Brønsted acid-mediated mono- and di-substitution of quinoxalines with indoles: a pathway to indolocarbazole-quinoxaline scaffolds

Abstract

A versatile and efficient protocol for the mono- and di-substitution of quinoxalines with indoles has been developed, offering a direct pathway to indolocarbazole-quinoxaline scaffolds. By optimizing the reaction conditions, selective coupling at the C-2 and C-3 positions of quinoxalines with diverse indole derivatives was achieved under transition metal-free conditions. Substrate scope evaluation revealed broad functional group tolerance and the synthetic utility was demonstrated by gram-scale synthesis and subsequent cyclization into novel indolocarbazole-quinoxalines. Mechanistic studies suggest an ionic pathway, highlighting the potential of this method for constructing biologically relevant heterocyclic architectures.

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Alkaloids are naturally occurring nitrogen-containing organic compounds found in diverse sources, including plants, fungi, animals, insects, marine organisms, and microorganisms.¹ These compounds are vital in modern medicinal applications, exhibiting diverse therapeutic properties. Among them, indolocarbazoles form an important class of alkaloids with a broad spectrum of biological activities, such as antimicrobial, anti-HIV, anticancer, protein kinase C inhibition, and antitumor effects.² Staurosporine, the first indolocarbazole family member to be successfully isolated, was obtained from Streptomyces staurosporeus in 1977 and demonstrated potent hypotensive and antimicrobial properties.³ Since then, extensive research has been focused on the synthesis of indolocarbazoles and explored their potential as drug candidates.⁴ Indolocarbazole-quinoxalines (ICQs) are a subclass of indolocarbazoles where the indole and quinoxaline units are connected through a fused ring system, typically forming a carbazole scaffold. These molecules act as synthetic receptors and chemosensors for anion recognition,⁵ which are critical in numerous chemical, biological, and environmental processes.⁶ The Yan group reported the synthesis of ICQs in 2008, and highlighted their efficiency in sensing acetate and fluoride anions.⁷ They initiated their reaction with indole and 2-(3-indolyl)-2-oxoacetyl chloride, to synthesize diindolylquinoxalines (DIQs) by following a previously reported procedure.⁸ The synthesis of the final ICQs involved a three-step reaction and purification process, which rendered the method both time-consuming and cost-inefficient.

In continuation of our efforts on the development of novel and efficient methods for the synthesis and functionalization of N-heterocycles,⁹ herein we report a Brønsted acid-mediated approach for the synthesis of biologically active indolocarbazole-quinoxaline scaffolds through the coupling of quinoxalines with indole derivatives. This method reduces the reaction to two steps, offering an improved yield and greater efficiency in the construction of these important molecules (Scheme 1).

Scheme 1 Synthesis of indolocarbazole-quinoxaline.

We initiated our studies by investigating the reaction of quinoxaline (1a) and indole (2a) under various conditions to optimize the synthesis of the desired products, as summarized in Table 1. Using HCl (1.2 equiv.) as a mediator in CH₃CN at 70 °C for 24 hours, the desired product, 2-(1H-indol-3-yl)quinoxaline (3aa), was isolated in 65% yield. We tested different acids and observed comparatively lower yields of product 3aa. In H₂SO₄ and HBr, we obtained 42% and 53% yields of 3aa, respectively, while with AlCl₃, no reaction was observed (Table 1, entries 2–4). Lowering the temperature to 15 °C and increasing HCl and indole to 2.0 equivalents each led to the formation of 2,3-di(1H-indol-3-yl)quinoxaline (4aa) with a 66% isolated yield, while 3aa was not formed under these conditions (Table 1, entry 5). We then explored alternative acids. Both H₂SO₄ and HBr provided lower yields of product 4aa. In H₂SO₄, no formation of 3aa was observed, while in HBr, only traces of 3aa were detected (Table 1, entries 6 and 7). Temperature variations significantly impacted the product distribution. At 0 °C, no formation of 3aa was observed, while 4aa was obtained in 47% yield (Table 1, entry 8). At 10 °C, traces of 3aa and a 48% yield of 4aa were observed (Table 1, entry 9). Increasing the temperature to 20 °C resulted in a 5% yield of 3aa and a 41% yield of 4aa (Table 1, entry 10). At room temperature, the reaction produced 49% of 3aa and 31% of 4aa (Table 1, entry 11). At 50 °C, 3aa was formed in 50% yield, with only trace amounts of 4aa detected (Table 1, entry 12). At 100 °C, the reaction yielded 59% of 3aa, but no formation of 4aa was observed (Table 1, entry 13). By adjusting the reaction temperature, the selectivity between products 3aa and 4aa could be effectively controlled. Detailed optimization studies showcasing the impact of solvent, temperature, catalyst, and reagents are available in the ESI† (see ESI,† for details).

