Aza-Annulation of 1,2,3,4-Tetrahydro-β-carboline Derived Enaminones and Nitroenamines: Synthesis of Functionalized Indolizino[8,7-b]indoles, Pyrido[1,2-a:3,4-b′]diindoles, Indolo[2,3-a]quinolizidine-4-ones and Other Tetrahydro-β-carboline Fused Heterocycles

Abstract

Aza-annulation of novel 1,2,3,4-tetrahydro-β-carboline derived enaminones and nitroenamines with various 1,2- and 1,3-bis electrophiles, such as oxalyl chloride, maleic anhydride, 1,4-benzoquinone, 3-bromopropionyl chloride, itaconic anhydride, and imines (from formaldehyde and primary amines), has been investigated. These methodologies provide simple one-step pathways for efficient construction of highly functionalized tetrahydro-β-carboline 1,2-fused, five- and six-membered heterocyclic frameworks, such as indolizino[8,7-b]indoles, pyrido[1,2-a:3,4-b′]diindoles, indolo[2,3-a]quinolizidines, and pyrimido[1′,6′:1,2]pyrido[3,4-b]indoles, which are core structures of many naturally occurring indole alkaloids with diverse bioactivity.

Introduction

The indole structure is regarded as one of the most privileged classes of heterocycles representing structural core of many natural and non-natural compounds with a range of biological activities.¹ The indole alkaloids have been a subject of intense structural, pharmacological, biosynthetic and synthetic studies, because of the structural diversity and complexity of many of its members, along with the important physiological and medicinal properties displayed by this class of compounds.² Similarly, natural product inspired compounds based on polycyclic indole alkaloids also exhibit interesting biological activities.³ Therefore, the development of new synthetic methods that allow rapid and efficient access to these natural and non-natural indole-containing scaffolds has attracted much attention for several decades among organic as well as medicinal chemists.³

Tetrahydro-β-carboline constitutes a recurring subunit in numerous indole alkaloids;⁴ besides, they are also templates for drug discovery and have been used as scaffolds for combinatorial libraries.^4a,4b Therefore, the construction of structurally novel, non-natural alkaloid type polycyclic heterocyclic scaffolds containing this subunit is a highly challenging and rewarding endeavor in the fast emerging area of oriented synthesis.

Enaminoketones, esters, and nitriles, including nitroenamines, have been shown to be versatile building blocks for the synthesis of various five- and six-membered heterocycles and are frequently used in domino and multicomponent reactions, because of the rich reactive sites present in these intermediates.⁵ However, the corresponding heterocyclic enaminones/esters derived from tetrahydro-β-carboline framework have not been much explored for the construction of indole-annulated heterocycles, despite their considerable synthetic potential,^4a,6 although acyclic β-enaminoesters generated from the reaction of tryptamine and alkyl propiolates have been frequently employed as useful building blocks for construction of indole-annulated heterocycles via the sequential Pictet–Spengler reaction.^7a,7b

We have previously reported an efficient general approach for the synthesis of 6,7-dimethoxytetrahydroisoquinoline-derived push-pull enaminones/esters/nitriles of the general structure 2 and their subsequent synthetic elaboration to tetrahydroisoquinoline-fused five- and six-membered heterocycles 3 (Scheme 1).⁸ The overall process involves the Bischler–Napieralski type cyclization of newly synthesized ketene N,S-acetals 1 derived from 3,4-dimethoxyphenylethylamine and polarized ketene dithioacetals^8a and subsequent aza-annulation of the resulting enaminones/nitroenamines 2 with two or three carbon 1,2- and 1,3-electrophilic species, affording highly functionalized isoquinoline-fused five- and six-membered heterocycles, such as pyrrolo[2,1-a]isoquinolines,^8b,8d indolo[2,1-a]isoquiolines,^8b and substituted benzo[a]quinolizidin-4-one and pyrimido[6,1-a]isoquinoline structural motifs^8d present in several naturally occurring alkaloids and physiologically active drugs (Scheme 1).

Scheme 1. Aza-Annulation of Isoquinoline-Derived Enaminones/Nitroenamine to Tetrahydroisoquinoline-Fused Five- and Six-Membered Heterocycles.

Our fascination with this class of molecules prompted us to extend these studies for the synthesis of tetrahydro-β-carboline-derived functionalized push-pull enamines, as potentially useful building blocks for the synthesis of 1,2-heteroannulated tetrahydro-β-carbolines-derived 5–6 membered heterocycles and related natural products. Thus, we had previously reported the synthesis of a series of functionalized 1,2,3,4-tetrahydro-β-carboline-derived enaminones/esters/nitriles 7 via trifluoroacetic acid-induced Bischler–Napieralski type cyclization of newly prepared polarized ketene N,S-acetals 6 from tryptamine 4 and polarized ketene S,S-acetals 5 (Scheme 2).^4a

Scheme 2. Synthesis of β-Carboline-Derived Push-Pull Enaminones.

On the basis of our previous studies, with tetrahydroisoquinoline-based enaminones (Scheme 1),^8b,8c we anticipated that tetrahydro-β-carboline-derived enaminones and nitroenamines such as 7 could also be employed in efficient aza-annulation reactions with various 1,2- and 1,3-biselectrophiles, leading to a variety of tetrahydro-β-carboline 1,2-annulated five- and six-membered heterocycles (Scheme 3). Also, because of our previous experience in exploring the reactivity and synthetic potential of polarized ketene N,S-acetals as functionalized enaminones,⁹ we also envisioned the possibility of synthesizing the target tetrahydro-β-carboline-fused heterocycles, directly from acyclic N,S-acetals 6,with concurrent formation of both tetrahydropyridine and 5/6 membered rings in a tandem one-pot operation (Scheme 3). The results of these studies have been presented in the following section, and we now report in the present paper, one-step synthetic elaboration of a few of these enaminones/nitroenamine 7 to 1,2-tetrahydro-β-carboline-annulated heterocycles, such as substituted dihydroindolizino[8,7-b]indoles, pyrido[1,2-a:3,4-b′]diindole, indolo-[2,3-a]quinolizidines, their benzo-fused analogues, and other novel heterocyclic scaffolds (Scheme 3).

Scheme 3. Synthesis of Tetrahydro-β-Carboline-Fused Five- and Six-Membered Heterocycles.

Results and Discussion

We first examined the cycloannulation of few β-carboline-derived enaminones and nitroenamine 7, with oxalyl chloride, and maleic anhydride with a view to synthesize dihydroindolizino[8,7-b]indole derivatives (Schemes 5 and 6). Indolizino[8,7-b]indole represents an important class of the indole-containing heterocyclic core present in several naturally occurring bioactive alkaloids, such as harmicine, pegaharmalines B, and also in synthetic pharmacologically active compounds such as human CCK₁ receptor antagonists (Figure 1).^7a−7c,10 Several synthetic approaches for the construction of this heterocyclic core have been reported in recent years.⁷⁻¹⁴ In most of these protocols, the pyrrole ring of the indolizino[8,7-b]indole framework has been constructed in various ways. Thus, Knolker and co-workers have reported a two-step procedure for the construction of the pyrrole ring by addition of a propargyl Grignard reagent to 3,4-dihydro-β-carboline and subsequent silver(I)-promoted oxidative cyclization of the resulting adduct.¹² The pyrrole ring has also been constructed by several research groups via 1,3-dipolar cycloaddition of tetrahydro-β-carboline-derived azomethine ylides^13a (or munchnones^13b) with various dipolarophiles, including a photoredox-catalyzed oxidation/1,3-dipolar cycloaddition reported by Maurya and co-workers.^13c Functionalized tetrahydroindolizino[8,7-b]indoles have also been obtained via a one-pot or stepwise reaction of tryptamine, alkyl propiolates, and β-nitroalkenes/α,β-unsaturated ketones via intermediacy of acyclic β-enaminoesters, with concomitant formation of both tetrahydropyridine and pyrrole ring in a domino fashion.^7a,7b Wu and co-workers have recently reported the acid-catalyzed multicomponent cyclization protocol for the synthesis of polyfunctional dihydroindolizino[8,7-b]indoles from readily available arylglyoxal monohydrates, tryptamine, and β-nitrostyrenes or malononitrile.^10,14

Scheme 5. Synthesis of 1-Acyl-6,11-dihydro-5H-indolizino [8,7-b]indole-2,3-diones.

Scheme 6. Cycloannulation of Enaminones 7 with Maleic Anhydride.

Figure 1.

Selected natural products and pharmaceutical compounds with dihydroindolizino [8,7-b]indole moiety.

