A facile, eco-friendly, proton donor-acceptor catalyzed, one-pot, three-component synthesis of tetrahydrobenzo[b]pyrans

Article abstract of DOI:10.14233/ajchem.2014.15657

Piperidinium acetate catalyzed, three-component cyclocondensation between aryl aldehydes (1), malononitrile (2) and cyclic-1,3-diketo compounds (3) in the eco-friendly and universal solvent water at room temperature for 0.5 h, resulted in the formation of tetrahydro[b]pyran derivatives (4). The formation of 4 could also be established in a step-wise fashion by isolating independently each of the intermediates benzylidine malononitrile 5 and benzylidine cyclohexane-1,3-dione 6 and treating them subsequently with 3 and 2, respectively to obtain benzopyrans 4. Mechanistic studies on the formation of 4 from 1, 2 and 3 are reported. Further, the synthesis of 4 could also be achieved in two variable but identical end-product giving tandem syntheses by treating 1 with 2 and then with 3 giving 4 or by treating 1 with 3 and then with 2 giving 4. All the products were suitably characterized using NMR, IR and Mass spectroscopy.

Full text of DOI:10.14233/ajchem.2014.15657

Asian Journal of Chemistry; Vol. 26, No. 8 (2014), 2221-2225
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.15657
A Facile, Eco-friendly, Proton Donor-Acceptor Catalyzed, One-Pot,
Three-Component Synthesis of Tetrahydrobenzo[b]pyrans
*
A. INDRASENA , SD. RIYAZ, A. NAIDU and P.K. DUBEY
Department of Chemistry, Jawaharlal Nehru Technological University Hyderabad College of Engineering, Kukatpally, Hyderabad-500 085, India
*Corresponding author: E-mail: adisherla.indrasena@gmail.com
Received: 18 April 2013;
Accepted: 17 August 2013;
Published online: 15 April 2014;
AJC-14996
Piperidinium acetate catalyzed, three-component cyclocondensation between aryl aldehydes (1), malononitrile (2) and cyclic-1,3-diketo
compounds (3) in the eco-friendly and universal solvent water at room temperature for 0.5 h, resulted in the formation of tetrahydro[b]pyran
derivatives (4). The formation of 4 could also be established in a step-wise fashion by isolating independently each of the intermediates
benzylidine malononitrile 5 and benzylidine cyclohexane-1,3-dione 6 and treating them subsequently with 3 and 2, respectively to obtain
benzopyrans 4. Mechanistic studies on the formation of 4 from 1, 2 and 3 are reported. Further, the synthesis of 4 could also be achieved
in two variable but identical end-product giving tandem syntheses by treating 1 with 2 and then with 3 giving 4 or by treating 1 with 3 and
then with 2 giving 4. All the products were suitably characterized using NMR, IR and Mass spectroscopy.
Keywords: Proton donor-acceptor catalyst, Piperidinium acetate, One-pot reaction, 4H-Benzo[b]pyrans.
C-C bond forming agents. Keeping this in view, it was intended
to use these proton donor acceptor catalysts in carrying out
the schemes shown below. Furthermore, the use of piperidi-
nium salts as a catalyst in the synthesis of 4H-benzo[b]pyrans
has not been studied extensively and intensively.
INTRODUCTION
4H-Benzo[b]pyrans are an important class of compounds
which have received considerable attention in recent years due
to their wide range of biological activities1. These include acti-
vities such as anticoagulant, anticancer, spasmolytic, diuretic,
antiancaphylactia2 etc. 4H-Pyrans also constitute the structural
unit of a series of natural products2. A number of 2-amino-
4H-pyrans are useful as photoactive materials2. Singh et al.
reported3 the synthesis of tetrahydrobenzo[b]pyrans from
benzaldehyde, malanonitrile and cyclic-1,3-diketo compounds
in refluxing acetonitrile containing catalytic amount of acetic
acid. Kaupp et al. prepared4 benzo[b]pyrans utilizing the
reactants in solid or molten state without using a solvent. Other
literature reported reactions has their own merits and limitations5.
