Styrylpyrazoles: Properties, Synthesis and Transformations

The pyrazole nucleus and its reduced forms, pyrazolines and pyrazolidine, are privileged scaffolds in medicinal chemistry due to their remarkable biological activities. A huge number of pyrazole derivatives have been studied and reported over time. This review article gives an overview of pyrazole derivatives that contain a styryl (2-arylvinyl) group linked in different positions of the pyrazole backbone. Although there are studies on the synthesis of styrylpyrazoles dating back to the 1970s and even earlier, this type of compound has rarely been studied. This timely review intends to summarize the properties, biological activity, methods of synthesis and transformation of styrylpyrazoles; thus, highlighting the interest and huge potential for application of this kind of compound.

This review is focused on a particular type of pyrazoles, the styrylpyrazoles (or styryl-1Hpyrazoles), which present a styryl (2-arylvinyl) group at one or more positions of the pyrazole nucleus. It is intended to give an overview of the styrylpyrazoles chemistry reported in the literature, including their properties and biological activity, since the 1970s to the present date. Despite the biological activities of styrylpyrazoles [12,14], their interesting physicochemical properties, such as tautomerism, isomerism [30,31] and conjugation extension, due to the presence of the 2-arylvinyl group, as well as the rich chemistry related to their synthesis and transformation, this type of pyrazole has rarely been studied, as can be seen from the low number of publications in a very large time span covered by this review. In our group, we have been working with styrylpyrazoles for a long time. Throughout this review, we intend to describe the properties and methods of synthesis of these compounds, and to show their great potential for application in different fields, from medicine to materials chemistry, including their use as key templates for transformation into other valuable compounds. The articles will be presented based on the molecules' structure, in particular, the position of the styryl group in the pyrazole scaffold.

