Preparation of Furo[3,2-c]coumarins from 3-Cinnamoyl-4-hydroxy-2H-chromen-2-ones and Acyl Chlorides: A Bu3P-Mediated C-Acylation/Cyclization Sequence

Article abstract of DOI:10.1002/anie.201502789

A Bu₃P-mediated cyclization reaction of 3-cinnamoyl-4-hydroxy-2H-chromen-2-ones though electrophilic addition of acyl chlorides towards the synthesis of highly functionalized furo[3,2-c]coumarins bearing a phosphorus ylide moiety is described. These unprecedented cyclization reaction proceeds under mild reaction conditions within short reaction times (1 min to 1 h), and can be further applied in the synthesis of alkenyl-substituted furo[3,2-c]coumarins by the treatment with carbonyl electrophiles under basic conditions.

Full text of DOI:10.1002/anie.201502789

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International Edition: DOI: 10.1002/anie.201502789
German Edition: DOI: 10.1002/ange.201502789
Heterocycles
Preparation of Furo[3,2-c]coumarins from 3-Cinnamoyl-4-hydroxy-2H-
chromen-2-ones and Acyl Chlorides: A Bu3P-Mediated C-Acylation/
Cyclization Sequence**
Chia-Jui Lee, Cheng-Che Tsai, Shao-Hao Hong, Geng-Hua Chang, Mei-Chun Yang,
Lennart Mçhlmann, and Wenwei Lin*
Abstract: A Bu3P-mediated cyclization reaction of 3-cinna-
moyl-4-hydroxy-2H-chromen-2-ones though electrophilic
addition of acyl chlorides towards the synthesis of highly
functionalized furo[3,2-c]coumarins bearing a phosphorus
ylide moiety is described. These unprecedented cyclization
reaction proceeds under mild reaction conditions within short
reaction times (1 min to 1 h), and can be further applied in the
synthesis of alkenyl-substituted furo[3,2-c]coumarins by the
treatment with carbonyl electrophiles under basic conditions.
pounds 4, whereas the treatment with carbonyl electrophiles
afforded the corresponding Wittig products 5.
A screening of the reaction conditions was pursued using
the coumarin derivative 1a and benzoyl chloride (2a) as
model substrates (Table 1). Initially, THF was used as the
solvent and Bu3P as the nucleophile. After 3.5 hours, the
reaction was quenched by saturated NaHCO3(aq), and the
desired product 4aa was obtained in 39% yield (NMR;
entry 1). When other solvents such as CH2Cl2 and toluene
were used, 4aa was provided in 33–35% yields (entries 2 and
3). To our delight, the reaction proceeded more effectively in
MeCN to afford 4aa in 59% yield (entry 4). After further
optimization of the reaction conditions (2.8 equiv of 2a and
3.4 equiv of Et3N), the adduct 4aa could be furnished in 82%
isolated yield within 1 hour (entries 4–6).[4] In addition,
different phosphine sources, such as iBu3P, Cy3P, MePh2P,
EtPh2P, and (OEt)3P, as well as DABCO, were examined
(entries 7–12). Presumably because of the lower nucleophi-
licity, only iBu3P could successfully promote the reaction to
provide 4aa, albeit with low yield (entry 7).[5] Therefore, the
G
iven their frequent appearance in drugs and natural
products, the efficient construction of highly functionalized
furans and furo[3,2-c]coumarins is a major challenge in
organic synthesis.[1] Tremendous work has been done within
this field of chemistry, and thus various transition-metal-
catalyzed cyclization reactions are well established.[2] As
cheap and metal-free alternatives, phosphine-triggered reac-
tions have recently attracted great interest.[3] Previously we
demonstrated that multifunctionalized furan derivatives were
afforded simply by treating Michael acceptors, such as a,b-
unsaturated carbonyl compounds, with tributylphosphine in
the presence of acyl chlorides using an intramolecular Wittig
reaction as the key step.[3d–g] Based on this discovery, we were
interested in the extension of this method to the application of
coumarin derivatives, such as 1, as Michael acceptors, which is
proposed to give access to 3-furyl-4-hydroxycoumarin deriv-
atives (I; Scheme 1).
