3,4-Dihydro-2(1H)-Pyridones as Building Blocks of Synthetic Relevance

3,4-Dihydro-2(1H)-pyridones (3,4-DHPo) and their derivatives are privileged structures, which has increased their relevance due to their biological activity in front of a broad range of targets, but especially for their importance as synthetic precursors of a variety of compounds with marked biological activity. Taking into account the large number of contributions published over the years regarding this kind of heterocycle, here, we presented a current view of 3,4-dihydro-2(1H)-pyridones (3,4-DHPo). The review includes general aspects such as those related to nomenclature, synthesis, and biological activity, but also highlights the importance of DHPos as building blocks of other relevant structures. Additional to the conventional multicomponent synthesis of the mentioned heterocycle, nonconventional procedures are revised, demonstrating the increasing efficiency and allowing reactions to be carried out in the absence of the solvent, becoming an important contribution to green chemistry. Biological activities of 3,4-DHPo, such as vasorelaxant, anti-HIV, antitumor, antibacterial, and antifungal, have demonstrated this heterocycle’s potential in medicinal chemistry.


Introduction
The study of determinate structures such as small molecules in drug discovery has increased, engaging most in medicinal chemistry. These structures, particularly those based on N-heterocycles, represent a class of molecules capable of binding to multiple receptors with high affinity, showing a broad range of biological activity.
Molecules 2022, 27, x FOR PEER REVIEW 2 of 26 anti-HIV [5], and other biological activities [6,7,8,9]. These results encourage many research groups to search for potentially active DHPo's analogs. At the same time, the 3,4-Dihydro-2(1H)-pyridones (3, and their derivatives are extensively used as precursors in the synthesis of bioactive molecules such as (±)-Andranginine ( Figure 2) [10], elective α1a adrenergic receptors [11], Rho-kinase inhibitors [12], and P2X7 receptor antagonists [13]. The synthesis of 3,4-DHPo derivatives has been important with the passing of time. Different synthetic procedures to synthesize them have taken place in the last years, although the most reported uses are the multicomponent reaction (MCR-4CR) of Meldrum's acid (1), a β-keto-ester derivative (2), and aromatic aldehydes (3) in the presence of ammonium acetate. Additionally, a broad range of techniques have been covered, from the conventional synthesis, through energy resources such as microwave, ultrasound, and infrared assisted to solid and liquid phase synthesis, and recently the chemoenzymatic assisted methodology applied to their asymmetric synthesis (Scheme 1).  The synthesis of 3,4-DHPo derivatives has been important with the passing of time. Different synthetic procedures to synthesize them have taken place in the last years, although the most reported uses are the multicomponent reaction (MCR-4CR) of Meldrum's acid (1), a β-keto-ester derivative (2), and aromatic aldehydes (3) in the presence of ammonium acetate. Additionally, a broad range of techniques have been covered, from the conventional synthesis, through energy resources such as microwave, ultrasound, and infrared assisted to solid and liquid phase synthesis, and recently the chemoenzymatic assisted methodology applied to their asymmetric synthesis (Scheme 1). anti-HIV [5], and other biological activities [6,7,8,9]. These results encourage many research groups to search for potentially active DHPo's analogs. At the same time, the 3,4-Dihydro-2(1H)-pyridones (3, and their derivatives are extensively used as precursors in the synthesis of bioactive molecules such as (±)-Andranginine ( Figure 2) [10], elective α1a adrenergic receptors [11], Rho-kinase inhibitors [12], and P2X7 receptor antagonists [13]. The synthesis of 3,4-DHPo derivatives has been important with the passing of time. Different synthetic procedures to synthesize them have taken place in the last years, although the most reported uses are the multicomponent reaction (MCR-4CR) of Meldrum's acid (1), a β-keto-ester derivative (2), and aromatic aldehydes (3) in the presence of ammonium acetate. Additionally, a broad range of techniques have been covered, from the conventional synthesis, through energy resources such as microwave, ultrasound, and infrared assisted to solid and liquid phase synthesis, and recently the chemoenzymatic assisted methodology applied to their asymmetric synthesis (Scheme 1).
Due to the importance of this N-heterocycle, this review aims to present the p characteristics of the 3,4-DHPo structures, the different procedures to synthesiz developed over time, the main biological application, and their crucial importance cursors to molecules with high relevance in medicinal chemistry. A critical over the advantages and disadvantages of the conventional and nonconventional sy methods to prepare 3,4-DHPo are highlighted.
The generally accepted mechanism follows a Hantzsch-like pathway with th ous formation of two intermediates; four from Knoevenagel condensation of Me acid (1) with the corresponding aromatic aldehyde (3), and five from the reaction ing β-keto-ester (2) and ammonia. A Michael-type addition of the enamine 5 on compound 5 gives rise to DHPo's (Scheme 3) [20]. The effort to synthesize 1,4-DHP derivatives by modifying its ring gave rise to the first accidental synthesis of 3,4-DHPo, as novel heterocycles that showed interesting biological activity [17]. The general synthesis of 3,4-DHPo as a racemic mixture involves a multicomponent reaction (MCR 4-CR) of Meldrum's acid (1), a β-keto-ester derivative (2), and an aromatic aldehyde (3) in the presence of ammonium acetate and with or without solvent (Scheme 2). Responsible for blocking the formation of the 1,4-dihydropyridines ring is the acidic character of Meldrum's acid (pKa = 9.97), which is higher than the β-keto-ester (pKa = 11.0) [18].
Due to the importance of this N-heterocycle, this review aims to present the principa characteristics of the 3,4-DHPo structures, the different procedures to synthesize them developed over time, the main biological application, and their crucial importance as pre cursors to molecules with high relevance in medicinal chemistry. A critical overview o the advantages and disadvantages of the conventional and nonconventional synthetic methods to prepare 3,4-DHPo are highlighted.
The generally accepted mechanism follows a Hantzsch-like pathway with the previ ous formation of two intermediates; four from Knoevenagel condensation of Meldrum's acid (1) with the corresponding aromatic aldehyde (3), and five from the reaction involving β-keto-ester (2) and ammonia. A Michael-type addition of the enamine 5 on ylidene compound 5 gives rise to DHPo's (Scheme 3) [20]. These kinds of molecules present a stereogenic center at C-4. Its absolute configuration (R-versus S-enantiomer) was a critical factor for the biological activity as an antagonist or agonist of calcium ions [19]. The mechanism reported in Scheme 3 has been demonstrated in synthesizing intermediaries and their characterization [20].
The generally accepted mechanism follows a Hantzsch-like pathway with the previous formation of two intermediates; four from Knoevenagel condensation of Meldrum's acid (1) with the corresponding aromatic aldehyde (3), and five from the reaction involving β-ketoester (2) and ammonia. A Michael-type addition of the enamine 5 on ylidene compound 5 gives rise to DHPo's (Scheme 3) [20].

