A green chemical synthesis of coumarin-3-carboxylic and cinnamic acids using crop-derived products and waste waters as solvents

Article abstract of DOI:10.1016/j.tetlet.2016.09.023

Crop-derived products, like juices obtained from edible fruits and vegetables, and waste waters deriving from agricultural and industrial processing have been recently exploited to efficiently promote several ‘classic’ and innovative synthetic organic reactions. Such a green chemical approach prevented the use of toxic, polluting, and hazardous materials and in the mean time allowed to increase the commercial values of crop products and industrial byproducts. Coumarin-3-carboxylic and cinnamic acids represent classes of naturally occurring and semi-synthetic compounds with interesting and promising pharmacological activities. In this Letter a new and improved methodology for the Knoevenagel condensation yielding the title compounds using juices from edible fruits and vegetables (lemon, grapefruit, carrot, pomegranate, kiwi, vinegar, tomato), liqueurs (limoncello), and waste waters (buttermilk and residues of olive processing) as solvents is described. Coumarin-3-carboxylic and cinnamic acids have been synthesized in excellent yields by ultrasound irradiation from differently substituted 2-hydroxybenzaldehydes, 2-hydroxyacetophenones, and benzaldehydes, and Meldrum's acid as starting substrates. The findings described herein enforce the concept of the usefulness of products and byproducts derived from agriculture and food industry to accomplish green chemical processes.

Full text of DOI:10.1016/j.tetlet.2016.09.023

Accepted Manuscript
A green chemical synthesis of coumarin-3-carboxylic and cinnamic acids using
crop-derived products and waste waters as solvents
Serena Fiorito, Vito Alessandro Taddeo, Salvatore Genovese, Francesco
Epifano
PII:
S0040-4039(16)31181-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.09.023
TETL 48093
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
30 July 2016
6 September 2016
7 September 2016
Please cite this article as: Fiorito, S., Taddeo, V.A., Genovese, S., Epifano, F., A green chemical synthesis of
coumarin-3-carboxylic and cinnamic acids using crop-derived products and waste waters as solvents, Tetrahedron
Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.09.023
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A green chemical synthesis of coumarin-3-carboxylic and cinnamic acids using crop-derived
products and waste waters as solvents
Serena Fiorito, Vito Alessandro Taddeo, Salvatore Genovese*, Francesco Epifano
1Department of Pharmacy, University “G. d’Annunzio” of Chieti-Pescara, Via dei Vestini 31,
66100 Chieti Scalo (CH), Italy, Email: s.genovese@unich.it, Phone: +3908713554654, Fax:
+3908713554912
Abstract
Crop-derived products, like juices obtained from edible fruits and vegetables, and waste waters
deriving from agricultural and industrial processing have been recently exploited to efficiently
promote several “classic” and innovative synthetic organic reactions. Such a green chemical
approach prevented the use of toxic, polluting, and hazardous materials and in the mean time
allowed to increase the commercial values of crop products and industrial byproducts. Coumarin-3-
carboxylic and cinnamic acids represent classes of naturally occurring and semi-synthetic
compounds with interesting and promising pharmacological activities. In this paper a new and
improved methodology for the Knoevenagel condensation yielding the title compounds using juices
from edible fruits and vegetables (lemon, grapefruit, carrot, pomegranate, kiwi, vinegar, tomato),
liqueurs (limoncello) and waste waters (buttermilk and residues of olive processing) as solvents is
described. Coumarin-3-carboxylic and cinnamic acids have been synthesized in excellent yields by
ultrasound irradiation from differently substituted 2-hydroxybenzaldehydes, 2-
hydroxyacetophenones, and benzaldehydes, and Meldrum’s acid as starting substrates. The findings
described herein enforce the concept of the usefulness of products and byproducts derived from
agriculture and food industry to accomplish green chemical processes.
Keywords: Cinnamic acids; Coumarin-3-carboxylic acids; Crop products; Knoevenagel
condensation; Waste water

