Bichalcophenes: A concise synthesis of formyl ester- and cyano ester-substituted bithiophenes, bifurans, and furanothiophenes

Article abstract of DOI:10.1002/jhet.295

(Chemical Equation Presented) Syntheses of new formyl ester- and cyano ester-substituted bithiophenes, bifurans, and furanothiophenes in good yield are described. The key synthetic step uses Stille coupling of appropriately substituted bromo 5-ring heterocycles with stannyl-substituted 5-ring heterocycles.

Full text of DOI:10.1002/jhet.295

January 2010
Bichalcophenes: A Concise Synthesis of Formyl Ester- and
Cyano Ester-Substituted Bithiophenes, Bifurans,
and Furanothiophenes
167
Abdelbasset A. Farahat,a,b Arvind Kumar,a Alaa El-Din M. Barghash,b
Fatma E. Goda,b Hassan M. Eisa,b and David W. Boykina*
aDepartment of Chemistry, Georgia State University, Atlanta, Georgia 30303
bDepartment of Pharmaceutical Organic Chemistry, Faculty of Pharmacy, University of Mansoura,
Mansoura 35516, Egypt
*E-mail: dboykin@gsu.edu
Received June 18, 2009
DOI 10.1002/jhet.295
Published online 8 January 2010 in Wiley InterScience (www.interscience.wiley.com).
Syntheses of new formyl ester- and cyano ester-substituted bithiophenes, bifurans, and furanothio-
phenes in good yield are described. The key synthetic step uses Stille coupling of appropriately substi-
tuted bromo 5-ring heterocycles with stannyl-substituted 5-ring heterocycles.
J. Heterocyclic Chem., 47, 167 (2010).
INTRODUCTION
RESULTS AND DISCUSSION
Compounds containing 2,20-bithiophene, 2,20-bifuran,
and 2,20-furanothiophene cores play important roles in
material sciences [1], pharmaceutical [2], and agrochem-
ical [3] fields. The various optical properties of these
bichalcophenes have lead to their extensive use in non-
linear optical devices [4], sensors [5], solar cells [6],
and other advanced materials. These bichalcophene units
appear in important medicinal chemical applications
including antibacterial [7] and anticancer studies [8].
Bichalcophene diamidines of the type I and II from our
laboratory have been shown to recognize G-quadruplex
DNA [9]. Formyl and ester bichalcophene analogs can
undergo a variety of condensation reactions to prepare
more complex molecules of importance to material sci-
ence as well as for the generation of useful bioactive
molecules. Cyano derivatives are key intermediates for
the preparation of amidine molecules, which are impor-
tant molecules in such diverse fields as medicinal chem-
istry [9] and catalyst development [10]. In view of our
interest in molecules that recognize G-quadruplex DNA
[9] and other ones, which can potentially function as
diagnostics [11] for parasites and as an expansion of our
previous bichalcophene work [12(a)]. We report here
the synthesis of new bichalcophenes with formyl, ester,
and cyano substituents that can serve as versatile build-
ing blocks for the preparation of more complex mole-
cules (Fig. 1).
Scheme 1 outlines our approach to the synthesis of both
the bichalcophene formyl and cyano esters. We chose to
use the Stille approach for the coupling to form the bichal-
cophene units because this methodology is known to be
quite robust and can be performed under neutral conditions
[1(b),2,12(b)]. For ease of preparation of the Stille reagent,
we chose to use the acetal-protected formyl furan 2a and
thiophene 2b. The commercially available aldehydes 1a, b
were converted into the previously unreported acetals 2a, b
in high yields (>95%) using NBS as a catalyst, under mild
conditions [13].The new tri-n-butylstannyl reagents were
prepared in good yields (>80%) by a conventional n-butyl-
lithium debromination of 2a, b at ꢀ78ꢁC followed by reac-
tion with tri-n-butyltin chloride. The Stille coupling reac-
tion between 3a, b and commercially available 4a, b in the
presence of a 5 mol % of Pd (PPh3)4 using 1,4-dioxane as
solvent at 100ꢁC for 24 h followed by deprotection of the
acetal product by stirring with conc. HCl solution for 6 h
gave the desired new formyl analogs 5a–d in good yields
(>73%). The formyl esters were converted into the cyano
esters (6a–d) using a conventional two-step process of con-
version of the aldehydes into the corresponding oximes fol-
lowed by acetic anhydride-facilitated dehydration to pro-
vide the new nitriles in good isolated yields (>75%).
