β-Functionalization of Indolin-2-one-Derived Aliphatic Acids for the Divergent Synthesis of Spirooxindole γ-Butyrolactones

Regioselective synthesis of enantiopure 1,2- and 1,3-dispirooxindoles along with a DFT study

In the present study, a library of important enantiopure dispirooxindole [indolizidine, pyrrolizidine, and pyrrolidine] derivatives with three or four contiguous and two quaternary stereogenic centers using different amino acids (pipecolic acid, sarcosine, proline and hydroxyproline) were synthesized in high yields (up to 96%) through a regio- and diastereoselective (up to 99 : 1) multicomponent 1,3-dipolar cycloaddition strategy. Based on the results, the alteration of amino acids led to a change in the regioselectivity and unusual regioisomers (pyrrolizidine versus indolizidine/pyrrolidine) were obtained to construct a novel enantiopure 1,3-dispirooxindole skeleton. The stereochemical outcome of the cycloaddition was determined by single crystal X-ray diffraction analysis and the self-disproportionation of enantiomers (SDE) test confirmed the enantiomeric purity of the desired products. The mechanism and differences in the regioselectivity of the 1,3-dipolar cycloaddition reactions between the stable azomethane ylides obtained from ninhydrin, pipecolinic acid, and proline with (E)-2-oxoindolin-3-ylideneacetyl sultam were theoretically studied through DFT calculations at the M06-2X/6-31G(d,p) level in methanol.

Enantioselective Organocatalytic Three-Component Vinylogous Michael/Aldol Tandem Reaction among 3-Alkylidene oxindoles, Methyleneindolinones, and Aldehydes

We reported a one-pot enantioselective three-component vinylogous Michael/aldol tandem reaction of prochiral 3-alkylidene oxindoles with methyleneindolinones and aldehydes using bifunctional organocatalysts. A variety of enantioenriched 3,3-disubstituted oxindoles 3 and spirolactones 4 were generated in moderate yields (up to 78%) with high stereoselectivities (up to >20:1 dr, >99% ee). Intriguingly, we observed that the aldol reaction with paraformaldehyde generates 3,3-disubstituted oxindoles 3 bearing a hydroxymethyl group, while the reaction with aliphatic aldehydes generates spirolactones 4.

A Review of the Pharmacological Activities and Recent Synthetic Advances of γ-Butyrolactones

γ-Butyrolactone, a five-membered lactone moiety, is one of the privileged structures of diverse natural products and biologically active small molecules. Because of their broad spectrum of biological and pharmacological activities, synthetic methods for γ-butyrolactones have received significant attention from synthetic and medicinal chemists for decades. Recently, new developments and improvements in traditional methods have been reported by considering synthetic efficiency, feasibility, and green chemistry. In this review, the pharmacological activities of natural and synthetic γ-butyrolactones are described, including their structures and bioassay methods. Mainly, we summarize recent advances, occurring during the past decade, in the construction of γ-butyrolactone classified based on the bond formation in γ-butyrolactone between (i) C5-O1 bond, (ii) C4-C5 and C2-O1 bonds, (iii) C3-C4 and C2-O1 bonds, (iv) C3-C4 and C5-O1 bonds, (v) C2-C3 and C2-O1 bonds, (vi) C3-C4 bond, and (vii) C2-O1 bond. In addition, the application to the total synthesis of natural products bearing γ-butyrolactone scaffolds is described.

Stereoselective synthesis and applications of spirocyclic oxindoles

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

Titanocene(III)-Mediated 5-exo-trig Radical Cyclization: En Route to Spirooxindole-Based Tetrahydrofuran and Bicyclic Lactone

The isatin core system is of immense importance due to the highly reactive prochiral C-3 position, which paves an easy way to construct large arrays of spirooxindole heterocyclic motifs. Herein, we depict an isatin-derived and 3,3'-disubstituted oxindole-appended epoxy-acrylate undergoing Cp₂Ti(III)Cl-mediated reductive oxirane-ring opening with concomitant intramolecular 5-exo-trig radical cyclization leading to tetrahydrofuran-based oxa-spirooxindole systems. The fused spirooxindole structural feature is embedded in many natural products and tends to exhibit a wide spectrum of biological activities. The presence of more than one quaternary center and the availability of multiple functional groups like hydroxyl, ester, or lactone in the resultant products expand the scope of synthetic applications of the newly acquired oxa-spirooxindole molecules.

