Synthesis and Biological Activities of Pyrazino[1,2-A]Indole and Pyrazino[1,2-A]Indol-1-One Derivatives

pharmaceuticals

Review Synthesis and Biological Activities of Pyrazino[1,2-a]indole and pyrazino[1,2-a]indol-1-one Derivatives

Kena Zhang, Christine Tran, Mouad Alami, Abdallah Hamze and Olivier Provot *

BioCIS, CNRS-Université Paris-Saclay, 92290 Châtenay-Malabry, France; [email protected] (K.Z.); [email protected] (C.T.); [email protected] (M.A.); [email protected] (A.H.) * Correspondence: [email protected]

Abstract: This review concerns the synthesis and biological activities of pyrazino[1,2-a]indoles and pyrazino[1,2-a]indol-1-ones reported since 1997 and the discovery of biological activity of pyrazinoindole derivatives. In the first part, we first presented the synthetic routes that have been reported from a methodological point of view to access the pyrazinoindole unit according to cyclization reactions using or not using metal catalysts. Then, syntheses and neuropsychiatric, auto-immune, anti-infectious and anti-cancer properties of pyrazinoindoles were detailed. In the second part, we first reported the main accesses to pyrazinoindol-1-one substrates according to Michael reactions, metal-catalyzed and metal-free cyclization reactions. The syntheses and anti- cancer, anti-infectious, anti-allergenic and neuropsychiatric properties of pyrazinoindolones were next described and discussed. Citation: Zhang, K.; Tran, C.; Alami, M.; Hamze, A.; Provot, O. Synthesis Keywords: pyrazinoindole; pyrazinoindolone; cyclization; catalysis; biological activity and Biological Activities of Pyrazino[1,2-a]indole and pyrazino[1,2-a]indol-1-one Derivatives. Pharmaceuticals 2021, 14, 1. Introduction 779. https://doi.org/10.3390/ The pyrazino[1,2-a]indole unit is a tricyclic aromatic nucleus combining an indole and ph14080779 a pyrazine linked by the N5 and C9a atoms (Figure1). The access to this substituted aromatic nucleus has been well studied since 1997 by the Academic Editors: Urszula chemist community from a synthetic point of view and for its potential in medicinal chem- K. Komarnicka, Monika Lesiów and istry [1,2]. In parallel, structural modifications of the pyrazino[1,2-a]indole nucleus showed Sabina Jaros that (3,4-dihydro)pyrazino[1,2-a]indoles (type A) and (3,4-dihydro)-pyrazino[1,2-a]indol-

Received: 16 July 2021 1-ones (type B) were efﬁcient pharmacophores used in a variety of diseases. To illustrate, Accepted: 6 August 2021 3,4-dihydropyrazinoindoles 1 [3] and 2 [4] (type A) have been showed to be effective at Published: 8 August 2021 melatonin and adenosine receptors, while 3,4-dihydropyrazinoindol-1-ones 3 [5] and 4 [6] (type B) have been studied for their anti-viral and anti-allergenic activities, respectively.

Publisher’s Note: MDPI stays neutral This review, which follows recent papers from our group dealing with the synthesis of with regard to jurisdictional claims in indole-fused heterocycles such as pyrido[1,2-a]indoles [7] and oxazino[4,3-a]indoles [8] is published maps and institutional afﬁl- divided into two parts. First, we compiled the recent syntheses and biological activities iations. of pyrazinoindoles A and then discussed on the biological properties of pyrazinoindol-1- ones B and on their preparation. It should be noted that patents dealing with this subject have not been mentioned in this review. Each part is preceded by a short introduction dealing with recent synthetic accesses to these types of heterocycles that have not been

Copyright: © 2021 by the authors. studied biologically. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Pharmaceuticals 2021, 14, 779. https://doi.org/10.3390/ph14080779 https://www.mdpi.com/journal/pharmaceuticals

Review Synthesis and Biological Activities of Pyrazino[1,2-a]Indole and Pyrazino[1,2-a]Indol-1-One Derivatives

Kena Zhang, Christine Tran, Mouad Alami, Abdallah Hamze and Olivier Provot *

Citation: Zhang, K.; Tran, C.; infectious, anti-allergenic and neuropsychiatric properties of pyrazinoindolones were next de- Alami, M.; Hamze, A.; Provot, O. scribed and discussed. Synthesis and Biological Activities of Pyrazino[1,2-a]Indole and Keywords: pyrazinoindole; pyrazinoindolone; cyclization; catalysis; biological activity Pyrazino[1,2-a]Indol-1-One Derivatives. Pharmaceuticals 2021, 14, x. https://doi.org/10.3390/xxxxx 1. Introduction Academic Editors: Urszula K. Ko- PharmaceuticalsThe2021 pyrazino[1,2-, 14, 779 a]indole unit is a tricyclic aromatic nucleus combining an indole2 of 24 marnicka, Monika Lesiów and and a pyrazine linked by the N5 and C9a atoms (Figure 1). Sabina Jaros

Received: 16 July 2021 pyrazino[1,2-a]indole Accepted: 6 August 2021 A B Published: date N C N R2 R2 99 10 3 8 R 8 10 O 1 1 Publisher’s Note: MDPI stays neu- R Targets of this review R1 1 7 N N 4 7 N N 4 6 R 6 R tral with regard to jurisdictional 4 4 3 3 R3 claims in published maps and institu- pyrazino[1,2-a]indoles ABpyrazino[1,2-a]indol-1-ones tional affiliations.

nPr Cl O HN F C 3 O N O N NH2 N O Copyright: © 2021 by the authors. H N S N Submitted for possible open access N N Me N N N Me O O publication under the terms and con- SMe Me ditions of the Creative Commons At- 1 melatonin analogue 2 allosteric enhancer of 3 anti-viral activity 4 histamine H3 receptor the A1 adenosine receptor tribution (CC BY) license (http://crea- inverse agonist tivecommons.org/licenses/by/4.0/). FigureFigure 1.1.Targets Targets of this of reviewthis review and selection and ofselection biologically of activebiologically tetrahydro-pyrazino[1,2- active tetrahydro-pyrazino[1,2-a]indoles 1a, 2a and dihydro-a]in- dolespyrazino[1,2- 1a, 2a aand]indol-1-ones dihydro-pyrazino[1,2-3a, 4a. a]indol-1-ones 3a, 4a. 2. Pyrazinoindoles A: Synthesis and Biological Properties The access to this2.1. substituted Recent Synthetic aromatic Approaches nu tocleus Variously has Substituted been well Pyrazinoindoles studied since and 1997 by the chemist community3,4-Dihydropyrazinoindoles from a synthetic point of view and for its potential in medicinal The creation of the pyrazino[1,2-a]indole nucleus was mainly achieved by cyclizing indole having various groups (CHO, ketone, imine, nitrile, etc.) on C2 with a nucleophile linked to the indole nitrogen atom, thus creating the pyrazino C-ring (Scheme1). For example, 2-substituted-1-(prop-2-yn-1-yl)-1H-indoles 5, 7, 9, 11, 13 transformed into pyrazi- Pharmaceuticals 2021, 14, x. https://doi.org/10.3390/xxxxx noindoles 6, 8, 10, 12 and 14 respectively www.by intramolecularmdpi.com/journal/p cyclizationharmaceuticals using NH3 in MeOH, [9,10] DBU under microwave irradiation, [11] AuCl3 [12] as triple bond activator, Ni(OAc)2 in the presence of hydroxylamine [13] or NaH in DMF [14]. The C-ring of the pyrazino[1,2-a]indole system has been also built by alcoholate promoted cyclization of indolodinitrile compound 15 [15] and by Curtius reaction using Morita–Baylis–Hillman derivatives 17 [16] with good to excellent yields. The synthesis of variously substituted pyrazinoindoles having a saturated C-ring has been more studied than that of their aromatic counterparts, probably because these compounds offer more functional diversity such as diastereoselective accesses, but mainly because they have shown superior efﬁcacy in medicinal chemistry. Among the simplest reactions described to prepare these compounds was the one proposed by Katritzky, who used a cycloaddition reaction between a N-ethylamine-indole 19 and formaldehyde in the presence of benzotriazole (Bt) (Scheme2). A subsequent nucleophilic substitution reaction of the benzotriazole gives rise to various N-substituted pyrazinoindoles 20 [17]. N-ethylamine-indoles 19 also reacted, in a complementary approach, with aldehydes and Bt in the presence of Lewis acids to give C1-substituted pyrazinoindoles 21 [18]. A Ugi-azide four component approach was recently published to prepare a series of N-substituted pyrazinoindoles 23 having on C1 a substituted tetrazole ring [19]. Leighton et al. proposed highly enantioselective iso-Pictet–Spengler reactions using the condensation of 2-(1H-indol-1-yl)ethanamine 24 with a variety of α-ketoamides, followed by the addition of a commercially available chiral silicon Lewis acid (L*) to give 1,1-disubstituted- tetrahydropyrazino[1,2-a]indoles 25 with good yields (55–90%) and high enantioselectivity (ee = 86–96%) [20]. Guinchard et al. also reported an Au(I)-catalyzed Pictet–Spengler reaction to prepare a variety of complex heterocyclic compounds including tetrahydro- Pharmaceuticals 2021, 14, x FOR PEER REVIEW 2 of 23

chemistry [1,2]. In parallel, structural modifications of the pyrazino[1,2-a]indole nucleus showed that (3,4-dihydro)pyrazino[1,2-a]indoles (type A) and (3,4-dihydro)-pyrazino[1,2- a]indol-1-ones (type B) were efficient pharmacophores used in a variety of diseases. To illustrate, 3,4-dihydropyrazinoindoles 1 [3] and 2 [4] (type A) have been showed to be Pharmaceuticals 2021, 14, 779 effective at melatonin and adenosine receptors, while 3,4-dihydropyrazinoindol-1-ones3 of 24 3 [5] and 4 [6] (type B) have been studied for their anti-viral and anti-allergenic activities, respectively. This review, which follows recent papers from our group dealing with the synthesispyrazinoindoles of indole-fused with good heterocycles yields ranging such from as pyrido[1,2- 43 to 93%a [21]indoles]. In 2021, [7] and Lacour oxazino[4,3- et al. a]indolesreported [8] that is dividedN-sulfonyltriazoles into two parts.26 and First, imidazolines we compiled27 reacted the recent under syntheses rhodium and cataly- biolog- icalsis activities to give a of variety pyrazinoindoles of hexahydro-pyrazinoindoles A and then discussed28 with on the excellent biologic yieldsal properties easily trans- of pyrazinoindol-1-onesformed in tetrahydropyrazinoindoles B and on their preparation.29 after a welcomeIt should rearrangement be noted that in patents triflic aciddealing withTfOH this [ 22subject]. Ghorai have et not al. been reported mentioned in 2018 in of th anis elegantreview. synthesis Each part of is 1,3-disubstituted preceded by a short introduction1,2,3,4-tetrahydropyrazino[1,2- dealing with recenta]indoles synthetic32 with acce excellentsses to these stereoselectivity types of heterocycles (de, ee >99%) that via have base-mediated ring opening of chiral aziridines 31 with skatoles 30 followed by BF -OEt not been studied biologically. 3 2 catalyzed Pictet–Spengler reaction [23]. Chandra group reported synthesis of di-substituted pyrazinoindol-4-ones 34 with an excellent diastereoselectivity (>99%) via a Pictet–Spengler 2. reactionPyrazinoindoles by mixing A: 3-substituted- SynthesisN and-acylindoles Biological33 andProperties aromatic aldehydes in the presence 2.1.of Recent hexafluoroisopropanol Synthetic Approaches (HFIP) to underVariously microwave Substituted irradiation Pyrazinoindoles [24]. and 3,4- DihydropyrazinoindolesAfter this overview of recent methodologies giving access to type-A heterocycles, we a willThe now creation examine of the the synthesis pyrazino[1,2- of biologicallya]indole active nucleus pyrazino[1,2- was mainly]indoles achieved which by will cyclizing be classified by their biological activities. indole having various groups (CHO, ketone, imine, nitrile, etc.) on C2 with a nucleophile linked2.2. Biologically to the indole Active nitrogen Pyrazino[1,2-a]indoles atom, thus creating the pyrazino C-ring (Scheme 1). For example,2.2.1. 2-substituted-1-(prop-2-yn-1-yl)-1 Neuropsychiatric Properties H-indoles 5, 7, 9, 11, 13 transformed into pyra- zinoindolesBos et 6 al.,, 8 in, 10 a, program 12 and dedicated14 respectively to the discovery by intramolecular of novel drugs cyclization for the treatment using NH of 3 in MeOH,neuropsychiatric [9,10] DBU disorders, under microwave synthesized irradiation, a variety of [11] pyrazino[1,2- AuCl3 [12]a ]indolesas triple36a–f bondwhich activator, Ni(OAc)were found2 in the as partialpresence agonist of hydroxylamine ligands at the 5HT [13]2C orreceptor NaH in (Scheme DMF [14].3)[25 ].

SchemeScheme 1. 1. SynthesisSynthesis of pyrazinoindoles.pyrazinoindoles.

