A revisit to the hantzsch reaction: Unexpected formation of tetrahydro-benzo[ b ]pyrans beyond polyhydroquinolines

Article abstract of DOI:10.1055/s-0030-1259924

A competitive multicomponent synthesis of tetrahydrobenzo[b]pyrans over polyhydroquinolines is achieved in the presence of ammonium acetate at ambient temperature in ethanol medium. The formation of tetrahydrobenzo[b]pyran instead of polyhydroquinoline in

Full text of DOI:10.1055/s-0030-1259924

LETTER
791
A Revisit to the Hantzsch Reaction: Unexpected Formation of Tetrahydro-
benzo[b]pyrans beyond Polyhydroquinolines
Unexpected
F
orm
a
e
tion of
T
et
d
rahydrobenz
a
o[
b
]pyrans
rbeyond PolyhyA
droquinolines . Undale,a YoonKook Park,b Kyungmoon Park,b Dilip H. Dagade,a Dattaprasad M. Pore*a,b
a
Department of Chemistry, Shivaji University, Kolhapur- 416004, India
Department of Biological and Chemical Engineering, Hongik University, Jochiwon, Chungnam-339-701, Korea
Fax +91(231)2692333; E-mail: p_dattaprasad@rediffmail.com
b
Received 15 December 2010
kinson’s disease, AIDS associated dementia and Down’s
syndrome as well as for the treatment of schizophrenia
and myoclonus.6 Some 2-amino-4H-pyrans could also be
used as photoactive materials.7 These pharmacological
Abstract: A competitive multicomponent synthesis of tetrahy-
drobenzo[b]pyrans over polyhydroquinolines is achieved in the
presence of ammonium acetate at ambient temperature in ethanol
medium. The formation of tetrahydrobenzo[b]pyran instead of
polyhydroquinoline in solution phase has been confirmed by spec- and technological properties of tetrahydrobenzo[b]pyrans
tral and single-crystal analysis. The mechanism for the above trans-
encourage the development of methods for their synthesis
formation is suggested and supported by semi-empirical
and functionalization.
calculations of heat of formation for the intermediates as well as
products.
In continuation of our interest in 1,3-dicarbonyl com-
pounds,2a,8 in this letter we describe competitive, multi-
component synthesis of tetrahydrobenzo[b]pyran over
polyhydroquinolines under Hantzsch reaction conditions.
Key words: multicomponent synthesis, tetrahydrobenzo[b]pyrans,
ammonium acetate, semi-emperical calculations
Recently, we have reported catalyst-free synthesis of
polyhydroquinolines (Hantzsch Reaction) via multicom-
ponent reaction of aldehyde, dimedone, ethylacetoacetate,
and ammonium acetate at ambient temperature.9 Inspired
by these results, our attention was then focused towards
the synthesis of polyhydroquinolines of another family by
replacing ethylacetoacetate with malononitrile. Initially,
the reaction of anisaldehyde, dimedone, malononitrile,
and ammonium acetate was carried out under catalyst-free
conditions in ethanol medium at room temperature to ob-
tain 4-(4¢-methoxyphenyl)-3-cyano-7,7-dimethyl-5-oxo-
1,4,5,6,7,8-Hexahydroquinoline (6, Scheme 1).
In recent years, use of multicomponent reactions (MCR)
have attracted a great deal of attention as they are per-
formed without a need to isolate any intermediate. These
are superior to two-component reactions, with respect to
several aspects including the simplicity of a one-pot pro-
cedure, possibly because of the ease in making structural
variations and building up complex molecules.1 However,
in case of MCR the prediction of the product is a difficult
task due to competitive existence of a variety of interme-
diates and formation of the byproducts in the reaction. The
use of computational tools for calculation of heat of for-
mation makes the task easier. MCR leading to interesting
heterocyclic scaffolds are particularly useful for the cre-
ation of diverse chemical libraries of ‘druglike’ mole-
cules. One prominent MCR that produces an interesting
class of oxygen heterocycles is the venerable tetrahy-
drobenzo[b]pyran synthesis.
