98-109 Published on the Web 4/18/2019 DOI: 10.15227/orgsyn.096.0098 Ó 2019 Organic Syntheses, Inc.
98
DiscussionAddendumfor:ConvenientPreparationof3-EthoxycarbonylBenzofuransfromSalicylaldehydesandEthylDiazoacetate Mizzanoor Rahaman1 and M. Mahmun Hossain*1 Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, 3210 North Cramer Street, Milwaukee, Wisconsin 53211-3029, United States Original Article: Dudley, M. E.; Morshed, M. M.; Hossain, M. M. Org. Synth. 2009, 86, 172–180.
The benzofuran scaffold is an important heterocyclic core component found in several natural products and in polymers.2,3 Typically, 2,3-disubstituted benzofurans are notable building blocks in many medicinal and biologically active compounds.4-9 In addition, 3-substituted benzofurans act as anticancer agents,10 antitubercular agents,11 antimicrobial agents,12
antiviral agents,13 and anti-inflammatory agents.14 They also act as enzyme inhibitors,15,16 ischemic cell death inhibitors,17 receptor agonist-antagonists,18
and use as diagnostic imaging agents targeting amyloid plaques in Alzheimer's disease.19 Although syntheses of 2-substituted or 2,3-disubstituted benzofurans are most common, the syntheses of 3-substituted benzofurans are rare.20-24 In 2004, our group reported an unprecedented reaction of acrylate formation from readily available aldehydes starting materials and ethyl diazoacetate (EDA) in the presence of Brønsted acid, HBF4· OEt2.25 While studying the substrate scope of benzaldehydes to prepare substituted 3-hydroxyacrylates, our group reacted salicylaldehyde with EDA and isolated a very low mass of acrylates with a larger portion of
CHO
OH O
COOEt1. N2CHCOOEt, HBF4, (10 mol%), CH2Cl2, rt
2. H2SO4, rt
ClCl
Org. Synth. 2019, 96, 98-109 DOI: 10.15227/orgsyn.096.0098 99
the hemiacetal, 3-ethoxycarbonyl-2-hydroxy-2,3-dihydrofuran. The hemiacetal underwent dehydration in the presence of concentrated H2SO4 to form the 3-ethoxycarbonyl benzofuran. The general scope of the reaction was also investigated using various commercially available substituted salicylaldehydes and EDA. The acid catalyzed dehydration via cyclization produced the products in excellent yields.26 Alternatemethodstoprepare3-alkoxycarbonylbenzofurans
Most alternate syntheses of 3-alkoxycarbonyl benzofuran involve
transition metal catalysis. In 1982, Ortar et al. reported the synthesis of 3-methoxycarbonyl benzofurans by the oxidation reaction of chromanones with excess thallium trinitrate (TTN) in methanol in the presence of trimethyl orthoformate (TMOF). The product was isolated as a pale-yellow liquid in only 23% yield (Scheme 1).27
O
O
TTN, TMOF
MeOH, 6 h O
OMeO
R R
Scheme 1. Synthesis by a thallium catalyzed oxidation annulation
Henke and coworkers reported a new synthetic route to 3-ethoxycarbonyl benzofuran (Scheme 2a).28 In this two-step procedure, the first step involved the Michael addition of 2-bromophenol with ethyl propionate in the presence of trimethylamine to prepare 3-(2-bromophenoxy)-acrylic acid ethyl ester. Second, a palladium-catalyzed intramolecular Heck coupling of the 3-(2-bromophenoxy)acrylic acid ester created 3-ethoxycarbonyl benzofuran as a yellow oil in 61% yield.
Later, Frontier et al. applied a similar synthetic strategy as reported by Henke et al.28 involving 3-(2-iodophenoxy)acrylic acid ethyl ester to prepare 3-ethoxycarbonyl benzofuran in 74% yield (Scheme 2b).29 The ester was produced from the reaction of 2-iodophenol and ethyl propionate in the presence of N-methylmorpholine (NMM).