In conclusion, we have reported a concise synthesis,
in good yields, of new cyano and formyl ester-substi-
tuted bichalcophenes, which can be used for the
C
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A. A. Farahat, A. Kumar, A. E.-D. M. Barghash, F. E. Goda, H. M. Eisa, and D. W. Boykin
Vol 47
slowly to a stirred solution of 2a, b (130 mmol) in 150 mL an-
hydrous THF under nitrogen atmosphere at –78ꢁC. After the
addition was completed, the mixture was stirred for 4 h and
then 40.6 mL tri-n-butyltin chloride (150 mmol) was added
slowly at –78ꢁC. After stirring overnight, the solution was
quenched with 50 mL water and solvents were removed under
reduced pressure. The residue was extracted with 200 mL
ether, the organic layer was washed with 30 mL 10% NaF so-
lution and 100 mL water, dried (sodium sulfate), and concen-
trated. The resulting oil was purified by distillation.
5-(Dimethoxymethyl)furan-2-yl-tri-n-butylstannane (3a). Yel-
low oil, yield 84%; bp 145ꢁC (0.6 mm Hg); 1H-NMR (400
MHz, CDCl3): d ¼ 6.53 (d, J ¼ 2.8 Hz, 1 H), 6.44 (d, J ¼ 2.8
Hz, 1 H), 5.50 (s, 1 H), 3.39 (s, 6 H). 1.64 (m, 6 H), 1.53 (m,
6 H), 1.25 (m, 6 H), 1.09 (m, 9 H); 13C-NMR (CDCl3): d ¼
12.5, 15.5, 26.9, 28.9, 52.4, 100.4, 126.4, 134.8, 135.1, 146.9;
ESI-MS: m/z calculated for C19H36O3Sn: 431.20, Found: 432.3
(Mþ þ 1); Anal. Calcd. for C19H36O3Sn: C, 52.92; H, 8.42;
Found: C, 52.71; H, 8.55.
Figure 1. Bichalcophenes which recognize G-quadruplex DNA.
5-(Dimethoxymethyl)thiophen-2-yl-tri-n-butylstannane
(3b). Yellow oil, yield 81%; bp 185ꢁC (0.7 mm Hg); 1H-
NMR (400 MHz, CDCl3): d ¼ 7.28 (d, J ¼ 2.8 Hz, 1 H), 7.15
(d, J ¼ 2.8 Hz, 1 H), 5.69 (s, 1 H), 3.48 (s, 6 H), 1.78 (m, 6
H), 1.60 (m, 6 H), 1.48 (m, 6 H), 1.17 (m, 9 H); 13C-NMR
(CDCl3): d ¼ 13.2, 17.3, 27.4, 28.5, 52.7, 101.1, 126.9, 134.2,
135.7, 146.3; ESI-MS: m/z calculated for C19H36O2SSn:
preparation of more complex molecules for multiple
applications.
EXPERIMENTAL
447.26, Found: 448.2 (Mþ
þ
1); Anal. Calcd. for
All commercial reagents were used without purification.
Melting points were determined on a Mel-Temp 3.0 melting
point apparatus and are uncorrected. TLC analysis was carried
out on silica gel 60 F254 precoated aluminum sheets using UV
light for detection. 1H- and 13C-NMR spectra were recorded
on a Bruker 400 MHz spectrometer using the indicated sol-
vents. Mass spectra were obtained from the Georgia State Uni-
versity Mass Spectrometry Laboratory, Atlanta, GA. Elemental
analysis were performed by Atlantic Microlab, Norcross, GA.