Synthetic Strategies
Different strategies have been reported to obtain 3,4-DHPo, conventional synthesis with or without solvent, using different energy resources such as microwave, ultrasound, and infrared to improve the yield and develop green chemistry reactions. Solid-phase organic synthesis (SPOS) and liquid-phase synthesis (LPS) on soluble polymers as supports have also been developed. Finally, chemoenzymatic to asymmetric synthesis has also been established.

Conventional Synthesis
The first strategy used to obtain the 3,4-DHPo involved the Multicomponent Reaction (MCR) using equimolecular amounts of starting compounds under ethanol reflux for six hours, allowing the synthesis of 3,4-DHPo derivatives with a 15-26% yield [21]. One year later [22], the synthesis of two 3-CN-DHPo with 51-70% yield from pyran derivatives previously synthesized was reported.
The Suárez group has improved the efficiency of this MCR procedure, replaced the ethanol with acetic acid, and allowed moderate to high yields (Scheme 3) [17][18][19][23][24][25][26][27][28][29]. The best results are due to the catalytic effect and the higher boiling point of acetic acid, which improve the decarboxylation step and increase the yields compared with previously reported methods [21,30].
In 2011, Sun et al. developed a protocol to synthesize structurally diverse 3,4-DHPo via MCR, similar to those previously described by the Suárez group but using arylamines, acetylenedicarboxylate, aromatic aldehydes, and Meldrum acid as starting reagents. The proposed mechanism involves a Michael addition of the enamino ester formed in situ from the reaction of arylamine with dimethyl acetylenedicarboxylate to arylidine cyclic 1,3-diketones [31].
An efficient one-pot synthesis of polysubstituted dihydropyridones derivatives was reported by Khazaei and Anary-Abbasinejad [32]. The reaction was achieved using cyanoacetamide (6), aryl aldehydes derivatives (3), ethyl acetoacetate (2), and ammonium acetate and using pyridine as the catalyst, in ethanol as solvent, and under reflux conditions obtaining 54-68% of yields (Scheme 4). The advantage of this method is the use of neutral conditions and the facility of mixing reagents without any previous activation or modification. Scheme 3. Proposed reaction mechanism for 3,4-DHPo synthesis.

Synthetic Strategies
Different strategies have been reported to obtain 3,4-DHPo, conventional synthesis with or without solvent, using different energy resources such as microwave, ultrasound, and infrared to improve the yield and develop green chemistry reactions. Solid-phase organic synthesis (SPOS) and liquid-phase synthesis (LPS) on soluble polymers as supports have also been developed. Finally, chemoenzymatic to asymmetric synthesis has also been established.