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The C-ring of the pyrazino[1,2-a]indole system has been also built by alcoholate promoted cyclization of indolodinitrile compound 15 [15] and by Curtius reaction using Morita–Baylis–Hillman derivatives 17 [16] with good to excellent yields. The synthesis of variously substituted pyrazinoindoles having a saturated C-ring has been more studied than that of their aromatic counterparts, probably because these compounds offer more functional diversity such as diastereoselective accesses, but mainly because they have shown superior efficacy in medicinal chemistry. Among the simplest reactions described to prepare these compounds was the one proposed by Katritzky, who used a cycloaddition reaction between a N-ethylamine-indole 19 and formaldehyde in the presence of benzotriazole (Bt) (Scheme 2). A subsequent nucleophilic substitution reaction

Pharmaceuticals 2021, 14, 779 of the benzotriazole gives rise to various N-substituted pyrazinoindoles 20 [17]. N-ethyl-4 of 24 amine-indoles 19 also reacted, in a complementary approach, with aldehydes and Bt in the presence of Lewis acids to give C1-substituted pyrazinoindoles 21 [18].

Scheme 2. Synthesis of 1,2,3,4-tetrahydro-pyrazinoindoles.

APyrazinoindole Ugi-azide four derivativescomponent approach36a–f were was prepared recently published according to to prepare a N-alkylation/ a series of Ncyclization/reduction-substituted pyrazinoindoles sequence 23 of having indoles on35 C1having a substituted an ester tetrazole function ring on C2 [19]. [25 ].Leighton Pyrazi- etnoindoles al. proposed36 were highly found enanti tooselective be partial iso-Pictet–Spengler agonists at the 5HT reactions2C receptor using subtype the condensa- binding withtion of a higher2-(1H-indol-1-yl)ethanamine affinity than for 5HT2A 24receptors. with a variety Best affinitiesof α-ketoamides, for 5HT2C followedreceptor by were the additionobserved of for a 10-methoxy-pyrazinoindolescommercially available chiral having silicon onLewis 6, 7, acid 8 or (L*) 9-position to give of 1,1-disubsti- the A-ring tuted-tetrahydropyrazino[1,2-bulky atoms (F < Me < Cl < Br).a]indoles In animals, 25 with36d goodshowed yields a (55–90%) 30-fold selectivity and high forenantiose- 5HT2C lectivityreceptors (ee compared = 86–96%) to 5HT [20].2A Guinchardreceptors and et anal. onlyalso 3-foldreported selectivity an Au(I)-catalyzed compared to 5HTPictet–1A 36d Spenglerreceptors. reactionIn vivo toresults prepare (rats a andvariety monkeys) of complex also demonstratedheterocyclic compounds that pyrazinoindole including tet- had a promising therapeutic potential for the treatment of various psychiatric disorders, rahydro-pyrazinoindoles with good yields ranging from 43 to 93% [21]. In 2021, Lacour et such as obsessive-compulsive disorders, panic anxiety or depression. al. reported that N-sulfonyltriazoles 26 and imidazolines 27 reacted under rhodium Imidazoline receptors exist in two forms, I1 and I2, for which there are very few ligands that are selective for one of the two forms. As a result, it is very difficult to assign a well-defined role to them even though I receptors have been described as involved in a 2 variety of CNS disorders. Tetrahydro-pyrazinoindoles 37a–c were evaluated by Glennon group for their potential as I2 imidazoline receptor ligands [26] due to their resemblance to β-carbolines [27] and imidazo-pyridoindoles (Figure2)[28]. Remarkably, 8-methoxypyrazinoindole 37c binds to I2 receptors with high affinity (Ki = 6.2 nM) and has a 1500-fold selectivity for I2 receptors compared to α2-adrenergic receptors (Ki = 9550 nM) and a 1000-fold selectivity for I1 receptors. A similar high selectivity for 37c was also observed towards I2 receptors compared to serotonin 5HT2A and 5HT2C receptors. Pharmaceuticals 2021, 14, x FOR PEER REVIEW 4 of 23

catalysis to give a variety of hexahydro-pyrazinoindoles 28 with excellent yields easily transformed in tetrahydropyrazinoindoles 29 after a welcome rearrangement in triflic acid TfOH [22]. Ghorai et al. reported in 2018 of an elegant synthesis of 1,3-disubstituted 1,2,3,4-tetrahydropyrazino[1,2-a]indoles 32 with excellent stereoselectivity (de, ee >99%) via base-mediated ring opening of chiral aziridines 31 with skatoles 30 followed by BF3- OEt2 catalyzed Pictet–Spengler reaction [23]. Chandra group reported synthesis of di-substituted pyrazinoindol-4-ones 34 with an excellent diastereoselectivity (>99%) via a Pictet– Spengler reaction by mixing 3-substituted-N-acylindoles 33 and aromatic aldehydes in the presence of hexafluoroisopropanol (HFIP) under microwave irradiation [24]. After this overview of recent methodologies giving access to type-A heterocycles, we will now examine the synthesis of biologically active pyrazino[1,2-a]indoles which will be classified by their biological activities.

2.2. Biologically Active Pyrazino[1,2-a]Indoles 2.2.1. Neuropsychiatric Properties Bos et al., in a program dedicated to the discovery of novel drugs for the treatment Pharmaceuticals 2021, 14, 779 of neuropsychiatric disorders, synthesized a variety of pyrazino[1,2-a]indoles5 of 24 36a–f which were found as partial agonist ligands at the 5HT2C receptor (Scheme 3) [25].

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group for their potential as I2 imidazoline receptor ligands [26] due to their resemblance Scheme 3. Synthesis of tetrahydro-pyrazinoindoles 36 and their binding data at the 5HT recep- to β-carbolines [27] andScheme imidazo-pyridoindoles 3. Synthesis of tetrahydro-pyrazinoindoles (Figure 2) [28]. 36 and their binding data at2C the 5HT2C receptor torsubtype. subtype.

Pyrazinoindole derivatives 36a–f were prepared according to a N-alkylation/cyclization/reduction sequence of indoles 35 having an ester function on C2 [25]. Pyrazinoindoles 36 were found to be partial agonists at the 5HT2C receptor subtype binding with a higher affinity than for 5HT2A receptors. Best affinities for 5HT2C receptor were observed for 10- methoxy-pyrazinoindoles having on 6, 7, 8 or 9-position of the A-ring bulky atoms (F < Me < Cl < Br). In animals, 36d showed a 30-fold selectivity for 5HT2C receptors compared to 5HT2A receptors and an only 3-fold selectivity compared to 5HT1A receptors. In vivo results (rats and monkeys) also demonstrated that pyrazinoindole 36d had a promising Figure 2. Tetrahydro-pyrazinoindolestherapeutic potential37 and for their the binding treatment data at of I2 variand αous-adrenergic psychiatric receptors. disorders, such as obsessive- Figure 2. Tetrahydro-pyrazinoindoles 37 and their binding data at I2 and α-adrenergic receptors. compulsive disorders, panic anxiety or depression. Zlotos et al. synthesized a series of C1-substituted tetrahydro-pyrazinoindoles 1 and Remarkably, 8-methoxypyrazinoindole39 as novel potent melatoninergic 37c ligands binds from to I382 receptorsin 4 steps (Scheme with 4high)[3]. affinity (Ki The afﬁnity of pyrazinoindoles 1, 39a,b for human MT1 and MT2 melatonin receptors = 6.2 nM) and has a 1500-fold selectivity for I2 receptors compared to α2-adrenergic receptors in Chinese Hamster Ovary (CHO) cells was measured by competition binding analysis (Ki = 9550 nM) and ausing 1000-fold 2-[125I]-iodomelatonin. selectivity for The I most1 receptors. active compound A similar39a washigh found selectivity to be an interesting for 37c was also observed towardsligand for I MT12 receptors and MT2 compared receptors with to excellentserotonin afﬁnity, 5HT but2A and with no5HT subtype2C receptors. selectivity Zlotos et al. synthesized(MT1: Ki = 6.6a series nM; MT2: of C1 Ki-substituted = 6.9 nM, respectively). tetrahyd Thisro-pyrazinoindoles tetrahydro-pyrazinoindole 1 and compound was found to be a partial agonist at MT receptors and possessed no intrinsic 39 as novel potent melatoninergic ligands from 38 in 4 steps1 (Scheme 4) [3]. activity at MT2 receptors. It is noteworthy that the treatment of pyrazinoindole 7 (R = H) with MeI gave the corresponding N-dimethyliodonium salt which was found to displace [3H]-cytisine from the nicotinic binding sites on rat cerebral cortex and was revealed to be a nicotinic agonist ligand [29].

Scheme 4. Synthesis of tetrahydro-pyrazinoindoles 1 and 39.

The affinity of pyrazinoindoles 1, 39a,b for human MT1 and MT2 melatonin receptors in Chinese Hamster Ovary (CHO) cells was measured by competition binding analysis using 2-[125I]-iodomelatonin. The most active compound 39a was found to be an interesting ligand for MT1 and MT2 receptors with excellent affinity, but with no subtype selectivity (MT1: Ki = 6.6 nM; MT2: Ki = 6.9 nM, respectively). This tetrahydro-pyrazinoindole compound was found to be a partial agonist at MT1 receptors and possessed no intrinsic activity at MT2 receptors. It is noteworthy that the treatment of pyrazinoindole 7 (R = H) with MeI gave the corresponding N-dimethyliodonium salt which was found to displace [3H]-cytisine from the nicotinic binding sites on rat cerebral cortex and was revealed to be a nicotinic agonist ligand [29].

2.2.2. Auto-Immune Properties Among the C1-substituted pyrazinoindoles, we can cite the work of Buzard et al. who prepared a series of C3-tetrahydro-pyrazinoindoles 42 from the same precursor 41 resulting from an intramolecular Michael reaction carried out on mesylate 40 in the presence of NH3 (Scheme 5) [30]. In a previous work, Buzard et al. showed that some cyclopenta[b]indoles were very potent agonists of the sphingosine 1-phosphate (S1P1) receptor that could

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Imidazoline receptors exist in two forms, I1 and I2, for which there are very few ligands that are selective for one of the two forms. As a result, it is very difficult to assign a well-defined role to them even though I2 receptors have been described as involved in a variety of CNS disorders. Tetrahydro-pyrazinoindoles 37a–c were evaluated by Glennon group for their potential as I2 imidazoline receptor ligands [26] due to their resemblance to β-carbolines [27] and imidazo-pyridoindoles (Figure 2) [28].

Figure 2. Tetrahydro-pyrazinoindoles 37 and their binding data at I2 and α-adrenergic receptors.

Remarkably, 8-methoxypyrazinoindole 37c binds to I2 receptors with high affinity (Ki = 6.2 nM) and has a 1500-fold selectivity for I2 receptors compared to α2-adrenergic receptors (Ki = 9550 nM) and a 1000-fold selectivity for I1 receptors. A similar high selectivity for 37c was also observed towards I2 receptors compared to serotonin 5HT2A and 5HT2C receptors. PharmaceuticalsZlotos2021, 14, 779et al. synthesized a series of C1-substituted tetrahydro-pyrazinoindoles 16 and of 24 39 as novel potent melatoninergic ligands from 38 in 4 steps (Scheme 4) [3].

Scheme 4. Synthesis of Schemetetrahydro-pyrazinoindoles 4. Synthesis of tetrahydro-pyrazinoindoles 1 and 39. 1 and 39. Pharmaceuticals 2021, 14, x FOR PEER REVIEW 6 of 23 2.2.2. Auto-Immune Properties The affinity of pyrazinoindolesAmong the C1-substituted 1, 39a,b pyrazinoindoles,for human MT1 we canand cite MT2 the work melatonin of Buzard receptors et al. who in Chinese Hamsterprepared Ovary a series(CHO) of C3-tetrahydro-pyrazinoindoles cells was measured by42 competitionfrom the same precursorbinding41 analysisresulting using 2-[125I]-iodomelatonin.from an intramolecular The most Michael active reaction compound carried out 39a on was mesylate found40 in to the be presence an interest- of NH3 be used for the treatment(Scheme5)[ of30 ].certain In a previous autoimmu work, Buzardne diseases et al. showed [31]. Due that someto the cyclopenta[ structuralb]indoles re- ingsemblance ligand for to MT1 thesewere and indoles, veryMT2 potent areceptors series agonists of withtric of theyclic exce sphingosine analoguesllent affinity, 1-phosphate (pyridoi butndoles,(S1P with1) receptorno oxazinoindoles subtype that could selec- be tivityand (MT1: pyrazinoindoles) Ki = 6.6used nM; forwere MT2: the designed, treatment Ki = 6.9 of nM,synthesi certain respectively). autoimmunezed, and evaluated. diseasesThis tetrahydro-pyrazinoindole [31 Pyridoindoles]. Due to the structural proved resemblance to these indoles, a series of tricyclic analogues (pyridoindoles, oxazinoindoles compoundto be the wasmost found promising to be compoundsa partial agonist in this at series MT1 receptorsof fused-indole and possessed compounds, no intrinsiceven if and pyrazinoindoles) were designed, synthesized, and evaluated. Pyridoindoles proved activitypyrazinoindoles at MT2 receptors. 42a–dto be the, preparedIt most is noteworthy promising from compounds 41 that in four the in stepstreatment this series (N-Boc of of fused-indole protection,pyrazinoindole compounds, O-debenzyla- 7 (R even = H) if withtion, MeI O gave-functionalization the correspondingpyrazinoindoles with42a–d variousN-dimethyliodonium, prepared benzyl from 41chloridesin four salt steps and which (N-Boc t-Butylester protection,was foundO hydrolysis)-debenzylation, to displace [3H]-cytisineshowed interesting from theO-functionalization activitiesnicotinic asbinding S1P with1 receptor various sites on benzyl agonistsrat chloridescerebral with and cortex nanomolart-Butylester and was hydrolysis) EC 50revealed values. showed toFor in-be the treatment of autoimmuneteresting activities disease as S1P1 asreceptor rheuma agoniststoid witharthritis, nanomolar Hill ECet 50al.values. synthesized For the treatment a py- a nicotinic agonist ligandof autoimmune [29]. disease as rheumatoid arthritis, Hill et al. synthesized a pyrazinoindole razinoindole derivativederivative having having on on C10 a a substituted substituted maleimide maleimide nucleus nucleus which was which unfortunately was unfor- found 2.2.2.tunately Auto-Immune found toto be Properties be poorly poorly activeactive as as protein protein kinase kinase C inhibitor C inhibitor (IC50 = 540 (IC nM)50 = [54032]. nM) [32]. Among the C1-substituted pyrazinoindoles, we can cite the work of Buzard et al. who prepared a series of C3-tetrahydro-pyrazinoindoles 42 from the same precursor 41 resulting from an intramolecular Michael reaction carried out on mesylate 40 in the presence of NH3 (Scheme 5) [30]. In a previous work, Buzard et al. showed that some cyclopenta[b]indoles were very potent agonists of the sphingosine 1-phosphate (S1P1) receptor that could