For our study, the desired enaminones 7a–f and nitroenamine 7g were synthesized according to our earlier reported procedure,^4a as shown in the Scheme 4. We first examined the reactions of enaminones 7a,b,d and nitroenamine 7g with oxalyl chloride, with a view to synthesize indolizino[8,7-b]indole-2,3-diones 8 (Scheme 5). Thus, under optimized reaction conditions, when the enaminone 7a was reacted with oxalyl chloride in the presence of triethylamine in tetrahydrofuran (THF) at room temperature, work-up and purification of the crude reaction mixture afforded a red crystalline solid, in 76% yield, which was characterized as the expected 1-benzoyl-6,11-dihydro-5H-indolizino[8,7-b]indole-2,3-dione 8a on the basis of its spectral and analytical data (Scheme 5). Similarly, the corresponding 4-chlorobenzoyl and acetyl-substituted enaminones 7b and 7d also reacted with oxalyl chloride under identical conditions, yielding the corresponding indolizino[8,7-b]indole-2,3-diones 8b and 8d in high yields. Alternatively, we also reacted the corresponding tryptamine-derived acyclic N,S-acetals 6a,b, 6d with oxalyl chloride under similar conditions, expecting the formation of the desired indolizino[8,7-b]indole-2,3-diones 8 in a domino fashion, with concomitant formation of both the rings via intramolecular cyclization of the initially formed N-substituted 4-benzoyl-5-methylthiopyrrolidin-2,3-diones 9 (Scheme 5). Indeed, the reactions proceeded as expected, yielding the desired tetracyclic indolizino[8,7-b]indole-2,3-diones 8a,b,d in comparatively better yields (route b, Scheme 5). However, the corresponding cyclic nitroenamine 7g or the acyclic nitroketene N,S-acetal 6g failed to furnish the desired 1-nitro-indolizino[8,7-b]indole-2,3-dione 8g under above described conditions, yielding only an intractable reaction mixture (Scheme 5).

Scheme 4. Synthesis of β-Carboline-Derived Enaminones 7a–f and Nitroenamine 7g.

With the successful isolation of indolizino[8,7-b]indole-2,3-diones 8 from the reactions of enaminones 7 with oxalyl chloride, we next investigated aza-annulation of enaminones 7 with maleic anhydride 10, with a view to synthesize functionalized indolizino[8,7-b]indoles such as 11 with an acetic acid side chain (Scheme 6). Aza-annulation of a few acyclic and cyclic enaminones/esters/nitriles with maleic anhydride/maleimide has been reported in the literature affording substituted monocyclic and bicyclic pyrrolidinones;^8b,15 however, cycloannulation of tetrahydro-β-carboline-derived enaminones such as 7 with maleic anhydride has not been investigated. Thus, when the enaminone 7a was reacted with an equimolar quantity of maleic anhydride in solvents such as benzene, toluene, and acetonitrile under reflux conditions, the product isolated after work-up was characterized as expected 2-(1-benzoyl-3-oxo-3,5,6,11-tetrahydro-2H-indolizino[8,7-b]indol-2-yl)acetic acid 11a, with the help of spectral and analytical data (Scheme 6). However, the best yield (60%) of 11a was obtained in refluxing acetonitrile, whereas in other solvents, formation of side products was observed along with 11a. Similarly the 4-chlorobenzoyl-substituted enaminone 7b also afforded the substituted indolizino[8,7-b]indole-2-acetic acid 11b in good yield (Scheme 6). However, the corresponding nitroenamine 7g, although reacted completely with maleic anhydride under identical conditions, the products could not be isolated in their pure form, even after repeated column chromatography. Similarly, attempted domino cyclization of N,S-acetal 6a with maleic anhydride was not successful and neither the tetracyclic product 11a nor the pyrrolidinone intermediate 12a could be isolated from the reaction mixture (Scheme 6).

We next extended our aza-annulation strategy for the synthesis of pyrido[1,2-a:3,4-b′]diindole analogues via a Nenitzescu type reaction of enaminones 7 with 1,4-benzoquinone (Scheme 7).^8b This pentacyclic pyrido[1,2-a:3,4-b′]diindole framework constitutes the core structure of several marine alkaloids, including red pigment fascaplysin, homofascaplysins B,C, and their bromo-analogues (Figure 2).¹⁶ Fascaplysin displays a broad range of biological activities, such as antibacterial, antifungal, antiviral, antimalarial, HIV-1-RT, and especially inhibition of cyclin-dependent kinase 4, which regulates the G₀–G₁/S checkpoint of the cell cycle.^16,17 Therefore, there is considerable interest in the synthesis and development of fascaplysin and its analogues, as lead compounds for potential anticancer drugs and for other therapeutic applications.^16,17 Similarly, naturally occurring alkaloids cladoniamide G possessing an unprecedented indolotryptoline skeleton have also shown to display significant toxicity against human colon and breast cancer (Figure 2).¹⁸

Scheme 7. Synthesis of 2-Hydroxy-13-acyl/nitro-12H-pyrido[1,2-a:3,4-b′]diindole by Cycloannulation of Enaminones 7 with 1,4-Benzoquinone.

Figure 2.

Natural products containing 12H-pyrido[1,2-a:3,4-b′]diindole skeleton.

Although the Nenitzescu reaction for the synthesis of 5-hydroxyindole has been widely studied and various acyclic and cyclic enaminoesters/enaminones have been employed as enamine components in this reaction,^5a the corresponding heterocyclic enaminones such as 7a–d or nitroenamine 7g derived from β-carboline have not been explored for the construction of the pyridodiindole framework. On the basis of our previous studies with tetrahydroisoquinoline-derived enaminones,^8b we have developed a new one-step procedure for the synthesis of the novel pentacyclic pyrido[1,2-a:3,4-b′]diindole framework through the Nenitzescu reaction of enaminones 7 with 1,4-benzoquinone (Scheme 7). Thus, the reaction of 7a with 1,4-benzoquinone in either refluxing acetic acid or in presence of ZnCl₂ catalyst (20 mol %)^5a in dichloromethane yielded only a complex mixture of products; however, when the enaminone 7a was stirred with 1,4-benzoquinone in nitromethane for 2 days under our earlier described conditions,^8b,8e the reaction mixture after a usual work-up and purification yielded a yellow solid (58%) characterized as 6,7-dihydro-2-hydroxy-13-bezoyl-12H-pyrido[1,2-a:3,4-b′]diindole 13a (Scheme 7). Similarly, the other substituted enaminones 7b,c also underwent cycloannulation with 1,4-benzoquinone under identical conditions furnishing the corresponding 2-hydroxy-13-aroyldihydropyridodiindoles 13b,c in moderate to good yields (Scheme 7). Interestingly, the nitroenamine 7g could also be reacted with benzoquinone, yielding the corresponding hitherto unreported 2-hydroxy-13-nitropyridodiindole analogue 13g, although in low yield (35%) (Scheme 7).

We next investigated aza-annulation of enaminones 7 with 3-bromopropionyl chloride and itaconic anhydride with a view to construct indolo[2,3-a]quinolizidine-4-one frameworks (Schemes 8 and 9). The indolo[2,3-a]quinolizidine structural motif is of significant importance, since this privileged structure is present in a plethora of numerous naturally occurring, bioactive indole alkaloids,^1d,6b,19 such as deplancheine, geissoschizine, dihydrocorynantheine, including ajmalicine, and yohimbane, a potent modulator of tubulin cytoskeleton, and important anticancer drugs (Figure 3).³ Because of their complex structures and pharmacological properties, new synthetic routes for the construction of this tetracyclic indolo[2,3-a]quinolizin-4-ones with diverse functionalities have attracted much attention, among synthetic as well as medicinal chemists.²⁰ Some of the recent approaches for the construction of this challenging heterocyclic target involve cyclization of N-acyliminium ion on the pendant indole ring,^20a,20b Bischler–Napieralski reaction,^21a and Fischer Indole synthesis.^21b

Scheme 8. Synthesis of 1-Acyl/nitro-indolo[2,3-a]quinolizidin-4-ones.

Scheme 9. Aza-Annulations of Enaminones 7 and N,S-Acetals 6 with Itaconic Anhydride.

Figure 3.

Naturally occurring indole alkaloids with indolo[2,3-a]quinolizidine framework.

Franzén²² and Wu’s²³ groups have recently developed facile organocatalytic enantioselective one-pot, three- component, cascade approaches for highly substituted indoloquinolizidines, involving a Michael addition-Pictet–Spengler sequence of β-ketoesters (or alkyl propiolates), α,β-unsaturated aldehydes, and tryptamine. Muller and co-workers^4b have reported a sequential, four-component synthesis of highly substituted indolo[2,3-a]quinolizidin-4-ones by Sonogashira coupling of acid chlorides, terminal alkynes, followed by amination with a tryptamine, aza-annulation-Pictet–Spengler sequence.