1. Introduction
In recent times, organic synthetic methodologies are mostly devoted to the development of greener
and more eco-friendly protocols involving the use of alternative solvents to replace toxic, hazardous,
polluting, and expensive solvents and catalysts. These latter are in the majority of cases represented
by heavy metals, that are notoriously very difficult to be recycled and eliminated. Currently several
organic reactions have been performed in water, being this latter readily available, inexpensive, and
safe.1 In parallel the applications of fruit and vegetable juices, and waste waters deriving from
agricultural and industrial processing recently attracted the attention of several research groups.2-4
Juices from fruits and vegetables and waste waters in fact can efficiently behave as biocatalysts and
solvents, do have environmentally beneficial characters, are non hazardous, non toxic, non polluting,
easy available, and generally cheap. Notable examples of organic reactions accomplished using
such materials as promoters include the Knoevenagel condensation,5 the Biginelli reaction,6 the
synthesis of Schiff bases7and triazoles,8 preparation of amides,9 reduction of carbonyls, hydrolysis
of esters and amides,10 and several other processes, most of which have been recently extensively
reviewed.11 Coumarin-3-carboxylic acids (synonym 3-carboxycoumarins) represent a wide group of
heterocyclic compounds with an extensive array of applications. Recent literature reports suggest
that coumarin-3-carboxylic acids exert valuable effects as anti-cancer,12 anti-diabetic,13 and
neuroprotectve agents.14 They have been also employed as key intermediates for the synthesis of
several pharmacologically active compounds like β-lactams,15 isoureas,16 and
tetrahydropyridones.17 3-Carboxycoumarin esters and amides have been shown to have anti-cancer
properties consisting in the inhibition of tumor cell growth and invasion in vitro and in vivo.18
Finally 3-carboxycoumarins have been frequently employed as fluorescent probes19 and triplet
oxygen sensitizers.20 An extensive review about the synthetic methodologies to provide coumarin-3-
carboxylic acids has been recently reported.21, 22 Cinnamic acids, like coumaric, ferulic, and caffeic
ones, are secondary metabolites ubiquitous in the plant kingdom. Naturally occurring and
semisynthetic derivatives exert beneficial effects for human health acting as anti-microbial,23 anti-

cancer,24 neuroprotective,25 anti-obesity,26 anti-oxidant,27 and anti-inflammatory agents.28 In the
context of a green chemical approach, only two methods for the synthesis of 3-carboxycoumarins
and /or cinnamic acids have been described so far. Maggi and coworkers in 2001 described the
uncatalysed Knoevenagel condensation of salicylaldehydes and Meldrum’s acid29. In 2013 Bandgar
and Chavan reported the development of this process using the same substrates and an aqueous
extract of Acacia concinna pods.30 These two processes however have some drawbacks such as
high temperatures, yields strictly depending on the structures of starting materials, and difficulties
in availability of the vegetable material. Thus the synthesis of coumarin-3-carboxylic and cinnamic
acids can be still considered a challenge to perform further research and development of greener
synthetic methods. As a continuation of our investigations devoted to draw and set up easy to
handle protocols for the synthesis of starting products and/or compounds of biological interest
employing means from the natural kingdom, we wish to report herein that coumarin-3-
carboxylic and cinnamic acids can be effectively obtained by ultrasound-assisted Knoevenagel
condensation of Meldrum’s acid and 2-hydroxybenzaldehydes or 2-hydroxyacetophenones and
benzaldehydes respectively using natural juices (lemon, grapefruit, carrot, pomegranate, kiwi,
vinegar, tomato), the liqueur “limoncello”, or waste waters deriving from agricultural and industrial
practices (processing of olives, milk, and cheese) (Figure 1).
R1
O
O
COOH
O
O
O
R1
+
R2
R2
O
OH
O
O
O
COOH
O
O
+
R
R
O
Figure 1. Knoevenagel condensation routes to coumarin-3-carboxylic (R1 = H, CH3) and cinnamic
acids

2. Results and Discussion
Preliminary tests of the Knoevenagel condensation have been accomplished using salicylaldehyde
and Meldrum’s acid as substrates, both suspended in the aqueous medium represented by lemon,
grapefruit, carrot, pomegranate, kiwi, vinegar, and tomato juices, limoncello (an hydroalcoholic
solution deriving from overnight maceration of lemon zest), or olive and buttermilk waste waters.
The processes were carried out for 24 h under magnetic stirring at room temperature. Employing
these experimental conditions, monitoring the reaction by thin layer chromatography (TLC) allowed
to assess that no or poor (< 10 %) conversions of the starting materials into the desired coumarin-3-
carboxylic acid was achieved in any of the listed media. Increasing temperature up to 60 °C,
resulted in a rapid darkening of the solution and in many instances large decomposition of the
starting products, even when shorter reaction times were applied. Ultrasounds have been frequently
recently employed to promote green chemical processes. We then decided to screen the efficiency
of ultrasound irradiation for the synthesis of coumarin-3-carboxylic acids using the same
stoichiometric ratios and volumes of solvent as above. The complete conversion of the substrates
into the desired products has been observed in every case after 5 min. irradiation. Pure coumarin-3-
carboxylic acid (entry 1, Table 1) has been isolated as a white solid after filtration in very good
yields (93 – 99 %) without the need of further purification. A blank experiment accomplished
employing pure water as the solvent under ultrasound irradiation for the same time resulted in a
poor conversion (yield < 20 %) of salicylaldehyde into 3-carboxycoumarin. Having optimized the
Knoevenagel condensation conditions, the same experimental protocol was applied to differently
substituted 2-hydroxybenzaldehydes and 2-hydroxyacetophenones and the corresponding 3-
carboxycoumarins have been synthesized in excellent yields as summarized in Table 1.
Independently from their individual structure, 2-hydroxybenzaldehydes and 2-
hydroxyacetophenones provided the desired compounds in yields ranging from 94% to 99%.

