Efficient Multicomponent Synthesis of Diverse Antibacterial Embelin-Privileged Structure Conjugates

A library of embelin derivatives has been synthesized through a multicomponent reaction from embelin (1), aldehydes and privileged structures such as 4-hydroxycoumarin, 4-hydroxy-2H-pyran-2-one and 2-naphthol, in the presence of InCl3 as catalyst. This multicomponent reaction implies Knoevenagel condensation, Michael addition, intramolecular cyclization and dehydration. Many of the synthesized compounds were active and selective against Gram-positive bacteria, including one important multiresistant Staphylococcus aureus clinical isolate. It was found how the conjugation of diverse privileged substructure with embelin led to adducts having enhanced antibacterial activities.


Introduction
Natural products continue to play a pivotal role in the search for new therapeutic drug leads. They have inherent bioactivities and high bioavailability, probably because of their specific interactions with target macromolecules in living organisms. Thus, the chemical space defined by natural products may nicely overlap with biological space [1]. Therefore, structural motifs and core skeletons from bioactive natural products can serve for the synthesis of novel core skeletons with high biological relevancy. In this context, the natural benzoquinone embelin (1) is an attractive molecule since displays a good number of biological activities such as antimicrobial [2], inhibition of X-chromosome-linked inhibitor of apoptosis protein (XIAPS) [3], inhibition of mortalin-p53 interactions, and activation of p53 protein in tumor cells [4], inhibition of 5-lipoxygenase [5], antitumoral activity via activation of p38/JNK pathway [6] and antidiabetic activity [7].
Thus, this benzoquinone represents a good starting scaffold for the preparation of a structurally diverse collection of embelin derivatives. To assure the biological relevancy of this library we combine the use of privileged structures and complexity-generating reactions such Scheme 1. Structure of embelin-privileged structure conjugates.
Since in the presence of an aldehyde both the embelin and the mentioned compounds (2-4) having nucleophilic carbons can react to afford the corresponding quinone methide intermediate via Knoevenagel condensation, we calculated the Fukui function in order to explore, which one shows the highest nucleophilicity. Fukui function is one of the widely used local density functional descriptor to model chemical reactivity and site-selectivity [16]. The local (condensed) Fukui functions (fk + , fk − , fk 0 ) are calculated using the procedure proposed by Yang and Mortier [17], employing equations such as fk + = [q(N + 1) − q(N)] for nucleophilic attack; fk − = [q(N) − q(N − 1)] for electrophilic attack and fk 0 = ½ [q(N + 1) − q(N − 1)] for radical attack, where N is the total numbers of electrons. When a molecule accepts electrons, the electrons tend to go to places were fk + is large because it is at these locations that the molecule is most able to stabilize additional electrons. Therefore a molecule is susceptible of an electrophilic attack at sites where fk − is large. The calculated values of the Fukui function (fk − ) are shown in Figure 1. As we can see, compounds 2-4 show higher values of (f − ) than embelin (1), which implies that the quinone methide intermediate is presumably formed from these compounds, and next the Scheme 1. Structure of embelin-privileged structure conjugates.
Since in the presence of an aldehyde both the embelin and the mentioned compounds (2-4) having nucleophilic carbons can react to afford the corresponding quinone methide intermediate via Knoevenagel condensation, we calculated the Fukui function in order to explore, which one shows the highest nucleophilicity. Fukui function is one of the widely used local density functional descriptor to model chemical reactivity and site-selectivity [16]. The local (condensed) Fukui functions (f k + , f k − , f k 0 ) are calculated using the procedure proposed by Yang and Mortier [17], employing equations such as f k + = [q(N + 1) − q(N)] for nucleophilic attack; f k − = [q(N) − q(N − 1)] for electrophilic attack and f k 0 = 1 2 [q(N + 1) − q(N − 1)] for radical attack, where N is the total numbers of electrons. When a molecule accepts electrons, the electrons tend to go to places were f k + is large because it is at these locations that the molecule is most able to stabilize additional electrons. Therefore a molecule is susceptible of an electrophilic attack at sites where f k − is large. The calculated values of the Fukui function (f k − ) are shown in Figure 1.
Molecules 2020, 25, x 2 of 22 combine the use of privileged structures and complexity-generating reactions such as multicomponent reactions [8][9][10][11][12]. Privileged structures are defined as a single molecular framework able to provide a series of ligands for diverse receptors and have been extensively utilized in rational drug design owing to their potent biological activities [13]. Thus, from a domino Knoevenagel-Michael addition-cyclization-dehydration reaction using embelin (1), aldehydes and antibacterial privileged structural motifs as source of nucleophilic carbons, we can access to a library of dihydropyranbenzoquinones embedded with privileged substructures (Scheme 1). Moreover, this library may provide new compounds with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms.
