Regioselective Access to Structurally Diverse Coumarin Analogues through Iron-Catalysed Annulation Reactions

Article abstract of DOI:10.1002/ejoc.201700999

A highly efficient iron-catalysed propargylation/alkyne oxacyclization/isomerization strategy is described. Biologically active furo[3,2-c]coumarins and pyrano[3,2-c]coumarins are expeditiously assembled in moderate to good yields and with a broad substrate scope. The regioselective access to different coumarins is mainly dependent on the terminal group of the secondary propargylic alcohol.

Full text of DOI:10.1002/ejoc.201700999

A Journal of
Accepted Article
Title: Regioselective Access to Structurally Diverse Coumarin
Analogues via Iron-catalyzed Annulation Reactions
Authors: Qiao Ren, Muyao Li, Lujiang Yuan, Jie Kang, Ruoyun Chen,
and Lei Wang
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700999
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10.1002/ejoc.201700999
European Journal of Organic Chemistry
FULL PAPER
Regioselective Access to Structurally Diverse Coumarin
Analogues via Iron-catalyzed Annulation Reactions
Qiao Ren,*[b] Jie Kang,[c] Muyao Li,[b] Lujiang Yuan,[b] Ruoyun Chen,[c] and Lei Wang*[a]
[a]
Prof. Dr. L. Wang
Department of Natural Medicine Chemistry Research Center
Institute of Medicinal Plant Development
Chinese Academy of Medical Science and Peking Union Medical College
Beijing 100193, P.R. China.
E-mail: siquanwl@gmail.com
[b]
[c]
Dr. Q. Ren, M. Li, L. Yuan
College of Pharmaceutical Sciences
Southwest University
No.2 Tiansheng Road, BeiBei District, Chongqing 400715, P. R. China
Email: qren2014@swu.edu.cn
J. Kang, R. Chen
State Key Laboratory of Bioactive Substance and Function of Natural Medicines
Institute of Materia Medica
Chinese Academy of Medical Sciences & Peking Union Medical College
No. 1 Xiannongtan Street, Beijing 100050, P.R. China.
Supporting information for this article is given via a link at the end of the document.
Abstract: A highly efficient iron-catalyzed propargylation-alkyne
oxacyclization-isomerization strategy is disclosed to expeditiously
assemble representative bioactive furo[3,2-c]coumarins and
pyrano[3,2-c]coumarins with moderate to good yields and a broad
substrate scope. The regioselective access to different coumarins is
mainly dependent on the terminal group of the secondary propargylic
alcohol.
Consequently, numerous elegant approaches have been
developed to access the furo[3,2-c]coumarin derivatives, such as
acid-promoted cascade addition-cyclization-oxidation, Bu3P-
mediated C-acylation/cyclization reaction, photoredox neutral
coupling and transition metal catalysis (Scheme 1).8 Notably, the
readily accessible propargylic alcohols have been employed as
the powerful substrates to construct the densely functionalized
furans by the aid of transition metal (Ru, Ca, Yb, Al and Cu).9
However, these typical methods always suffer from expensive
transition metal catalysts, harsh conditions and limited functional
group tolerance. In this regards, the development of novel
approaches is still highly desirable to expeditiously assemble
diverse coumarin derivatives from readily available starting
materials. Recent studies indicated the iron catalysis could be
employed as an important alternative to trigger the
transformations in comparison with using expensive transition
metals catalysts.10 To the best of our knowledge, the iron-
catalyzed synthesis of furo[3,2-c]coumarins and related coumarin
analogues from the readily available starting materials is less
explored.11 Herein, we disclosed an expeditious assembly of
valuable furo[3,2-c]coumarins and pyrano[3,2-c]coumarins with
high regioselectivities via iron-catalyzed annulation reactions.
Introduction
The coumarin ring, a privileged oxygen-containing heterocycle
structural motif, is ubiquitous in a diverse range of natural
products and pharmaceuticals. Among them, furo[3,2-c]coumarin,
the well-known tricyclic compound in which a coumarin unit is
angularly fused to the furan scaffold, has drawn considerable
attention due to its broad spectrum biological activities on cell
cycle, apoptosis and differentiation (Fig. 1).1 For example, neo-
tanshinlactone is
a natural product isolated from Salvia
miltiorrhiza and selectively inhibits the proliferation of estrogen
receptor positive breast cancer cells through transcriptional down-
regulation of ESR1 mRNA.1,2 Additionally, many naturally
occurring coumestans demonstrate intriguing anti-inflammatory,
estrogenic, antitumor, antioxidant and antihepatotoxic activities,
such as glycyrol3, coumestrol4, medicagol5, plicadin6 and
wedelolactone7. Therefore, the efficient construction of furo[3,2-
c]coumarin skeleton is of great significance in chemical and
biological community.
