DABCO-promoted efficient and convenient synthesis of benzofurans

Article abstract of DOI:10.1080/00397911.2010.542862

An efficient and convenient synthesis of benzofurans has been described from phenacyl halides and o-hydroxy benzaldehyde in the presence of DABCO. The procedure is applicable for a variety of phenacyl halides and provide a variety of benzofurans. DABCO ac

Full text of DOI:10.1080/00397911.2010.542862

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DABCO-Promoted Efficient and
Convenient Synthesis of Benzofurans
H. M. Meshram a , B. Chennakesava Reddy a , B. R. V. Prasad a , P.
Ramesh Goud a , G. Santosh Kumar a & R. Naveen Kumar a
a Organic Chemistry Division I, Indian Institute of Chemical
Technology, Hyderabad, India
Accepted author version posted online: 17 Nov 2011.Version of
record first published: 03 Feb 2012.
To cite this article: H. M. Meshram, B. Chennakesava Reddy, B. R. V. Prasad, P. Ramesh Goud, G.
Santosh Kumar & R. Naveen Kumar (2012): DABCO-Promoted Efficient and Convenient Synthesis
of Benzofurans, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 42:11, 1669-1676
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Synthetic Communications1, 42: 1669–1676, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.542862
DABCO-PROMOTED EFFICIENT AND CONVENIENT
SYNTHESIS OF BENZOFURANS
H. M. Meshram, B. Chennakesava Reddy, B. R. V. Prasad,
P. Ramesh Goud, G. Santosh Kumar, and R. Naveen Kumar
Organic Chemistry Division I, Indian Institute of Chemical Technology,
Hyderabad, India
GRAPHICAL ABSTRACT
Abstract An efficient and convenient synthesis of benzofurans has been described from phe-
nacyl halides and o-hydroxy benzaldehyde in the presence of DABCO. The procedure is
applicable for a variety of phenacyl halides and provide a variety of benzofurans. DABCO
act as a base and as well as nucleophile.
Keywords DABCO; phenacyl halide; salicylaldehyde
INTRODUCTION
Many biologically active natural products
[1]
contain a benzofuran ring as a
core unit. Some of the benzofuran derivatives[2] find applications as pharmaceuti-
cals,[3] antifungal agents,[4] antitumor agents,[5] antioxidants, and as antagonists of
angoitension II.[6] In addition to this, a few benzofuran derivatives are used in cos-
metic formulations[7] and act as brightening agents.[8] Because of the biological
importance of benzofurans, there is continuous interest in the development of con-
venient and efficient synthesis of benzofurans. There are various approaches[9] for
the construction of benzofuran ring from different starting materials, but these meth-
ods are not popular because starting material is not readily available and use of Pd
catalyst[10] is expensive. Most commonly, the Rap–Stoermer reaction[11] has been
employed for the synthesis of benzofuran, which involves the cyclization of phenacyl
bromide with o-hydroxy benzaldehyde in the presence of a base. This strategy is
widely accepted because of the easy availability of starting material, which may
Received October 11, 2010.
Address correspondence to H. M. Meshram, Organic Division I, Indian Institute of Chemical
Technology, Hyderabad 500 007, India. E-mail: hmmeshram@yahoo.com
1669

1670
H. M. MESHRAM ET AL.
Scheme 1. DABCO-mediated efficient and convenient synthesis of benzofurans.
generate a large number of benzofuran derivatives. However, the reported methods
have some disadvantages such as use of corrosive strong bases,[12] which are detri-
mental to other functionalities such as ester and nitrile. In addition to this, some
of the methods require expensive palladium catalyst, high boiling solvents (dimethyl-
formamide, DMF),[13] and longer reaction time (24ꢀ72 h).[14] In view of this, there is
a need to develop efficient, convenient, and safe general method for the synthesis of
benzofurans that avoids the use of harmful solvents.
1,4-Diazabicyclo[2,2,2]octane (DABCO) has been employed as
a
organic-hindered base for various organic reactions such as deprotection of pep-
tides,[15] as a catalyst for the Baylis–Hillman reaction,[16] isoxazole preparation[17]
o-alkylation of phenols,[18] and deprotection of benzylic trimethylsilyl ethers.[19] To
the best of our knowledge, there is no report on the synthesis of benzofuran using
organic bases. In continuation of our research in the development of nonmetallic
reagents,[20] herein we report a simple, efficient, and general method for the prep-
aration of substituted benzofurans by the reaction of salicylaldehyde with phenacyl
bromide (Scheme 1).
Scheme 2. Plausible mechanism for the preparation of benzofuran.