Conventional Synthesis
The first strategy used to obtain the 3,4-DHPo involved the Multicomponent Reaction (MCR) using equimolecular amounts of starting compounds under ethanol reflux for six hours, allowing the synthesis of 3,4-DHPo derivatives with a 15-26% yield [21]. One year later [22], the synthesis of two 3-CN-DHPo with 51-70% yield from pyran derivatives previously synthesized was reported.
The Suárez group has improved the efficiency of this MCR procedure, replaced the ethanol with acetic acid, and allowed moderate to high yields (Scheme 3) [17][18][19][23][24][25][26][27][28][29]. The best results are due to the catalytic effect and the higher boiling point of acetic acid, which improve the decarboxylation step and increase the yields compared with previously reported methods [21,30].
In 2011, Sun et al. developed a protocol to synthesize structurally diverse 3,4-DHPo via MCR, similar to those previously described by the Suárez group but using arylamines, acetylenedicarboxylate, aromatic aldehydes, and Meldrum acid as starting reagents. The proposed mechanism involves a Michael addition of the enamino ester formed in situ from the reaction of arylamine with dimethyl acetylenedicarboxylate to arylidine cyclic 1,3-diketones [31].
An efficient one-pot synthesis of polysubstituted dihydropyridones derivatives was reported by Khazaei and Anary-Abbasinejad [32]. The reaction was achieved using cyanoacetamide (6), aryl aldehydes derivatives (3), ethyl acetoacetate (2), and ammonium acetate and using pyridine as the catalyst, in ethanol as solvent, and under reflux conditions obtaining 54-68% of yields (Scheme 4). The advantage of this method is the use of neutral conditions and the facility of mixing reagents without any previous activation or modification. An improvement of this method was reported by Dehghan et al. They used similar general conditions, introducing some variation [33]. The researchers changed ammonium acetate to ammonium carbonate and did not use pyridine. Additionally, they also used and compared ethanol and water as reaction solvents. A significant yield improvement  An improvement of this method was reported by Dehghan et al. They used similar general conditions, introducing some variation [33]. The researchers changed ammonium acetate to ammonium carbonate and did not use pyridine. Additionally, they also used and compared ethanol and water as reaction solvents. A significant yield improvement was obtained in water (90-96%) compared with ethanol (55-75%).
Furthermore, the Hakimi research group reported a new one-pot, four-component synthesis of 3,4-dihydro-2-pyridone derivatives (3,. The reaction of Meldrum's acid (1), methyl acetoacetate (2), benzaldehyde derivatives (3), and ammonium acetate and using SiO 2 -Pr-SO 3 H as an efficient catalyst under solvent-free conditions was reported (Scheme 5) [34]. The advantages of this methodology are high product yields (78-93%), being environmentally benign, short reaction times, and easy handling. An improvement of this method was reported by Dehghan et al. They used similar general conditions, introducing some variation [33]. The researchers changed ammonium acetate to ammonium carbonate and did not use pyridine. Additionally, they also used and compared ethanol and water as reaction solvents. A significant yield improvement was obtained in water (90-96%) compared with ethanol (55-75%).
Instead, Dostanic et al. published about synthesizing (substituted phenylazo)-pyridones in the presence of KOH and acetone to obtain 11-61% yield [42]. These were used dyeing polyester fabrics as yellow dyes. Another report described the synthesis of 3,4dihydropyridones derivatives in the presence of Cs 2 CO 3 and toluene with 53-66% yield [43]. The synthesis of aza-analogs of 3,4-DHPo with anticancer activity was reported by Bariwal et al. using benzoylacetone, substituted aldehyde, urea or thiourea with HCl, and ethanol as a solvent with 55-77% yield [44].
Besides, the conventional reaction has been improved by using different catalysts. Zhiqiang et al. reported a three-component cascade reaction to achieve 3,4-DHPo derivatives using imidazole as a catalyst with ethylene glycol as solvent [45]. Later, Bhattacharyya et al. reported a greener method to obtain 3,4-DHPo derivatives using a one-pot multicomponent reaction in aqueous media catalyzed by nanostructured ZnO [46]. Additionally, Khazaei et al. used ZnO nanoparticles to give 3,4-DHPo derivatives under ethanol as solvent [44]. Ziarani et al. published the synthesis of 3,4-DHPo derivatives by sulphonicacid-functionalized ordered nanoporous SBA-15 as a nano heterogeneous catalyst via one-pot, four-component reaction under solvent-free conditions [47]. Further, Pradhan et al. presented green protocols to achieve 3,4-DHPo derivatives using two catalysts such as the vitamin B1 or PEG-SO 3 H in water as solvent [48]. All these described reactions showed moderate to good yields. Besides, those reactions where a catalyst was attached to solid or polymeric supports showed better results due to the possibility of the most efficient purification procedures.
Besides, the conventional reaction has been improved by using different catalysts. Zhiqiang et al. reported a three-component cascade reaction to achieve 3,4-DHPo derivatives using imidazole as a catalyst with ethylene glycol as solvent [45]. Later, Bhattacharyya et al. reported a greener method to obtain 3,4-DHPo derivatives using a one-pot multicomponent reaction in aqueous media catalyzed by nanostructured ZnO [46]. Additionally, Khazaei et al. used ZnO nanoparticles to give 3,4-DHPo derivatives under ethanol as solvent [44]. Ziarani et al. published the synthesis of 3,4-DHPo derivatives by sulphonic-acid-functionalized ordered nanoporous SBA-15 as a nano heterogeneous catalyst via one-pot, four-component reaction under solvent-free conditions [47]. Further, Pradhan et al. presented green protocols to achieve 3,4-DHPo derivatives using two catalysts such as the vitamin B1 or PEG-SO3H in water as solvent [48]. All these described reactions showed moderate to good yields. Besides, those reactions where a catalyst was attached to solid or polymeric supports showed better results due to the possibility of the most efficient purification procedures.

Scheme 9. Tandem olefins isomerization-RCM to obtain 3,4-DHPo.
The conventional synthesis has been offered a broad possibility to obtain the desired product; however, nonconventional energy sources as microwave, infrared, and ultraviolet have also been incorporated to improve the efficiency of this MCR.

Microwave-Assisted Synthesis
In the last decades, microwave-assisted organic synthesis has been used as a tool for many known and new organic reactions. In general, its application allowed to reduce reaction times; increase efficiency; selective heating; excellent reproducibility; and, in some cases, avoid or minimize the use of solvents, contributing to green procedure development [54].

Scheme 9. Tandem olefins isomerization-RCM to obtain 3,4-DHPo.
The conventional synthesis has been offered a broad possibility to obtain the des product; however, nonconventional energy sources as microwave, infrared, and ultra let have also been incorporated to improve the efficiency of this MCR.

Microwave-Assisted Synthesis
In the last decades, microwave-assisted organic synthesis has been used as a too many known and new organic reactions. In general, its application allowed to reduce action times; increase efficiency; selective heating; excellent reproducibility; and, in s cases, avoid or minimize the use of solvents, contributing to green procedure deve ment [54].
The conventional synthesis has been offered a broad possibility to obtain the desired product; however, nonconventional energy sources as microwave, infrared, and ultraviolet have also been incorporated to improve the efficiency of this MCR.

