One-pot Three-component Synthesis of Furan-based Heterocycles from Benzyl Halides

Article abstract of DOI:10.1002/jhet.2806

An efficient and a novel approach for the synthesis of furochromene and furopyran scaffolds are described. Benzyl halides undergo oxidation in Kornblum conditions to give the corresponding aldehydes, which in turn undergo [4?+?1] cycloaddition reaction with isocyanide to afford the corresponding furan derivatives. The significant attraction of this protocol is simple procedure, mild reaction condition, and good yield.

Full text of DOI:10.1002/jhet.2806

Month 2017
One-pot Three-component Synthesis of Furan-based Heterocycles from
Benzyl Halides
*
Mallappa Beerappa and Kalegowda Shivashankar
PG Department of Chemistry, Central College Campus, Bangalore University, Bangalore 560 001, Karnataka, India
*
E-mail: shivashankark@gmail.com
Additional Supporting Information may be found in the online version of this article.
Received June 9, 2016
DOI 10.1002/jhet.2806
Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com).
An efficient and a novel approach for the synthesis of furochromene and furopyran scaffolds are described.
Benzyl halides undergo oxidation in Kornblum conditions to give the corresponding aldehydes, which in turn
undergo [4 + 1] cycloaddition reaction with isocyanide to afford the corresponding furan derivatives. The
significant attraction of this protocol is simple procedure, mild reaction condition, and good yield.
J. Heterocyclic Chem., 00, 00 (2017).
INTRODUCTION
only destroys the active center of aldehydes but also
interferes with reaction [27,28]. Therefore, despite a
number of precedents, an efficient, ecofriendly, practical,
and facile method for these transformations is still desired.
As part of an ongoing development of efficient protocol
for the preparation of furan derivatives and in continuation
of our work on MCRs [29–33], herein, we report a new and
straightforward method for the synthesis of furan
derivatives which involved an in situ oxidation of benzyl
halides into corresponding benzaldehydes under mild
Multicomponent reactions (MCRs) are very important
for the synthesis of various heterocyclic scaffolds in
organic chemistry. Multicomponent reactions with
isocyanide are particularly the most significant in this area
[1]. Furochromene and furopyran derivatives are certainly
one of the most important core structures in nature. Furans
were found in medicinal applications because they can
bind with various receptors [2]. Furan analogs are well-
known structural components in natural products such as
erysenegalensein J [3], isocoumestan [4], mycorrhizin A
[5], coniofurol A [6], plicadin [7], phellifuropyranone A
[8], and neotanshinlactone [9] (Fig. 1).
Kornblum condition. Then, they undergo
a three-
component reaction with α-hydroxy C─H acids and tert-
butyl isocyanide without adding any catalyst. Although
DMSO-potassium carbonate (K2CO3) is
a familiar
Many furan derivatives exhibited useful biological
activities such as anticancer [10], antimicrobial [11],
antineoplastic [12], and antidiabetic [13]. Because of
these properties, furan derivatives have attracted much
attention in synthetic organic chemistry. In recent years,
few synthetic procedures for synthesis of furans have
been well documented in the literature [14–21]. Some of
the protocols have been developed for the synthesis of
furan with the use of various catalysts such as I2 [22], Pd0
[23], and many more [24–26]. Although several synthetic
methodologies have been reported, these methods suffer
from 1 or more disadvantages such as the use of costly and
less easily available reagents, harsh reaction conditions,
and low yields. Aldehydes are the most common substrates
in many syntheses. They are not only toxic but also
unstable particularly when exposed to air and lead to
polymerization. Therefore, the presence of impurities not
oxidizing agent for the synthesis of many heterocyclic
compounds such as benzothiazoles [34], quinazolinones
[35], oxadiazoles [36], and imidazo[1,2-a]pyridines [37],
the broad scope and synthetic application of this reagent
for the construction of furan analogs in MCR have not
been reported. This protocol constructed the well-known
reaction but is not restricted to aldehyde substrate.
RESULT AND DISCUSSION
First, the reaction of 2,6-dichlorobenzyl bromide 1a,
4-hydroxycoumarin 2a, and tert-butyl isocyanide 3 was
selected as the model reaction for the optimization of the
reaction condition (Scheme 1). It is known that under
mild Kornblum condition [38], benzyl halide oxidized
into benzaldehyde within 2 hours as indicated by TLC
© 2017 Wiley Periodicals, Inc.