Figure 1. Representative bioactive natural products built on the furo[3,2-
c]coumarin core scaffold.
Scheme 1. Routes for the construction of the coumarin core scaffold.
1
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10.1002/ejoc.201700999
European Journal of Organic Chemistry
FULL PAPER
Results and Discussion
yield (3ka, 79%). Conversely, the presence of either an electron–
withdrawing group (Cl) or an electron–donating group (OMe) in
the 7-position resulted in relatively lower yields of the desired
products (3fa and 3la, 58% and 59%, respectively), indicating that
the steric hindrance of the substituents could reduce the reaction
rate. It is noteworthy that 4-hydroxybenzo[h]coumarin and 4-
hydroxybenzo[g]coumarin also gave the corresponding products
3na and 3oa in 78% and 72% yields respectively. Then, we
examined the scope of this strategy with respect to the terminal
propargylic alcohol 2. Moderate to good yields were generally
obtained with various terminal alkynols bearing one or two
electron–withdrawing/donating groups (2a―2g, 52―87%). To
our delight, this domino transformation smoothly accommodated
diverse propargylic alcohols bearing 2-naphthyl and 1-naphthyl,
affording the expected furo[3,2-c]coumarins 3ah and 3ai (83%
and 79%, respectively).
Preliminary studies were conducted with 4-hydroxycoumarin 1a
and 1-phenylprop-2-yn-1-ol 2a as the model substrates in the
presence of FeCl3·6H2O in MeNO2 at 80 oC. Gratifyingly,
FeCl3·6H2O exhibited an excellent catalytic activity, resulting in
the construction of the desired furo[3,2-c]coumarin product 3aa in
80% yield (Table 1, entry 1). To seek a more efficient metal
catalyst, we then examined some other Fe(III) and Fe(II) salts
(entries 2―7). As a result, the iron(III) perchlorate monohydrate
enabled the model reaction to give the best yield (entry 3, 87%).
Some other transition metal catalysts , such as Cu, Ag and Zn
salts, were also evaluated under identical reaction conditions, but
no improvement was achieved (entries 8―10, <43%). Further
exhaustive optimization was also performed by evaluating other
parameters, such as solvent as well as temperature (entries
11―17). Results revealed that the best catalytic activity was
obtained with MeNO2 at 80 oC. It is notable that a slightly lower
yield was achieved when the catalyst loading was reduced to 5
mol% (entry 18, 75%).
Table 2. Substrate scope[a]
.
Table 1. Reaction Development[a]
.
Entry
Solvent
Catalyst
Temp (°C)
Yield[b] (%) of 3aa
1
2
MeNO2
MeNO2
FeCl3·6H2O
FeBr3
80
80
80
60
3
4
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
DCE
Fe(ClO4)3·H 2O
Fe(NO3)3·9H2O
FeCl2·4H2O
80
80
80
80
80
80
80
80
80
80
80
100
60
40
RT
80
87
56
5
26
6
Fe(BF4)2·4H2O
Fe(ClO4)2·6H2O
CuI
30
7
57
8
NR
NR
43
9
AgOAc
10
11
12
13
14
15
16
17
18[c]
Zn(CF3SO3)2
Fe(ClO4)3·H 2O
Fe(ClO4)3·H 2O
Fe(ClO4)3·H 2O
Fe(ClO4)3·H 2O
Fe(ClO4)3·H 2O
Fe(ClO4)3·H 2O
Fe(ClO4)3·H 2O
Fe(ClO4)3·H 2O
65
DMF
NR
37
[a] Reaction conditions: 4-hydroxycoumarin 1 (0.30 mmol), 1-phenylprop-2-yn-
1-ol 2 (0.20 mmol), Fe(ClO4)3·H2O (10 mol%), MeNO2 (2.0 ml), 80 °C. [b]
Isolated yield.