Following our previously established protocol using 3-
cinnamoyl-4-hydroxy-2H-chromen-2-ones 1 and acyl chlor-
ides 2 as substrates in the presence of Et3N as base, the
expected adducts I could not be obtained (Scheme 1).
Instead, the phosphorus ylides 3 were determined to be the
major products. Remarkably, these ylide intermediates
showed good reactivity for undergoing additional transfor-
mations in a one-pot approach: addition of a saturated
NaHCO3 solution directly led to the dephosophorated com-
Scheme 1. Unexpected formation of the products 4 and 5 by treating
1 with Bu3P, acyl chlorides 2, and Et3N.
use of MeCN as solvent, 1.1 equivalents of Bu3P, 2.8 equiv-
alents of acyl chloride, and 3.4 equivalents of Et3N were
selected as the optimal reaction conditions.[6]
With the optimized reaction conditions in hand, the
substrate scope of this novel cyclization was investigated
(Table 2). A wide range of aryl-substituted acyl chlorides was
screened, and good to excellent results could be achieved
within short reaction times (entries 1–8). Heteroaryl-substi-
tuted (entries 9 and 10) as well as aliphatic acyl chlorides
(entries 11 and 12) were also employed successfully for this
[*] C.-J. Lee, C.-C. Tsai, S.-H. Hong, G.-H. Chang, M.-C. Yang,
Dr. L. Mçhlmann, Prof. Dr. W. Lin
Department of Chemistry, National Taiwan Normal University
88, Sec. 4, Tingchow Road, Taipei 11677 (Taiwan, R.O.C.)
E-mail: wenweilin@ntnu.edu.tw
[**] The authors thank the Ministry of Science and Technology of the
Republic of China (MOST 101-2113M-003-001-MY3) for financial
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502789.
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Table 1: Optimization of reaction conditions.[a]
Table 2: Synthesis of furo[3,2-c]coumarin derivatives 4 from 1, Bu3P, acyl
chlorides 2 and Et3N.[a]
Entry Solvent Nu
2a (equiv) Et3N (equiv) t [h]
Yield [%][b]
Entry
R1
R2
Yield [%][b]
1
2
3
4
5
6
7
8
THF
Bu3P
2.2
2.2
2.2
2.2
2.8
1.3
2.8
2.8
2.3
2.3
2.3
2.3
3.4
1.5
3.4
3.4
3.4
3.4
3.4
3.4
3.5
3.5
3.5
3.5
1
39
35
33
59
82[c]
26
1
2
3
4
5
6
7
8
Ph (1b)
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph (2a)
86 (4ba)[c]
83 (4bb)
77 (4bc)
79 (4bd)
94 (4be)
85 (4bf)
80 (4bg)
82 (4bh)
40 (4bi)
33 (4bj)
74 (4bk)
49[d] (4bl)
82 (4aa)
81 (4ab)
92 (4ac)
86 (4ad)
90 (4ae)
60 (4af)
83 (4ag)
52 (4cb)
82 (4 cd)
79 (4cg)
54 (4ck)
74[e] (4da)
toluene Bu3P
CH2Cl2 Bu3P
MeCN Bu3P
MeCN Bu3P
MeCN Bu3P
MeCN iBu3P
MeCN Cy3P
MeCN MePh2P 2.8
MeCN EtPh2P 2.8
MeCN (OEt)3P 2.8
MeCN DABCO 2.8
p-MeOC6H4 (2b)
p-NO2C6H4 (2c)
p-ClC6H4 (2d)
m-ClC6H4 (2e)
o-ClC6H4 (2 f)
p-BrC6H4 (2g)
o-BrC6H4 (2h)
2-thienyl (2i)
2-furyl (2j)
1
1 (12) 25 (31)
1 (12) 0 (0)
1 (12) 2 (3)
1 (12) 1 (2)
1 (12) 0 (0)
1 (12) 0 (0)
9
9
10
11
12
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Ph
Ph
Ph
Cy (2k)
iPr (2l)
Ph (2a)
[a] Reaction conditions: Step 1: 0.2 mmol of 1a and 1.1 equiv of the
nucleophile (Nu) were utilized in dry solvent (1 mL) under N2
atmosphere. Step 2: Saturated NaHCO3(aq) (3 mL) was added and stirred
for additional 30 min. [b] Yield determined by NMR analysis of the crude
reaction mixture using CHPh3 as an internal standard. Values within
parentheses represent the yield of the product after a reaction time of 12
hours. [c] Yield of isolated product. DABCO=1,4-iazabicyclo-
[2.2.2]octane, THF=tetrahydrofuran.
