A [3 + 2] Dipolar Cycloaddition Route to 3-Hydroxy-3-alkyl Oxindoles: An Approach to Pyrrolidinoindoline Alkaloids

Manganese-Catalyzed 5-Endo-trig Oxygenative Cyclization of α,β-Unsaturated Oximes under Air and Ambient Conditions for the Synthesis of 4,5-Dihydroisoxazoles

The stereoselective 5-endo-trig oxygenative cyclization of α,β-unsaturated oximes was achieved using molecular oxygen (O2) and a manganese catalyst. Several 4-hydroxy-4,5-dihydroisoxazoles were obtained in high yields by directly incorporating O2 from the atmosphere (eliminating the necessity for a pure oxygen environment) and using an unprecedentedly low loading of Mn(acac)3 (as little as 0.020 mol %) without additional additives. Because of its desirable features, such as operational simplicity, inexpensive catalyst, mild reaction conditions (open flask conditions at room temperature), and broad substrate compatibility, this novel reaction provides an attractive synthetic approach to producing 4-hydroxy-4,5-dihydroisoxazoles.

Direct organocatalytic transfer hydrogenation and C-H oxidation: high-yielding synthesis of 3-hydroxy-3-alkyloxindoles

Readily available 3-alkylideneoxindoles were effectively reduced to 3-alkyloxindoles through transfer hydrogenation using Hantzsch ester as a reducing agent at ambient temperature and the greenness/sustainability of this protocol was assessed by correlation with Pd/C-mediated hydrogenation with hydrogen gas. Furthermore, an organocatalytic method was developed to access drug-like 3-alkyl-3-hydroxyoxindoles by C-H oxidation of 3-alkyl-indolin-2-one, using a catalytic amount of 1,1,3,3-tetramethylguanidine (TMG) as an organic base and dissolved oxygen in THF as an oxidant at room temperature. Key reaction intermediates were observed by controlled on-line ESI-HRMS experiments and identified by their corresponding mass (m/z) analysis. This two-step high-yielding transfer hydrogenation/C-H oxidation protocol was used for the total synthesis of medicinally important 3-cyanomethyl-3-hydroxyoxindole and formal total synthesis of (±)-alline and (±)-CPC-I in very good overall yields compared to previous methods.

Protein Modification via Nitrile Oxide-Dehydroalanine Cycloaddition: Formation of Isoxazoline Ring on the Protein Backbone

Here we describe a novel catalyst-free 1,3-dipolar cycloaddition bioconjugation approach for chemical modification of proteins. The dehydroalanine (Dha)-containing protein reacts with nitrile oxides generated in situ through 1,3-dipolar cycloaddition in fully aqueous-buffered systems. This leads to the formation of a new isoxazoline ring at a pre-defined site (Dha) of the protein. Furthermore, the 1-pyrene isoxazoline-installed annexin V acts as a fluorescent probe, which successfully labels the outer cellular membranes of human cholangiocarcinoma (HuCCA-1) cells for detection of apoptosis.

Asymmetric Rubottom-Type Oxidation Catalyzed by Chiral Calcium Phosphates

A highly efficient catalytic asymmetric Rubottom-type oxidation is described. Using meta-chloroperbenzoic acid (m-CPBA) as the oxidant and chiral calcium phosphate as the catalyst, the facile transformation enables direct hydroxylation of N-Boc oxindoles and β-ketoesters in high yields (up to 99 %) and in a highly enantioenriched fashion (up to >99 % ee). The application of the established method was demonstrated by the synthesis of a pharmaceutically important 3-hydroxyoxindole with excellent enantiocontrol.

Expeditious synthesis of highly substituted 3(2H)-furanone and quinoline by microwave assisted reaction between aldonitrones and dibenzoylacetylene

Microwave assisted reaction between various aldonitrones and dibenzoylacetylene gave 2-hydroxy-2,5-diphenyl-4-(phenylamino)furan-3(2H)-one (5) and (3-hydroxy-2-phenylquinolin-4-yl)(phenyl)methanone (8) arising through two distinct reaction pathways in good yields. Compared to reactions carried out under thermal...

