Continuous Flow Synthesis of Heterocycles: A Recent Update on the Flow Synthesis of Indoles

Continuous flow synthesis of N,N-dimethyltryptamine (DMT) analogues with therapeutic potential

Herein, we describe the continuous flow synthesis and in-line extraction of N,N-dimethyltryptamine (DMT) and several of its analogues using a Fischer indole reaction, along with a larger gram scale synthesis (4.75 g) of the model compound. These products could then be quickly transformed into their respective fumarate salts, making them easier to handle and stable for long time storage using a straightforward batch procedure. Additionally, the commercially available drug rizatriptan benzoate could be synthesised with high purity using this setup. The presented method employs relatively green solvents both for the synthesis and purification of the target products.

Recent advances in electrochemically enabled construction of indoles from non-indole-based substrates

Indole motifs are important heterocycles found in natural products, pharmaceuticals, agricultural chemicals, and materials. Although there are well-established classical name reactions for indole synthesis, these transformations often require harsh reaction conditions, have a limited substrate scope, and exhibit poor regioselectivity. As a result, organic synthesis chemists have been exploring efficient and practical methods, leading to numerous strategies for synthesizing a variety of functionalized indoles. In recent years, electrochemistry has emerged as an environmentally friendly and sustainable synthetic tool, with widespread applications in organic synthesis. This technology allows for elegant synthetic routes to be developed for the construction of indoles under external oxidant-free conditions. This feature article specifically focuses on recent advancements in indole synthesis from non-indole-based substrates, as well as the mechanisms underlying these transformations.

Iodine-mediated oxidative triple functionalization of indolines with azoles and diazonium salts

We report an innovative synthetic strategy for the generation of polysubstituted indoles from indolines, aryldiazonium salts, and azoles. The methodology encompasses an electrophilic substitution reaction affording C5-indoline intermediates which undergo an iodine-mediated oxidative transformation coupled with C-H functionalization to yield the indole derivatives.

Exploration of Indolo[3,2c]isoquinoline derived triazoles as potential antimicrobial and DNA cleavage agents: Synthesis, DFT calculations, and molecular modeling studies

Indole and its derivatives are well-known assorted motif in drug design and development. We here in reporting synthesis of new 9-chloro-1-(4-substituted phenyl)-12H-indolo[2,3-c][1,2,4]triazolo[3,4-a]isoquinolines 7 (a-h). Structures of the newly synthesized compounds were confirmed by making use of spectroscopic techniques like IR, NMR and Mass. The DFT calculations were taken for the selected molecules using CAM-B3LYP hybrid functional with a 6-31 + g(d) all-electron basis set using the Gaussian 09 package. The drug-likeness predictions were described for the synthesized derivatives. The In vitro antimicrobial and DNA cleavage activities were reported for all compounds 7 (a-h). The compounds 7a, 7b, and 7h showed excellent microbial inhibition and DNA cleavage activity as compared to standard drugs. Furthermore, the docking studies for the newly synthesized molecules were carried out by Auto dock software with two molecular targets Epidermal Growth Factor Receptor tyrosine kinase (1 M17) and C-kit Tyrosine Kinase (1 T46) exhibited better binding affinity of all synthesized compounds. In addition, the docking results were observed to be in full agreement with the in vitro DNA cleavage assay suggesting the potential of synthesized metal complexes in biological applications. Lastly, the protein stability, fluctuations of APO-Protein, and protein-ligand complexes were investigated through Molecular Dynamics (MD) simulations studies using Desmond Maestro 11.3 and potential lead molecules were identified.

Rapid and efficient syntheses of tryptophans using a continuous-flow quaternization-substitution reaction of gramines with a chiral nucleophilic glycine equivalent

A continuous-flow quaternization reaction of gramines with MeI (<1 min) followed by a substitution reaction with a chiral nucleophilic glycine-derived Ni-complex (S)-2 (<1 min) has successfully been developed to afford the corresponding alkylated Ni-complexes 3 in good yields with excellent diastereoselectivity, based on the results of a one-pot quaternization-substitution reaction of gramines with (S)-2 in a batch process. The continuous-flow process allowed the safe and efficient scale-up synthesis of 3j (84% yield, 99% de, 540 g h^-1) to give 7-azatryptophan derivative (S)-4j readily by an acid-catalyzed hydrolysis reaction followed by protection with an Fmoc group. The present method for the rapid and efficient syntheses of enantiopure unnatural tryptophan derivatives from various gramines and (S)-2 will be useful to further promote peptide and protein drug discovery and development research.

Intermolecular C-H Aminocyanation of Indoles via Copper-iodine Cocatalyzed Tandem C-N/C-C Bond Formation

An efficient copper-iodine cocatalyzed intermolecular C-H aminocyanation of indoles with a broad substrate scope has been developed for the first time. This method enables highly step-economic access to 2-amino-3-cyanoindoles in moderate to good yields and provides a complementary strategy for the regioselective difunctionalization of carbon═carbon double bonds of interest in organic synthesis and related areas. Mechanistic studies suggest that these transformations are initiated by iodine-mediated C2-H amination with azoles, followed by copper-catalyzed C3-H cyanation with ethyl cyanoformate.

