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MISHRA et al.
latter undergoes cyclization to tetrahydro-1-benzo-
pyran. This successfully confirms the key role of
ZrRHSi in the synthesis of various tetrahydro-1-
benzopyrans.
and the solid precipitate was filtered off. The catalyst
was recovered by washing the solid residue with ethyl
acetate. The crude product obtained from the ethyl
acetate layer was subjected to silica gel column chro-
matography (ethyl acetate–petroleum ether, 3:7). The
recovered solid catalyst was kept aside for reuse.
In summary, a reliable and practical method has
been developed for the synthesis of tetrahydro-1-
benzopyran derivatives. Environmentally benign nature
of ZrRHSi, economic aspects, reusability, and multi-
component reaction conditions are the other remarkable
advantages of the proposed methodology. We feel that
this economically viable methodology will prove to be
a greener and good alternative to the existing methods
for the synthesis of tetrahydro-1-benzopyrans.
2-Amino-4-(4-methoxyphenyl)-7,7-dimethyl-5-
oxo-5,6,7,8-tetrahydro-4H-1-benzopyran-3-carboni-
1
trile (4e). Yield 89%, white solid, mp 193°C. H NMR
spectrum, δ, ppm: 1.03 s (3H), 1.11 s (3H), 2.21 d (2H,
J = 4 Hz), 2.44 s (2H), 3.76 s (3H), 4.36 s (2H), 6.81 d
(
7
3
2H, Harom, J = 4 Hz), 7.15 d (2H, Harom, J = 8 Hz),
1
3
.26 s (2H). C NMR spectrum, δ , ppm: 27.66,
C
2.16, 34.74, 40.67, 50.69, 55.19, 63.88, 76.68, 113.97,
EXPERIMENTAL
118.67, 128.6, 135.45, 157.33, 158.61, 161.19, 195.89.
2
-Amino-4-(3,5-dibromophenyl)-7,7-dimethyl-5-
All chemicals and solvents used were commercial
products and were used without further purification.
The crystal structure of the catalyst was characterized
by X-ray powder diffraction using a PANalytical Xpert
oxo-5,6,7,8-tetrahydro-4H-1-benzopyran-3-carboni-
trile (4j). Yield 92%, white solid, mp 217°C. H NMR
spectrum, δ, ppm: 0.99–1.22 m (6H), 2.39 s (2H),
,59–2.71 m (2H), 4.07 d (1H, J = 4 Hz), 5.83 s (1H,
1
2
Pro X-ray diffractometer (Cu K radiation, λ 0.154 nm;
α
NH ), 6.23 s (1H, Harom), 7.42 s (1H, Harom), 7.69 s
2
scanning rate 2 deg/min). Scanning electron micro-
graph was obtained from Quanta ESCM, FEI instru-
ment with EDAX. Thermal analysis of the catalyst was
performed in a DSC-TG Analyzer Standard instrument
1
3
(
2
1
1
1H, Harom). C NMR spectrum, δ , ppm: 26.93,
7.35, 32.14, 41.23, 44.69, 50.52, 76.68, 108.53,
12.23, 116.13, 121.98, 129.90, 135.52, 146.9, 164.53,
67.31, 197.01.
C
(
1
SDT Q600, V20.9, Build 20) at a heating rate of
0 deg/min in a nitrogen atmosphere. Thin layer chro-
2-Amino-7,7-dimethyl-4-(naphthalen-1-yl)-5-
matography (silica gel F254 plates) was used to monitor
the reaction progress; TLC plates were visualized
oxo-5,6,7,8-tetrahydro-4H-1-benzopyran-3-carbo-
nitrile (4m). Yield 85%, white solid, mp 253°C.
H NMR spectrum, δ, ppm: 1.07 s (3H), 1.13 s (3H),
1
13
1
under a UV lamp. The H and C NMR spectra were
recorded on a Bruker AMX instrument (400 MHz for
2.16–2.27 m (2H), 2.51–2.53 m (2H), 4.48 s (2H),
1
H and 100 MHz for 13C) using DMSO-d as solvent
13
5.27 s (1H), 7.23–7.81 m (7H). C NMR spectrum, δ ,
6
C
and tetramethylsilane as reference.
ppm: 27.83, 28.85, 30.66, 32.19, 40.73, 50.57, 64.20,
7
1
6.68, 114.64, 118.54, 122.28, 123.26, 129.41, 134.89,
57.39, 161.73, 195.76.
Zirconia supported on rice husk silica (ZrRHSi).
The raw material, rice husk, was collected from a local
rice mill (Bangalore, India). The husk was mixed with
concentrated aqueous HCl, and the mixture was
refluxed for 3 h. The treated husk was washed with
water, dried at 110°C for 12 h, and burned at 600°C in
a muffle furnace for 6 h to get amorphous white silica.
Colloidal silica was mixed with a required amount of
a zirconyl nitrate solution, and aqueous ammonia (1:1)
was added dropwise to this mixture until precipitation
was complete. The precipitate was washed, dried, and
calcined at 500°C for 5 h.
2-Amino-4-(2-hydroxyphenyl)-7,7-dimethyl-5-
oxo-5,6,7,8-tetrahydro-4H-1-benzopyran-3-carbo-
nitrile (4o). Yield 87%, white solid, mp 214°C.
1
H NMR spectrum, δ, ppm: 1.03 s (3H), 1.11 s (3H),
2
.21 d (2H, J = 4 Hz), 2.44 s (2H), 4.35 s (2H), 4.85 s
(1H), 6.73 d (2H, Harom, J = 8 Hz), 7.10 d (2H, Harom,
13
J = 8 Hz), 7.26 s (2H). C NMR spectrum, δ , ppm:
C
2
1
1
7.67, 28.84, 30.9, 34.74, 40.69, 50.7, 76.68, 114.64,
18.54, 122.28, 123.26, 129.41, 134.89, 157.39,
61.73, 195.76.
General procedure for the synthesis of tetra-
hydro-1-benzopyrans. A mixture of aromatic alde-
hyde (1 mmol), dimedone (2 mmol), malononitrile
ACKNOWLEDGMENTS
(
1 mmol), ZrRHSi (0.1 g), and ethanol (15 mL) was
The authors gratefully acknowledge CHRIST (Deemed
stirred at 80°C for 2 h. The progress of the reaction was
monitored by TLC using ethyl acetate–hexane (3:7) as
eluent. The mixture was diluted with 10 mL of water,
to be University) for the support and for the facilities to carry
out this project. We are thankful for IISc and St. Joseph’s
College, Bangalore, for NMR, SEM, and XRD analysis.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 10 2020