Microwave-Assisted Synthesis
In the last decades, microwave-assisted organic synthesis has been used as a tool for many known and new organic reactions. In general, its application allowed to reduce reaction times; increase efficiency; selective heating; excellent reproducibility; and, in some cases, avoid or minimize the use of solvents, contributing to green procedure development [54].
Besides, Hernandez et al. published the oxidation reaction of 4H-pyrans derivatives (19) to obtain 3-cyano-2-pyridones (20) in ethanol, using H2SO4 catalyst source and MW irradiation (Scheme 11) [57]. These compounds are hybrid milrinone-enifedipine analogs. Scheme 11. Microwave-Assisted oxidation of 4H-pyrans derivatives (19) to 3,4-Dihydro-2(1H)-pyridones (20). This research group compared the different energy sources for oxidation; carried out the reaction at room temperature, at ethanol reflux, under infrared and microwave irradiation; and obtained 8, 72, 80, and 86% yields, respectively. Furthermore, the reaction times decreased from seven hours at room temperature until seven and five minutes for IR and MW irradiation, respectively. These comparisons show the importance of using different energy resources and their potential. Besides, ultrasound has been the other nonconventional energy source used to efficiently synthesize 3,4-DHPo. -

Ultrasound-Assisted Synthesis
The ultrasonic activation is based on cavitation effects, allowing this technique to improve the mass transfer in several organic reactions reported. In 2011, our group published the synthesis of 4-aryl 3,4-dihydropyridone derivatives (3,4-DHPo) by ultrasoundassisted technique, through the one-step condensation of Meldrum's acid (1), alkyl acetoacetates (2), aromatic aldehydes (3), and ammonium acetate, using glacial acetic acid as solvent, at room temperature, and obtained high yields (Scheme 12) [58]. This research group compared the different energy sources for oxidation; carried out the reaction at room temperature, at ethanol reflux, under infrared and microwave irradiation; and obtained 8, 72, 80, and 86% yields, respectively. Furthermore, the reaction times decreased from seven hours at room temperature until seven and five minutes for IR and MW irradiation, respectively. These comparisons show the importance of using different energy resources and their potential. Besides, ultrasound has been the other nonconventional energy source used to efficiently synthesize 3,4-DHPo. -

Ultrasound-Assisted Synthesis
The ultrasonic activation is based on cavitation effects, allowing this technique to improve the mass transfer in several organic reactions reported. In 2011, our group published the synthesis of 4-aryl 3,4-dihydropyridone derivatives (3,4-DHPo) by ultrasound-assisted technique, through the one-step condensation of Meldrum's acid (1), alkyl acetoacetates (2), aromatic aldehydes (3), and ammonium acetate, using glacial acetic acid as solvent, at room temperature, and obtained high yields (Scheme 12) [58].
The main advantages of ultrasound-assisted synthesis compared with conventional procedures are the milder conditions, the shorter reaction times, and the higher yields that improve the efficiency of the organic synthesis of these heterocycles. In addition, another energy source, such as the infrared-assisted technique, has also been explored. -
The main advantages of ultrasound-assisted synthesis compared with conventional procedures are the milder conditions, the shorter reaction times, and the higher yields that improve the efficiency of the organic synthesis of these heterocycles. In addition, another energy source, such as the infrared-assisted technique, has also been explored. -

Infrared-Assisted Synthesis
Parallel to the MW-assisted synthesis design of 3,4-DHPo derivatives, our group also reported the preparation of 3,4-Dihydro-2(1H)-pyridones derivatives (3,4-DHPo) by infrared-assisted method of the same multicomponent reaction under solvent-free conditions and similar reagents with moderate yields (Scheme 13) [59]. energy source, such as the infrared-assisted technique, has also been explored. -

Infrared-Assisted Synthesis
Parallel to the MW-assisted synthesis design of 3,4-DHPo derivatives, our group also reported the preparation of 3,4-Dihydro-2(1H)-pyridones derivatives (3,4-DHPo) by infrared-assisted method of the same multicomponent reaction under solvent-free conditions and similar reagents with moderate yields (Scheme 13) [59]. To summarize, different energy sources have been used to prepare 3,4-DHPo, allowing a broad range of products with varying substituent patterns. Particularly, nonconventional techniques such as MW and IR lead to high yields, short reaction times, and safe and straightforward work-up, constituting a notable improvement and involving green chemistry to synthesize these organic molecules. The support (insoluble or soluble)-assisted synthesis of 3,4-DHPo has also been a successful development.

Solid-Phase Organic Synthesis (SPOS)
One of the most commonly used techniques in combinatorial chemistry is Solid-Phase Organic Synthesis (SPOS), because it allows the rapid synthesis of many structurally diverse molecules in a short time. In 2006, our group published the first SPOS of 3,4-DHPo derivatives following a solid-support assisted synthetic strategy (Scheme 14) [60]. The immobilized acetoacetate (21) was obtained by reaction of 2,2,6-trimethyl-1,3-dioxin-4-one (1) and Wang resin (0.92 mmol OH/g); the further reaction of 21 in the presence of NH4OAc and HOAc led to the corresponding immobilized enamine (22), which reacted with the Knovenagel derivatives (23) to afford the expected immobilized 3,4-DHPo (24). The heterocycle was cleaved from the resin with 71-85% overall yield (Scheme 14) [60]. To summarize, different energy sources have been used to prepare 3,4-DHPo, allowing a broad range of products with varying substituent patterns. Particularly, nonconventional techniques such as MW and IR lead to high yields, short reaction times, and safe and straightforward work-up, constituting a notable improvement and involving green chemistry to synthesize these organic molecules. The support (insoluble or soluble)-assisted synthesis of 3,4-DHPo has also been a successful development.