M. Beerappa and K. Shivashankar
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Figure 1. Natural products containing furan component. [Color figure can be viewed at wileyonlinelibrary.com]
monitoring. Subsequently, the in situ produced
benzaldehyde condensed with 4-hydroxycoumarin,
followed by cycloaddition reaction with tert-butyl
isocyanide at 90°C for 1 hour to afford the corresponding
product 4a in good yield. A preliminary examination
showed that DMSO-K2CO3 effectively oxidized and
promoted cyclocondensation in the model reaction.
Then, a range of different temperatures were examined
to improve the yield, and 90°C was found to afford the
best yield of compound 4a. Time taken to achieve
complete conversions and the isolated yields are recorded
in Table 1. Proton nuclear magnetic resonance and ESI-
MS analysis of 4a clearly indicated the formation of
furan derivative. All the products (4a-p) were
condensation under similar reaction condition to give the
corresponding furan analogs in excellent yields. Reactions
were also carried out by replacing 4-hydroxycoumarin
with 2-hydroxynaphthalene-1,4-dione (Table 1, entries 15
and 16). The reactions underwent smooth condensation to
give respective furan derivatives in high yields.
Under the optimized reaction condition, both electron
withdrawing and electron-donating groups were well
tolerated. In general, the overall yield ranged from 96%
for 2-(tert-butylamino)-3-(3-nitrophenyl)-4H-furo[3,2-c]
chromen-4-one (4f) (Table 1, entry 6) to 84% for 2-(tert-
butylamino)-3-(3-chlorophenyl)-6-methyl-4H-furo[3,2-c]
pyran-4-one (4n) (Table 1, entry 14).
The proposed mechanism (Scheme 2) is similar to the
well-known mechanism reported in the literatures [39–43].
The first step is the oxidation of benzyl halide into
benzaldehyde under Kornblum condition. In the
second step, K2CO3 abstracted the acidic proton of
4-hydroxycoumarin to generate anion. In the third step, the
anion of 4-hydroxycoumarin reacted with benzaldehyde,
followed by protonation and dehydration to afford the
Knoevenagel condensation adduct. In the last step, the
adduct underwent [4 + 1] cycloaddition reaction with
isocyanide to give imino lactone. The tautomerization of
imino lactone to furan derivative is drived by the stability
of the conjugated amino furan heteroatomic moiety.
1
characterized by melting point determination, H-NMR,
and 13C-NMR spectral data.
With the optimized conditions in hand, the generality and
scope of the present protocol were further investigated by
the replacement of 4-hydroxycoumarin (Table 1, entries
1-10) with 4-hydroxy-6-methyl-2H-pyran-2-one (Table 1,
entries 11-14). The reaction underwent successful
Scheme 1. Synthesis of 2-(tert-butylamino)-3-(2,6-dichlorophenyl)-4H-
furo[3,2-c]chromen-4-one (4a) under mild Kornblum condition. [Color
figure can be viewed at wileyonlinelibrary.com]
CONCLUSION
In summary, we have developed a novel method for
assembling furochromenes and furopyrans from benzyl
halides, α-hydroxy C─H acids, and tert-butyl
isocyanide in 1 pot under Kornblum condition. The use of
benzyl halides in place of benzaldehydes, mild reaction
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Synthesis of furan-based heterocycles from benzyl halides
Table 1
Synthesis of Furan Derivatives (4a-p).
Entry
Benzyl halides
α-Hydroxy C─H acids
Products
Time (h)
Yield (%)
1
3.0
92
2
3.4
2.5
3.0
86
3
94
4
92
(Continued)
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M. Beerappa and K. Shivashankar
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Table 1
(Continued)
Entry
Benzyl halides
α-Hydroxy C─H acids
Products
Time (h)
Yield (%)
5
3.0
90
96
92
6
7
8
2.5
3.0
3.2
90
(Continued)
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Synthesis of furan-based heterocycles from benzyl halides
Table 1
(Continued)
Entry
Benzyl halides
α-Hydroxy C─H acids
Products
Time (h)
Yield (%)
9
3.0
88
10
3.4
86
11
3.0
90
12
2.8
92
(Continued)
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M. Beerappa and K. Shivashankar
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Table 1
(Continued)
Entry
Benzyl halides
α-Hydroxy C─H acids
Products
Time (h)
Yield (%)
13
3.0
86
14
3.0
84
15
3.0
90
16
3.0
90
conditions, short reaction time, ease of workup, and good
yields are the noteworthy features of this protocol.