The use of piperidinium acetate as catalyst in the most
eco-friendly solvent water at room temperature involving
shorter reaction times giving better yields for the synthesis of
benzo[b]pyrans has not been studied. Furthermore, mecha-
nistic studies involving isolation of intermediates and reactions
involving tandem syntheses of intermediates as well as end-
products has not been explored so far. These studies are being
reported in this communication for the first time.
As part of our continued interest in the green develop-
ment10-12 of expedient methods for the synthesis of heterocyclic
compounds of biological importance, we report herein a very
simple and highly efficient method for the synthesis of 4H-
benzo[b]pyrans (Scheme-I).
+
N
–
O
Ar
O
O
O
CH3COO
H
H
CN
CN
CN
NH
Ar-CHO
H O- 30 min / R.T
2
R
R
R
1
2
1
2
R
1
1
3a)
3b)
R=R = H
1
4a-o
R=R = CH
Scheme-I
EXPERIMENTAL
Melting points are uncorrected and were determined in
open capillary tubes in sulphuric acid bath. Thin-layer chroma-
tography (TLC) was performed on silica gel G and spotting
was done using iodine or UV light. IR spectra were recorded
with Jasco FT-IR 5300, 1H NMR on Varian 400-MHz instru-
ment and Mass spectra on anAgilent LC-MS instrument giving
only M+. values in Q+1 mode.
Proton donor-acceptor catalysts5-9 have been playing a
vital role in the synthesis of Knoevenagel condensation
derivatives since a long time. Their, ease of synthesis and easy
availability makes the synthetic chemists use them as effective

2222 Indrasena et al.
Asian J. Chem.
General procedure for preparation of 4: A mixture of
appropriate benzaldehyde (1a, 3.0 mmol), malononitrile (2,
3.0 mmol) and the appropriate 1,3-dicarbonyl compound (3,
3.0 mmol), piperidinium acetate (catalytic amount-10 mg) and
water (25 mL) was stirred at room temperature for 20-30 min.
At the end of this period, the separated solid was collected by
filtration, washed with water (2 mL × 25 mL) and dried in a
vaccum oven to obtain crude 4a. The latter, were recrystallized
from EtOH to get the pure 4a (Scheme-I).
2-Amino-4-(4-dimethylamino-phenyl)-5-oxo-5,6,7,8-
tetrahydro-4H-chromene-3-carbonitrile (4j): IR (KBr, νmax,
cm-1): 3415, 3325 (unequal doublet, asymmetric and symmetric
stretching of -NH2), 2195 (-CN group, sharp), 1710 (-C=O of
1
chromene moiety, sharp); H NMR (400 MHz, DMSO-d6/
TMS): δ 1.86 -2.62 (m,6H –(CH2)3), 2.95 (6H, s, 2 N-CH3),
4.18 (s, 1H) 6.14 (S, 2H, -NH2, D2O exchangeble), 7.15-7.30
(4 H, m, ArH): Anal. calcd. (%) for (C18H19N3O2) requires C,
69.88; H, 6.19; N, 13.58; found (%) C, 69.58; H, 6.49; N,
13.28; LC-MS: m/z: (M+ + 1): 310.
2-Amino-4-(3-ethoxy-4-methoxy-phenyl)-7,7-dimethyl-
5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4e):
IR (KBr, νmax, cm-1): 3415, 3325 (unequal doublet, asymmetric
and symmetric stretching of -NH2), 2195 (-CN group, sharp),
1710 (-C=O of chromene moiety, sharp); 1H NMR (400 MHz,
DMSO-d6/TMS): δ 0.98 (s,3H,-CH3) 1.09 (s, 3H,-CH3), 1.46
(t, 3H,-OCH2-CH3), 1.99-2.17 (m, 2H), 2.35-2.73 (m, 2H),
3.82 (s, 3H, -OCH3), 4.07 (q, 2H, -OCH2), 6.14 (S, 2H, -NH2,
D2O exchangeble), 7.15–7.30 (3H, m, ArH): Anal. calcd. (%)
for (C21H24N2O4) requires C, 68.46; H, 6.57; N, 7.60; found
(%) C, 68.49; H, 6.67; N, 7.50; LC-MS:m/z: (M+ + 1): 367.
