# L-Tyrosine as an Eco-Friendly and Efficient Catalyst for Knoevenagel Condensation of Arylaldehydes with Meldrum’s Acid in Solvent-Free Condition under Grindstone Method

Authors: G. Thirupathi; M. Venkatanarayana; P. K. Dubey; Y. Bharathi Kumari
Journal: Organic Chemistry International
 (2012)

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## Abstract

We investigate L-Tyrosine as an efficient catalyst for the Knoevenagel condensation of arylaldehydes with meldrum’s acid containing cyclic active methylene group in solvent-free condition under grindstone method at room temperature to produce substituted-5-benzylidene-2,2-dimethyl-[1,3]dioxane-4,6-diones3(a–j).


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## Body

## 1. Introduction

Toda introduced a method called the grindstone method [1, 2]. In this method, solids are grounded together using a pestle and mortar to get the products. These reactions are more efficient selective than those carried out in the corresponding solutions.Developing green chemical methods is one of the most important purposes of organic synthesis. Organic synthesis in the absence of solvent is a powerful tool for the generation of structurally different molecules whose special selectivity arises great interest. Moreover, solvent-free reactions are faster, taking just a few minutes. This aspect is coupled with the lowering of the total costs of running a reaction without solvent and specially designed equipment, which could become a very impressive factor in industry.Carbon–carbon bond formation reaction is the most important reaction in organic synthesis [3–6]. The Knoevenagel condensation is one such reaction which facilitates C–C double-bond formation and has been widely used in synthesis of alkenes of biological significance [7–11]. These reactions are usually catalyzed by bases [12–17] such as primary and secondary amines and their corresponding ammonium salts; Lewis acids [18–20], zeolite [21–23], and ionic liquids [24–27] have also been added to the existing list of substances that assisted Knoevenagel condensation in organic synthesis.Knoevenagel condensation of meldrum’s acid and aldehydes gives rise to substrates for variety of reactions [28]. They are used in cycloaddition reactions [29], 1,4-conjugate addition reactions and preparation of mono alkyl meldrum’s acid derivatives [30], and preparation of deuterated carboxylic acid derivatives [31]. These derivatives are also used in the preparation of ketenes by α,β-pyrolysis [32], which are then used for preparation of different compounds such as cyclobutadiene derivatives [33], α,β-unsaturated esters [34], and α,β-unsaturated amides [35, 36].Tyrosine is known to be an efficient, bifunctional, zwitterionic, and ecofriendly catalyst. It is available in both the enantiomeric, (S)-Tyrosine and (R)-Tyrosine, forms. The two functional groups of tyrosine enable it to act both as an acid as well as a base catalyst in chemical condensation reactions.In this paper, we highlight our findings on the L-tyrosine catalyzed condensation of arylaldehydes with meldrum’s acid containing cyclic active methylene group in solvent-free condition under grindstone method at room temperature to produce substituted -5-benzylidene-2,2-dimethyl-[1,3]dioxane-4,6-diones in solvent-free condition under grindstone method at room temperature.