Wang and coworkers reported the formation of 3-ethoxycarbonyl benzofuran in 81% yield from (E)-3-phenoxyacrylates through the direct oxidative cyclization by a palladium catalyst (Scheme 2c).30 The
Org. Synth. 2019, 96, 98-109 DOI: 10.15227/orgsyn.096.0098 100
corresponding acrylates were prepared from phenol and propynoic acid ethyl ester in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO).
Scheme 2. Syntheses by a palladium-catalyzed cyclization
Morice et al. reported the preparation of several 3-methoxycarbonyl benzofurans based on the conversion of 3-coumaranones into their corresponding triflates, followed by palladium assisted CO insertion reactions in methanol (Scheme 3).31 3-Coumaranones were prepared from o-methoxybenzoic acids with oxalyl chloride and diazomethane followed by decomposition of the diazo ketone in acetic acid/sodium acetate solution.
Scheme 3. Synthesis by a palladium assisted CO insertion
Karchava et al. reported the preparation of 3-methoxycarbonyl benzofuran from hydroxyacrylates. First, the methyl-2-bromophenylacetates
Br
OH Et3N, THF, rt
Br
O
O
OEt
O
OOEt
Pd(OAC)2, PPh3
NaHCO3, DMF, 110 °CR R R
I
OH NMM, THF, rt
I
O
O
OEt
O
OOEt
Pd(OAC)2, PPh3
Et3N, CH3CN, 80 °CR R R
OH O
O
OEt
O
OOEt
Pd(OAC)2, PPh3
CF3CO2Ag, C6H6, 110 °CDABCO, CH2Cl2RRR
a.
b.
c.
O
O
O
O
O
O
O
O
O
OOMe
DIPEA, Tf2O
CH2Cl2, rt, 16 h
OMe
O
OHNaOAc, HOAc
OMe
O
O
OTfPd(OAc)2, dppp, NEt3, MeOH
CO (1 atm), DMF, 80 °C, 2 h
1. (COCl)2
2. CH2N2, Et2O
N2
R R R
R R
Org. Synth. 2019, 96, 98-109 DOI: 10.15227/orgsyn.096.0098 101
were treated with methyl formate in the presence of sodium hydride and after acidic work up, the mixture provided the acrylate product. Later, a copper-catalyzed cyclization of the acrylate produced 3-methoxycarbonyl benzofuran in 88% yield (Scheme 4).32
Br
1. NaH, HCOOMe, 1 h
2. 10% HCl, NaHCO3 Br
OMeO
CuI, K2CO3, DMF
100 °C, 2 h O
OMeO
OMe
OOH
R R R
Scheme 4. Synthesis by a copper-catalyzed cyclization
Recently, in 2016, Yao et al. reported the synthesis of 3-ethoxycarbonyl benzofuran by a Rh(III)-catalyzed reaction between salicylaldehyde and ethyl 2-diazo-3-oxopropanoate in dichloroethane (DCE).33 Silver triflimide (AgNTf2) favored the formation of 3-ethoxycarbonyl benzofuran via a tandem C-H activation/decarbonylation/annulation process in 72% yield (Scheme 5).
OH O
OOEt
[RhCp*(Cl)2]2, PPh3
AgNTf2, DCE, 50 °CH
O
N2
O
OEtR
R
H
O
- CO
Scheme 5. Synthesis by a rhodium-catalyzed annulation Applicationsofourmethodinorganicsynthesis
Compared to the alternative syntheses of 3-alkoxycarbonyl benzofuran, our synthetic method is simple, less time consuming, and high yielding with inexpensive and commercially available starting materials. This one-pot synthetic method has been used by us and by other groups in the preparation of several biologically active compounds as described herein.
Telvekar and coworkers synthesized N’-benzylidene benzofuran-3-carbohydrazides from 3-ethoxycarbonyl benzofurans (Scheme 6).34 All these compounds were found to be active against tuberculosis and showed antifungal activity against Candida albicans.