p-MeOC6H4 (1a)
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
p-MeOC6H4
p-MeC6H4 (1c)
p-MeC6H4
p-MeC6H4
p-MeC6H4
p-BrC6H4 (1d)
p-MeOC6H4 (2b)
p-NO2C6H4 (2c)
p-ClC6H4 (2d)
m-ClC6H4 (2e)
o-ClC6H4 (2 f)
p-BrC6H4 (2g)
p-MeOC6H4 (2b)
p-ClC6H4 (2d)
p-BrC6H4 (2g)
Cy (2k)
protocol, albeit in slightly reduced yields. Furthermore,
different coumarin derivatives (1a, 1c, and 1d) have been
examined with various acyl chlorides 2. Delightfully, under
the given reaction conditions, the corresponding products 4
were provided in moderate to excellent yields (entries 13–24).
Further transformation of the phosphorus ylide 3 was
realized through intermolecular Wittig reactions.[8] In the
presence of additional carbonyl electrophiles (6), the corre-
sponding adducts 5 and 5’ could be furnished in one-pot
reactions (Table 3). Moderate to excellent yields of 5 and 5’
were obtained using ethyl glyoxylate (6a) and ninhydrin (6b),
respectively, as trapping reagents.
A control experiment was carried out to demonstrate the
existence of the phosphorus zwitterion A-d as the key
intermediate for the generation of 4da (Scheme 2). The
zwitterionic adduct A-d resulting from 1d and Bu3P was
afforded in 91% isolated yield, and characterized by X-ray
analysis.[7] It is worth mentioning that, compared to the work
of Kwon and co-workers, we also did not observe any
intramolecular electrostatic interaction in the zwitterion A-
d.[7,9] After treatment of A-d with benzoyl chloride (2a) in the
presence of Et3N, and subsequent quenching with saturated
NaHCO3(aq), the product 4da was furnished in 70% yield.
Ph (2a)
[a] Reaction conditions: Step 1: 1 (0.2 mmol), Bu3P (1.1 equiv), 2
(2.8 equiv), and Et3N (3.4 equiv) in dry MeCN (1 mL) were stirred for 1 h
under N2 atmosphere. Step 2: Saturated NaHCO3(aq) (3 mL) was added
and stirred for additional 30 min. [b] Yield of isolated product. [c] The
structure of 4ba was determined by X-ray analysis.[7] [d] Reaction time:
3 h. [e] Reaction time: 10 min.
As presented in Tables 1–3, 2.8 equivalents of acyl chlo-
ride 2 was necessary to obtain reasonable yields of 4, although
only one acyl group is incorporated into the product.[4] To
understand the exact function of 2, additional investigations
were carried out (Scheme 3). The substrate 1b was treated
with 1.1 equivalents of Bu3P and 0.9 equivalents of benzoyl
chloride (2a) [Eq. (1)]. After 10 minutes, 1.8 equivalents of 4-
nitrobenzoyl chloride (2c) and Et3N (3.4 equiv) were added,
and the reaction mixture was stirred for another hour. By
1
quenching with NaHCO3(aq), H NMR analysis of the crude
reaction mixture revealed that only trace amounts of the
benzoyl-substituted product 4ba was formed. Instead, 4bc
could be identified as the major product. The other control
experiment with reversed addition sequence of 2a and
2cshowed comparable results with contrary product ratio
[Eq. (2)].
Based on these results, we propose a plausible reaction
mechanism (Scheme 4). First, the phosphorus zwitterion A is
formed by Michael addition of Bu3P to 1. The zwitterionic
intermediate A then undergoes acylation with 2 (RCOCl) to
provide the intermediate B. Under basic conditions, the
elimination reaction of B takes place, thus leading to the
formation of the corresponding allene intermediate C.[10]
Scheme 2. Confirmation of the phosphorus zwitterion A-d as the
reaction intermediate in the control experiment.