Solid-Phase Organic Synthesis (SPOS)
One of the most commonly used techniques in combinatorial chemistry is Solid-Phase Organic Synthesis (SPOS), because it allows the rapid synthesis of many structurally diverse molecules in a short time. In 2006, our group published the first SPOS of 3,4-DHPo derivatives following a solid-support assisted synthetic strategy (Scheme 14) [60]. The immobilized acetoacetate (21) was obtained by reaction of 2,2,6-trimethyl-1,3-dioxin-4one (1) and Wang resin (0.92 mmol OH/g); the further reaction of 21 in the presence of NH 4 OAc and HOAc led to the corresponding immobilized enamine (22), which reacted with the Knovenagel derivatives (23) to afford the expected immobilized 3,4-DHPo (24). The heterocycle was cleaved from the resin with 71-85% overall yield (Scheme 14) [60]. This technique has been used in response to the increment of target molecule thesis to combinatorial chemistry. SPOS of 3,4-DHPo derivatives presents good and opens the way to synthesize other molecules with biological activity. Additi the synthesis of 3,4-DHPo derivatives using soluble polymers as support has als studied [61].

Liquid-Phase Organic Synthesis (LPOS)
The employment of soluble polymers as supports in organic synthesis is kno This technique has been used in response to the increment of target molecules synthesis to combinatorial chemistry. SPOS of 3,4-DHPo derivatives presents good results and opens the way to synthesize other molecules with biological activity. Additionally, the synthesis of 3,4-DHPo derivatives using soluble polymers as support has also been studied [61].

Liquid-Phase Organic Synthesis (LPOS)
The employment of soluble polymers as supports in organic synthesis is known as liquid-phase synthesis (LPS). In 2008, Fu et al. reported the LPS of 4-substituted-5-methoxycarbonyl-6-methyl-3,4-dihydropyridones on polyethylene glycol (PEG) 4000 assisted by MW irradiation (Scheme 15) [62]. First, the acetoacetylation of PEG was realized to obtain the immobilized acetoacetate (21); further, condensation of 21 with aldehyde derivative (3), Meldrum's acid in the presence of ammonium acetate, and solvent-free assisted by microwave irradiation allowed to obtain the immobilized 3,4-DHPo (24). The target compound 3,4-DHPo was obtained after cleavage using NaOMe in MeOH with 88-95% yield. The LPS of 3,4-DHPo derivatives showed excellent results and improved the overall results with the solid phase, allowing the one-step condensation, which was not possible in the SPOS procedure. On the other hand, all synthesized 3,4-DHPo showed at least one chiral center at C4, and all previously reported procedures allowed to obtain the corresponding racemic mixtures. Hence, efforts have been made to search for the chemo-selective synthesis of these derivatives.