Thermal-mediated [1,3]-hydrogen transfer as the key step: access to oxindole-chromone hybrid collection with structural diversity

Inspired by the chemistry and biology of chromone and oxindole derivatives, herein we report the first example of thermal-mediated [1,3]-hydrogen transfer as the key step for the efficient synthesis of oxindole-chromone hybrid collections 2, which avoids additional catalyst and solvent conditions. All the oxindole-chromones 2 are smoothly obtained in >99% yields in all cases, avoiding column chromatography purification. In particular, the products 2 can act as potential synthons for further elaboration in structural diversity, which might be valuable in organic and medicinal chemistry.

Application of isatin-derived saturated esters in the synthesis of 3,3'-spirooxindole γ-butyrolactams

Stable while reactive isatin-derived saturated esters have been utilized as 3-carbon synthons in a base-promoted formal [3 + 2] annulation with N-Boc imines. The developed protocol offers a direct pathway for the rapid and divergent construction of two classes of 3,3'-spirooxindole γ-butyrolactam skeletons that are recognized as the privileged structures of various bioactive compounds. This protocol also has the advantages of mild reaction conditions, scalability and wide reaction scope.

Enantioselective Synthesis of 2-Oxindole Spirofused Lactones and Lactams by Heck/Carbonylative Cylization Sequences: Method Development and Applications

An efficient one-pot assembly of all-carbon spiro-oxindole compounds from non-oxindole-based materials has been developed through a palladium-catalyzed asymmetric Heck/carbonylative lactonization and lactamization sequence. Diversified spirooxindole γ-and δ-lactones/lactams were accessed in high yields with good to excellent enantioselectivities (up to 99 % ee) under mild reaction conditions. The natural product coixspirolactam A was conveniently synthesized by applying the current methodology, and thus its absolute configuration was elucidated for the first time. Asymmetric synthesis of an effective CRTH2 receptor antagonist has also been demonstrated utilizing this method in the key step.

Organocatalytic, Enantioselective, Polarity-Matched Ring-Reorganization Domino Sequence Based on the 3-Oxindole Scaffold

A one-pot squaramide-catalyzed enantioselective ring-reorganization domino sequence (Michael addition/intramolecular ring-opening/lactamization) of 3-hydroxyoxindole and methyleneindolinone, which can be readily derived from 3-oxindole, has been established in this work. As a result, novel polycyclic quinolinone-spirooxindoles bearing three contiguous chiral centers were efficiently and step-economically assembled under mild conditions in high yields (up to 97%) with excellent enantioselectivities (up to >99% ee) and moderate to good diastereoselectivities (up to >95:5 dr).

Synthesis of CF3CH2-Containing Indolines by Transition-Metal-Free Aryltrifluoromethylation of Unactivated Alkenes

With an unactivated double bond as the radical acceptor, allyl amines underwent a metal-free trifluoromethylation/cyclization cascade with CF₃SO₂Na (Langlois' reagent), affording CF₃CH₂-containing indolines and tetrahydroisoquinolines, whose practical syntheses are significant challenges. This protocol features mild conditions, low cost, and a broad substrate scope.

Highly Efficient and Diastereoselective Construction of Trifluoromethyl-Containing Spiro[pyrrolidin-3,2'-oxindole] by a Catalyst-free Mutually Activated [3+2] Cycloaddition Reaction

A catalyst-free self-catalyzed [3+2] cycloaddition reaction of isatin-derived α-(trifluoromethyl)imines with vinylpyridines is reported. The reaction offers a straightforward and atom-economical procedure for the preparation of a series of 5'-trifluoromethyl-spiro[pyrrolidin-3,2'-oxindoles] in excellent yields and diastereoselectivities. The reaction mechanism has been investigated by control experiments, DFT calculation of pK_a values and the kinetic profiles, revealing that this reaction featured the mutual activation between isatin-derived α-(trifluoromethyl)imines and vinylpyridine to generate the reactive species.

A continuing challenge: N-heterocyclic carbene-catalyzed syntheses of γ-butyrolactones

Catalytic, stereoselective N-heterocyclic carbene-catalyzed reactions facilitate efficient construction of many different heterocyclic compounds, such as the enantioenriched 5-membered (γ) lactones highlighted in this tutorial review. Herein, various strategies to enable formal [3+2] type annulations between electrophilic carbonyl equivalents and homoenolate nucleophiles for the synthesis of γ-lactones are summarized.