A few of the indoloquinolizidin-4-ones have also been synthesized by aza-annulation of β-carboline-derived enaminoester with acrylate derivatives in moderate to good yields.^6a Du and co-workers have recently reported a novel synthetic approach to functionalized indolo[2,3-a]quinolizidones via N-heterocyclic carbene-catalyzed annulations of β-carboline-derived enamino ester with enals.^6b,24

In our study, we first examined cycloannulation of enaminones 7a, 7d, and nitroenamine 7g with 3-bromopropionoyl chloride 15 with a view to synthesize 1-aroyl/nitro-tetrahydroindolo[2,3-a]quinolizin-4-ones 14 (Scheme 8). Thus, under optimized reactions conditions, when 7a was reacted with 3-bromopropionoyl chloride 15 in refluxing THF and triethylamine, the reaction proceeded smoothly, yielding the desired indoloquinolizidin-4-one 14a in 65% yield. Alternatively, we also attempted one-pot tandem cyclization of open-chain N,S-acetal 6a with 15, under identical conditions, and to our delight, indoloquinolizidinone 14a was obtained in increased yield of 75%, without isolation of the corresponding tetrahydro-2-pyridone intermediate 16a (Scheme 8). The corresponding 1-acetyl and hitherto unreported 1-nitroindoloquinolizidinone 14d and 14g were similarly obtained in good yields from respective cyclic enaminone 7d, 7g or the corresponding N,S-acetals 6d and 6g under identical conditions (Scheme 8).

With the successful synthesis of 1-substituted tetrahydroindolo[2,3-a]quinazolidones 14 by cycloannulation of enaminones 7 with 3-bromopropionyl chloride, we next examined the aza-annulation of enaminones 7a–c and nitroenamine 7g with itaconic anhydride 18, with anticipation to synthesize functionalized indoloquinolizidin-4-ones 17, bearing an acetic acid side chain (Scheme 9). There are very few reports of aza-annulation of enamine substrates with exocyclic anhydrides, like itaconic anhydride 18.^4a,9a Thus, when enaminone 7a was reacted with itaconic anhydride in refluxing acetonitrile, under our previously described conditions,^4a,9a work-up and purification of the reaction mixture yielded a single product, which was found to be the expected 1-benzoyl-indolo[2,3-a]quinolizin-4-one-3-acetic acid 17a (62%) on the basis of its spectral and analytical data (Scheme 9). Similarly, the enaminones 7b,c and the nitroenamine 7g also reacted with itaconic anhydride 18 under identical conditions furnishing the corresponding 1-aroyl- and 1-nitroindolo[2.3-a]quinolizido-4-one-3-acetic acids 17b,c, 17g in good yields (Scheme 9). In an alternative procedure, the acyclic N,S-acetal 6a was reacted with itaconic anhydride 18 with a view to obtain the target functionalized indolo[2,3-a]quinolizinone 17a in a tandem one-pot operation. However, when 6a was reacted with 18 in refluxing acetotonitrile, the product isolated was found to be only acyclic substituted dihydropyridone 19a (70%), which did not cyclize to indoloquinolizidone 17a even on prolonged heating of the reaction mixture (Scheme 9). However, treatment of the isolated dihydropyridone 19a with trifluoroacetic acid at room temperature furnished the quinolizidone 17a in good yield (64%, Scheme 9). In fact, it was not necessary to purify the pyridone 19a and the crude reaction mixture after evaporation of acetonitrile (from the reaction of 6a and itaconic anhydride), affording 17a in comparable yield, on treatment with trifluoroacetyl (TFA).

We also subjected 2-chloro/bromobenzoylenaminones, such as 7c, 7e,f, to intramolecular nucleophilic aromatic substitution (S_NAr), with a view to synthesize pentacyclic dihydroindolo[2′,3′:3,4]pyrido[1,2-a]quinolin-2-ones 20 (Scheme 10).²⁵ Thus, when o-chlorobenzoylenaminone 7c was heated in in either dimethylformamide or dimethyl sulfoxide (DMSO) in the presence of bases like K₂CO_3, Cs₂CO₃, or sodium t-butoxide at higher temperature for a prolonged time, the desired product 20c was formed in varying yields; however, the best yield (71%) of 20c was obtained when N,S-acetal 7c was heated in DMSO for 12h at 120 °C (Scheme 10). Similarly, the other substituted o-halobenzoyl enaminones 7e,f also underwent intramolecular nucleophilic substitution to give the corresponding indolo-fused dihydroindolopyridoquinolin-2-ones 20e,f in high yields. However, the products 20e,f were found to be highly insoluble and 20e could be characterized by ¹H NMR/high-resolution mass spectrometry (HRMS) data, whereas 20f, only by HRMS.

Scheme 10. Synthesis of Pentacyclic Dihydroindolo[2′,3′:3,4]pyrido[1,2-a]quinolin-2-ones.

Finally, we also synthesized a few of the 2,3,4,6,7,12-hexahydropyrimido[1′,6′:1,2]pyrido[3,4-b]indoles 21a–c via cycloannulation of enaminones and nitroenamines 7a, 7g via the double Mannich reaction with formaldehyde and primary amines (Scheme 11). Thus, when the enaminone 7a was stirred with formaldehyde and benzylamine at room temperature in solvents like THF, CH₂Cl₂, acetonitrile, and benzene, starting materials remained unchanged whereas in methanol as solvent, the corresponding 1-benzoyl-3-N-benzylhexahydropyrimido[1′,6′:1,2]pyrido[3,4-b]indole 21a was obtained in 65% yield. On the other hand, annulation of nitroenamine 7e with formaldehyde and benzyl or furfuryl amines was found to be very facile, providing the corresponding, 1-nitro-3-benzyl/furfuryl-hexahydrpyrimido[1′,6′:1,2]pyrido[3,4-b]indoles 21b,c in 80 and 91% yields, respectively (Scheme 11). Some of these compounds are shown to be a potent inhibitor of lipid peroxidation.²⁶

Scheme 11. Synthesis of Hexahydropyrimido [1′,6′:1,2]pyrido[3,4-b]indoles.

Conclusions

In summary, we have carried out a detailed study of aza-annulation of newly synthesized β-carboline-derived enaminones and nitroenamines with various 1,2- or 1,3-biselectrophilic species, like oxalyl chloride, maleic anhydride, 1,4-benzoquinone, 3-bromopropionyl chloride, itaconic anhydride, etc. and successfully developed convenient one-pot protocols for the construction of a variety of novel β-carboline 1,2-fused highly functionalized five- and six-membered tetra- and pentacyclic heterocyclic motifs, in reasonable yields. It should be noted that, while there are few reports of aza-annulation of β-carboline-derived enaminoesters (or in situ generated acyclic enaminoester from tryptamine and ethyl propiolate) furnishing tetrahydroindolizino[8,7-b]indoles or indolo[2,3-a]quinolizidines derivatives, the synthesis and reactivity of the corresponding β-carboline-derived enaminones and especially nitroenamines have not been explored. Also, the aza-annulations of these β-carboline-derived enamines with maleic anhydride, itaconic anhydride, and 1,4-bezoquinone have not been reported in the literature. These novel protocols provide a rapid and efficient access to biologically important non-natural indole alkaloids in a highly concise fashion. The overall study reveals the possibility of construction of a range of novel substituted β-carboline-fused heterocyclic scaffolds with potential biological activity employing this protocol.

Experimental Section

General Information

All reagents were purchased from commercial suppliers and used without further purification. Solvents were dried according to the standard procedures. All reactions were monitored by thin layer chromatography using standard thin-layer chromatography (TLC) Silica gel plates and visualized with UV light. Column chromatography was performed using silica gel (100–200 mesh) or neutral alumina wherever mentioned. Nuclear magnetic resonance spectra were recorded on Brucker (400 MHz) ultrashield plus and Jeol (600 MHz) ECZ 600R FT-NMR spectrometer with CDCl₃, DMSO-d₆, or CD₃OD as solvent. Chemical shifts were reported in δ ppm using residual solvent protons as internal standard (δ 7.26 for CDCl₃, δ 2.50 for DMSO-d₆, and δ 3.31 for CD₃OD in ¹H NMR, δ 77.16 for CDCl₃, δ 39.52 for DMSO-d₆, and δ 49.01 for CD₃OD in ¹³C NMR). Coupling constants were reported as J values in hertz (Hz). Splitting patterns are designated as s (singlet), d (doublet), t (triplet), q (quartet), dd (doublet of doublet), dt (doublet of triplet), td (triplet of doublet), ddd (doublet of doublet of doublet), m (multiplet), and br (broad). Infrared spectra of neat samples were recorded in attenuated total reflectance mode using Fourier transform infrared instrument (Agilent technologies) and HRMS on a 6538 UHD accurate mass Q-TOF LC/MS spectrometer through electrospray ionization (ESI) mode. Melting points were recorded using an electrothermal capillary melting point apparatus and are uncorrected. All tetrahydro-β-carboline derived enaminones 7a–f and nitroenamine 7g were prepared according to our earlier reported procedure from the respective N,S-acetals 6a–g.^4a The spectral data of the known enaminones 7a–d and 7g has been reported earlier,^4a whereas spectral and analytical data of unknown enaminones 7e,f is given below.