2-Amino-4-biphenyl-4-yl-7,7-dimethyl-5-oxo-5,6,7,8-
2-Amino-4-(4-nitro-phenyl)-5-oxo-5,6,7,8-tetrahydro-
4H-chromene-3-carbonitrile (4k): IR (KBr, νmax, cm-1): 3415,
3325 (unequal doublet, asymmetric and symmetric stretching
of -NH2), 2195 (-CN group, sharp), 1710 (-C=O of chromene
moiety, sharp); 1H NMR (400 MHz, DMSO-d6/TMS): δ 1.86
-2.62 (m,6H -(CH2)3), 4.18 (s,1H,) 6.14 (S, 2H, -NH2, D2O
exchangeble), 7.15-7.30 (4 H, m, ArH): Anal. calcd. (%) for
(C16H13N3O4) requires C, 61.73; H, 4.2 found (%) C, 61.43; H,
4.11; N, 13.60; LC-MS:m/z: (M++1): 312.
2-Amino-4-(3-nitro-phenyl)-5-oxo-5,6,7,8-tetrahydro-
4H-chromene-3-carbonitrile (4l): IR (KBr, νmax, cm-1): 3415,
3325 (unequal doublet, asymmetric and symmetric stretching
of -NH2), 2195 (-CN group, sharp), 1710 (-C=O of chromene
moiety,sharp) ; 1H NMR (400 MHz, DMSO-d6/TMS): δ 1.86
-2.62 (m, 6H -(CH2)3), 4.18 (s,1H,) 6.14 (S, 2H, -NH2, D2O
exchangeble), 7.15-7.30 (4 H, m, ArH): Anal. calcd. (%) for
(C16H13N3O4) requires C, 61.73; H, 4.21 found (%) C, 61.53;
H, 4.31; N, 13.40; LC-MS:m/z: (M++1): 312.
tetrahydro-4H-chromene-3-carbonitrile (4f): IR (KBr, νmax
,
cm-1): 3415, 3325 (unequal doublet, asymmetric and
symmetric stretching of -NH2), 2195 (-CN group, sharp), 1710
1
(-C=O of chromene moiety, sharp); H NMR (400 MHz,
DMSO-d6/TMS): δ 0.98 (s, 3H, -CH3) 1.05 (s, 3H, -CH3), 2.10-
2.14 (m, 2H), 2.25-2.29 (m, 2H), 7.04 (S, 2H, -NH2, D2O
exchangeble), 7.2-7.64 (9 H, m, ArH): Anal. calcd. (%) for
(C24H22N2O2) requires C, 77.81; H, 5.99; N, 7.56; found (%)
C, 77.85; H, 5.97; N, 7.66; LC-MS m/z: (M+ + 1: 371.
2-Amino-4-(4-methoxy-phenyl)-5-oxo-5,6,7,8-tetra-
hydro-4H-chromene-3-carbonitrile(4m): IR (KBr, νmax, cm-1):
3415, 3325 (unequal doublet, asymmetric and symmetric
stretching of -NH2), 2195 (-CN group, sharp), 1710 (-C=O of
2-Amino-4-(4-chloro-phenyl)-5-oxo-5,6,7,8-tetrahydro-
4H-chromene-3-carbonitrile (4g): IR (KBr, νmax, cm-1): 3415,
3325 (unequal doublet, asymmetric and symmetric stretching
of -NH2), 2195 (-CN group, sharp), 1710 (-C=O of chromene
moiety, sharp); 1H NMR (400MHz, DMSO-d6/TMS): δ 1.86 -
2.62 (m, 6H-(CH2)3), 4.18 (s, 1H,) 7.00 (S, 2H, -NH2, D2O
exchangeble), 7.15-7.30 (5 H, m, ArH): Anal. calcd. (%) for
(C16H13N2O2Cl) requires C, 63.90; H, 4.36; N, 9.31; found (%)
C, 63.60; H, 4.76; N, 9.31;LC-MS:m/z: (M++1): 301.