An update on the progress of cycloaddition reactions of 3-methyleneindolinones in the past decade: versatile approaches to spirooxindoles

Cycloaddition reactions are of great interest due to their potential and rapid construction of optically enriched spiro-cyclic products. 3-Methyleneindolinones have been proven to be a valuable precursor in cycloaddition reactions for the construction of diverse 3,3'-spirocyclic oxindoles. Their versatile reactivity has provided a new forum for the development of a variety of building blocks and synthetic compounds, including bioactive molecules. Herein, significant accomplishments in the cycloaddition reactions of 3-methyleneindolinones for the synthesis of spirooxindoles have been summarised and elaborated. The review is outlined according to the type of cycloaddition such as [2 + 1], [2 + 2], [3 + 2], [4 + 2] and [5 + 2] cycloaddition reactions.

Preparation of spiro[indole-3,5′-isoxazoles] via Grignard conjugate addition/spirocyclization sequence

A highly efficient one-pot procedure combining conjugate addition of Grignard reagents to (2-nitroalkenyl)indoles and sub-sequent Brønsted acid-assisted spirocyclization allowed for preparation of 4′H-spiro[indole-3,5′-isoxazoles] in a diastereomerically selective fashion. Utilization of alkyl Grignard reagents provided an easy access to 4′-alkylsubstituted derivatives hardly available by other means.

One-pot procedure combining conjugate addition of Grignard reagents to (2-nitroalkenyl)indoles and subsequent acid-assisted spirocyclization allowed for diastereoselective preparation of 4′H-spiro[indole-3,5′-isoxazoles].

Di-tert-butyl Ethynylimidodicarbonate as a General Synthon for the β-Aminoethylation of Organic Electrophiles: Application to the Formal Synthesis of Pyrrolidinoindoline Alkaloids (±)-CPC-1 and (±)-Alline

The reagent di-tert-butyl ethynylimidodicarbonate is demonstrated as a β-aminoethyl anion synthetic equivalent. It can be used to install ethyleneamine groups by exploiting its terminal alkyne reactivity with common organic electrophiles. Reactions exemplified with this terminal ynimide reagent include additions to imines, aldehydes, ketones, pyridinium salts, Michael acceptors, epoxides, and Pd-catalyzed Sonogashira couplings. Subsequent regioselective [3 + 2] cycloadditions of the alkynyl-imides (ynimides) generate N,N-di-Boc imide-functionalized triazole and isoxazole heterocycles. Reduction of the ynimides with Pd-catalyzed hydrogenation generates ethyleneimides with easily removable N,N-di-Boc-carbamate protecting groups, allowing for a flexible ynimide-based approach to ethyleneamine installation. The utility of this two-step aminoethylation strategy was demonstrated in the short formal syntheses of pyrrolidinoindoline alkaloids (±)-CPC-1 and (±)-alline. Analogously, the reagent (N,N,N')-tri-Boc 2-ethynylhydrazine serves as a β-hydrazinoethyl anion synthetic equivalent.

One-step assembly of alkoxypyrroloindolines via iodine-catalyzed alkoxycyclization of indole derivatives

Herein we report an iodine-catalyzed alkoxycyclization of tryptamine derivatives under mild reaction conditions. This method distinguished itself by providing a catalytic, one-step assembly of diversely functionalized C3a-alkoxypyrroloindolines as well as dihydrofuran and lactone fused indolines. Mechanistic studies suggest that an ionic pathway is operative and this probably accounts for the diastereospecificity of all isolated cycloadducts.

Ru-Catalyzed dehydrogenative synthesis of antimalarial arylidene oxindoles

Ru(ii)-NHC catalyzes α-olefination of 2-oxindoles using diaryl methanols in the absence of an acceptor. A wide array of symmetrical and unsymmetrical diaryl methanols undergoes dehydrogenative coupling with 2-oxindole selectively to generate various substituted 3-(diphenylmethylene)indolin-2-one derivatives in good yields and produces environmentally benign by-products, H2 and H2O. This methodology was successfully applied for the synthesis of a bioactive drug i.e. TAS-301. The biological activities of the synthesized 3-(diphenylmethylene)indolin-2-one derivatives were screened against the Plasmodium falciparum parasite and found to exhibit a significant activity with IC50 = 2.24 μM.

