Synthesis of the oxindole alkaloid (−)-horsfiline

Indole C5-Selective Bromination of Indolo[2,3-a]quinolizidine Alkaloids via In Situ-Generated Indoline Intermediate

There are many indole alkaloids that contain diverse functional groups attached to the benzene ring on the indole core. Promising biological activities of these alkaloids have been reported. Herein, we report the indole C5-selective bromination of indolo[2,3-a]quinolizidine alkaloids by adding nearly equimolar amounts of Br3 ⋅ PyH and HCl in MeOH. The resulting reaction plausibly proceeds through an indoline intermediate by the nucleophilic addition of MeOH to the C3-brominated indolenine intermediate. Data support the intermediacy of a C3-, C5-dibrominated indolenine intermediate as a brominating agent. These conditions demonstrate excellent selectivity for indole C5 bromination of natural products and their derivatives. Thus, these simple, mild, and metal-free conditions allow for selective, late-stage bromination followed by further chemical modifications. The utility of the brominated product prepared from naturally occurring yohimbine was demonstrated through various derivatizations, including a bioinspired heterodimerization reaction.

Copper-Catalyzed Tunable Oxygenative Rearrangement of Tetrahydrocarbazoles

Herein, we report a general copper-catalyzed method for the tunable oxygenative rearrangement of tetrahydrocarbazoles to cyclopentyl-bearing spiroindolin-2-ones and spiroindolin-3-ones. The method demonstrates excellent chemoselectivity, regioselectivity, and product control simply by using the H2O and O2 as oxygen source, respectively. This open-flask method is safe and simple to operate, and no other chemical oxidants are required. Besides, inspired from the unique pathway of 1, 2-migration rearrangement, a highly controllable hydroxylation of indoles for the construction of C3a-hydroxyl iminium indolines was also developed. Mechanistic experiments suggest that a single-electron transfer-induced oxidation process is responsible for the tunable selectivity control.

Chemo-/Regio-Selective Synthesis of Novel Functionalized Spiro[pyrrolidine-2,3'-oxindoles] under Microwave Irradiation and Their Anticancer Activity

A novel series of nitrostyrene-based spirooxindoles were synthesized via the reaction of substituted isatins 1a-b, a number of α-amino acids 2a-e and (E)-2-aryl-1-nitroethenes 3a-e in a chemo/regio-selective manner using [3+2] cycloaddition (Huisgen) reaction under microwave irradiation conditions. The structure elucidation of all the synthesized spirooxindoles were done using 1H and 13C NMR and HRMS spectral analysis. The single crystal X-ray crystallographic study of compound 4l was used to assign the stereochemical arrangements of the groups around the pyrrolidine ring in spiro[pyrrolidine-2,3'-oxindoles] skeleton. The in vitro anticancer activity of spiro[pyrrolidine-2,3'-oxindoles] analogs 4a-w against human lung (A549) and liver (HepG2) cancer cell lines along with immortalized normal lung (BEAS-2B) and liver (LO2) cell lines shows promising results. Out of the 23 synthesized spiro[pyrrolidine-2,3'-oxindoles], while five compounds (4c, 4f, 4m, 4q, 4t) (IC50 = 34.99-47.92 µM; SI = 0.96-2.43) displayed significant in vitro anticancer activity against human lung (A549) cancer cell lines, six compounds (4c, 4f, 4k, 4m, 4q, 4t) (IC50 = 41.56-86.53 µM; SI = 0.49-0.99) displayed promising in vitro anticancer activity against human liver (HepG2) cancer cell lines. In the case of lung (A549) cancer cell lines, these compounds were recognized to be more efficient and selective than standard reference artemisinin (IC50 = 100 µM) and chloroquine (IC50 = 100 µM; SI: 0.03). However, none of them were found to be active as compared to artesunic acid [IC50 = 9.85 µM; SI = 0.76 against lung (A549) cancer cell line and IC50 = 4.09 µM; SI = 2.01 against liver (HepG2) cancer cell line].

Copper-Catalyzed Aerobic Selective Oxidation of Tetrahydrocarbolines

We report a safe and convenient open-flask copper-catalyzed selective oxidation/functionalization methodology for tetrahydrocarbolines and tetrahydro-β-carbolines that employs atmospheric O₂ as the terminal oxidant. The system is applicable to oxidative rearrangement of tetrahydro-β-carbolines, tetrahydrocarboline oxidation to α-alkoxy carbazoles and spirooxindoles, and Witkop oxidation. Mechanistic experiments indicated that a single-electron oxidation process is responsible for the tunable selectivity control. This copper-catalysis protocol represents a significant advance in the field of indole oxidation.