SchemeScheme 5. 5.Synthesis Synthesis of tetrahydro-pyrazinoindole of tetrahydro-pyrazinoindole41 and human 41 and S1P1 humancAMP EC S1P50 values1 cAMP of derivatives EC50 values42a–d of. derivatives 42a–d.

2.2.3. Anti-Bacterial and Anti-Fungal Properties A series of 15 pyrazinoindoles 43 were prepared according to Refs. [17,18] (see Scheme 2) by Verma group and evaluated for their anti-bacterial properties (Table 1) [33]. The in vitro antibacterial activity was evaluated by disc diffusion assay (DDA) using pathogenic strains of Staphylococcus aureus, Salmonella typhi, Streptomyces thermonitrificans, Pseudomonas aeruginosa and Escherichia coli. It was demonstrated that 43a was only active on P. aeruginosa and, similarly, a significant activity on P. aeruginosa and S. thermonitrificans was noticed with 43b. Pyrazinoindoles 43c–e were found to be active against all tested strains but with a relatively modest efficacy when compared with gentamycin. From these results, it seems that the presence of substituents on the nitrogen atom of pyrazinoindoles is deleterious for a satisfactory anti-bacterial activity. Pyrazinoindoles having an aromatic C-ring were not active against all tested strains.

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2.2.3. Anti-Bacterial and Anti-Fungal Properties A series of 15 pyrazinoindoles 43 were prepared according to Refs. [17,18] (see Scheme2) by Verma group and evaluated for their anti-bacterial properties (Table1)[ 33]. The in vitro antibacterial activity was evaluated by disc diffusion assay (DDA) using pathogenic strains of Staphylococcus aureus, Salmonella typhi, Streptomyces thermonitrificans, Pseudomonas aeruginosa and Escherichia coli. It was demonstrated that 43a was only active on P. aeruginosa and, similarly, a significant activity on P. aeruginosa and S. thermonitrificans was noticed with 43b. Pyrazinoindoles 43c–e were found to be active against all tested Pharmaceuticals 2021, 14, x FOR PEER REVIEW strains but with a relatively modest efficacy when compared with gentamycin.7 of From 23 these results, it seems that the presence of substituents on the nitrogen atom of pyrazinoindoles is deleterious for a satisfactory anti-bacterial activity. Pyrazinoindoles having an aromatic C-ring were not active against all tested strains. Table 1. Anti-bacterial properties of tetrahydro-pyrazinoindoles 43a–f. Table 1. Anti-bacterial properties of tetrahydro-pyrazinoindoles 43a–f.

CpndCpnd DDA DDAMinimum Minimum Inhibitory Inhibitory ConcentrationsConcentrations (µg/disc) (μg/disc) S. aureusS. aureus S. S.typhi typhi P. aeruginosa P. aeruginosa S. thermonitrificans S. thermonitriﬁcans 43a 43a -- - - 3.75 3.75 - - 43b - - 15 3.75 43c 43b 30- - 30 15 30 3.75 7.5 43d 43c 1530 30 60 30 60 7.5 60 43e 15 30 60 30 43f 43d -15 60 - 60 60 60 60 Gentamycin43e 115 30 1 60 0.5 30 1 43f - - 60 60 Gentamycin 1 Tetrahydro-pyrazinoindoles 1 43 were 0.5 also evaluated for their 1anti-fungal activity against Aspergillus ﬂavus, Aspergillus fumigatus, Aspergillus. niger and Candida albicans Tetrahydro-pyrazinoindoles(Table2)[ 34]. The 43 anti- wereAspergillus also evaluatedactivity for was their evaluated anti-fungal by disc activity diffusion against assay (DDA) and the anti-Candida activity was investigated by microbroth dilution assay. The more Aspergillus flavus, Aspergillus fumigatus, Aspergillus. niger and Candida albicans (Table 2) [34]. active tetrahydro-pyrazinoindoles 43 presented in Table2 displayed a mild to moderate The anti-Aspergillusanti-fungal activity activity, was evaluated even if these by pyrazinoindoles disc diffusion were assay found (DDA) to be, andin vitro the, lessanti- cytotoxic Candida activity wasthan investigated Amphotericin by B whenmicrobroth used at highdilution concentrations. assay. The SARs more with active compounds tetrahy-43 were dro-pyrazinoindolessimilar 43 presented for both anti-bacterial in Table 2 and displayed anti-fungal a mild activities. to moderate anti-fungal activity, even if these pyrazinoindoles were found to be, in vitro, less cytotoxic than Am- 2.2.4. Anti-Arrhythmic, Anti-Lipolytic, Neuro- and Cardio-Protective Properties photericin B when used at high concentrations. SARs with compounds 43 were similar for In a program dedicated to the discovery of ligands able to activate the A AR adenosine both anti-bacterial and anti-fungal activities. 1 receptor, Romagnoli et al. proposed some derivatizations on PD81,723, an allosteric modulator acting at the A1AR receptor, enhancing the functional effects of adenosine Table 2. Anti-fungalreceptor properties subtype of tetrahydro-pyrazinoindoles (Scheme6)[4]. 43g–I and 43c.

Cpnd DDA Minimum Inhibitory Concentrations (μg/disc) A. flavus A. fumigatus A. niger C. albicans 43g 11.7 5.8 11.7 15.6 43h 47 23 47 62.5 43i 187 94 187 125 43c 47 47 47 125 Gentamycin 1 1 0.5 1

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Table 1. Anti-bacterial properties of tetrahydro-pyrazinoindoles 43a–f.

Cpnd DDA Minimum Inhibitory Concentrations (μg/disc) S. aureus S. typhi P. aeruginosa S. thermonitrificans 43a - - 3.75 - 43b - - 15 3.75 43c 30 30 30 7.5 43d 15 60 60 60 43e 15 30 60 30 43f - - 60 60 Gentamycin 1 1 0.5 1

Tetrahydro-pyrazinoindoles 43 were also evaluated for their anti-fungal activity against Aspergillus flavus, Aspergillus fumigatus, Aspergillus. niger and Candida albicans (Table 2) [34]. The anti-Aspergillus activity was evaluated by disc diffusion assay (DDA) and the anti- Candida activity was investigated by microbroth dilution assay. The more active tetrahydro-pyrazinoindoles 43 presented in Table 2 displayed a mild to moderate anti-fungal activity, even if these pyrazinoindoles were found to be, in vitro, less cytotoxic than Am- photericin B when used at high concentrations. SARs with compounds 43 were similar for Pharmaceuticals 2021both, 14, 779 anti-bacterial and anti-fungal activities. 8 of 24

Table 2. Anti-fungal properties of tetrahydro-pyrazinoindoles 43g–I and 43c. Table 2. Anti-fungal properties of tetrahydro-pyrazinoindoles 43g–I and 43c.

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CpndCpnd DDA DDA Minimum Minimum Inhibitory Inhibitory Concentrations Concentrations (µg/disc) (μg/disc) 2.2.4. Anti-Arrhythmic, A. ﬂavus A.Anti-Lipolytic, flavus A. A. fumigatus Neuro- fumigatus and Cardio-Protective A. A. niger niger Properties C. C. albicans albicans 43g In a program43g dedicated11.711.7 to the discover 5.85.8y of ligands able 11.7to 11.7 activate the A1AR 15.6 adeno- 15.6 sine43h receptor, Romagnoli47 et al. proposed 23 some derivatizations 47 on PD81,723, an allosteric 62.5 43i 43h 18747 9423 187 47 125 62.5 modulator43c acting43i at47 the A1187AR receptor, enhancing 4794 the functional 47 187 effects of adenosine 125 125 re- Gentamycinceptor subtype (Scheme 1 6) [4]. 1 0.5 1 43c 47 47 47 125 Gentamycin 1 1 0.5 1

SchemeScheme 6. Synthesis 6. Synthesis of tetrahydro-pyrazinoindoles of tetrahydro-pyrazinoindoles 2, 246a–c, 46a–cand and effect effect in cAMP in cAMP assay assay in hA1 inCHO hA cells.1CHO cells.

Thus, pyrazinoindoles 2, 46a–d were synthesized from dibromothiophene 45 and Thus, pyrazinoindoles 2, 46a–d were synthesized from dibromothiophene 45 and 8- 8-substituted pyrazinoindoles 44 in 3 steps (SN2 reaction, debromination, phthalimide substituted pyrazinoindoleshydrolysis). 44 Pyrazinoindoles in 3 steps (S2N, 246a–c reaction,were next debromination, evaluated in a functionalphthalimide assay hy- for their drolysis). Pyrazinoindolesability to inhibit2, 46a–c forskolin were next stimulated evaluated cAMP in production a functional via the assay hA1-AR for intheir intact abil- Chinese hamster ovary (CHO) cells. The four pyrazinoindoles 2, 46a–c were found to be significantly ity to inhibit forskolin stimulated cAMP production via the hA1-AR in intact Chinese ham- more active than the reference PD 81,723. The best compound 8-fluorated pyrazinoindole ster ovary (CHO) cells. The four pyrazinoindoles 2, 46a–c were found to be significantly 46a inhibited the percentage of cAMP production by 69% vs. 18% for PD 81,723. It was more active than the reference PD 81,723. The best compound 8-fluorated pyrazinoindole also shown that these derivatives significantly inhibited antagonist binding at the hA1AR, 46a inhibited the percentagehA2AR or hA of3AR cAMP receptors. production by 69% vs. 18% for PD 81,723. It was also shown that these derivatives significantly inhibited antagonist binding at the hA1AR, 2.2.5. Anti-Cancer Properties hA2AR or hA3AR receptors. Romagnoli et al. studied in 2009 the antiproliferative properties of a series of pyrazinoindoles 17 which were prepared from the reduction/cyclization of N-cyanomethyl 2.2.5. Anti-Cancer Properties derivatives 47 followed by an oxidation reaction using MnO2 (Scheme7)[35]. Romagnoli et al. studied in 2009 the antiproliferative properties of a series of pyrazinoindoles 17 which were prepared from the reduction/cyclization of N-cyanomethyl derivatives 47 followed by an oxidation reaction using MnO2 (Scheme 7) [35].

Scheme 7. Synthesis and anti-cancer properties of a selection of pyrazinoindoles 49a–c.

It was shown that pyrazinoindole 49a was the more cytotoxic derivative against human leukemia K562 cancer cells with a promising IC50 value of 0.07 μM. However, this strong cytotoxicity was not observed in other cell lines such as murine leukemia (L1210),

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2.2.4. Anti-Arrhythmic, Anti-Lipolytic, Neuro- and Cardio-Protective Properties

In a program dedicated to the discovery of ligands able to activate the A1AR adenosine receptor, Romagnoli et al. proposed some derivatizations on PD81,723, an allosteric modulator acting at the A1AR receptor, enhancing the functional effects of adenosine receptor subtype (Scheme 6) [4].

Scheme 6. Synthesis of tetrahydro-pyrazinoindoles 2, 46a–c and effect in cAMP assay in hA1CHO cells.

Thus, pyrazinoindoles 2, 46a–d were synthesized from dibromothiophene 45 and 8- substituted pyrazinoindoles 44 in 3 steps (SN2 reaction, debromination, phthalimide hydrolysis). Pyrazinoindoles 2, 46a–c were next evaluated in a functional assay for their abil- ity to inhibit forskolin stimulated cAMP production via the hA1-AR in intact Chinese hamster ovary (CHO) cells. The four pyrazinoindoles 2, 46a–c were found to be significantly more active than the reference PD 81,723. The best compound 8-fluorated pyrazinoindole 46a inhibited the percentage of cAMP production by 69% vs. 18% for PD 81,723. It was also shown that these derivatives significantly inhibited antagonist binding at the hA1AR, hA2AR or hA3AR receptors.