Toluene
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
87
In addition, the gram-scale synthesis of 3aa was also
evaluated to investigate the practicability of this process. 0.92 g
of 1a reacted smoothly with 0.50 g of 2a in 7 h to construct 1.05 g
of the corresponding furo[3,2-c]coumarin 3aa (91%). Meanwhile,
we anticipated that this methodology could be extended to a more
versatile substrate scope from coumarin derivatives. Therefore,
the general cyclic 1,3-diketone was examined under identical
reaction conditions. Gratifying, the fused furan derivative 4 was
generated in 40% yield (Scheme 2). It is noteworthy that the less
reactive β-keto ester could be well employed as the substrate to
assemble the densely functionalized furan core structure 5 in 85%
yield (Scheme 2).
57
10
Trace
75
[a] Reaction conditions: 4-hydroxycoumarin 1a (0.30 mmol, 1.5 equiv), 1-
phenylprop-2-yn-1-ol 2a (0.20 mmol, 1.0 equiv) and the indicated catalyst (10
mol%) in MeNO2 (2.0 ml) was stirred at 80 °C for 5 h. [b] Isolated yield. [c]
Catalyst (5 mol%).
With the optimal conditions in hand, the generality of this
methodology was investigated with various representative 4-
hydroxycoumarins 1 (Table 2). A number of electron–withdrawing
(F, Cl, Br and Ph) and electron–donating groups (Me, Et, i-Pr and
OMe) in the 6-position of the 4-hydroxycoumarins were well
tolerated to furnish the corresponding furo[3,2-c]coumarins in
moderate to good yields (3ba―3ea and 3ga―3ja, 66―85%).
Additionally, the substituent in the 5-position had no impact on the
2
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10.1002/ejoc.201700999
European Journal of Organic Chemistry
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To confirm the reaction pathway of this protocol, we next
attempted to synthesize and isolate the 3-substituted coumarins
9 (Table 4).13 The efficiency of iron(III) perchlorate monohydrate
allowed the reaction to tolerate a variety of secondary and primary
alcohols in moderate to excellent yields (9aa―9ae, 42-89%).
Noticeably, the 1-phenylpropan-1-ol afforded the effective long-
acting oral anticoagulant drug phenprocoumon 9aa in 75% yield.
Scheme 2. Synthesis of the densely functionalized furans 4 and 5.
To emphasize the generality of our strategy, we next
investigated the feasibility of utilizing internal propargylic alcohols
as substrates in this iron-catalyzed propargylation-alkyne
oxacyclization-isomerization strategy. The reaction of 4-
hydroxycoumarin 1a with 1,3-diphenylprop-2-yn-1-ol 6a was
performed in MeNO2 at 80 oC in the presence of Fe(ClO4)3·H 2O.
Surprisingly, the novel functionalized 2,4-diphenyl-4H,5H-
pyrano[3,2-c]coumarin 7aa was obtained in 75% yield with high
regioselectivity (Table 3), instead of the corresponding furo[3,2-
c]coumarin complex. We next examined the generality of this
domino catalytic process. Remarkably, the process not only
allowed for diverse structural variations of 4-hydroxycoumarins 1,
but also tolerated a variety of the internal secondary propargylic
alcohols. Fe(ClO4)3·H 2O smoothly promoted this domino reaction
to afford diverse pyrano[3,2-c]coumarins with moderate to good
yields (40―75%). This methodology accommodated a large
number of 4-hydroxycoumarin substrates bearing neutral
functional groups (7oa, 55%), electron–withdrawing groups
(7ba―7da, 43―54%) and electron–donating groups (7ga―7la,
49―70%) on the benzene ring. Moreover, versatile 1,3-
Table 4. Substrate scope[a]
.