General procedure for the synthesis of 2a, b. A 1.06 g
NBS (6 mmol) was added in portions to a stirred mixture of
1a, b (130 mmol) and 19.55 mL trimethyl orthoformate (190
mmol) in 150 mL anhydrous CH2Cl2 and 15 mL anhydrous
CH3OH at 0ꢁ–5ꢁ. After the addition was completed, the result-
ing mixture was stirred overnight at room temperature,
quenched with water, and the organic layer was separated,
washed with water again, dried (sodium sulfate), and evapo-
rated. The resulting oil was purified by distillation.
C19H36O2SSn: C, 51.02; H, 8.11; Found: C, 51.17; H, 7.99.
General procedure for the synthesis of 5a–d. A 1.15 g
tetrakis-triphenyl-phosphine palladium (1 mmol) was added to
a stirred mixture of 3a, b (20 mmol) and 4a, b (20 mmol) in
deaerated dry 40 mL dioxane under nitrogen atmosphere. The
vigorously stirred mixture was warmed to 90–100ꢁC for 24 h.
The solvent was removed under reduced pressure; the residue
was dissolved in 150 mL dichloromethane containing 5 mL of
concentrated ammonia, then washed with water, and passed
through celite. The solution was mixed with conc. 5 mL HCl,
stirred for 6 h, separated and washed with water, dried (so-
dium sulfate), and evaporated. The product was purified by
column chromatography on silica gel, using dichloromethane
as eluent.
Methyl 50-formyl-2,20-bifuran-5-carboxylate (5a). Yellow
solid, yield (two steps) 79%; mp 129–130ꢁC; 1H-NMR (400
MHz, CDCl3): d ¼ 9.65 (s, 1 H), 7.69 (d, J ¼ 3.6 Hz, 1 H),
7.49 (d, J ¼ 3.6 Hz, 1 H), 7.24 (brs, 1 H), 7.25 (brs, 1 H),
3.86 (s, 3 H); 13C-NMR (CDCl3): d ¼ 53.1, 111.2, 125.6,
127.6, 133.8, 135.2, 137.4, 152.3, 152.7, 161.9, 178.5; ESI-
2-Bromo-5-(dimethoxymethyl) furan (2a). Colorless liquid,
yield 95%; bp 45ꢁC (0.35 mmHg); 1H-NMR (400 MHz, CDCl3):
d ¼ 6.91 (d, J ¼ 2.8 Hz, 1 H), 6.85 (d, J ¼ 2.8 Hz, 1 H), 5.38 (s,
1 H), 3.36 (s, 6 H); 13C-NMR (CDCl3): d ¼ 52.3, 99.6, 112.7,
125.7, 130.4, 143.1; ESI-MS: m/z calculated for C7H9BrO3:
221.05, Found: 222.1 (Mþ þ 1), 223.1 (Mþ þ 2); Anal. Calcd.
for C7H9BrO3: C, 38.03; H, 4.10; Found: C, 37.81; H, 4.25.
2-Bromo-5-(dimethoxymethyl) thiophene (2b). Colorless liq-
MS: m/z calculated for C11H8O5: 220.18, Found: 221 (Mþ
1); Anal. Calcd. for C11H8O5: C, 60.00; H, 3.66; Found: C,
60.28; H, 3.79.
þ
Methyl 5-(5-formylfuran-2-yl) thiophene-2-carboxylate
(5b). Yellowish white solid, yield (two steps) 85%; mp 143–
1
143.5ꢁC. H-NMR (400 MHz, CDCl3): d ¼ 9.62 (s, 1 H), 7.85
1
uid, yield 97%; bp 69ꢁC (0.25 mm Hg). H-NMR (400 MHz,
(d, J ¼ 4 Hz, 1 H), 7.79 (d, J ¼ 4 Hz, 1 H), 7.68 (d, J ¼ 4
Hz, 1 H), 7.34 (d, J ¼ 4, 1 H), 3.91 (s, 3 H); 13C-NMR
(CDCl3): d ¼ 53.1, 111.3, 125.7, 127.8, 133.8, 135.1, 137.9,
152.3, 152.9, 162.1, 177.3; ESI-þMS: m/z calculated for
C11H8O4S: 236.24, Found: 237 (M þ 1); Anal. Calcd. for
C11H8O4S: C, 55.92; H, 3.41; Found: C, 56.21; H, 3.48.