Table 1. Juices or waste waters promoted synthesis of coumarin-3-carboxylic acids
Entry Carbonyl compound
Product
Yield (%)a
R1
O
R2
R3
COOH
O
R4
A
B
C
D
E
F
G
H
I
L
1
Salicylaldehyde
R1 = R2 = R3 = R4 = H
99
99
98
99
97
96
99
97
97
97
99
97
95
94
96
98
97
92
97
99
95
97
99
98
98
99
98
96
93
98
99
98
99
99
98
97
96
94
95
94
99
98
98
95
96
98
99
99
97
95
96
98
95
98
99
98
99
97
99
98
95
96
99
97
99
98
99
98
98
96
99
92
97
98
99
95
99
96
99
99
99
99
94
91
96
98
98
97
97
96
97
98
97
99
96
97
98
99
98
98
98
96
95
95
2
4-(Diethylamino)salicylaldehyde
4-Nitrosalicylaldehyde
R1 = R2 = R4 = H, R3 = N(Et)2
R1 = R2 = R4 = H, R3 = NO2
R1 = R3 = R4 = H, R2 = Br
R1 = R2 = R4 = H, R3 = OH
3
4
5-Bromosalicylaldehyde
5
2,4-Dihydroxybenzaldehyde
2-Hydroxy-5-nitroacetophenone
5-Chloro-2-hydroxyacetophenone
2-Hydroxy-4-methoxyacetophenone
2,5-Dihydroxyacetophenone
2,3,5-Trihydroxyacetophenone
2-Hydroxyacetophenone
6
R1 = Me, R2 = NO2, R3 = R4 = H 96
R1 = Me, R2 = Cl, R3 = R4 = H
98
R1 = Me, R3 = OMe, R2 = R4 = H 96
7
8
9
R1 = Me, R2 = OH, R3 = R4 = H
R1 = Me, R2 = R4 = OH, R3 = H
R1 = Me, R2 = R3 = R4 = H
94
95
94
10
11
aYields of pure isolated products. Structural characterization by IR, GC-MS, 1H NMR, and 13C NMR Purity degree > 97.8 % assessed by HPLC.
A = lemon juice, B = grapefruit juice, C = carrot juice, D = pomegranate juice, E = kiwi juice, F = vinegar, G = tomato juice, H = limoncello, I = olive mil waste water, L =
buttermilk

Ultrasound irradiation has been subsequently applied to the synthesis of cinnamic acids using
benzaldehydes and Meldrum’s acid. The only difference in performing such experiments was the
reaction time that has to be increased to 15 min. to observe the complete conversion of the starting
material into the desired adducts (Table 2).

