Received Date: 12/12/2017 Accepted Date: 04/03/2017 Published Date: 08/03/2017
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This research aims to study the reactions of Diaminohexanoic Acid – an essential amino acid- also known as lysine at various pHs and further observe its absorption in a cactus species known as Euphorbia trigona. Lysine absorption was studied by observing physical changes in the Euphorbia trigona. Three pH studies were completed in solvents such as Methylene chloride, methanol, and water in the 25-40°C temperature range. This study was a small part of our on-going effort to develop assays for essential amino acid detections.
Acetanilide, β-cyclodextrin-SO3H, Aromatic amines, Green synthesis
In recent years, thiazolidinone and their derivatives have become among the most extensively investigated compounds [1-5]. They constituted a very important group of heterocyclic compounds, having valuable biological activities in the areas of medicine as well as agriculture. The thiazolidinone scaffold is a central part of biologically active compounds with various applications and uses [6-8] such as antibacterial [9-11], antifungal [12-14], anticancer [15,16], anti-tubercular [17,18], antimalarial , anticonvulsant [20,21], anti-inflammatory [22,23], anti-HIV [24,25] and analgesic . The 4-thiazolidinone moiety is very versatile and has featured in many drugs (Figure 1) and several compounds with 4-thiazolidinone core structure were found to kill selectively drug resistant cancer cells and induce cell death . The peptidoglycan is an essential component of the cell wall of both Gram-positive and Gramnegative bacteria. The 4-Thiazolidinones have been reported as novel inhibitors of the bacterial enzyme Mur B which is precursor acting during the biosynthesis of peptidoglycan .
The Vilsmeier-Haack reagent has been proud to be a versatile reagent capable of exacting a large variety of synthesis transformation . It finds application in formylation , cyclonaloaddition  and cyclisation . Recently, its potentiality wax explored in the synthesis of substituted aldehydes from acetanilide, N,N-dimethylformamide (DMF) and phosphoryl trichloride (POCl3) to develop novel quinolone based, fused, heterocyclic systems as potential anticancer agents [33-39], a quinolone nucleus with different substituent’s at 2 and 3 positions was required which afforded a versatile synthon for farther hetero annulations. For the past few years, our group has also been preparing and evaluating biologically important compounds.
The most common approach for the synthesis of the thiazolidinone ring involves substituted aldehyde, substituted amine and thioglycolic acid in one step or two steps. So many researchers have dedicated much effort to synthesis of 4-thiazolidinones using benzene or toluene [40,41], PEG , ionic liquids [43,44], Bi(SCH2COOH)3 , ZnCl2 , protic acid , zeolite , silica gel , and Saccharomyces cerevisiae  as catalysts. However, some of these procedures have major disadvantages, for example prolonged reaction time, use of special apparatus (Dean and Stark assembly), use of much expensive and environmentally very toxic catalysts and solvents, and low or moderate yields. Therefore, because of recent interest in green chemistry, there is a need to develop a simple and highly efficient procedure using a reusable and environment-friendly catalyst and Multicomponent reaction [51-55].
In continuation of our work [56-63], on the synthesis of bioactive compounds, we have synthesized some thiazolidinone analogues. Here, we wish to mention the development and implementation of a methodologies allowing for the synthesis of 2-chloro-(substituted) quinoline-3-carbaldehyde (2a-d) and 2-((substituted)-2-chloroquinolin-3-yl)-3-((substituted) phenyl) thiazolidin-4-one (5a-p).
Substituted acetanilide, β-cyclodextrin-SO3H, aromatic amines, DMF, POCl3 and various solvents were commercially available. The major chemicals were purchased from Sigma Aldrich and Avra labs. Reaction courses were monitored by TLC on silica gel precoated F254 Merck plates. Developed plates were examined with UV lamps (254 nm). The IR spectra were recorded on FT-IR (Bruker). The melting points were recorded on SRS Optimelt, melting point apparatus and these are uncorrected. The 1H NMR spectra were recorded on a 400 MHz Bruker spectrometer. The chemical shifts are reported as δppmunits. The 13C NMR spectra were recorded on a 100 MHz Bruker spectrometer. Chemical shifts are reported as δppm units. The following abbreviations are used; singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m) and broad (br). The mass spectra were taken with micromass- QUATTRO-II of water mass spectrometer.