CDCl3): d ¼ 6.96 (d, J ¼ 4 Hz, 1 H), 6.83 (dd, J ¼ 1.2, 4 Hz,
1 H), 5.54 (d, J¼ 1.2, 1 H), 3.33 (s, 6 H); 13C-NMR (CDCl3):
d ¼ 52.5, 99.7, 112.5, 125.9, 131.1, 142.8; ESI-MS: m/z calcu-
lated for C7H9BrO2S: 237.11, Found: 238.3 (Mþ þ 1), 239.3
(Mþ þ 2); Anal. Calcd. for C7H9BrO2S: C, 35.46; H, 3.83;
Found: C, 35.22; H, 4.15.
Methyl-50-formyl-2,20-bithiophene-5-carboxylate (5c). White
1
solid, yield (two steps) 73%; mp 161ꢁC. H-NMR (400 MHz,
General procedure for the synthesis of 3a, b. A 101 mL
1.6M n-butyllithium solution in hexane (150 mmol) was added
CDCl3): d ¼ 9.93 (s, 1 H), 8.05 (d, J ¼ 4 Hz, 1 H), 7.82 (d, J
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

January 2010
Bichalcophenes: A Concise Synthesis of Formyl Ester- and Cyano Ester-Substituted
Bithiophenes, Bifurans, and Furanothiophenes
169
Scheme 1
¼ 4 Hz, 1 H), 7.79 (d, J ¼ 4, 1 H), 7.50 (d, J ¼ 4, 1 H), 3.89
(s, 3 H); 13C-NMR (CDCl3): d ¼ 51.5, 111.6, 121.9, 126.4,
139.3, 139.4, 142.2, 144.8, 150.7, 162.4, 181.1; ESI-MS: m/z
calculated for C11H8O3S2: 252.31, Found: 253 (Mþ þ 1);
Anal. Calcd. for C11H8O3S2: C, 52.36; H, 3.2; Found: C,
52.46; H, 3.13.
a stirred solution of 5a–d (10 mmol) in 25 mL methanol and
heated at reflux for 24 h. Evaporation of the solvent under
reduced pressure to yield a precipitate that was partitioned
between water and 100 mL ethyl acetate, and the organic layer
was dried (sodium sulfate) and then concentrated to dryness
under reduced pressure. The crude oxime was allowed to
reflux in 20 mL acetic anhydride for 24 h, and the solvent was
evaporated under reduced pressure. The product was purified
by column chromatography on silica gel, using hexanes/ethyl
acetate (80/20, v/v) as eluent.
Methyl 50-cyano-2,20-bifuran-5-carboxylate (6a). Yellowish
brown solid, yield (two steps) 81%; mp 99–100ꢁC. 1H-NMR (400
MHz, DMSO-d6): d ¼ 7.77 (d, J ¼ 3.6 Hz, 1 H), 7.46 (d, J ¼ 3.6
Hz, 1 H), 7.20 (brs, 2 H), 3.85 (s, 3 H); 13C-NMR (DMSO-d6): d
¼ 52.5, 108.5, 111.3, 114.5, 121, 126.3, 138.5, 140.6, 144.2,
150.6, 158.4; ESI-MS: m/z calculated for C11H7NO4: 217.18,
Found: 218 (Mþ þ 1); Anal. Calcd. for C11H7NO4: C, 60.83; H,
3.25; N, 6.45; Found: C, 60.88; H, 3.38; N, 6.08.