Org. Synth. 2019, 96, 98-109 DOI: 10.15227/orgsyn.096.0098 102
Scheme 6. Synthesis of N’-benzylidene benzofuran-3-carbohydrazide
Eccles and coworkers synthesized several leukotriene A4 hydrolase (LTA4H) inhibitors from 3-ethoxycarbonyl benzofuran (Scheme 7).35 LTA4H inhibitors are used in inflammatory diseases, such as bowel disease, rheumatoid arthritis, chronic obstructive pulmonary disease, and asthma.
Scheme 7. Synthesis of leukotriene A4 hydrolase (LTA4H) inhibitor
Morrow et al. synthesized pterocarpene and coumestan-type
heterocycles by the Mitsunobu coupling of 3-(hydroxymethyl)benzofurans with o-iodophenols (Scheme 8).36 Pterocarpans have been shown to exhibit broad spectrum activity against gram-positive bacteria and vancomycin-resistant strains of enterococci. Coumestans, such as coumestrol and flemmichapparin C, have also been shown to display antibacterial, antifungal, and antimyotoxic effects.
1. N2CHCOOEt, HBF4•OEt2, CH2Cl2, rt
2. H2SO4, rtOH
R1
O
R1
O
HOEt
O
O
R1
HN
O
R1 = H, R2 = HR1 = Cl, R2 = ClR = Ar
R2R2
H2N NH2 . H2O
C2H5OH, reflux5-7 h
O
R1
HN
O
R2
NH2 O Ar
C2H5OH, reflux5-7 h
NAr
R2
OH
1. N2CHCOOEt, HBF4•O(Me)2, CH2Cl2
2. toluene, reflux O
O
H
OMeO
BBr3, CH2Cl2
-78 °CMeO MeO O
OMeO
HO
S
NCl
Cs2CO3CH3CN, 50 °C O
OMeO
O
N
S
DIBALH or LAH
CH2Cl2, 0 °CO
OH
O
N
S
HNR1R2
K2CO3, DMFO
NR1R2
O
N
S
NR1R2 = Piperidine, morpholine, isonepicotic acid, N-(piperidin-4-yl)acetamide, 1-(piperidin-4-yl)pyrrolidin-2-one, 1-(2,5-diazabicyclo[2.2.1]heptan-2-yl)ethanone
Org. Synth. 2019, 96, 98-109 DOI: 10.15227/orgsyn.096.0098 103
OH
1. N2CHCOOEt, HBF4CH2Cl2, rt
2. H2SO4, rt O
O
HOEt
O
O
OH
DIAD, Ph3P, THF
DIBAL-H
CH2Cl2
HO
I
O
O
IO
Pd(OAc)2, KOAc
Bu4NBr, DMF
O
O
O
PCCCH2Cl2
OR1 R1 R1
R2
R1 R1R2
R2 R2
R1Pterocarpenes Coumestans
Scheme 8. Synthesis of pterocarpenes and coumestans
Tolstikov et al. reported several regioselective Diels–Alder reactions of Danishefsky’s diene with 3-ethoxycarbonyl benzofurans (Scheme 9).37 These reactions provided effective method for the construction of the heterocyclic skeleton of hexahydrodibenzofuran-7-one and tetrahydrodibenzofuran-7-one. These tricyclic fragments are the structural motifs of many pharmacologically vital substances, such as plant alkaloids morphine, galanthamine, lycoramine, and lunarine, linderol A, and several selective estrogen receptor b -agonists.
Scheme 9. Construction of heterocyclic skeleton
Elofsson and coworkers constructed a library based on the 3-carboxy 2-aryl benzofuran scaffold from 3-ethoxycarbonyl benzofuran (Scheme 10).38 These two scaffolds are core components in many biologically active natural and synthetic compounds of which many display a wide range of activities including antiviral, antibacterial, anti-inflammatory, antiangiogenic, and antimitotic activities.