Progress toward a biomimetic synthesis of pegaharmaline A

Efforts towards a biomimetic synthesis of the alkaloid pegaharmaline A began with attempted validation of the putative biosynthesis described in the isolation report. The reaction between vasicinone-derived pyrroloquinazoline 1 and tryptamines 2 and 9 proceeded under aqueous conditions at ambient temperature, forming the 1,6,10-triazaspiro[4.5]dec-7-anes 7 and 8. Alternative pyrroloquinazoline precursors were subsequently investigated; the reaction between dehydrodimethylisovasicinone (10) and tryptamine (9) led to the ring-opened product 13 that could not be converted into pegaharmaline A scaffold under Bischler-Napieralski conditions. The Pictet-Spengler reaction between a model isovasicinone (22) and tryptamine (9) was successful, but the resulting tetrahydro-β-carboline could not be converted into the natural product. These studies suggest an alternative biosynthetic pathway is potentially operating, while structural revision of the natural product cannot be ruled out at this time. As vasicinones and tryptamines are widely distributed throughout Nature, the novel scaffolds reported herein may be undiscovered natural products.

Design and synthesis of spirooxindole–pyrrolidines embedded with indole and pyridine heterocycles by multicomponent reaction: anticancer and in silico studies

We report the synthesis of spirooxindole–pyrrolidines tethered with indole and pyridine heterocycles using 1,3-dipolar cycloaddition, and their anticancer activities and molecular docking studies.

Theoretical Investigation on the Elusive Reaction Mechanism of Spirooxindole Formation Mediated by Cytochrome P450s: A Nascent Feasible Charge-Shift C-O Bond Makes a Difference

Spirooxindoles are pivotal biofunctional groups widely distributed in natural products and clinic drugs. However, construction of such subtle chiral skeletons is a long-standing challenge to both organic and bioengineering scientists. The knowledge of enzymatic spirooxindole formation in nature may inspire rational design of new catalysts. To this end, we presented a theoretical investigation on the elusive mechanism of the spiro-ring formation at the 3-position of oxindole mediated by cytochrome P450 enzymes (P450). Our calculated results demonstrated that the electrophilic attack of CpdI, the active species of P450, to the substrate, shows regioselectivity, i.e., the attack at the C9 position forms a tetrahedral intermediate involving an unusual feasible charge-shift C9^δ+-O^δ- bond, while the attack at the C1 position forms an epoxide intermediate. The predominant route is the first route with the charge-shift bonding intermediate due to holding a relatively lower barrier by >5 kcal mol^-1 than the epoxide route, which fits the experimental observations. Such a delocalized charge-shift bond facilitates the formation of a spiro-ring mainly through elongation of the C1-C9 bond to eliminate the aromatization of the tricyclic beta-carboline. Our theoretical results shed profound mechanistic insights for the first time into the elusive spirooxindole formation mediated by P450s.

TCCA-mediated oxidative rearrangement of tetrahydro-β-carbolines: facile access to spirooxindoles and the total synthesis of (±)-coerulescine and (±)-horsfiline

Multi-reactive centered reagents are beneficial in chemical synthesis due to their advantage of minimal material utilization and formation of less by-products. Trichloroisocyanuric acid (TCCA), a reagent with three reactive centers, was employed in the synthesis of spirooxindoles through the oxidative rearrangement of various N-protected tetrahydro-β-carbolines. In this protocol, low equivalents of TCCA were required to access spirooxindoles (up to 99% yield) with a wide substrate scope. Furthermore, the applicability and robustness of this protocol were proven for the gram-scale total synthesis of natural alkaloids such as (±)-coerulescine (1) and (±)-horsfiline (2) in excellent yields.

Three-reactive centered reagent (TCCA) mediated construction of spirooxindoles through an oxidative rearrangement of various N-protected tetrahydro-β-carbolines and total synthesis of natural alkaloids (±)-coerulescine and (±)-horsfiline.

Green oxidation of indoles using halide catalysis

Oxidation of indoles is a fundamental organic transformation to deliver a variety of synthetically and pharmaceutically valuable nitrogen-containing compounds. Prior methods require the use of either organic oxidants (meta-chloroperoxybenzoic acid, N-bromosuccinimide, t-BuOCl) or stoichiometric toxic transition metals [Pb(OAc)₄, OsO₄, CrO₃], which produced oxidant-derived by-products that are harmful to human health, pollute the environment and entail immediate purification. A general catalysis protocol using safer oxidants (H₂O₂, oxone, O₂) is highly desirable. Herein, we report a unified, efficient halide catalysis for three oxidation reactions of indoles using oxone as the terminal oxidant, namely oxidative rearrangement of tetrahydro-β-carbolines, indole oxidation to 2-oxindoles, and Witkop oxidation. This halide catalysis protocol represents a general, green oxidation method and is expected to be used widely due to several advantageous aspects including waste prevention, less hazardous chemical synthesis, and sustainable halide catalysis.