[a] Reaction conditions: 4-hydroxycoumarin 1 (0.20 mmol), alcohol 8 (0.30
mmol), Fe(ClO4)3·H2O (10 mol%), MeNO2 (2.0 ml), 80 °C. [b] Isolated yield.
diphenylprop-2-yn-1-ols
withdrawing groups and electron–donating groups afforded
moderate to good yields (45―63%). Noticeably, when
6 incorporated with both electron–
Therefore, we postulate this one-pot domino process
involves the initial propargylic substitution of the 4-
hydroxycoumarins 1 by the secondary alkynols 2, affording the
resulting γ-ketoalkynes 9 in the presence of the iron(III) Lewis acid
(Scheme 3).8n, 9a-9c The subsequent π-coordination and 5-exo dig
cyclization occurs through the intramolecular regioselective
nucleophilic attack of the 4-hydroxy group at the β-carbon position
of the coordinated alkyne moiety with terminal propargylic
alcohols as substrates. Eventually, the protonolysis of the alkenyl-
a
heterocyclic thiophene or a cyclopentane moiety was introduced
to the internal secondary alkynol backbone, the reaction still
provided a moderate yield (40% and 45%, respectively). The
molecular structures of 3aa and 7ab were unequivocally
confirmed by means of single-crystal X-ray analysis.12
Table 3. Substrate scope[a]
.
iron intermediates
B
constructs the 2-methylene-2,3-
dihydrofuro[3,2-c]coumarins C along with the regeneration of the
iron salt. Additionally, these key intermediates C could be
aromatized to the thermodynamically stable furo[3,2-c]coumarins
3 in the presence of the iron(III) salts. Remarkably, when the
protocol is performed with internal alkynols, the 6-endo dig
cyclization is preferred to generate the pyrano[3,2-c]coumarins 7
instead of the corresponding furans, probably due to the
conjugation effect of the aryl group attached to the alkyne moiety.
Conclusions
In summary, we have developed an iron-catalyzed domino
propargylation-alkyne oxacyclization-isomerization reaction of
readily available 4-hydroxycoumarins to secondary propargylic
alcohols. The densely functionalized furo[3,2-c]coumarins and
pyrano[3,2-c]coumarins were generated in a one-pot and elegant
manner in moderate to high yields (40―87%) with high
regioselectivities. The different outcome of this catalytic protocol
is ascribed to the terminal group attached to the alkyne moiety.
Further application of this iron(III) catalytic system to novel
reactions and elaboration of the final products are now ongoing in
our laboratory.
[a] Reaction conditions: 4-hydroxycoumarin 1 (0.30 mmol), 6 (0.20 mmol),
Fe(ClO4)3·H2O (10 mol%), MeNO2 (2.0 ml), 80 °C. [b] Isolated yield.
3
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10.1002/ejoc.201700999
European Journal of Organic Chemistry
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Scheme 3. A plausible reaction pathway.
Experimental Section
General Procedure for the Synthesis of Furo[3,2-c]coumarins: To a
solution of 4-hydroxy-2H-chromen-2-one 1a (48.6 mg, 0.30 mmol, 1.5
equiv) and 1-phenylprop-2-yn-1-ol 2a (26.4 mg, 0.20 mmol, 1.0 equiv) in
2.0 mL nitromethane, iron(III) perchlorate hydrate (7.4 mg, 0.02 mmol, 0.1
equiv) as a catalyst was added. The reaction mixture was stirred at 80 oC
for 5 h. The crude product was purified by column chromatography on
silica gel, eluted by hexane/EtOAc=15:1 then 10:1 to afford 47.6 mg (86 %
yield) of the desired product 3aa as white solid.
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General Procedure for the Synthesis of Pyrano[3,2-c]coumarins: To
a solution of 4-hydroxy-2H-chromen-2-one 1a (48.6 mg, 0.30 mmol, 1.5
equiv) and 1,3-diphenylprop-2-yn-1-ol 6a (41.7 mg, 0.20 mmol, 1.0 equiv)
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0.1 equiv) as a catalyst was added. The reaction mixture was stirred at 80
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yield) of the desired product 7aa as white solid.
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Acknowledgements
We are grateful for financial support from the National Natural
Science Foundation of China (21602179, and 81602977) and the
Opening fund of Hubei Key Laboratory of Bioinorganic Chemistry
& Materia Medica (BCMM201702).
[11] K. Lee, J. Kim, KNU-Industry Cooperation Foundation, KR 1505097,
2015.
Keywords: iron catalysis • regioselectivity • domino reaction •
furo[3,2-c]coumarin synthesis • pyrano[3,2-c]coumarin sythesis
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4
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Entry for the Table of Contents
A highly efficient iron-catalyzed propargylation-alkyne oxacyclization-isomerization strategy is disclosed to expeditiously assemble
representative bioactive furo[3,2-c]coumarins and pyrano[3,2-c]coumarins with high regioselectivities.
5
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