Tannic Acid an Efficient Catalyst for the Synthesis of 12-aryl-8 , 9 , 10 , 12-tetrahydrobenzo [ a ] xanthen-11-one Derivatives

Tannic acid explore a highly efficient catalytic activity for the synthesis of 12-aryl-8,9,10,12tetrahydrobenzo[a]xanthen-11-one derivatives in excellent yields via cyclocondensation of aromatic aldehyde, β-naphthol and dimedone. Catalyst having advantages such as it is cheap and biodegradable and the protocol avoids the use of expensive catalyst and toxic solvent. We believe that this methodology is an efficient, simple, highly yielding, time saving and environmentally friendly.


INTRODUCTION
In recent year paramount importance has been devoted to explore new methodologies for synthesis of bioactive heterocyclic under solvent free conditions.Many organic solvents have toxicity and volatile in nature particularly chlorinated hydrocarbon that are wildly used in large amounts for the organic transformation causes adverse effects to the environment [1,2].Therefore the development of solvent free catalytic reaction, high yielding and ecofriendly approach is highly desirable [3,4].Tertrahydrobenzo[a]xanthene motif exhibit good biological and pharmaceutical activities such as antibacterial, antiviral and anti-inflammatory activities.They have also employed as dye and p H sensitive fluorescent material to monitor changes in intracellular pH [5][6][7][8].Owing to the wide range of pharmacological and biological activities, the development of newer synthetic method enabling facile access to this heterocycle is still desirable.Several methods have been reported for the synthesis of xanthenes which includes by using poly(4-vinyl pyridinium) perchlorate [9], task specific acidic ionic liquid [NMP] H2SO4 [10], nano silica phosphoric acid [11], HClO4-SiO2 [12], proline triflate in water [13], Iodine [14], InCl3 [15], Sr(OTf)2 [16] and p-TSA [17].Despite of the available methods less importance has been paid on the synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-one derivatives.However, these methods have a number of drawbacks which includes poor yields of product, harsh reaction condition, prolonged reaction times, high cost of catalyst and use of toxic organic solvents.
In recent year, Tannic acid has received considerable attention as an efficient catalyst for various organic transformations such as synthesis of imidazole derivatives [18], 1-amidoalkyl-2naphthols, 19 and synthesis of benzodiepines derivatives [20].
Tannic acid is specific commercial form of tannin.Tannic acid is a naturally occurring plant polyphenol possing central glucose molecule, derivatized at its hydroxyl groups [19].Tannic acid also employed as reducing agent, hydrogen donor and quinches of singlet oxygen. 19It is weak acidity (pKa around 6) is due to the numerous phenol group in the structure.

MATERIAL AND METHODS
All chemical were purchased from Aldrich chemical company and used without further purification. . 1 H NMR spectra were recorded on Bruker Advance 400, in DMSO in presence TMS as an internal standard. 13C NMR spectra recorded on Bruker DRX-300 in DMSO as solvent.Mass spectra were recorded on water UPLC TQD Mass spectrometer, showing M +. peak.Melting points were recorded in open capillary method and uncorrected.
Spectral data of some representative compounds are given here.

RESULTS AND DISCUSSION
Initially, the reaction of benzaldehyde (1a, 1mmol), β-naphthol (2, 1mmol) and dimedone (3, 1mmol) was considered as model reaction, to optimize the reaction condition at 90 °C.The results are summarized in Table 1.The model reaction was first carried out in absence of catalyst in water the reaction was not gave desired product even after prolonged reaction time, then we employed different catalysts for model reaction some catalysts (Table 1) can catalyze this reaction with moderate yields but we found that using tannic acid the desire product formed with higher yield that is 96% yield within 45 minutes (Table 1, entry 9).In addition ethanol, methanol, DMF, toluene, acetonitrile were also employed as solvent for model reaction.In these cases product (4a) was formed in very low yields (Table 1  In recent year, more emphasis has been paid on solvent free synthesis, so we performed model reaction under solvent free condition we observed that the reaction was completed within 45 min and gave excellent yields of the products when using tannic acid under solvent free condition (Table1, entry 9).
Then to optimize the amount of catalyst, the model reaction was tested at different mol% of catalyst, summarized in Table 2.They reveal that at 10 mol% of tannic acid was employed for model reaction we observed that the reaction preceded smoothly and gave product (4a) in highest yield (Table 2, entry 2).Further increases the mol% of tannic acid did not lead to considerable change in the yield and reaction times.To find out optimum reaction temperature, we performed the model reaction at different temperature for the synthesis of (4a) with 10 mol% tannic acid under neat condition.(Table 3), We found that, the reaction did not proceeded at room temperature, then we increase temperature by 40, 50, 60, 70, 80, and 90°C for model reaction.We found that the model reaction proceeded smoothly and complete conversation of reactant to desire product (4a) in 96% yield within 45 min.(Table 3 entry 6).Further increasing in temperature did not affect the product yield.
Under optimized condition, we performed series of 12-aryl-8,9,10,12tetrahydrobenzo[a]xanthen-11-one derivatives.The results are summarized in Table 4.In order to furnish the versatility of this protocol, various types of aromatic aldehydes were used to synthesize corresponding xanthenes derivatives.The nature of electron withdrawing or donating substituent on the aromatic ring did not show any significant difference in the yield of xanthene derivatives.
The plausible mechanism for catalytic activity of tannic acid in the synthesis of compound (4a-o) should be postulated, as shown in (Scheme 3).The reaction is thought to proceed; firstly, the tannic acid increases the electrophilicity of carbonyl carbon of aromatic aldehyde that will enhance the rate of condensation with β-naphthol, which leads to formation of ortho-quinone methides intermediate.Then it will reacted with dimedone via Michael addition followed by addition of phenolic hydroxyl group to the carbonyl carbon lead to cyclic hemiketal which on dehydration gave desire product (4a).

CONCLUSION
In conclusion, we explore an efficient methodology for the synthesis of 12-aryl-8,9,10,12tetrahydrobenzo[a]xanthen-11-one derivatives in presence of tannic acid as catalyst under the solvent free condition.The advantage of this method over existing one are high excellent yields, easy work up procedure, low cost of tannic acid and reduced reaction time.This methodology provides access to compound that is useful in heterocyclic synthesis

ACKNOWLEDMENTS
The Emeritus Scientist Scheme awarded to MSS by the Council of Scientific and Industrial Research, New Delhi is gratefully acknowledge.The DSK also thanks to Head, Department of Chemistry, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad for providing laboratory facilities and SAIF, CDIR, Lukhnow for providing spectral data.

Table 1 .
Optimization of catalysts and solvents for model reaction a .

Table 2 .
Effect of mol% of Tannic acid on model reaction a Scheme 2. Standard model reaction.