O
R
CN
O
N
H
NH2
grinding
path 1
R
H
O
1
6
The chemistry and pharmacology of tetrahydroben-
zo[b]pyrans are receiving considerable interest2 which
can be readily realized from the appearance of vast num-
ber of articles dealing with synthesis and biological activ-
ities of tetrahydrobenzo[b]pyran derivatives.3–6 4H-Pyran
nucleus constitute structural features of a broad range of
natural products.4 The tetrahydrobenzo[b]pyran ring is of-
ten encountered as a structural component of compounds
possessing biological activity as anticoagulant, spas-
molytic, anticancer, anti-anaphylactin5 agents. Also, they
can be used as cognitive enhancers for the treatment of
neurodegenerative disease including Alzheimer’s disease,
amyoprophic lateral sclerosis, Huntington’s disease, Par-
NH4OAC
+
4
EtOH
r.t.
O
R
O
CN
CN
2
path 2
CN
3
O
NH2
5
Scheme 1 Competative multicomponent synthesis of tetrahydro-
benzo[b]pyran; path 110
Surprisingly, in contrast to our expectation instead of
polyhydroquinoline corresponding tetrahydrobenzo[b]
pyran (5, Scheme 1, path 2) was obtained in excellent
yield. Recently, Kapoor10 et al. reported multicomponent
synthesis of polyhydroquinolines 6 in solid phase
(Scheme 1, path 1) without any catalyst using grinding
SYNLETT 2011, No. 6, pp 0791–0796
x
x.
x
x.
2
0
1
1
Advanced online publication: 16.03.2011
DOI: 10.1055/s-0030-1259924; Art ID: U11110ST
© Georg Thieme Verlag Stuttgart · New York

792
K. A. Undale et al.
LETTER
O
R
O
conditions. It is noteworthy that in solution phase tetrahy-
drobenzo[b] pyrans were formed instead of the expected
polyhydroquinolines. This result made us to think about
the mechanism of the reaction. It was noted that ammoni-
um acetate acts as a catalyst for present transformation in-
stead of a reactant. Ionization of ammonium acetate in
CN
NH2
O
R
O
5
H
CN
NH
+
ethanol medium generates NH4 ions that coordinates
strongly to the O atom of 1,3-diketone to form its enolate
ion causing rapid enolization of 1,3-diketone. Its subse-
quent Michael addition with cyanoolefin (8, Scheme 2)
formed in situ from Knoevenagel condensation of alde-
hyde 1 and malononitrile (3) followed by cyclocondensa-
R
R
O
+
CN
H
H
1
O
CN
CN
–
8
CN
tion
results
into
the
corresponding
O
tetrahydrobenzo[b]pyran instead of polyhydroquinoline.
For formation of polyhydroquinolines enaminone inter-
mediate 7 is essential. Hence, to ascertain formation of in-
termediate, we have carried out reaction of dimedone and
ammonium acetate (1:1) in ethanol at room temperature
for 1 hour, however, we obtained starting 1,3-diketone as
it is. Our earlier report on the synthesis of
polyhydroquinolines9 also supported that the enaminone
of b-keto ester and not the 1,3-diketone was formed in so-
lution phase.
NH4OAc
+
NH4
3
4
O
CN
H
+
–OAc
NH4
+
CN
2
O
3
R
O
R
H
O
CN
H
1
+
To understand the feasibility of formation of product 5 in-
stead of product 6 in solution phase, we have carried out
RHF semi-empirical calculations using MNDO-PM311
Hamiltonian method for these species along with the cor-
responding possible intermediates required for the two
separate paths. The computational outputs are summa-
rized in Figure 1. It is observed that the heat of formation
of product 6 is highly positive, that is, endothermic com-
pared to product 5 (Figure 1), which illustrate that the
product 5 should be formed instead of 6. However, the
formation of product 5 proceeds via enolization of dime-
done and that of product 6 via enaminone. Amongst these
two intermediates of the two different paths, the heat of
CN
NH2
7
8
X
O
R
CN
N
H
NH2
6
Scheme 2 Mechanism of formation of tetrahydrobenzo[b]pyrans
vs. polyhydroquinolines
formation of the enol form is highly negative than that of
enaminone (Figure 1). This also confirms that the enoliza-
tion is highly feasible in solution phase (in protic solvents)
than enaminone formation. Hence, product 5 is formed in
solution phase and structure of which has been confirmed
with spectral analysis. However, in solid phase, the syn-
thesis of product 6 is reported.10 The heat of formation of
product 6 is highly positive indicating that the species can
only be formed when extra heat is supplied externally.