1-(2,4-Dichlorobenzoyl)methylene-1,2,3,4-tetrahydro-β-carboline (7e)

Obtained from 6e; yellow solid (542 mg, 76%); mp 192–194 °C; R_f 0.26 (2:8 (EtOAc)/hexane); ¹H NMR (600 MHz, DMSO-d₆) δ 11.64 (s, 1H, NH), 10.34 (s, 1H, indole NH), 7.67 (br s, 1H, ArH), 7.61 (d, J = 7.6 Hz, 1H, ArH), 7.55 (d, J = 8.4 Hz, 1H, ArH), 7.50 (dd, J = 8.4, 1.2 Hz, 1H, ArH), 7.40 (d, J = 7.6 Hz, 1H, ArH), 7.25 (t, J = 7.6 Hz, 1H, ArH), 7.08 (t, J = 7.6 Hz, 1H, ArH), 5.90 (s, 1H, =CH), 3.67–3.65 (m, 2H, NCH₂), 3.00 (t, J = 7.0 Hz, 2H, CH₂); ¹³C{¹H} NMR (150 MHz, DMSO-d₆) δ 186.7, 152.2, 140.5, 137.8, 133.8, 130.9, 130.2, 129.3, 127.2, 127.0, 125.2, 124.6, 119.8, 119.75, 116.4, 112.1, 89.3, 39.0, 19.7; IR (neat, cm^–1) 3020, 1725, 740; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₁₉H₁₅Cl₂N₂O [M + H]⁺ 357.0561, found 357.0551

1-(2-Bromo-5-methoxybenzoyl)methylene-1,2,3,4-tetrahydro-β-carboline (7f)

Obtained from 6f; yellow solid (416 mg, 70%); mp 198–200 °C; R_f 0.26 (4:6 EtOAc/hexane); ¹H NMR (400 MHz, CDCl₃) δ 10.69 (s, 1H, NH), 9.35 (s, 1H, indole NH), 7.71 (d, J = 7.2 Hz, 1H, ArH), 7.49–7.39 (m, 3H, ArH), 7.29–7.25 (m, 1H, ArH), 7.15 (d, J = 2.8 Hz, 1H, ArH), 7.79 (dd, J = 8.8, 3.2 Hz, 1H, ArH), 5.89 (s, 1H, =CH), 3.89 (s, 3H, OCH₃), 3.73 (dt, J = 7.2, 2.8 Hz, 1H, NCH₂), 3.15 (t, J = 7.2 Hz, CH₂); ¹³C{¹H} NMR (100 MHz, CDCl₃) δ 190.3, 158.7, 152.8, 144.3, 137.9, 133.9, 127.2, 125.9, 125.2, 120.5, 119.7, 117.5, 116.4, 114.2, 112.1, 109.9, 89.7, 55.5, 40.2, 20.3; IR (neat, cm^–1) 3059, 1722, 642; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₀H₁₈BrN₂O₂ [M + H]⁺ 397.0552, found 397.0531.

General Procedure for the Reaction of Enaminones 7a,b, 7d or N,S-Acetals 6a,b, 6d with Oxalyl Chloride

Synthesis of 1-Aroyl/nitro-5,6-dihydro-2H-indolizino[8,7-b]indole-2,3(11H)-diones 8

To a stirred solution of the appropriate enaminones 7 (1.09 mmol) or the N,S-acetal 6 and triethylamine (0.38 mL, 2.7 mmol) in dry THF (15 mL) under a nitrogen atmosphere, oxalyl chloride (0.95 mL, 1.1 mmol) was added at 0 °C. After stirring the reaction mixture for 4 h (monitored by TLC), the solvent was evaporated under reduced pressure and the residue dissolved in CH₂Cl₂ and washed three times with water (3 × 10 mL). The organic layer was dried over anhydrous Na₂SO₄ and evaporated under reduced pressure, and the crude residue was purified by silica gel chromatography using (2:8) EtOAc/hexane as eluent to give pure 8.

1-Benzoyl-5,6-dihydro-2H-indolizino[8,7-b]indole-2,3(11H)-dione (8a)

Obtained from 6a or 7a; red solid (328 mg, 88% from 6a; 283 mg, 76% from 7a); mp 202–204 °C; R_f 0.2 (3:7 EtOAc/hexane); ¹H NMR (600 MHz, CDCl₃) δ 11.96 (s, 1H, NH), 7.73–7.71 (m, 3H, ArH), 7.59–7.56 (m, 1H, ArH), 7.53–7.51 (m, 2H, ArH), 7.48–7.44 (m, 2H, ArH), 7.24–7.22 (m, 1H, ArH), 4.16 (t, J = 7.8 Hz, 2H, NCH₂), 3.42 (t, J = 7.8 Hz, 2H, CH₂); ¹³C{¹H} NMR (150 MHz, CDCl₃) δ 190.4, 177.7, 159.9, 158.6, 141.1, 138.6, 132.6, 130.4, 129.3, 127.1, 125.6, 123.2, 122.1, 121.5, 113.5, 105.2, 37.8, 20.4; IR (neat, cm^–1) 3040, 1746, 1688, 1608; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₁H₁₅N₂O₃ [M + H]⁺ 343.1083, found 343.1063.

1-(4-Chlorobenzoyl)-5,6-dihydro-2H-indolizino[8,7-b]indole-2,3(11H)-dione (8b)

Obtained from 6b or 7b; orange solid (294 mg, 78% from 6b; 299 mg 73% from 7b); mp 197–198 °C; R_f 0.2 (2:8 EtOAc/hexane); ¹H NMR (600 MHz, CDCl₃) δ 11.94 (s, 1H, NH), 7.73 (d, J = 7.8 Hz, 1H, ArH), 7.67 (d, J = 8.4 Hz, 2H, ArH), 7.55–7.51 (m, 2H, ArH), 7.45 (d, J = 8.4 Hz, 2H, ArH), 7.24–7.23 (m, 1H, ArH), 4.16 (t, J = 7.0 Hz, 2H, NCH₂), 3.42 (t, J = 7.0 Hz, 2H, CH₂); ¹³C{¹H} NMR (150 MHz, CDCl₃) δ 187.5, 178.1, 159.6, 158.8, 141.0, 137.6, 136.9, 131.1, 129.5, 127.9, 124.8, 122.6, 121.6, 121.4, 114.2, 104.7, 37.3, 19.7; IR (neat, cm^–1) 2921, 1749, 1688, 1611, 742; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₁H₁₄ClN₂O₃ [M + H]⁺ 377.0637, found 377.0616.

1-Acetyl-5,6-dihydro-2H-indolizino[8,7-b]indole-2,3(11H)-dione (8d)

Obtained from 6d or 7d; red flaky solid (238 mg, 76% from 6d; 220 mg, 72% from 7d); mp 195–197 °C; R_f 0.3 (3:7 EtOAc/hexane); ¹H NMR (400 MHz, CDCl₃) δ 12.56 (s, 1H, NH), 7.71 (d, J = 8.0 Hz, 1H, ArH), 7.57–7.50 (m, 2H, ArH), 7.24–7.22 (m, 1H, ArH), 4.14 (t, J = 6.8 Hz, 2H, NCH₂), 3.40 (t, J = 6.8 Hz, 2H, CH₂), 2.69 (s, 3H, COCH₃); ¹³C{¹H} NMR (100 MHz, CDCl₃) δ 194.2, 190.3, 179.8, 159.3, 158.9, 141.5, 130.8, 127.4, 125.9, 123.5, 122.4, 121.8, 114.0, 38.1, 30.4, 20.7; IR (neat, cm^–1) 3100, 1742, 1704, 1611; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₁₆H₁₃N₂O₃ [M + H]⁺ 281.0926, found 281.0906.

General Procedure for the Reaction of Enaminones 7a,b with Maleic Anhydride

Synthesis of 2-(1-Aroyl-3-oxo-3,5,6,11-tetrahydro-2H-indolizino[8,7-b]indol-2-yl)acetic Acids 11

A solution of 7 (1.09 mmol) and maleic anhydride (107 mg, 1.1 mmol) in dry acetonitrile (15 mL) was refluxed for 8 h (monitored by TLC). The reaction mixture was then brought to room temperature and evaporated under reduced pressure, and the residue was dissolved in EtOAc. The organic layer was washed with water, dried (anhydrous Na₂SO₄), and evaporated to afford the crude product, which was purified by column chromatography over a neutral-alumina column using (9:1) EtOAc/hexane as eluent.