Since in the presence of an aldehyde both the embelin and the mentioned compounds (2-4) having nucleophilic carbons can react to afford the corresponding quinone methide intermediate via Knoevenagel condensation, we calculated the Fukui function in order to explore, which one shows the highest nucleophilicity. Fukui function is one of the widely used local density functional descriptor to model chemical reactivity and site-selectivity [16]. The local (condensed) Fukui functions (fk + , fk − , fk 0 ) are calculated using the procedure proposed by Yang and Mortier [17], employing equations such as fk + = [q(N + 1) − q(N)] for nucleophilic attack; fk − = [q(N) − q(N − 1)] for electrophilic attack and fk 0 = ½ [q(N + 1) − q(N − 1)] for radical attack, where N is the total numbers of electrons. When a molecule accepts electrons, the electrons tend to go to places were fk + is large because it is at these locations that the molecule is most able to stabilize additional electrons. Therefore a molecule is susceptible of an electrophilic attack at sites where fk − is large. The calculated values of the Fukui function (fk − ) are shown in Figure 1. As we can see, compounds 2-4 show higher values of (f − ) than embelin (1), which implies that the quinone methide intermediate is presumably formed from these compounds, and next the As we can see, compounds 2-4 show higher values of (f − ) than embelin (1), which implies that the quinone methide intermediate is presumably formed from these compounds, and next the nucleophilic attack of embelin will take place on the more electrophilic α, β-unsaturated carbonyl, followed of intramolecular cyclization with loss of H 2 O to yield the corresponding conjugates (Scheme 2).
Molecules 2020, 25, x 3 of 22 nucleophilic attack of embelin will take place on the more electrophilic , -unsaturated carbonyl, followed of intramolecular cyclization with loss of H2O to yield the corresponding conjugates (Scheme 2).

Scheme 2. Formation of embelin-conjugates.
Furthermore, for assessing the molecular diversity of the devised molecular framework, electrostatic polar surface area of energy-minimized conformers as well as the isosurface diagram of each adduct (R=H) were obtained by the calculation of electrostatic polar potentials and electron density [18]. As shown in Scheme 2, these three conjugates have a distinguishable display of electrostatic polar surface area because of the differentiation in electronic properties of each privileged substructure. The further expansion of molecular diversity can be achieved via the introduction of various moieties at the dihydropyran such as aliphatic and aromatic groups with electron donating and electron withdrawing substituents.
First, we decided to study the multicomponent reaction of embelin, 4-hydroxy-6-methyl-2H-pyran-2-one (2) and 4-bromobenzaldehyde. We used different reaction conditions and several catalysts employed in multicomponent reactions of 1,3-dicarbonyl compounds such as EDDA [19], PTSA [20], Sc(OTf)3 [21], Yb(OTf)3 [22], and InCl3 [23]. Some results are shown in Table 1. Furthermore, for assessing the molecular diversity of the devised molecular framework, electrostatic polar surface area of energy-minimized conformers as well as the isosurface diagram of each adduct (R=H) were obtained by the calculation of electrostatic polar potentials and electron density [18]. As shown in Scheme 2, these three conjugates have a distinguishable display of electrostatic polar surface area because of the differentiation in electronic properties of each privileged substructure. The further expansion of molecular diversity can be achieved via the introduction of various moieties at the dihydropyran such as aliphatic and aromatic groups with electron donating and electron withdrawing substituents.
First, we decided to study the multicomponent reaction of embelin, 4-hydroxy-6-methyl-2H-pyran-2-one (2) and 4-bromobenzaldehyde. We used different reaction conditions and several catalysts employed in multicomponent reactions of 1,3-dicarbonyl compounds such as EDDA [19], PTSA [20], Sc(OTf) 3 [21], Yb(OTf) 3 [22], and InCl 3 [23]. Some results are shown in Table 1. The use of ethylendiamine diacetate (EDDA) as an effective organocatalyst for the initial Knoevenagel condensation did not produce the desired adduct 3a (entries 1-3). When InCl 3 (10 mol%) was used in EtOH under reflux compound 3a was obtained in low yield (32%, entry 4). The yield was improved when the reaction was carried out without solvent (52%, entry 5) at 120 • C. The use of other Lewis acids (entries 6 and 7) and p-toluenesulfonic acid (PTSA) (entry 8) under neat conditions at 120 • C, did not improved the yields. Increasing the load of InCl 3 (20 mol%) gave higher yield (58%). We also carried out the multicomponent reaction without catalyst (entry 9) and adduct 3a was achieved in low yield (11%). Thus, we selected the reaction conditions of entry 10 and the scope of this multicomponent process was then assessed through the variation of diverse aromatic and aliphatic aldehydes ( Table 2). Diversely substituted tricyclic embelin adducts (3a-3l) could be prepared in moderated yields, demonstrating the versatility of this domino process. As a general trend, the multicomponent reaction is tolerant to a large variety of aryl-substituted aldehydes with electron-donating and electron-withdrawing groups, and also the reaction proceeds with aliphatic aldehydes. Table 2. Synthesis of novel tricyclic embelin adducts (3a-3l).
3a was achieved in low yield (11%). Thus, we selected the reaction conditions of entry 10 and the scope of this multicomponent process was then assessed through the variation of diverse aromatic and aliphatic aldehydes ( Table 2). Diversely substituted tricyclic embelin adducts (3a-3l) could be prepared in moderated yields, demonstrating the versatility of this domino process. As a general trend, the multicomponent reaction is tolerant to a large variety of aryl-substituted aldehydes with electron-donating and electron-withdrawing groups, and also the reaction proceeds with aliphatic aldehydes. Table 2. Synthesis of novel tricyclic embelin adducts (3a-3l). scope of this multicomponent process was then assessed through the variation of diverse aromatic and aliphatic aldehydes ( Table 2). Diversely substituted tricyclic embelin adducts (3a-3l) could be prepared in moderated yields, demonstrating the versatility of this domino process. As a general trend, the multicomponent reaction is tolerant to a large variety of aryl-substituted aldehydes with electron-donating and electron-withdrawing groups, and also the reaction proceeds with aliphatic aldehydes. Table 2. Synthesis of novel tricyclic embelin adducts (3a-3l).