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Table 3: Synthesis of either 5 or 5’ by using 1, Bu3P, 2, Et3N, and 6.[a]
Entry R1
R2
t1 [min] t2 [h] Yield [%][b]
1
2
3
4
5
6
7
8
p-MeOC6H4 Ph
60
60
10
60
10
30
1
1
0.5
1
0.5
1
1.5
3
5
1
5
1
1
3
0.5
0.5
1
1
1
88 (3.7:1)[c] (5aa)
91 (3.5:1)[c] (5bd)
75 (3.2:1)[c] (5ea)
77 (3.6:1)[c] (5ca)
75 (3.0:1)[c] (5da)[d]
86 (5’aa)[d]
41 (5’ab)
78 (5’ag)
61 (5’bb)
84 (5’ba)
75 (5’bc)
90 (5’bd)
45 (5’bl)
88 (5’ca)
71 (5’da)
66 (5’ea)
48 (5’eb)
74 (5’ed)
34 (5’el)
Ph
p-ClC6H4
p-NO2C6H4 Ph
Scheme 3. Control experiments: elucidating the roles of acyl chlorides
2 in the reaction mechanism. Reaction conditions: Step 1: The reaction
of 1b (0.2 mmol) and all the indicated reagents were carried out in dry
MeCN (1 mL) at 308C under N2 atmosphere. Step 2: Saturated
NaHCO3(aq) (3 mL) was added and stirred for additional 30 min.
p-MeC6H4
p-BrC6H4
Ph
Ph
p-MeOC6H4 Ph
p-MeOC6H4 p-MeOC6H4 30
p-MeOC6H4 p-BrC6H4 30
p-MeOC6H4 30
Ph 30
p-NO2C6H4 30
9
Ph
Ph
10
11
12
13
14
15
16
17
18
19
Ph
Ph
Ph
p-ClC6H4
iPr
30
30
30
10
5
5
1
5
p-MeC6H4
p-BrC6H4
Ph
Ph
p-NO2C6H4 Ph
p-NO2C6H4 p-MeOC6H4
p-NO2C6H4 p-ClC6H4
p-NO2C6H4 iPr
[a] Reaction conditions: Step 1: 1 (0.2 mmol), Bu3P (1.1 equiv), 2
(2.8 equiv), and Et3N (3.0 equiv) were stirred in dry MeCN (1 mL) under
N2 atmosphere. Step 2: Et3N (1.0 equiv) and 6 (1.3 equiv) were added to
the reaction mixture. [b] Yield of the isolated product. [c] The isomeric
ratio of 5 (Z/E). [d] The structures of 5da and 5’aa were determined by X-
ray analysis.[7]
Because of their instability and high reactivity we were not
able to isolate B and C. However, their appearance could be
verified by in situ ESI-HRMS analysis of the reaction
mixture.[11] The intermediate C then undergoes C-acylation
with 2 (R2COCl), thus furnishing the corresponding phos-
phonium chloride D. Thereafter, the intramolecular cycliza-
tion of D proceeds to afford E, which further isomerizes to
generate the ylide precursor F.[12] The phosphonium chloride
F can be activated in the presence of Et3N to react with
a carbonyl electrophile (6), thus giving either the correspond-
ing adduct 5 or 5’.
Scheme 4. Proposed mechanism for the Bu3P-mediated cyclization
reaction.
With regard to the proposed reaction mechanism, we
anticipated that this protocol is not only limited to coumarin
derivatives but also could be extended to a more versatile
substrate scope. Hence, in our preliminary study, the acyclic
compound 7 was examined under the given reaction con-
ditions (Scheme 5). Without further optimization, moderate
yields of 8a and 8b could be achieved after 1 hour, thus
demonstrating the generality of this novel phosphine-trig-
gered reaction.
Scheme 5. Preliminary results for the synthesis of 8a,b by using our
one-pot approach.