2-(1-Benzoyl-3-oxo-3,5,6,11-tetrahydro-2H-indolizino[8,7-b]indol-2-yl)acetic Acid (11a)

Obtained from 7a; yellow solid (230 mg, 60%); mp 185–187 °C; R_f 0.13 (4:6 EtOAc/hexane); ¹H NMR (600 MHz, DMSO-d₆) δ 11.92 (s, 1H, CO₂H), 7.74–7.70 (m, 4H, ArH), 7.57 (t, J = 7.6 Hz,1H, ArH), 7.50 (t, J = 7.6 Hz, 2H, ArH), 7.32 (t, J = 7.6 Hz, 1H, ArH) 7.14 (t, J = 7.8 Hz, 1H, ArH), 4.20–4.18 (m, 2H, NCH₂), 3.58–3.53 (m, 1H, ArCHH-6), 3.26 (dt, J = 16.8, 4.2 Hz, 1H, ArCHH-6), 3.13–3.08 (m, 1H, =CCH-2), 2.60 (dd, J = 16.8, 3.0 Hz, 1H, CHHCO₂H), 1.87 (dd, J = 16.8, 6.0 Hz, 1H, CHHCO₂H); ¹³C{¹H} NMR (150 MHz, DMSO-d₆) δ 189.9, 177.5, 171.7, 146.5, 141.6, 137.5, 131.6, 129.1, 127.7, 126.2, 125.5, 124.2, 120.8, 120.5, 119.1, 113.8, 110.6, 44.7, 38.4, 34.2, 19.8; IR (neat, cm^–1) 3100 (br), 1725, 1610, HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₃H₁₉N₂O₄ [M + H]⁺ 387.1308, found 387.1338.

2-(1-(4-Chlorobenzoyl)-3-oxo-3,5,6,11-tetrahydro-2H-indolizino[8,7-b]indol-2-yl)acetic Acid (11b)

Obtained from 7b; yellow solid (233 mg, 62%); mp 188–190 °C; R_f 0.13 (4:6 EtOAc/hexane); ¹H NMR (600 MHz, DMSO-d₆) δ 12.25 (br s, 1H, CO₂H), 11.83 (s, 1H, NH), 7.72 (d, J = 8.4 Hz, 2H, ArH), 7.69–7.66 (m, 2H, ArH), 7.53 (d, J = 8.4 Hz, 2H, ArH), 7.31 (t, J = 7.8 Hz, 1H, ArH), 7.12 (t, J = 7.8 Hz, 1H, ArH), 4.15–4.12 (m, 2H, NCH₂), 3.60–3.50 (m, 1H, ArCHH-6), 3.23 (dt, J = 16.2, 4.8 Hz, 1H, ArCHH-6), 3.10–3.08 (m, 1H, =CCH-2), 2.62 (dd, J = 16.8, 3.6 Hz, 1H, CHHCO₂H), 1.94 (dd, J = 16.8, 5.4 Hz, 1H, CHHCO₂H); ¹³C{¹H} NMR (150 MHz, CDCl₃) δ 187.9, 176.8, 171.1, 146.3, 139.7, 137.0, 135.8, 129.2, 128.6, 125.8, 124.9, 123.5, 120.2, 120.0, 118.8, 113.3, 109.7, 44.0, 37.9, 33.8, 19.2; IR (neat, cm^–1) 2995 (br), 1720, 1650, 746; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₃H₁₈ClN₂O₄ [M + H]⁺ 421.0955, found 421.0956.

General Procedure for the Reaction of Enaminones 7a–c and Nitroenamine 7g with 1,4-Benzoquinone

Synthesis of 2-Hydroxy-7,12-dihydro-6H-indolo[2,1-a]β-carbolin-13-yl-aryl/nitro Methanones 13

To a stirred solution of the appropriate enaminones 7a–c, 7g (1.6 mmol) in nitromethane, p-benzoquinone (237 mg, 2.2 mmol) was added under a nitrogen atmosphere and the reaction was stirred at 25 °C for 1.5–2 days (monitored by TLC). The reaction mixture was concentrated under reduced pressure, residue was dissolved in EtOAc, and the organic layer was washed with water (3 × 10 mL) and dried (anhydrous Na₂SO₄). The solvent was evaporated under reduced pressure to afford the crude products, which were purified by silica-gel column chromatography using (2:8) EtOAc/hexane as eluent.

2-Hydroxy-7,12-dihydro-6H-indolo[2,1-a]β-carbolin-13-yl-phenylmethanone (13a)

Obtained from 7a; yellow solid (180 mg, 58%); mp 230–231 °C; R_f 0.40 (3:7 EtOAc/hexane); ¹H NMR (400 MHz, DMSO-d₆) δ 11.36 (s, 1H, NH), 8.93 (s, 1H, OH), 7.73–7.65 (m, 5H, ArH), 7.59 (t, J = 7.2 Hz, 2H, ArH), 7.47 (d, J = 8.8 Hz, 1H, ArH-4), 7.21 (t, J = 7.2 Hz, 1H, ArH), 7.12 (t, J = 7.2 Hz, 1H, ArH), 6.74 (dd, J = 8.8, 2.4 Hz, 1H, ArH-3), 6.11 (d, J = 2.4 Hz, 1H, ArH-1), 4.47 (t, J = 7.6 Hz, 2H, NCH₂), 3.33 (t, J = 7.6 Hz, 2H, CH₂); ¹³C{¹H} NMR (100 MHz, DMSO-d₆) δ 191.1, 152.6, 140.9, 136.5, 136.2, 131.5, 131.1, 128.4, 128.3, 127.7, 125.5, 125.4, 123.4, 119.7, 118.8, 112.7, 111.6, 110.9, 109.1, 105.2, 41.4, 19.6; IR (neat, cm^–1) 3289, 2921, 1700; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₅H₁₉N₂O₂ [M + H]⁺ 379.1447; found 379.1540.

(4-Chlorophenyl)(2-hydroxy-7,12-dihydro-6H-pyrido[1,2-a:3,4-b′]diindol-13-yl)methanone (13b)

Obtained from 7b; yellow solid (380 mg, 62%); mp 230–231 °C; R_f 0.34 (3:7 EtOAc/hexane); ¹H NMR (600 MHz, DMSO-d₆) δ 11.28 (s, 1H, NH), 8.99 (s, 1H, OH), 7.70 (d, J = 5.6 Hz, 2H, ArH), 7.65–7.59 (m, 4H, ArH), 7.45 (d, J = 8.4 Hz, 1H, ArH-4), 7.19 (t, J = 7.6 Hz, 1H, ArH), 7.08 (t, J = 7.6 Hz, 1H, ArH), 6.71 (dd, J = 8.4, 2.4 Hz, 1H, ArH-3), 6.13 (d, J = 2.4 Hz, 1H, ArH-1), 4.43 (t, J = 7.2 Hz, 2H, NCH₂), 3.26 (t, 2H, J = 7.2 Hz, CH₂); ¹³C{¹H} NMR (150 MHz, DMSO-d₆) δ 190.4, 153.3, 140.2, 137.2, 136.8, 131.7, 130.9, 129.2, 128.1, 126.1, 126.0, 124.0, 120.3, 119.4, 113.3, 112.5, 111.7, 109.5, 105.5, 41.9, 20.1; IR (neat, cm^–1) 3274, 2922, 1606, 760; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₅H₁₈ClN₂O₂ [M + H]⁺ 413.0934; found 413.0908.

(2-Chlorophenyl)(2-hydroxy-7,12-dihydro-6H-pyrido[1,2-a:3,4-b′]diindol-13-yl)methanone (13c)

Obtained from 7c; yellow solid (173 mg, 54%); mp 197–198 °C; R_f 0.36 (3:7 EtOAc/hexane); ¹H NMR (400 MHz, DMSO-d₆) 11.91 (s, 1H, NH), 9.01 (s, 1H, OH), 7.76 (d, J = 8.4 Hz, 1H, ArH-4), 7.70–7.67 (m, 2H, ArH), 7.63 (t, J = 8.4 Hz, 1H, ArH), 7.55 (t, J = 7.6 Hz, 1H,ArH), 7.47 (t, J = 7.6 Hz, 2H, ArH), 7.26 (t, J = 7.2 Hz, 1H, ArH), 7.13 (t, J = 7.6 Hz, 1H, ArH), 6.75 (br d, J = 8.4 Hz, 1H, ArH-3), 5.56 (brs, 1H, ArH-1), 4.47 (t, J = 7.4 Hz, 2H, NCH₂), 3.38 (t, J = 7.4 Hz, 2H, CH₂); ¹³C{¹H} NMR (100 MHz, CDCl₃) δ 188.6, 153.2, 141.6, 137.3, 136.0, 131.3, 131.0, 129.9, 129.0, 127.8, 127.6, 125.4, 125.3, 123.8, 119.9, 119.0, 112.9, 112.8, 112.4, 111.2, 109.4, 104.3, 41.5, 19.5; IR (neat, cm^–1) 3387, 2923, 1698, 760; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₅H₁₈ClN₂O₂ [M + H]⁺ 413.0939; found 413.0910.