In situ generation of nitrile oxides from copper carbene and tert-butyl nitrite: synthesis of fully substituted isoxazoles

Herein, we present a novel [3 + 2] cycloaddition reaction of β-keto esters with nitrile oxides, which were generated in situ from copper carbene and tert-butyl nitrite. This three-component reaction provides new methodology for the direct synthesis of fully substituted isoxazole derivatives, featuring mild reaction conditions, readily accessible starting materials and simple operation. The experimental studies and DFT calculations suggest that the reaction starts with the generation of the key intermediate nitrile oxides, followed by a [3 + 2] cycloaddition reaction of β-keto esters to give the final isoxazole products.

Nitrostyrenes as 1,4-CCNO-dipoles: diastereoselective formal [4+1] cycloaddition of indoles

An unusual reactivity of nitrostyrenes in phosphorous acid was discovered, which permits the employment of these readily available synthons as 1,4-CCNO-dipole surrogates in a highly diastereoselective (4+1)-cycloaddition of indoles to afford 4′H-spiro[indole-3,5′-isoxazoles] in a diastereomerically pure form.

Stereoselective Synthesis of 3-(1,3-Diarylallylidene)oxindoles via a Palladium-Catalyzed Tandem Reaction

We have developed an efficient three-component tandem reaction for the synthesis of 3-(1,3-diarylallylidene)oxindoles combining three palladium-catalyzed reactions: the Sonogashira, Heck, and Suzuki-Miyaura reactions. This method allows a stereoselective approach to each (E)- and (Z)-isomer by ligand change and controlling the reaction temperature.

Enolate-mediated 1,3-dipolar cycloaddition reactions of allyl ketones with nitrile oxides: direct access to 3,5-disubstituted isoxazolines

TMG-catalyzed [3 + 2] organocatalytic 1,3-dipolar cycloaddition reactions of allyl ketones with in situ generated nitrile oxides have been developed. This strategy could generate 3,5-disubstituted isoxazolines in high yields and regioselectivities.

Synthesis of 3,4,5-trisubstituted isoxazoles via 1,3-dipolar cycloaddition/SO2 extrusion of benzoisothiazole-2,2-dioxide-3-ylidenes with nitrile oxides

An straightforward synthesis of 3,4,5-trisubstituted isoxazole derivatives via 1,3-dipolar cycloaddtion/SO₂ extrusion of benzoisothiazole-2,2-dioxide-3-ylidenes with nitrile oxides is reported using 4 Å molecular sieves as dehydrochlorinating agent.

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.

An efficient aldol-type direct reaction of isatins with TMSCH2CN

Cesium fluoride catalyzed direct cyanomethylation of various isatins by using trimethylsilyl acetonitrile (TMSAN) as a nucleophile has been developed. The reaction has been explored for a number of isatins, with various substitutions on its aromatic ring. Further, the versatility of the reaction is demonstrated by converting the direct aldol adducts to various corresponding intermediates.

Unprecedented formation of spiro[indoline-3,7′-pyrrolo[1,2-a]azepine] from multicomponent reaction of l-proline, isatin and but-2-ynedioate

Spiro[indoline-3,3′-pyrrolizines] and spiro[indoline-3,7′-pyrrolo[1,2-a]azepines] can be selectively produced from three-component reaction of α-amino acids, isatins and but-2-ynedioates.

Synthesis and Tautomerism of Spiro-Pyrazolines

An experimental study on the synthesis, tautomerism and acid promoted structural changes of spiro-pyrazolines is described. The target was achieved through a [3+2]-dipolar cycloaddition of an alkene with nitrile imines generated in situ and was isolated in high yield. The synthesized cycloadduct displayed a tendency to exhibit an imine-enamine type of tautomerism as evidenced by X-ray crystal and NMR studies. Furthermore, addition of an acid resulted in the transformation of an imine tautomer to an enamine. The current report constitutes a first formal observation of this kind of tautomerism observed in spiro-indoline pyrazolines.