1
chromene moiety, sharp); H NMR (400 MHz, DMSO-d6/
TMS): δ 1.86 -2.62 (m,6H –(CH2)3), 3.82 (s, 3H, -OCH3), 4.18
(s,1H,) 6.14 (S, 2H, -NH2, D2O exchangeble), 7.15-7.30 (4 H,
m, ArH): Anal. calcd. (%) for (C17H16N2O3) requires C, 68.91;
H, 5.44; N, 9.45; found (%) C, 68.94; H, 5.24; N, 9.55; LC-
MS:m/z: (M++1): 297.
2-Amino-5-oxo-4-p-tolyl-5,6,7,8-tetrahydro-4H-
chromene-3-carbonitrile (4n): IR (KBr, νmax, cm-1): 3415, 3325
(unequal doublet, asymmetric and symmetric stretching of
-NH2), 2195 (-CN group, sharp), 1710 (-C=O of chromene
moiety, sharp); 1H NMR (400 MHz, DMSO-d6/TMS): δ 1.86
-2.62 (m, 6H -(CH2)3), 3.82 (s, 3H, -OCH3), 4.18 (s,1H,) 6.14
(S, 2H, -NH2, D2O exchangeble), 7.15-7.30 (4 H, m, ArH):
Anal. calcd. (%) for (C17H16N2O2) requires C, 72.84; H, 5.75;
N, 9.99; found (%) C, 725.84; H, 5.45; N, 9.59; LC-MS:m/z:
(M++1): 281.
2-Amino-4-(4-ethoxy-3-methoxy-phenyl)-5-oxo-
5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4h): IR
(KBr, νmax, cm-1): 3415, 3325 (unequal doublet, asymmetric
and symmetric stretching of -NH2), 2195 (-CN group, sharp),
1710 (-C=O of chromene moiety,sharp); 1H NMR (400MHz,
DMSO-d6/TMS): δ 0.98 (s,3H,-CH3) 1.09 (s, 3H, -CH3), 1.99-
2.17 (m, 2H), 2.35-2.73 (m, 2H), 6.14 (S, 2H, -NH2, D2O
exchangeble), 7.15-7.30 (5 H, m, ArH): Anal. calcd. (%) for
(C19H20N2O4) requires C, 67.05; H, 5.92; N, 8.23; found 9 %)
C, 67.08; H, 5.72; N, 8.63; LC-MS:m/z: (M+ + 1): 341.
2-Amino-4-(4-hydroxy-phenyl)-5-oxo-5,6,7,8-tetra-
hydro-4H-chromene-3-carbonitrile (4i): IR (KBr, νmax, cm-1):
3415, 3325 (unequal doublet, asymmetric and symmetric
stretching of -NH2), 2195 (-CN group, sharp), 1710 (-C=O of
2-Amino-4-biphenyl-4-yl-5-oxo-5,6,7,8-tetrahydro-
4H-chromene-3-carbonitrile (4o): IR (KBr, νmax, cm-1): 3415,
3325 (unequal doublet, asymmetric and symmetric stretching
of -NH2), 2195 (- CN group, sharp), 1710 (-C=O of chromene
moiety, sharp); 1H NMR (400MHz, DMSO-d6/TMS): δ 1.86-
2.62 (m, 6H -(CH2)3), 4.18 (s,1H,) 6.14 (S, 2H, -NH2, D2O
exchangeble), 7.15-7.30 (9 H, m, ArH): Anal. calcd. (%) for
(C22H18N2O2) requires C, 77.17; H, 5.30; N, 8.18; found (%)
C, 77.47; H, 5.10; N, 8.48; LC-MS:m/z: (M+ + 1): 343.