Properties of Styrylpyrazoles
The pharmacological activity of styrylpyrazoles and related compounds has been known for a long time. A study published in 1970 described the anti-inflammatory activity of 1-phenyl-2-styryl-3,5-dioxopirazolidines (6). Among these compounds, derivatives 6a (R 1 = Me, R 2 = R 3 = H, R 4 = n-C4H9) and 6b (R 1 = Cl, R 2 = 4-OMe, R 3 = H, R 4 = n-C4H9) ( Figure 2) were more active than phenylbutazone or oxyphenbutazone in the carrageenin-induced foot edema test in rats [32]. It was found that a pmethylphenyl ring increased the intrinsic activity in general, while a p-chlorophenyl substituent decreases the activity, except for compound 6b. The p-methoxy or 3,4-methylenedioxy substituents of the styryl group tend to increase both toxicity and inhibitory activity, while p-alkyl substituents (Me or i-Pr) have the opposite effect.
Several studies have highlighted the antiproliferative activity of 12a in PC3 cells, with growth inhibitory concentration (GI 50 ) values of 5.6 µM [43] and in breast cancer cell lines, MCF-7 and SKBR3, with GI 50 values of 4.19 µM and 0.25 µM, respectively [42]. Liao and coworkers have also shown the antiproliferative activity of 3,5-bis(styryl)pyrazoles 12a and 13 in the androgen-independent PC3 prostate cancer cell line with GI 50 values in the low micromolar range. In particular, compound 13 caused significant effects on the PC3 cell cycle, inducing cell death and binding to tubulin (Kd 0.4 ± 0.1 µM), inhibiting tubulin polymerization in vitro, being competitive with paclitaxel for binding to tubulin, and leading to microtubule depolymerization in PC3 cells. Therefore, 13 was considered as a lead compound for the treatment of castration-resistant prostate cancer (CRPC) [44]. shown interesting results in studies related to neuroprotection and Alzheimer's disease, and was able to restore membrane homeostasis disrupted after brain trauma, being a promising compound for therapeutic use in the treatment of Parkinson's disease [37]. In combination with tissue plasminogen activator, CNB-001 is efficient in the treatment of stroke [38]. Compound 12a also acted as inhibitor of β-amyloid secretion and of protein kinases involved in neuronal excitotoxicity [39]. Some pyrazoles that are analogues of 12a displayed good inhibition of γ-secretase activity, tau aggregation, and/or affinity to fibrillar Aβ42 aggregates [40].
Later, the same authors reported the strong binding interactions of DSDP with two serum transport proteins, human serum albumin (HSA) and bovine serum albumin (BSA) [50]. Exploiting multi-spectroscopic techniques together with in silico molecular docking simulation, they have demonstrated that DSDP is a potent fluorophore. It binds with both proteins with the formation of a 1:2 protein-probe complex at a lower protein concentration range and a 1:1 complex at a higher protein concentration range. The calculated binding constants for the 2:1 DSDP-protein complexes were found to be 1.37 × 10 10 M −2 and 1.47 × 10 10 M −2 for HSA and BSA, respectively, while for the 1:1 complexation process, the constants were 1.85 × 10 5 M −1 and 1.73 × 10 5 M −1 for DSDP-HSA and DSDP-BSA systems, respectively. Moreover, they have demonstrated that hydrogen bonding and hydrophobic interactions are the forces primarily responsible for both types of binding. The information gathered from these studies may be useful for the rational design of drugs looking at a greater clinical efficacy.
1,3,5-Trisubstituted pyrazolines, analogues of DSDP, exhibited large fluorescence quantum yields (Φ f = 0.6-0.8), suited for the design of energy-transfer-based fluorescent probes [51]. In DSDP's structure, two of the aryl rings (phenyl and styryl) are interconnected electronically through the pyrazoline π-system while the 5-phenyl ring is electronically decoupled and can behave as an electron donating receptor for cations or electron deficient centers. The lone pair of electrons on the N1 atom of the pyrazoline ring also takes part in the conjugation, facilitating the ICT process. Thus, DSDP acts as a D-π-A system and the introduction of an electron withdrawing group (such as -CN, -NO 2 , -COOEt) on the styryl moiety can enhance the push-pull D-π-A capacity of DSDP-like molecular systems [52]. However, a drastic modification of the photophysical properties of DSDP is observed upon dehydrogenation of the pyrazoline ring with subsequent formation of the corresponding pyrazole (DSP) 16 ( Figure 6). While DSDP gives dual absorption and dual emission bands corresponding to the locally excited (LE) and ICT species, DSP yields single absorption and emission bands for the LE species only. Comparative steady state and time resolved fluorometric studies have revealed that aromatization of the pyrazoline ring completely inhibits the ICT process. These results were also corroborated by quantum chemical calculations [52]. binding mode with no conformational change. These findings were also supported by molecular docking simulation. Later, the same authors reported the strong binding interactions of DSDP with two serum transport proteins, human serum albumin (HSA) and bovine serum albumin (BSA) [50]. Exploiting multi-spectroscopic techniques together with in silico molecular docking simulation, they have demonstrated that DSDP is a potent fluorophore. It binds with both proteins with the formation of a 1:2 protein-probe complex at a lower protein concentration range and a 1:1 complex at a higher protein concentration range. The calculated binding constants for the 2:1 DSDP-protein complexes were found to be 1.37 × 10 10 M −2 and 1.47 × 10 10 M −2 for HSA and BSA, respectively, while for the 1:1 complexation process, the constants were 1.85 × 10 5 M −1 and 1.73 × 10 5 M −1 for DSDP-HSA and DSDP-BSA systems, respectively. Moreover, they have demonstrated that hydrogen bonding and hydrophobic interactions are the forces primarily responsible for both types of binding. The information gathered from these studies may be useful for the rational design of drugs looking at a greater clinical efficacy.
1,3,5-Trisubstituted pyrazolines, analogues of DSDP, exhibited large fluorescence quantum yields (Φf = 0.6-0.8), suited for the design of energy-transfer-based fluorescent probes [51]. In DSDP's structure, two of the aryl rings (phenyl and styryl) are interconnected electronically through the pyrazoline π-system while the 5-phenyl ring is electronically decoupled and can behave as an electron donating receptor for cations or electron deficient centers. The lone pair of electrons on the N1 atom of the pyrazoline ring also takes part in the conjugation, facilitating the ICT process. Thus, DSDP acts as a D-π-A system and the introduction of an electron withdrawing group (such as -CN, -NO2, -COOEt) on the styryl moiety can enhance the push-pull D-π-A capacity of DSDP-like molecular systems [52]. However, a drastic modification of the photophysical properties of DSDP is observed upon dehydrogenation of the pyrazoline ring with subsequent formation of the corresponding pyrazole (DSP) 16 ( Figure 6). While DSDP gives dual absorption and dual emission bands corresponding to the locally excited (LE) and ICT species, DSP yields single absorption and emission bands for the LE species only. Comparative steady state and time resolved fluorometric studies have revealed that aromatization of the pyrazoline ring completely inhibits the ICT process. These results were also corroborated by quantum chemical calculations [52].