3a was achieved in low yield (11%). Thus, we selected the reaction conditions of entry 10 and the scope of this multicomponent process was then assessed through the variation of diverse aromatic and aliphatic aldehydes ( Table 2). Diversely substituted tricyclic embelin adducts (3a-3l) could be prepared in moderated yields, demonstrating the versatility of this domino process. As a general trend, the multicomponent reaction is tolerant to a large variety of aryl-substituted aldehydes with electron-donating and electron-withdrawing groups, and also the reaction proceeds with aliphatic aldehydes. Table 2. Synthesis of novel tricyclic embelin adducts (3a-3l).
3a was achieved in low yield (11%). Thus, we selected the reaction conditions of entry 10 and the scope of this multicomponent process was then assessed through the variation of diverse aromatic and aliphatic aldehydes ( Table 2). Diversely substituted tricyclic embelin adducts (3a-3l) could be prepared in moderated yields, demonstrating the versatility of this domino process. As a general trend, the multicomponent reaction is tolerant to a large variety of aryl-substituted aldehydes with electron-donating and electron-withdrawing groups, and also the reaction proceeds with aliphatic aldehydes. Table 2. Synthesis of novel tricyclic embelin adducts (3a-3l).
3a was achieved in low yield (11%). Thus, we selected the reaction conditions of entry 10 and the scope of this multicomponent process was then assessed through the variation of diverse aromatic and aliphatic aldehydes ( Table 2). Diversely substituted tricyclic embelin adducts (3a-3l) could be prepared in moderated yields, demonstrating the versatility of this domino process. As a general trend, the multicomponent reaction is tolerant to a large variety of aryl-substituted aldehydes with electron-donating and electron-withdrawing groups, and also the reaction proceeds with aliphatic aldehydes. Table 2. Synthesis of novel tricyclic embelin adducts (3a-3l).
3a was achieved in low yield (11%). Thus, we selected the reaction conditions of entry 10 and the scope of this multicomponent process was then assessed through the variation of diverse aromatic and aliphatic aldehydes ( Table 2). Diversely substituted tricyclic embelin adducts (3a-3l) could be prepared in moderated yields, demonstrating the versatility of this domino process. As a general trend, the multicomponent reaction is tolerant to a large variety of aryl-substituted aldehydes with electron-donating and electron-withdrawing groups, and also the reaction proceeds with aliphatic aldehydes. Table 2. Synthesis of novel tricyclic embelin adducts (3a-3l).
3a was achieved in low yield (11%). Thus, we selected the reaction conditions of entry 10 and the scope of this multicomponent process was then assessed through the variation of diverse aromatic and aliphatic aldehydes ( Table 2). Diversely substituted tricyclic embelin adducts (3a-3l) could be prepared in moderated yields, demonstrating the versatility of this domino process. As a general trend, the multicomponent reaction is tolerant to a large variety of aryl-substituted aldehydes with electron-donating and electron-withdrawing groups, and also the reaction proceeds with aliphatic aldehydes. Table 2. Synthesis of novel tricyclic embelin adducts (3a-3l).
3a was achieved in low yield (11%). Thus, we selected the reaction conditions of entry 10 and the scope of this multicomponent process was then assessed through the variation of diverse aromatic and aliphatic aldehydes ( Table 2). Diversely substituted tricyclic embelin adducts (3a-3l) could be prepared in moderated yields, demonstrating the versatility of this domino process. As a general trend, the multicomponent reaction is tolerant to a large variety of aryl-substituted aldehydes with electron-donating and electron-withdrawing groups, and also the reaction proceeds with aliphatic aldehydes. Table 2. Synthesis of novel tricyclic embelin adducts (3a-3l).
3a was achieved in low yield (11%). Thus, we selected the reaction conditions of entry 10 and the scope of this multicomponent process was then assessed through the variation of diverse aromatic and aliphatic aldehydes ( Table 2). Diversely substituted tricyclic embelin adducts (3a-3l) could be prepared in moderated yields, demonstrating the versatility of this domino process. As a general trend, the multicomponent reaction is tolerant to a large variety of aryl-substituted aldehydes with electron-donating and electron-withdrawing groups, and also the reaction proceeds with aliphatic aldehydes. The reaction can be rationalized via the formation of a conjugated electron-deficient enone (A) through a Knoevenagel condensation of 2 and an aldehyde. The next step of this mechanism could involve a Michael addition of embelin (1) to the reactive quinone methide intermediate to yield the intermediate (B), which can undergo an intramolecular cyclization through carbonyl a to give the para-quinone adduct or through carbonyl b to yield the ortho-quinone adduct (Scheme 3).
The process is regioselective since only the 1,4-benzoquinone adduct is obtained. A plausible explanation for this regioselectivity is that the reaction takes place through a more electron deficient carbonyl moiety next to another carbonyl group. Two new fused rings next to the benzoquinone core and three σ bonds (two C-C σ bonds and one C-O σ bond) were formed in this multicomponent reaction. The regiosubstitution of the corresponding adducts was confirmed by the three-bond correlations detected in the HMBC spectrum and also by the 13 C NMR values of the quinone carbonyls [10][11][12] (Supplementary Materials). The InCl3 would promote the generation of the key quinone methide through dehydration of the alcohol formed in the Knoevenagel condensation and furthermore it could activate the quinone methide intermediates A. The reaction can be rationalized via the formation of a conjugated electron-deficient enone (A) through a Knoevenagel condensation of 2 and an aldehyde. The next step of this mechanism could involve a Michael addition of embelin (1) to the reactive quinone methide intermediate to yield the intermediate (B), which can undergo an intramolecular cyclization through carbonyl a to give the para-quinone adduct or through carbonyl b to yield the ortho-quinone adduct (Scheme 3).