2.2.5. Anti-Cancer Properties Romagnoli et al. studied in 2009 the antiproliferative properties of a series of pyra- Pharmaceuticalszinoindoles2021, 14, 779 17 which were prepared from the reduction/cyclization of N-cyanomethyl9 ofde- 24 rivatives 47 followed by an oxidation reaction using MnO2 (Scheme 7) [35].

Scheme 7. Synthesis and anti-cancer properties of a selection of pyrazinoindoles 49a–c. Scheme 7. Synthesis and anti-cancer properties of a selection of pyrazinoindoles 49a–c. Pharmaceuticals 2021, 14, x FOR PEER REVIEW 9 of 23 It was shown that pyrazinoindole 49a was the more cytotoxic derivative against It was shown humanthat pyrazinoindole leukemia K562 cancer 49a cells was with the a promisingmore cytotoxic IC50 value derivative of 0.07 µM. However,against hu- this strong cytotoxicity was not observed in other cell lines such as murine leukemia (L1210), man leukemiamurine K562mammarymurine cancer carcinoma mammary cells (FM3A),with carcinoma a humanpromising (FM3A), T-lymphoblastoid human IC50 T-lymphoblastoidvalue (Molt/4 of 0.07 and μ (Molt/4 CEM)M. However, and and CEM) this and strong cytotoxicityhuman cervicalhuman was carcinoma not cervical observed (HeLa) carcinoma cells in (HeLa)withother IC cellscell50 values withlines superior IC such50 values toas 20 superiormurine μM. to leukemia 20 µM. (L1210), In view of preparingIn view ofpyrazinoindoles preparing pyrazinoindoles 51 as anti-cancer51 as agents, anti-cancer Kumar agents, et al. mixed Kumar N- et al. mixed propargyl indolesN-propargyl 50 having indoles an aldehyde50 having function an aldehyde on C2 function with (2-ami on C2nophenyl)methanol with (2-aminophenyl)methanol derivatives inderivatives the presence in theof a presence catalytic ofamount a catalytic of AgNO amount3 (Scheme of AgNO 8) [36].3 (Scheme After the8)[ re-36]. After the action of δ-alkynylreaction aldehydes of δ-alkynyl and aldehydes nucleophilic and anilines, nucleophilic the alcohol anilines, function the alcohol adds function on the adds on the imine thus creatingimine thus a second creating bond a second (C-O). bond The third (C-O). bond The creation third bond (C-N) creation of this (C-N) process of this process occurs with occursthe nitrogen with theatom nitrogen of the imine atom which of the reacts imine with which the reactsalkyne with triple the bond alkyne acti- triple bond vated by AgNOactivated3 in a 6- byexo AgNO-dig manner3 in a 6- (76–84%).exo-dig manner (76–84%).

SchemeScheme 8. Synthesis 8. Synthesis of pyrazinoindoles of pyrazinoindoles51 and 51 and their their IC50 ICvalues50 values against against three three human human cancer cancer cell cell lines. lines.

PyrazinoindolesPyrazinoindoles 51 were next51 evaluatedwere next evaluatedagainst 3 againstcancer 3cell cancer lines cell (K562 lines leukemia (K562 leukemia cells, cells, BT-474BT-474 human human breast breast cells; cells;MCF-7 MCF-7 breast breast cancer cancer cells). cells). As Asit can it can be be seen seen in in Scheme Scheme 8 , the more 8, the more cytotoxiccytotoxic compound compound was was 51a51a againstagainst K562 K562 and and BT-474 BT-474 cancer cancer cells. cells. This This pyra- pyrazinoindole zinoindole waswas significantly signiﬁcantly more activeactive thanthan 4OH-tamoxifene,4OH-tamoxifene, used used as as reference reference compound, com- against pound, againstK562 K562 and and BT-474 BT-474 cells cells but displayedbut displayed a lower a lower IC50 value IC50 value against against MCF-7 MCF-7 cancer cells. This cancer cells. resultThis result is interesting is interesting as 51a asexhibited 51a exhibited maximum maximum cytotoxicity cytotoxicity in p53-deﬁcient in p53-defi- cell lines K562 cient cell linesand K562 BT-474 and cells BT-474 but notcells in but p53 no wildtypet in p53 wildtype MCF-7 cells. MCF-7 It would cells. certainlyIt would becer- interesting to tainly be interesting to perform SARs on these structures and to evaluate them on a panel of human cancer lines resistant to the usual treatments. After discussing the syntheses of pyrazinoindoles of type A and their biological activities, we will now detail the access to pyrazinoindol-1-ones B which have been more studied than pyrazinoindoles, probably because they are active on a larger number of biological targets.

3. Pyrazinoindol-1-Ones B: Synthesis and Biological Properties 3.1. Recent Synthetic Approaches to Variously Substituted Pyrazinoindol-1-Ones of Type B One of the easiest methods to prepare tetrahydro-pyrazinoindol-1-one derivatives 54 was proposed by Chen and Xiao group in 2011 [37]. In the presence of vinylsulfonium salt 53, the authors showed that variety of (1H-indol-2-yl)methanols were transformed with high yields into corresponding oxazinoindoles usable in medicinal chemistry [38]. It was next demonstrated that this easy-to-implement process was efficiently transposed to the synthesis of tetrahydro-pyrazinoindoles 54a,b by replacing (1H-indol-2-yl)methanols, as nucleophiles, by indole-2-carboxamides 52a,b (Scheme 9). The reaction proceeds via a Mi- chael addition of the indole nitrogen anion on the electrophilic sulfonium salt followed by

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perform SARs on these structures and to evaluate them on a panel of human cancer lines resistant to the usual treatments. After discussing the syntheses of pyrazinoindoles of type A and their biological activities, we will now detail the access to pyrazinoindol-1-ones B which have been more studied than pyrazinoindoles, probably because they are active on a larger number of biological targets.

3. Pyrazinoindol-1-Ones B: Synthesis and Biological Properties 3.1. Recent Synthetic Approaches to Variously Substituted Pyrazinoindol-1-Ones of Type B One of the easiest methods to prepare tetrahydro-pyrazinoindol-1-one derivatives 54 Pharmaceuticals 2021, 14, x FOR PEER REVIEW 10 of 23 was proposed by Chen and Xiao group in 2011 [37]. In the presence of vinylsulfonium salt 53, the authors showed that variety of (1H-indol-2-yl)methanols were transformed with high yields into corresponding oxazinoindoles usable in medicinal chemistry [38]. It was next demonstrated that this easy-to-implement process was efﬁciently transposed to Pharmaceuticalsa SN2 2021 substitution, 14, x FOR PEER of REVIEW the amide group, after prototropy, to give the expected pyrazino[1,2-10 of 23 the synthesis of tetrahydro-pyrazinoindoles 54a,b by replacing (1H-indol-2-yl)methanols, a]indol-1-ones 54a,bas in nucleophiles, excellent yields by indole-2-carboxamides together with Ph52a2S.,b (Scheme9). The reaction proceeds via a Michael addition of the indole nitrogen anion on the electrophilic sulfonium salt followed bya S aN2 S Nsubstitution2 substitution of the of the amide amide group, group, after after prototropy, prototropy, to to give give the the expected expected pyrazino[1,2- pyrazino[1,2- a]indol-1-ones]indol-1-ones 54a54a,,bb inin excellentexcellent yieldsyields togethertogether withwith PhPh22S.

Scheme 9. Synthesis of pyrazinoindolones 54a,b. Scheme 9. Synthesis of pyrazinoindolones 54a,,b.. The Michael reaction was also applied few years after by Bagnoli et al. under same The Michael reaction was also applied few years after by Bagnoli et al. under same conditions using secondaryThe Michael amides reaction and vinylselenones was also applied (Schemefew years 10)after [39]. by Bagnoli et al. under same conditions usingusing secondarysecondary amidesamides andand vinylselenonesvinylselenones (Scheme(Scheme 1010))[ [39].39].

Scheme 10. Synthesis of pyrazinoindolones 61 and selected examples.

Scheme 10. SynthesisScheme of pyrazinoindolonesAs 10. itSynthesis can be ofseen pyrazinoindolones with 61 and 61a selected and 6161band examples, electron-donating selected examples. . and electron-withdrawing groupsAs are it can welcome be seen on withthe indole61a and nucleus.61b, electron-donatingIt should be noticed and that electron-withdrawing pyrazinoindoles hav- As it can be seengroupsing on with C4 are alkyl welcome61a substituentsand on 61b the, indoleelectron-donating(hexyl, nucleus. butyl, methyl) It should andwere be electron-withdrawing obtained noticed thatin moderate pyrazinoindoles to good groups are welcomehavingyields on the (40–70%, on indole C4 alkyl e.g., nucleus. substituents 61d 50%), It probably should (hexyl, butyl, duebe noticed to methyl) steric hindrancethat were pyrazinoindoles obtained considerations. in moderate hav-Lastly, to good the ing on C4 alkyl substituentsyieldsreactivity (40–70%, of primary(hexyl, e.g., 61d amidesbutyl,50%), withmethyl) probably vinylselen duewere toones stericobtained was hindrance not studiedin moderate considerations. under these to good conditions Lastly, the in this work. yields (40–70%, e.g.,reactivity 61d 50%), of primary probably amides due with to steric vinylselenones hindrance was considerations. not studied under Lastly, these conditions the in thisAnother work. aza-Michael version was proposed by the team of Bandini and Umani-Ron- reactivity of primarychi amides who considered with vinylselen the intramolecones wasular notcyclization studied of under compounds these conditions62 to give pyra- in this work. zinoindol-1-ones 63 (Scheme 11) [40]. Another aza-Michael version was proposed by the team of Bandini and Umani-Ron- chi who considered the intramolecular cyclization of compounds 62 to give pyrazinoindol-1-ones 63 (Scheme 11) [40].

Scheme 11. Synthesis of pyrazinoindolones 63 and selected examples.

After a base and solvent screening study, it was demonstrated that the best combination is usage of K2CO3 (10 mol%) and DMSO as the solvent. Electron-donating and electron-withdrawing substituents on the indole led to desired pyrazinoindoles with comparable yields. The presence of aromatic substituents (phenyl, naphtyl) on the C3 position Scheme 11. Synthesis of pyrazinoindolones 63 and selected examples.

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a SN2 substitution of the amide group, after prototropy, to give the expected pyrazino[1,2- a]indol-1-ones 54a,b in excellent yields together with Ph2S.

Scheme 9. Synthesis of pyrazinoindolones 54a,b.

The Michael reaction was also applied few years after by Bagnoli et al. under same conditions using secondary amides and vinylselenones (Scheme 10) [39].

Scheme 10. Synthesis of pyrazinoindolones 61 and selected examples.

As it can be seen with 61a and 61b, electron-donating and electron-withdrawing groups are welcome on the indole nucleus. It should be noticed that pyrazinoindoles having on C4 alkyl substituents (hexyl, butyl, methyl) were obtained in moderate to good yields (40–70%, e.g., 61d 50%), probably due to steric hindrance considerations. Lastly, the Pharmaceuticalsreactivity2021, 14 ,of 779 primary amides with vinylselenones was not studied under these conditions11 of 24 in this work. Another aza-Michael version was proposed by the team of Bandini and Umani-Ron- chi who considered theAnother intramolec aza-Michaelular version cyclization was proposed of compounds by the team of Bandini62 to andgive Umani-Ronchi pyra- who considered the intramolecular cyclization of compounds 62 to give pyrazinoindol-1- zinoindol-1-ones 63ones (Scheme63 (Scheme 11) [40]. 11)[40 ].

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Scheme 11. SynthesisScheme of pyrazinoindolones 11. Synthesis of pyrazinoindolones 63 and selected63 and examples. selected examples. After a base and solvent screening study, it was demonstrated that the best combi- of indolesAfter a62 base did and notnation solvent affect is usage the screening outcome of K2CO study,3 (10 of mol%)the it was cyclization and demonstrated DMSO asreaction the solvent.that (e.g., the Electron-donating best 63d combina- 80%). Finally, and enantiometriction is usage of pure K2COelectron-withdrawing acrylate3 (10 mol%) containing andsubstituents DMSO a (S)-phenylethylamine as onthe the solv indoleent. led Electron-donating to desired unit pyrazinoindolesgave the and expected elec- with com- py- tron-withdrawing substituentsparable yields. on The the presence indole of aromaticled to desired substituents pyrazinoindoles (phenyl, naphtyl) with on the compa- C3 position razinoindole 63c inof a indoles good62 yielddid not but affect with the a outcome modest of thediastereoselectivity. cyclization reaction (e.g., 63d 80%). Finally, rable yields. The presence of aromatic substituents (phenyl, naphtyl) on the C3 position In addition toenantiometric Michael addition pure acrylate reactions containing giving a (S)-phenylethylamine access to pyrazinoindolones, unit gave the expected methods using catalyticpyrazinoindole amounts of63c organopalladiuin a good yield butm withcatalyst a modest have diastereoselectivity. been described to prepare In addition to Michael addition reactions giving access to pyrazinoindolones, methods these substrates. For example, the group of Broggini studied the cyclization reactions of a using catalytic amounts of organopalladium catalyst have been described to prepare these variety of indoylallylaminessubstrates. For 64 example, in the the presence group of Broggini of PdCl studied2(MeCN) the cyclization2 and CuCl reactions2 [41] of a to variety pro- of indoylallylamines 64 in the presence of PdCl (MeCN) and CuCl [41] to produce duce pyrazinoindolones 66 or with Pd(OAc)2, Na2CO23, and 2 Bu4NCl 2 to give pyra- pyrazinoindolones 66 or with Pd(OAc)2, Na2CO3, and Bu4NCl to give pyrazinoindolones zinoindolones 65 [42]65 [42 (Scheme] (Scheme 1212).).