Synthesis of Styrylpyrazoles
In this section, the methods for the synthesis of styrylpyrazoles are described in a systematic way, based on the compounds' structure, considering the position of the styryl group in the pyrazole scaffold, starting from C-1 to C-5. Bis(styryl)pyrazoles, substituted at C-3 and C-5, are also considered. Moreover, in each subheading, the methods will be presented by alphabetic order. Single examples of a certain reaction will be considered at the end of each section in the miscellaneous subheading.

Synthesis of Styrylpyrazoles
In this section, the methods for the synthesis of styrylpyrazoles are described in a systematic way, based on the compounds' structure, considering the position of the styryl group in the pyrazole scaffold, starting from C-1 to C-5. Bis(styryl)pyrazoles, substituted at C-3 and C-5, are also considered. Moreover, in each subheading, the methods will be presented by alphabetic order. Single examples of a certain reaction will be considered at the end of each section in the miscellaneous subheading.

N-Cross-Coupling Reaction of Pyrazoles with Styrylboronic Acid
In 2010, Kantam et al. synthesized (E)-1-styrylpyrazoles in a simple and efficient way, by the cross-coupling reaction of pyrazoles 17 with styrylboronic acid 18 using a recyclable heterogeneous Cu-exchanged fluorapatite (CuFAP) catalyst, under base-free conditions (Scheme 1) [53]. The lower yield (70%) for the coupling with 3,5-dimethyl-1H-pyrazole 17 (R 1 = R 2 = Me) was attributed to the steric hindrance caused by the methyl groups. When the reaction was performed in the absence of air, no coupled product was obtained, since O 2 is involved in the oxidation of Cu(I) to Cu(II), which is the active species in the reaction mechanism [53].  [53]. The lower yield (70%) for the coupling with 3,5-dimethyl-1H-pyrazole 17 (R 1 = R 2 = Me) was attributed to the steric hindrance caused by the methyl groups. When the reaction was performed in the absence of air, no coupled product was obtained, since O2 is involved in the oxidation of Cu(I) to Cu(II), which is the active species in the reaction mechanism [53].
Tsuchimoto et al. reported a similar single addition of the N-H bond of pyrazoles to phenylacetylene and prop-1-yn-1-ylbenzene in the presence of a silver catalyst (AgNO3 or AgOTf) to produce a regioisomeric mixture of 1-substituted pyrazoles 22 and 23 (Scheme 2) [55]. Complete stereoselectivity was achieved, since only the (Z)-isomer of 1-styrylpyrazole was formed, indicating that anti-addition, via a concerted mechanism, in which pyrazole nitrogen attacks the alkyne from the side opposite to a coordinated Lewis acid (LA), is involved as a key step in the reaction process [55].

