Synthesis of (±)-Strychnofoline via a Highly Convergent Selective Annulation Reaction

Computational Insights into "Lone Pair-Lone Pair Interaction-Controlled" Isomerization in the Asymmetric Total Syntheses of (+)-3-(Z)-Laureatin and (+)-3-(Z)-Isolaureatin

Described herein is our computational study to rationalize the stereoselective epimerization of α,α'-cis-disubstituted oxolane and oxetane ketones 6 and 7 to the corresponding α,α'-trans ketones 8 and 9 reported in our previous total syntheses of (+)-3-(Z)-isolaureatin (1) and (+)-3-(Z)-laureatin (2). Density functional theory (DFT) calculations using appropriately truncated models revealed that the α,α'-trans ketones are more stable than the α,α'-cis ketones, in very good agreement with experimental results. The computational results showed that the isomer with a longer interatomic distance between the two ring oxygen atoms was lower in energy, which suggested the presence of repulsive interactions between those oxygen atoms. Support for these distance-dependent repulsive interactions came from the observation that the energy differences between the two isomers correlated with the solvent polarity. Most importantly, a larger interoxygen distance difference could be responsible for the enhanced stereoselectivity for epimerization of oxetane ketone 7 compared to oxolane ketone 6 [0.25 Å (trans only) vs 0.13 Å (trans/cis = 4:1)].

Unified Strategy Enables the Collective Syntheses of Structurally Diverse Indole Alkaloids

Natural products and their analogues are significant sources of therapeutic lead compounds. However, synthetic strategies for generating large collections of these molecules remain a significant challenge. The most difficult step in their synthesis is the design of a common intermediate that can be easily transformed into natural products belonging to different families. This study demonstrates the evolution of synthetic tactics designed to assemble the functionalized piperidines present in indole alkaloids from a common intermediate. More importantly, we also report a previously unknown Ir- and Er-catalyzed dehydrogenative spirocyclization reaction that enables direct access to spirocyclic oxindole alkaloids. As a practical application, the asymmetric total syntheses of 29 natural alkaloids belonging to different families were accomplished by following a uniform synthetic route. The proposed methodology extends the capability of the iridium-catalyzed dehydrogenative coupling reaction to the realm of indole-alkaloid synthesis and provides new opportunities for the efficient preparation of natural product-like molecules.

Discovery and Biomimetic Semisynthesis of Spirophyllines A-D from Uncaria rhynchophylla

Spirophyllines A-D (1-4), four new spirooxindole alkaloids all characterized by the spiro[pyrrolidin-3,3'-oxindole] core and a rare isoxazolidine ring, were isolated from Uncaria rhynchophylla. Their structures were determined by spectroscopic methods and confirmed by X-ray crystallography. Based on the biomimetic semisynthesis strategy, compounds 1-8 were synthesized in three steps via the key reactions of 1,3-dipolar cycloaddition and Krapcho decarboxylation from corynoxeine. Interestingly, compound 3 showed moderate inhibitory activity against the Kv1.5 potassium channel (IC50 = 9.1 μM).

Substrate-Controlled Regioselectivity Switchable [3 + 2] Annulations To Access Spirooxindole Skeletons

The additive-free [3 + 2] annulation from isatins, amino acids with 2-styrylbenzoxazoles, was described, providing a series of functional and structurally complex 3,3'-pyrrolidinyl-spirooxindole derivatives containing four contiguous and two quaternary stereogenic centers in high yields (up to 95%) and excellent diastereoselectivities (up to >25:1 dr). Interestingly, the reaction exhibits switchable regioselectivity depending on the substrate of amino acids. With proline or thioproline as the substrate, the reaction afforded α-regioselective spirooxindole skeletons. In contrast, when piperidine acid is the substrate, the reaction provided γ-regioselective spirooxindole skeletons.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.

I(III)-Catalyzed Oxidative Cyclization-Migration Tandem Reactions of Unactivated Anilines

An I(III)-catalyzed oxidative cyclization-migration tandem reaction using Selectfluor as the oxidant was developed that converts unactivated anilines into 3H-indoles is reported herein. The reaction requires as little as 1 mol % of the iodocatalyst and is mild, tolerating pyridine and thiophene functional groups, and the dependence of the diastereoselectivity of the process on the identity of the iodoarene or iodoalkane precatalyst suggests that the catalyst is present for the stereochemical determining C-N bond forming step.

Divergent synthesis of oxindole derivatives via controllable reactions of isatin-derived para-quinone methides with sulfur ylides

A catalyst-free and controllable reaction of isatin-derived para-quinone methides with sulfur ylides was developed, and it enables the divergent synthesis of two valuable oxindoles.

