# Green Synthesis of Acid Esters from Furfural via Stobbe Condensation

Authors: Shubhra Banerjee; Ravibabu A. Tayade; Bhagyashree D. Sharma
Journal: Journal of Chemistry
 (2013)

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## Abstract

Solvent-free Stobbe condensation of furfural1 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. These acid ester products were saponified to the corresponding dicarboxylic acids 6a–f which are useful in the synthesis of photochromic fulgides.


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## Body

## 1. Introduction

Fulgides are important for their photochromic properties [1–3]. Stobbe condensation [4–10], a C–C bond forming reaction, is a pathway to fulgides. A lot of research has been done on photochromic fulgides; among them, furan ring containing fulgides [11] have maintained the interest of researchers through decades of development of organic synthesis.During the last few years, solvent-free reactions [12, 13] for the different organic synthesis have been a field of increasing interest in synthetic organic chemistry because of their ease to occur. The 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. The reaction is feasible in a dry agate mortar at room temperature, avoiding hazards of using any solvent.


## 2. Experimental

All chemicals were of reagent grade quality and used without further purification. Melting points were measured on a melting point apparatus and are uncorrected. NMR spectra were recorded on a Bruker 300 MHz spectrometer. Chemical shifts are reported in ppm relative to tetramethyl silane as an internal standard. The infrared spectra were obtained on a Bruker IFS 66 V Fourier transform spectrometer using KBr pellets. The compounds were scanned for UV-visible spectra [14] using Perkin Elmer spectrophotometer.Procedure for Preparation of 3-Carbomethoxy,4-furyl,3-butenoic 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).Scheme 1
One-pot synthesis of acid ester3.Yield 83%,1H NMR (300 MHz, CDCl3) (δ/ppm): 1.25 (s, 2H), 3.04(s, 3H), 7.5 (s, 1H), 7.5701–7.580 (AMX, 3H). FTIR (KBr, cm−1) 1170 (C–O), 1475.26 (aliphatic CH2) 1654 (C=O acid), 1700 (C=O ester), 2511 (–OCH3). UV λmax (EtOH) 211.95 nm (logε3.131). eq.wt. (found) 205.95 required for C10H10O5 eq. wt. 210.18.Procedure for Preparation of Methyl 3-Carbomethoxy,4-furyl,3-butenoate
(4). 3-Carbomethoxy,4-furyl, 3-butenoic acid3(1 g) in ethereal solution was esterified by diazomethane [16] being generated from nitrosomethylurea (0.6 g) added to a reaction mixture of 50% KOH solution (4.0 mL) and ether (10.0 mL). After 15 min, the reaction mixture was washed with 10% Na2CO3 solution three times. The organic layer was dried over anhydrous sodium sulphate. Removal of solvent gave the crude semisolid diester, namely, methyl 3-carbomethoxy,4-furyl, 3-butenoate, 4 (1.2 g, 90% yield) (see Scheme 2).Scheme 2
Methylation.1,1-Cyclopentylidene-1′-carbomethoxy,3(2-furyl)-prop-2-ene-2-oic Acid(5a).  To a neat mixture of the diester 4 (2.24 g, 0.01 mole) and cyclopentanone (0.84 g, 0.01 mole), was added dried potassium tertiary butoxide (1.13 g, 0.01 mole). The reaction mixture was well mixed and ground for 10 min. It was further exposed to air. Acidification followed by usual workup [15] gave the acid ester 5a recrystallised from benzene-pet. ether as reddish brown crystals.Yield 78%,1H NMR (300 MHz, DMSO) (δ/ppm): 2.58–2.59 (m, 8H, alicyclic CH2), 3.02 (s, 3H, methoxy protons), 7.5523–7.5552 (AMX, 3H, furyl), 7.6 (s, 1H, vinyl). FTIR (KBr, cm−1) 1174 (C–O), 1480 (alicyclic CH2) 1715 (C=O acid), 1735 (C=O ester), 2511 (–OCH3). UV λmax (EtOH) 350 nm (logε4.1). eq. wt. (found) 273.44 required for C15H16O5 eq. wt. 276.29.1,1-Cyclohexylidene-1′-carbomethoxy,3(2-furyl)-prop-2-ene-2-oic Acid(5b).  1H NMR (300 MHz, DMSO) (δ/ppm): 1.66 (s, 6H, alicyclic CH2), 2.1 (s, 2H, alicyclic CH2), 2.94 (s, 2H, alicyclic CH2), 3.02 (s, 3H, methoxy protons), 6.392–6.457, 6.675–6.723, 7.662–7.674 (AMX, 3H, furyl), 7.7 (s, 1H, vinyl). FTIR (KBr, cm−1) 1260 (C–O), 1480 (alicyclic CH2) 1614 (C=C), 1684 (C=O acid), 1712 (C=O ester). UV λmax (EtOH) 294 nm (logε3.32). eq. wt. (found) 292.20 required for C16H18O5 eq. wt. 290.30.1,1-Isopropylidene-1′-carbomethoxy,3(2-furyl)-prop-2-ene-2-oic Acid(5c).   1H NMR (300 MHz, DMSO) (δ/ppm): 1.75 (s, 3H, aliphatic CH2), 2.2 (s, 3H, aliphatic CH2), 3.7 (s, 3H, methoxy protons), 6.429–6.442, 6.59–6.60, 7.468–7.491 (AMX, 3H, furyl), 7.6 (s, 1H, vinyl). FTIR (KBr, cm−1) 1258 (C–O), 1664 (C=C), 1685 (C=O acid), 1715 (C=O ester). UV λmax (EtOH) 290 nm (logε3.2). eq. wt. (found) 252.30; required for C13H14O5 eq. wt. 250.42.1-Furyl-3-carbomethoxy-4-phenyl Buta-1,3-diene-2-oic Acid