Preparation of 5a from 1a and 2: A mixture of benzal-
dehyde (1a, 3.0 mmol), malononitrile (2, 3.0 mmol) and
1
chromene moiety,sharp) ; H- NMR (400 MHz, DMSO-d6/
TMS): δ 1.86 -2.62 (m, 6H-(CH2)3), 4.18 (s,1H,) 6.14 (S, 2H,
-NH2, D2O exchangeble), 6.82 (s, 1H, -OH), 7.15–7.30 (4 H,
m, ArH):, 7.15–7.30 (4H, m, ArH): Anal. calcd. (%) for
(C16H14N2O3) requires C, 68.07; H, 5.00; N, 9.92; found (%)
C, 68.27; H, 5.08; N, 9.62; LC-MS:m/z: (M++1): 283.

Vol. 26, No. 8 (2014)
Three-Component Synthesis of Tetrahydrobenzo[b]pyrans 2223
piperidinium acetate(catalytic amount) in water (25 mL) was
stirred at R.T for 20-30 min. At the end of this period, the
separated solid was collected by filtration, washed water (50
mL) and dried in a vacuum oven to obtain 5a. The latter, was
used as such in the next step. The structure of the product was
established on the basis of IR, NMR and Mass spectroscopy,
which are in congruence with the reported method13. Yield
(%): 93; m.p. 49-50 ºC.
friendly and universal solvent i.e., water, was stirred together,
for 20-30 min, in the presence of catalytic amount of piperi-
dinium acetate (10 mg) at room temperature. The corres-
ponding 4H-benzo[b]pyrans (4a-o) were obtained in excellent
yields.The results are summarized in Table-1.
As shown in Table-1, the reaction seems to go well with a
range of benzaldehydes containing electron-withdrawing groups
(such as nitro, halide etc.) or even those containing electron-
donating groups (such as hydroxyl, alkoxy, dimethylamino
etc.).
Preparation of 4a from 5a:A mixture of 5a (1, 3 mmol),
cyclic-1,3-diketo compound (3, 3.0 mmol) and piperidinium
acetate(catalytic amount) in water (25 mL) was stirred at room
temperature for 20-30 min. At the end of this period. The sepa-
rated solid was collected by filtration, washed with water
(50 mL) and dried in a vaccum oven to obtain 4a. The latter,
was recrystallized from EtOH to get the pure 4a.
The catalyst plays a vital role in determining the success
of the reaction in terms of rate and yields. In the absence of
catalyst, there was hardly any progress in the reaction even
after stirring the reactants for 2-3 h. Various proton donor-
acceptor catalysts have been screened for this reaction in the
present work. These include piperidinium benzoate, piperi-
dinium p-toluenesulfonate and pyridine p-toluenesulfonate.
Among all the catalysts used in the present work piperidinium
acetate proved to be most effective as far as completion of
reaction in short time and yields are concerned (Fig. 1).
Preparation of 6a from 1a and 3: A mixture of benzal-
dehyde (1, 3.0 mmol), cyclic-1,3-diketo compound (3, 3 mmol)
and piperidinium acetate (catalytic amount) in water (25 mL)
was stirred at room temperature for 20-30 min. At the end of
this period, the separated solid was collected by filtration,
washed with water (50 mL) and dried in a vaccum oven to
obtain the 6a. The latter was used as such in the next step. The
structure of the product was established on the basis of IR,
NMR and mass spectroscopy, which are in congruence with
the reported method13. Yield (%): 93; m.p. 49-50 ºC.
100
90
80
70
60
50
40
30
20
10
0
Preparation of 4a from 6a:A mixture of 6a (1a, 3 mmol),
malononitrile (2, 3.0 mmol) and piperidinium acetate (catalytic
amount -10 mg) and water (25 mL) was stirred at room tempe-
rature for 20-30 min. At the end of this period, the separated
solid was collected by filtration, washed with water (50 mL)
and dried in a vaccum oven to obtain crude 4a. The latter, was
recrystallized from EtOH to get the pure 4a.