Table 1 Optimization of the conditions^a

Entry	Deviations from standard conditions	Yield (%)
		3aa	4aa
a Reaction conditions: quinoxaline (1a) (0.3 mmol), indole (2a) (1 equiv.) and HCl (1.2 equiv.) in solvent at 70 °C, isolated yield w.r.t. 1a. b 2.0 equiv. HCl was used.
1	No deviations	65	—
2	H2SO4	42	—
3	HBr	53	—
4	AlCl3	nr	nr
5	15 °C and 2.0 equiv. 2a	—	66
6	H2SO4, 15 °C and 2.0 equiv. 2a	—	53
7	HBr, 15 °C and 2.0 equiv. 2a	Trace	30
8	0 °C and 2.0 equiv. 2a	Trace	47
9	10 °C and 2.0 equiv. 2a	Trace	48
10	20 °C and 2.0 equiv. 2a	5	41
11	rt and 2.0 equiv. 2a	49	31
12	50 °C and 2.0 equiv. 2a	50	Trace
13	100 °C and 2.0 equiv. 2a	59	—

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With the optimized reaction conditions established (Table 1 entry 1), we explored the substrate scope of quinoxaline and indole derivatives, as summarized in Scheme 2. Starting with indoles, we investigated substitutions at different positions. At the C-4 position, indoles bearing H, OH, and Br substitutions yielded the products 3aa–3ac in moderate to good yields (42–78%). Substituents at the C-5 position of indoles, whether electron-donating or electron-withdrawing, also resulted in good yields (59–83%) of products (3ad–3ag), while the nitro group significantly lowered the product (3ah) yield to 38%. Indoles substituted at the C-6 position with –OMe, –F, or –Cl gave the corresponding products 3ai–3ak in yields ranging from 54% to 71%. Similarly, C-7-substituted indoles with fluoro, chloro, or bromo groups provided the products 3al–3an in 54%, 40%, and 50% yields, respectively. Indoles having naphthalene, methyl, and phenyl substituents at the C-2 position also produced the products 3ao–3aq in 60%, 67%, and 49% yields, respectively. Di-substituted indoles, such as 1-methyl-1H-indole-5-carboxylic acid and 1-ethyl-2-methyl-1H-indole, yielded 3ar and 3as in 36% and 67% yields, respectively. Benzyloxy substitutions at the C-4 and C-7 positions afforded the products 3at and 3au in 76% and 68% yields. The structure of product 3au was further validated by single-crystal XRD analysis (CCDC 2410123†). Substituents at the N1 position of indole, including methyl, butyl, propionitrile and phenyl, provided the products 3av–3ay in moderate yields (39–62%). Moving to quinoxaline derivatives, which have methyl and hydroxy groups at the C-5 position provided the products 3az and 3ba in 44% and 42% yields. Quinoxalines having methyl, and fluoro, substituents at the C-6 position, produced the products 3bb and 3bc in 71% yields in each case. We further extended the same reaction conditions to quinoxaline-2(1H)-one (1b), and obtained the C-3 indole-coupled product 3bd in 57% yield. Recognizing the importance of C-3-hetero-substituted quinoxaline-2(1H)-ones,¹⁰ we examined the derivatives of quinoxaline-2(1H)-ones with N − 1 substituents such as methyl, ethyl, and benzyl. These derivatives provided the desired products 3be–3bg in good to excellent yields (73–80%). Substituted indoles with 1-methylquinoxaline-2(1H)-one (3c) were also tested. Methoxy (−OMe) and trifluoromethyl (−CF₃) groups at the C-5 position of indole gave the corresponding products 3bh and 3bi in 60% and 79% yields. Benzyloxy substitution at C-7 and naphthalene at the C-2 position of indoles afforded 3bj and 3bk in 51% and 53% yields, respectively. Finally, we attempted to extend the reaction conditions to other N-heterocycles, including 2-phenylimidazo[1,2-a] pyridine, 1H-benzo[d]imidazole, pyrazine, and benzo[d]thiazole; unfortunately, these substrates were unsuccessful, highlighting the specificity of the reaction conditions.

Scheme 2 Substrate scope of mono-substitution.^{a a}Reaction conditions: 1a (0.3 mmol), 2a (1 equiv.), and HCl (1.2 equiv.) in acetonitrile at 70 °C for 24 h, isolated yield. ^bThe reaction temp. is 80 °C and time 48 h.