recorded on a Bruker spectrometer (Billerica, MA) in
CDCl3/DMSO-d6 by using TMS as an internal standard. The
chemical shift values are recorded on δ scale. The infrared
spectra were obtained by using Agilent Cary 630 FTIR
spectrophotometer (Santa Clara, CA). The mass spectrum
was recorded on Thermo LCQ Fleet (Arcade, NY). The
elemental analyses were carried on an Elemental Vario
Micro Cube rapid analyzer (Donaustr, Hanau, Germany).
EXPERIMENTAL
General information. The melting points were measured
by electric melting point equipment and are uncorrected.
Proton (400 MHz) and 13C (100 MHz) NMR spectra were
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Scheme 2. A plausible reaction mechanism for the formation of furan derivative. [Color figure can be viewed at wileyonlinelibrary.com]
Typical procedure for the synthesis of 2-(tert-butylamino)-3-
(2,6-dichlorophenyl)-4H-furo[3,2-c]chromen-4-one (4a).
to 7.81 (m, 7H, Ar-H) ppm; 13C-NMR (100 MHz,
CDCl3): δ = 30.4, 54.2, 98.7, 111.5, 113.0, 117.2,
119.9, 124.3, 128.2, 128.6, 129.3, 130.1, 136.8, 148.2,
151.2, 151.9, 154.9, and 157.4 ppm; ESI-MS: m/z
402.1 (M+); Anal. Calcd for C21H17Cl2NO3: C, 62.70;
A
mixture of 2,6-dichlorobenzyl bromide (0.29 g, 1.2 mmol)
and K2CO3 (0.37 g, 2.7 mmol) in DMSO (3 mL) was
taken in a pressure tube stirred for 2 hours at 90°C. The
resulting mixture was allowed to cool at room temperature.
4-Hydroxycoumarin (0.2 g, 1.2 mmol) and tert-butyl
isocyanide (0.1 g, 1.2 mmol) were added to the reaction
mixture, and stirring was continued at 90°C for 1 hour. The
progress of the reaction was monitored by TLC. After
completion of the reaction, the reaction mixture was cooled
at room temperature, and water (10 mL) was added.
Stirring was continued for 10 minutes at ambient
temperature. The resulting precipitate was filtered, washed
with water, dried, and recrystallized from a mixture of
H, 4.26; N, 3.48 found: C, 62.62; H, 4.20; N, 3.40%.
2-(tert-Butylamino)-3-(2,4,6-trimethoxyphenyl)-4H-furo[3,2-
c]chromen-4-one (4b). Yield 0.36 g (86%); yellow solid;
mp 186 to 188°C; IR (ATR, cmÀ1): 3320 (N─H), 1720
1
(C═O); H-NMR (400 MHz, CDCl3): δ = 1.26 (s, 9H,
tert-butyl), 3.78 to 3.86 (m, 10H, 3-OCH3 and ─NH),
6.24 to 7.79 (m, 6H, Ar-H) ppm; 13C-NMR (100 MHz,
CDCl3): δ = 30.2, 53.6, 55.4, 56.0, 91.3, 97.4, 100.7,
112.4, 113.5, 116.9, 119.9, 124.0, 128.0, 130.1, 151.3,
151.8, 155.1, 157.8, 158.9, and 161.4 ppm; Anal. Calcd
for C24H25NO6: C, 68.07; H, 5.95; N, 3.31 found: C,
diethyl ether and n-hexane (1:3) to obtain pure product.
2-(tert-Butylamino)-3-(2,6-dichlorophenyl)-4H-furo[3,2-c]
chromen-4-one (4a). Yield 0.36 g (92%); yellow solid;
68.01; H, 5.90; N, 3.30%.