(5d)
.
1H NMR (300 MHz, DMSO) (δ/ppm): 3.7 (s, 3H, methoxy protons), 6.376–6.389, 6.28–6.63, 7.35–7.46 (AMX, 3H, furyl), 7.5 (s, 5H, phenyl), 7.7 (s, 1H, vinyl), 7.8 (s, 1H, vinyl). FTIR (KBr, cm−1) 1174 (C–O), 1480 (alicyclic CH2) 1715 (C=O acid), 1735 (C=O ester), 2511 (-OCH3). UV λmax (EtOH) 350 nm (logε4.1). eq. wt. (found) 295.44 required for C17H14O5 eq. wt. 298.1-Furyl-3-carbomethoxy-4,4-diphenyl Buta-1,3-diene-2-oic Acid
(5e)
. 1H NMR (300 MHz, DMSO) (δ/ppm): 2.99 (s, 3H, methoxy protons), 6.276–6.289, 6.38–6.63, 6.55–6.76 (AMX, 3H, furyl), 7.1 (s, 5H, phenyl), 7.5 (s, 5H, phenyl), 7.52 (s, 1H, vinyl). FTIR (KBr, cm−1) 634, 712 (mono-substituted Ph rings), 1172 (C–O), 1660 (C=O acid), 1710 (C=O ester), 2510 (–OCH3). UV λmax (EtOH) 254 nm (logε4.2). eq. wt. (found) 372.00 required for C23H18O5 eq. wt. 374.39.1,4-Difuryl-3-carbomethoxy Buta-1,3-diene-2-oic Acid
(5f). 1H NMR (300 MHz, DMSO) (δ/ppm): 3.72 (s, 3H, methoxy protons), 6.385–6.409, 6.632–6.663, 7.422–7.447 (AMX pattern repeated, 6H, furyl protons), 7.67 (s, 1H, vinyl), 7.77 (s, 1H, vinyl). FTIR (KBr, cm−1) 634, 712 (mono-substituted Ph rings), 1200 (C–O), 1625 (C=C), 1660 (C=O acid), 1720 (C=O ester), 2500 (OCH3). UV λmax (EtOH) 333 nm (logε4.6). eq. wt. (found) 290.00 required for C15H12O6 eq. wt. 288.24 (see Scheme 3).Scheme 3
Synthesis of butadienes.Procedure for Preparation of 1,1-Cyclopentylidene-1′,2-dicarboxy, 3 (2-furyl)-2-propene(6a). Acid ester 5a(1 g) was dissolved in 30 mL 8% alcoholic KOH and refluxed for 5 hr. It was filtered hot and cooled. Acidification with ice-cool conc. HCl gave the diacid 6a as dark brown crystals on recrystallization from benzene pet. ether.1H NMR (300 MHz, DMSO) (δ/ppm): 1.25–2.17 [m, 8H, alicyclic CH2] 7.20–7.26 [AMX, 3H, furyl], 7.5 [s, 1H, vinyl]. FTIR. (KBr, cm−1) 1174.80 (C–O), 1488.10 (alicyclic CH2), 1694.00 and 1710.00 (C=O unsaturated acid). UV λmax (EtOH): 357.00 nm (logε4.6). eq. wt. (found) 130.00 required for C14H14O5 eq. wt. 131.13.1,1-Cyclopentylidene-1′,2-dicarboxy, 3 (2-furyl)-2-propene(6b). 1H NMR (300 MHz, DMSO) (δ/ppm): 1.55 (s, 6H, alicyclic CH2), 2.1 (s, 2H, alicyclic CH2), 2.81 (s, 2H, alicyclic CH2), 6.383–6.396, 6.62–6.63, 7.43–7.44 (AMX, 3H, furyl), 7.69 (s, 1H, vinyl). FTIR (KBr, cm−1) 1214 (C–O), 1600 (C=C), 1688 (C=O acid). UV λmax (EtOH) 278 nm (logε4.6). eq. wt.(found) 140.64 required for C15H16O5eq. wt. 138.14.1,1-Cyclopentylidene-1′,2-dicarboxy, 3 (2-furyl)-2-propene(6c). 1H NMR (300 MHz, DMSO) (δ/ppm): 1.75 (s, 3H, aliphatic CH2), 2.32 (s, 3H, aliphatic CH2), 6.448–6.592, 6.712-6.718, 7.468-7.48 (AMX, 3H, furyl), 7.5 (s, 1H, vinyl). FTIR (KBr, cm−1) 1218 (C–O), 1600 (C=C), 1698 (C=O acid). UV λmax (EtOH) 280 nm (logε4.5). eq. wt. (found) 121.23 required for C12H12O5 eq. wt. 118.11.1-Furyl-2,3-dicarboxy-4 Phenyl Buta-1,3-diene
(6d). 1H NMR (300 MHz, DMSO) (δ/ppm): 6.376–7.01, 7.331-7.429, 7.35–7.46 (AMX, 3H, furyl), 7.5 (s, 5H, phenyl), 7.72 (s, 1H, vinyl), 7.89 (s, 1H, vinyl). FTIR (KBr, cm−1) 1228 (C–O), 1628 (C=C), 1690 (C=O acid). UV λmax (EtOH) 370 nm (logε4.6). eq. wt. (found) 143.28; required for C16H12O5 eq. wt. 142.31.1-Furyl–2,3-dicarboxy-4,4′-diphenyl Buta-1,3-diene(6e).1H NMR (300 MHz, DMSO) (δ/ppm): 6.71–6.73, 6.80–6.85, 6.87–6.89 (AMX, 3H, furyl), 7.65 (s, 5H, phenyl), 7.75 (s, 5H, phenyl), 7.81 (s, 1H, vinyl). FTIR (KBr, cm−1) 613, 620 (mono-substituted Ph rings), 1170 (C–O), 1648 (C=O acid), 1700 (C=O acid). UV λmax (EtOH) 270 nm (logε4.6). eq. wt. (found) 179.95 required for C22H16O5 eq. wt. 180.19.1,4-Difuryl−2,3-dicarboxy Buta-1,3-diene
(6f).   1H NMR (300 MHz, DMSO) (δ/ppm): 6.2981–6.304, 6.623–6.631, 7.434–7.438 (AMX pattern repeated, 6H, furyl protons), 7.67 (s, 1H, vinyl), 7.77 (s, 1H, vinyl). FTIR (KBr, cm−1) 1250 (C–O), 1635 (C=C), 1690 (C=O acid). UV λmax (EtOH) 332 nm (logε4.76). eq. wt. (found) 138.40 required for C14H10O6 eq. wt. 137.11.