This is being provided through mechanical energy in
terms of sufficient grinding which may result in the for-
mation of product 6.
OMe
O
O
CN
NH2
N
NH2
H
6
7
ΔH = –47.68
ΔH = 31.88
OMe
Based on the above experimental results and the compu-
tational analysis, a plausible mechanism for the competi-
O
O
CN
tive
multicomponent
synthesis
of
tetra-
hydrobenzo[b]pyran in ethanol is depicted in Scheme 2.
OH
O
NH2
Tetrahydrobenzo[b]pyrans of structurally diverse alde-
hydes (Table 1) were prepared under the same conditions
(1/2/3/4 = 1:1:1:1, Scheme 1) to support the above-men-
tioned fact. It is worthy of note that synthesis of tetrahy-
drobenzo[b]pyrans involving organometallic moiety
3
5
ΔH = –90.11
ΔH = –2.21
Figure 1 Heat of formation for the product and the intermediates
(Scheme 2) calculated using RHF semi-empirical method using
MNDO-PM3 Hamiltonian.
Synlett 2011, No. 6, 791–796 © Thieme Stuttgart · New York

LETTER
Unexpected Formation of Tetrahydrobenzo[b]pyrans beyond Polyhydroquinolines
793
remained ignored, and hence we have focused our atten-
tion towards ferrocene carboxylaldehyde. Gratifyingly,
we achieved tetrahydrobenzo[b]pyran of ferrocene-2-
carboxylaldehyde (Scheme 3) in good yield (Table 1, en-
try 11).
Fe
CHO
O
O
CN
CN
CN
NH4OAc
+
+
Fe
EtOH
r.t.
R
R
O
NH2
O
Table 1 Synthesis of Tetrahydrobenzo[b]pyran Using Equimolar
R
R
Amount of Ammonium Acetate at Ambient Temperature
Scheme 3 Tetrahydrobenzo[b]pyran of ferrocene carboxylaldehyde
Entry Product 5a
Time (min)Yield (%)b,c
R1
Ammonium acetate acts as a catalyst for the present trans-
formation instead of a reactant, which led us to the devel-
opment of an alternative and more economical protocol
for the multicomponent synthesis of tetrahydroben-
zo[b]pyran using ammonium acetate as an inexpensive,
highly efficient, and easy to handle catalyst. Then efforts
have been taken for optimization of amount of ammonium
acetate, and it is found that 0.20 mmol of ammonium ace-
tate was required as a catalyst for the present transforma-
tion (entry 4, Table 2).
O
CN
R2
O
NH2
R2
1
2
3
4
5
6
7
R1 = H, R2 = Me
5a
5b
5c
5d
5e
5f
30
35
35
45
45
40
40
96
96
95
91
92
92
91
R1 = 4-MeO, R2 = Me
R1 = 4-Cl, R2 = Me
Having established the reaction conditions, various tet-
rahydrobenzo[b]pyrans were synthesized in excellent
yields. Several representative examples are summarized
in Table 3. In all cases, tetrahydrobenzo[b]pyran was the
only product and no byproduct was observed. Aromatic
aldehydes with substituents carrying either electron-
donating or electron-withdrawing groups reacted success-
fully and gave the products in high yields. To demonstrate
the scope of the procedure, the reaction of the heteroaro-
matic aldehyde viz. thiophene-2-carboxaldehyde was
studied, and the results are summarized in Table 3 (entries
8 and 9). It is noteworthy that variation in the 1,3-diketone
has also been successfully used for the synthesis of tet-
rahydrobenzo[b]pyrans.
R1 = 3,4-(MeO)2, R2 = Me
R1 = 4-i-Pr, R2 = Me
R1 = 3-O2N, R2 = Me
R1 = 2,5- Me 2, R2 = Me
5g
O
CN
8
9
R2
O
NH2
R2
R2 = Me
R2 = H
5h
5i
50
50
93
95
Table 2 Effect of Amount of NH4OAc on the Yield of Tetra-
hydrobenzo[b]pyran
Entry
Catalyst mmol
Time (min)
Yield (%)a
1
2
3
4
5
6
1
35
35
45
45
45
45
96
96
95
95
89
85
0.5
O
10
CN
0.25
0.20
0.15
0.10
R2
O
NH2
R2
R2 = Me
5j
50
85
Fe
a Reaction conditions: anisaldehyde (1 mmol), malononitrile (1
mmol), dimedone (1 mmol), EtOH (5 mL), r.t.
