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Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Harvesting bio-waste: A sustainable route towards water promoted synthesis of 2-amino-4H-chromene using Annona squamosa peel ash as a heterogeneous catalyst

Uttam P. Patil,*a Suresh S. Patil b and Author Full Name c
Natural feedstock, Annona squamosa peel ash a heterogeneous catalyst alternative to toxic and corrosive reagents for room temperature synthesis of 2-amino-4H-chromene derivatives in water is described. A. squamosa peel ash has been characterized by FT-IR, XRD, SEM, EDS, and DSC-TGA. These analytical tools prominently highlighted the basic sites in the catalyst that may be intensely responsible to accelerate the reactions. Clean and excellent quality products with higher yield are the characteristic features of present methodology.

Annona squamosa peel ash, natural catalyst, water, 2-amino-4H-chromene
Water has a significant role as it is universal solvent for all chemical reactions of life.1 The major aspect for water as a reaction media came from the studies on ‘Hydrophobic effects on simple organic reactions in the water’ by Ronald Breslow in 1980s.2 Sharpless and co-workers recently defined ‘on water’ conditions using water as a solvent for the reaction of water-insoluble reactants without the use of organic cosolvent.3 Another study done by Engberts has shown that the Diels-Alders cycloaddition reaction accelerated very rapidly in water.4 Notably, use of water as a solvent has attracted much interest in recent years. Indeed, water offers many advantages because it is a cheap, readily available, non-toxic and non-flammable solvent, thus, being very attractive from both an economic and environmental point of view.5
One–pot multicomponent reaction strategies offer significant advantages over conventional linear-type syntheses by virtue of their convergence, productivity, facile execution and high yield.6 Because of these advantages, developing the new reaction (MCR) with environmentally benign methods has been recognized as one of the most important topics of green chemistry.7
Sustainability has become a watchword and guiding principle for modern society and with it a growing appreciation that anthropogenic ‘waste’, in all its manifold forms, can offer a valuable source of energy, construction materials, chemicals and high-value functional products. In the context of chemical transformations, waste materials not only provide alternative renewable feedstock, but also a resource from which to create catalysts. Such waste-derived heterogeneous catalysts serve to improve the overall energy and atom-efficiency of existing and novel chemical processes.8
In order to achieve the goal of green synthetic schemes, the implication of natural feedstock as a catalyst can become an alternative to toxic and corrosive reagents and can alter the rate of reaction efficiently and tend to the desired product within a short time. The natural feedstock is unprocessed or minimally processed bio-waste material. Recently, it has been successfully employed as catalysts by many researchers in organic transformations. Foster A. Agblevor used red mud as a bulk catalyst to replace zeolites in making of crude oil from pyrolyzed biomass.9 Water extract of banana peel ash, 10 water extract of rice straw ash, 11 Lucas reagent prepared from zinc hyper accumulating plant Thlaspi caerulescens,12 lemon juice,13 orange peel,14 papaya plant bark ash extract,15 etc. have been used as catalysts from natural feedstock material.
2-amino-4H-chromene derivatives find a broad range of applications in the field of biology, medicinal chemistry and pharmacology. 4H-pyrans and 4H-pyran-annulated heterocyclic scaffolds, viz; 4H-chromene moieties are the key building block of the numerous oxygen containing heterocyclic natural compounds possessing distinct properties of general interest.16 This structural motif is broadly represented by several types of alkaloids manifesting diverse biological and pharmacological activities including antitumor,17 anti-HIV,18 antimicrobial and anti-fungal,19 anti-inflammatory,20 anti-allergenic,21 and anti-neurodegenerative disorders such as Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s diseases (HD).22 Besides, several of these functionalized 4H-pyran derivatives exhibit anti-cancer activities and act as anti-cancer agent.23 The chromene moiety widely used for the treatment of rheumatoid, cancer and psoriasis,24 in pigment and dye industries these derivatives used in laser dyes,25 optical brighteners,26 fluorescence makers27 and potent biodegradable agrochemicals.28 These attractive features of 4-H-pyran motivated scientist community to design and develop natural product derived analogues of alkaloids for examples, arisugacin A,29 amarogentin,30 veprisine,31 ophioglonin,32 hyperxanthone33 (Scheme 1) and many others.
In pursuance of widespread applicability of 2-amino-4H-chromene derivatives, considerable attention has been focused on the development of clean, efficient and environmentally friendly methodologies to synthesize 2-amino-4H-chromene derivatives via one-pot three-component reactions. Literature survey reveals that, very limited methodologies have been reported for the synthesis of 2-amino-4H-chromene derivatives using various catalysts such as piperidine,34 Triethylamine,35 H6P2W18O62.18H2O,36 DBU,37 TBAB,38 (CTA)3SiW14-Li+-MMT,39 Al2O3,40 MgO,41 AuCl3,42 aqueous K2CO3,43 ultrasound irradiation in absence of catalyst. 44
The findings for more general, clean, efficient, environmentally friendly and high yielding routes using a safer catalyst to this class of oxygen containing heterocycles remains a valid exercise. With this broad view in mind and as a part of our continuing efforts to generate green synthetic route convenient and suitable for organic transformations, herein, we wish to report a simple, straightforward, clean, efficient and high yielding one pot multi-component protocol for facile synthesis of densely functionalized 2-amino-4H-chromene derivatives from the reaction of aromatic aldehydes, malononitrile or ethyl cyanoacetate and enolizable C-H activated acidic compounds in water at room temperature using Annona squamosa peel ash as an inexpensive and environmentally safe industrial waste natural feedstock as a heterogeneous catalyst (Scheme 2.). To the best of our knowledge, this is the first method employed for the synthesis of 2-amino-4H-chromene derivatives using bio-waste, mild, efficient Annona squamosa peel ash as a catalyst in water at room temperature without the addition of any external base, promoter or additives. In this communication, we report on the use of A. squamosa peel ash as an all in one mixture of catalyst, base, additives/ promoters for the synthesis2-amino-4H-chromene derivatives.