## 3. 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. This 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 refluxing 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. The 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 environment-friendly reaction conditions.A well ground mixture of furfural1 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. After neutralization with dil. HCl, crude 3-carbomethoxy,4-furyl, 3-butenoic acid, 3which could be easily crystallized from benzene-petroleum ether. The structure of the acid ester 3, has been analyzed by UV, IR, and NMR spectra. It is confirmed that, from both the routes, namely, classical Stobbe condensation [15] and the solvent-free reaction, the same acid ester 3 is obtained.The acid ester product upon methylation with diazomethane [16] afforded diester, namely, methyl 3-carbomethoxy,4-furyl, 3-butenoate, 4 in 95% yield.The diester4 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).Table 1
Synthesis of Stobbe compounds using Diester4.
Entry
Ketone/aldehyde
Half-ester
Diacid
1
Cyclopentanone
5a
6a
2
Cyclohexanone
5b
6b
3
Acetone
5c
6c
4
Benzaldehyde
5d
6d
5
Benzophenone
5e
6e
6
Furfural
5f
6fFigure 1
Stobbe products5a–fand diacids6a–f.The results indicated that, all the reactions were performed under solvent-free conditions in good yields of 70–90% (please see Table2).  The structures of these compounds were analyzed by NMR spectroscopy [19].  All the 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 7.6δ.  Aromatic protons are depicted well in the aromatic zone of 7.2–7.5δ for the compound 5d and 6d. Similarly, the aliphatic and alicyclic protons are exhibited in their characteristic positions. Further, FTIR spectra [20] support the structures of the compounds.  The typical acid carbonyl is found in the range 1670–1690 cm−1, and the ester C=O appears at near 1715 cm−1.Table 2
Yield and melting points of Stobbe products3, 5a–f.
Compound
Yield %
mp (°C)
Classical
Green
3
65
83
111
5a
63
78
162
5b
75
82
120
5c
65
70
102
5d
62
74
194
5e
60
71
128
5f
69
85
160It is interesting to note that the saponification [15] reactions of the acid esters 5a–5f afforded the diacids 6a–6f in good yields (please see Table 3).Table 3
Yield and melting points of dicarboxylic acids6a–f.
Compound
Yield %
mp (°C)
6a
85
168
6b
85
177
6c
86
180
6d
90
202
6e
88
132
6f
89
230The 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.


## 4. 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.



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*Source: 152370-2012-08-07.xml*