OH
1. N2CHCOOEt, HBF4•OEt2, CH2Cl2
2. H2SO4, rt O
O
H
OEtO
PhMe, 170 °C, 48-52 hor, PhMe, microwave
O
MeO OSiMe3
CH2
O
EtO OMe
H
1. H4NF, MeOH, -15 °C2. TsOH, MeOH, rt3. CF3COOH, CH2Cl2, rt4. TsOH, PhMe, 110 °C O
O
EtO OMe
H O
O
EtO OMe
H O
O
EtO
H
O O O
OSiMe3
R R R
R RR
Org. Synth. 2019, 96, 98-109 DOI: 10.15227/orgsyn.096.0098 104
Scheme 10. Synthesis of 2-arylbenzofuran-3-carboxamide derivatives
Zhao et al. reported a total synthesis of paeoveitol, the norditerpene natural product which has antidepressant ability, from 3-ethoxycarbonyl benzofuran (Scheme 11).39 Our published procedure was employed to synthesize 3-ethoxycarbonyl benzofuran, which was reduced to paeoveitol D. Paeoveitol was synthesized by an unusual intermolecular ortho-quinone methide cycloaddition with paeoveitol D with excellent regio- and diastereoselectivity.
Scheme 11. Total synthesis of paeoveitol via paeoveitol D
Later, Chen and coworkers reported the first catalytic asymmetric total synthesis of (+)-paeoveitol and (-)-paeoveitol from 3-ethoxycarbonyl
1. N2CHCOOEt, HBF4•OEt2, CH2Cl2, rt
2. H2SO4, rtOH O
O
HOEt
O
R1R1 O
OEtO
R1
B(OH)2
O
OEtO
R1
O
NO
R1
Ar
R3
R4
O
OHO
R1
O
OEtO
R1
I
B(OCH3)3, LDA
THF, -78 °C
Ar
Ar
I2, LDA, THF
-78 °C, rt
boronic acidPdCl2(dppf).DCM
Na2CO3, DME/H2O
iodo derivativesPdCl2(dppf).DCMNa2CO3, DME/H2O
amine TBTU, DMF
NaOH
THF/H2O
OH
1. N2CHCOOEt, HBF4•OEt2CH2Cl2, rt
2. H2SO4, rt O
O
H
OEtO
O
OH
ZnCl2, CH2Cl2
DIBAL-H
CH2Cl2
O
O
HO
Me
HO
Me
HO
Me
HO Me
OH HO
Me H
OH
Me
Me
OHHO
Me
PaeoveitolPaeoveitol D
Org. Synth. 2019, 96, 98-109 DOI: 10.15227/orgsyn.096.0098 105
benzofuran via a biomimetic hetero-Diels-Alder reaction in the presence of chiral phosphoric acids as catalysts (Scheme 12).40
Scheme 12. Asymmetric synthesis of (+)-paeoveitol and (-)-paeoveitol
Bongen et al. reported an efficient asymmetric synthesis of 7-benzoyl-2,3-
dihydro-1-benzofuran-3-carboxylic acid, BRL-37959 (Scheme 13).41 3-Ethoxycarbonyl benzofuran was reduced by magnesium turnings in methanol to form 2,3-dihydrobenzofuran-3-carboxylic acid ethyl ester, which was resolved by dynamic kinetic resolution. Friedel-Crafts acylation reaction of the enantiopure product followed by acidic hydrolysis produced (R)-BRL-37959, which acts as analgesic agents with low gas irritancy.5
Scheme 13. Synthesis of enantiopure (R)-BRL-37959
Recently, in 2018, our group reported the synthesis of 7-benzoyl-2,3-dihydro-1-benzofuran-3-carboxylic acid, BRL-37959 and its analogs from 3-ethoxycarbonyl benzofuran (Scheme 14).42 To synthesize BRL-37959,
OH
1. N2CHCOOEt, HBF4•Et2OCH2Cl2, rt
2. H2SO4, rt O
O
H
OEtO
O
OH
chiral phosphoric acid cat.