Scheme 2.
Addition of pyrazole 20 to alkynes to produce (Z)-1-styrylpyrazoles 21-23 [54,55].   [55]. Complete stereoselectivity was achieved, since only the (Z)-isomer of 1-styrylpyrazole was formed, indicating that anti-addition, via a concerted mechanism, in which pyrazole nitrogen attacks the alkyne from the side opposite to a coordinated Lewis acid (LA), is involved as a key step in the reaction process [55].  [53]. The lower yield (70%) for the coupling with 3,5-dimethyl-1H-pyrazole 17 (R 1 = R 2 = Me) was attributed to the steric hindrance caused by the methyl groups. When the reaction was performed in the absence of air, no coupled product was obtained, since O2 is involved in the oxidation of Cu(I) to Cu(II), which is the active species in the reaction mechanism [53].
Tsuchimoto et al. reported a similar single addition of the N-H bond of pyrazoles to phenylacetylene and prop-1-yn-1-ylbenzene in the presence of a silver catalyst (AgNO3 or AgOTf) to produce a regioisomeric mixture of 1-substituted pyrazoles 22 and 23 (Scheme 2) [55]. Complete stereoselectivity was achieved, since only the (Z)-isomer of 1-styrylpyrazole was formed, indicating that anti-addition, via a concerted mechanism, in which pyrazole nitrogen attacks the alkyne from the side opposite to a coordinated Lewis acid (LA), is involved as a key step in the reaction process [55].
Recently, Garg et al. reported a transition-metal-free chemo-, regio-, and stereoselective synthesis of (Z)-and (E)-1-styrylpyrazoles by the addition of pyrazoles 26 onto functionalized terminal alkynes using a super basic solution of KOH/dimethyl sulfoxide (DMSO) [31]. The nature of the base seems to be crucial for the reaction, which does not occur in the presence of an organic base, such as triethylamine (Et 3 N). The reaction stereoselectivity is governed by time and amount of the base (Scheme 3). Deuterium labeling and control experiments highlighted the role of the KOH/DMSO catalytic system in the cis→trans isomerization, which was further supported by comparative 1 H-NMR spectrum studies in DMSO/DMSO-d 6 . This method is of wide scope and several functionalities, such as OH or Me, NH 2, present both in the alkyne and pyrazole are well-tolerated [31].  [31]. The nature of the base seems to be crucial for the reaction, which does not occur in the presence of an organic base, such as triethylamine (Et3N). The reaction stereoselectivity is governed by time and amount of the base (Scheme 3). Deuterium labeling and control experiments highlighted the role of the KOH/DMSO catalytic system in the cis→trans isomerization, which was further supported by comparative 1 H-NMR spectrum studies in DMSO/DMSO-d6. This method is of wide scope and several functionalities, such as OH or Me, NH2, present both in the alkyne and pyrazole are welltolerated [31].
The formation of pyrazolines 42 occurred through oxirane ring-opening at the α-carbon, followed by the rearrangement of the azadiene intermediate (A) through a hydride [1,5] sigmatropic shift to 1,3-diketone monohydrazone (B, Scheme 9). Finally, the intramolecular cyclization led to 5hydroxypyrazolines 42. These compounds are only stable if they have an electron-withdrawing group, such as 1-acyl, or if a perfluoroalkyl group is present at the C-5 of the pyrazoline ring [70]. The condensation of acetylenic ketones 44 with aryl hydrazines 45 produces (E)-3(5)styrylpyrazoles (46 and/or 47) in fair to good yield. The substitution pattern in these pyrazoles depends on the nature of the substituents and mainly on the reaction conditions (Scheme 10) [71]. When methanol is used as solvent and the reaction is stirred at room temperature for a period prior to the addition of acid and heating, (E)-5-styrylpyrazoles 47 were obtained as the major products. However, if acid is present and heat applied from the onset, a mixture of (E)-3-and 5-styrylpyrazoles 46 and 47 is obtained. Regioselectivity, in this case, varies from 39:61% to 83:17% of 46:47, depending on the nature of the substituents. For instance, when R 1 = MeO, R 2 = H and R 3 = MeSO2, the regioisomer 46 is obtained in higher amount. On contrary, when R 1 = R 2 = H and R 3 = NO2, 46 is the minor regioisomer. In the absence of acid, the initial reaction step involves Michael addition of the more basic terminal nitrogen of the hydrazine derivative to the terminal acetylenic carbon to form enamine, which exists in tautomeric equilibrium with the isomeric hydrazone. The cyclization of the hydrazone in the presence of added acid subsequently afforded only (E)-5-styrylpyrazoles 46. The