Piperidinium
p-toluene-
sulfonate
Piperidinium
benzoate
Piperidinium
acetate
Pyridine
p-toluene-
sulfonate
Catalyst
Preparation of 4a via first tandem synthesis:A mixture
of benzaldehyde (1a, 3.0 mmol), malononitrile (2, 3 mmol),
piperidinium acetate(catalytic amount-10 mg) and water (25
mL) was stirred at room temperature for 20-30 min. The
completion of the reaction was monitored by the TLC for the
disappearance of 1a. To the same reaction mixture was added
cyclic-1,3-diketo (3 mmol) compound and continued stirring
for another 15-20 min. At the end of this period, the separated
solid was collected by filtration, washed with water (50 mL)
and dried in a vaccum oven to obtain the 4a. The latter, require
no further recrystallization.
Fig. 1. Reaction of 1a with 2 and 3a in water at room temperature for 0.5 h
Two possible mechanisms have been proposed to account
for the formation of 4 from 1, 2 and 3. In the first possible
mechanism (Scheme-II), initially, piperidinium acetate
protonates the carbonyl group of 1 to form reversibly 1A. Then,
the acetate anion of piperidinium acetate abstracts a proton
from the active methylene group of 2 to afford 2A. 2A attacks
the carbocation species 1B to form the intermediate 2B which
loses a molecule of water to form the α,β-unsaturated cyano
compound 5. The cyclic-1,3-diketo compound 3 reacts through
its carbanion 3A with 5 to form the intermediate 3B. Then,
the intermediate 3B is cyclized by nucleophilic attack of OH
group on to the cyano moiety to afford the target molecule 4.
Evidence for the above mechanism comes from the fact that,
the intermediate 5 [condensation product of reaction between
aromatic aldehyde (1) and malononitrile (2)] was prepared
separately, isolated, characterized and treated with 3 subse-
quently to obtain the target moiety i.e. 4.
Preparation of 4a via second tandem synthesis:A mixture
of benzaldehyde (1a, 3.0 mmol), cyclic-1,3-diketo (2, 3 mmol),
piperidinium acetate(catalytic amount, 10 mg) and water (25
mL) was stirred at room temperature for 20-30 min. The
completion of the reaction was monitored by the TLC for the
disappearance of 1a. To the same reaction mixture was added
malononitrile (3 mmol) and the mixture was stirred for addi-
tional 15-20 min. At the end of this period, the separated solid
was collected by filtration, washed with water (50 mL) and
dried in a vaccum oven to obtain the 4a. The latter, require no
further recrystallization.
In the second possible mechanism (Scheme-III), initially,
piperidinium acetate protonates the carbonyl group of 1 to
form reversibly 1A. Then, the acetate anion of piperidinium
acetate abstracts a proton from the active methylene group of
3 to afford 3′. 3′ attacks the carbocation species 1B to form
the intermediate 3″ which loses a molecule of water to form
the α,β-unsaturated cyano compound 6. The di cyano compound
RESULTS AND DISCUSSION
A mixture of aromatic aldehyde (1), malononitrile (2)
and 5,5-dimethyl-1,3-cyclohexanedione (3) in the most eco-

2224 Indrasena et al.
Asian J. Chem.
Scheme-II
Scheme-III
TABLE-1
PIPERIDINIUM ACETATE CATALYZED REACTION OF 1 WITH 2 AND 3 GIVING 4
3 Used
S.
No.
