Nitroketene dithioacetal chemistry: Synthesis of coumarins incorporating nitrothiophene moiety

Article abstract of DOI:10.1007/s12039-011-0102-7

Alkylation of dipotassium 2-nitro-1,1-ethylenedithiolate 1 with ethyl 4-chloroacetoacetate 6 in a mixture of MeOH-H₂O (2:1) furnished highly functionalized 3-nitrothiophene 5. Pechmann condensation of 5 with 2-hydroxybenzaldehydes 8a-i, 10 furnished coumarin-3-nitrothiophene conjugates 9a-i, 11. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-011-0102-7

c
J. Chem. Sci. Vol. 123, No. 4, July 2011, pp. 411–420. ꢀ Indian Academy of Sciences.
Nitroketene dithioacetal chemistry: Synthesis of coumarins
incorporating nitrothiophene moiety
H SURYA PRAKASH RAO∗ and K VASANTHAM
Department of Chemistry, Pondicherry University, Pondicherry 605 014, India
e-mail: hspr.che@pondiuni.edu.in
MS received 7 September 2010; revised 14 December 2010; accepted 22 December 2010
Abstract. Alkylation of dipotassium 2-nitro-1,1-ethylenedithiolate 1 with ethyl 4-chloroacetoacetate 6 in a
mixture of MeOH-H2O (2:1) furnished highly functionalized 3-nitrothiophene 5. Pechmann condensation of 5
with 2-hydroxybenzaldehydes 8a–i, 10 furnished coumarin-3-nitrothiophene conjugates 9a–i, 11.
Keywords. Nitroketenedithioacetals; 3-nitrothiophenes; 2-hydroxy benzaldehydes; coumarins;
Pechmann condensation.
1. Introduction
reaction provided a single product, ethyl 4-{[4-(2-
ethoxy-2-oxoethyl)-3-nitrothiophen-2-yl]sulfanyl}-3-
oxobutanoate 5 (R = COOEt; scheme 1). The product
was obviously formed via route d (scheme 1). In this
paper, we give details of this study for the synthesis
of highly functionalized 3-nitrothiophene 5. Thio-
phenes in general6 and 3-nitrothiophenes7 in particular
have found several applications in pharmaceutical and
technological fields. Further to isolation and character-
ization of 3-nitrothiophene 5, we describe its transfor-
mation into two more 3-nitrothiophene derivatives and
nine coumarin 3-nitrothiophene conjugates.
Nitroketene dithioacetals 2, prepared from carbon disul-
fide, nitromethane and alkylating agents in a two-step
process, are extremely useful two carbon synthons for
the synthesis of heterocyclic compounds incorporating
diverse functional groups.1 We have been investigat-
ing on the alkylation of dipotassium salt of 2-nitro-1,
1-ethylenedithiolate 1 with a variety of alkyl halides
and found that product formation is critically depen-
dent on the nature of the alkyl halide and conditions.
We found that while alkylation of the salt 1 with sim-
ple alkyl halides provided bis-alkylated products of the
type 2 (route a, scheme 1),2 alkylation with sterically
hindered alkyl halides or with propargyl bromide pro-
vided 1,3-dithioles of the type 3 (route b, scheme 1)
and 4 (route c, scheme 1) respectively.3,4 Recently we
found that alkylation of the salt 1 with acyl methyl chlo-
rides furnished 3-nitrothiophenes of the type 5 (route d,
scheme 1).5
In continuation of above studies, we considered the
reaction of the salt 1 with an alkyl halide having mul-
tiple functionalities, like ethyl 4-chloroacetoacetate 6,
which is having two carbonyls, two active methylenes
and two leaving groups namely chloro and ethoxy. The
reaction could produce thiophene 5 going via the route
d in analogy with our previous findings involving
acyl methyl chlorides or it could produce mono- or
bis-alkylated products (route a or b) followed by
cyclization. To evaluate these interesting possibilities,
we performed alkylation of 6 with the salt 1. The
2. Experimental
2.1 General
Reactions were performed in oven-dried glassware
(150◦C). Dichloromethane, tetrahydrofuran, hexanes,
ethyl acetate obtained commercially were distilled
before use. Thin layer chromatography (TLC) was car-
ried out using silica gel 60 F254 plates. The developed
chromatogram was analysed by UV lamp (254 nm)
or iodine vapors. The crude compounds were puri-
fied by flash chromatography on silica gel (230–
400 mesh) using hexanes-EtOAc mixture (10% to
50% EtOAc) as eluent. Ethyl 4-chloroacetoacetate,
2-hydroxybenzaldehyde (salicylaldehyde) and piperi-
dine were purchased from Sigma Aldrich and used
as received. Substituted 2-hydroxybenzaldehydes 8b–i
were prepared following the procedure described by
Wynberg.8 The 1H NMR (400 MHz/300 MHz/60 MHz)
∗For correspondence
411

412
H Surya Prakash Rao and K Vasantham
reaction mixture separated into two phases on dilu-
S
S
tion with dichloromethane (45 mL). The organic layer
was washed with water (3 × 25 mL) and brine (2 ×
15 mL) and dried over anhydrous Na2SO4. Evapora-
tion of the solvent under reduced pressure resulted
in the crude product as dark brown pasty mass. The
crude product was subjected to column chromatogra-
phy on silica gel by using increasing amounts of EtOAc
(5% to 40%) in hexanes as eluent. Evaporation of the
pooled fraction having required 3-nitrothiophene fur-
nished 1.14 g of ethyl 4-{[4-(2-ethoxy-2-oxoethyl)-3-
nitro-2-thienyl]sulfanyl}-3-oxobutanoate 5a as yellow
crystalline solid in 57% yield. Mp: 79–80◦C (MeOH);
UV λmax (MeOH): 281 nm (log ε = 4.1), 370 nm (log
ε = 3.6); IR νmax (KBr): 1722 (CO), 1547, 1489, 1370
(NO2), 1081, 768 cm−1. 1H NMR δ (400 MHz; CDCl3;
Me4Si): 7.01 (s, 1H, CH), 4.23–4.14 (m, 4H, OCH2),
4.11 (s, 2H, CH2), 3.85 (s, 2H, CH2), 3.66 (s, 2H, CH2),
1.29 (t, J = 7.2 Hz, 3H, CH3), 1.27 (t, J = 7.2 Hz, 3H,
CH3); 13C NMR δ (100 MHz; CDCl3; Me4Si): 195.7
(C), 169.7 (C), 166.4 (C), 147.5 (C), 141.8 (C), 131.5
(C), 121.5 (CH), 61.7 (CH2), 61.1 (CH2), 47.3 (CH2),
44.9 (CH2), 36.2 (CH2), 14.2 (CH3), 14.1 (CH3); HRMS
(ESI+): calcd for C14H17NNaO7S2 (MNa+), 398.0344;
found, 398.0344. Anal. Calcd. for C14H17NO7S2: C
44.71; H, 4.56; N, 3.71; S, 17.08; found: C 44.68; H,
4.52; N, 3.69; S, 17.06.