The formation of pyrazolines 42 occurred through oxirane ring-opening at the α-carbon, followed by the rearrangement of the azadiene intermediate (A) through a hydride [1,5] sigmatropic shift to 1,3-diketone monohydrazone (B, Scheme 9). Finally, the intramolecular cyclization led to 5hydroxypyrazolines 42. These compounds are only stable if they have an electron-withdrawing group, such as 1-acyl, or if a perfluoroalkyl group is present at the C-5 of the pyrazoline ring [70]. The condensation of acetylenic ketones 44 with aryl hydrazines 45 produces (E)-3(5)styrylpyrazoles (46 and/or 47) in fair to good yield. The substitution pattern in these pyrazoles depends on the nature of the substituents and mainly on the reaction conditions (Scheme 10) [71]. When methanol is used as solvent and the reaction is stirred at room temperature for a period prior to the addition of acid and heating, (E)-5-styrylpyrazoles 47 were obtained as the major products. However, if acid is present and heat applied from the onset, a mixture of (E)-3-and 5-styrylpyrazoles 46 and 47 is obtained. Regioselectivity, in this case, varies from 39:61% to 83:17% of 46:47, depending on the nature of the substituents. For instance, when R 1 = MeO, R 2 = H and R 3 = MeSO2, the regioisomer 46 is obtained in higher amount. On contrary, when R 1 = R 2 = H and R 3 = NO2, 46 is the minor regioisomer. In the absence of acid, the initial reaction step involves Michael addition of the more basic terminal nitrogen of the hydrazine derivative to the terminal acetylenic carbon to form enamine, which exists in tautomeric equilibrium with the isomeric hydrazone. The cyclization of the hydrazone in the presence of added acid subsequently afforded only (E)-5-styrylpyrazoles 46. The Scheme 9. Mechanism of formation of (E)-3-styrylpyrazolines 42 and (E)-3-styrylpyrazoles 43 [70].
The condensation of acetylenic ketones 44 with aryl hydrazines 45 produces (E)-3(5)-styrylpyrazoles (46 and/or 47) in fair to good yield. The substitution pattern in these pyrazoles depends on the nature of the substituents and mainly on the reaction conditions (Scheme 10) [71]. When methanol is used as solvent and the reaction is stirred at room temperature for a period prior to the addition of acid and heating, (E)-5-styrylpyrazoles 47 were obtained as the major products. However, if acid is present and heat applied from the onset, a mixture of (E)-3-and 5-styrylpyrazoles 46 and 47 is obtained. Regioselectivity, in this case, varies from 39:61% to 83:17% of 46:47, depending on the nature of the substituents. For instance, when R 1 = MeO, R 2 = H and R 3 = MeSO 2 , the regioisomer 46 is obtained in higher amount. On contrary, when R 1 = R 2 = H and R 3 = NO 2 , 46 is the minor regioisomer. In the absence of acid, the initial reaction step involves Michael addition of the more basic terminal nitrogen of the hydrazine derivative to the terminal acetylenic carbon to form enamine, which exists in tautomeric equilibrium with the isomeric hydrazone. The cyclization of the hydrazone in the presence of added acid subsequently afforded only (E)-5-styrylpyrazoles 46. The formation of this isomer seems to be promoted by the strong electron-donating and electron-withdrawing effect of the substituents.
Hydrogenolysis of (E)-5-styrylisoxazole 53, using Mo(CO)6 in the presence of water (1.0 equiv), followed by ring closure with hydrazine also produced (E)-3-styrylpyrazole 7 [33]. In a first step, a ketone is obtained (Scheme 14, i), which after reaction with 97% hydrazine in acetic acid gave (E)-3styrylpyrazole 7 (Scheme 14, ii). Deshayes et al. reported the use of 3-bromomethylpyrazole 54 as template for the synthesis of (E)-3-styrylpyrazole 55. In a first step, this compound was treated with triethyl phosphite, in the Arbusov reaction, affording the corresponding diethylphosphonomethylpyrazole, which then reacted with benzaldehyde in the presence of sodium hydride to give the corresponding (E)-3-
Although the biological activity of 4-styrylpyrazoles has rarely been studied, Silva et al. have shown that some derivatives of 4-styrylpyrazoles 77 and 79 with long alkyl chains of ten or twelve carbons on the N-1 or linked at the oxygen of the 2′-hydroxyphenyl moiety present affinity for CB1 type cannabinoid receptors in the micromolar range, or even in the nanomolar range, as observed for the (E)-4-(4-chlorostyryl)-3(5)-(2-decyloxyphenyl)-1H-pyrazole (Ki = 53 ± 33 nM) [14].
Although the biological activity of 4-styrylpyrazoles has rarely been studied, Silva et al. have shown that some derivatives of 4-styrylpyrazoles 77 and 79 with long alkyl chains of ten or twelve carbons on the N-1 or linked at the oxygen of the 2 -hydroxyphenyl moiety present affinity for CB 1 type cannabinoid receptors in the micromolar range, or even in the nanomolar range, as observed for the (E)-4-(4-chlorostyryl)-3(5)-(2-decyloxyphenyl)-1H-pyrazole (K i = 53 ± 33 nM) [14].