Scheme 12. SynthesisScheme of 12. pyrazinoindolonesSynthesis of pyrazinoindolones 65 and65 66and and66 andselected selected examples. examples. The reaction producing 66 starts by the halogenation of the indoyl C3-position by The reaction CuXproducing2 without 66 involvement starts by of the palladium halogenation catalyst. of Then, the an indoyl amino-palladation C3-position of the by π CuX2 without involvementoleﬁns -bond of occursthe palladium followed by catalyst. a chlorine Then, transfer an on amino-palladation the Pd-species to give pyrazi- of the noindolones 66 rather than β-hydride elimination products. This domino process, which olefins π-bond occurs followed by a chlorine transfer on the Pd-species to give pyrazinoindolones 66 rather than β-hydride elimination products. This domino process, which allows the synthesis of di-halogenated pyrazinoindolones in very good yields, seems to be very suitable for the synthesis of complex and functionalized pyrazinoindolones. Brog- gini et al. also reported the synthesis of pyrazinoindolones 68 by using palladium-catalyzed cyclization of N-allylindoles 67 in the presence of Pd(OAc)2 as catalyst, Na2CO3 as base, benzoquinone as oxidant, and tetra-butyl ammonium chloride as additive in DMF (Scheme 13) [43].

Scheme 13. Synthesis of pyrazinoindolones 68 and selected examples.

Wolfe et al. proposed later an extension of this amino-palladation method by adding aromatic bromides to the re-examined reaction medium (Scheme 14) [44]. In this process, the catalytic cycle is initiated by oxidative addition of the Ar-Br on Pd(0) and, the resulting Pd(II) species coordinated the alkene double bond. After deprotonation of the amide, an

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of indoles 62 did not affect the outcome of the cyclization reaction (e.g., 63d 80%). Finally, enantiometric pure acrylate containing a (S)-phenylethylamine unit gave the expected pyrazinoindole 63c in a good yield but with a modest diastereoselectivity. In addition to Michael addition reactions giving access to pyrazinoindolones, methods using catalytic amounts of organopalladium catalyst have been described to prepare these substrates. For example, the group of Broggini studied the cyclization reactions of a variety of indoylallylamines 64 in the presence of PdCl2(MeCN)2 and CuCl2 [41] to produce pyrazinoindolones 66 or with Pd(OAc)2, Na2CO3, and Bu4NCl to give pyrazinoindolones 65 [42] (Scheme 12).

Scheme 12. Synthesis of pyrazinoindolones 65 and 66 and selected examples.

The reaction producing 66 starts by the halogenation of the indoyl C3-position by CuX2 without involvement of the palladium catalyst. Then, an amino-palladation of the

Pharmaceuticalsolefins2021 ,π14-bond, 779 occurs followed by a chlorine transfer on the Pd-species to give pyra-12 of 24 zinoindolones 66 rather than β-hydride elimination products. This domino process, which allows the synthesis of di-halogenated pyrazinoindolones in very good yields, seems to be very suitable forallows the synthesis the synthesis of complex of di-halogenated and functionalized pyrazinoindolones pyrazinoindolones. in very good yields, Brog- seems gini et al. also reportedto be verythe suitablesynthesis for theof pyrazinoindolones synthesis of complex and68 by functionalized using palladium-cata- pyrazinoindolones. 68 lyzed cyclization ofBroggini N-allylindoles et al. also 67 reported in the the presence synthesis of of Pd(OAc) pyrazinoindolones2 as catalyst,by usingNa2CO palladium-3 as N 67 base, benzoquinonecatalyzed as oxidant, cyclization and oftetra-allylindoles-butyl ammoniumin the presence chloride of Pd(OAc) as additive2 as catalyst, in DMF Na2 CO3 as base, benzoquinone as oxidant, and tetra-butyl ammonium chloride as additive in DMF (Scheme 13) [43]. (Scheme 13)[43].

Scheme 13. SynthesisScheme of pyrazinoindolones 13. Synthesis of pyrazinoindolones 68 and selected68 and examples. selected examples. Pharmaceuticals 2021, 14, x FOR PEER REVIEW 12 of 23 Wolfe et al. proposed later an extension of this amino-palladation method by adding Wolfe et al. proposedaromatic bromideslater an extension to the re-examined of this reactionamino-palladation medium (Scheme method 14)[44 ].by In adding this process, aromatic bromidesthe to catalyticthe re-examined cycle is initiated reaction by oxidative medium addition (Scheme of the 14) Ar-Br [44]. on Pd(0)In this and, process, the resulting Pd(II) species coordinated the alkene double bond. After deprotonation of the amide, an amino-palladationthe catalytic cycle is occurred initiated toby give,oxidative after additionreductive of elimination,the Ar-Br on 70Pd(0) and and, regeneration the resulting of Pd(II) species coordinatedamino-palladation the alkene occurred double to give, bond. after After reductive deprotonation elimination, 70 ofand the regeneration amide, an of the the Pd(0) catalyst. Pd(0) catalyst.

Scheme 14. SynthesisScheme of pyrazinoindolones 14. Synthesis of pyrazinoindolones 70 and selected70 and examples. selected examples. Laliberté et al. prepared a series of pyrazinoindolones 73 with good yields arising from Laliberté et al.the prepared Pd-coupling a series reaction of between pyrazinoindolones indole-hydroxamates 73 with71 and good electrophilic yields arising (Z)-but-2-ene from the Pd-coupling(1,4- reactionbiscarbonate) between72 (Scheme indole-hydroxamates 15)[45]. 71 and electrophilic (Z)-but- 2-ene (1,4-biscarbonate) 72 (Scheme 15) [45].

Pd2(dba)3CHCl3 5mol% O P(Oi-Pr)3 30 mol% O R BocO OBoc R 1 DCM N NHOR N 1 H NOR

71 72 73 (58%-88%) Br MeO Cl O O F O O

N N N N N Oi-Pr N Ot-Bu N Ot-Bu N Ot-Bu

73a (78%) 73b (70%) 73c (75%) 73d (60%) Scheme 15. Synthesis of pyrazinoindolones 73 and selected examples.

Electron-rich and electron-poor substituents are well tolerated at each position of the indole ring and gave the expected pyrazinoindoles in comparable yields. The R1 substituent of the hydroxamate was studied and when R1 = Bn, i-Pr, and t-Bu, the pyrazinoindolones were obtained in 72–81% yields, whereas with R1 = Me, the reaction is somewhat less efficient (58%). It would be interesting to study a chiral version of this reaction using suitable ligands to observe their effects on enantioselectivity. A chiral elegant access to various nitrogen-containing heterocycles was proposed by B. Trost during the transformation of vinyl aziridines with indoles and pyrroles (Scheme 16) [46]. Among the heterocycles evaluated as nucleophiles (pyrroles and indoles) in the presence of vinyl aziridines, it was showed that the use of Pd2(dba)3⋅CHCl3 with the chiral ligand L* in dichloroethane (DCE) provided access to the N-alkylation products in excellent yields and enantiomeric excesses in a regioselective manner. When these experimental conditions were applied to indolyl-methylcarboxylate 74 in the presence of vinylaziridines 75a,b, pyrazinoindolones 76a,b were obtained in good yields with excellent enantiomeric excess after opening of the aziridine followed by cyclization.

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amino-palladation occurred to give, after reductive elimination, 70 and regeneration of the Pd(0) catalyst.

Scheme 14. Synthesis of pyrazinoindolones 70 and selected examples.

Laliberté et al. prepared a series of pyrazinoindolones 73 with good yields arising Pharmaceuticalsfrom the2021 ,Pd-coupling14, 779 reaction between indole-hydroxamates 71 and electrophilic (Z)-but-13 of 24 2-ene (1,4-biscarbonate) 72 (Scheme 15) [45].

Pd2(dba)3CHCl3 5mol% O P(Oi-Pr)3 30 mol% O R BocO OBoc R 1 DCM N NHOR N 1 H NOR

71 72 73 (58%-88%) Br MeO Cl O O F O O

N N N N N Oi-Pr N Ot-Bu N Ot-Bu N Ot-Bu

73a (78%) 73b (70%) 73c (75%) 73d (60%) Scheme 15. SynthesisScheme of pyrazinoindolones 15. Synthesis of pyrazinoindolones 73 and selected73 and examples. selected examples. Electron-rich and electron-poor substituents are well tolerated at each position of the Electron-rich andindole electron-poor ring and gave the substituents expected pyrazinoindoles are well tolerated in comparable at each yields. position The R1 substituent of the indole ring and gaveof the the hydroxamateexpected pyrazinoindoles was studied and when in comparable R1 = Bn, i-Pr, and yields.t-Bu, theThe pyrazinoindolones R1 substitu- were obtained in 72–81% yields, whereas with R1 = Me, the reaction is somewhat less ent of the hydroxamate was studied and when R1 = Bn, i-Pr, and t-Bu, the pyra- efﬁcient (58%). It would be interesting to study a chiral version of this reaction using 1 zinoindolones weresuitable obtained ligands in 72–81% to observe yields, their effects whereas on enantioselectivity. with R = Me, the reaction is somewhat less efficient (58%).A chiralIt would elegant be accessinteresting to various to study nitrogen-containing a chiral version heterocycles of this was reaction proposed using suitable ligandsby B.to Trostobserve during their the effects transformation on enantioselectivity. of vinyl aziridines with indoles and pyrroles (Scheme 16)[46]. Among the heterocycles evaluated as nucleophiles (pyrroles and in- A chiral elegant access to various nitrogen-containing heterocycles was proposed by B. doles) in the presence of vinyl aziridines, it was showed that the use of Pd2(dba)3·CHCl3 Trost during the transformationwith the chiral ligand of vinyl L* in aziri dichloroethanedines with (DCE) indoles provided and access pyrroles to the N (Scheme-alkylation 16) prod- [46]. Among the heterocyclesucts in excellent evaluated yields and as enantiomeric nucleophiles excesses (pyrroles in a regioselective and indoles) manner. in the When pres- these experimental conditions were applied to indolyl-methylcarboxylate⋅ 74 in the presence of Pharmaceuticalsence of 2021 vinyl, 14, x FOR aziridines, PEER REVIEW it was showed that the use of Pd2(dba)3 CHCl3 with the chiral13 of 23 vinylaziridines 75a,b, pyrazinoindolones 76a,b were obtained in good yields with excellent ligand L* in dichloroethaneenantiomeric (DCE) excess provided after opening access of the to aziridine the N followed-alkylation by cyclization. products in excellent yields and enantiomeric excesses in a regioselective manner. When these experimental conditions were applied to indolyl-methylcarboxylate 74 in the presence of vinylaziridines 75a,b, pyrazinoindolones 76a,b were obtained in good yields with excellent enantiomeric excess after opening of the aziridine followed by cyclization.

Scheme 16.16. Synthesis of optically activeactive pyrazinoindolones 34a34a,,bb..

A singlesingle exampleexample ofof gold-promoted gold-promoted cyclization cyclization of ofN -propargylamidolindoleN-propargylamidolindole76 76giving giv- pyrazinoindoloneing pyrazinoindolone78a was78a was proposed proposed by A. by Padwa A. Padwa (Scheme (Scheme 17)[ 17)47]. [47]. In this In this reaction, reaction, the gold-catalystthe gold-catalyst (5 mol%) (5 mol%) ﬁrst first activates activates the alkynethe alkyne triple triple bond bond which which is then is then attacked attacked by the by nucleophilicthe nucleophilic amide amide group. group. An An additional additional hydration hydration of theof the other other triple triple bond bond could could occur oc- bycur re-using by re-using the gold-catalyst, the gold-catalys to promotet, to promote pyrazinoindolone pyrazinoindolone78a. It 78a should. It should be noted be thatnoted it isthat also it is possible also possible to access to access with lowwith yields low yields to N- substituted-3-methylto N-substituted-3-methyl derivatives derivatives78b–d 78b–by mixingd by mixingN-propargyl-indole-2-carbaldehyde N-propargyl-indole-2-carbaldehyde77 in the 77 presencein the presence of amines of (MeNHamines2 (MeNH, BnNH2, BnNH2 and hexNH2) as nucleophiles and Cs2CO3 as a base without the need of any metal. The mechanism of this cyclization was discussed in detail by the authors [48].

Scheme 17. Synthesis of pyrazinoindolone 78a–d.