O2N
SR
3
b
RX
S- K+
S- K+
S
S
O2N
SR
SR
RX
a
HC CCH2Br
c
O2N
O2N
1
2
4
d
RCOCH2Cl
O2N
R
R
O
S
S
5
Scheme 1. Reaction of dipotassium salt 1 with different
alkyl halides.
and 13C NMR (100 MHz / 75 MHz), DEPT spectra were
recorded in CDCl3/CDCl3:CCl4 (1:1)/DMSO-d6:CCl4
(1:1) on Bruker 400 MHz, JEOL 300 MHz or JEOL
60 MHz FT-NMR spectrometers with tetramethylsilane
(TMS; 0 ppm) as internal standard. CHN analysis were
performed on PerkinElmer elemental analyzer.
2.2 Preparation of ethyl 4-{[4-(2-ethoxy-2-oxoethyl)-
3-nitro-2-thienyl]sulfanyl}-3-oxobutanoate 5a
2.2a X-ray crystal structure of ethyl 4-[4-(2-ethoxy-
2-oxoethyl)-3-nitro-2-thienyl]sulfanyl-3-oxobutanoate
5a:
O2N
S
COOEt
EtOOC
S
O
5a
To a stirred suspension of freshly prepared dipotassium
salt of nitroketene dithioacetate 1 (0.5 g, 2.35 mmol)
in a mixtureof MeOH and water (2:1; 10 mL) at
0◦C a dilute solution of ethyl 4-chloro-3-oxobutanoate
6 (0.77 g, 4.70 mmol) in aqueous MeOH (15 mL)
was added by using pressure equalizer funnel at 0◦C
during 45 min. The resulting reaction mixture was
then stirred vigorously at room temperature (rt) for
6 h. After the completion of the reaction (TLC; hex-
anes − EtOAc = 6:4), the mixture was transferred
into a beaker containing 20 g of crushed ice. The
acidic (pH = 5) reaction mixture was carefully neu-
tralized with 0.1 N NaHCO3. The contents of the
Crystal data and structure refinement data for 5a.
Identification
code
: CCDC 633059
Empirical
formula
: ‘C14H17NO7S2’
Formula weight
Temperature
: 375.41
: 273(2)
´
Wavelength
: 0.71073 Å
Crystal system,
space group
: Monoclinic P21

Nitroketene dithioacetal chemistry: Synthesis of coumarins incorporating nitrothiophene moiety
413
´
To a stirred solution of ethyl 4-{[4-(2-ethoxy-2-
oxoethyl)-3-nitro-2-thienyl]sulfanyl}-3-oxobutanoate 5a
(0.05 g, 0.13 mmol) in EtOH (1 mL), 1 drop of dil.
H2SO4 (1 drop of conc. H2SO4 in 1 mL EtOH) was
added. The resulting reaction mixture was refluxed on
a pre-heated oil bath maintained at 100◦C for 18 h
for complete transformation (TLC hexanes - EtOAc
7:3). Excess acid was quenched with Na2CO3. Evapo-
ration of the solvent under reduced pressure furnished
0.04 g of ethyl 2-{4-nitro-5-[(2-oxopropyl)sulfanyl]-
3-thienyl}acetate 7 in 99% yield. Mp: 134–136◦C
(MeOH); UV λmax (MeOH): 238 nm (log ε = 4.1),
377 nm (log ε = 3.6); IR νmax (KBr): 1735 (CO), 1718
(CO), 1546, 1492, 1319 (NO2), 1093, 863, 738 cm−1.
1H NMR δ (400 MHz; CDCl3; Me4Si): 6.99 (s, 1H,
CH), 4.17 (q, J = 7.2 Hz, 2H, CH2), 3.92 (s, 1H, OH),
3.87 (s, 3H, CH3), 2.37 (s, 3H, CH3), 1.27 (t, J =
7.2 Hz, 3H, CH3); 13C NMR δ (100 MHz; CDCl3;
Me4Si): 200.6 (C), 169.8 (C), 154.0 (C), 148.2 (C),
131.6 (C), 121.3 (CH), 61.2 (CH2), 45.6 (CH2), 36.3
(CH2), 28.6 (CH3) 14.2 (CH3). HRMS (ESI+): calcd for
C11H13NNaO5S2 (MNa+), 326.0133; found, 326.0132.
Anal. Calcd. for C11H13NO5S2: C, 43.50; H, 4.32; N,
4.62; S, 21.14; found: C 43.49; H, 4.30; N, 4.60; S,
21.12.
Unit cell
dimensions
: a = 14.9609(16) Å;
α = 90 deg.
´
: b = 4.9763(5) Å;
β = 92.706(4) deg.
´
: c = 23.301(3) Å;
γ = 90.00 deg.
´
Volume
Z, Calculated
density
Absorption
coefficient
F(000)
Crystal size
Theta range
for data
: 1732.8(3) Å3
: 4, 1.525 Mg/m3
: 0.342 mm−1
: 784
: 0.26 × 0.11 × 0.08 mm
: 2.63 to 18.97 deg.
collection
Limiting indices
: −16<=h<=20,
−4<=k<=6,
−33<=l <=31
: 16260 / 2949
Reflections
collected /
[R (int) = 0.0338]
unique
Completeness to
theta = 25.00
Absorption
correction
: 99.9 %
: Semi-empirical from
equivalents
Max. and min.