Chameleon-Like Activating Nature of the Spirooxindole Group in Donor-Acceptor Cyclopropanes

The concept of a chameleon activating group is considered in the context of donor-acceptor cyclopropane chemistry. When spiro-conjugated with cyclopropane, oxindole can act as an acceptor or a donor depending on the electronic nature of vicinal substituents. This dichotomy is reflected in the alteration of chemoselectivity of spiro[oxindole-1,3'-cyclopropane] ring opening with nucleophiles.

Diastereoselective Sonochemical Synthesis of Spirocyclopropaneoxindoles and Evaluation of Their Antioxidant and Cytotoxic Activities

An efficient diastereoselective synthesis of spirocyclopropaneoxindoles is reported using three-component reactions of various phenacylidenetriphenylphosphorane, isatins and phenacyl bromide under ultrasonic irradiation. The structures of synthesized spirocyclopropaneoxindoles were characterized by their spectral data. The antioxidant activities of the synthesized compounds were evaluated by 1,1-diphenyl-2-picrylhydrazyl radical scavenging assay. Among the products, those with NH group in their structure exhibited higher antioxidant activities than other derivatives. Also, in vitro cytotoxicity of compounds 4b, 4e, 4j, 4k were examined against heLa cancer cell lines using MTT assay. The results revealed that compound 4j with chlorine substituent on phenyl group displayed higher cytotoxicity activity (IC₅₀ =4.50±0.30 μg/mL) after 48 h.

Stereocontrolled [3+2] Cycloaddition of Donor-Acceptor Cyclopropanes to Iminooxindoles: Access to Spiro[oxindole-3,2'-pyrrolidines]

A novel stereocontrolled assembly of spiro[oxindole-3,2'-pyrrolidines] via [3+2]-cycloaddition of donor-acceptor cyclopropanes to electron-poor ketimines, iminooxindoles, was developed. The method allows for efficient employment of common readily available donor-acceptor cyclopropanes, functionalized with ester, keto, nitro, cyano etc. groups, and N-unprotected iminooxindoles. The stereospecificity of the initial S_N2-like imine attack on a cyclopropane molecule together with a high diastereoselectivity of further C-C bond formation facilitate a rapid access to spiro[oxindole-3,2'-pyrrolidines] in their optically active forms. Preliminary in vitro testing of the synthesized compounds against LNCaP (p53+) and PC-3 (p53-) cells revealed good antiproliferative activities and p53-selectivity indices for several compounds that are intriguing in terms of their further investigation as inhibitors of MDM2-p53 interaction.

Lewis acid catalyzed annulation of spirocyclic donor–acceptor cyclopropanes with exo-heterocyclic olefins: access to highly functionalized bis-spirocyclopentane oxindole frameworks

Oxindole-activated spiro-DAC is used as synthon to prepare complex bis-spirocyclopentane oxindole frameworks via ring-enlargement.

Solvent-Controlled, Tunable Domino Reaction of 3-Ylideneoxindoles with in Situ-Generated α-Aryldiazomethanes: A Facile Access to 3-Spirocyclopropyl-2-oxindole and Pyrazoloquinazolinone Scaffolds

A mild and efficient solvent-controlled, metal-free switchable 1,3-dipolar cycloaddition/ring contraction or ring expansion domino reaction of 3-ylideneoxindoles with in situ-generated α-aryldiazomethanes has been developed. This domino reaction provided a series of aryl-substituted 3-spirocyclopropyl-2-oxindoles and pyrazoloquinazolinones with excellent regio- and diastereoselectivity from common substrates under varying solvent conditions.

An unusual stereoretentive 1,3-quaternary carbon shift resulting in an enantioselective RhII-catalyzed formal [4+1]-cycloaddition between diazo compounds and vinyl ketenes

Enantioselective quaternary carbon construction in the assembly of cyclopentenones employing a RhII-catalyzed, formal [4+1]-cycloaddition is described. A Rh2(S-TCPTTL)4-catalyzed cyclopropanation of a vinyl ketene with a disubstituted diazo compound initiates a stereoretentive, accelerated ring expansion to provide the cycloadduct in good to excellent yields and enantioselectivity.

Nine-step total synthesis of (−)-strychnofoline

The asymmetric synthesis of an anti-cancer spirooxindole alkaloid, (−)-strychnofoline, has, for the first time, been accomplished in only nine steps.