EFFICIENT SYNTHESIS OF BENZOFURANS
1671
Table 1. Synthesis of benzofuran compounds from o-hydroxy salicylic acid and phenacyl halides
aAll the exhibited physical and spectral (NMR, mass, IR) properties in accordance with the assigned
structure.
bIsolated yield.

1672
H. M. MESHRAM ET AL.
Initially we have carried out the reaction of salicylaldehyde (1 equi.) and
a-bromo acetophenones (1 equi.) in the presence of 10 mol% of DABCO at room
temperature and the expected product benzofuran was isolated in 60% yield
(15 min). To improve the yield, the reaction was performed with increased DABCO
(15, 20 mol%). It was noted that 20 mol% DABCO was optimal. Decrease in (10
mol%) or increase (25 mol%) did not improve the yield. Next, the reaction was stud-
ied in different solvents such as benzene, toluene, dichloromethane, tetrahydrofuran
THF and acetonitrile. THF was the solvent of choice in terms of reaction time and
yield. We presume that initially phenacyl bromide (2) reacts with DABCO (1) and
forms the quaternary salt 3.[21] Later, it reacts with salicylaldehyde. Subsequent
cyclization dehydration gives the expected product (7). DABCO act as a base as well
as nucleophile[22] (Scheme 2).
Next we investigated various substituted salicylaldihyde and phenacyl bromide
in the optimized condition. It was observed that salicylaldehyde with electron-with-
drawing substituents (entries 1ꢀ20) reacted effectively with phenacyl bromide and
led to good yields of benzofuran, whereas the reaction of benzaldehyde bearing elec-
tron-donating substituents (entries 21–50) gave relatively poor yields. The present
protocol was very efficient and applicable for a variety of phenacyl halides. The
importance of the present procedure is that phenacyl chloride also reacts efficiently
and results in corresponding benzofurans. This method may be applicable for the
preparation of large number of benzofurans because of the easy availability of phe-
nacyl chloride and salicyladehyde. Some of the reported methods require corrosive
base, higher temperature and longer reaction time. However, the present procedure
using DABCO was very efficient at room temperature and resulted in good yield.
From Table 1, it can be observed that substituted phenacyl halides such as bromo
and chloro also react smoothly to give benzofuran in good yield.
In conclusion, we have developed an efficient, convenientm and mild protocol
for the synthesis of functionalized benzofurans using DABCO. The method is appli-
cable for a variety of functionalized aldehyde as well as phenacyl halides. Moreover,
the use of organic base, operational simplicity, and efficient reaction are distinct
advantages of the protocol.
EXPERIMENTAL
All reactions were carried out without any special precautions in an atmos-
phere of air. Chemicals were purchased from Fluka and S. D. Fine Chemicals.
thin-layer chromatography (TLC) used precoated silica-gel plates (60 F254, 0.2 mm
layer; E. Merk 1HNMR spectra were measured on Varian 200 or Bruker 300 spectro-
meters in CDCl3; d in ppm, J in hertz. Mass spectra were determined on VG Auto-
spec; in m=z.
General Procedure
A mixture of salicylaldehyde (1 equi.), phenacyl halide (1 equi.), and DABCO
(20 mol% in 2 ml of tetrahydrofuran, THF) was stirred at room temperature for the
stipulated time period (see Table 1). After completion of the reaction as indicated by
TLC, it was diluted with water and extracted with ethyl acetate. The combined