transmission
Refinement
method
Data / restraints /
parameters
Goodness-of-fit
on Fˆ2
Final R indices
[I>2sigma(I)]
R indices
: 0.9112 and 0.8730
2.2c Methyl 2-{4-nitro-5-[(2-oxopropyl)sulfanyl]-3-
thienyl}acetate 7a:
: Full-matrix
least-squares on F2
: 6821/1/437
O2N
COOMe
Me
S
S
: 0.971
O
7a
:R1 = 0.0715,
wR2 = 0.1752
:R1 = 0.0765,
wR2 = 0.1772
: 1.152 and
Following
the
above
procedure
ethyl
4-
{[4-(2-ethoxy-2-oxoethyl)-3-nitro-2-thienyl]sulfanyl}-
3-oxobutanoate 5a (0.13 mmol) in MeOH (1 mL),
was transformed into 0.036 g of methyl 2-{4-nitro-5-
[(2-oxopropyl)sulfanyl]-3-thienyl}acetate 7a in 96%
yield with dil. H2SO4. Mp: 108–110◦C (MeOH); UV
λmax (MeOH): 238 nm (log ε = 4.1), 377 nm (log ε =
3.6); IR νmax (KBr): 1735 (COOEt), 1718 (CO), 1546,
(all data)
Largest diff. peak
´
and hole
−0.425 e.Å−3
1
1492, 1319 (NO2), 1093, 863, 738 cm−1. H NMR δ
2.2b Ethyl 2-{4-nitro-5-[(2-oxopropyl)sulfanyl]-3-
thienyl}acetate 7:
(400 MHz; CDCl3; Me4Si): 6.98 (s, 1H, CH), 3.89
(s, 2H, CH2), 3.87 (s, 2H, CH2), 3.72 (s, 3H, CH3),
2.37 (s, 3H, CH3); 13C NMR δ (100 MHz; CDCl3;
Me4Si): 200.2 (C), 170.0 (C × 2), 147.7 (C), 131.4
(C), 121.2 (CH), 52.2 (OCH3), 45.5 (CH2), 36.0 (CH2),
28.5 (CH3). HRMS (ESI+): calcd for C10H11NNaO5S2
(MNa+), 311.9976; found, 311.9976. Anal. Calcd. for
O2N
COOEt
Me
S
S
O
7

414
H Surya Prakash Rao and K Vasantham
C10H11NO5S2: C, 41.51; H, 3.83; N, 4.84; S, 22.17; 2.3a Ethyl 2-{5-[(6-chloro-2-oxo-2H-3-
found: C 41.49; H, 3.80; N, 4.82; S, 22.12.
chromenyl) sulfanyl]-4-nitro-3-thienyl}acetate 9b:
NO2
Cl
S
2.3 Representative procedure for the synthesis
of coumarins: Preparation of ethyl 2-{4-nitro-5-[(2-
oxo-2H-3-chromenyl)sulfanyl]-3-thienyl}acetate 9a
S
COOEt
O
O
9b
Following the general procedure described above,
the reaction of 5-chloro-2-hydroxybenzaldehyde 8b
(0.024 g, 0.16 mmol) and ethyl 4-{[4-(2-ethoxy-
2-oxoethyl)-3-nitro-2-thienyl]sulfanyl}-3-oxobutanoate
5a (0.05 g, 0.13 mmol) in the presence of piperi-
dine (0.001 g, 0.1 mol%) stirred at rt for 34 h
furnished 0.037 g of ethyl 2-{5-[(6-chloro-2-oxo-2H-
NO2
S
S
COOEt
O
O
9a
To a homogenous solution of 2-hydroxybenzaldehyde
8a (0.02 g, 0.16 mmol) in THF, piperidine (0.001 g,
0.1 mol%) was added and stirred for 10 min at rt.
The reaction mixture became brown in colour. To
this solution, ethyl 4-{[4-(2-ethoxy-2-oxoethyl)-3-
nitro-2-thienyl]sulfanyl}-3-oxobutanoate 5a (0.05 g,
0.13 mmol) was added and the reaction mixture was
allowed to stir at rt for 32 h for completion of the
reaction (TLC: hexanes- EtOAc 8:2). The reaction mix-
ture was then diluted with dichloromethane (25 mL)
and the organic layer was washed sequentially with
water (3 × 25 mL), brine and dried over anhydrous
sodium sulfate. Evaporation of the solvent under
reduced pressure resulted in crude ethyl 2-{4-nitro-
5-[(2-oxo-2H-3-chromenyl)sulfanyl]-3-thienyl}acetate
9a as gummy liquid. The crude product was subjected
to column chromatography on SiO2 (35 g, 15 cm ×
1 cm) using increasing amounts of ethyl acetate in
hexanes as eluent. Evaporation of the pooled fractions
having the required product resulted in 0.042 g of 9a
as a light yellow colour solid in 69% yield. M.p.: 188–
190◦C (MeOH); Rf : 0.68 (hexanes − EtOAc; 8:2); UV
λmax (MeOH): 278 nm (log ε = 4.2), 374 nm (log ε =
3.4); IR νmax (KBr): 1735 (CO), 1541, 1492, 1319
3-chromenyl)sulfanyl]-4-nitro-3-thienyl}acetate
9b
as yellow colour solid in 66% yield. Mp: 160–161◦C
(MeOH); UV λmax(MeOH): 271 nm (log ε = 4.3),
379 nm (log ε = 3.8); IR νmax (KBr): 1740 (CO), 1546,
1
1492, 1319 (NO2), 1091, 883, 740 cm−1. H NMR δ
(300 MHz, CDCl3: DMSO-d6; Me4Si): 8.35 (s, 1H,
CH), 7.72 (s, 1H, CH), 7.63 (d, J = 8.7 Hz, 1H, CH),
7.39 (d, J = 9 Hz, 1H, CH), 7.27 (s, 1H, CH), 4.15
(q, J = 7.2 Hz, 2H, OCH2), 3.90 (s, 2H, CH2), 1.26 (t,
J = 7.2 Hz, 3H, CH3); 13C NMR δ (300 MHz, CDCl3:
DMSO-d6; Me4Si): 169.3 (C), 157.4 (C), 152.2 (C),
148.6 (CH), 143.7 (C), 142.5 (C), 132.9 (CH), 130.7
(C), 129.7 (C), 127.6 (CH), 124.2 (CH), 121.7 (C),
119.5 (C), 117.9 (CH), 60.6 (CH2), 35.6 (CH3), 13.8
(CH3). HRMS (ESI+): calcd for C17H12ClNNaO6S2
(MNa+), 447.9692; found, 447.9677. Anal. Calcd. for
C17H12ClNO6S2: C, 47.95; H, 2.84; Cl, 8.32; N, 3.29;
S, 15.06; found: C, 47.91; H, 2.79; Cl, 8.30; N, 3.31; S,
15.09.