The process is regioselective since only the 1,4-benzoquinone adduct is obtained. A plausible explanation for this regioselectivity is that the reaction takes place through a more electron deficient carbonyl moiety next to another carbonyl group. Two new fused rings next to the benzoquinone core and three σ bonds (two C-C σ bonds and one C-O σ bond) were formed in this multicomponent reaction. The regiosubstitution of the corresponding adducts was confirmed by the three-bond correlations detected in the HMBC spectrum and also by the 13 C NMR values of the quinone carbonyls [10][11][12] (Supplementary Materials). The InCl3 would promote the generation of the key quinone methide through dehydration of the alcohol formed in the Knoevenagel condensation and furthermore it could activate the quinone methide intermediates A. The reaction can be rationalized via the formation of a conjugated electron-deficient enone (A) through a Knoevenagel condensation of 2 and an aldehyde. The next step of this mechanism could involve a Michael addition of embelin (1) to the reactive quinone methide intermediate to yield the intermediate (B), which can undergo an intramolecular cyclization through carbonyl a to give the para-quinone adduct or through carbonyl b to yield the ortho-quinone adduct (Scheme 3).
The process is regioselective since only the 1,4-benzoquinone adduct is obtained. A plausible explanation for this regioselectivity is that the reaction takes place through a more electron deficient carbonyl moiety next to another carbonyl group. Two new fused rings next to the benzoquinone core and three σ bonds (two C-C σ bonds and one C-O σ bond) were formed in this multicomponent reaction. The regiosubstitution of the corresponding adducts was confirmed by the three-bond correlations detected in the HMBC spectrum and also by the 13 C NMR values of the quinone carbonyls [10][11][12] (Supplementary Materials). The InCl3 would promote the generation of the key quinone methide through dehydration of the alcohol formed in the Knoevenagel condensation and furthermore it could activate the quinone methide intermediates A. The reaction can be rationalized via the formation of a conjugated electron-deficient enone (A) through a Knoevenagel condensation of 2 and an aldehyde. The next step of this mechanism could involve a Michael addition of embelin (1) to the reactive quinone methide intermediate to yield the intermediate (B), which can undergo an intramolecular cyclization through carbonyl a to give the para-quinone adduct or through carbonyl b to yield the ortho-quinone adduct (Scheme 3).
The process is regioselective since only the 1,4-benzoquinone adduct is obtained. A plausible explanation for this regioselectivity is that the reaction takes place through a more electron deficient carbonyl moiety next to another carbonyl group. Two new fused rings next to the benzoquinone core and three σ bonds (two C-C σ bonds and one C-O σ bond) were formed in this multicomponent reaction. The regiosubstitution of the corresponding adducts was confirmed by the three-bond correlations detected in the HMBC spectrum and also by the 13 C NMR values of the quinone carbonyls [10][11][12] (Supplementary Materials). The InCl3 would promote the generation of the key quinone methide through dehydration of the alcohol formed in the Knoevenagel condensation and furthermore it could activate the quinone methide intermediates A. The reaction can be rationalized via the formation of a conjugated electron-deficient enone (A) through a Knoevenagel condensation of 2 and an aldehyde. The next step of this mechanism could involve a Michael addition of embelin (1) to the reactive quinone methide intermediate to yield the intermediate (B), which can undergo an intramolecular cyclization through carbonyl a to give the para-quinone adduct or through carbonyl b to yield the ortho-quinone adduct (Scheme 3).
The process is regioselective since only the 1,4-benzoquinone adduct is obtained. A plausible explanation for this regioselectivity is that the reaction takes place through a more electron deficient carbonyl moiety next to another carbonyl group. Two new fused rings next to the benzoquinone core and three σ bonds (two C-C σ bonds and one C-O σ bond) were formed in this multicomponent reaction. The regiosubstitution of the corresponding adducts was confirmed by the three-bond correlations detected in the HMBC spectrum and also by the 13 C NMR values of the quinone carbonyls [10][11][12] (Supplementary Materials). The InCl3 would promote the generation of the key quinone methide through dehydration of the alcohol formed in the Knoevenagel condensation and furthermore it could activate the quinone methide intermediates A. The reaction can be rationalized via the formation of a conjugated electron-deficient enone (A) through a Knoevenagel condensation of 2 and an aldehyde. The next step of this mechanism could involve a Michael addition of embelin (1) to the reactive quinone methide intermediate to yield the intermediate (B), which can undergo an intramolecular cyclization through carbonyl a to give the para-quinone adduct or through carbonyl b to yield the ortho-quinone adduct (Scheme 3).