3-(2-(tert-Butylamino)-4-oxo-4H-furo[3,2-c]chromen-3-yl)
benzonitrile (4c). Yield 0.33 g (94%); yellow solid; mp 140
mp 171 to 173°C; IR (ATR, cmÀ1): 3263 (N─H), 1724
to 142°C; IR (ATR, cmÀ1): 3340 (N─H), 2182 (CN), 1730
1
(C═O); H-NMR (400 MHz, CDCl3): δ = 1.34 (s, 9H,
1
(C═O); H-NMR (400 MHz, CDCl3): δ = 1.34 (s, 9H,
tert-butyl), 3.60 (s, 1H, ─NH, D2O exchangeable), 7.26
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M. Beerappa and K. Shivashankar
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tert-butyl), 3.6 (s, 1H, NH), 7.26 to 7.82 (m, 8H, Ar-H) ppm;
13C-NMR (100 MHz, CDCl3): δ = 30.3, 54.1, 97.0, 107.3,
113.6, 117.3, 120.6, 120.8, 122.7, 124.8, 128.8, 128.8,
130.9, 132.9, 153.0, 154.5, 156.0, 158.3, 158.5, and
161.0 ppm; Anal. Calcd for C22H18N2O3: C, 73.73; H,
5.06; N, 7.82 found: C, 73.70; H, 5.00; N, 7.78%.
130.0, 133.2, 136.8, 139.2, 150.6, 151.9, and
154.3 ppm; Anal. Calcd for C18H17Cl2NO3: C, 59.03;
H, 4.68; N, 3.82 found: C, 58.93; H, 4.52; N, 3.78%.
3-(2-(tert-Butylamino)-6-methyl-4-oxo-4H-furo[3,2-c]pyran-
3-yl)benzonitrile (4l). Yield 0.29 g (92%); yellow solid;
mp 146 to 148°C; IR (ATR, cmÀ1): 3313 (N─H), 2189
(CN), 1715 (C═O); 1H-NMR (400 MHz, CDCl3):
δ = 1.36 (s, 9H, tert-butyl), 2.33 (s, 3H, ─CH3), 3.36 (s,
1H, NH), 5.96 (s, 1H, pyran), 7.20 to 7.78 (m, 4H, Ar-H)
ppm; 13C-NMR (100 MHz, CDCl3): δ = 20.2, 30.6, 54.6,
94.5, 98.4, 107.7, 113.7, 117.4, 121.81, 122.7, 124.8,
130.5, 132.9, 143.1, 153.7, 158.3, and 161.1 ppm; Anal.
Calcd for C19H18N2O3: C, 70.79; H, 5.63; N, 8.69 found:
2-(tert-Butylamino)-3-(2,6-dichlorophenyl)-7,8-dimethyl-4H-
furo[3,2-c]chromen-4-one (4d).
Yield 0.39 g (92%);
yellow solid; mp 161 to 163°C; IR (ATR, cmÀ1): 3280
(N─H), 1726 (C═O); 1H-NMR (400 MHz, CDCl3):
δ = 1.32 (s, 9H, tert-butyl), 2.35 (s, 6H, 2-CH3) 3.53 (s,
1H, ─NH), 7.19 to 7.53 (m, 5H, Ar-H) ppm; 13C-NMR
(100 MHz, CDCl3): δ = 19.5, 20.5, 30.5, 54.2, 104.6,
117.1, 117.7, 120.0, 124.2, 128.1, 128.3, 128.7, 128.8,
130.8, 132.6, 134.5, 143.4, 154.9, and 157.4 ppm; Anal.
Calcd for C23H21Cl2NO3: C, 64.19; H, 4.92; N, 3.25
C, 70.71; H, 5.58; N, 8.61%.
2-(tert-Butylamino)-3-(4-chlorophenyl)-6-methyl-4H-furo[3,2-c]
pyran-4-one (4m). Yield 0.28 g (86%); yellow solid; mp
115 to 117°C; IR (ATR, cmÀ1): 3324 (N─H), 1705
found: C, 64.10; H, 4.83; N, 3.20%.