2.3b Ethyl 2-{5-[(6-bromo-2-oxo-2H-3-
chromenyl) sulfanyl]-4-nitro-3-thienyl}acetate 9c:
NO2
1
(NO2), 1096, 881, 729 cm−1. H NMR (300 MHz,
Br
S
CDCl3; Me4Si): δ 8.22 (s, 1H, CH), 7.68–7.35 (m,
4H, CH), 7.02 (s, 1H, CH), 4.18 (q, J = 7.2 Hz, 2H,
OCH2), 3.89 (s, 2H, CH2), 1.27 (t, J = 7.2 Hz, 3H,
CH3); 13C NMR (300 MHz, CDCl3; Me4Si): δ 169.8
(C), 158.2 (C), 154.5 (C), 150.5 (CH), 148.7 (C),
142.2 (C), 133.6 (CH), 131.2 (C), 128.5 (CH), 125.1
(CH), 123.1 (CH), 120.8 (C), 118.7 (C), 117.0 (CH),
61.3 (CH2), 36.2 (CH2), 14.1 (CH3). HRMS (ESI+):
calcd for C17H13NNaO6S2 (MNa+), 414.0082; found,
414.0079. Anal. calcd. for C17H13NO6S2: C, 52.16; H,
3.35; N, 3.58; S, 16.38; found: C 52.19; H, 3.31; N,
3.59; S, 16.36.
S
COOEt
O
O
9c
Following the general procedure described above,
the reaction of 5-bromo-2-hydroxybenzaldehyde 8c
(0.032 g, 0.16 mmol) and ethyl 4-{[4-(2-ethoxy-
2-oxoethyl)-3-nitro-2-thienyl]sulfanyl}-3-oxobutanoate
5a (0.05 g, 0.13 mmol) in the presence of piperidine
(0.001 g, 0.1 mol%) stirred at rt for 34 h furnished
0.028 g of ethyl 2-{5-[(6-bromo-2-oxo-2H-3-chromenyl)

Nitroketene dithioacetal chemistry: Synthesis of coumarins incorporating nitrothiophene moiety
415
sulfanyl]-4-nitro-3-thienyl}acetate 9c as yellow colour C18H15NO7S2: C, 51.30; H, 3.59; N, 3.32; S, 15.22;
solid in 48 % yield. Mp: 170–172◦C (MeOH); UV found: C, 50.29; H, 3.62; N, 3.35; S, 15.19.
λmax (MeOH): 276 nm (log ε = 4.3), 374 nm (log ε =
3.8); IR νmax (KBr): 1740 (CO), 1546, 1492, 1319
2.3d Ethyl 2-{5-[(6-methyl-2-oxo-2H-3-
chromenyl) sulfanyl]-4-nitro-3-thienyl}acetate 9e:
1
(NO2), 1091, 883, 740 cm−1. H NMR δ (300 MHz,
CDCl3: DMSO-d6; Me4Si): 8.2 (s, 1H, CH), 7.73 (d,
J = 9.0 Hz, 1H, CH), 7.53 (s, 1H, CH), 7.32 (d, J =
NO2
Me
S
8.4 Hz, 1H, CH), 7.19 (s, 1H, CH), 4.17 (q, J = 7.2 Hz,
2H, CH2), 3.91 (s, 2H, CH2), 1.27 (t, J = 7.2 Hz, 3H,
CH3); 13C NMR δ (300 MHz, CDCl3; Me4Si): 169.9
(C), 157.6 (C), 153.0 (C), 150.9 (CH), 146.4 (C), 141.3
(C), 136.2 (CH), 131.6 (CH), 130.7 (C), 128.6 (CH),
125.0 (CH), 120.4 (C), 120.3 (C), 118.7 (CH), 60.5
(OCH2), 35.4 (CH2), 13.8 (CH3). HRMS (ESI+): calcd
for C17H12BrNNaO6S2 (MNa+), 491.9187; found,
491.9172. Anal. Calcd. for C17H12BrNO6S2: C, 43.41;
H, 2.57; Br, 16.99; N, 2.98; S, 13.64; found: C, 43.40;
H, 2.59; Br, 17.01; N, 2.99; S, 13.68.
S
COOEt
O
O
9e
Following the general procedure described above,
the reaction of 5-methyl-2-hydroxybenzaldehyde 8e
(0.044 g, 0.32 mmol) and ethyl 4-{[4-(2-ethoxy-
2-oxoethyl)-3-nitro-2-thienyl]sulfanyl}-3-oxobutanoate
5a (0.1 g, 0.27 mmol) in the presence of piperi-
dine (0.001 g, 0.1 mol%) stirred at rt for 36 h
furnished 0.078 g of ethyl 2-{5-[(6-methyl-2-oxo-2H-
3-chromenyl)sulfanyl]-4-nitro-3-thienyl}acetate 9e as
yellow colour solid in 67% yield. Mp: 186–188◦C
(MeOH); UV λmax (MeOH): 276 nm (log ε = 4.3),
374 nm (log ε = 3.8); IR νmax (KBr): 1740 (CO),
2.3c Ethyl 2-{5-[(6-methoxy-2-oxo-2H-3-
chromenyl) sulfanyl]-4-nitro-3-thienyl}acetate 9d:
NO2
1546, 1492, 1319, 1091, 883, 740 cm−1. H NMR δ
1
MeO
S
(400 MHz, CDCl3; Me4Si): 8.2 (s, 1H, CH), 7.46 (d,
J = 8.1 Hz, 1H, CH), 7.33 (d, J = 8.7 Hz, 1H, CH),
7.28 (d, J = 6.6 Hz, 1H, CH), 7.0 (s, 1H, CH), 4.15
(q, J = 7.2 Hz, 2H,OCH2), 3.97 (s, 2H, CH2), 2.44 (s,
3H, CH3), 1.23 (t, J = 7.2 Hz, 3H, CH3) ; 13C NMR δ
(100 MHz, CDCl3; Me4Si): 169.9 (C), 158.1 (C), 152.3
(CH), 148.2 (C), 140.7 (C), 135.0 (CH), 134.6 (C × 2),
130.8 (C), 128.8 (CH), 124.6 (CH), 118.5 (C), 118.4
(C), 116.2 (CH), 60.4 (OCH2), 35.4 (CH2), 20.1 (CH3),
14.1 (CH3). HRMS (ESI+): calcd for C18H15NNaO6S2
(MNa+), 428.0238; found, 428.0229. Anal. Calcd. for
C18H15NO6S2: C, 53.32; H, 3.73; N, 3.43; S, 15.82;
found: C, 53.34; H, 3.71; N, 3.39; S, 15.79.