Cyclocondensation Reactions
One of the first reports about the synthesis of styrylpyrazoles dates back to 1978 [83]. At that time, Soliman et al. described the reaction of diketoester 86 with aryl/hetaryl hydrazines in ethanol to prepare (E)-5-styrylpyrazoles 87 (Scheme 27).

Cyclocondensation Reactions
One of the first reports about the synthesis of styrylpyrazoles dates back to 1978 [83].

Cyclocondensation Reactions
One of the first reports about the synthesis of styrylpyrazoles dates back to 1978 [83]. At that time, Soliman et al. described the reaction of diketoester 86 with aryl/hetaryl hydrazines in ethanol to prepare (E)-5-styrylpyrazoles 87 (Scheme 27).
Molecules 2020, 25, x FOR PEER REVIEW  19 of 34 monosubstituted hydrazines, such as phenyl hydrazine, although apparently simple, conceal a complex mechanistic problem. Diketones 88 have two tautomeric forms (88 and 88′) and phenyl hydrazine can react initially through NH or NH2. When the reaction is carried out in methanol in neutral conditions, a nucleophilic attack of the primary amine (NH2) to the more electrophilic position of the diketone (C-1) occurs and only pyrazoles 91 were obtained in low yields. Using acidic conditions (AcOH as solvent), the more basic amine (NH2) was protonated and subsequently the nucleophilic attack at the more electrophilic position was through the NH affording pyrazoles 90 [84]. performed the reaction of enamine 92 with phenyl hydrazine in refluxing ethanol, overnight, to produce 4-ethoxycarbonyl-1-phenyl-5-styrylpyrazole 93. In the same conditions, the reaction with benzyl hydrazine afforded a 3:1 mixture of two pyrazole isomers, the 1-benzyl-4-ethoxycarbonyl-5-styrylpyrazole 94 as the major product together with 1-benzyl-4-ethoxycarbonyl-3-styrylpyrazole 95 (Scheme 29) [85].
Molecules 2020, 25, x FOR PEER REVIEW  19 of 34 monosubstituted hydrazines, such as phenyl hydrazine, although apparently simple, conceal a complex mechanistic problem. Diketones 88 have two tautomeric forms (88 and 88′) and phenyl hydrazine can react initially through NH or NH2. When the reaction is carried out in methanol in neutral conditions, a nucleophilic attack of the primary amine (NH2) to the more electrophilic position of the diketone (C-1) occurs and only pyrazoles 91 were obtained in low yields. Using acidic conditions (AcOH as solvent), the more basic amine (NH2) was protonated and subsequently the nucleophilic attack at the more electrophilic position was through the NH affording pyrazoles 90 [84].

Cyclocondensation Reaction
Typically, 3,5-bis(styryl)pyrazole curcumin analogue 12a and N-aryl derivatives have been obtained by treatment of curcumin 11 with hydrazine hydrate or aryl hydrazines in ethanol [42] or toluene [40] at reflux for a long reaction time of 24-40 h. Using glacial acetic acid at reflux, the reaction time can be reduced to 6-8 h [33,39,41]. Room temperature reactions have also been reported but required a longer reaction time [40]. In 2015, Sherin et al. reported a solvent-free, mechanochemical method for the synthesis of curcumin 11 derived 3,5-bis(styryl)pyrazoles 12a-f [36] (Scheme 33). The heterocyclization of 11 with hydrazine or hydrazine derivatives was performed with vigorous grinding, using an agate mortar and pestle, at room temperature, in the presence of a catalytic amount of acetic acid. A very short reaction time was necessary, in comparison with the previously referred methods that use conventional heating [39][40][41][42]. The reaction scope seems to be broad since phenyl hydrazine, p-methoxy, p-chloro, p-nitro, and p-carboxyphenyl hydrazines gave bis(styryl)pyrazoles 12b-f in good yields (79-84%). One year later, the same authors performed the mechanochemical synthesis of 1-phenyl-3,5-bis(styryl)pyrazoles 13a-f, varying the substituents present in the aromatic ring of both styryl groups [90]. Recently, Liao et al. described a rapid synthesis of similar 3,5-bis(styryl)pyrazole curcumin analogues 13 by using microwave irradiation conditions [44].