The last example of this section concerns the synthesis of pyrazinoindolones 80 obtained in two steps from an Ugi condensation followed by a base-mediated cyclization on Ugi intermediates 79 (Scheme 18) [49].

Scheme 18. Synthesis of pyrazinoindolones 80 and selected examples.

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Scheme 16. Synthesis of optically active pyrazinoindolones 34a,b.

SchemeA single 16. Synthesis example of opticallyof gold-promoted active pyrazinoindolones cyclization of 34a N,b-propargylamidolindole. 76 giving pyrazinoindolone 78a was proposed by A. Padwa (Scheme 17) [47]. In this reaction, A single example of gold-promoted cyclization of N-propargylamidolindole 76 giv- the gold-catalyst (5 mol%) first activates the alkyne triple bond which is then attacked by ing pyrazinoindolone 78a was proposed by A. Padwa (Scheme 17) [47]. In this reaction, the nucleophilic amide group. An additional hydration of the other triple bond could oc- the gold-catalyst (5 mol%) first activates the alkyne triple bond which is then attacked by cur by re-using the gold-catalyst, to promote pyrazinoindolone 78a. It should be noted the nucleophilic amide group. An additional hydration of the other triple bond could oc- that it is also possible to access with low yields to N-substituted-3-methyl derivatives 78b– Pharmaceuticalscur by2021 re-using, 14, 779 the gold-catalyst, to promote pyrazinoindolone 78a. It should be noted 14 of 24 d by mixing N-propargyl-indole-2-carbaldehyde 77 in the presence of amines (MeNH2, that it is also possible to access with low yields to N-substituted-3-methyl derivatives 78b– BnNH2 and hexNH2) as nucleophiles and Cs2CO3 as a base without the need of any metal. d by mixing N-propargyl-indole-2-carbaldehyde 77 in the presence of amines (MeNH2, The mechanism of this cyclization was discussed in detail by the authors [48]. BnNH2 and hexNH2) as nucleophiles and Cs2CO3 as a base without the need of any metal. and hexNH2) as nucleophiles and Cs2CO3 as a base without the need of any metal. The The mechanism of thismechanism cyclization of this was cyclization discussed was discussedin detail inby detail the authors by the authors [48]. [48].

Scheme 17. Synthesis of pyrazinoindolone 78a–d. Scheme 17. 78a–d Scheme 17. Synthesis of pyrazinoindoloneSynthesis 78a–d of. pyrazinoindolone . The last example of this section concerns the synthesis of pyrazinoindolones 80 ob- The last example of this section concerns the synthesis of pyrazinoindolones 80 ob- tainedThe in twolast stepsexample fromtained of anthis in twoUgi section stepscondensation from concer an Ugins followed condensationthe synthesis by followeda base-mediatedof pyrazinoindolones by a base-mediated cyclization 80 cyclization ob- on on Ugitained intermediates in two steps 79 fromUgi (Scheme intermediates an Ugi 18) condensation [49].79 (Scheme 18followed)[49]. by a base-mediated cyclization on Ugi intermediates 79 (Scheme 18) [49].

Scheme 18. SynthesisScheme of pyrazinoindolones 18. Synthesis of pyrazinoindolones 80 and selected80 andexamples. selected examples. Scheme 18. Synthesis of pyrazinoindolonesAfter this overview 80 ofand the selected synthetic examples. routes to pyrazinoindolones published from a methodological point of view, we will now list the main accesses to biologically active pyrazinoindolones.

3.2. Biologically Active Pyrazino[1,2-a]indol-1-ones 3.2.1. Anti-Cancer Properties

MAPK-activated protein kinase 2 (MAPK2), a serine/threonine-protein kinase known as the best understood downstream partner of p38 MAP kinase play an essential role in signal transduction pathways involved in cell proliferation, differentiation, and death. To prepare a series of MAPK Activated Protein Kinase 2 (MAPK2) inhibitors, Goldberg et al. used reductive conditions (H2/PtO2 or CoCl2/NaBH4) to indoyl-1-cyanomethyl derivatives 81 to afford, after cyclization, pyrazinoindolone platforms 82 (Scheme 19)[50]. Chemical modiﬁcations of the ethylester (3 steps) led to a series of twenty pyrazinoindolones 83 which were evaluated as MAPK2 inhibitors and best compounds are presented in Scheme 19. Pyrazinoindolones 83a and 83b were found to provide best combination of molecular, cellular, and physicochemical properties with nanomolar IC50 values. Pyrazi- noindolone 83b was also found to have a negligible effect on a wide range of other kinases, showing selectivity, and its efﬁcacy was also demonstrated through various in vitro and in vivo tests. Pharmaceuticals 2021, 14, x FOR PEER REVIEW 14 of 23

After this overview of the synthetic routes to pyrazinoindolones published from a methodological point of view, we will now list the main accesses to biologically active pyrazinoindolones.

3.2. Biologically Active Pyrazino[1,2-a]Indol-1-Ones 3.2.1. Anti-Cancer Properties MAPK-activated protein kinase 2 (MAPK2), a serine/threonine-protein kinase known as the best understood downstream partner of p38 MAP kinase play an essential role in signal transduction pathways involved in cell proliferation, differentiation, and death. To prepare a series of MAPK Activated Protein Kinase 2 (MAPK2) inhibitors, Gold- berg et al. used reductive conditions (H2/PtO2 or CoCl2/NaBH4) to indoyl-1-cyanomethyl derivatives 81 to afford, after cyclization, pyrazinoindolone platforms 82 (Scheme 19) [50]. Chemical modifications of the ethylester (3 steps) led to a series of twenty pyrazinoindolones 83 which were evaluated as MAPK2 inhibitors and best compounds are presented in Scheme 19. Pyrazinoindolones 83a and 83b were found to provide best combination of molecular, cellular, and physicochemical properties with nanomolar IC50 values. Pyrazinoindolone 83b was also found to have a negligible effect on a wide range of Pharmaceuticalsother2021 kinases,, 14, 779 showing selectivity, and its efficacy was also demonstrated through various 15 of 24 in vitro and in vivo tests.

H /PtO 2 or 2 CoCl , NaBH 2 4 O 2-3 steps O CO2Et N H EtO2C N N EtO C N NH N NH R1 CN 2 R R2 R1 S O R1 R2 R2 81 82 (54%-95%) 83 O O

NH NH N N H H N N N N O Me O O O S S NH NH N N 83a (MK2) IC =3nM t-Bu 50 t-Bu 83b (MK2) IC50 =2nM Scheme 19. SynthesisScheme of pyrazinoindolones 19. Synthesis of pyrazinoindolones 83 as MAPK283 inhibitors.as MAPK2 inhibitors. To counteract the deleterious effects of free radicals on DNA, which cause cancers, To counteractBukhari the deleterious et al. recently effects prepared of fr andee evaluatedradicals aon small DNA, library which of pyrazinoindoles cause cancers,85 having Bukhari et al. recentlyon C3 prepared benzyl substituents and evaluated (Scheme a 20small)[51]. library Pyrazinoindolones of pyrazinoindoles85 which were 85 hav- found to be ing on C3 benzyl substituentsnontoxic against (Scheme human 20) mammary [51]. Pyrazinoindolones gland epithelial cells displayed85 which a were micromolar found level of to be nontoxic againstcytotoxicity human against mammary PC-3 prostate glan cancerd epithelial cells, MCF-7 cells breastdisplayed cancer cells,a micromolar PaCa-2 pancreatic carcinoma cells, A-549 epithelial cancer cells and HT-29 colon cancer cells. It was shown level of cytotoxicity against PC-3 prostate cancer cells, MCF-7 breast cancer cells, PaCa-2 that 85a–c exhibited a low inﬂuence on tubulin assembly but inhibited EGFR with IC50 pancreatic carcinomavalues cells, from A-549 1.7 to 3.9epithelialµM. These cancer compounds cells and were HT-29 also found colon to becancer inhibitors cells. of It reactive was shown that 85a–coxygen exhibited species (ROS)a low and influence showed on noticeable tubulin antioxidant assembly activity. but inhibited Histopathological EGFR and Pharmaceuticals 2021, 14, x FOR PEER REVIEW 15 of 23 with IC50 values fromimmunohistochemical 1.7 to 3.9 μM. These studies compounds showed that, were when also chlorpyrifos, found to be a inhibitors ROS enhancer, of was reactive oxygen speciesassociated (ROS) to and pyrazinoindolone showed noticeable85b on antioxidant the testis of male activity. mice, Histopatholog- testicular damage was signiﬁcantly decreased. ical and immunohistochemical studies showed that, when chlorpyrifos, a ROS enhancer, was associated to pyrazinoindolone 85b on the testis of male mice, testicular damage was significantly decreased.

Scheme 20.Scheme Synthesis 20. Synthesis and evaluation and evaluation of pyrazinoindolones of pyrazinoindolones 8585 asas anti-cancer anti-cancer agents.agents.

Kwak group synthesized a wide range of enantiopure (R)- and (S)-3-substituted pyrazinoindolones 88 from an intramolecular Mitsunobu cyclization of 86 using DEAD, PPh3 in THF (Scheme 21) [52].

Scheme 21. Synthesis and evaluation of optically active 3-substituted pyrazinoindolones 88.

Compounds 88 were evaluated for their anti-proliferative activity on MCF-7 breast cancer cells and on triple negative MDA-MB-468 breast cancer cells. It was showed that the stereochemistry on C3 has not a significant impact on the cytotoxicity. Interestingly, it was observed that the most cytotoxic compounds 88a–c presented in Scheme 21 were more effective than the reference compound gefinitib. Furthermore, when 88b was combined with gefinitib, a synergistic effect was observed, and the level of cytotoxicity was increased in the MDA-MB-468 cell line. Using Western blot analysis, the authors confirmed that best pyrazinoindolones inhibited phosphorylation of Akt signaling pathway in the MDA-MB-468 cell line. In a work dedicated on the synthesis of Longamide B analogs, Fujimoto et al. prepared and evaluated C4-substituted pyrazinoindolone 90 as a potential inhibitor of indoleamine 2,3-dioxygenase 1 (IDO1, an enzyme overexpressed in certain cancers such as colon and stomach cancers (Scheme 22) [53].

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Pharmaceuticals 2021, 14, 779 16 of 24 Scheme 20. Synthesis and evaluation of pyrazinoindolones 85 as anti-cancer agents.

Kwak group synthesized a wide range of enantiopure (R)- and (S)-3-substituted py- Kwak group synthesized a wide range of enantiopure (R)- and (S)-3-substituted razinoindolones 88 from an intramolecular Mitsunobu cyclization of 86 using DEAD, PPh3 pyrazinoindolones 88 from an intramolecular Mitsunobu cyclization of 86 using DEAD, in THF (Scheme 21) [52]. PPh3 in THF (Scheme 21)[52].

Scheme Scheme21. Synthesis 21. Synthesis and evaluation and evaluation of optically of optically active active 3-substituted 3-substituted pyrazinoindolones pyrazinoindolones88. 88.

Compounds 88 were evaluated for their anti-proliferative activity on MCF-7 breast Compounds cancer88 were cells evaluated and on triple for negative their anti-proliferative MDA-MB-468 breast activity cancer on cells. MCF-7 It was showedbreast that cancer cells and onthe triple stereochemistry negative onMDA-MB-468 C3 has not a significant breast cancer impact cells. on the It cytotoxicity. was showed Interestingly, that it the stereochemistrywas on observed C3 has that not the a significant most cytotoxic impact compounds on the88a–c cytotoxicity.presented inInterestingly, Scheme 21 were it more was observed thateffective the most than cytotoxic the reference compounds compound gefinitib.88a–c presented Furthermore, in whenScheme88b 21was were combined more effective thanwith the gefinitib, reference a synergistic compound effect gefinitib. was observed, Furthermore, and the level when of cytotoxicity 88b was was com- increased in the MDA-MB-468 cell line. Using Western blot analysis, the authors confirmed that best bined with gefinitib,pyrazinoindolones a synergistic inhibitedeffect wa phosphorylations observed, and of Akt the signaling level of pathway cytotoxicity in the MDA-MB-was increased in the MDA-MB-468468 cell line. cell line. Using Western blot analysis, the authors confirmed that best pyrazinoindolonesIn a work dedicated inhibite on thed synthesis phosphorylation of Longamide of B Akt analogs, signaling Fujimoto pathway et al. prepared Pharmaceuticals 2021, 14, x FOR PEER REVIEW 16 of 23 in the MDA-MB-468and cell evaluated line. C4-substituted pyrazinoindolone 90 as a potential inhibitor of indoleamine In a work dedicated2,3-dioxygenase on the 1 synthesis (IDO1, an enzymeof Longamide overexpressed B analogs, in certain Fujimoto cancers such et al. as pre- colon and stomach cancers (Scheme 22)[53]. pared and evaluated C4-substituted pyrazinoindolone 90 as a potential inhibitor of indoleamine 2,3-dioxygenase 1 (IDO1, an enzyme overexpressed in certain cancers such as colon and stomach cancers (Scheme 22) [53].

Scheme 22. SynthesisScheme 22.andSynthesis evaluation and of evaluation optically of active optically pyrazinoindolone active pyrazinoindolone 48. 48.