S
COOEt
O
O
9d
Following the general procedure described above,
the reaction of 5-methoxy-2-hydroxybenzaldehyde
8d (0.025 g, 0.17 mmol) and ethyl 4-{[4-(2-ethoxy-
2-oxoethyl)-3-nitro-2-thienyl]sulfanyl}-3-oxobutanoate
5a (0.052 g, 0.14 mmol) in the presence of piperidine
(0.001 g, 0.1 mol%) stirred at rt for 32 h furnished
0.032 g of Ethyl 2-{5-[(6-methoxy-2-oxo-2H-3-
chromenyl)sulfanyl]-4-nitro-3-thienyl}acetate 9d as
yellow colour solid in 53% yield. Mp: 170–172◦C
(MeOH); UV λmax (MeOH): 273 nm (log ε = 4.3),
374 nm (log ε = 3.8); IR νmax (KBr): 1736 (CO), 2.3e Ethyl 2-{5-[(6-ethyl-2-oxo-2H-3-
1546, 1492, 1319, 1091, 883, 740 cm−1. H NMR δ chromenyl) sulfanyl]-4-nitro-3-thienyl}acetate 9f:
1
(400 MHz, CDCl3; Me4Si): 8.17 (s, 1H, CH), 7.27 (s,
NO2
1H, CH), 7.18 (d, J = 8.0 Hz, 1H, CH), 7.11 (d, J =
Et
S
7.8 Hz, 1H, CH), 7.0 (s, 1H, CH), 4.17 (q, J = 7.2 Hz,
2H, CH2), 4.0 (s, 3H, CH3), 3.88 (s, 2H, CH2), 1.27
(t, J = 7.2 Hz, 3H, CH3) ; 13C NMR δ (300 MHz,
CDCl3; Me4Si): 169.7 (C), 157.7 (C), 150.4 (CH),
147.3 (C), 145.4 (C), 144.2 (C), 142.9 (C), 131.2 (C),
125.0 (CH), 123.0 (CH), 121.3 (C), 119.6 (CH), 119.3
(C), 115.2 (CH), 61.2 (CH2), 56.3 (OCH3), 36.1 (CH2),
14.1 (CH3). HRMS (ESI+): calcd for C18H15NNaO7S2
(MNa+), 444.0188; found, 444.0179. Anal. Calcd. for
S
COOEt
O
O
9f
Following the general procedure described above, the
reaction of 5-ethyl-2-hydroxybenzaldehyde 8f (0.024 g,
0.16 mmol) and ethyl 4-{[4-(2-ethoxy-2-oxoethyl)-3-
nitro-2-thienyl]sulfanyl}-3-oxobutanoate 5a (0.05 g,

416
H Surya Prakash Rao and K Vasantham
1
0.13 mmol) in the presence of piperidine (0.001 g, 1492, 1319 (NO2), 1091, 883, 740 cm−1. H NMR δ
0.1 mol%) stirred at rt for 36 h furnished 0.04 g of (400 MHz, CDCl3; Me4Si): 8.21 (s, 1H, CH), 7.48 (d,
ethyl 2-{5-[(6-ethyl-2-oxo-2H-3-chromenyl)sulfanyl]- J = 8.1 Hz, 1H, CH), 7.32 (d, J = 8.7 Hz, 1H, CH),
4-nitro-3-thienyl}acetate 9f as yellow colour solid 7.29 (d, J = 8.6 Hz, 1H, CH), 6.98 (s, 1H, CH), 4.14
in 69% yield. Mp: 174–176◦C (MeOH); UV λmax (q, J = 7.2 Hz, 2H, OCH2), 3.98 (s, 2H, CH2), 1.27 (s,
(MeOH): 274 nm (log ε = 4.1), 372 nm (log ε = 3.6); 9H, CH3), 1.23 (t, J = 7.2 Hz, 3H, CH3); 13C NMR δ
IR νmax (KBr): 1738 (CO), 1546, 1492, 1319 (NO2), (100 MHz, CDCl3; Me4Si): 169.9 (C), 158.1 (C), 153.1
1
1091, 883, 740 cm−1. H NMR δ (400 MHz, CDCl3; (CH), 152.3 (C), 148.2 (C), 140.7 (C), 135.0 (CH),
Me4Si): 8.17 (s, 1H, CH), 7.48 (d, J = 8.0 Hz, 1H, CH), 134.6 (C), 130.8 (C), 128.8 (CH), 124.6 (CH), 118.5
7.35 (d, J = 8.0 Hz, 1H, CH), 7.32 (d, J = 8.0 Hz, 1H, (C), 118.4 (C), 116.2 (CH), 60.4 (OCH2), 35.4 (CH2),
CH), 7.0 (s, 1H, CH), 4.18 (q, J = 7.2 Hz, 2H, OCH2), 31.6 (CH3 × 3), 26.3 (C), 14.0 (CH3). HRMS (ESI+):
3.88 (s, 2H, CH2), 2.73 (q, J = 7.6 Hz, 2H, CH2), 1.29 calcd for C21H21NNaO6S2 (MNa+), 470.0708; found,
(t, J = 6.0 Hz, 3H, CH3), 1.27 (t, J = 5.2 Hz, 3H, CH3); 470.0699. Anal. Calcd. for C21H21NO6S2: C, 56.36; H,
13C NMR δ (100 MHz, CDCl3; Me4Si): 169.7 (C), 4.73; N, 3.13; S, 14.33; found: C, 56.33; H, 4.72; N,
158.5 (C), 152.9 (C × 2), 150.7 (CH), 141.3 (C), 133.7 3.09; S, 14.31
(CH), 131.2 (C), 127.0 (CH), 122.9 (CH), 120.6 (C),
118.6 (C × 2), 116.8 (CH), 61.2 (OCH2), 36.2 (CH2),
calcd for C19H17NNaO6S2 (MNa+), 442.0395; found,
2.3g Ethyl 2-{5-[(7-methoxy-2-oxo-2H-3-
chromenyl) sulfanyl]-4-nitro-3-thienyl}acetate 9h:
28.1 (CH2), 15.5 (CH3), 14.2 (CH3). HRMS (ESI+):
442.0382. Anal. Calcd. for C19H17NO6S2: C, 54.40; H,
4.08; N, 3.34; S, 15.29; found: C, 54.42; H, 4.11; N,
3.37; S, 15.26.