O
CN
11
R2
The identification of tetrahydrobenzo[b]pyrans 5a–k
(Table 1, entries 1–3) is unequivocally ascertained by 1H
NMR, 13C NMR, HRMS, and FT-IR spectral analysis.
The structure of 5i (Table 1, entry 9) was further con-
firmed by X-ray crystallography (Figure 2), which re-
vealed a unique characteristic of this type of compounds
in the solid state. According to the X-ray single-crystal
diffraction study, the geometry of crystal is monoclinic.
O
NH2
R2
R2 = Me
5k
90
82
a Reaction conditions: aldehyde:1,3-diketone/malononitrile/
NH4OAc = 1:1:1:1, r.t., EtOH (method A).
b All products showed satisfactory spectroscopic data (IR, 1H and 13
NMR, HRMS).
C
c Yields refer to pure, isolated products.
Synlett 2011, No. 6, 791–796 © Thieme Stuttgart · New York

794
K. A. Undale et al.
LETTER
The cyclohexane ring from the aldehyde adopts a chair
conformation while the cyclohexenone ring adjacent to
the pyran ring adopts a boat conformation, and the plane
of the cyclohexyl ring is orthogonal to central pyran ring
(the cyclohexyl ring makes a dihedral angle of 87.03u
with the plane through the pyran moiety).
Figure 4 Unit-cell structure of compound 5i (Table 1, entry 9)
Table 3 Ammonium Acetate Catalyzed Synthesis of Tetrahy-
drobenzo[b]pyran at Ambient Temperature
Entry Product 5a
Time (min) Yield (%)b,c
Figure 2 ORTEP diagram of 5i (Table 1, entry 9).
R1
O
CN
R2
O
NH2
R2
1
2
3
4
5
6
7
R1 = H, R2 = Me
5a
5l
40
50
92
92
89
90
93
88
89
R1 = 4- MeC6H4, R2 = Me
R1 = 3-ClC6H4, R2 = H
5m 40
R1 = 2,5-Me2C6H3, R2 = H
R1 = 4-OMeC6H4, R2 = Me
R1 = 4-i-PrC6H4, R2 = Me
R1 = 2-O2NC6H4, R2 = Me
5n
5b
5e
5f
40
45
50
50
S
O
CN
Figure 3 Hydrogen bonding in 5i (Table 1, entry 9)
R2
R2
O
NH2
Figure 4 indicates a unit cell structure of compound 5i
(Table 1, entry 9) involving eight molecules of it. It also
shows all possible H-bonding sites for the molecule. Sig-
nificant formation of the 12-membered macrocyclic ring
at the center takes place due to (N–H) H bonding
(Figure 3). N–H Hydrogen bonds possess a bond length of
2.185 Å, and the bond length of the O–H bond are 1.902
Å. These two types of hydrogen bonds are shown in
Figure 3
8
9
R1 = 2-thienyl, R2 = Me
5o
5p
40
50
90
88
R1 = 2-thienyl, R2 = H
a Reaction conditions: aldehyde/1,3-diketone/malononitrile/
NH4OAc = 1:1:1:0.20, r.t., EtOH (method B).
b All products showed satisfactory spectroscopic data (IR, 1H and 13
NMR, HRMS).
C
c Yields refer to pure, isolated products.
Synlett 2011, No. 6, 791–796 © Thieme Stuttgart · New York

LETTER
Unexpected Formation of Tetrahydrobenzo[b]pyrans beyond Polyhydroquinolines
795
H), 1.13 (s, 3 H), 2.17–2.24 (m, 2 H), 2.49–2.51 (m, 2 H), 4.53 (s, 1
H), 4.68 (s, 2 H, NH2), 7.49 (t, 1 H, J = 7.8 Hz), 7.70 (dd, 1 H,
J = 1.2, 7.8 Hz), 8.03–8.10 (m, 2 H).