Indole oxidation represents a fundamental organic transformation delivering valuable nitrogen compounds. Here, the authors report a general halide catalysis protocol applied to three classes of oxidation reactions of indoles with oxone as a sustainable terminal oxidant.

Synthesis of 1′-tetrazolylmethyl-spiro[pyrrolidine-3,3′-oxindoles] via two coupled one-pot processes Ugi-azide/Pictet–Spengler and oxidative spiro-rearrangement

Synthesis of bis- and tris-heterocycles via one-pot I-MCR/PS cyclization/further one-pot oxidative spiro-rearrangement.

Spiro[pyrrolidine-3,3'-oxindoles] and Their Indoline Analogues as New 5-HT6 Receptor Chemotypes

Synthetic derivatives of spiro[pyrrolidinyl-3,3′-oxindole] alkaloids (coerulescine analogues) were investigated as new ligands for aminergic G-protein coupled receptors (GPCRs). The chemical starting point 2′-phenylspiro[indoline-3,3′-pyrrolidin]-2-one scaffold was identified by virtual fragment screening utilizing ligand- and structure based methods. As a part of the hit-to-lead optimization a structure-activity relationship analysis was performed to explore the differently substituted 2′-phenyl-derivatives, introducing the phenylsulphonyl pharmacophore and examining the corresponding reduced spiro[pyrrolidine-3,3′-indoline] scaffold. The optimization process led to ligands with submicromolar affinities towards the 5-HT₆ receptor that might serve as viable leads for further optimization.

Spiro[pyrrolidine-3, 3´-oxindole] as potent anti-breast cancer compounds: Their design, synthesis, biological evaluation and cellular target identification

The spiro[pyrrolidine-3, 3´-oxindole] moiety is present as a core in number of alkaloids with substantial biological activities. Here in we report design and synthesis of a library of compounds bearing spiro[pyrrolidine-3, 3´-oxindole] motifs that demonstrated exceptional inhibitory activity against the proliferation of MCF-7 breast cancer cells. The synthesis involved a one pot Pictet Spengler-Oxidative ring contraction of tryptamine to the desired scaffolds and occurred in 1:1 THF and water with catalytic trifluoroacetic acid and stoichiometric N-bromosuccinimide as an oxidant. Phenotypic profiling indicated that these molecules induce apoptotic cell death in MCF-7 cells. Target deconvolution with most potent compound 5l from the library, using chemical proteomics indicated histone deacetylase 2 (HDAC2) and prohibitin 2 as the potential cellular binding partners. Molecular docking of 5l with HDAC2 provided insights pertinent to putative binding interactions.

Synthesis, and QSAR analysis of anti-oncological active spiro-alkaloids

QSAR study describes the anti-neoplastic spiro-alkaloids with relevant molecular descriptors using CODESSA III software. The dispiro[3H-indole-3,2'-pyrrolidine-3',3"-piperidines] 24-48 were synthesized via [3 + 2]-cycloaddition reaction of azomethine ylides, (generated in situ via decarboxylative condensation of isatins 21-23 with sarcosine) and 3E,5E-1-alkyl-3,5-bis(arylmethylidene)-4-piperidones 10-20. Some of the synthesized analogues exhibited promising antitumor properties against HELA (cervical), HEPG2 (liver), T-47D, MCF7 (breast), and HCT116 (colon) human tumor cell lines, demonstrating activity close to or even better than the standard Doxorubicin, based on in vitro Sulfo-Rhodamine-B bio-assay.

Asymmetric organocatalyzed Michael addition of nitromethane to a 2-oxoindoline-3-ylidene acetaldehyde and the three one-pot sequential synthesis of (-)-horsfiline and (-)-coerulescine

(-)-Horsfiline and (-)-coerulescine were synthesized through three one-pot operations in 33 and 46% overall yield, respectively. Key to the success was the efficient use of a diarylprolinol silyl ether to catalyze the asymmetric Michael addition of nitromethane to a 2-oxoindoline-3-ylidene acetaldehyde. This allowed the all-carbon quaternary, spirocyclic carbon stereocenter to be constructed in good yield with excellent enantioselectivity.