NO2
S
S
COOEt
MeO
O
O
2.3f Ethyl 2-(5-{[6-(tert-butyl)-2-oxo-2H-3-
9h
chromenyl] sulfanyl}-4-nitro-3-thienyl)acetate 9g:
Following the general procedure described above,
the reaction of 4-methoxy-2-hydroxybenzaldehyde
8h (0.049 g, 0.32 mmol) and ethyl 4-{[4-(2-ethoxy-
2-oxoethyl)-3-nitro-2-thienyl]sulfanyl}-3-oxobutanoate
5a (0.1 g, 0.27 mmol) in the presence of piperidine
(0.001 g, 0.1 mol%) stirred at rt for 34 h furnished
0.085 g of ethyl 2-{5-[(7-methoxy-2-oxo-2H-3-
chromenyl)sulfanyl]-4-nitro-3-thienyl}acetate 9h as
yellow colour solid in 73% yield. Mp: 182–183◦C
(MeOH); UV λmax (MeOH): 276 nm (log ε = 4.3),
372 nm (log ε = 3.8); IR νmax (KBr): 1740 (CO), 1546,
NO2
t-Bu
S
S
COOEt
O
O
9g
Following the general procedure described above,
the reaction of 5-tert-butyl-2-hydroxybenzaldehyde
8g (0.057 g, 0.32 mmol) and ethyl 4-{[4-(2-ethoxy-
2-oxoethyl)-3-nitro-2-thienyl]sulfanyl}-3-oxobutanoate
5a (0.1 g, 0.27 mmol) in the presence of piperidine
(0.001 g, 0.1 mol%) stirred at rt for 36 h furnished
0.076 g of ethyl 2-(5-{[6-(tert-butyl)-2-oxo-2H-3-
chromenyl]sulfanyl}-4-nitro-3-thienyl)acetate 9g as
yellow colour solid in 64% yield. Mp: 154–156◦C
(MeOH); UV λmax (MeOH): 274 nm (log ε = 4.1),
372 nm (log ε = 3.7); IR νmax (KBr): 1738 (CO), 1546,
1
1492, 1319 (NO2), 1091, 883, 740 cm−1. H NMR δ
(400 MHz, CDCl3; Me4Si): 8.14 (s, 1H, CH), 7.34 (d,
J = 8.8 Hz, 1H, CH), 7.21 (d, J = 6.4 Hz, 1H,
CH), 7.0 (s, 1H, CH), 6.95 (s, 1H, CH), 4.18 (q,
J = 7.2 Hz, 2H, OCH2), 3.89 (s, 2H, CH2), 3.87 (s,
3H, CH3), 1.28 (t, J = 7.2 Hz, 3H, CH3); 13C NMR δ
(100 MHz, CDCl3; Me4Si): 171.7 (C), 169.8 (C), 158.7
MeOH, H2O
(2:1)
O2N
COOEt
S-
S-
K+
K+
O2N
O
EtOOC
+
Cl
EtOOC
0 oC
rt
S
S
O
4 h, 57%
1
6
5
Scheme 2. Reaction of dipotassium salt 1 with ethyl 4-chloroacetoacetate 6.

Nitroketene dithioacetal chemistry: Synthesis of coumarins incorporating nitrothiophene moiety
417
EtOOC
O2N
MeOOC
O2N
EtOOC
O2N
H2SO4
EtOH
H2SO4
MeOH
EtOOC
S
S
S
S
O
S
S
O
O
5
7
6
Scheme 3. Solvolysis of 3-nitrothiophene 5.
(C), 156.5 (C), 149.9 (CH), 149.0 (C), 131.2 (C), 123.2 (C), 157.6 (C), 153.5 (C), 146.6 (CH), 144.5 (C), 134.4
(CH), 121.7 (CH), 120.9 (C), 119.0 (C), 118.1 (CH), (C), 133.9 (CH), 131.3 (C), 130.9 (C), 130.5 (C), 123.4
109.9 (CH), 106.8 (C), 61.3 (OCH2), 55.9 (OCH3), (CH), 121.6 (C), 118.8 (C), 115.9 (CH), 61.3 (OCH2),
36.2 (CH2), 14.2 (CH3). HRMS (ESI+): calcd for 36.1 (CH2), 15.6 (CH3), 14.2 (CH3). HRMS (ESI+):
C18H15NNaO7S2 (MNa+), 444.0188; found, 444.0179. calcd for C18H14ClNNaO6S2 (MNa+), 461.9849; found,
Anal. Calcd. for C18H15NO7S2: C, 51.30; H, 3.59; N, 461.9832. Anal. Calcd. for C18H14ClNO6S2: C, 49.15;
3.32; S, 15.22; found: C, 50.29; H, 3.62; N, 3.35; S, H, 3.21; Cl, 8.06; N, 3.18; O, 21.82; S, 14.58; found:
15.19.
C, 49.11; H, 3.20; Cl, 8.07; N, 3.19; O, 21.78; S, 14.56.
2.3h Ethyl 2-{5-[(6-chloro-7-methyl-2-oxo-2H-3-
chromenyl)sulfanyl]-4-nitro-3-thienyl}acetate 9i:
2.3i Ethyl 2-{4-nitro-5-[(3-oxo-3H-benzo[f]chromen-
2-yl)sulfanyl]-3-thienyl}acetate 11:
NO2
NO2
S
Cl
S
S
COOEt
Me
O
O
S
COOEt
O
O
9i
11
Following the general procedure described above, the
reaction of 3-chloro-6-hydroxy-2-methylbenzaldehyde Following the general procedure described above, the
8i (0.024 g, 0.16 mmol) and ethyl 4-{[4-(2-ethoxy- reaction of 2- hydroxy-1-napthaldehyde 10 (0.055 g,
2-oxoethyl)-3-nitro-2-thienyl]sulfanyl}-3-oxobutanoate 0.32 mmol) and ethyl 4-{[4-(2-ethoxy-2-oxoethyl)-
5a (0.05 g, 0.13 mmol) in the presence of piperidine 3-nitro-2-thienyl]sulfanyl}-3-oxobutanoate 5a (0.1 g,
(0.001 g, 0.1 mol%) stirred at rt for 36 h furnished 0.27 mmol) in the presence of piperidine (0.001 g,
0.026 g of ethyl 2-{5-[(6-chloro-7-methyl-2-oxo-2H- 0.1 mol%) stirred at rt for 36 h furnished 0.067 g
3-chromenyl)sulfanyl]-4-nitro-3-thienyl}acetate 9i as of ethyl 2-{4-nitro-5-[(3-oxo-3H-benzo[ f ]chromen-2-
yellow colour solid in 43% yield. Mp: 189–190◦C yl)sulfanyl]-3-thienyl}acetate 11 as yellow colour solid
(MeOH); UV λmax (MeOH): 275 nm (log ε = 4.3), in 57% yield. Mp: 196–198◦C (MeOH); UV λmax
382 nm (log ε = 3.8); IR νmax (KBr): 1740 (CO), 1546, (MeOH): 276 nm (log ε = 4.3), 394 nm (log ε =
1
1492, 1319 (NO2), 1091, 883, 740 cm−1. H NMR δ 3.8); IR νmax (KBr): 1740 (CO), 1546, 1492, 1319
1
(400 MHz, CDCl3; Me4Si): 8.4 (s, 1H, CH), 7.59 (d, (NO2), 1091, 883, 740 cm−1. H NMR δ (400 MHz,
J = 9.2 Hz, 1H, CH), 7.21 (d, J = 8.8 Hz, 1H, CH), 7.0 CDCl3; Me4Si): 9.0 (s, 1H, CH), 8.26 (d, J =
(s, 1H, CH), 4.18 (q, J = 7.2 Hz, 2H, OCH2), 3.89 (s, 8.0 Hz, 1H, CH), 8.11 (d, J = 12.0 Hz, 1H,
2H, CH2), 2.59 (s, 3H, CH3), 1.28 (t, J = 7.2 Hz, 3H, CH), 8.0 (d, J = 8.0 Hz, 1H, CH), 7.76 (t,
CH3); 13C NMR δ (100 MHz, CDCl3; Me4Si): 169.7 J = 8.0 Hz, 1H, CH), 7.64 (t, J = 8.0 Hz, 1H, CH),
NO2
O2N
S
COOEt
CHO
OH
C5H11N (0.1 mol%)
S
EtOOC
+
THF, 0 oC g rt
32 h, 69%
S
COOEt
S
O
O
O
5
8a
9a
Scheme 4. Reaction of 2-hydroxybenzaldehyde 8a with β-keto ester unit in parent thiophene 5a.