In summary, we have developed a novel phosphine-
promoted C-acylation/cyclization reaction of 1, 2, and Bu3P in
the presence of Et3N to access to the phosphorus ylides 3
under very mild reaction conditions. The in situ generated
ylide intermediates 3 can efficiently react with carbonyl
thus furnishing either the corresponding furo[3,2-c]coumarin
derivatives 5, 5’, or 4, respectively, in moderate to excellent
yields. Furthermore, the substrate scope can be further
extended to the acyclic precursor 7 according to our
developed concept. A plausible reaction mechanism is also
proposed and supported by control experiments. Further
electrophiles 6 or be quenched with saturated NaHCO3(aq)
,
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5853; Angew. Chem. 2013, 125, 5965; d) X. Cui, X. Xu, L. Wojtas,
investigations on this chemistry are currently ongoing in our
laboratory.
M. M. Kim, X. P. Zhang, J. Am. Chem. Soc. 2012, 134, 19981; for
the recent example using Brønted acid as catalyst, see: e) J. S.
Clark, F. Romiti, K. F. Hogg, M. H. S. A. Hamid, S. C. Richter,
A. Boyer, J. C. Redman, L. J. Farrugia, Angew. Chem. Int. Ed.
2015, 54, 5744; Angew. Chem. 2015, 127, 5836.
Experimental Section
Typical procedure for the preparation of 4: A dry and nitrogen-
flushed 10 mL Schlenk flask, equipped with a magnetic stirring bar
and a septum, was charged with 1 (0.2 mmol), dry MeCN (1 mL),
Bu3P (1.1 equiv), acyl chloride 2 (2.8 equiv), and Et3N (3.4 equiv).
The reaction mixture was stirred for 1 h at 308C. Then the reaction
was quenched by sat. NaHCO3 solution (3 mL), stirred for an
additional 30 min, and extracted with EtOAc (3 ” 20 mL). Thereafter,
the solvent was removed by evaporation in vacuo. The crude reaction
mixture was purified by flash column chromatography to provide 4.
Typical procedure for the preparation of 5 or 5’: A dry and
nitrogen-flushed 10 mL Schlenk flask, equipped with a magnetic
stirring bar and a septum, was charged with 1 (0.2 mmol), dry MeCN
(1 mL), Bu3P (1.1 equiv), acyl chloride 2 (2.8 equiv), and Et3N
(3.4 equiv). The reaction mixture was stirred at 308C until 1 was fully
consumed. Then Et3N (1.2 equiv) and either ethyl glyoxalate (6a) or
ninhydrin (6b; 1.3 equiv) were added, and the resulting reaction
mixture was stirred until the reaction was completed. Thereafter, the
solvent was removed by evaporation in vacuo. The crude reaction
mixture was purified by flash column chromatography to provide
either 5 or 5’.
[3] For selected reviews, see: a) L.-W. Ye, J. Zhou, Y. Tang, Chem.
Soc. Rev. 2008, 37, 1140; b) S. Xu, Z. He, RSC Adv. 2013, 3,
16885; c) Y. C. Fan, O. Kwon, Chem. Commun. 2013, 49, 11588;
d) S. Xu, Y. Tang, Lett. Org. Chem. 2014, 11, 524; e) Z. Wang, X.
Xu, O. Kwon, Chem. Soc. Rev. 2014, 43, 2927; for synthesis of
pyrroles, see: f) Y. Lu, B. A. Arndtsen, Org. Lett. 2009, 11, 1369;
for synthesis of furans, see: g) U. Das, Y.-L. Tsai, W. Lin, Org.
Biomol. Chem. 2014, 12, 4044; h) K.-W. Chen, S.-e. Syu, Y.-J.
Jang, W. Lin, Org. Biomol. Chem. 2011, 9, 2098; i) T.-T. Kao, S.-e.
Syu, Y.-W. Jhang, W. Lin, Org. Lett. 2010, 12, 3066; j) C.-J. Lee,
Y.-J. Jang, Z.-Z. Wu, W. Lin, Org. Lett. 2012, 14, 1906; k) J.-C.
Deng, S.-C. Chuang, Org. Lett. 2014, 16, 5792; for other selected
examples, see: l) G. Cheng, Y. Hu, J. Org. Chem. 2008, 73, 4732;
m) C.-K. Jung, J.-C. Wang, M. J. Krische, J. Am. Chem. Soc. 2004,
126, 4118; n) S. Xu, L. Zhou, R. Ma, H. Song, Z. He, Chem. Eur.