The process is regioselective since only the 1,4-benzoquinone adduct is obtained. A plausible explanation for this regioselectivity is that the reaction takes place through a more electron deficient carbonyl moiety next to another carbonyl group. Two new fused rings next to the benzoquinone core and three σ bonds (two C-C σ bonds and one C-O σ bond) were formed in this multicomponent reaction. The regiosubstitution of the corresponding adducts was confirmed by the three-bond correlations detected in the HMBC spectrum and also by the 13  Next, we decided to synthesize more complex embelin adducts by reacting embelin (1), aldehydes and the privileged structure 4-hydroxycoumarin (4). These tetracyclic adducts (4a-4l) compared to adducts 3a-3l present an extension of the structure by the introduction of an aromatic ring fused to the 2H-pyran-2-one nucleus, which results very attractive for the establishment of structure-activity relationships after biological evaluation. The same reaction conditions for the synthesis of adducts 3a-3l were used. Table 3 shows the structures and the yields of the obtained conjugates (4a-4l). As we can see improved yields were achieved by using 4-hydroxycoumarin as nucleophile component in the initial Knoevenagel condensation. Table 3. Synthesis of novel embelin-coumarin conjugates (4a-4l).

Scheme 3. Plausible formation of adducts 3a-3l.
The process is regioselective since only the 1,4-benzoquinone adduct is obtained. A plausible explanation for this regioselectivity is that the reaction takes place through a more electron deficient carbonyl moiety next to another carbonyl group. Two new fused rings next to the benzoquinone core and three σ bonds (two C-C σ bonds and one C-O σ bond) were formed in this multicomponent reaction. The regiosubstitution of the corresponding adducts was confirmed by the three-bond correlations detected in the HMBC spectrum and also by the 13 C NMR values of the quinone carbonyls [10][11][12] (Supplementary Materials). The InCl 3 would promote the generation of the key quinone methide through dehydration of the alcohol formed in the Knoevenagel condensation and furthermore it could activate the quinone methide intermediates A.
Next, we decided to synthesize more complex embelin adducts by reacting embelin (1), aldehydes and the privileged structure 4-hydroxycoumarin (4). These tetracyclic adducts (4a-4l) compared to adducts 3a-3l present an extension of the structure by the introduction of an aromatic ring fused to the 2H-pyran-2-one nucleus, which results very attractive for the establishment of structure-activity relationships after biological evaluation. The same reaction conditions for the synthesis of adducts 3a-3l were used. Table 3 shows the structures and the yields of the obtained conjugates (4a-4l). As we can see improved yields were achieved by using 4-hydroxycoumarin as nucleophile component in the initial Knoevenagel condensation. compared to adducts 3a-3l present an extension of the structure by the introduction of an aromatic ring fused to the 2H-pyran-2-one nucleus, which results very attractive for the establishment of structure-activity relationships after biological evaluation. The same reaction conditions for the synthesis of adducts 3a-3l were used. Table 3 shows the structures and the yields of the obtained conjugates (4a-4l). As we can see improved yields were achieved by using 4-hydroxycoumarin as nucleophile component in the initial Knoevenagel condensation. Table 3. Synthesis of novel embelin-coumarin conjugates (4a-4l). compared to adducts 3a-3l present an extension of the structure by the introduction of an aromatic ring fused to the 2H-pyran-2-one nucleus, which results very attractive for the establishment of structure-activity relationships after biological evaluation. The same reaction conditions for the synthesis of adducts 3a-3l were used. Table 3 shows the structures and the yields of the obtained conjugates (4a-4l). As we can see improved yields were achieved by using 4-hydroxycoumarin as nucleophile component in the initial Knoevenagel condensation. Table 3. Synthesis of novel embelin-coumarin conjugates (4a-4l). compared to adducts 3a-3l present an extension of the structure by the introduction of an aromatic ring fused to the 2H-pyran-2-one nucleus, which results very attractive for the establishment of structure-activity relationships after biological evaluation. The same reaction conditions for the synthesis of adducts 3a-3l were used. Table 3 shows the structures and the yields of the obtained conjugates (4a-4l). As we can see improved yields were achieved by using 4-hydroxycoumarin as nucleophile component in the initial Knoevenagel condensation. Table 3. Synthesis of novel embelin-coumarin conjugates (4a-4l). compared to adducts 3a-3l present an extension of the structure by the introduction of an aromatic ring fused to the 2H-pyran-2-one nucleus, which results very attractive for the establishment of structure-activity relationships after biological evaluation. The same reaction conditions for the synthesis of adducts 3a-3l were used. Table 3 shows the structures and the yields of the obtained conjugates (4a-4l). As we can see improved yields were achieved by using 4-hydroxycoumarin as nucleophile component in the initial Knoevenagel condensation. Table 3. Synthesis of novel embelin-coumarin conjugates (4a-4l). compared to adducts 3a-3l present an extension of the structure by the introduction of an aromatic ring fused to the 2H-pyran-2-one nucleus, which results very attractive for the establishment of structure-activity relationships after biological evaluation. The same reaction conditions for the synthesis of adducts 3a-3l were used. Table 3 shows the structures and the yields of the obtained conjugates (4a-4l). As we can see improved yields were achieved by using 4-hydroxycoumarin as nucleophile component in the initial Knoevenagel condensation. Table 3. Synthesis of novel embelin-coumarin conjugates (4a-4l). compared to adducts 3a-3l present an extension of the structure by the introduction of an aromatic ring fused to the 2H-pyran-2-one nucleus, which results very attractive for the establishment of structure-activity relationships after biological evaluation. The same reaction conditions for the synthesis of adducts 3a-3l were used. Table 3 shows the structures and the yields of the obtained conjugates (4a-4l). As we can see improved yields were achieved by using 4-hydroxycoumarin as nucleophile component in the initial Knoevenagel condensation. The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Table 4. Synthesis of novel tetracyclic embelin adducts (5a-5l). The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Table 4. Synthesis of novel tetracyclic embelin adducts (5a-5l). The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Table 4. Synthesis of novel tetracyclic embelin adducts (5a-5l). The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Table 4. Synthesis of novel tetracyclic embelin adducts (5a-5l).