In summary, we have devised a competitive multicompo-
nent synthesis of tetrahydrobenzo[b]pyrans over polyhy-
droquinolines in ethanol at ambient temperature under
Hantzsch reaction conditions. The methodology adopted
is justified on the basis of thinking and visualizing the re-
action pathways. The semi-empirical calculations of heat
of formation for the products and the intermediates have
been estimated to confirm the formation of tetrahydroben-
zo[b]pyrans instead of polyhydroquinolines in protic me-
dium. The detailed mechanism for competitive formation
of tetrahydrobenzo[b]pyrans over polyhydroquinolines is
suggested.
Table 1, Entry 10, 5j
IR (KBr): 3386, 3297, 3246, 3183, 2960, 2185, 1681, 1651, 1607,
1373, 1217, 1141 cm–1. 1H NMR (300 MHz, CDCl3): d = 1.09 (s, 3
H), 1.12 (s, 3 H), 2.29 (s, 2 H), 2.39 (s, 2 H), 4.08 (d, 1 H), 4.57 (br
s, 2 H, NH2), 6.12 (dd, 1 H, J = 6.9, 15.9 Hz), 6.50 (d, 1 H, J = 15.9
Hz), 7.20–7.63 (m, 5 H).
Method B (for Table 3)
Ammonium Acetate Catalyzed Synthesis of Tetrahydrobenzo-
[b]pyrans
A mixture of aldehyde (1 mmol), malononitrile (1 mmol), and 1, 3-
diketone (1 mmol), NH4OAc (0.20 mmol) in EtOH (5 mL) was
stirred at r.t. for the time indicated in Table 3. The reaction mixture
was poured into ice water and just filtered to yield corresponding
tetrahydrobenzo[b]pyran. The residue was purified by recrystalliza-
tion in EtOH to get the desired product, i.e., tetrahydrobenzo[b]-
pyran (5).
General
IR spectra were recorded on a Perkin-Elmer FT-IR 783 spectropho-
tometer. NMR spectra were recorded on a BrukerAC-300 spectro-
meter in CDCl3 using TMS as internal standard, HRMS spectra
were recorded on Q-TOF mass spectrometer.
Spectral Data of Compounds Synthesized by Method B
New Compounds
Table 3, Entry 3, 5m
Typical Procedure
Method A (for Table 1)
Competitive Synthesis of Tetrahydrobenzo[b]pyrans
A mixture of aldehyde (1 mmol), malononitrile (1 mmol), and 1,3-
diketone (1 mmol), NH4OAc (1 mmol) in EtOH (5 mL) was stirred
at r.t. for the time indicated in Table 1. The reaction mixture was
poured into ice water and just filtered to yield corresponding tet-
rahydrobenzo[b]pyran. The residue was purified by recrystalliza-
tion in EtOH to obtain the desired product, i.e.,
tetrahydrobenzo[b]pyran 5.
IR (KBr): 3311, 3152, 2926, 2196, 1681, 1643, 1364, 1213, 1165,
1
1001 cm–1. H NMR (300 MHz, CDCl3): d = 2.05–2.11 (m, 2 H),
2.37–2.41 (m, 2 H), 2.60 (m, 2 H), 4.41 (s, 1 H), 4.58 (s, 2 H, NH2),
7.19–7.26 (m, 4 H). 13C NMR (75 MHz, CDCl3): d = 19.51, 26.46,
34.70, 36.18, 99.45, 125.69, 126.93, 126.99, 129.28, 162.91,
195.29. HRMS (TOF-MSES+): m/z [M
+
Na]+ calcd for
C16H13O2N2ClNa: 323.0563; found: 323.0564.