Product
obtained
Yield
(%)
1 Used
m.p. (ºC)
R1
R2
CH3
CH3
CH3
CH3
CH3
CH3
H
1
2
3
4
CH3
CH3
CH3
CH3
CH3
CH3
H
92
95
90
94
89
91
93
94
95
95
96
92
90
89
89
225-227(Lit. m.p. 229-231)4
175-177 (Lit. m.p. 177-178)4
209-211 (Lit. m.p. 208-210)5
217-218 (Lit. m.p. 214-215)5
1a(Ar = C6H5)
1b (Ar= 4-NO2-C6H4)
1c (Ar= 4-Cl-C6H4)
4a
4b
4c
4d
4e
4f
1d (Ar= 4-OH -C6CH4)
1e (Ar=(3-OCH34-C2H5)-C6H3
1f (Ar= 4-C6H4-C6H5)
1g (Ar= 4-Cl-C6H4)
5
6
208-210
>250
7
240-242
4g
4h
4i
8
H
H
198-200
1h (Ar=(3-OCH34-C2H5)-C6H3
1i (Ar=4-OH-C6H4)
9
H
H
185-187
10
11
12
13
14
15
H
H
>250
1j (Ar= 4-Me2NC6H4)
1k (Ar= 4-NO2-C6H4)
1l (Ar= 3-NO2-C6H4)
1m (Ar= 4-OCH3-C6H4)
1n (Ar= 4-CH3C6H4)
1o (Ar= 4-C6H4-C6H5)
4j
H
H
228-230
4k
4l
H
H
210-213
H
H
196-199
4m
4n
4o
H
H
>188-190
220-222
H
H
2 reacts through its carbanion 2A with 6 to form the
intermediate 4′. Then the intermediate 4′ is cyclized by the
nucleophilic attack of OH group on to the cyano moiety to
afford the target molecule 4. Evidence for the above mechanism
comes from the fact that, the intermediate 6 [condensation
product of reaction between aromatic aldehyde (1) and malo-
nonitrile (3)] was prepared separately, isolated, characteriszed
and treated with 2 subsequently to obtain the target moiety
i.e., 4.
between the aldehyde (1) and the malononitrile (2) followed
by Michael addition of the intermediate to the cyclic-1,3-diketo
compound, whereas the Scheme-III involves an initial
formation of intermediate between aldehyde (1) and the cyclic-
1,3-diketo compound (3), followed by Michael addition of
the intermediate to the active methylene compound 2 i.e.,
malononitrile to yield the final product 4, in both the cases.
It appears that the reaction involving condensation of 1
with 2 and 3 giving 4 is occuring by both the mechanisms
simultanesously and concurrently in the reaction vessel. This
is further proved by the tandem experiments described below.
The synthesis of 4 could also be achieved in two variable but
In substance, it can be said that the two mechanisms
(Schemes II and III) differ from one another, in that, the
Scheme-II involves, initial formation of an intermediate

Vol. 26, No. 8 (2014)
Three-Component Synthesis of Tetrahydrobenzo[b]pyrans 2225
identical end-product giving tandem syntheses. Thus, a mixture
of benzaldehyde (1a) (i.e., 1, R=H), malononitrile (2) and
piperidinium acetate was stirred in water at room temperature
for 20 min. The reaction was monitored on TLC for the dis-
appearance of 1a. To the resulting mixture, 3 was added in a
tandem way without isolating 5 and then stirred at room
temperature for further 20 min. At the end of this period, the
mixture was processed to obtain the final product 4. Similarly,
a mixture of benzaldehyde (1a) (i.e., 1, R=H), cyclic-1,3-diketo
compound. (3) and piperidinium acetate was stirred in water
at room temperature for 20 min. The reaction was monitored
on TLC for the disappearance of 1a.To the resulting mixture,
2 was added in a tandem way without isolating 6 and then
stirred at room temperature for further 20 min. At the end of
this period, the mixture was processed to obtain the final
product 4.
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In this work, a simple and eco friendly methodology has
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cheap and easily available proton-donor catalyst piperidinium
acetate. The simple work-up and reaction condition makes this
route more attractive and economically viable. Furthermore,
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ACKNOWLEDGEMENTS
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They authors are thankful to the authorities of Jawaharlal
Nehru Technological University, Hyderabad, for providing
laboratory facilities.