We have developed the protocol for synthesis and screening of model reaction of 2-chloroquinoline-3-carbaldehyde (2a) (Scheme 1) by cyclisation between acetanilide towards DMF and POCl3. In terms of the effect on the yield compounds 2a-d upon varying the molar proportions of POCl3 from 3 moles to 18 moles (Table 1). The Vilsmeier-Haack reaction of acetanilide 1a (1 mmol), DMF (3 mmol) and POCl3 (12 mmol) without solvent and various proportions was selected as a model reaction to optimize the reaction conditions. In terms of the effect various molar proportions on the cyclisation reaction, acetanilide 1a (1 mmol), DMF (3 mmol) and POCl3 (12 mmol) molar proportions was found to be the best molar proportions for the reaction (Table 1); other molar proportions, were less efficient (Table 1).
aAll the reaction was carried out in various molar proportion of compounds, bIsolated yield.
Table 1. Screening of the molar proportions, reaction time and yield (2a)a.
The DMF and POCl3 molar proportions 3:3 and 3:6 gave the corresponding product in a 27% and 25% yield, which was the worst among these molar proportions (Table 1). Nevertheless, these yields were generally low before further optimizations. To increase the efficiency of the reaction, the effects of different molar proportions were investigated (Table 1). The entry 4 exhibited the best performance with molar proportions of DMF and POCl3 3:12 (96%). The molar proportions 3:15 and 3:18 gives good yields (Table 1). All the reactions were carried out in various molar proportions of each compound. Among these reactions, 3:12 molar proportions turned out to be the best choice with yields of 98%. We would like to mention here this molar proportion was the best choice with a better yield and less time required for the completion of the reaction (Table 1).
Thus, we decided to carry out the reactions in 3:12 molar proportions. As shown in Table 1, optimal conventional method reaction conditions were obtained using molar proportional ratio of 1:3:12 of acetanilide: DMF: POCl3 respectively in the absence of the solvent. This optimal reaction conditions, the synthesis of the compound (2a–d) by Vilsmeier-Haack reaction showed in Scheme 2 and Table 2.
|Entry||Compounds (2a-d)||DMF : POCl3||Product||Time (h)||Yieldb (%)||M.P. (Ã‚ÂºC)|
aReaction conditions: 2a-d (1 mmol), DMF (3 mmol), POCl3 (12 mmol), Temperature 80-90Ã‚Â°C, bIsolated yield
Table 2. Physical data of 2-chloro-(substituted) quinoline-3-carbaldehyde (2a-d)a.
Further, we synthesized title compounds of 2-((substituted)-2-chloroquinolin-3-yl)-3-((substituted) phenyl) thiazolidin-4-one (5a-p). But before that we were screening catalyst, solvents, reaction time and yield of model reaction 2-(2-chloroquinolin-3-yl)-3- phenylthiazolidin-4-one (5a) (Schemes 3 and 4; Tables 3 and 4).
|Entry||Catalyst||Temperature (Ã‚Â°C)||Time (h)||Yieldb (%)|
|Ã‚Â Ã‚Â Ã‚Â 1||Sulfamic acid||Reflux||Ã‚Â Ã‚Â Ã‚Â Ã‚Â 8||20|
|6||Piperidine ammonium acetate||Reflux||11||30|
|7||Pyrrolidine ammonium acetate||Reflux||10||38|
aReaction conditions: 2a (1 mmol), 3a (1 mmol), thioglycolic acid 4 (1.5 mmol), water (5 ml), catalyst (10 mol %), bIsolated yield.
Table 3. Screening of catalyst for the model reaction (5a)a.
|Entry||Solvent (5 ml)||Temperature (Ã‚Â°C)||Time (h)||Yieldb (%)|
|7||Solvent- free||Ã‚Â Reflux||Ã‚Â Ã‚Â Ã‚Â Ã‚Â Ã‚Â Ã‚Â Ã‚Â 1||98|
aReaction conditions: 2a (1 mmol), 3a (1 mmol), thioglycolic acid 4 (1.5 mmol), ÃƒÂŸ-cyclodextrin-SO3H (10 mol %), aIsolated yield.
Table 4. Screening of solvents for the model reaction (5a)a.
In view of green chemistry, it was decided to prefer water as solvent in our initial study for optimization of the catalyst. During this study, the model reaction was performed using water as a reaction medium at reflux temperatures. The desired product 5a was obtained in 20% only, even after reaction for 9h in water under reflux (Table 4).