418
H Surya Prakash Rao and K Vasantham
Table 1. Transformation of β-keto ester 5a to coumarins 9a–i.
Entry
2-Hydroxy benzaldehydes 8a-i
4-Nitro-3-thienyl-2H-chromenes 9a-i
Time (h)
Yield (%)
NO2
S
CHO
S
OH
COOEt
O
O
8a
9a
1
2
32
34
69
66
NO2
Cl
Br
CHO
OH
S
Cl
Br
S
S
COOEt
COOEt
O
O
O
8b
8c
8d
9b
NO2
CHO
OH
S
O
9c
3
4
34
34
48
53
NO2
MeO
Me
CHO
S
MeO
Me
S
OH
COOEt
O
O
9d
NO2
CHO
OH
S
S
COOEt
O
O
8e
8f
9e
5
6
36
38
67
69
NO2
NO2
CHO
OH
S
S
COOEt
COOEt
O
O
9f
S
CHO
OH
S
O
O
8g
9g
7
8
36
32
64
73
NO2
CHO
OH
S
S
MeO
Cl
COOEt
COOEt
MeO
Cl
O
O
8h
9h
Me
Me
NO2
S
CHO
OH
S
O
O
8i
9i
9
34
42

Nitroketene dithioacetal chemistry: Synthesis of coumarins incorporating nitrothiophene moiety
419
EtOOC
O2N
C5H11
(0.1 mol%)
N
NO2
S
CHO
OH
+
S
S
THF, 0 oC rt
36 h, 57%
O
S
COOEt
O
O
EtOOC
5
11
10
Scheme 5. Reaction of 2-hydroxy-1-napthaldehyde 10 with β-keto ester unit in parent thiophene 5a.
7.52 (t, J = 8.0 Hz, 1H, CH), 6.99 (s, 1H, CH), 4.19 hydroxybenzaldehyde. Many natural products with
(q, J = 8.0 Hz, 2H, OCH2), 3.90 (s, 2H, CH2), 1.28 coumarin (benzopyrone) motif have been isolated
(t, J = 8.0 Hz, 3H, CH3); 13C NMR δ (100 MHz, from plant sources and such molecules show immense
CDCl3; Me4Si): 169.9 (C), 158.6 (C), 155.2 (C), 147.1 biological activity.10 In addition, coumarin and its
(CH), 146.8 (C), 135.5 (CH), 131.3 (C × 2), 130.4 (C), C3-substitution products found clinical medical appli-
129.3 (CH), 129.2 (CH), 128.8 (C), 126.8 (CH), 122.8 cations as blood thinners or as anticoagulants.11 Some
(CH), 121.4 (CH), 119.1 (C), 116.8 (CH), 113.2 (C), coumarin derivatives are triplet sensitizers and are
61.3 (OCH2), 36.3 (CH2), 14.2 (CH3). HRMS (ESI+): in use as dye lasers.12 Classical routes to coumarins
calcd for C21H15NNaO6S2 (MNa+), 464.0238; found, include Pechmann,13,14 Knoevenagel,15 Perkin,16
464.0224. Anal. Calcd. for C21H15NO6S2: C, 57.13; H, Reformatsky17 and Wittig18 condensation reactions.
3.42; N, 3.17; S, 14.53; found: C, 57.11; H, 3.39; N, Thus, thiophene 5a was subjected to Pechmann conden-
3.14; S, 14.49.
sation with 2-hydroxybenzaldehyde (salicylaldehyde)
8a to produce a C3-nitrothiophene substituted coumarin
9a. The reaction conducted in presence of piperidine
in THF produced the coumarin, ethyl 2-{4-nitro-5-[(2-
oxo-2H-3-chromenyl)sulfanyl]-3-thienylacetate 9a as
3. Results and discussion
The reaction of ethyl chloroacetoacetate 6 with the only product in 69% yield (scheme 4). By changing
dipotassium nitroketenedithioacetate 1 provided 3- solvents and bases systematically, we found that the
nitrothiopene 5a as the only product in 57% yield transformation works well in presence of a catalytic
(scheme 2). The structure of the thiophene 5a was amount of piperidine (0.1 mol %) in THF at rt. Even
established on the basis of spectroscopic (IR, 1H NMR, though catalytic amount of piperidine demanded longer
13C NMR, 2D NMR and HRMS), analytical data and reaction time (table 1), the reaction was cleaner and
single crystal X-ray structure determination.9 As antic- product isolation was facile. The coumarin 9a was
1
1
ipated from the assigned structure, H NMR spectrum characterized on the basis of spectroscopic (IR, H
of 5a displayed four singlets for three methylenes (δ NMR, 13C NMR, 2D NMR and HRMS) and analytical
3.66, 3.85, 4.11 ppm) and one for C5H (δ 7.01 ppm). data. Three singlets at δ 3.89 ppm for two hydrogens,
1
The H NMR spectrum also revealed occurrence of at δ 7.02 for one hydrogen and at 8.22 ppm for one
the keto-enol tautomerism to the extent of 85:15 where hydrogen assignable to methylene, thiophene C5H and
keto-form predominated.