Methyl-5-(5-formylthiophen-2-yl)
(5d). Yellowish brown solid, yield (two steps) 78%; mp 178–
furan-2-carboxylate
1
180ꢁC. H-NMR (400 MHz, CDCl3): d ¼ 9.96 (s, 1 H), 8.07
(d, J ¼ 4 Hz, 1 H), 7.78 (d, J ¼ 4 Hz, 1 H), 7.48 (d, J ¼ 4, 1
H), 7.30 (d, J ¼ 4, 1 H), 3.86 (s, 3 H); 13C-NMR (CDCl3): d
¼ 52.5, 111.6, 121.2, 127.1, 139.1, 139.5, 143.4, 144.2, 151.3,
158.4.1, 184.7. ESI-MS: m/z calculated for C11H8O4S: 236.24,
Found: 237 (Mþ þ 1); Anal. Calcd. for C11H8O4S: C, 55.92;
H, 3.41; Found: C, 56.01; H, 3.44.
General procedure for the synthesis of 6a–d. A 10 mL
aqueous solution of 0.7 g hydroxylamine hydrochloride (10
mmol) and 1.06 g sodium carbonate (10 mmol) was added to
Journal of Heterocyclic Chemistry
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A. A. Farahat, A. Kumar, A. E.-D. M. Barghash, F. E. Goda, H. M. Eisa, and D. W. Boykin
Vol 47
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Methyl 5-(5-cyanofuran-2-yl)thiophene-2-carboxylate (6b). Yel-
lowish brown solid, yield (two steps) 78%; mp 115-116ꢁC.
1H-NMR (400 MHz, DMSO-d6): d ¼ 7.85 (d, J ¼ 4 Hz, 1 H),
7.80 (d, J ¼ 4 Hz, 1 H), 7.70 (d, J ¼ 3.6, 1 H), 7.31 (d, J ¼
3.6, 1 H), 3.86 (s, 3 H); 13C-NMR (DMSO-d6): d ¼ 49.9,
107.1, 111.9, 113.2, 121, 126.1, 138.9, 139.2, 145.8, 151.9,
165.9; ESI-MS: m/z calculated for C11H7NO3S: 233.24, Found:
234 (Mþ þ 1); Anal. Calcd. for C11H7NO3S: C, 56.64; H,
3.02; N, 6.01; Found: C, 56.95; H, 3.16; N, 5.77.
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Methyl 50-cyano-2,20-bithiophene-5-carboxylate (6c). Yellow
solid, yield (two steps) 75%; mp 142-144ꢁC. 1H-NMR (400
MHz, DMSO-d6): d ¼ 8.01 (d, J ¼ 4 Hz, 1 H), 7.71 (d, J ¼ 4
Hz, 1 H), 7.45 (d, J ¼ 3.6, 1 H), 7.28 (d, J ¼ 3.6, 1 H), 3.85
(s, 3 H); 13C-NMR (DMSO-d6): d ¼ 55.1, 105.2, 110.2, 113.8,
121.2, 126.7, 134.5, 137.2, 147.1, 152.4, 162.4; ESI-MS: m/z
calculated for C11H7NO2S2: 249.31, Found: 250.2 (Mþ þ 1);
Anal. Calcd. for C11H7NO2S2: C, 52.99; H, 2.83; N, 5.62;
Found: C, 53.12; H, 3.06; N, 5.39.
Methyl 5-(5-cyanothiophen-2-yl) furan-2-carboxylate
(6d). Brown solid, yield (two steps) 78%; mp 126-126.5ꢁC.
1H-NMR (400 MHz, DMSO-d6): d ¼ 7.61 (d, J ¼ 4 Hz, 1 H),
7.54 (d, J ¼ 4 Hz, 1 H), 7.25 (d, J ¼ 4, 1 H), 6.80 (d, J ¼ 4,
1 H), 3.94 (s, 3 H); 13C-NMR (DMSO-d6): d ¼ 51.3, 109.2,
111.9, 115.4, 120.7, 124.9, 137.7, 140.1, 144.7, 151.1, 161.8;
ESI-MS: m/z calculated for C11H7NO3S: 233.24, Found: 234
(Mþ þ 1); Anal. Calcd. for C11H7NO3S: C, 56.64; H, 3.02; N,
6.01; Found: C, 56.67; H, 3.09; N, 5.98.
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