13-Nitro-7,12-dihydro-6H-pyrido[1,2-a:3,4-b′]diindol-2-ol (13g)

Obtained from 7g; yellow solid (180 mg, 35%); mp 215–217 °C; R_f 0.40 (3:7 EtOAc/hexane); ¹H NMR (400 MHz, DMSO-d₆) δ 11.33 (s, 1H, NH), 9.56 (s, 1H, OH), 7.76 (d, J = 8 Hz, 1H, ArH), 7.68 (d, J = 7.6 Hz, 1H, ArH), 7.63 (d, J = 2.4 Hz, 1H, ArH-1), 7.57 (d, J = 8.8 Hz, 1H, ArH-4), 7.27 (t, J = 7.6 Hz, 1H, ArH), 7.13 (t, J = 7.6 Hz, 1H, ArH), 6.88 (dd, J = 8.8, 2.4 Hz, 1H, ArH-3), 4.46 (t, J = 7.6 Hz, 2H, NCH₂), 3.36 (t, J = 7.6 Hz, 2H,CH₂); ¹³C{¹H} NMR (100 MHz, DMSO-d₆) δ 155.2, 137.2, 132.5, 129.3, 124.7, 124.5, 123.1, 122.2, 121.8, 120.1, 119.3, 115.1, 114.3, 113.3, 112.0, 104.7, 41.6, 19.4; IR (neat, cm^–1) 3395, 2922, 1454; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₁₈H₁₄N₃O₃ [M + H]⁺ 320.1035; found 320. 1011.

General Procedure for the Reaction of Enaminones 7a, 7d and Nitroenamine 7g or N,S-Acetals 6a, 6d, 6g with 3-Bromopropionyl Chloride

Synthesis of 1-Acyl/nitro-2,3,6,7-tetrahydroindolo[2,3-a]quinolizin-4(12H)-ones 14

To a stirred solution of the appropriate enaminones 7 (1.46 mmol) or N,S-acetal 6 and triethylamine (0.5 mL, 3.64 mmol) in dry THF (15 mL) under a nitrogen atmosphere, 3-bromopropionyl chloride (0.17 mL, 1.75 mmol) was added at 0 °C. The mixture was allowed to stir for 3h (monitored by TLC). The solvent was evaporated under reduced pressure, and the residue, dissolved in CH₂Cl₂ (15 mL) and washed with water (3 × 10 mL). The organic layer was dried over anhydrous Na₂SO₄, the solvent was removed under reduced pressure, and the crude residue was purified by silica gel chromatography using (3:7) EtOAc/hexane as eluent.

1-Benzoyl-2,3,6,7-tetrahydroindolo[2,3-a]quinolizin-4(12H)-one (14a)

Yellow solid (375 mg, 75% from 6a, 327 mg, 65% from 7a); mp 124–126 °C; R_f 0.3 (3:7 EtOAc/hexane); ¹H NMR (400 MHz, CDCl₃) δ 10.39 (s, 1H, NH), 7.79–7.76 (m, 2H, ArH), 7.58–7.55 (m, 2H, ArH), 7.52–7.48 (m, 2H, ArH), 7.35 (br d, J = 8.0 Hz, 1H, ArH), 7.29–7.25 (m, 1H, ArH), 7.14–7.11 (m, 1H, ArH), 4.32 (t, J = 6.0 Hz, 2H, NCH₂), 3.06 (t, J = 6.0 Hz, 2H, ArCH₂), 2.71–2.63 (m, 4H, COCH₂, =CCH₂); ¹³C{¹H} NMR (100 MHz, CDCl₃) δ 198.5, 170.5, 139.4, 139.0, 137.0, 132.6, 128.6, 128.5, 126.7, 125.3, 120.2, 119.3, 118.3, 113.8, 112.1, 40.7, 31.9, 26.0, 20.8; IR (neat, cm^–1) 2916, 1672, 1356, HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₂H₁₉N₂O₂ [M + H]⁺ 343.1447, found 343.1437.

1-Acetyl-2,3,6,7-tetrahydroindolo[2,3-a]quinolizin-4(12H)-one (14d)

Yellow solid (301 mg, 74% from 6d, 247 mg, 60% from 7d); mp 138–139 °C; R_f 0.32 (3:7 EtOAc/hexane); ¹H NMR (400 MHz, CDCl₃) δ 12.19 (s, 1H, NH), 7.58 (d, J = 8.0 Hz, 1H, ArH), 7.46 (d, J = 8.0 Hz, 1H, ArH), 7.31 (t, J = 7.2 Hz, 1H, ArH), 7.13 (t, J = 7.2 Hz, 1H, ArH), 4.28 (t, J = 6.0 Hz, 2H, NCH₂), 3.02 (t, J = 6.0 Hz, 2H, ArCH₂), 2.83–2.80 (m, 2H, COCH₂), 2.65–2.62 (m, 2H, =CCH₂), 2.47 (s, 3H, COCH₃); ¹³C{¹H} NMR (100 MHz, CDCl₃) δ 199.3, 170.5, 140.0, 136.4, 126.8, 125.4, 125.0, 120.1, 119.4, 118.5, 114.4, 112.4, 41.2, 31.5, 30.7, 24.1, 21.0; IR (neat, cm^–1) 3200, 1697, 1626, HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₁₇H₁₇N₂O₂ [M + H]⁺ 281.1290, found 281.1277.

1-Nitro-2,3,6,7-tetrahydroindolo[2,3-a]quinolizin-4(12H)-one (14g)

Red solid (384 mg, 80% from 6g; 316 mg, 76% from 7g); mp 165–166 °C; R_f 0.45 (2:8 EtOAc/hexane); ¹H NMR (400 MHz, CDCl₃) δ 10.45 (s, 1H, NH), 7.61 (d, J = 8.0 Hz, 1H, ArH), 7.44–7.37 (m, 2H, ArH), 7.18 (br t, J = 7.6 Hz, 1H, ArH), 4.31 (t, J = 6.4 Hz, 2H, NCH₂), 3.20 (t, J = 7.6 Hz, 2H, =CCH₂), 3.06 (t, J = 6.4 Hz, 2H, ArCH₂), 2.76 (t, J = 7.6 Hz, 2H, COCH₂); ¹³C{¹H} NMR (100 MHz, CDCl₃) δ 169.6, 138.6, 137.8, 128.6, 127.4, 124.7, 123.5, 121.0, 120.1, 112.4, 42.1, 30.2, 23.4, 20.9; IR (neat, cm^–1) 2922, 1686, 1560, 1257; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₁₅H₁₄N₃O₃ [M + H]⁺ 284.1028, found 284.0999.

General Procedure for the Reaction of Enaminones 7a–c and Nitroenamine 7g with Itaconic Anhydride

Synthesis of 2-(1-Aroyl/nitro-4-oxo-2,3,4,6,7,12-hexahydroindolo[2,3-a]quinolizin-3-yl)acetic Acids 17

A solution of the appropriate enaminone 7 (1.5 mmol) and itaconic anhydride (180 mg, 1.6 mmol) in dry acetonitrile (15 mL) was refluxed for 10 h (monitored by TLC). The reaction mixture was then brought to room temperature and evaporated under reduced pressure, and the residue was dissolved in EtOAc (15 mL). The organic layer was washed with water, dried (anhydrous Na₂SO₄), and evaporated to afford the crude product, which was purified by column chromatography over a neutral alumina column using (8:2) EtOAc/hexane as eluent.

2-(1-Benzoyl-4-oxo-2,3,4,6,7,12-hexahydroindolo[2,3-a]quinolizin-3-yl)acetic Acid (17a)

Obtained from 7a; yellow solid (200 mg, 64%); mp 170–172 °C; R_f 0.18 (8:2 EtOAc/hexane); ¹H NMR (400 MHz, DMSO-d₆) δ 12.20 (br s, 1H, NH), 10.33 (s, 1H, CO₂H), 7.84 (d, J = 7.2 Hz, 2H, ArH), 7.59–7.46 (m, 4H, ArH), 7.39 (d, J = 8.0 Hz, 1H, ArH), 7.16 (t, J = 7.6 Hz, 1H, ArH), 7.03 (t, J = 7.6 Hz, 1H, ArH), 4.78 (dt, J = 12.8, 4.8 Hz, 1H, NCHH-6), 3.43–3.36 (m, 1H, NCHH-6), 3.01 (dt, J = 16.4, 4.0 Hz, 1H, ArCHH-7), 2.93–2.88 (m, 2H, ArCHH-7, H-3), 2.77–2.68 (m, 2H, =CH₂), 2.53–2.52 (m, merged with DMSO signal, 1H, CHHCO₂H), 2.34 (dd, J = 16.4, 6.4 Hz, 1H, CHHCO₂H); ¹³C{¹H} NMR (100 MHz, DMSO-d₆) δ 196.0, 172.8, 170.9, 138.3, 137.2, 136.1, 132.5, 128.9, 128.4, 126.1, 124.7, 124.1, 119.5, 119.0, 116.7, 112.9, 112.3, 40.3, 37.0, 34.0, 30.0, 20.3; IR (neat, cm^–1) 3400, 2923, 1708, HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₄H₂₁N₂O₅ [M + H]⁺ 401.1628; found 401.1607.