EFFICIENT SYNTHESIS OF BENZOFURANS
1673
organic layer was washed with water and dried over Na2SO4. Removal of solvent
under reduced pressure gave crude products, which were purified through a column
(hexane–EtoAc) to get the pure product. All the products were characterized by
infrared (IR), mass, and NMR spectroscopy.
Characteristic Data of New Compounds
1
Entries (21 and 22). Solid; mp 152–154 ꢁC; H NMR (300 MHz, CDCl3): d
(ppm) 1.56 (t, 3H, J ¼ 6.7 Hz), 4.30 (q, 2H, J ¼ 6.7 Hz), 7.11ꢀ7.45 (m, 4H), 7.66ꢀ
7.69 (m, 5H); FABMS: m=z 267 (Mþ). IR (KBR) n ¼ 3022, 2999, 1670, 1512,
1388, 1040, 817, 715 cmꢀ1. HRMS calculated for C17H14O3: 267.0945; gound:
267.0927. Anal. calcd. for C17H14O3: C, 76.7; H, 5.29; O, 18.02. Found: C, 76.83;
H, 5.38, O; 18.01.
1
Entries (23 and 24). Solid; mp 168ꢀ174 ꢁC; H NMR (300 MHz, CDCl3): d
(ppm) 1.55 (t, 3H, J ¼ 6.7 Hz), 4.31 (q, 2H, J ¼ 6.7 Hz), 6.90 (d, 1H, J ¼ 7.5 Hz), 7.20
(t, 1H, J ¼ 7.5 Hz), 7.31 (d, 1H, J ¼ 3.0 Hz), 7.50 (d, 2H, J ¼ 8.3 Hz), 7.62 (s, 1H),
8.22 (d, 2H, J ¼ 8.3 Hz); FABMS: m=z 301 (Mþ þ 1), 300, 284, 279. IR (KBR)
n ¼ 3020, 2998, 1672, 1500, 1380, 1040, 807, 725 cmꢀ1. HRMS calculated for
C17H13ClO3: 300.0553, found: 300.0327. Anal. calcd. for C17H13ClO3: C, 67.89; H,
4.36; O, 15.96. Found: C, 67.83; H, 4.38; O, 15.88.
1
Entries (25 and 26). Solid; mp 176-178 ꢁC; H NMR (300 MHz, CDCl3): d
(ppm) 1.52 (t, 3H, J ¼ 6.7 Hz), 4.22 (q, 2H, J ¼ 6.7 Hz), 6.99 (1H, d, J ¼ 7.1 Hz),
7.12 (t, 1H, J ¼ 7.1 Hz), 7.32ꢀ7.70 (m, 4H), 8.1 (d, 2H, J ¼ 8.7 Hz); FABMS: m=z
346, 344 (Mþ). IR (KBR) n ¼ 3012, 2989, 1679, 1380, 1044, 819, 735 cmꢀ1. HRMS
calculated for C17H13BrO3: 344.0048; found: 344.0042. Anal. calcd. for C17H13BrO3:
C, 59.15; H, 3.80; O, 13.90. Found: C, 59.13; H, 3.78; O, 13.77.
1
Entries (27 and 28). Solid; mp 192–194 ꢁC; H NMR (300 MHz, CDCl3): d
(ppm) 1.56 (t, 3H, J ¼ 6.7 Hz), 3.78 (s, 3H), 4.30 (q, 2H, J ¼ 6.7 Hz), 7.00 (m, 3H),
7.2 (s, 1H), 7.77–8.10 (m, 4H); FABMS: m=z 319 (MþþNa). IR (KBR) n ¼ 3010,
1677, 1520, 1368, 1060, 827, 725 cmꢀ1. HRMS calculated for C18H16O4Na:
319.0946; found: 319.0732. Anal. calcd. for C18H16O4: C, 72.96; H, 5.44; O, 21.60.
Found: C, 72.93; H, 5.43; O, 21.55.
1
Entries (29 and 30). Solid; mp 186–188 ꢁC ; H NMR (300 MHz, CDCl3): d
(ppm) 1.55 (t, 3H, J ¼ 6.7 Hz), 2.31 (s, 3H), 4.31 (q, 2H, J ¼ 6.7 Hz), 6.90 (d, 1H,
J ¼ 7.5 Hz), 7.20 (t, 1H, J ¼ 7.5 Hz), 7.11–7.75 (m, 6H); FABMS: m=z 281
(Mþ þ 1). IR (KBR) n ¼ 3022, 2999, 1667, 1377, 1050, 817, 733 cmꢀ1. HRMS calcu-
lated for C18H16O3: 280.1099; found: 280.1097. Anal. calcd. for C18H16O3: C, 77.12;
H, 5.75; O, 17.12. Found: C, 77.10; H, 5.68; O, 17.11.
1
Entries (35 and 36). Solid; mp 176–178 ꢁC; H NMR (300 MHz, CDCl3): d
(ppm) 3.81 (s, 3H), 7.15 (d, 1H, J ¼ 3.14 Hz), 7.28 (d, 2H, J ¼ 7.6 Hz), 7.90–8.11
(m, 5H); FABMS: m=z 352 (MþþNa). IR (KBR) n ¼ 3000, 2977, 1667, 1522,
1378, 1020, 827 cmꢀ1. HRMS calculated for C16H11BrO3Na: 352.9789; found:
352.9777. Anal. calcd. for C16H11BrO3: C, 58.03; H, 3.35; O, 14.49. Found: C,
58.11; H, 3.32; O, 14.41.