Table 3, Entry 4, 5n
Spectral Data of Compounds Synthesized by Method A
New Compounds
IR (KBr): 3474, 3313, 3211, 3179, 2924, 2189, 1683, 1661, 1363,
1206, 1161 cm–1. 1H NMR (300 MHz, CDCl3): d = 2.00–2.09 (m, 2
H), 2.25 (s, 3 H), 2.32–2.38 (m, 2 H), 2.53 (s, 3 H), 2.58–2.67 (m, 2
H), 4.46 (s, 2 H, NH2), 4.65 (s, 1 H), 6.73 (s, 1 H), 6.88 (dd, 1 H,
J = 1.2, 7.8 Hz), 7.00 (d, 1 H, J = 7.5 Hz). 13C NMR (75 MHz,
CDCl3): d = 18.28, 19.39, 20.37, 26.16, 30.31, 35.97, 63.07, 99.19,
115.10, 127.02, 127.23, 129.59, 131.93, 134.81, 140.86, 156.21,
162.39, 195.22. HRMS (TOF-MSES+): m/z [M + Na]+ calcd for
C18H18O2N2Na: 317.1266; found: 317.1269.
Table 1, Entry 9, 5i
IR (KBr): 3358, 3171, 2921, 2187, 1679, 1651, 1384, 1212, 999 cm–1.
1H NMR (300 MHz, CDCl3): d = 0.87–1.76 (m, 11 H), 1.96–2.11
(m, 2 H), 2.28–2.58 (m, 4 H), 3.32 (s, 1 H), 4.50 (br s, 2 H, NH2).
13C NMR (75 MHz, CDCl3): d = 20.19, 26.18, 26.28, 26.53, 26.99,
2755, 30.39, 34.62, 36.96, 43.73, 59.03, 115.23, 159.71, 164.78,
196.57. HRMS (TOF-MSES+): m/z [M
+
Na]+ calcd for
C16H20O2N2Na: 295.1422; found: 295.1379.
Table 3, Entry 9, 5p
Table 1, Entry 11, 5k
IR (KBr): 3364, 3320, 3177, 2956, 2924, 2191, 1682, 1651, 1664,
1360, 1209, 1163 cm–1. 1H NMR (300 MHz, CDCl3): d = 2.03–2.60
(m, 2 H), 4.60 (s, 2 H, NH2), 4.80 (s, 1 H), 6.90–6.99 (m, 2 H), 7.14
(d, 1 H). 13C NMR (75 MHz, CDCl3): d = 19.76, 26.71, 30.00,
36.45, 62.83, 99.72, 115.00, 118.24, 124.14, 124.64, 126.75,
147.43, 157.60, 162.89, 195.60. HRMS (TOF-MSES+): m/z [M +
Na]+ calcd for C14H12O2N2SNa: 295.0517; found: 295.0513.
IR (KBr): 3378, 3177, 2959, 2185, 1678, 1645, 1370, 1216, 1030,
1002 cm–1. 1H NMR (300 MHz, CDCl3): d = 0.96 (s, 3 H), 1.08 (s,
3 H), 2.26 (s, 2 H), 2.33 (s, 2 H), 3.87–4.21 (m, 9 H), 4.36 (s, 1 H),
4.62 (s, 2 H, NH2). 13C NMR (75 MHz, CDCl3): d = 27.21, 28.43,
29.05, 32.15, 40.60, 50.74, 60.99, 65.75, 66.71, 66.99, 68.12, 69.15,
93.11, 99.99, 116.29, 159.90, 161.67, 196.28. HRMS (TOF-
MSES+): m/z [M + Na]+ calcd for C22H22O2N2FeNa: 425.0928;
found: 425.0925.
Acknowledgment
Known Compounds
Table 1, Entry 4, 5d
One of the authors DMP wish to thank DST [New Delhi], Govt. of
India for award of BOYSCAST Fellowship [SR/BY/C-02/09].
IR (KBr): 3389, 3329, 3255, 3220, 2958, 2940, 2894, 2840, 2198,
1678, 1657, 1365, 1218, 1163 cm–1. 1H NMR (300 MHz, CDCl3):
d = 1.06 (s, 3 H), 1.12 (s, 3 H), 2.24 (s, 2 H), 2.45 (s, 2 H), 2.84 (s,
3 H), 3.87 (s, 3 H), 4.36 (s, 1 H), 4.50 (s, 2 H, NH2), 6.73–6.81 (m,
3 H).
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1349, 1251, 1137 cm–1. 1H NMR (300 MHz, CDCl3): d = 1.05 (s, 3
Synlett 2011, No. 6, 791–796 © Thieme Stuttgart · New York

796
K. A. Undale et al.
LETTER
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Synlett 2011, No. 6, 791–796 © Thieme Stuttgart · New York

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