coumarin C4H respectively, were notable signals in the
The thiophene 5a, prepared in this study pos- 1H NMR spectrum.
sesses multiple functional groups like ketone, ester
The Pechmann transformation of thiophene 5a into
and nitrogroups. Particularly, there are three active coumarin 9a proved to be quite general for nine 2-
methylenes flanked by three carbonyl carbons. Ini- hydroxybenzaldehydes 8a–i and the coumarin products
tially, we treated thiophene 5a with EtOH in pres- 9a–i were obtained in 42–73% yield (table 1).
ence of a catalytic amount of H2SO4. The product
Scope of the present 3-nitrothiophene substituted
was the anticipated methyl ketone 7 (scheme 3). Reac- coumarin synthesis could be further extended by mak-
tion of with MeOH in presence of catalytic amount ing 2-hydroxy-1-naphthaldehyde 10 participate in the
of H2SO4 provided 7a, the decarboethoxylated and condensation reaction (scheme 5). This reaction pro-
trans-esterification product (scheme 3).
vided coumarin 11 in 57% yield. Spectroscopic data
Next, we considered transformation of 5a into supported structure. Particularly noteworthy is the sin-
1
coumarin and 3-nitrothiophene conjugates by reac- glet for C4H of the coumarin ring in the H NMR
tion of the β-keto ester moiety in 5a with 2- spectrum which appeared at δ 9.1 ppm.

420
H Surya Prakash Rao and K Vasantham
2. Rao H S P, Sakthikumar L and Shreedevi S 2002 Sulfur
Mechanistically, the transformation is interesting as
Lett. 25 207
one acetate unit in 5a was lost during coumarin forma-
tion.19 Moreover, the active methylene adjacent to C2S,
instead of that of the β-keto ester moiety was utilized
in the coumarin synthesis. This is in contrast to gen-
eral expectation that β-keto ester moiety in 5a would
react in preference over the methylene flanked by C2S
and CO groups during coumarin formation. Perhaps,
under the reaction conditions, the carbanion generated
on C2S methylene is more reactive than the carbanion
generated on the β-keto ester moiety.
3. Rao H S P, Sakthikumar L, Vanitha, S and Sivakumar, S
2003 Tetrahedron Lett. 44 4701
4. Rao H S P 2008 Indian J. Chem., Sect B. 47B 272
5. Rao H S P and Vasantham K 2009 J. Org. Chem. 74
6847
6. Batchu C 2008 J. Sulfur Chem. 29 187–240 (b)
Russell R K 1996 Comprehensive Heterocyclic Chem-
istry II (ed.) (New York: Pergamon Press) 2 pp. 679–729
(c) Rajappa S and Natekar M V 1996 Comprehensive
Heterocyclic Chemistry II A R Katritzky, C W Rees, E
F V Scriven (eds) (New York: Pergamon Press) 2 pp 491
(d) Blicke F F 1952 Biological and Pharmacological
Activity of Thiophene and its Derivatives H D Hartough
(ed.) (New York: Interscience) p. 29
4. Conclusions
7. (a) Morley J O and Matthews T P 2006 Org. Biomol.
Chem. 4 359 (b) Campaigne E 1994 Comprehensive
Heterocyclic Chemistry 4 922
Our studies clearly delineated the alkylation of the salt
1 with ethyl 4-chloroacetoacetate 6 to provide the thio-
phene 5a which could be utilized for the synthesis of
novel coumarins 9 and 11 with 3-nitrothiophene moiety
at C3 position. Overall, we have demonstrated a facile
two-step synthesis of hybrid heterocyles incorporating
biologically important 3-nitrothiophene and coumarin
units starting from inexpensive and readily available
chemicals and reagents. Furthermore, we have demon-
strated that the one of the two esters in 5a can be selec-
tively decarboethoxylated to provide methyl ketones as
well trans-esterification products.
8. (a) Wynberg
H 1960 Chem. Rev. 60 164 (b)
Wynberg H and Meijer E 1982 Org. Reactions 28 1–
36 (c) Wynberg H 1991 The Reimer–Tiemann reaction
in comp. Org. Synth. (eds). Trost, B. M.; Fleming, I
(Oxford: Pergamon) 2 269
9. Details of X-ray single crystal structure determination of
5 was deposited at CCDC (deposition No. 633059)
10. (a) Hepworth J D Gabbutt C D Heron B M in 1996
Comprehensive heterocyclic chemistry-II A R Katritzky,
C W Rees and E F V Scriven (eds). 5 417 (b)
Hepworth J D in 1984 Comprehensive heterocyclic
chemistry A R Katritzky, C W Rees (eds). 3 799 (c)
Murray R D H Mendez Jand Brown S A 1982 The natu-
ral coumarins: Occurrence, chemistry and biochemistry
(New York: John Wiley and Sons) 227
11. O’Kennedy R and Thornes R D 1997 Coumarins: Biol-
ogy, applications and mode of action (Chichester UK:
Wiley)
12. (a) Fluorescent indicators: Brun M P Bischoff L and
Garbay C 2004 Angew Chem. Int. Ed. 43 3432 (b) Laser
technology: Sekar N 2003 Colourage 50 55 (c) Fluores-
cent chemo-sensors: Chen C-T and Huang W-P 2002 J.
Am. Chem. Soc. 124 6246
13. (a) von Pechmann and H Duisberg C 1883 Ber. Dtsch.
Chem. Ges. 16 2119 (b) von Pechmann Hand Duisberg
C 1884 Chem. Ber. 17 929 (c) Russel A and Fyre J R
1941 Org. Synth. 21 22 (d) Sethna S and Phadke R 1953
Org. React. 7 1 (e) Rabjohn N 1976 Org. React. 24 261
(e) Sugino T and Tanaka K 2001 Chemistry Lett. 513
14. Rao H S P and Sivakumar S 2006 J. Org. Chem. 71 8715
15. (a) Jones G 1967 Org. React. 15 204 (b) Brufola G,
Fringuelli F, Piermatti O and Pizzo F 1996 Heterocycles
43 1257
Acknowledgements
HSPR thanks University Grants Commission for UGC-
SAP, Department of Science and Technology for Fund
for Improvement of S&T Infrastructure in Universities
and Higher Educational Institutions (FIST) and Council
of Scientific and Industrial Research (CSIR) for finan-
cial support. VK thanks CSIR for research fellowship.
We thank Prof. A Srikrishna, Indian Institute of Science
(IISc), Bangalore, for providing spectral data.
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