Table 2. Juices or waste waters promoted synthesis of cinnamic acids
Entry Aldehyde
Product
Yield (%)a
COOH
R1
R2
A
B
C
D
E
F
G
H
I
L
1
Benzaldehyde
R1 = R2 = H
98
95
97
92
93
95
97
98
98
99
95
91
92
97
95
95
95
94
99
98
95
99
96
94
94
94
98
99
94
96
96
97
97
92
91
95
95
95
98
99
99
99
93
92
98
94
96
97
96
99
95
97
97
98
99
92
93
95
97
95
91
94
98
99
94
96
96
97
92
93
98
99
92
91
95
97
97
98
98
97
98
99
97
95
98
94
98
97
95
93
98
97
99
98
99
96
99
99
96
94
98
96
99
95
97
97
97
94
97
99
2
Vanillin
R1 = OH R2 = OCH3
R1 = OH R2 = H
R1 = F, R2 = H
R1 = Cl, R2 = H
R1 = Br, R2 = H
R1 = I, R2 = H
R1 = R2 = OH
R1 = NO2, R2 = H
R1 = NH2, R2 = H
3
p-OH-Benzaldehyde
p-F-Benzaldehyde
p-Cl-Benzaldehyde
p-Br-Benzaldehyde
p-I-Benzaldehyde
3,4-Dihydroxybenzaldehyde
p-NO2-Benzaldehyde
p-NH2-Benzaldehyde
4
5
6
7
8
9
10
11
4-(3',3'-dimethylallyloxy)-3-methoxybenzaldehyde R1 = 3,3-dimethylallyloxy R2 =
OCH3
aYields of pure isolated products. Structural characterization by IR, GC-MS, 1H NMR, and 13C NMR. Purity degree > 98.4 % assessed by HPLC.
A = lemon juice, B = grapefruit juice, C = carrot juice, D = pomegranate juice, E = kiwi juice, F = vinegar, G = tomato juice, H = limoncello, I = olive mil waste water, L =
buttermilk

Pure cinnamic acids have been isolated as solids after filtration in very good yields also in this latter
case (91 – 99 %). Juices and waste waters were recovered after filtration from all reaction
media and re-used to accomplish other condensation processes without significant loss of
efficiency in terms of yields of carboxylic acids. Thus the reaction leading to coumarin-3-
carboxylic acid (entry 1, Table 3) has been carried out six additional times with lemon juice, olive
mil waste water, and buttermilk providing the final product in 92 % - 99 % yields. Similarly the
step leading to ferulic acid (entry 2, Table 3) has been repeated six additional times using the same
media listed above providing the desired adduct in 95 % - 98 % yields.

Table 3. Examples of recyclability of juices and waste waters as promoters of Knoevenagel condensation.
Entry Aldehyde
Product
Yield (%)
Lemon juice
1
Salicylaldehyde
Coumarin-3-carboxylic acid
95
96
98
95
98
98
95
92
99
95
96
98
94
95
99
98
92
92
97
95
99
Olive mil waste water
99
93
93
94
95
Buttermilk
92
92
Lemon juice
2
Vanillin
Ferulic acid
95
96
Olive mil waste water
96
Buttermilk
97 97
96
97
98

It can be hypothesized that the strong capacity of fruits and vegetable juices and waste waters to
promote the Knoevenagel condensation can be ascribed to the presence of organic acids (citric,
tartaric, malic, oxalic, succinic, lactic, pyruvic, and several others) in all the used aqueous media.
Such acids in turn may act as effective catalysts of the coupling reactions. pH measurements for
juices and waste waters in fact revealed values in the range 4.2 – 5.1. Thus from a mechanistic point
of view, these acids may be able to protonate the oxygen atom of the carbonyl function of
benzaldehydes or acetophenones enhancing its reactivity towards the nucleophilic attack by
Meldrum’s acid in a way similar to that already depicted in the literature.31
3. Conclusions
In this manuscript we have demonstrated that non toxic, safe, non polluting, cheap, easy to obtain
and handle juices and waste waters deriving from agricultural practices and industrial processes are
efficient and high yielding promoters for the Knoevenagel condensation of 2-
hydroxybenzaldehydes, 2-hydroxyacetophenones, and benzaldehydes with Meldrum’s acid leading
to 3-carboxycoumarins and cinnamic acids respectively. The main advantages of our process are the
simple work-up, mild reaction conditions, short times, and excellent yields. Thus the reaction
reported herein can be regarded as a valid and additional example to the already described
methodologies that employ crop-derived products and byproducts as solvent and/or catalysts to
promote organic reactions. The efficient use described herein of water residues of industrial
processing of olives, milk, and cheese in particular represents also an example of useful waste
management and recycling. Further investigation into the potentialities, applications, and general
scopes of the above listed and other media of natural origin are now in progress in our laboratories
and will be reported in due course.
Acknowledgements

Financial support to this research from University “G. d’Annunzio” of Chieti-Pescara (Fondi FAR
2015) is gratefully acknowledged.
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Graphical abstract
O
O
O
O
2-Hydroxybenzaldehydes
2-Hydroxyacetophenones +
Benzaldehydes
Juices
3-Carboxycoumarins
Cinnamic acids
91 - 99%
Waste waters
Ultrasounds
5-15 min.

Highlights
1) Natural juices or waste waters used for a green Knoevenagel condensation
2) 3-Carboxycoumarins and cinnamic acids have been obtained in very good yields
3) Easy to handle and powerful technology have been applied