1
(C═O); H-NMR (400 MHz, CDCl3): δ = 1.39 (s, 9H,
2-(tert-Butylamino)-3-(4-chlorophenyl)-4H-furo[3,2-c]chromen-
4-one (4e). Yield 0.33 g (90%); yellow solid; mp 132 to
tert-butyl), 2.52 (s, 3H, ─CH3), 3.51 (s, 1H, NH), 5.91 (s,
1H, pyran), 7.14 to 7.67 (m, 4H, Ar-H) ppm; 13C-NMR
(100 MHz, CDCl3): δ = 20.1, 30.7, 54.4, 94.5, 99.1,
118.4, 129.5, 130.7, 131.5, 131.8, 146.7, 150.2, 150.9, and
158.5 ppm; Anal. Calcd for C18H18ClNO3: C, 65.16; H,
1
134°C; IR (ATR, cmÀ1): 3320 (N─H), 1721 (C═O); H-
NMR (400 MHz, DMSO-d6): δ = 1.39 (s, 9H, tert-butyl),
4.09 (s, 1H, ─NH), 7.26 to 7.80 (m, 8H, Ar-H) ppm; 13C-
NMR (100 MHz, CDCl3): δ = 30.3, 54.9, 99.0, 108.1,
113.9, 117.2, 117.9, 118.2, 121.0, 122.2, 131.1, 142.2,
145.8, 149.6, 149.8, 152.4, and 157.4 ppm; Anal. Calcd
for C21H18ClNO3: C, 68.57; H, 4.93; N, 3.81 found: C,
68.41; H, 4.86; N, 3.68%.
2-(tert-Butylamino)-3-(3-chlorophenyl)-4H-furo[3,2-c]chromen-
4-one (4g). Yield 0.33 g (92%); yellow solid; mp 121 to
5.47; N, 4.22 found: C, 65.10; H, 5.38; N, 4.14%.
2-(tert-Butylamino)-3-(3-chlorophenyl)-6-methyl-4H-furo[3,2-c]
pyran-4-one (4n).
Yield 0.27 g (84%); yellow solid; mp
105 to 107°C; IR (ATR, cmÀ1): 3318 (N─H), 1712
1
(C═O); H-NMR (400 MHz, CDCl3): δ = 1. 34 (s, 9H,
tert-butyl), 2.38 (s, 3H, ─CH3), 3.60 (s, 1H, NH), 6.14 (s,
1H, pyran), 7.26 to 7.46 (m, 4H, Ar-H) ppm; 13C-NMR
(100 MHz, CDCl3): δ = 20.1, 30.3, 54.2, 95.2, 99.2,
108.8, 116.5, 119.8, 121.1, 132.3, 141.8, 146.5, 150.8,
158.1, and 159.3 ppm; Anal. Calcd for C18H18ClNO3: C,
65.16; H, 5.47; N, 4.22 found: C, 65.10; H, 5.32; N, 4.14%.
123°C; IR (ATR, cmÀ1): 3290 (N─H), 1722 (C═O);
1H-NMR (400 MHz, CDCl3): δ = 1.37 (s, 9H, tert-butyl),
3.68 (s, 1H, ÀNH), 7.26 to 7.74 (m, 8H, Ar-H) ppm;
13C-NMR (100 MHz, CDCl3): δ = 30.4, 54.2, 98.3, 109.9,
111.0, 117.0, 118.5, 120.7, 122.0, 129.8, 130.9, 131.5,
141.8, 142.8, 145.0, 150.9, 158.1, and 159.1 ppm; Anal.
Calcd for C21H18ClNO3: C, 68.57; H, 4.93; N, 3.81found:
Acknowledgments. We are thankful to the authorities of the
Council of Scientific and Industrial Research (CSIR), New
Delhi, India, for financial assistance (grant no. 02[0172]/13/
EMR-II. Date: October 21, 2013, May 05, 2014 and May 12, 2016).
C, 68.50; H, 4.73; N, 3.72%.
3-(4-Bromophenyl)-2-(tert-butylamino)-4H-furo[3,2-c]chromen-
4-one (4h). Yield 0.36 g (90%); yellow solid; mp 121 to
123°C; IR (ATR, cmÀ1): 3305 (N─H), 1719 (C═O); H-
1
NMR (400 MHz, CDCl3): δ = 1.38 (s, 9H, tert-butyl),
3.98 (s, 1H, ─NH), 7.26 to 7.79 (m, 8H, Ar-H) ppm; 13C-
NMR (100 MHz, CDCl3): δ = 30.1, 54.4, 99.4, 108.1,
114.0, 114.1, 117.2, 117.9, 121.1, 129.1, 132.1, 140.8,
140.9, 145.8, 149.7, 150.5, 152.5, and 161.0 ppm; Anal.
Calcd for C21H18BrNO3: C, 61.18; H, 4.40; N, 3.40 found:
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet