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                As our
environment is endowed by nature, needs to be protected from growing production
of large amount of waste and toxic by-products which sequentially leads to chemical
pollution. Therefore, synthetic chemists have earned tremendous interest to
develop relatively safe technologies which play a vital role in green chemistry.
By concerning above fact, establishing newer chemical transformations should satisfy
green principles such as non-toxicity, non-flammability, easy work-up, eco-friendly
medium, separation and recycling of the catalysts. Since, from the last decade more
efforts were devoted towards the design of an environment friendly chemical
synthesis with respect to reagents, environmentally benign solvents that could
be easily biodegradable 1, 2. Multi-component reaction (MCR) strategies which
have been widely used in the convergent synthesis of complex organic molecules
from simple and readily available starting materials with high atom economy and
high selectivity is one of the tool to achieve both economic and environmental
goals. Therefore, synthesis of heterocyclic compounds with significant
bioactivities with MCR support is an immensely important pursuit in organic
synthesis.

                Synthesis of
acridines
is an enormous area of interest due to polyfunctionalized group with wide range
of biological activities as well as an ecological point of view 3. Among
them, 1, 8-dioxodecahydroacridines is an important class of aza-heterocycles in
which a phenyl substituted pyridine ring is fused with two cyclohexanone rings.
These structure contains 1, 4-dihydropyridine (1,4-DHP) as a parent nucleus
which acts as fluorescent probes in bioanalytical chemistry 4 and also used as
potential drug candidates for the treatment of cardiovascular diseases. Some of
the representative compounds of this class are used in dye-sensitized solar
cells and also in the preparation of blue light-emitting devices 5-6. In
addition, the 9-aryl-decahydroacridine-1,8-dione derivatives have been widely
employed as DNA intercalator, SIRT1 inhibitors, calcium and potassium channel
modulators 7-8. Several studies reveal that these heterocycles exhibits copious
medicinal applications which include antitumor, calcium -blockers, antileukimic,
antifungal, anticancer, anti-atherosclerotic, and bronchodilator 9-13. These
are also used as, laser dyes, chemosensors and initiators in photo
polymerization process. These derivatives are important due to their structural
similarity with the coenzyme nicotinamide adenine dinucleotide (NADH), which
acts as coenzyme in biological systems.

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                The
most common route for the synthesis of 1,8-dioxodecahydroacridines includes
condensation of diverse aldehydes, dimedone or cyclic 1, 3-dicarbonyl compounds
with various nitrogen source such as ammonium acetate, urea, ammonium
hydroxide, ammonium bicarbonate, and hydroxylamine 14-18. A variety of
catalysts such as sulfonated polyethylene glycol
(PEG-OSO3H), Silzic (SiO2-ZnCl2), silica
boron-sulfuric acid, Proline, Zn(OAc)2, nano nickel cobalt ferrite(Ni0.5Co0.5Fe2O4),
Carbon based solid acid, Bronsted acidic imidazolium salts, Ascorbic acid,
Acetic acid, Tris(pentafluorophenyl)borane/B(C6F5)3,
Silica-Supported polyphosphoric acid,
ammonium chloride, Silica-supported preyssler
nanoparticles 19-32 etc have been reported to accomplish this
transformation. However, most of these reported methodologies have certain drawbacks such
as use of toxic and corrosive solvent, use of expensive chemicals, tedious
preparation of catalyst, prolonged reaction times, tedious work-up, harsh
reaction conditions and low yields of the desired product. Therefore, a great demand
still exists for utilization of an efficient, simple and eco-friendly process
especially by using organocatalyst is highly desirable.

                Citric
acid (2-hydroxy-propane-1, 2, 3-tricarboxylic acid) is a weak organic acid with
the formula C6H8O7 and was first isolated and
crystallized from lemon juice in 1784. It is found as natural preservative and
antioxidant in variety of citrus fruits orange, lemon, pineapple, peach and
pear. This organic acid is a nearly universal intermediate product of
metabolism. Furthermore, citric acid is also used for the preparation of salt
and form complex with many metals such as magnesium, iron, manganese, calcium
and copper. Widespread presence, non-toxic nature and chemical stability of
this acid, it has been used as sequestering in industrial process, as softener
in detergent, as an anticoagulant blood preservative and as a complexing agent
in metal treatment. Other industrial and pharmaceutical applications of citric
acid include antioxidant in cosmetics, cleaning, buffering. Despite its huge
industrial and pharmaceutical importance, only a few reports exemplify its catalytic
application in organic synthesis.

                As part of our research work in the development of sustainable
methodologies for the synthesis of bioactive moiety 33-38, herein we report
green protocol for the synthesis of 1,8-dioxodecahydroacridines
from one pot multi-component reaction
of dimedone and NH4OAc with diverse aryl aldehydes in the presence
of inexpensive and highly efficient citric acid as organocatalyst (Scheme 1).