## 2. Results and Discussion

Arylaldehydes1(a–j) were subjected to condensation with meldrum’s acid containing cyclic active methylene group in the presence L-tyrosine in solvent-free condition under grindstone method at room temperature for 7–15 min. resulting in the formation of substituted -5-benzylidene-2,2-dimethyl-[1,3]dioxane-4,6-diones3(a–j) in 91–94% yields (Table 1 and Scheme 1). This method is very convenient for the preparation of large amount of substituted -5-benzylidene-2,2-dimethyl-[1,3]dioxane-4,6-diones with high yields in less time. L-Tyrosine as an ecofriendly and efficient catalyst to induce the reaction.Table 1
Synthesisofsubstituted -5-benzylidene-2,2-dimethyl-[1,3]dioxane-4,6-dione with Knoevenagel condensation in solvent-free condition under grindstone method at room temperature.
S. Number
R
Condition
Time (min)
Yield (%)
mp (°C)
(Lit.mp., 28, 37, 38, 39 & 40) (°C)
a
C6H5
r.t.
7
93
84-85
85-86
b
4-CH3–C6H4
r.t.
15
91
117-118
116–118
c
4-OCH3–C6H4
r.t.
15
91
124-125
126-127
d
p-(Me2N)–C6H4
r.t.
14
92
163–165
162–164
e
p-(OH)–C6H4
r.t.
13
92
195-196
192–194
f
p-(Cl)–C6H4
r.t.
8
94
157-158
158–160
g
p-(NO2)–C6H4
r.t.
7
94
127-128
217–129
h
O-(NO2)–C6H4
r.t.
8
93
118–120
119-120
i
O-(Cl)–C6H4
r.t.
8
93
133–135
133-134
j
M-(OH)–C6H4
r.t.
13
92
173-174
175-176Scheme 1
Knoevenagel condensation of arylaldehydes with meldrum’s acid in the presence of L-tyrosine in solvent-free condition under grindstone method. (a) R = C6H5, (b) R = CH3–C6H4, (c) R = OCH3–C6H4, (d) P-(Me2N)–C6H4, (e) P-(OH)–C6H4, (f) P-(Cl)–C6H4, (g) P-(NO2)–C6H4, (h) O-(NO2)–C6H4, (i) O-(Cl)–C6H4, and (j) M-(OH)–C6H4.In the absence of L-tyrosine, the reaction does not proceed the reactants in the solvent-free condition under grindstone method at room temperature for 4 h. The use of L-tyrosine as a catalyst helps to avoid the use of environmentally unfavourable organic solvents as reaction medium.The above reactions of1(a–j) with meldrum’s acid were attempted in the presence of various amino acids like valine, glycine, alanine, and Lysine in the solvent-free condition under grindstone method at room temperature for 5 h. but there was not much progress in the reactions as seen by TLC examination of crude reaction mixtures. In the presence of phenyl alanine and L-tryptophan in the solvent-free condition under grindstone method at room temperature, there was a little bit of progress, but the reaction was not completed for 5 hrs.The above reactions of arylaldehydes1(a–j) with meldrum’s acid containing cyclic active methylene group were attempted in the presence of L-proline low yield which was obtained for 1 h in the solvent-free condition under the grindstone method at room temperature.The above reactions of arylaldehydes1(a–j) with meldrum’s acid containing cyclic active methylene group were attempted in the presence of various bases like NaOH, KOH which were too strong bases to result in more byproducts. Low yield was obtained and 3 h reaction time is needed using K2CO3, ammonium acetate, piperidine, and triethylamine as catalyst for condensation of arylaldehydes 1(a–j) with meldrum’s acids containing cyclic active methylene group (2) in the solvent-free condition under the grindstone method at room temperature.From Table1, it was shown that the condensation of arylaldehydes with electron withdrawing group such as –NO2 and –Cl at paraposition with meldrum’s acid containing cyclic active methylene compounds can be carried out in relatively shorter time and higher yield than with electron donating group such as –OH, –OCH3, and N, N-dimethyl arylaldehydes in the solvent-free condition under the grindstone method at room temperature.A plausible mechanism for the formation of3 from 1 and 2 in the presence of L-tyrosine as catalyst is shown in Scheme 2. The reaction mechanism is supported by the literature reference [37].Scheme 2
Plausible mechanism for the formation of3 from 1 and 2 in the presence of L-tyrosine under solvent-free condition at room temperature.In the mechanism shown in Scheme2, L-tyrosine, in its zwitterionic form (Xb), abstracts a proton from meldrum’s acid containing cyclic active methylene group (2) forming the carbanion of meldrum’s acid (2I) which then attacks the protonated arylaldehydes (1I) forming the corresponding intermediate (1II) that loses water to form the end product 3.In summary, L-Tyrosine as an efficient catalyst for the preparation of substituted-5-benzylidene-2,2-dimethyl-[1, 3]dioxane-4,6-dione by Knoevenagel condensation in the solvent free condition under the grind stone method at room temperature. This method is applicable to a wide range of arylaldehydes 1(a–j) and meldrum’s acid (2) containing active methylene group to produce substituted -5-benzylidene-2,2-dimethyl-[1,3]dioxane-4,6-diones 3(a–j) in solvent-free condition under grindstone method at room temperature.The attractive features of this procedure are the mild reaction conditions, high conversions, operational simplicity, and inexpensive and ready availability of the catalyst, all of which make it a useful and attractive strategy for the preparation of substituted -5-benzylidene-2,2-dimethyl-[1,3]dioxane-4,6-dione in solvent-free condition under grindstone method at room temperature.


## 3. Experimental Section

Melting points were measured in open capillary tubes and are uncorrected. TLC was done on plates coated with silica gel-G and spotting was done using iodine or UV lamp. IR-spectra were recorded using FT-IR in KBr phase.1H-NMR spectra were recorded at 400 MHz, respectively. Compounds are known, and products were identified by spectral and melting-point comparison with the authentic samples.General Procedure for the Preparation of3(a–j) from 1(a–j) and Meldrum’s Acid
A mixture of1 (10 mmol), meldrum’s acid 2 (10 mmol), and L-tyrosine (2 mmol) was physically grinded in solvent-free condition under grindstone method at room temperature for a specified period of time (Table 1). After completion of reaction (as shown by TLC checking), the mixture was poured into ice-cold water (50 mL). The separated solid was filtered, washed with water (100 mL), and dried to obtain crude 3(a–j). The latter were then recrystallised from ethanol to afford pure 3(a–j). Compounds are known, and products were identified by spectral and melting-point comparison with the authentic samples.



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*Source: 191584-2012-11-13.xml*