Building on our optimized conditions (Table 1, entry 1), we proceeded to synthesize diindolylquinoxaline derivatives, as shown in Scheme 3. Quinoxaline reacted with indole to yield the disubstituted product 4aa in 66% yield. Under these conditions, to show the efficacy of the method, a few representative examples were presented (Scheme 3). Indoles having substituent groups (hydroxy, methoxy, bromo, chloro and methyl) at the C-4, C-5, C-6 and C-7 positions, reacted well and provided the corresponding products 4ab–4af in moderate to good yields (34–69%). One of the products 4aa has been further confirmed through single crystal XRD analysis (CCDC 2408003†). Extending the conditions to 1H-benzo[g]indole produced the product 4ag in 43% yield. We also explored the substituted quinoxalines. A 5-methylquinoxaline derivative reacted with indole to produce 4ah in 30% yield, while coupling with 5-methoxyindole yielded the desired product 4ai in 43% yield. Quinoxaline with electron-withdrawing substituents also performed well in this transformation, yielding the corresponding product 4aj in 80% yield.

Scheme 3 Substrate scope of di-substitution.^{a a}Reaction conditions: 1a (0.3 mmol), 2a (2.0 equiv.), and HCl (2 equiv.) in acetonitrile at 15 °C for 24 h, isolated yield.

After successfully establishing the substrate scope, we explored the synthetic utility of this methodology by aiming to cyclize diindolylquinoxaline derivatives into indolocarbazole-quinoxalines. Starting with 2,3-di(1H-indol-3-yl)-1,2,3,4-tetrahydroquinoxaline (4aa), we optimized the various parameters, including catalysts, oxidants, solvents, reaction time, and temperature (for details see Tables S4–S6 in the ESI†). Using 2.0 equivalents of DDQ as an oxidant, and 50 mol% p-TSA as a mediator, in benzene as the solvent at 80 °C, we obtained 86% isolated yield of the desired product 5,6-dihydrodiindolo[3,2-a:2′,3′-c] phenazine (5aa) in 12 hours (Scheme 4). Furthermore, the cyclization of other derivatives with substitutions on both indole as well as quinoxaline provided the desired products 5ab–5ad in good yields (58–75%) demonstrating the versatility of the reaction conditions. To evaluate the practical utility of this protocol, we scaled up the synthesis of both the mono-substituted product (3aa) and the disubstituted product (4aa) to gram-scale quantities. The reactions yielded 47% and 37%, respectively, as shown in Scheme 5A.

Scheme 4 Substrate scope of indolocarbazole-quinoxalines.^{a a}Reaction conditions: 4aa (0.1 mmol), DDQ (2 equiv.), p-TSA (0.5 equiv.) in benzene at 80 °C for 12 h, isolated yield.

Scheme 5 (A) Gram-scale synthesis and (B) control experiments.

To gain insight into the reaction mechanism, we carried out control experiments (Scheme 5B). The use of radical scavengers such as TEMPO and BHT slightly affected the reaction outcome, resulting in 60% and 55% yields of 3aa, respectively. This suggests that the reaction likely proceeds through an ionic pathway rather than a radical one.

Based on control experiments and literature reports,¹¹ we proposed a plausible reaction mechanism (Scheme 6). In the presence of HCl, compound 1a forms an intermediate A. A nucleophilic attack by 2a on A leads to the formation of intermediate B, which is confirmed by HRMS analysis (see Fig S2 under ESI†). Upon oxidation of intermediate B, the desired product 3aa was produced. At low temperatures, an additional molecule of 2a reacts with intermediate B, resulting in the formation of 2,3-di(1H-indol-3-yl)-1,2,3,4-tetrahydroquinoxaline (4aa) as the final product. Furthermore, product 4aa can be readily converted into the cyclized product 5aa through oxidative cyclization.

Scheme 6 Plausible reaction mechanism.

This study provides a practical and scalable method for synthesizing mono- and di-indole-substituted quinoxalines and their cyclized indolocarbazole derivatives. The developed protocol exhibits high functional group compatibility, enabling the synthesis of complex heterocyclic frameworks with moderate to good yields. Gram-scale synthesis underscores the method's applicability, while mechanistic investigations confirm the ionic nature of the transformation. This work lays a foundation for further exploration of indolocarbazole-quinoxalines in medicinal and material science applications, showcasing the power of this straightforward and adaptable approach.

CSIR-CSMCRI, Communication No. 207/2024. We are thankful to “Marine Element & Marine Environment and Centralized Instrumental Facilities” for providing instrumentation facilities. M. B. V. is thankful to the University Grants Commission, New Delhi, India for the award of a Senior Research Fellowship. We are also thankful to CSIR-CSMCRI (MLP-074 and HCP-049) for partial financial support.

Data availability

The data supporting this article have been included as part of the ESI† (NMR and HRMS spectra). Crystallographic data for the 3au and 4aa compounds has been deposited at the CCDC under 2410123 and 2408003 correspondingly.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2410123 and 2408003. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4cc06606e

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