Annona squamosa L. (Fig. 1a) is known as sugar apple or custard apple and it belongs to the annonaceae family which has tremendous medicinal applications. Fruit is edible and consumed on large scale. Its pulp has the worldwide market. The fruit processing industries extract out the pulp of the fruits and generate a huge amount of waste such as peel, stalk, seeds, and other residual parts. This large volume of bio-waste is dumped on soil near the production site, incinerated or burnt, which can cause environmental pollution. Considering outstanding properties of the plant parts and the waste disposal problem, in continuation of our ongoing research work, we thought to use this bio-waste material in the development of the green synthetic route for organic transformations.

In this study, the fruit waste material collected from the fruit processing industry site, separated peels were washed with dist. water and sun dried. The dried peels (Figure. 1b) cut into pieces and burnt in air. The peel ash (fig. 1c) of A. squamosa was characterized by FT-IR, XRD, SEM-EDS, and DSC-TGA.
All reagents were purchased from Sigma-Aldrich and used as received without further purification. FT-IR spectra were recorded on Bruker (Alpha 100508) and measured using KBr plate and wave numbers (?) are reported in cm-1. 1H and 13C NMR spectra of compounds were recorded on AVANCE-300 spectrometer. Chemical shifts (?) introduced in parts per million (ppm) using the residue solvent peaks as the reference relative to TMS. Pre-coated plates of Silica gel 60 F254 were used for thin layer chromatography (TLC). Powder XRD measurement recorded on XPERT-PRO X-Ray Powder Diffractometer (PANalytical, Pretoria, S.A.) using Cu K? radiation (?=1.5406Å) with scattering angle (2 theta). The microscopic morphology of ash was examined by JEOL 6380A Scanning electron microscopy (SEM-EDS)
Preparation of catalyst
Collected Annona squamosa fruit peels from local fruit processing industry were thoroughly washed with distilled water and sun dried. It was then cut into small pieces and burnt in the open air until the maximum reduction of carbonaceous material from the ash and obtained brown coloured ash ground to develop ash catalyst.
General procedure for the synthesis of 2-amino-4H-chromene derivatives
A mixture of enolizable compound (1 mmol), aromatic aldehyde (1 mmol), malononitrile or ethyl cyanoacetate (1.2 mmol) and catalyst (75 mg) in water (3 mL) was stirred at room temperature. After completion of the reaction (indicated by TLC), the reaction mixture was filtered off and washed with water. The obtained product was recrystallized from ethanol to get pure product. Synthesized compounds were characterized by 1H-NMR, 13C-NMR and FT-IR spectra.