CH2Cl2, rt
DIBAL-H
CH2Cl2
O
O
MeO
Me
MeO
Me
HO
Me
HO Me
OH HO
Me H
OH
Me
Me
OHHO
Me
(+)-Paeoveitol
BBr3
CH2Cl2 O
OEtHO
Me
O
O
OHO
Me H
OH
Me
Me
OH
(-)-Paeoveitol
, or
Paeoveitol D
OH
1. N2CHCOOEt, HBF4•Et2O, CH2Cl2
2. H2SO4, rt
O
HCAL-B, 36 °C
KPi buffer
(100 mm, pH 8.5)
O
OEtO
O
OEtO
O
OEtO
1. Mg/MeOH
2. EtOH, H2SO4
CS2, AlCl340 °C, 72 h
Cl Cl
Cl H2O, H2SO4
rtO
Cl
OEt
O
O
O
Cl
OH
O
O
Cl
O
Cl
Org. Synth. 2019, 96, 98-109 DOI: 10.15227/orgsyn.096.0098 106
incorporation of a benzoyl group at the C-6 position of benzofuran ring by the Friedel-Crafts acylation reaction was the main challenge. Our recent method demonstrates that bismuth (III) trifluoromethanesulfonate can be used as a catalyst for the Friedel-Crafts acylation reaction with good yield. This efficient method allowed the production of many analogs of BRL-37959 in high yield.
Scheme 14. Synthesis of BRL-37959 and its analogs
In summary, 3-ethoxycarbonyl benzofuran plays a pivotal role in the field of medicinal and pharmaceutical chemistry. The concise synthesis of 3-ethoxycarbonyl benzofuran was reported by our group with excellent yield from commercially available starting materials. In this addendum, we discussed several alternate methods for the preparation of 3-ethoxycarbonyl benzofuran and applications of our published method to synthesize important biologically active compounds. In the future, our developed method of preparing 3-ethoxycarbonyl benzofuran could be a valuable procedure in making benzofuran-ring containing natural and unnatural products. References 1. Department of Chemistry and Biochemistry, University of Wisconsin-
Milwaukee, 3210 North Cramer Street, Milwaukee, Wisconsin 53211-3029, United States. Email: [email protected]
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OH
R11. N2CHCOOEt,
HBF4•Et2O, CH2Cl2, rt
2. H2SO4, rt O
R1O
H
OEtO
Mg, I2
MeOH, 72 h O
R1OMe
O
Cl
OR2
Bi(OTf)3nitrobenzene, 90 °C
O
R1
OMe
ONaOH
MeOH, 12 h
O
R2
O
R1
OH
O
O
R2
Org. Synth. 2019, 96, 98-109 DOI: 10.15227/orgsyn.096.0098 107
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M. Mahmun Hossain received his MSc degree in chemistry from the University of Dhaka, Bangladesh. In 1985, he received his PhD from the University of South Carolina. After about three years of postdoctoral study with Professor Jack Halpern at the University of Chicago, he joined the Department of Chemistry & Biochemistry at the University of Wisconsin-Milwaukee as an Assistant Professor. Later, he was promoted to Professor, and received a research award from the UWM Foundation for his outstanding research and creativity.
Mizzanoor Rahaman received his BS and MS degree in Chemistry from the University of Dhaka, Bangladesh. He began PhD studies in Organic Chemistry at the University of Wisconsin-Milwaukee in 2013. Since then he has been an active member in Professor Hossain group and has received multiple Chancellor's Fellowship Awards. He was awarded an ACS Milwaukee Section Travel Grant in 2018. He was also honored with the University’s Outstanding Teaching Assistant Award for 2017.