In a subsequent experimental detailed, various acid catalysts bearing sulfonated functionality, for example sulfamic acid, sulfanilic acid, p-toluenesulfonic acid (p-TSA), and β-cyclodextrin-SO3H and other catalyst such as Hydrochloric acid, Piperidine ammonium acetate, Pyrrolidine ammonium acetate were screened under reflux conditions with water as a solvent. Reactions conducted in the presence of Hydrochloric acid, Piperidine ammonium acetate, Pyrrolidine ammonium acetate, sulfamic acid, p-TSA and sulfanilic acid given less yield even after completion of reaction with 50%, 30%, 38%, 20%, 30%, and 45% yields respectively.
(Table 3). When β-cyclodextrin-SO3H was used, the reaction afforded 5a in 80% yield within 1 h (Table 3).
The investigation toward the effect of solvent, the model reaction was further performed with β-cyclodextrin-SO3H in different solvents (acetic acid, DMF, methanol, toluene, ethanol, and Isopropanol) (Table 4). Use of these solvents failed to improve the yield and rate of the reaction. However, considering the increasing importance of solvent-free reactions in organic synthesis, we examined the catalytic efficiency of β-cyclodextrin-SO3H in the absence of the solvent. The rate of the reaction increased substantially and the desired product 5a was obtained in higher yield 98% (Table 4). Thus, β-cyclodextrin-SO3H proved the most efficient acidic catalyst in terms of time and yield.
We like to mention here in solvent-free were the best choice and less time required for reaction completion. We decided to carry out the further reaction in solvent-free condition with β-cyclodextrin-SO3H catalyst (Scheme 5; Table 5). However, the reaction provides cleaner reaction, short reaction time, and the products were only required to be washed with ice-cold water. The yields are good to excellent. To assess the generality of this reaction, substrate scope was investigated under the optimized conditions; the results are listed in Table 5. The various aldehyde and anilines were cyclocondensed with thioglycolic acid to afford a variety of 4-thiazolidinones (Table 5). Aromatic aldehydes with electron withdrawing or donating substituents reacted very well, affording good to excellent yields of 4-thiazolidinones.
|Entry||Compounds (2a-d)||Compounds (3a-d)||Product||Time (h)||Yieldb (%)||M.P.
aReaction condition (5a-p): 2a-d (1 mmol), 3a-d (1 mmol), thioglycolic acid 4 (1.5 mmol), ÃƒÂŸ-cyclodextrin-SO3H (10 mol %), reflux for 2-5 h, bIsolated yields.
Table 5. Physical data of 2-((substituted)-2-chloroquinolin-3-yl)-3-((substituted) phenyl)thiazolidin-4-one(5a-p)a.
For reasons of economy, the stability and sustained activity of the catalysts are extremely important. Thus, recovery and reuse of β-cyclodextrin-SO3H in the model reaction were also studied. After completion of the reaction, the mixture was cooled to room temperature and ethyl acetate was added. The solid catalyst was isolated by filtration of the reaction mixture, washed with ethyl acetate, dried in an oven then reused for further catalytic cycles.
A plausible mechanism for the reaction is proposed as shown in Scheme 6. The catalyst β-cyclodextrin-SO3H activates to the aldehyde by increasing the electrophilicity of its carbonyl group to facilitate the formation of the imine intermediate (I). The anion of β-cyclodextrin-SO3H might enhance the nucleophilicity of the mercapto group of thioglycolic acid, causing its facile addition to the imine intermediate generated in situ. The β-cyclodextrin-SO3H then activates the carbonyl group of acid of intermediate (II), leading to intramolecular cyclization then dehydration to afford the thiazolidinone (III).
We have successfully developed a β-cyclodextrin-SO3H catalyzed an easy access to a new series of a facile one pot three component synthesis of 2-((substituted)-2-chloroquinolin-3-yl)-3-((substituted) phenyl) thiazolidin-4-one. This catalyst is highly active, mild reaction conditions, good to excellent yields, easy work-up, reused without significant loss of activity and easily available substrate make this reaction an attractive method for the preparation of compounds 5a-p. This procedure has many advantages over other methods, for example shorter reaction times, recycling and reuse of a heterogeneous biopolymer-based solid-acid catalyst, convenient one-pot operation, and simple experimental procedure.
The authors thankful to Head, Department of Chemical Technology, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad 431004 (MS), India for providing the laboratory facility. The authors are also thankful to the principal, Head, Department of Chemistry, Deogiri college, Station road, Aurangabad 431 005, MS, India, for providing the laboratory facility.