Result and discussion
Catalyst characterization
a) FT-IR analysis
For identification of functional groups, FT-IR spectra of Annona squamosa peel ash were recorded (figure 2). The characteristic absorption band in the region 3742 and 3672 cm-1 are assigned to O-H stretching mode of freely vibrating hydroxyl groups on the surface of metals that may be attributed to K-O, Al-O, and Mg-O. The bands observed at 1699 and 1651 cm-1 are assigned to the C-O stretching vibration due to the presence of metal carbonates probably by absorption of atmospheric CO2 on the surface of metal oxides.

The band at 1448 cm-1 corresponds to O-H bending. Compared with standard data, the peaks in the region 1364 and 672 cm-1 corresponds to K2CO3 and peaks at 1106, 1017 and 876 cm-1 can be attributed to oxides of Al, Mg, and Zr. FT-IR absorption spectra highlight the presence of metal hydroxides and carbonates in the A. squamosa ash catalyst.
b) SEM-EDS analysis of Annona squamosa peels ash.

The morphology of A. squamosa peel ash was performed by scanning electron microscope (SEM) (figure 3). It showed irregular particles with heterogeneous morphology. The ash particles have irregular cavities, porous surface with smooth edges, agglomerated and flower like appearance. Material with this type of morphology generally has a high specific surface which provides good absorption capacity. This type of nature of peel ash facilitates organic transformation very smoothly and effectively.

The EDS spectrum of peel ash is depicted in figure 4. The EDS analysis of peel ash has shown that the K (66.09%) is a major element while Pt (1.39%), Al (0.90%) and Mg (0.78%) are minor elements in peel ash. It strongly supports basic sites in the catalyst.

c) XRD analysis of Annona squamosa peels ash.

The XRD of A. squamosa peel ash (figure 5) has shown the strong characteristic peaks of potassium oxide, potassium carbonate and other potassium salts at 2? = 24.14, 28.39, 29.87, 32.15, 33.88, 37.04, 39.27, 44.89, 50.21, 62.37, 66.40, 73.68, 87.60 (COD Ref. Code. 96-151-8208, 96-900-9646). The peaks at 2? = 20.63, 30.42, 32.15, 33.88, 40.54, 41.48, 44.18, 49.26, 56.42, 58.62, 62.37, 70.27, 88.83 (COD Ref. Code. 96-210-0389, 96-900-6791, 96-110-1169, 96-151-2557) (XRD pattern was verified using XPert Highscore Plus software) correspond to ZrO2, Al2O3, MgO and PtO2 in ash catalyst. Thus, XRD analysis indicated that the catalyst contains oxides and carbonates of K, Al, Zr, Mg, and Pt, which was also strongly supported by EDS analysis that probably responsible for striking basicity of the solid-catalyst.

d) DSC-TGA analysis of Annona squamosa peels. Thermogravimetric analysis was carried out to examine the thermal degradation of A. squamosa peel powder and its blend to determine the composition of the feedstock. DSC-TGA curves of peel powder are depicted in figure 6. According to the obtained thermal profile, the degradation of peel powder occurred in three steps associated with hemicellulose, cellulose and lignin which can be clearly distinguished up to 536.82 oC. The sample mass decreases continuously between room temperature to 536.82 oC which is related to dehydration and degradation of peel powder. The weight loss at 184.73 oC, 362.29 oC, and 536.82 oC can be assigned to hemicellulose, cellulose, and pectin. Thermal degradation after 536.82 oC was not observed in the graph which demonstrates the left part of the sample is containing ash associated with minerals in the form of oxides, carbonates or other salts in higher concentrations.
In order to access the efficiency of A. squamosa ash catalyst and to screen the reaction conditions for the synthesis of 2-amino-4-(4-chlorophenyl)-5-oxo-4H,5H-pyrano3,2-cchromene-3-carbonitrile (4h) under different variables affecting the yield, we have chosen a reaction of 4-hydroxycoumarin (1) with 4-chlorobenzaldehyde (2) and malononitrile (3) (molar ratio: 1:1:1.2) as a model reaction. The results are shown in Table 1. In our preliminary exercise, the model reaction was performed in absence of solvent and catalyst and the reaction was run at room temperature (Table1, entry 1, 2) however, the product was not detected even after heating at 100oC. Further, the trial reaction was carried out in water as a solvent at room temperature and also by refluxing it; a trace amount of yield was obtained (Table 1, entry 3, 4). Then, the reaction was run in various solvents such as toluene, DCM, CHCl3 and CH3CN (Table 1, entry 5-7) and the reaction mixture was refluxed. Unfortunately, the resulting yield was very poor. Moreover, Annona squamosa peel ash was introduced and the model reaction was carried out at room temperature in solvent-free condition and reflux condition; yield was increased at a certain level (Table 1, entry 8, 9). But when the reaction was run in water, the yield was increased from moderate to a good level. Then, to optimize the reaction condition, various amounts of catalyst was loaded. It was found that 75 mg amount of catalyst in 3 mL water and room temperature stirring conditions are effective to model reaction (94% yield) (Table 1, entry 12). Further increase in the amount of catalyst found to be less effective, it may be due to the formation of stable protonated species owing to strong alkaline nature of the catalyst.