Transformations of Styrylpyrazoles
Styrylpyrazoles are interesting templates for synthetic manipulation towards new heterocycles. Nevertheless, to date, only a small number of transformations involving the 2-arylvinyl moiety of styrylpyrazoles have been reported in the literature. In this section, we describe the most common transformations of the 2-arylvinyl moiety of styrylpyrazoles.

Transformations of Styrylpyrazoles
Styrylpyrazoles are interesting templates for synthetic manipulation towards new heterocycles. Nevertheless, to date, only a small number of transformations involving the 2-arylvinyl moiety of styrylpyrazoles have been reported in the literature. In this section, we describe the most common transformations of the 2-arylvinyl moiety of styrylpyrazoles.

Diels-Alder Cycloadditions
Starting from 3-and 5-styrylpyrazoles 115 and 116, Silva et al. developed a synthetic route to prepare naphthylpyrazoles [84]. These styrylpyrazoles, which can behave as dienophiles, undergo a Diels-Alder cycloaddition with o-benzoquinodimethane (117), the diene, which was formed in situ by the thermal extrusion of sulfur dioxide from 1,3-dihydrobenzo[c]thiophene 2,2-dioxide. N-Substituted 3-styrylpyrazoles 115 reacted with diene 117 at 250 °C in 1,2,4-trichlorobenzene giving the corresponding 3-[2-(3-aryl-1,2,3,4-tetrahydronaphthyl)]-5-(2-hydroxyphenyl)-1-phenylpyrazoles (118) in good yields (69-91%) (Scheme 36, i). The presence of an electron-withdrawing substituent on the p-position of the phenyl ring increases the reactivity of the styryl double bond [84]. These authors also performed the Diels-Alder reaction of (E)-3(5)-(2-hydroxyphenyl)-5(3)-styrylpyrazoles (116) with diene (117), but in this case longer reaction times were necessary and the expected cycloadducts were obtained in lower yields (24-48%). The efficiency of this Diels-Alder reaction increased by using a LA, aluminum(III) chloride, which increased the reactivity of the styryl double bond, probably through the formation of an aluminum(III) complex involving the oxygen of the 2′-hydroxy group and the free nitrogen of the pyrazole moiety. Under these conditions, the cycloadduct 119 containing an electron-donating substituent at the p-position of styryl moiety was obtained in better yield. On the contrary, the cycloadduct 119 containing the p-nitro group as substituent was obtained in better yield without addition of aluminum(III) chloride (Scheme 36, iii) [84]. The formation of naphthylpyrazoles 120 and 121 occurred by dehydrogenation of the corresponding cycloadducts with DDQ (2-6 days). The naphthylpyrazole 120 bearing an electron-donating substituent (R 1 = OMe) at the p-position of the phenyl group linked to the hydroaromatic ring was obtained in a shorter time (2 days) and with better yield (59%) than the other derivatives (R 1 = H (25%) and R 1 = NO2 (17%)). The presence of the p-methoxy substituent stabilizes the carbocation formed through a hydride transfer from compound 118 to DDQ. To increase the yield of this oxidation, p-toluenesulfonic acid was added and the products were obtained in a shorter time (2-7 h) and with better yields, especially for the derivative containing the nitro group (R 1 = NO2, 36%) (Scheme 36, ii). The oxidation of cycloadducts Scheme 35. Transformation of (E)-4-styrylpyrazole 108 into cycloadducts 112, 113 and pyrazole 114 [92].