Development and Characterization of Indole-Responsive Whole-Cell Biosensor Based on the Inducible Gene Expression System from Pseudomonas putida KT2440

Indole is a biologically active compound naturally occurring in plants and some bacteria. It is an important specialty chemical that is used as a precursor by the pharmaceutical and chemical industries, as well as in agriculture. Recently, indole has been identified as an important signaling molecule for bacteria in the mammalian gut. The regulation of indole biosynthesis has been studied in several bacterial species. However, this has been limited by the lack of in vivo tools suitable for indole-producing species identification and monitoring. The genetically encoded biosensors have been shown to be useful for real-time quantitative metabolite analysis. This paper describes the identification and characterization of the indole-inducible system PpTrpI/P_{PP_RS00425} from Pseudomonas putida KT2440. Indole whole-cell biosensors based on Escherichia coli and Cupriavidus necator strains are developed and validated. The specificity and dynamics of biosensors in response to indole and its structurally similar derivatives are investigated. The gene expression system PpTrpI/P_{PP_RS00425} is shown to be specifically induced up to 639.6-fold by indole, exhibiting a linear response in the concentration range from approximately 0.4 to 5 mM. The results of this study form the basis for the use of whole-cell biosensors in indole metabolism-relevant bacterial species screening and characterization.

Recent advances in continuous flow synthesis of heterocycles

In the current scenario, flow chemistry is emerging as a significant technology in the field of organic synthesis. This miniaturized protocol including microreactors facilitates excellent heat transfer, low solvent wastage, lesser reaction time, a safer environment for reagent handling and appreciable yields of desired products. Thus, this "enabling technology" has a great scope in the synthesis and preparation of a variety of heterocycles that require toxic reagents as starting materials. This review discusses the recent advances (2020-2021) in continuous flow strategy for synthesis and derivatization of variety of heterocyclic entities, of different ring size, using different approaches. This also highlights the advantages of different combined techniques like Microwave assisted heating, electrochemical flow cell, LED light source, NMR and FT-IR analysis, etc., that enables utilization of various mechanisms and real-time monitoring of reactions leading to improved results.

Development of a Continuous Flow Synthesis of 2-Substituted Azetines and 3-Substituted Azetidines by Using a Common Synthetic Precursor

The generation and functionalization, under continuous flow conditions, of two different lithiated four-membered aza-heterocycles is reported. N-Boc-3-iodoazetidine acts as a common synthetic platform for the genesis of C3-lithiated azetidine and C2-lithiated azetine depending on the lithiation agent. Flow technology enables easy handling of such lithiated intermediates at much higher temperatures compared to batch processing. Flow technology combined with cyclopentylmethyl ether as an environmentally responsible solvent allows us to address sustainability concerns.

Synthesis of 2-Substitued Indoles via Pd-Catalysed Cyclization in an Aqueous Micellar Medium

The synthesis of 2-substituted indoles starting from the corresponding unprotected 2-alkynylanilines was made possible in 3% TPGS-750-M water using Pd(OAc)2 alone as the catalyst. The reaction was sensitive to the heating mode respect to the nature of the starting material as, in many cases, convectional heating was better than microwave dielectric heating. The MW (microwave) delivery mode had also an influence in the formation of by-products and, consequently, product yields. A tandem Sonogashira-cyclisation reaction was also accomplished using Pd(OAc)2/Xphos in the nanomicellar water environment.

Design, Synthesis, Anticancer Activity, and Solid Lipid Nanoparticle Formulation of Indole- and Benzimidazole-Based Compounds as Pro-Apoptotic Agents Targeting Bcl-2 Protein

Cancer is a multifactorial disease necessitating identification of novel targets for its treatment. Inhibition of Bcl-2 for triggered pro-apoptotic signaling is considered a promising strategy for cancer treatment. Within the current work, we aimed to design and synthesize a new series of benzimidazole- and indole-based derivatives as inhibitors of Bcl-2 protein. The market pan-Bcl-2 inhibitor, obatoclax, was the lead framework compound for adopted structural modifications. The obatoclax’s pyrrolylmethine linker was replaced with straight alkylamine or carboxyhydrazine methylene linkers providing the new compounds. This strategy permitted improved structural flexibility of synthesized compounds adopting favored maneuvers for better fitting at the Bcl-2 major hydrophobic pocket. Anti-cancer activity of the synthesized compounds was further investigated through MTT-cytotoxic assay, cell cycle analysis, RT-PCR, ELISA and DNA fragmentation. Cytotoxic results showed compounds 8a, 8b and 8c with promising cytotoxicity against MDA-MB-231/breast cancer cells (IC₅₀ = 12.69 ± 0.84 to 12.83 ± 3.50 µM), while 8a and 8c depicted noticeable activities against A549/lung adenocarcinoma cells (IC₅₀ = 23.05 ± 1.45 and 11.63 ± 2.57 µM, respectively). The signaling Bcl-2 inhibition pathway was confirmed by molecular docking where significant docking energies and interactions with key Bcl-2 pocket residues were depicted. Moreover, the top active compound, 8b, showed significant upregulated expression levels of pro-apoptotic/anti-apoptotic of genes; Bax, Bcl-2, caspase-3, -8, and -9 through RT-PCR assay. Improving the compound’s pharmaceutical profile was undertaken by introducing 8b within drug-solid/lipid nanoparticle formulation prepared by hot melting homogenization technique and evaluated for encapsulation efficiency, particle size, and zeta potential. Significant improvement was seen at the compound’s cytotoxic activity. In conclusion, 8b is introduced as a promising anti-cancer lead candidate that worth future fine-tuned lead optimization and development studies while exploring its potentiality through in-vivo preclinical investigation.