Asymmetric Synthesis
The 3,4-DHPo structure is closely related to the configuration of its chiral center at C4, bringing biological activity. The chemoenzymatic synthesis can search the specific chiral center configuration and, at the same time, use an ecofriendly procedure. Torres et al. The LPS of 3,4-DHPo derivatives showed excellent results and improved the overall results with the solid phase, allowing the one-step condensation, which was not possible in the SPOS procedure. On the other hand, all synthesized 3,4-DHPo showed at least one chiral center at C4, and all previously reported procedures allowed to obtain the corresponding racemic mixtures. Hence, efforts have been made to search for the chemoselective synthesis of these derivatives.
On the other side, Huang et al. reported the first asymmetric synthesis of 3,4-DHPo derivatives [66]. The formation of monoacid (R)-30 was carried out by desymmetrization or asymmetric methanolysis of prochiral anhydride (29) using the organocatalyst 28, 2-Me-THF, and MeOH as solvents (Scheme 17). The conversion was 100%, and the bestobserved ee of 80%. The next step was the selective formylation to obtain the intermediate 31, and finally, the cyclization of 31 with ammonium acetate using acetic acid achieved the final product (R)-32, with an overall yield of three steps 48% and >95% ee (Scheme 17) [66]. The same group also reported the pilot-scale enantioselective synthesis of 32 and kilogram-scale production of N-methyl derivative of 32 in an excellent overall yield of ~22% Another enzymatic multicomponent reaction (EMCR) was reported using benzaldehyde, cyanoacetamide, ketone, and Acylaze Amano (AA)-catalyzed. This single enzymatic catalyzed reaction is attractive due to its high atom economy, easy work-up process, tolerance to a wide range of substituted reagents (benzaldehyde and ketones), and all mild conditions [65]. The best result was shown in the enzymatic hydrolysis of 4-aryl-5-(tertbutoxycarbonyl)-6-methyl-3,4-dihydro-2(1H)-pyridones with CAL-B enzyme and TMBE as a solvent to obtain high ee (93-95%) and moderate yields (30-31%) of (S)-derivative (26).
On the other side, Huang et al. reported the first asymmetric synthesis of 3,4-DHPo derivatives [66]. The formation of monoacid (R)-30 was carried out by desymmetrization or asymmetric methanolysis of prochiral anhydride (29) using the organocatalyst 28, 2-Me-THF, and MeOH as solvents (Scheme 17). The conversion was 100%, and the best-observed ee of 80%. The next step was the selective formylation to obtain the intermediate 31, and finally, the cyclization of 31 with ammonium acetate using acetic acid achieved the final product (R)-32, with an overall yield of three steps 48% and >95% ee (Scheme 17) [66]. The same group also reported the pilot-scale enantioselective synthesis of 32 and kilogram-scale production of N-methyl derivative of 32 in an excellent overall yield of~22% with excellent stereochemical purity (97% ee) [13]. (26).
On the other side, Huang et al. reported the first asymmetric synthesis of 3,4-DHPo derivatives [66]. The formation of monoacid (R)-30 was carried out by desymmetrization or asymmetric methanolysis of prochiral anhydride (29) using the organocatalyst 28, 2-Me-THF, and MeOH as solvents (Scheme 17). The conversion was 100%, and the bestobserved ee of 80%. The next step was the selective formylation to obtain the intermediate 31, and finally, the cyclization of 31 with ammonium acetate using acetic acid achieved the final product (R)-32, with an overall yield of three steps 48% and >95% ee (Scheme 17) [66]. The same group also reported the pilot-scale enantioselective synthesis of 32 and kilogram-scale production of N-methyl derivative of 32 in an excellent overall yield of ~22% with excellent stereochemical purity (97% ee) [13].
As time went on, different strategies were developed to obtain 3,4-DHPo derivatives in higher yields. Using nonconventional sources such as microwave and infrared radiation increased the yield; reduced the reaction time; and, in many cases, avoided the use of solvent, approaching green chemistry. However, access to the equipment can become a handicap. On the other hand, the main disadvantage of described conventional and nonconventional methodologies is the lack of stereoselectivity, which only shows in the asymmetrical synthesis. Nevertheless, a broad range of procedures described shows the importance of these heterocycles, not only because of their biological activity but also because they are crucial starting materials for synthesizing other more complex entities.
through N-Heterocyclic carbene (NHC)-catalyzed aza-Claisen reaction. Vellalath et al. described the enantioselective nucleophile catalyzed Michael/proton transfer/lactamization cascade with 3,4-difluorocinnamoyl chloride (37) and the enamine (39) in the presence of 37 as a catalyst with a nonpolar solvent and LiCl as an additive, which affected enantioselectivity; this mild process delivered 3,4-DHPo derivative in 78% yield and 92% ee (Scheme 19) [67]. As time went on, different strategies were developed to obtain 3,4-DHPo derivatives in higher yields. Using nonconventional sources such as microwave and infrared radiation increased the yield; reduced the reaction time; and, in many cases, avoided the use of solvent, approaching green chemistry. However, access to the equipment can become a handicap. On the other hand, the main disadvantage of described conventional and nonconventional methodologies is the lack of stereoselectivity, which only shows in the asymmetrical synthesis. Nevertheless, a broad range of procedures described shows the importance of these heterocycles, not only because of their biological activity but also because they are crucial starting materials for synthesizing other more complex entities.