Diels-Alder Cycloadditions
Starting from 3-and 5-styrylpyrazoles 115 and 116, Silva et al. developed a synthetic route to prepare naphthylpyrazoles [84]. These styrylpyrazoles, which can behave as dienophiles, undergo a Diels-Alder cycloaddition with o-benzoquinodimethane (117), the diene, which was formed in situ by the thermal extrusion of sulfur dioxide from 1,3-dihydrobenzo[c]thiophene 2,2-dioxide. N-Substituted 3-styrylpyrazoles 115 reacted with diene 117 at 250 • C in 1,2,4-trichlorobenzene giving the corresponding 3-[2-(3-aryl-1,2,3,4-tetrahydronaphthyl)]-5-(2-hydroxyphenyl)-1-phenylpyrazoles (118) in good yields (69-91%) (Scheme 36, i). The presence of an electron-withdrawing substituent on the p-position of the phenyl ring increases the reactivity of the styryl double bond [84]. These authors also performed the Diels-Alder reaction of (E)-3(5)-(2-hydroxyphenyl)-5(3)-styrylpyrazoles (116) with diene (117), but in this case longer reaction times were necessary and the expected cycloadducts were obtained in lower yields (24-48%). The efficiency of this Diels-Alder reaction increased by using a LA, aluminum(III) chloride, which increased the reactivity of the styryl double bond, probably through the formation of an aluminum(III) complex involving the oxygen of the 2 -hydroxy group and the free nitrogen of the pyrazole moiety. Under these conditions, the cycloadduct 119 containing an electron-donating substituent at the p-position of styryl moiety was obtained in better yield. On the contrary, the cycloadduct 119 containing the p-nitro group as substituent was obtained in better yield without addition of aluminum(III) chloride (Scheme 36, iii) [84]. The formation of naphthylpyrazoles 120 and 121 occurred by dehydrogenation of the corresponding cycloadducts with DDQ (2-6 days). The naphthylpyrazole 120 bearing an electron-donating substituent (R 1 = OMe) at the p-position of the phenyl group linked to the hydroaromatic ring was obtained in a shorter time (2 days) and with better yield (59%) than the other derivatives (R 1 = H (25%) and R 1 = NO 2 (17%)). The presence of the p-methoxy substituent stabilizes the carbocation formed through a hydride transfer from compound 118 to DDQ. To increase the yield of this oxidation, p-toluenesulfonic acid was added and the products were obtained in a shorter time (2-7 h) and with better yields, especially for the derivative containing the nitro group (R 1 = NO 2 , 36%) (Scheme 36, ii). The oxidation of cycloadducts 119 was also tried, using both methods (Scheme 36, ii) but only decomposition products were obtained, save the case of 3-(2-hydroxyphenyl)-5-{2-[3-(4-methoxyphenyl)]naphthyl}pyrazole 121 (R 1 = OMe), which was obtained in low yield (13%, method B) [84].

Conclusions
Styrylpyrazoles have shown remarkable biological activities, namely as anti-inflammatory, antimicrobial (antibacterial and antifungal) and antioxidant agents. Among these compounds, the 3,5-bis(styryl)pyrazoles, curcumin analogues, should be highlighted herein due to their significant antioxidant, neuroprotective, antimalarial, antimycobacterial, antiangiogenic, cytotoxic and antiproliferative activities. Additionally, styrylpyrazoles present interesting photophysical properties suitable for metal ion sensing, for the design of energy-transfer-based fluorescent probes and are also good DNA groove binders. Although structure-activity relationship studies have been reported for some styrylpyrazole derivatives, there is a gap in the literature regarding a detailed investigation of the specific properties due to the presence of the styryl group, or the different properties that may appear by exchanging the styryl group position. As far as we know, there are no comparative data between styrylpyrazoles and analogous non-styrylpyrazoles, which are very important in order to understand the role of the styryl moiety in the pyrazoles' properties.
Only Funding: The authors would like to thank the University of Aveiro and FCT/MEC for the financial support to the LAQV-REQUIMTE (UIDB/50006/2020) research project, financed by national funds and, when appropriate, co-financed by FEDER under the PT2020 Partnership Agreement of the Portuguese NMR network. Vera L. M. Silva thanks the assistant professor position (within CEE-CINST/2018; since 01/09/2019) and the integrated programme of SR&TD "pAGE-Protein Aggregation Across the Lifespan" (reference CENTRO-01-0145FEDER-000003), co-funded by the Centro 2020 program, Portugal 2020 and European Union through the European Regional Development Fund.

Conflicts of Interest:
The authors declare no conflict of interest.