2-(1-(4-Chlorobenzoyl)-4-oxo-2,3,4,6,7,12-hexahydroindolo[2,3-a]quinolizin-3-yl)acetic Acid (17b)

Obtained from 7b; yellow solid (617 mg, 58%); mp 217–218 °C; R_f 0.18 (8:2 EtOAc/hexane); ¹H NMR (600 MHz, DMSO-d₆) δ 10.36 (s, 1H, NH), 7.80 (d, J = 8.4 Hz, 2H, ArH), 7.50–7.47 (m, 3H, ArH), 7.32 (d, J = 8.4 Hz, 1H, ArH), 7.10 (t, J = 7.8 Hz, 1H, ArH), 6.97 (t, J = 7.8 Hz, 1H ArH), 4.71 (dt, J = 12.0, 4.0 Hz, 1H, NCHH-6) 3.36 (td, J = 12.0, 4.1 Hz, 1H, NCHH-6), 2.96 (dt, J = 16.2, 3.8 Hz,1H,ArCHH-7), 2.92–2.85 (m, 2H, ArCHH-7, H-3), 2.70–2.65 (m, 2H, =CH₂), 2.51–2.48 (dd, J = 16.8, 5.4 Hz, 1H, CHHCO₂H), 2.34 (dd, J = 16.8, 7.2 Hz, 1H, CHHCO₂H); ¹³C{¹H} NMR (150 MHz, DMSO-d₆) δ 195.1, 173.5, 171.6, 137.9, 137.7, 137.7 137.2, 131.3, 129.0, 126.6, 125.2, 124.8, 120.1, 119.7, 117.5, 112.8, 112.7, 40.8, 37.6, 34.7, 31.8, 30.3, 20.9; IR (neat, cm^–1) 3445, 2921, 1701, 1678, 746; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₄H₂₀ClN₂O₄ [M + H]⁺ 435.1112; found 435.1121.

2-(1-(2-Chlorobenzoyl)-4-oxo-2,3,4,6,7,12-hexahydroindolo[2,3-a]quinolizin-3-yl)acetic Acid (17c)

Obtained from 7c; yellow solid (392 mg, 54%); mp 217–218 °C; R_f 0.18 (8:2 EtOAc/hexane); ¹H NMR (400 MHz, DMSO-d₆) δ 12.18 (br s, 1H, NH), 11.67 (s, 1H, CO₂H), 7.62 (d, J = 8 Hz, 1H, ArH), 7.58–7.44 (m, 5H, ArH), 7.28 (t, J = 7.6 Hz, 1H, ArH), 7.10 (t, J = 7.6 Hz, 1H, ArH), 4.83 (dt, J = 12.8, 4.0 Hz, 1H, NCHH-6), 3.52 (td, J = 12.8, 4.0 Hz, 1H, NCHH-6), 3.05 (dt, J = 16.8, 6.0 Hz, 1H, ArCHH-7), 2.96–2.86 (m, 2H, ArCHH-7, H-3), 2.65 (dd, J = 16.8, 6.0 Hz, 1H, =CHH-2), 2.55 (d, J = 16.8 Hz, 1H, =CHH-2), 2.45 (dd, J = 16.8, 6.8 Hz, 1H, CHHCO₂H), 2.34 (dd, J = 16.4, 6.8 Hz, 1H, CHHCO₂H); ¹³C{¹H} NMR (100 MHz, DMSO-d₆) δ 193.6, 172.7, 171.1, 140.7, 140.1, 136.6, 131.3, 129.7, 129.4, 128.9, 127.4, 126.2, 125.2, 124.5, 119.9, 119.5, 118.9, 112.6, 112.3, 41.0, 36.8, 33.8, 28.8, 20.2; IR (neat, cm^–1) 3200, 2922, 1687, 745; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₄H₂₀ClN₂O₄ [M + H]⁺ 435.0994; found 435.0966.

2-(1-Nitro-4-oxo-2,3,4,6,7,12-hexahydroindolo[2,3-a]quinolizin-3-yl)acetic Acid (17g)

Obtained from 7g; red solid (326 mg, 59%); mp 206–207 °C; R_f 0.16 (8:2 EtOAc/hexane); ¹H NMR (400 MHz, DMSO-d₆) δ 12.34 (br s, 1H, NH), 11.05 (s, 1H, CO₂H), 7.63 (d, J = 7.6 Hz, 1H, ArH), 7.52 (d, J = 8.0 Hz, 1H, ArH), 7.30 (t, J = 7.2 Hz, 1H, ArH), 7.09 (t, J = 7.2 Hz, 1H, ArH), 4.58 (br d, J = 12.4 Hz, 1H, NCHH-6), 3.48–3.40 (m, 1H, NCHH), 3.28 (dd, J = 13.6, 5.6 Hz, 1H, =CHH-2), 3.87–2.85 (m, 4H, =CHH-2, ArCH₂-7, H-3), 2.77 (dd, J = 16.8, 5.6 Hz, 1H, CHHCO₂H), 2.60–2.45 (m, merged with DMSO signal, 1H, CHHCO₂H); ¹³C{¹H} NMR (100 MHz, DMSO-d₆) δ 172.6, 170.7, 138.1, 136.9, 125.9, 125.9, 123.8, 123.5, 121.7, 119.9, 119.8, 112.6, 40.7, 36.4, 34.0, 27.7, 20.0; IR (neat, cm^–1) 3383, 2920, 1690, 1330, 747; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₁₇H₁₆N₃O₅ [M + H]⁺ 342.1090; found 342.1083.

Procedure for the Reaction of N,S-Acetal 6a with Itaconic Anhydride

Synthesis of 2-(1-(2-(1H-indol-3-yl)ethyl)-5-benzoyl-6-(methylyhio)-2-oxo-1,2,3,4-tetrahydropyridin-3-yl)acetic Acid (19a)

A solution of the N,S-acetal 6a (509 mg, 1.5 mmol) and itaconic anhydride (180 mg, 1.6 mmol) in dry acetonitrile (15 mL) was refluxed for 8 h (monitored by TLC). The reaction mixture was then brought to room temperature, the solvent was evaporated under reduced pressure, and the residue was dissolved in EtOAc (15 mL). The organic layer was washed with water, dried (anhydrous Na₂SO₄), and evaporated to afford the crude 19a, which was purified by column chromatography over a neutral-alumina column using (9:1) EtOAc/hexane as eluent.

2-(1-(2-(1H-indol-3-yl)ethyl)-5-benzoyl-6-(methylthio)-2-oxo-1,2,3,4-tetrahydropyridin-3-yl)acetic Acid (19a)

Obtained from 6a; yellow solid (474 mg, 70%); mp 140–142 °C; R_f 0.15 (9:1 EtOAc/hexane); ¹H NMR (600 MHz, DMSO-d₆) δ 12.21 (s, 1H, NH), 10.86 (s, 1H, CO₂H), 7.59–7.56 (m, 4H, ArH), 7.47 (t, J = 7.2 Hz, 2H, ArH), 7.34 (d, J = 8.4 Hz, 1H, ArH), 7.17 (d, J = 2.4 Hz, 1H, indole H-2), 7.05 (t, J = 7.2 Hz, 1H, ArH), 6.97 (t, J = 7.2 Hz, 1H, ArH), 4.23–4.22 (m, 1H, NCHH), 3.94–3.91 (m, 1H, NCHH), 2.99–2.89 (m, 3H,ArCH₂, H-3), 2.67 (dd, J = 17.4, 6.6 Hz, 1H,=CHH-4), 2.39–2.33 (m, 3H, CH₂CO₂H, =CHH-4), 1.98 (s, 3H, SCH₃); ¹³C{¹H} NMR (150 MHz, DMSO-d₆) δ 195.5, 173.5, 171.7, 137.7, 137.2, 136.7, 133.6, 129.3, 127.9, 126.9, 123.7, 121.5, 118.9, 118.8, 111.9, 111.3, 44.1, 37.3, 34.4, 28.9, 24.8, 18.4; IR (neat, cm^–1) 3104 (br), 3450, 1707, 1661, 742; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₅H₂₅N₂O₄S [M+1]⁺ 449.1540; found 449.1565.

Conversion of 19a to 17a

To a solution of 19a (450 mg, 1 mmol) in dichloromethane (10 mL), TFA (0.23 mL, 3 mmol) was added and the reaction mixture was stirred at room temperature for 5h (monitored by TLC). After evaporation of solvent, it was neutralized with sat.NaHCO₃ (15 mL), extracted with EtOAc (3 × 10 mL).The organic layer was washed with water, dried (anhydrous Na₂SO₄), and evaporated to afford the crude product, which was purified by column chromatography over a neutral-alumina column using (8:2) EtOAc/hexane as eluent to give pure 17a; (yield, 256 mg), 64%; spectral and analytical data as mentioned above.

General Procedure for Base-Mediated Intramolecular Nucleophilic Substitution of 7c, 7e, and 7f

Synthesis of Dihydroindolo[2′,3′:3,4]pyrido[1,2-a]quinolin-2-ones 20

To a stirring solution of enaminones 7 (1.0 mmol) in DMSO (10 mL) in a sealed tube, K₂CO₃ (414 mg, 3.0 mmol) was added and the reaction mixture was heated to 120 °C for 12 h (monitored by TLC). For the product 19f, the enaminone 7f was heated in N-methylpyrrolidine, at 140 °C for 36 h in a sealed tube. The reaction mixture was cooled to room temperature and was diluted with sat. NH₄Cl solution (15 mL). The precipitated product was filtered and washed with water and hexane. The crude product was purified by column chromatography using (6:4) EtOAc/hexane as eluent.