Introduction:

                As our
environment is endowed by nature, needs to be protected from growing production
of large amount of waste and toxic by-products which sequentially leads to chemical
pollution. Therefore, synthetic chemists have earned tremendous interest to
develop relatively safe technologies which play a vital role in green chemistry.
By concerning above fact, establishing newer chemical transformations should satisfy
green principles such as non-toxicity, non-flammability, easy work-up, eco-friendly
medium, separation and recycling of the catalysts. Since, from the last decade more
efforts were devoted towards the design of an environment friendly chemical
synthesis with respect to reagents, environmentally benign solvents that could
be easily biodegradable 1, 2. Multi-component reaction (MCR) strategies which
have been widely used in the convergent synthesis of complex organic molecules
from simple and readily available starting materials with high atom economy and
high selectivity is one of the tool to achieve both economic and environmental
goals. Therefore, synthesis of heterocyclic compounds with significant
bioactivities with MCR support is an immensely important pursuit in organic
synthesis.

                Synthesis of
acridines
is an enormous area of interest due to polyfunctionalized group with wide range
of biological activities as well as an ecological point of view 3. Among
them, 1, 8-dioxodecahydroacridines is an important class of aza-heterocycles in
which a phenyl substituted pyridine ring is fused with two cyclohexanone rings.
These structure contains 1, 4-dihydropyridine (1,4-DHP) as a parent nucleus
which acts as fluorescent probes in bioanalytical chemistry 4 and also used as
potential drug candidates for the treatment of cardiovascular diseases. Some of
the representative compounds of this class are used in dye-sensitized solar
cells and also in the preparation of blue light-emitting devices 5-6. In
addition, the 9-aryl-decahydroacridine-1,8-dione derivatives have been widely
employed as DNA intercalator, SIRT1 inhibitors, calcium and potassium channel
modulators 7-8. Several studies reveal that these heterocycles exhibits copious
medicinal applications which include antitumor, calcium -blockers, antileukimic,
antifungal, anticancer, anti-atherosclerotic, and bronchodilator 9-13. These
are also used as, laser dyes, chemosensors and initiators in photo
polymerization process. These derivatives are important due to their structural
similarity with the coenzyme nicotinamide adenine dinucleotide (NADH), which
acts as coenzyme in biological systems.

                The
most common route for the synthesis of 1,8-dioxodecahydroacridines includes
condensation of diverse aldehydes, dimedone or cyclic 1, 3-dicarbonyl compounds
with various nitrogen source such as ammonium acetate, urea, ammonium
hydroxide, ammonium bicarbonate, and hydroxylamine 14-18. A variety of
catalysts such as sulfonated polyethylene glycol
(PEG-OSO3H), Silzic (SiO2-ZnCl2), silica
boron-sulfuric acid, Proline, Zn(OAc)2, nano nickel cobalt ferrite(Ni0.5Co0.5Fe2O4),
Carbon based solid acid, Bronsted acidic imidazolium salts, Ascorbic acid,
Acetic acid, Tris(pentafluorophenyl)borane/B(C6F5)3,
Silica-Supported polyphosphoric acid,
ammonium chloride, Silica-supported preyssler
nanoparticles 19-32 etc have been reported to accomplish this
transformation. However, most of these reported methodologies have certain drawbacks such
as use of toxic and corrosive solvent, use of expensive chemicals, tedious
preparation of catalyst, prolonged reaction times, tedious work-up, harsh
reaction conditions and low yields of the desired product. Therefore, a great demand
still exists for utilization of an efficient, simple and eco-friendly process
especially by using organocatalyst is highly desirable.

                Citric
acid (2-hydroxy-propane-1, 2, 3-tricarboxylic acid) is a weak organic acid with
the formula C6H8O7 and was first isolated and
crystallized from lemon juice in 1784. It is found as natural preservative and
antioxidant in variety of citrus fruits orange, lemon, pineapple, peach and
pear. This organic acid is a nearly universal intermediate product of
metabolism. Furthermore, citric acid is also used for the preparation of salt
and form complex with many metals such as magnesium, iron, manganese, calcium
and copper. Widespread presence, non-toxic nature and chemical stability of
this acid, it has been used as sequestering in industrial process, as softener
in detergent, as an anticoagulant blood preservative and as a complexing agent
in metal treatment. Other industrial and pharmaceutical applications of citric
acid include antioxidant in cosmetics, cleaning, buffering. Despite its huge
industrial and pharmaceutical importance, only a few reports exemplify its catalytic
application in organic synthesis.

                As part of our research work in the development of sustainable
methodologies for the synthesis of bioactive moiety 33-38, herein we report
green protocol for the synthesis of 1,8-dioxodecahydroacridines
from one pot multi-component reaction
of dimedone and NH4OAc with diverse aryl aldehydes in the presence
of inexpensive and highly efficient citric acid as organocatalyst (Scheme 1).

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