The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Table 4. Synthesis of novel tetracyclic embelin adducts (5a-5l).
The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Table 4. Synthesis of novel tetracyclic embelin adducts (5a-5l).
The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Table 4. Synthesis of novel tetracyclic embelin adducts (5a-5l).
The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Table 4. Synthesis of novel tetracyclic embelin adducts (5a-5l).
The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Table 4. Synthesis of novel tetracyclic embelin adducts (5a-5l).
The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Table 4. Synthesis of novel tetracyclic embelin adducts (5a-5l).
The last series was synthesized using 2-naphthol as nucleophilic component, in this case the tetracyclic adducts do not present the lactone ring of the previous series and they were obtained with higher yields than those from 4-hydroxy-2H-pyran-2-one and 4-hydroxycoumarin ( Table 4). The conjugates synthesized from the aliphatic aldehydes (5j-5l) were achieved with the lowest yields. Since, the conjugation of embelin with other anti-bacterial moieties could provide new candidates with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms, all synthesized conjugates were tested for antimicrobial activity. The compounds had no effect on the growth of the assayed Gram-negative bacteria Escherichia coli and on the growth of the yeast Saccharomyces cerevisiae (MIC > 128 μM). By contrast, many compounds were selectively active against the three Gram-positive bacteria tested: the methicillin-sensitive Staphylococcus aureus (MSSA) ATCC25923 strain, the methicillin-resistant S. aureus NRS402 strain, which is also intermediate resistant to vancomycin (VISA), and the Enterococcus faecalis ATCC29212 strain (Table 5), and they were more active than embelin (1). This fact is interesting since bacterial infections, caused by Gram-positive pathogens such as Staphylococcus are account for the majority of opportunistic community-acquired and hospital-acquired infections. Since, the conjugation of embelin with other anti-bacterial moieties could provide new candidates with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms, all synthesized conjugates were tested for antimicrobial activity. The compounds had no effect on the growth of the assayed Gram-negative bacteria Escherichia coli and on the growth of the yeast Saccharomyces cerevisiae (MIC > 128 μM). By contrast, many compounds were selectively active against the three Gram-positive bacteria tested: the methicillin-sensitive Staphylococcus aureus (MSSA) ATCC25923 strain, the methicillin-resistant S. aureus NRS402 strain, which is also intermediate resistant to vancomycin (VISA), and the Enterococcus faecalis ATCC29212 strain (Table 5), and they were more active than embelin (1). This fact is interesting since bacterial infections, caused by Gram-positive pathogens such as Staphylococcus are account for the majority of opportunistic community-acquired and hospital-acquired infections. Since, the conjugation of embelin with other anti-bacterial moieties could provide new candidates with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms, all synthesized conjugates were tested for antimicrobial activity. The compounds had no effect on the growth of the assayed Gram-negative bacteria Escherichia coli and on the growth of the yeast Saccharomyces cerevisiae (MIC > 128 μM). By contrast, many compounds were selectively active against the three Gram-positive bacteria tested: the methicillin-sensitive Staphylococcus aureus (MSSA) ATCC25923 strain, the methicillin-resistant S. aureus NRS402 strain, which is also intermediate resistant to vancomycin (VISA), and the Enterococcus faecalis ATCC29212 strain (Table 5), and they were more active than embelin (1). This fact is interesting since bacterial infections, caused by Gram-positive pathogens such as Staphylococcus are account for the majority of opportunistic community-acquired and hospital-acquired infections. Since, the conjugation of embelin with other anti-bacterial moieties could provide new candidates with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms, all synthesized conjugates were tested for antimicrobial activity. The compounds had no effect on the growth of the assayed Gram-negative bacteria Escherichia coli and on the growth of the yeast Saccharomyces cerevisiae (MIC > 128 μM). By contrast, many compounds were selectively active against the three Gram-positive bacteria tested: the methicillin-sensitive Staphylococcus aureus (MSSA) ATCC25923 strain, the methicillin-resistant S. aureus NRS402 strain, which is also intermediate resistant to vancomycin (VISA), and the Enterococcus faecalis ATCC29212 strain (Table 5), and they were more active than embelin (1). This fact is interesting since bacterial infections, caused by Gram-positive pathogens such as Staphylococcus are account for the majority of opportunistic community-acquired and hospital-acquired infections.  compounds 1, 3a-l, 4a-l, and 5a- Since, the conjugation of embelin with other anti-bacterial moieties could provide new candidates with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms, all synthesized conjugates were tested for antimicrobial activity. The compounds had no effect on the growth of the assayed Gram-negative bacteria Escherichia coli and on the growth of the yeast Saccharomyces cerevisiae (MIC > 128 μM). By contrast, many compounds were selectively active against the three Gram-positive bacteria tested: the methicillin-sensitive Staphylococcus aureus (MSSA) ATCC25923 strain, the methicillin-resistant S. aureus NRS402 strain, which is also intermediate resistant to vancomycin (VISA), and the Enterococcus faecalis ATCC29212 strain (Table 5), and they were more active than embelin (1). This fact is interesting since bacterial infections, caused by Gram-positive pathogens such as Staphylococcus are account for the majority of opportunistic community-acquired and hospital-acquired infections.  