Compound 90 resulted from a nucleophilic reaction of methyl indole-carboxylate Compound 90 resulted from a nucleophilic reaction of methyl indole-carboxylate on on 89 followed by protecting group removal and cyclization (80%; overall yield). The 89 followed by protectingIDO1 inhibitory group activity removal was and evaluated cyclization at a concentration(80%; overall of yield). 1 mM The but, unfortunately,IDO1 inhibitory activitypyrazinoindolone was evaluated90 atshowed a concentration a poor IDO1 of inhibitory 1 mM but, activity unfortunately, (20%). pyrazinoindolone 90 showedBoger a poor et al. IDO1 serendipitously inhibitory prepared activity symmetrical (20%). diketopiperazine 92 by reacting Boger et al. serendipitouslyindole-2-carboxylic prepared acid 49 with symmetrical 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide diketopiperazine 92 by reacting (EDCI) indole-2-carboxylicin DMAPacid 49 (Scheme with 1-ethyl-3-(3-dimethylami 23)[54]. After Boc-deprotectionnopropyl)carbodiimide of 92, a series of lactam (EDCI) derivatives was in DMAP (Schemesynthesized 23) [54]. After and evaluated Boc-deprotection against a mouse of 92,lymphocytic a series of lactam leukemia derivatives cell line (L1210) was and the best compound 93a was found to be highly cytotoxic with a IC value of 34 nM. synthesized and evaluated against a mouse lymphocytic leukemia cell line50 (L1210) and the best compound 93a was found to be highly cytotoxic with a IC50 value of 34 nM.

Scheme 23. Synthesis and cytotoxic properties of symmetrical and unsymmetrical diketopiperazines 95 and 95.

A few years later from the above-mentioned study, Montalbano et al., with the same reaction conditions, has synthesized various unsymmetrical diketopiperazines 95 with variable yields. Unfortunately, these derivatives exhibited modest cytotoxicity against a panel of 60 human cancer cell lines [55]. The group of Vigusin and Moody also prepared various pyrazinoindole-1,4-diones as gliotoxin analogues from the reaction of indole-2- carboxylic acid with sarcosine ethyl ester hydrochloride in the presence of EDCI (Scheme not shown). Unfortunately, pyrazinoindole-dione [56] compounds were found to be poorly active as inhibitors of farnesyltransferase (FTase) and geranylgeranyltransferase (GGTase I) [57]. Diketopiperazines have been isolated as secondary metabolites from the marine fungus Neosartorya pseudofischeri (Scheme 24) [58]. Of the five pyrazinoindole-diones isolated,

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Scheme 22. Synthesis and evaluation of optically active pyrazinoindolone 48.

Compound 90 resulted from a nucleophilic reaction of methyl indole-carboxylate on 89 followed by protecting group removal and cyclization (80%; overall yield). The IDO1 inhibitory activity was evaluated at a concentration of 1 mM but, unfortunately, pyrazinoindolone 90 showed a poor IDO1 inhibitory activity (20%). Boger et al. serendipitously prepared symmetrical diketopiperazine 92 by reacting indole-2-carboxylic acid 49 with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDCI) in DMAP (Scheme 23) [54]. After Boc-deprotection of 92, a series of lactam derivatives was Pharmaceuticals 2021, 14, 779 17 of 24 synthesized and evaluated against a mouse lymphocytic leukemia cell line (L1210) and the best compound 93a was found to be highly cytotoxic with a IC50 value of 34 nM.

SchemeScheme 23. Synthesis 23. Synthesis and cytotoxic and cytotoxic properties properties of symmetrical of symmetrical and unsymmetrical and unsymmetrical diketopiperazines diketopipera-95 and 95. zines 95 and 95. A few years later from the above-mentioned study, Montalbano et al., with the same A few yearsreaction later from conditions, the above-mentioned has synthesized study, various Montalbano unsymmetrical et al., diketopiperazines with the same 95 with variable yields. Unfortunately, these derivatives exhibited modest cytotoxicity against a reaction conditions, has synthesized various unsymmetrical diketopiperazines 95 with panel of 60 human cancer cell lines [55]. The group of Vigusin and Moody also prepared variable yields. variousUnfortunately, pyrazinoindole-1,4-diones these derivatives as exhibited gliotoxin analoguesmodest cytotoxicity from the reaction against of a indole-2- panel of 60 humancarboxylic cancer acid cell withlines sarcosine [55]. Th ethyle group ester of hydrochloride Vigusin and inMoody the presence also prepared of EDCI (Scheme Pharmaceuticals 2021, 14, x FOR PEER REVIEWvarious pyrazinoindole-1,4-dionesnot shown). Unfortunately, as gliotoxin pyrazinoindole-dione analogues from [56the] compoundsreaction of wereindole-2-17 found of 23 to be

carboxylic acid withpoorly sarcosine active as ethyl inhibitors ester of hy farnesyltransferasedrochloride in the (FTase) presence and of geranylgeranyltransferase EDCI (Scheme not shown). Unfortunately,(GGTase I) [57 ].pyrazinoindole-dione [56] compounds were found to be Diketopiperazines have been isolated as secondary metabolites from the marine onlypoorly compound active as 97 inhibitors was found of farnesyltransferase to be cytotoxic agai(FTase)nst theand humangeranylgeranyltransferase colon cancer lines (GGTase I) [57].fungus Neosartorya pseudoﬁscheri (Scheme 24)[58]. Of the ﬁve pyrazinoindole-diones HCT116 and RKOisolated, with IC only50 values compound of 10 and97 was 33 foundμM, respectively. to be cytotoxic Moreover, against the 97 human was found colon cancer Diketopiperazines have been isolated as secondary metabolites from the marine fun- to possess anti-bacteriallines HCT116 activity and RKO and with inhibited IC50 values the of 10growth and 33 µofM, Staphylococcus respectively. Moreover, aureus97 was gus Neosartorya pseudofischeri (Scheme 24) [58]. Of the five pyrazinoindole-diones isolated, ATCC29213 and R3708found towith possess MIC anti-bacterial values of 283 activity and and70 μ inhibitedM. the growth of Staphylococcus aureus ATCC29213 and R3708 with MIC values of 283 and 70 µM.

Scheme 24.Scheme Secondary 24. Secondary metabolites metabolites 96–9896 –from98 from the the marine marine fungus fungus NeosartoryaNeosartorya pseudoﬁscheri. pseudofischeri.

3.2.2. Anti-Infectious Properties Zoidis et al., in a program dedicated to the synthesis of novel indole-flutimide having activity against influenza PA endonuclease and Hepatitis C virus, prepared and evaluated five novel N-hydroxyimides 102 (Scheme 25) [59]. Intermediate O-benzyl-compounds 101 were easily obtained from the condensation of indoles 99 with O-benzyl hydroxylamine 100 in the presence of HOBt and EDCI. The authors demonstrated that incorporating the 2,6- diketopiperazine moiety of flutamide into the pharmacophore ring of indole, the resulting hydroxyimides 102 displayed potent inhibitory against influenza PA endonuclease with micromolar IC50 values. Compound 102a was found to be as potent as 2,4-dioxo-4-phenylbutanoic acid (DPBA), a known PAN inhibitor used as reference. Interestingly, hydroxyimide 102c showed notable anti-hepatitis C virus (HCV) activity (EC50 = 10.5 μM) in Huh5-2 cell line harboring the firefly luciferase-expressing subgenomic replicon of the HCV genotype 1b Con1 strain. These compounds were next evaluated by Tavis group as possible inhibitors of Hepatitis B virus but, unfortunately, were found to be ineffective [60].

Scheme 25. Synthesis and evaluation of a selection of hydroxyimides 102 as antiviral compounds.

In 2005, Ivachtchenko and co-workers proposed an efficient three-component synthesis of 3-substituted pyrazinoindoles from a Ugi condensation of ketoacids with anilines and isonitriles [61]. This protocol was used a few years later by Yokokawa group in a program dedicated to the synthesis of dengue inhibitors, to prepare pyrazinoindolone 3 using ketoacid 103 with 3-thiomethylaniline 104 and isocyanide 105 (Scheme 26) [5].

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only compound 97 was found to be cytotoxic against the human colon cancer lines HCT116 and RKO with IC50 values of 10 and 33 μM, respectively. Moreover, 97 was found to possess anti-bacterial activity and inhibited the growth of Staphylococcus aureus ATCC29213 and R3708 with MIC values of 283 and 70 μM.

Scheme 24. Secondary metabolites 96–98 from the marine fungus Neosartorya pseudofischeri.

Pharmaceuticals3.2.2.2021 Anti-Infectious, 14, 779 Properties 18 of 24 Zoidis et al., in a program dedicated to the synthesis of novel indole-flutimide having activity against influenza3.2.2. Anti-Infectious PA endonuclease Properties and Hepatitis C virus, prepared and evaluated five novel N-hydroxyimidesZoidis et102 al., (Scheme in a program 25) dedicated [59]. Intermediate to the synthesis O-benzyl-compounds of novel indole-flutimide 101 having were easily obtainedactivity from againstthe condensation influenza PA endonucleaseof indoles 99 and with Hepatitis O-benzyl C virus, hydroxylamine prepared and evaluated 100 in the presence of HOBtfive novel andN -hydroxyimidesEDCI. The auth102ors(Scheme demonstrated 25)[59]. Intermediate that incorporatingO-benzyl-compounds the 2,6- 101 diketopiperazine moietywere easily of flutamide obtained from into the the condensation pharmacophore of indoles ring99 ofwith indole,O-benzyl the hydroxylamineresulting hydroxyimides 102100 displayedin the presence potent of inhibitory HOBt and EDCI.against The influenza authors demonstrated PA endonuclease that incorporating with mi- the 2,6-diketopiperazine moiety of flutamide into the pharmacophore ring of indole, the re- cromolar IC50 values.sulting Compound hydroxyimides 102a was102 displayed found to potent be as inhibitory potent as against 2,4-dioxo-4-phenylbuta- influenza PA endonuclease noic acid (DPBA), witha known micromolar PAN inhibitor IC50 values. used Compound as reference.102a was Interestingly, found to be as hydroxyimide potent as 2,4-dioxo- 102c showed notable4-phenylbutanoic anti-hepatitis acid C virus (DPBA), (HCV) a known activity PAN inhibitor(EC50 = 10.5 used μ asM) reference. in Huh5-2 Interestingly, cell line harboring the fireflyhydroxyimide luciferase-expressing102c showed notable subgenomic anti-hepatitis replicon C virus (HCV) of the activity HCV (EC genotype50 = 10.5 µM) in Huh5-2 cell line harboring the firefly luciferase-expressing subgenomic replicon of the 1b Con1 strain. TheseHCV compounds genotype 1b were Con1 next strain. ev Thesealuated compounds by Tavis were group next as evaluated possible by inhibitors Tavis group as of Hepatitis B viruspossible but, unfortunately, inhibitors of Hepatitis were found B virus but,to be unfortunately, ineffective were[60]. found to be ineffective [60].

SchemeScheme 25. Synthesis 25. Synthesis and andevaluation evaluation of of a a selection selection of of hydroxyimides hydroxyimides102 as 102 antiviral as antiviral compounds. compounds. In 2005, Ivachtchenko and co-workers proposed an efﬁcient three-component synthesis In 2005, Ivachtchenkoof 3-substituted and pyrazinoindolesco-workers proposed from a Ugi an condensation efficient three-component of ketoacids with anilines syn- and Pharmaceuticals 2021, 14, x FOR PEER REVIEWthesis of 3-substitutedisonitriles pyrazinoindoles [61]. This protocol from was a Ugi used condensation a few years later of by ketoacids Yokokawa with group anilines in a18 program of 23

and isonitriles [61].dedicated This prot toocol the synthesis was used of dengue a few inhibitors,years later to prepareby Yokokawa pyrazinoindolone group in3 usinga ketoacid 103 with 3-thiomethylaniline 104 and isocyanide 105 (Scheme 26)[5]. program dedicated to the synthesis of dengue inhibitors, to prepare pyrazinoindolone 3 using ketoacid 103 with 3-thiomethylaniline 104 and isocyanide 105 (Scheme 26) [5].

Scheme 26. Synthesis of a dengueScheme inhibitor 26. Synthesis 3. of a dengue inhibitor 3.

After a chiral separation, each enantiomer of 3 were evaluated separately in an A549 After a chiral separation,cell-based ﬂavivirus each immunodetectionenantiomer of (CFI)3 were assay. evaluated A marked differenceseparately in EC in50 anvalues A549 was cell-based flavivirusobserved immunodetection between the (3R )(CFI) and (3 Sassay.) enantiomers A marked highlighting difference the importance in EC of50 chirality values in was observed betweencompound the (33Rfor) and this type (3S) of enantiomers tropical disease. highlighting In all DENV 1–4 the serotypes, importance (3S)-3 was of foundchi- rality in compound 3 for this type of tropical disease. In all DENV 1–4 serotypes, (3S)-3 was found to display very low EC50 values ranging from 0.01 to 0.09 μM and was found to be up to 50 times more effective than the (3R)-enantiomer.