Structural Characterization
The structure of 3,4-Dihydro-2(1H)-pyridones has been determined for physical and analytical techniques such as melting point (Table 1), 1 H-and 13 C-NMR spectroscopy, mass spectrometry (electron impact (EI) and electrospray ionization (ESI)), and X-ray. These techniques allowed to obtain a broad range of databases of physical properties to synthesize 3,4-DHPo [20,28,59].  The technique most used in the characterization of 3,4-DHPo derivatives has been 1 H-NMR spectroscopy, allowing us to corroborate the ring formation through the ABX pattern, as was explained by our group [68], which showed the protons H1, H3a, H3b, and H4 of the heterocycle ring (Table 2 and Figure 4). The technique most used in the characterization of 3,4-DHPo derivatives has been 1 H-NMR spectroscopy, allowing us to corroborate the ring formation through the ABX pattern, as was explained by our group [68], which showed the protons H1, H3a, H3b, and H4 of the heterocycle ring (Table 2 and Figure 4). The technique most used in the characterization of 3,4-DHPo derivatives has been 1 H-NMR spectroscopy, allowing us to corroborate the ring formation through the ABX pattern, as was explained by our group [68], which showed the protons H1, H3a, H3b, and H4 of the heterocycle ring (Table 2 and Figure 4). Electron impact (EI) and electrospray ionization (ESI) have also been used to characterize the 3,4-DHPo. Our group reported the fragmentation patterns of the even-electron ions formed under ESI conditions and the odd-electron ions generated under EI conditions from substituted 3,4-DHPo [20]. The characteristic fragmentation pattern under EI conditions was also established (Scheme 20) [65].    Electron impact (EI) and electrospray ionization (ESI) have also been used to characterize the 3,4-DHPo. Our group reported the fragmentation patterns of the even-electron ions formed under ESI conditions and the odd-electron ions generated under EI conditions from substituted 3,4-DHPo [20]. The characteristic fragmentation pattern under EI conditions was also established (Scheme 20) [65].    Different authors have confirmed the structure of 3,4-DHPo with different su ents by X-ray diffraction [17,29,30,[69][70][71][72][73], semiempirical (AM1) calculations [25] spectroscopic including NOE experiments, and coupling constants to determin structural conformation in solution [17].
There are some structural requirements to the 3,4-DHPo derivatives conform such as the absolute configuration at C-4 (R-versus S-enantiomer) acting as a mo switch, the substituted phenyl ring occupies an axial position perpendicularly bi the boatlike DHPo ring in a synperiplanar orientation, and the cis-carbonyl ester o tion concerning the olefinic double bond [17]. The semiempirical (AM1) calculatio NOE experiments defined two conformational structures; a first presents the aryl s uent at C-4 extended in a pseudoaxial position, and a second in which the aryl subs is in a pseudoequatorial position; besides, the first conformation was 2-4 kcal/mo stable than second structure. In both arrangements, the pyridone ring presented a t boat [17,19,23,24]. The X-ray studies confirmed the pseudoaxial disposition (Fig  which was stable in a solid state after the crystallization of ethanol [17]. Different authors have confirmed the structure of 3,4-DHPo with different substituents by X-ray diffraction [17,29,30,[69][70][71][72][73], semiempirical (AM1) calculations [25], NMR spectroscopic including NOE experiments, and coupling constants to determinate the structural conformation in solution [17].
There are some structural requirements to the 3,4-DHPo derivatives conformation, such as the absolute configuration at C-4 (R-versus S-enantiomer) acting as a molecular switch, the substituted phenyl ring occupies an axial position perpendicularly bisecting the boatlike DHPo ring in a synperiplanar orientation, and the cis-carbonyl ester orientation concerning the olefinic double bond [17]. The semiempirical (AM1) calculations and NOE experiments defined two conformational structures; a first presents the aryl substituent at C-4 extended in a pseudoaxial position, and a second in which the aryl substituent is in a pseudoequatorial position; besides, the first conformation was 2-4 kcal/mol more stable than second structure. In both arrangements, the pyridone ring presented a twisted boat [17,19,23,24]. The X-ray studies confirmed the pseudoaxial disposition ( Figure 5), which was stable in a solid state after the crystallization of ethanol [17]. tion concerning the olefinic double bond [17]. The semiempirical (AM1) calculations and NOE experiments defined two conformational structures; a first presents the aryl substituent at C-4 extended in a pseudoaxial position, and a second in which the aryl substituent is in a pseudoequatorial position; besides, the first conformation was 2-4 kcal/mol more stable than second structure. In both arrangements, the pyridone ring presented a twisted boat [17,19,23,24]. The X-ray studies confirmed the pseudoaxial disposition ( Figure 5), which was stable in a solid state after the crystallization of ethanol [17].
3,4-DHPo has been used as an intermediate for the formation of β-lactams derivatives through photochemical cycloaddition (Figure 6h) [87]. Besides, this heterocycle has been incorporated in the diastereoselective synthesis of 3-Oxo-14,15-dihydroandranginine, a unique indole alkaloid with an unusual ring system that includes a tetrahydroazepine unit condensed with an hexahydroquinoline entity in a trans-trans fashion (Figure 6i) [34]. The α 1a receptor antagonist from 3,4-DHPo was also synthesized, and its efficacy was demonstrated in a screen of prostate contraction model in rats (Figure 6j) [11].
Additionally, in 2019, simple 2-pyridones were applied in synthesizing alkaloids alkaloid-inspired compounds based on the piperidine or pyridine framework demonstrating the versatility of the revised scaffold.

Vasorelaxant activity
The regulation of blood pressure depends on vascular tone. In addition, nitric oxide (NO) is an excellent vasodilator molecule, but a low production of endothelium-derived NO causes a diminished vasodilator tone. This increases vascular resistance, which contributes to elevated blood pressure [96]. Therefore, various research groups are focused on finding compounds with vasorelaxant activity. A recent study demonstrated that 3-cyano-pyridin-2-ones ( Figure 9) show a significant vasorelaxant. Three of them are the most potent and revealed an endothelium-independent effect.

Vasorelaxant activity
The regulation of blood pressure depends on vascular tone. In addition, nitr (NO) is an excellent vasodilator molecule, but a low production of endothelium-NO causes a diminished vasodilator tone. This increases vascular resistance, wh tributes to elevated blood pressure [96]. Therefore, various research groups are on finding compounds with vasorelaxant activity. A recent study demonstrated cyano-pyridin-2-ones ( Figure 9) show a significant vasorelaxant. Three of them most potent and revealed an endothelium-independent effect. In addition, 3-cyano-2-pyridone derivatives were synthesized as calcium blockers and probable PD3 and PD4 inhibitors, taken as comparison; nifedipine fo calcium channel blocker, milrinone and amrinone for PD3 and PD4 inhibition. The pounds also have antihypertensive and vasorelaxant activity [57]. -

HIV-1 inhibitors
Non-nucleoside reverse transcriptase inhibitors (NNRTIs) play an essentia treating HIV infections. They have been used as the main target in the attack aga virus, and most of them present butterfly-like conformation [94,97]. This confo facilitates the intramolecular interactions between receptor and ligand. The NNRT acts specifically with the HIV reverse transcriptase (RT) substrate-binding site and its replication. Pyridone derivatives act in this way due to their favorable confor Hence, they are highly active against HIV-1. Parreira et al. reported a series of 22 derivatives such as 50 (Figure 10), which are inhibitors of HIV-1 [94]. In 2001, they sized 32 DHPo and proved their HIV-1 inhibitor activity; one of these compoun ( Figure 10) showed higher activity [90]. Additionally, another research group de the use of pyridine cocktails to attack viral variants that exhibit drug resistance [9 In addition, 3-cyano-2-pyridone derivatives were synthesized as calcium channel blockers and probable PD3 and PD4 inhibitors, taken as comparison; nifedipine for L-type calcium channel blocker, milrinone and amrinone for PD3 and PD4 inhibition. These compounds also have antihypertensive and vasorelaxant activity [57]. -