1674
H. M. MESHRAM ET AL.
1
Entries (39 and 40). Solid; mp 155–157 ꢁC; H NMR (300 MHz, CDCl3): d
(ppm) 2.30 (s, 3H), 3.81 (s, 3H), 7.15 (s, 1H), 7.32 (d, 2H, J ¼ 7.6 Hz), 7.70 (s,
1H), 7.88–8.01 (m, 4H); FABMS: m=z 267 (Mþ þ 1). IR (KBR) n ¼ 3022, 1667,
1532, 1049 cmꢀ1. HRMS calculated for C17H14O3: 266.0942; found: 266.0927.
Anal. calcd. for C17H14O3: C, 76.68; H, 5.30; O, 18.02. Found: C, 76.60; H, 5.28;
O, 18.11.
1
Entries (43 and 44). Solid; mp 222–224 ꢁC; H NMR (300 MHz, CDCl3): d
(ppm) 7.11 (s, 1H), 7.27 (d, 1H, J ¼ 7.7 Hz), 7.41 (d, 1H, J ¼ 8.1 Hz), 7.66 (s, 1H),
8.1 (d, 2H, J ¼ 8.6 Hz), 8.21 (d, 2H, J ¼ 8.6 Hz); FABMS: m=z 295 (MþþNa). IR
(KBR) n ¼ 3002, 2979, 1690, 1502, 1050, 715 cmꢀ1. HRMS calculated for
C15H9ClO3Na: 295.01378; found: 295.01372. Anal. Calcd. for C15H9ClO3: C,
66.07; H, 3.33; O, 17.60. Found: C, 66.10; H, 3.38; O, 17.61.
1
Entries (45 and 46). Solid; mp 212–214 ꢁC; H NMR (300 MHz, CDCl3): d
(ppm) 7.05–7.40 (m, 3H), 7.76 (s, 1H), 7.9–8.0 (m, 4H); FABMS: m=z 318, 316
(Mþ). IR (KBR) n ¼ 3012, 2999, 1677, 1512, 1488, 1041, 715 cmꢀ1. HRMS calcu-
lated for C15H9BrO3: 315.9735; found: 315.9722. Anal. calcd. for C15H9BrO3: C,
56.81; H, 2.86; O, 15.13. Found: C, 56.80; H, 2.88; O, 15.11.
1
Entries (47 and 48). Solid; mp 201–203 ꢁC; H NMR (300 MHz, CDCl3): d
(ppm) 3.80 (s, 3H), 6.95 (m, 2H), 7.41 (s, 1H), 7.55 (d, 1H, J ¼ 6.7 Hz), 7.77 (d,
1H, J ¼ 7.9 Hz), 7.97 (s, 1H), 7.99 (m, 2H); FABMS: m=z 269 (Mþ þ 1). IR
(KBR) n ¼ 3020, 1667, 1522, 1388, 1034 cmꢀ1. HRMS calculated for C16H12O4:
268.0735; found: 268.0732. Anal. calcd. for C16H12O4: C, 71.64; H, 4.51; O, 23.86.
Found: C, 71.60; H, 4.48; O, 23.81.
1
Entries (49 and 50). Solid; mp 212–214 ꢁC; H NMR (300 MHz, CDCl3): d
(ppm) 2.42 (s, 3H), 7.20–7.41 (m, 3H), 7.66 (s, 1H), 7.81–8.12 (m, 4H); FABMS:
m=z 253 (Mþ þ 1). IR (KBR) n ¼ 3002, 2999, 1660, 1378, 1040, cmꢀ1. HRMS calcu-
lated for C16H12O3: 252.0786, Found: 252.0778. Anal. Calcd. for C16H12O3: C,
76.18; H, 4.79; O, 19.03. Found: C, 76.10; H, 4.78; O, 19.01.
ACKNOWLEDGMENTS
B. C. K. R., B. R. V. P., and P. R. G. thank the Council of Scientific and Indus-
trial Research and University Grants Commission, New Delhi, for the award of fel-
lowships and J. S. Yadav, director, Indian Institute for Chemical Technology, for his
support and encouragement.
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