Efficient enantioselective total synthesis of (-)-horsfiline

A new efficient and concise enantioselective synthetic method for (-)-horsfiline is reported. (-)-Horsfiline could be obtained from diphenylmethyl tert-butyl malonate in 9 steps (32%,>99% ee) by using the enantioselective phase-transfer catalytic allylation (91% ee) as the key step. This approach can be applied as a practical route for the large-scale synthesis of spirooxindole natural products, which enables a systematic investigation of their biological activity to be performed.

Asymmetric allylic alkylation of isatin-derived Morita-Baylis-Hillman carbonates with nitroalkanes

A stereoselective allylic alkylation of isatin-derived Morita-Baylis-Hillman (MBH) carbonates with nitroalkanes has been developed. In the presence of 10 mol % β-isocupreidine (β-ICD), 3,3'-disubstituted oxindoles were prepared with moderate diastereoselectivities and excellent enantioselectivities.

Synthesis and QSAR study of novel cytotoxic spiro[3H-indole-3,2'(1'H)-pyrrolo[3,4-c]pyrrole]-2,3',5'(1H,2'aH,4'H)-triones

1,3-Dipolar cycloaddition reaction of 1-aryl-1H-pyrrole-2,5-diones 1a-e with non-stabilized azomethine ylides, generated in situ via decarboxylative condensation of isatins 2a-c and sarcosine (3) in refluxing ethanol, afforded 4'-aryl-5'a,6'-dihydro-1'-methyl-spiro[3H-indole-3,2'(1'H)-pyrrolo[3,4-c]pyrrole]-2,3',5'(1H,2'aH,4'H)-triones 4a-o in good yields. Compound 4l exhibited high anti-tumor activity against HEPG2 (liver cancer) cell line (IC(50) = 12.16 μM) compared to that of Doxorubicin (IC(50) = 7.36 μM), and the other synthesized compounds revealed moderate anti-tumor properties against HCT116 (colon), MCF7 (breast) and HEPG2 (liver) human tumor cell lines. 3D-Pharmacophore modeling and quantitative structure-activity relationship (QSAR) analysis were combined to explore the structural requirements controlling the observed anti-tumor properties. It was found that the major structural factors affecting potency of these compounds were related to their basic skeleton.

Total synthesis of (±)-coerulescine and (±)-horsfiline

Wittig olefination-Claisen rearrangement protocol was applied to obtain 3-allyl oxindole. This oxindole was then converted to (±)-coerulescine and (±)-horsfiline.

Tandem intramolecular photocycloaddition-retro-Mannich fragmentation as a route to spiro[pyrrolidine-3,3'-oxindoles]. Total synthesis of (+/-)-coerulescine, (+/-)-horsfiline, (+/-)-elacomine, and (+/-)-6-deoxyelacomine

Irradiation of a tryptamine linked through its side-chain nitrogen to an alkylidene malonate residue results in an intramolecular [2 + 2] cycloaddition to the indole 2,3-double bond. The resultant cyclobutane undergoes spontaneous retro-Mannich fission to produce a spiro[indoline-3,3'-pyrrolenine] with relative configuration defined by the orientation of substituents in the transient cyclobutane. The tandem intramolecular photocycloaddition-retro-Mannich process, abbreviated as TIPCARM, leads to a spiropyrrolidine which is poised to undergo a second retro-Mannich fragmentation that expels the malonate unit and generates transiently an indolenine. The latter undergoes rearrangement to a beta-carboline, which upon brominative oxidation undergoes further rearrangement to an oxindole. With tryptamine as starting material, the entire sequence leads to the alkaloid (+/-)-coerulescine. Starting from 5-methoxytryptamine, a parallel series affords (+/-)-horsfiline. Modification of the malonylidene unit to include an isobutyl substituent at C3 affords a photosubstrate which also undergoes the TIPCARM process. In this case, a 2'-isobutyl-substituted spiro[indoline-3,3'-pyrrolenine] results. This undergoes stereoselective hydride reduction to give a product with relative orientation at the spiro carbon and the new stereocenter bearing the isobutyl appendage corresponding to that of the alkaloid elacomine. From tryptamine, the sequence paralleling that leading to coerulescine and horsfiline terminates at 6-deoxyelacomine, whereas 6-methoxytryptamine as starting material affords (+/-)-elacomine itself.

Pyrrolidinyl-spirooxindole natural products as inspirations for the development of potential therapeutic agents

The 3,3'-pyrrolidinyl-spirooxindole unit is a privileged heterocyclic motif that forms the core of a large family of alkaloid natural products with strong bioactivity profiles and interesting structural properties. Significant recent advances in the synthesis of this fused heterocyclic system have led to intense interest in the development of related compounds as potential medicinal agents or biological probes.