HIV-1 inhibitors
Non-nucleoside reverse transcriptase inhibitors (NNRTIs) play an essential role in treating HIV infections. They have been used as the main target in the attack against this virus, and most of them present butterfly-like conformation [94,97]. This conformation facilitates the intramolecular interactions between receptor and ligand. The NNRTI interacts specifically with the HIV reverse transcriptase (RT) substrate-binding site and inhibits its replication. Pyridone derivatives act in this way due to their favorable conformation. Hence, they are highly active against HIV-1. Parreira et al. reported a series of 22 DHPo derivatives such as 50 (Figure 10), which are inhibitors of HIV-1 [94]. In 2001, they synthesized 32 DHPo and proved their HIV-1 inhibitor activity; one of these compounds (51) (Figure 10) showed higher activity [90]. Additionally, another research group described the use of pyridine cocktails to attack viral variants that exhibit drug resistance [98]. acts specifically with the HIV reverse transcriptase (RT) substrate-binding site and inhibits its replication. Pyridone derivatives act in this way due to their favorable conformation. Hence, they are highly active against HIV-1. Parreira et al. reported a series of 22 DHPo derivatives such as 50 (Figure 10), which are inhibitors of HIV-1 [94]. In 2001, they synthesized 32 DHPo and proved their HIV-1 inhibitor activity; one of these compounds (51) (Figure 10) showed higher activity [90]. Additionally, another research group described the use of pyridine cocktails to attack viral variants that exhibit drug resistance [98]. -

Antibacterial and antifungal activity
DHPos derivatives present favorable properties as antibacterial agents against multidrug-resistant bacteria such as streptococci and anaerobic microorganisms [4]. In 2018, in vitro antimicrobial activity was reported in some 4-(biphenyl-4-yl)-1,4-dihydropyridine and 4-(biphenyl-4-yl)pyridine derivatives, followed by molecular docking and DFT studies [101]. Ahamed et al. have tested in vitro antibacterial activity of some 1,4dihydropyridine derivatives against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus epidermidis, Staphylococcus aureus, and Klebsiella pneumoniae. Most of these compounds were highly active against E. coli, and some even showed antifungal activity [102]. -
NMDA inhibitors are highly interesting in pharmaceutical research due to their application in treating moderate to severe Alzheimer's disease [103]. A series of 122 DHPos derivatives have been proven to be NMDA inhibitors through QSAR methodology [8]. Five of them, whose structures have a DHPo ring fused to a substituted aromatic ring (55 to 59), showed the highest inhibitory capacity (Figure 12). -
Ahamed et al. have tested in vitro antibacterial activity of some 1,4-dihydropyridine derivatives against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus epidermidis, Staphylococcus aureus, and Klebsiella pneumoniae. Most of these compounds were highly active against E. coli, and some even showed antifungal activity [102]. -
NMDA inhibitors are highly interesting in pharmaceutical research due to their application in treating moderate to severe Alzheimer's disease [103]. A series of 122 DHPos derivatives have been proven to be NMDA inhibitors through QSAR methodology [8]. Five of them, whose structures have a DHPo ring fused to a substituted aromatic ring (55 to 59), showed the highest inhibitory capacity (Figure 12). most potent activity (IC50 = 29.12 μM). This inhibitory activity grows with the increase in the electron-withdrawing ability of the groups [34].
NMDA inhibitors are highly interesting in pharmaceutical research due to their application in treating moderate to severe Alzheimer's disease [103]. A series of 122 DHPos derivatives have been proven to be NMDA inhibitors through QSAR methodology [8]. Five of them, whose structures have a DHPo ring fused to a substituted aromatic ring (55 to 59), showed the highest inhibitory capacity (Figure 12). Additionally, some dihydropyridines act as calcium channel blockers, potential candidates for schizophrenia and antihypertensive treatment [104,105]. Additionally, some dihydropyridines act as calcium channel blockers, potential candidates for schizophrenia and antihypertensive treatment [104,105].

Conclusions
Since the first accidental 3,4-DHPo synthesis, the most extensive method reported to obtain these derivatives proceeded via a multicomponent reaction (MCR-4CR) of Meldrum's acid, a β-keto-ester, and an aromatic aldehyde in the presence of ammonium acetate. This experimental procedure is simple and allows a wealth of molecular diversity depending on substituents in the starting reagents. Besides, this base strategy has been extended to nonconventional methods such as Microwave-, Ultrasound-, or Infrared-assisted reactions, allowing us to increase the efficiency by reducing the reaction time; increasing the yields; and, for some of them, eliminating the reaction solvents, as an important contribution to Green Chemistry. SPOS and LPOS have also been applied, allowing for obtaining a combinatorial library of these structures.
All 3,4-DHPo synthesized showed a broad range of biological activity highlighted as vasorelaxants and antihypertensives due to its structural similarity with 1,4-DHPs. Many of them also showed activity as antitumors, HIV-inhibitors, or antibacterial and antifungal, among others. On the other hand, 3,4-DHPo have become excellent synthetic precursors of many different complex structures with exciting applications.
High expectations still surround the next generation and further evolution of 3,4-DHPo. The improvement of asymmetric synthesis methodologies will allow the enantioselective preparation of this heterocycle, opening up the possibilities of enhancing biological activities. In the short and long term, more complex structures can be synthesized using DHPo as base molecular skeletons. In addition, the multicomponent platforms that characterize DHPo synthesis will promote the use of virtual screening tools and combinatorial chemistry, allowing us access to leading compounds based on this heterocycle. All described advancements contribute to an in-depth understanding of the potential of this scaffold and pave the way to apply novel 3,4-DHPo-based derivatives for further rational development in drug discovery.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.