8,9-Dihydroindolo[2′,3′:3,4]pyrido[1,2-a]quinolin-2(14H)-one (20c)

Obtained from 7c; yellow solid (203 mg, 71%); mp 345–347 °C; R_f 0.2 (6:4 EtOAc/hexane); ¹H NMR (600 MHz, DMSO-d₆) δ 11.73 (s, 1H, NH), 8.18 (dd, J = 7.8, 1.8 Hz, 1H, ArH), 7.98 (d, J = 8.4 Hz, 1H, ArH), 7.72 (dt, J = 7.8, 1.8 Hz, 1H, ArH), 7.64 (d, J = 7.8 Hz, 1H, ArH), 7.42 (d, J = 7.8 Hz, 1H, ArH), 7.32 (t, J = 7.2 Hz, 1H, ArH), 7.24 (t, J = 7.8 Hz, 1H, ArH), 7.07 (t, J = 7.8 Hz, 1H, ArH), 6.69 (s, 1H, =CH), 4.51 (t, J = 6.6 Hz, 2H, NCH₂), 3.23 (t, J = 6.6 Hz, 2H, CH₂); ¹³C{¹H} NMR (150 MHz, DMSO-d₆) δ 176.1, 142.3, 142.1, 138.8, 132.8, 128.1, 126.9, 126.1, 125.7, 124.9, 123.3, 120.3, 120.2, 117.0, 114.4, 112.5, 103.8, 44.3, 20.2; IR (neat, cm^–1) 3215, 1592, 1293; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₁₉H₁₅N₂O [M + H]⁺ 287.1157; found 287.1128.

5-Chloro-8,9-dihydroindolo[2′,3′:3,4]pyrido[1,2-a]quinolin-2(14H)-one (20e)

Obtained from 7e; yellow solid (200 mg, 73%); mp 350–352 °C; R_f 0.2 (4:6 EtOAc/hexane); ¹H NMR (600 MHz, CD₃OD) δ 11.75 (s, 1H, NH), 8.15 (d, J = 8.4 Hz, 1H, ArH-3), 8.09 (d, J = 1.8 Hz, 1H, ArH-6), 7.65 (d, J = 7.8 Hz, 1H, ArH), 7.41 (d, J = 8.4 Hz, 1H, ArH), 7.37 (dd, J = 8.4, 1.8 Hz, 1H, ArH-4), 7.24 (dt, J = 7.8, 1.2 Hz, 1H, ArH), 7.08 (dt, J = 7.8, 1.2 Hz, 1H, ArH), 6.65 (s, 1H, =CH), 4.50 (t, J = 7.2 Hz, 2H, NCH₂), 3.22 (t, J = 7.2 Hz, 2H, CH₂); IR (neat, cm^–1) 3200, 1612, 743; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₁₉H₁₄ClN₂O [M + H]⁺ 321.0795; found 321.0778.

4-Methoxy-8,9-dihydroindolo[2′,3′:3,4]pyrido[1,2-a]quinolin-2(14H)-one (20f)

Obtained from 7f; (202 mg, 64%); brown solid; mp 300–302 °C; R_f 0.2 (3:7 EtOAc/hexane); insoluble in most of the solvents; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₀H₁₇N₂O₂ [M + H]⁺ 317.1290; found 317.1274.

General Procedure for the Synthesis of 1-Benzoyl/nitro-2,3,4,6,7,12-hexahydropyrimido[1′6′:1,2]pyrido[3,4-b]indoles 21

A solution of the enaminone 7a (288 mg, 1 mmol), or nitroenamine 7g (230 mg,1 mmol), formaldehyde (37–41% w/v aq. solution) (0.3 mL, 8 mmol), and the appropriate amine (1.5 mmol) in methanol(15 mL) was stirred at room temperature for 4 h (monitored by TLC). The solvent was evaporated under reduced pressure, and the residue was dissolved in EtOAc (20 mL); the organic layer was washed with water, dried (anhydrous Na₂SO₄), and evaporated to afford the crude product, which was purified by column chromatography over silica gel using (1:9) EtOAc/hexane as eluent.

3-Benzyl-1-benzoyl-2,3,4,6,7,12-hexahydropyrimido[1′6′:1,2]pyrido[3,4-b]indole (21a)

Obtained from 7a and benzylamine; yellow solid (297 mg, 71%); mp 128–130 °C; R_f 0.35 (1:9 EtOAc/hexane); ¹H NMR (400 MHz, CDCl₃) δ 12.93 (s, 1H, NH), 7.57 (d, J = 8.0 Hz, 1H, ArH), 7.43 (d, J = 7.2 Hz, 4H, ArH), 7.34–7.26 (m, 8H, ArH), 7.11 (t, J = 7.2 Hz, 1H, ArH), 4.24 (s, 2H, NCH₂N), 3.78 (s, 2H, =CCH₂N), 3.68 (s, 2H, CH₂Ph), 3.49 (t, J = 6.4 Hz, 2H, NCH₂-6), 3.09 (t, J = 6.4 Hz, 2H, ArCH₂-7); ¹³C{¹H} NMR (100 MHz, CDCl₃) δ 194.1, 149.2, 143.2, 137.7, 136.1, 129.0, 129.0, 128.2, 128.1, 127.7, 127.4, 127.1, 125.0, 124.6, 119.8, 119.0, 114.9, 112.8, 97.6, 70.4, 56.3, 54.1, 49.0, 20.3; IR (neat, cm^–1) 3051, 1558, 1271; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₈H₂₆N₃O [M + H]⁺ 420.2076; found 420.2081.

3-Benzyl-1-nitro-2,3,4,6,7,12-hexahydropyrimido[1′6′:1,2]pyrido[3,4-b]indole (21b)

Obtained from nitroenamine 7g and benzyl amine; yellow solid (288 mg, 80%); mp 175–177 °C; R_f 0.35 (1:9 EtOAc/hexane); ¹H NMR (400 MHz, DMSO-d₆) δ 11.30 (s, 1H, NH), 7.72 (d, J = 8.0 Hz, 1H, ArH), 7.42 (d, J = 8.4 Hz, 1H, ArH), 7.34–7.15 (m, 7H, ArH), 5.25 (s, 2H, NCH₂N), 4.16 (s, 2H, =CCH₂N), 3.74 (br t, J = 7.2 Hz 2H, NCH₂-6), 3.67 (s, 2H, CH₂Ph), 3.08 (t, J = 7.2 Hz, 2H, ArCH₂-7); ¹³C{¹H} NMR (100 MHz, DMSO-d₆) δ 148.9, 138.2, 137.8, 128.4, 128.2, 127.2, 126.4, 125.6, 124.0, 120.5, 120.3, 120.2, 115.7, 110.8, 64.8, 54.4, 52.1, 40.5, 19.5; IR (neat, cm^–1) 3058, 1446, 1365; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₂₁H₂₁N₄O₂ [M + H]⁺ 361.1665; found 361.1657.

3-(Furan-2-ylmethyl)-1-nitro-2,3,4,6,7,12-hexahydropyrimido[1′6′:1,2]pyrido[3,4-b]indole (21c)

Obtained from 7g and 2-furfurylamine; yellow solid (318 mg, 91%); mp 63–65 °C; R_f 0.35 (1:9 EtOAc/hexane); ¹H NMR (400 MHz, CDCl₃) δ 11.07 (s, 1H, NH), 7.56 (d, J = 7.2 Hz, 1H, ArH), 7.39–7.14 (m, 4H, ArH, furylH), 6.30 (d, J = 2.8 Hz, 2H, furylH), 4.22 (s, 2H, NCH₂N), 4.07 (s, 2H, =CCH₂N), 3.76 (s, 2H, CH₂furyl), 3.67 (br s, 2H, NCH₂-6), 3.11 (br s, 2H, ArCH₂-7); ¹³C NMR (100 MHz, CDCl₃) δ 150.6, 147.6, 142.8, 137.4, 126.6, 124.8, 124.5, 120.7, 119.9, 119.8, 116.4, 112.7, 110.4, 109.4, 70.0, 52.3, 50.3, 49.2, 20.2; IR (neat, cm^–1) 3304, 1490, 1330, 1081; HRMS (ESI-Q-TOF) m/z: [M + H]⁺ calcd for C₁₉H₁₉N₄O₃ [M + H]⁺ 351.1457; found 351.1443.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b02957.

• Copies of ¹H NMR, ¹³C NMR, and HRMS spectra of compounds 7e, 7f, 8a,b, 8d, 11a,b, 13a–c, 13g, 14a, 14d, 14g, 17a–c, 17g, 19a, 20c, 20e, 21a–c (PDF)

Author Contributions

^§ A.A. and P.I. contributed equally to this work.

The authors declare no competing financial interest.