Lanthanide amide-catalyzed one-pot functionalization of isatins: synthesis of spiro[cyclopropan-1,3′-oxindoles] and 2-oxoindolin-3-yl phosphates

A lanthanide-catalyzed one-pot reaction using readily accessible substrates was developed affording spiro[cyclopropan-1,3′-oxindoles] in good yields and with high stereoselectivity.

Organoiodine reagent-promoted intermolecular oxidative amination: synthesis of cyclopropyl spirooxindoles

An organoiodine-promoted intramolecular direct oxidative C(sp²)–N cross-coupling reaction was developed for the preparation of cyclopropyl spirooxindoles from readily available secondary cyclopropyl carboxamides.

Bifunctional thiourea catalyzed asymmetric Michael addition of anthrone to methyleneindolinones

Bifunctional thiourea catalyzed asymmetric conjugate addition of anthrones to methyleneindolinones were developed to prepare biologically significant hybrid derivatives of oxindoles and anthrones in high yield, high regioselectivity and stereoselectivity.

Facile synthesis of aza-spirocyclopropanyl oxindoles by the reaction of 3-(2-bromoethyl)-indole with 2,3-dimethylimidazole-1-sulfonyl azide triflate

3-(2-Bromoethyl)indole reacts with 2,3-dimethylimidazole-1-sulfonyl azide triflate and then with an alcohol or amine to afford the corresponding aza-spirooxindole sulfonate and sulfonamide.

Molecular diversity of spirooxindoles. Synthesis and biological activity

Spirooxindoles are important synthetic targets possessing extended biological activity and drug discovery applications. This review focuses on the various strategies for the enantioselective synthesis of spirocyclic oxindoles relying on reports over the past decade and from earlier work. The spirooxindoles in this review are separated into three structural classes, and then further categorized into the method type from which the spirocycle is generated.

Highly diastereoselective and enantioselective formal [4 + 3] cycloaddition of donor-acceptor cyclobutanes with nitrones

The first highly diastereoselective and enantioselective catalytic formal [4 + 3] cycloaddition of 1,1-cyclobutane diester with nitrone has been developed. Sterically hindered chiral SaBOX/Cu(II) complex promotes the reaction efficiently with a broad substrate scope, producing a range of multifunctionalized optically active 1,2-oxazepanes with excellent stereocontrol (up to >99/1 dr and 97% ee).

Rapid and efficient one-pot synthesis of spiro[indoline-3,4'-pyrano[2, 3-c]pyrazole] derivatives catalyzed by l-proline under ultrasound irradiation

A rapid and efficient ultrasound-promoted one-pot synthesis of spiro[indoline-3,4'-pyrano[2,3-c]pyrazole] derivatives is described here. The reaction of substituted phenylhydrazine and dialkyl acetylenedicarboxylate with substituted isatin and malononitrile is catalyzed by l-proline to give good-to-excellent yields in water/ethanol (v/v, 1:1) at room temperature.

Diastereoselective synthesis of spiro[cyclopropane-1,3′-indolin]-2′-ones through metal-free cyclopropanation using tosylhydrazone salts

Anticancer spiro[cyclopropane-1,3′-indolin]-2′-ones are accessible through a transition metal-free diastereoselective cyclopropanation using in situ generated diazo-compounds.

1,3-Carbon D–A strategy for [3 + 2] cycloadditions/annulations with imines: synthesis of functionalized pyrrolidines and related alkaloids

[3 + 2] Cycloaddition/annulation: this article summarize the recent developments in synthesis of pyrrolidines and related alkaloids through complementary [3 + 2] cycloaddition/annulation of 1,3-carbon D–A precursors with imines.

Multicomponent reaction to construct spirocyclic oxindoles with a Michael (triple Michael)/cyclization cascade sequence as the key step

Multicomponent cycloadditions with readily available isocyanides, allenoates, and isatylidene malononitriles are disclosed. This reaction, which does not require the aid of any catalyst, allows the efficient syntheses of spirocyclic oxindoles with excellent regioselectivity. Reactions with ethyl 2,3-butadienoate and various structurally diverse α- and γ-substituted allenoates are also fully explored. Remarkably, we have shown that the usual three-component process can be further developed into an unprecedented four-component cycloaddition in the presence of water, which provides a new strategy to access highly unusual tricyclic oxindoles. From a synthetic point of view, this protocol is very interesting considering the high level of complexity reached in one step. The mechanism is thought to proceed by a triple Michael/cyclization process by using allenoate as a three carbon atom component (3 C). Furthermore, multicomponent reaction with γ-substituted allenoate also results in a very interesting conversion. In such cases, the unusual cleavage of the "C=C" double bond of isatylidene malononitrile and one of the "C=C" double bonds of allenoate is always observed.