To account the scope and generality of the protocol, the reaction of 4-hydroxycoumarin (1) with malononitrile or ethyl cyanoacetate (2) and a broad range of electron withdrawing and electron donating groups were performed in presence of A. squamosa peel ash under optimized reaction conditions. The results are exhibited in Table 2. The reactions were carried out on 1 mmol scale and it was found that there was no change in yield when reactions were carried out on 5 mmol scale. This highlights present protocol can be applied to large scale production. Another remarkable aspect is that use of malononitrile (Table 2, entry 4a-4t) gives the product in short time span than that of ethyl cyanoacetate (Table 2, entry 4u-4y).
In order to expand the applicability of present catalyst to the other reactions of these categories, a series of 2-amino-4H-chromene derivatives (4aa-4ap) were prepared from the reaction of resorcinol (1), aromatic aldehydes (2) and malononitrile (3) under same reaction conditions. The results are depicted in Table 3.

Retention and ability of recyclability is the major challenge of the catalytic system containing metal hydroxides, which finds the significant application and avoids metal contamination to the final products. The reusability and efficiency of the catalyst were investigated on the model reaction (Table 4). The reaction was carried out for four consecutive cycles and some loss of its catalytic activity was observed from 3rd cycle onwards. The loss in activity may be due to leaching of metal hydroxides and carbonates from active sites of the catalyst.

Furthermore, for investigation of merit of the present catalytic system and its efficiency in the synthesis of pyrano3-2-cchromene derivatives, it was compared with recently reported methodologies (Table 3). From the overall study, it is clear that, till date, no reported methodology for pyran derivatives makes use of such a very mild and green protocol and leads to such a high product yields. Consequently, the present findings appear to be the greenway for the synthesis of pyrano 3-2-cchromene derivatives and its analogues.
A plausible mechanism (Scheme 3.) has been suggested for one pot three component condensation of 4-hydroxycoumarin with aldehydes and malononitrile in water in presence of the catalyst at room temperature. Initially, the formation of Knoevenagel product from aldehydes and malononitrile in presence of A. squamosa ash, in next step, 4-hydroxycoumarin reacted with cyanocinnamonitrile by Michael addition, and finally, cyclization and tautomerization of intermediates to afford the desired product have been illustrated.
In conclusion, we have introduced a simple, clean and environmentally friendly protocol for the one-pot three-component synthesis of water promoted broad range of biologically and pharmacologically significant densely functionalized pyrano3-2-cchromene and 2-amino-4H-chromene derivatives in presence of Annona squamosa peel ash a waste-derived catalyst without using external base, promoter or additives. This method offers several advantages including room temperature synthesis, short reaction time with excellent yield, operational simplicity, ease of separation of product, recyclability of catalyst, ability to tolerate wide range of electron withdrawing and donating groups and as guiding principles of green chemistry ‘Use catalysts, not stoichiometric reagents’ herein, we have used industrial waste as catalyst through convenient way into organic transformations. On estimating all these advantages, it is worth noting that, Annona squamosa peel ash can become a competitive catalyst and can be efficient and suitable alternative for several organic transformations which are important industrially.

Conflicts of interest
There are no conflicts to declare.
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