compounds 1, 3a-l, 4a-l, and 5a- Since, the conjugation of embelin with other anti-bacterial moieties could provide new candidates with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms, all synthesized conjugates were tested for antimicrobial activity. The compounds had no effect on the growth of the assayed Gram-negative bacteria Escherichia coli and on the growth of the yeast Saccharomyces cerevisiae (MIC > 128 μM). By contrast, many compounds were selectively active against the three Gram-positive bacteria tested: the methicillin-sensitive Staphylococcus aureus (MSSA) ATCC25923 strain, the methicillin-resistant S. aureus NRS402 strain, which is also intermediate resistant to vancomycin (VISA), and the Enterococcus faecalis ATCC29212 strain (Table 5), and they were more active than embelin (1). This fact is interesting since bacterial infections, caused by Gram-positive pathogens such as Staphylococcus are account for the majority of opportunistic community-acquired and hospital-acquired infections.  compounds 1, 3a-l, 4a-l, and 5a- Since, the conjugation of embelin with other anti-bacterial moieties could provide new candidates with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms, all synthesized conjugates were tested for antimicrobial activity. The compounds had no effect on the growth of the assayed Gram-negative bacteria Escherichia coli and on the growth of the yeast Saccharomyces cerevisiae (MIC > 128 μM). By contrast, many compounds were selectively active against the three Gram-positive bacteria tested: the methicillin-sensitive Staphylococcus aureus (MSSA) ATCC25923 strain, the methicillin-resistant S. aureus NRS402 strain, which is also intermediate resistant to vancomycin (VISA), and the Enterococcus faecalis ATCC29212 strain (Table 5), and they were more active than embelin (1). This fact is interesting since bacterial infections, caused by Gram-positive pathogens such as Staphylococcus are account for the majority of opportunistic community-acquired and hospital-acquired infections.  compounds 1, 3a-l, 4a-l, and 5a- Since, the conjugation of embelin with other anti-bacterial moieties could provide new candidates with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms, all synthesized conjugates were tested for antimicrobial activity. The compounds had no effect on the growth of the assayed Gram-negative bacteria Escherichia coli and on the growth of the yeast Saccharomyces cerevisiae (MIC > 128 μM). By contrast, many compounds were selectively active against the three Gram-positive bacteria tested: the methicillin-sensitive Staphylococcus aureus (MSSA) ATCC25923 strain, the methicillin-resistant S. aureus NRS402 strain, which is also intermediate resistant to vancomycin (VISA), and the Enterococcus faecalis ATCC29212 strain (Table 5), and they were more active than embelin (1). This fact is interesting since bacterial infections, caused by Gram-positive pathogens such as Staphylococcus are account for the majority of opportunistic community-acquired and hospital-acquired infections.  compounds 1, 3a-l, 4a-l, and 5a-l against the three selected Gram-positive bacterial strains. Since, the conjugation of embelin with other anti-bacterial moieties could provide new candidates with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms, all synthesized conjugates were tested for antimicrobial activity. The compounds had no effect on the growth of the assayed Gram-negative bacteria Escherichia coli and on the growth of the yeast Saccharomyces cerevisiae (MIC > 128 μM). By contrast, many compounds were selectively active against the three Gram-positive bacteria tested: the methicillin-sensitive Staphylococcus aureus (MSSA) ATCC25923 strain, the methicillin-resistant S. aureus NRS402 strain, which is also intermediate resistant to vancomycin (VISA), and the Enterococcus faecalis ATCC29212 strain (Table 5), and they were more active than embelin (1). This fact is interesting since bacterial infections, caused by Gram-positive pathogens such as Staphylococcus are account for the majority of opportunistic community-acquired and hospital-acquired infections.  compounds 1, 3a-l, 4a-l, and 5a-l against the three selected Gram-positive bacterial strains.
Since, the conjugation of embelin with other anti-bacterial moieties could provide new candidates with great potency against both drug sensitive and drug-resistant Gram-positive and Gram-negative organisms, all synthesized conjugates were tested for antimicrobial activity. The compounds had no effect on the growth of the assayed Gram-negative bacteria Escherichia coli and on the growth of the yeast Saccharomyces cerevisiae (MIC > 128 µM). By contrast, many compounds were selectively active against the three Gram-positive bacteria tested: the methicillin-sensitive Staphylococcus aureus (MSSA) ATCC25923 strain, the methicillin-resistant S. aureus NRS402 strain, which is also intermediate resistant to vancomycin (VISA), and the Enterococcus faecalis ATCC29212 strain (Table 5), and they were more active than embelin (1). This fact is interesting since bacterial infections, caused by Gram-positive pathogens such as Staphylococcus are account for the majority of opportunistic community-acquired and hospital-acquired infections.  compounds 1, 3a-l, 4a-l, and 5a- As we can see, the less active compounds turned out to be the conjugates with the naphthalene scaffold since most of them have MIC > 128 M (entries [26][27][28][29][30][31][32][33][34][35][36][37]. Thus, the presence of a 2H-pyran-2-one moiety seems to be important for the antibacterial activity. In the series from 4-hydroxy-2H-pyran-2-one good values were achieved with both aromatic and aliphatic substituents at the dihydropyran ring (entries 2-13). Embelin-coumarin conjugates with aliphatic substituents at the dihydropyran ring were inactive while those with aromatic substituents showed high activity. Regarding the influence of the nature of the substituents on the aromatic ring in the activity, in the series from 4-hydroxy-6-methyl-2H-pyran-2-one, halogen substituents in para position afforded the lowest MIC values (entries 2-4). In the embelin-coumarin series the best results were obtained with 4-fluorphenyl and 3,4-dimetoxyphenyl groups (entries 4 and 8).

