Green Synthesis of Acid Esters from Furfural via Stobbe Condensation

Solvent-free Stobbe condensation of furfural 1 with dimethyl succinate 2 under anhydrous conditions at room temperature using dry-solid potassium tertiary butoxide gave 3-carbomethoxy, 4-furyl-3-butenoic acid 3, which upon methylation followed by Stobbe condensation reaction with different aldehydes and/or ketones under anhydrous conditions at room temperature afforded substituted carbomethoxy acids 5a–f.ese acid ester products were saponi�ed to the corresponding dicarboxylic acids 6a–fwhich are useful in the synthesis of photochromic fulgides.

During the last few years, solvent-free reactions [12,13] for the different organic synthesis have been a �eld of increasing interest in synthetic organic chemistry because of their ease to occur.e present work depicts a unique one-pot synthesis method in which solvent-free conditions allow improving the yield and expediting the reaction.Here, different substituted carbonyl compounds including aldehydes, aromatic, and alicyclic, aliphatic ketones and an active methylene compound, namely, dimethyl succinate, were condensed in anhydrous condition.e reaction is feasible in a dry agate mortar at room temperature, avoiding hazards of using any solvent.

Experimental
All chemicals were of reagent grade quality and used without further puri�cation.Melting points were measured on a melting point apparatus and are uncorrected.NMR spectra were recorded on a Bruker 300 MHz spectrometer.Chemical shis are reported in ppm relative to tetramethyl silane as an internal standard.e infrared spectra were obtained on a Bruker IFS 66 V Fourier transform spectrometer using KBr pellets.e compounds were scanned for UV-visible spectra [14] using Perkin Elmer spectrophotometer.Procedure for Preparation of 3-Carbomethoxy,4-furyl,3butenoic Acid (3).A homogenous mixture of furfural 1 (0.96 g, 0.01 mole) and dimethyl succinate 2 (1.46 g, 0.01 mole) was added to potassium tert.butoxide (1.13 g, 0.01 mole) and well ground with a pestle for 10 min.It was exposed to air for another 15 min.Upon neutralization with dil.HCl at 0 ∘ C and usual work up [15], crude product, namely, 3-carbomethoxy,4-furyl, 3-butenoic acid 3 was obtained, which was recrystallized from benzene-pet.Ether (see Scheme 1).

Results and Discussion
In the previous research [17], it was found that potassium tertiary butoxide can be smoothly used as a catalyst in Stobbe condensation under solvent-free conditions.is discovery stimulated us to explore whether it could be developed into a green [18] procedure for Stobbe condensation.
Stobbe condensation generally involves the use of metal alkoxide as a catalyst in re�uxing alcohol, and particularly, butanol.On the other hand, in the present paper, the use of butanol is discarded and instead, dry solid potassium tertiary butoxide is taken for the reaction.e advantages are inexpensive and easily available materials, have short reaction time, excellent yields (the yields of the products obtained by solvent-free method were compared with the yields of the products obtained by classical method which was done simultaneously for a comparative study), and environmentfriendly reaction conditions.
A well ground mixture of furfural 1 and dimethyl succinate 2 in 1 : 1 molar ratio with catalytic amount of potassium tertiary butoxide without any solvent was exposed to atmosphere at room temperature in a dry agate mortar.Aer neutralization with dil.HCl, crude 3-carbomethoxy,4furyl, 3-butenoic acid, 3 which could be easily crystallized from benzene-petroleum ether.e structure of the acid ester 3, has been analyzed by UV, IR, and NMR spectra.It is con�rmed that, from both the routes, namely, classical Stobbe condensation [15] and the solvent-free reaction, the same acid ester 3 is obtained.
e diester 4 is a starting material for the synthesis of various butadienes via Stobbe condensation with different aldehydes (furfural, benzaldehyde) and ketones (acetone, benzophenone, cyclopentanone, and cyclohexanone) using the base, that is, dry-solid potassium tert.butoxide in an oven-dried mortar at room temperature (see Figure 1 and Table 1).
e results indicated that, all the reactions were performed under solvent-free conditions in good yields of 70-90% (please see Table 2).e structures of these compounds were analyzed by NMR spectroscopy [19].All the  characteristic positions.Further, FTIR spectra [20] support the structures of the compounds.e typical acid carbonyl is found in the range 1670-1690 cm −1 , and the ester C=O appears at near 1715 cm −1 .It is interesting to note that the saponi�cation [15] reactions of the acid esters 5a-5f afforded the diacids 6a-6f in good yields (please see Table 3).e presence of different substituents does not really affect the product yield.However, during cyclization of these diacids, difficulty is faced with the bulkier phenyl or naphthyl substituents, which would be discussed elsewhere.

Conclusion
It was concluded that, the solvent-free condensations of substituted ketones and/or aldehyde with dimethyl succinate at room temperature occurred smoothly to give substituted acid esters.As compared to the classical condensation method done by previous workers [13], wherein was required, plenty of solvents and chemicals to proceed, the green method needed much less amount of dry solid reagents; which indicates that the method is efficient from the economical point of view.No heat energy is required for the formation of acid esters, thus, proving the reaction to be feasible at ecofriendly reaction conditions.Further, this method improved the yields and shortened the reaction time.

3, 5a-f.
butadienes, being derived from furfural, show a common AMX pattern in their high resolution NMR spectra, which can be seen for three furyl hydrogens nearly at 6.3, 6.6, and 7.6.Aromatic protons are depicted well in the aromatic zone of 7.2-7.5for the compound 5d and 6d.Similarly, the aliphatic and alicyclic protons are exhibited in their T 3: Yield and melting points of dicarboxylic acids 6a-f.