J. 2009, 15, 8698.
[4] The anhydride byproduct resulting from benzoyl chloride (2a)
and benzoate was observed, which indicates the excess amount
of 2a is necessary.
[5] B. Kempf, H. Mayr, Chem. Eur. J. 2005, 11, 917.
[6] For a detailled overview of all screening experiments, please see
the Supporting Information.
[7] The structures of 4ba, 5da, 5’aa, 8b and A-d were confirmed by
X-ray analysis (CCDC 883501, 1034356, 1034255, 1034256 and
883382 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.)
Typical procedure for the preparation of 8: A dry and nitrogen-
flushed 10 mL Schlenk flask, equipped with a magnetic stirring bar
and a septum, was charged with 7 (0.2 mmol), dry MeCN (1 mL),
Bu3P (1.1 equiv), the acyl chloride 2 (2.8 equiv), Et3N (4.6 equiv), and
ninhydrin (6b; 2.2 equiv, dissolved in 2 mL MeCN). The reaction
mixture was stirred for 1 h at 308C. Thereafter, the solvent was
removed by evaporation in vacuo. The crude reaction mixture was
purified by flash column chromatography to provide 8.
[8] a) G. Wittig, G. Geissler, Justus Liebigs Ann. Chem. 1953, 580, 4;
for selected reviews, see: b) B. E. Maryanoff, A. B. Reitz, Chem.
Rev. 1989, 89, 863; c) P. A. Byrne, D. G. Gilheany, Chem. Soc.
Rev. 2013, 42, 6670.
Keywords: acylation · cyclizations · heterocycles · ylides ·
zwitterion
[9] X.-F. Zhu, C. E. Henry, O. Kwon, J. Am. Chem. Soc. 2007, 129,
6722.
How to cite: Angew. Chem. Int. Ed. 2015, 54, 8502–8505
Angew. Chem. 2015, 127, 8622–8625
[10] For the selected example of generation of electron-deficient
allenes, see: a) M. Yoshida, Y. Hidaka, Y. Nawata, J. M.
Rudzinski, E. Osawa, K. Kanematsu, J. Am. Chem. Soc. 1988,
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[1] For selected examples with furan moiety, see: a) A. Boto, L.
Alvarez in Heterocycles in Natural Product Synthesis (Eds.: K. C.
Majumdar, S. K. Chattopadhyay), Wiley-VCH, Weinheim, 2011;
b) J. B. Sperry, D. L. Wright, Curr. Opin. Drug Discov. Devel.
2011, 8, 723; for selected examples with furo[3,2-c]coumarin
moiety, see: c) X. H. Wang, K. F. Bastow, C. M. Sun, Y. L. Lin,
H. J. Yu, M. J. Don, T. S. Wu, S. Nakamura, K. H. Lee, J. Med.
Chem. 2004, 47, 5816; d) L. Santana, E. Uriarte, F. Roleira, N.
Milhazes, F. Borge, Curr. Med. Chem. 2004, 11, 3239.
[2] For selected examples, see: a) Y. Xia, Y. Xia, R. Ge, Z. Liu, Q.
Xiao, Y. Zhang, J. Wang, Angew. Chem. Int. Ed. 2014, 53, 3917;
Angew. Chem. 2014, 126, 3998; b) Y. Ma, S. Zhang, S. Yang, F.
Song, J. You, Angew. Chem. Int. Ed. 2014, 53, 7870; Angew.
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Calleja, L. A. López, R. Vicente, Angew. Chem. Int. Ed. 2013, 52,
[11] For the details of ESI-HRMS results, please see the Supporting
Information.
[12] The original phosphonium salt F with chloride as the counterion
is too unstable to be identified. We therefore quenched the
reaction by HBF4 to afford the more stable intermediate F (R1 =
À
p-MeOC6H4, R2 = Ph) with BF4 as the counterion, which was
confirmed by NMR and HRMS analysis. For the experimental
details, please see the Supporting Information.
Received: March 26, 2015
Published online: June 1, 2015
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