3.2.3. Neuropsychiatric Properties One of the oldest examples of biologically active pyrazinoindolone derivatives was reported in 1994 by Mokrosz et al. who synthetized a series of new antagonist ligands of 5-HT1A-, 5-HT2-, and D2-receptors (Scheme 27) [62].

Scheme 27. Synthesis of 108a,b as 5-HT1A and 5-HT2 ligands.

A SN2 reaction between 3,4-dihydropyrazino[1,2-a]indol-1(2H)-one 106 and 1-aryl-4- chloropropylpiperazines 107 gave a rapid access to 108a and 108b with good yields (77% and 93%, respectively). As shown, pyrazinoindolones 108a and 108b displayed good affinities for 5-HT1A receptor with Ki values of 15 and 40 nM. It was also shown that 108a had a better affinity than 63b on 5-HT2 receptor. However, compound 108a did not show significant selectivity for 5-HT1A/5-HT2, and chemical transformations could perhaps overcome this problem. A decade after, Campiani et al. re-studied effects of pyrazinoindolone 108b on a variety of D1-3 dopamine, 5-HT1A serotonin, and alpha-noradrenaline receptor subtypes [63]. It was demonstrated that 108b showed D3 receptor sub-nanomolar affinity (KiD3 = 0.87 nM) with an excellent D2/D3 affinity ratio of 52 and a good D1/D3 affinity of 1801. This study demonstrated that pyrazinoindolone 108b represented a very promising lead for the generation of novel D2/D3 receptor ligands potentially active against cocaine craving.

3.2.4. Anti-Allergenic Properties The last example of this review concerns the synthesis and the biological properties of a series of substituted pyrazinoindolones 4 and 112 which were evaluated as histamine H3 receptor inverse agonists (Scheme 28) [6].

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Scheme 26. Synthesis of a dengue inhibitor 3.

After a chiral separation, each enantiomer of 3 were evaluated separately in an A549 cell-based flavivirus immunodetection (CFI) assay. A marked difference in EC50 values was observed between the (3R) and (3S) enantiomers highlighting the importance of chi- Pharmaceuticalsrality 2021in ,compound14, 779 3 for this type of tropical disease. In all DENV 1–4 serotypes, (3S19)-3 of 24 was found to display very low EC50 values ranging from 0.01 to 0.09 μM and was found to be up to 50 times more effective than the (3R)-enantiomer. to display very low EC50 values ranging from 0.01 to 0.09 µM and was found to be up to 50 times more effective than the (3R)-enantiomer. 3.2.3. Neuropsychiatric Properties One of the oldest3.2.3. examples Neuropsychiatric of biologically Properties active pyrazinoindolone derivatives was One of the oldest examples of biologically active pyrazinoindolone derivatives was reported in 1994 byreported Mokrosz in 1994et al. by who Mokrosz synthe et al.tized who synthetized a series of a seriesnew antagonist of new antagonist ligands ligands of of 1A 2 2 5-HT -, 5-HT -, and5-HT D -receptors1A-, 5-HT2-, and(Scheme D2-receptors 27) [62]. (Scheme 27)[62].

Scheme 27. Synthesis ofScheme 108a,b 27. asSynthesis 5-HT1A of and108a 5-HT,b as 5-HT2 ligands.1A and 5-HT2 ligands.

ASN2 reaction between 3,4-dihydropyrazino[1,2-a]indol-1(2H)-one 106 and 1-aryl- A SN2 reaction 4-chloropropylpiperazinesbetween 3,4-dihydropyrazino[1,2-107 gave a rapida]indol-1(2 access to 108aH)-oneand 106108b andwith 1-aryl-4- good yields chloropropylpiperazines(77% and 107 93%, gave respectively). a rapid access As shown, to 108a pyrazinoindolones and 108b with108a goodand yields108b displayed (77% and 93%, respectively).good As affinities shown, for 5-HTpyrazinoindolones1A receptor with Ki 108a values and of 15 108b and 40displayed nM. It was good also shown af- that 108a had a better affinity than 63b on 5-HT2 receptor. However, compound 108a finities for 5-HT1A receptor with Ki values of 15 and 40 nM. It was also shown that 108a did not show significant selectivity for 5-HT1A/5-HT2, and chemical transformations had a better affinitycould than perhaps 63b on overcome 5-HT2 receptor. this problem. However, A decade compound after, Campiani 108a et al. did re-studied not show effects significant selectivityof pyrazinoindolone for 5-HT1A/5-HT108b2, onand a variety chemical of D1- 3transformationsdopamine, 5-HT1A serotonin,could perhaps and alpha- overcome this problem.noradrenaline A decade receptor after, subtypes Campia [63ni]. et It wasal. re-studied demonstrated effects that 108b of pyrazinoindo-showed D3 receptor sub-nanomolar affinity (KiD3 = 0.87 nM) with an excellent D2/D3 affinity ratio of 52 and lone 108b on a varietya good of D1/D3D1-3 dopamine, affinity of 1801. 5-HT This1A serotonin, study demonstrated and alpha-noradrenaline that pyrazinoindolone re-108b ceptor subtypes [63].represented It was demonstrated a very promising that lead 108b for the showed generation D3 ofreceptor novel D2 /Dsub-nanomolar3 receptor ligands affinity (KiD3 = 0.87 potentiallynM) with active an excellent against cocaine D2/D craving.3 affinity ratio of 52 and a good D1/D3 affinity of 1801. This 3.2.4.study Anti-Allergenic demonstrated Properties that pyrazinoindolone 108b represented a very promising lead for theThe generation last example of of thisnovel review D concerns2/D3 receptor the synthesis ligands and the potentially biological properties active of against cocaine craving.a series of substituted pyrazinoindolones 4 and 112 which were evaluated as histamine H3 receptor inverse agonists (Scheme 28)[6]. 3.2.4. Anti-Allergenic PropertiesAlkylation of 5-benzyloxy-1H-indole-2-carboxylic acid ethyl ester 109 with various 2,2- dioxo[1–3]oxathiazolidine-3-carboxylic acid tert-butyl esters 110 gave pyrazinoindolones The last example111 inof highthis yields.review In caseconcerns of chiral the 5-substituted synthesis sulfamidates and the biological 110, the alkylation properties reaction of a series of substitutedproceeded pyrazinoindolones under inversion of configuration 4 and 112 which with an were enantiomeric evaluated excess as of histamine more than 98% ee. After adequate transformations (O-debenzylation followed by Mitsonobu reaction), all H3 receptor inverse compoundsagonists (Scheme were tested 28) in a[6]. functional GTPgS assay and were characterized as potent full inverse agonists at the human H3 receptor (hH3-R) with Ki values at a nanomolar level. It was shown that the introduction of a methyl substituent was interesting on C3 rather than on C4, and the 3-Me (S) analogue 112a displayed highest affinity with its enantiomer 112b. In general, most compounds 112 which showed moderate affinity against hH2 and hH4 receptors were selective against hH3 receptor subtype. Oral administration in rat showed that compound 4 was tolerated with no significant side-effects, was well absorbed, and penetrated in brain successfully.

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SchemeScheme 28. Synthesis 28. Synthesis of pyrazinoindolones of pyrazinoindolones 4, 112,112, and and afﬁnity affinity over over histamine histamine H3 receptor H3 receptor subtype. subtype.

4. Conclusions Alkylation of 5-benzyloxy-1H-indole-2-carboxylic acid ethyl ester 109 with various In conclusion, it appears that the synthesis of pyrazinoindolones seems easier than 2,2-dioxo[1–3]oxathiazolidthat of theirine-3-carboxylic pyrazinoindole counterparts, acid tert probably-butyl becauseesters indole110 precursorsgave pyra- bearing on zinoindolones 111C2 in amides,high yields. esters orIn acidcase groups of chiral 2 are 5-substituted abundant and sulfamidates commercially available.110, the al- Figure3 Pharmaceuticals 2021, 14, x FOR PEER REVIEW 20 of 23 kylation reaction proceededhighlights the under various inversion biological of properties configuration of tetrahydro-pyrazinoindoles, with an enantiomeric excess such as anti- of more than 98%fungal, ee. After anti-bacterial, adequate anti-cancer, transformations anti-arrhythmia, (O-debenzylation and others. followed by

Mitsonobu reaction), all compounds were tested in a functional GTPgSBr assay and were CO H characterized as potent full inverse agonists at the2 human H3 receptor (hH3-R) with Ki Me Me values at a nanomolar level. It was shown that the introduction of a methyl substituentCl NH F3C was interesting on C3 rather thanO on C4, andN the 3-Me (S) analogue 112a displayed highest N NH affinity with its enantiomerO 112b. In general, most compounds 112 which showedN NH moderate affinity against hH2 and hH4 receptors were selective against hH3 receptor subtype. Autoimmune Anti-fungal Oral administration in rat showeddiseases that compound 4 was tolerated with no significant side- effects, was well absorbed, and penetrated in brain successfully. R1 MeO Anti-bacterial Me 4. Conclusions R N 3 N NH N R CN In conclusion, it appears that the synthesis of pyrazinoindolones seems NeasierN than that of their pyrazinoindole counterparts,Psychiatry probably becauseR2 indole precursors bearing on Pyrazino[1,2-a]indoles C2 amides, esters or acidMe groups 2 are abundant and commercially available. Figure 3 Anti-cancer Anti-arrhythmy highlights the various biological propertiesOMe of tetrahydro-pyrazinoindoles,Cl such as anti- fungal, anti-bacterial, anti-cancer, anti-arrhythmia, and others. N O F It would certainly be interestingNH to develop asymmetric pathways allowing access to N O N N C1-, C3- or C4-substituted compounds in an enantioselective manner to testN their affinity and selectivity for the considered target(s). NH2 For pyrazinoindol-1-one derivatives whose biological activities are grouped inS Figure 4, some of these compounds with an extremely simple chemical structure have been shown to be biologically veryFigureFigure active, 3.3.Overview Overview especially ofof tetrahydro-pyrazino[1,2-tetrahydro-pyrazino[1,2- as anti-cancer aagentsa]indoles]indoles with having having nanomolar biological biological properties. properties. level of cytotoxicity.

Figure 4. Overview of pyrazino[1,2-a]indol-1-ones having biological properties.

We now hope that all the observations reported in this review could open very interesting perspectives in medicinal chemistry (anti-cancer, anti-infectious, anti-allergenic, and others). These outlooks will quickly incite medicinal chemists to design new effective and selective ligands based on these types of chemical structures.

Author Contributions: Conceptualization, O.P.; writing—original draft preparation, O.P., C.T., and A.H.; writing review and editing, O.P., M.A., and A.H.; bibliography, O.P. and K.Z. All authors have read and agreed to the published version of the manuscript.

Pharmaceuticals 2021, 14, x FOR PEER REVIEW 20 of 23

Br CO2H Me Me Cl NH F3C O N N NH O N NH

Autoimmune Anti-fungal diseases

R1 MeO Anti-bacterial Me R N 3 N NH N R CN N N Psychiatry R2 Pyrazino[1,2-a]indoles

Me Anti-arrhythmy Pharmaceuticals 2021, 14, 779 Anti-cancer 21 of 24 OMe Cl

O F N NH It would certainly be interesting to developN asymmetric pathways allowing access toO N N C1-, C3- or C4-substituted compounds in an enantioselective manner to testN their afﬁnity and selectivity for the considered target(s). NH2 For pyrazinoindol-1-one derivatives whose biological activities are grouped in FigureS 4 , some of these compounds with an extremely simple chemical structure have been shown to be biologically very active, especially as anti-cancer agents with nanomolar level ofFigure cytotoxicity. 3. Overview of tetrahydro-pyrazino[1,2-a]indoles having biological properties.

FigureFigure 4. 4.Overview Overview of pyrazino[1,2- of pyrazino[1,2-a]indol-1-onesa]indol-1-ones having biologicalhaving bi properties.ological properties.

We now hope that all the observations reported in this review could open very We now hope that all the observations reported in this review could open very inter- interesting perspectives in medicinal chemistry (anti-cancer, anti-infectious, anti-allergenic, andesting others). perspectives These outlooks in medicinal will quickly chemistry incite medicinal (anti-cancer, chemists to anti-infectious, design new effective anti-allergenic, andand selective others). ligands These basedoutlooks on these will types quickly of chemical incite medicinal structures. chemists to design new effective and selective ligands based on these types of chemical structures. Author Contributions: Conceptualization, O.P.; writing—original draft preparation, O.P., C.T., and A.H.; writing review and editing, O.P., M.A., and A.H.; bibliography, O.P. and K.Z. All authors have Author Contributions: Conceptualization, O.P.; writing—original draft preparation, O.P., C.T., and read and agreed to the published version of the manuscript. A.H.; writing review and editing, O.P., M.A., and A.H.; bibliography, O.P. and K.Z. All authors have Funding:read andThis agreed research to the received publis nohed external version funding. of the manuscript. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable.

Data Availability Statement: Data sharing not applicable. Acknowledgments: The authors gratefully acknowledge support of this project by CNRS and Uni- versité Paris-Saclay. Kena Zhang thanks the CSC (Chinese Scholarship Council) for a Ph.D. funding. Conﬂicts of Interest: The authors declare no conﬂict of interest. Pharmaceuticals 2021, 14, 779 22 of 24

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