General Methods
Commercial reagents were purchased from Sigma-Aldrich (Darmstadt, Germany) and Alfa Aesar (Lancashire, UK) and were used without further purification. Analytical thin-layer chromatography was performed on Polygram SIL G/UV254 silica gel plates and chromatograms were visualized under UV light (254 and 360 nm). Pre-coated TLC plates SIL G-100 UV254 (Macherey-Nagel) and SILICA GEL GF plates (1000 µm, Analtech) were used for preparative TLC purification. 1 H and 13 C NMR spectra were acquired in CDCl 3 (0.03% v/v TMS) DMSO-d 6 or C 6 D 6 at room temperature using Bruker Avance instruments (Bruker, Billarica, MA, USA) (400 or 500 MHz for 1 H NMR and 100 or 125 MHz for 13 C NMR). Chemical shifts are reported in parts per million (ppm). For 1 H NMR data are reported in the following manner: Chemical shift (integration, multiplicity, coupling constant where applicable). The following abbreviations are used: s (singlet), br (broad), d (doublet), t (triplet), dd (double doublet), td (triplet of doublets), and m (multiplet). Coupling constants (J) are given in Hertz (Hz). 13 C NMR were obtained with complete proton decoupling. MS and HRMS data were recorded in a VG Micromass ZAB-2F spectrometer and an ESI instrument LCT Premier XE Micromass (ESI-TOF). IR spectra were recorded on a Bruker IFS 28/55 spectrophotometer. All compounds were named using the ACD40 Name-Pro program, which is based on IUPAC rules. The embelin (1) used in the reactions was obtained from Oxalis erythrorhiza Gillies ex Hook. & Arn. following the procedure described in reference [24].

Biological Assays
Antibacterial activity was determined using the standard broth microdilution method as recommended by the National Committee for Clinical Laboratory Standards [8,11,25]. We used three Gram-positive bacterial strains; methicillin-sensitive Staphylococcus aureus ATCC25923 (MSSA), methicillin-resistant vancomycin-intermediate resistant Staphylococcus aureus NRS402 (VISA), and Enterococcus faecalis ATCC29212; as well as the Gram-negative Escherichia coli ATCC35218. Bacterial strains stored at −80 • C were first plated on brain heart infusion (BHI) agar and incubated at 37 • C overnight followed by a second overnight growth in cation-adjusted Mueller-Hinton (MH) broth. Bacterial suspensions were then normalized in fresh MH broth and added to premade 1:2 serial dilutions of each tested compounds and control antibiotics in the same media. The range of concentrations was from 0.5 to 128 (µM for the tested compounds and µg/mL for the reference antibiotics) and the final volume was 200 µL. The expected initial concentration in all wells was 1 × 10 5 cells/mL. The minimum inhibitory concentration (MIC) was estimated by eye after 24 h of incubation at 37 • C without shaking.
A similar procedure was used for the yeast Saccharomyces cerevisiae BY4741 wild-type strain [26]. In this case, the growing media was YPD and the inoculum was~2 × 10 4 cells/mL. The growth was measured at 30 • C after 24 h and 48 h.

Calculation of Electrostatic Polar Potentials, Electron Density, and Fukui Indices
The calculations of Density Functional Theory (DFT) was employed for optimization and minimization of geometry of the compounds shown in Scheme 2, as well as to examine the reactivity of the calculated compounds, their structural and electronic properties were obtained by parameters of reactivity and theoretical properties such as the energy values of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), electronic density and electrostatic potential using the B3LYP functional and the 6-31+G(d,p) basis set implemented in Jaguar-v.10.6 computational program [27,28]. Atomic Fukui indices were derived from Mulliken population of the highest occupied molecular orbital (HOMO) and the LUMO, were used to quantify electrophilicity of a molecule at a particular atomic site. The default convergence criterion implemented in Jaguar was used for self-consistent field (SCF) calculations (accuracy level = quick, convergence criteria: maximum iteration = 48, and energy change = 5 × 10 −5 hartree) and optimization (maximum steps = 100, convergence criteria = default, initial Hessian = Schlegel guess [29,30].

Conclusions
In summary, three series of new embelin conjugates were obtained through an InCl 3 catalyzed three component reaction from embelin (1), aldehydes and privileged substructures of antimicrobial interest such as 4-hydroxy-2H-pyran-2-one, 4-hydroxy-coumarin, and 2-naphthol. This MCR implies Knoevenagel condensation, Michael addition, intramolecular cyclization, and dehydration. Most part of the conjugates synthesized from 4-hydroxy-2H-pyran-2-one and 4-hydroxy-coumarin resulted to be active and selective toward Gram positive bacteria. Some structure-activity relationships were outlined and the most active compounds were 3a-3c, 4c, 4d, and 4g with MICs around 1-2 µM. The present results encourage further research with these compounds in order to develop novel antibiotic agents against Gram-positive bacteria.