Dipolarophile-Controlled Regioselective 1,3-Dipolar Cycloaddition: A Switchable Divergent Access to Functionalized N-Fused Pyrrolidinyl Spirooxindoles

Switchable Skeletal Rearrangement of Hexahydro-4H-indol-4-ones: Divergent Synthesis of Dihydroxy-4H-cyclopenta[b]pyridin-4-ones and 8-Alkenyl Oxepane-2,6-diones

An unprecedented base-controlled selective skeletal rearrangement reaction of hexahydro-4H-indol-4-ones has been developed. In this protocol, highly functionalized dihydroxy-4H-cyclopenta[b]pyridin-4-ones and 8-alkenyl oxepane-2,6-diones were prepared with a broad substrate scope and high chemoselectivity in moderate to excellent yields selectively by modulating LiOH and Et3N. In addition, the newly formed 8-alkenyl oxepane-2,6-dione scaffolds could be easily further derivatized to 5-(pyrrol-2-yl)dihydrofuran-2(3H)-ones through a rare intramolecular rearrangement reaction.

Diastereoselective Three-Component 1,3-Dipolar Cycloaddition to Access Functionalized β-Tetrahydrocarboline- and Tetrahydroisoquinoline-Fused Spirooxindoles

A chemselective catalyst-free three-component 1,3-dipolar cycloaddition has been described. The unique polycyclic THPI and THIQs were creatively employed as dipolarophiles, which led to the formation of functionalized β-tetrahydrocarboline- and tetrahydroisoquinoline-fused spirooxindoles in 60-94% of yields with excellent diastereoselectivities (10: 1->99: 1 dr). This reaction not only realizes a concise THPI- or THIQs-based 1,3-dipolar cycloaddition, but also provides a practical strategy for the construction of two distinctive spirooxindole skeletons.

Direct synthesis of N-functionalized indoles through isomerization of azomethine ylides

An unexpected isomerization of azomethine ylides generated in situ from isatin with indoline-2-carboxylic acid has been disclosed, providing direct access to N-functionalized indole scaffolds. This protocol has good functional group tolerance and provides various 3-(1H-indol-1-yl)indolin-2-one derivatives in moderate to high yields simply by using alcohol as the solvent, with no additional additive being required.

[3+2]-Cycloaddition of Nitrile Imines to Parabanic Acid Derivatives-An Approach to Novel Spiroimidazolidinediones

Approximately 1,3-Dipolar cycloaddition of imidazolidine derivatives containing exocyclic double bonds is a convenient method of creating spiro-conjugated molecules with promising anticancer activity. In this work, the derivatives of parabanic acid (2-thioxoimidazolidine-4,5-diones and 5-aryliminoimidazolidine-2,4-diones) were first investigated as dipolarophiles in the reactions with nitrile imines. The generation of nitrile imines was carried out either by the addition of tertiary amine to hydrazonoyl chlorides «drop by drop» or using the recently proposed diffusion mixing technique, which led to ~5-15% increases in target compound yields. It was found that the addition of nitrile imines to C=S or C=N exocyclic double bonds led to 1,2,4-thiazolines or triazolines and occurred regioselectively in accordance with the ratio of FMO coefficients of reactants. The yield of the resulting spiro-compound depended on the presence of alkyl substituents in the nitrile imine structure and was significantly decreased in reactions with imines with strong electron donor or electron-withdrawing groups. Some of the obtained compounds showed reasonable in vitro cytotoxicity. IC50 values were calculated for HCT116 (colon cancer) cells using the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) test.

Diastereoselective Synthesis of Dispiro[Imidazothiazolotriazine-Pyrrolidin-Oxindoles] and Their Isomerization Pathways in Basic Medium

Highly diastereoselective methods for the synthesis of two series of regioisomeric polynuclear dispyroheterocyclic compounds with five or six chiral centers, comprising moieties of pyrrolidinyloxindole and imidazo[4,5-e]thiazolo[3,2-b]-1,2,4-triazine of linear structure or imidazo[4,5-e]thiazolo[2,3-c]-1,2,4-triazine of angular structure, have been developed on the basis of a [3+2] cycloaddition of azomethine ylides to functionalized imidazothiazolotriazines. Depending on the structure of the ethylenic component, cycloaddition proceeds as an anti-exo process for linear derivatives, while cycloaddition to angular ones resulted in a syn-endo diastereomer. Novel pathways of isomerization for the synthesized anti-exo products upon treatment with sodium alkoxides have been found, which resulted in two more series of diastereomeric dispiro[imidazothiazolotriazine-pyrrolidin-oxindoles] inaccessible with the direct cycloaddition reaction. For the first series, the inversion of the configuration of one stereocenter, i.e., C-4' atom of the pyrrolidine cycle, (epimerization) was established. For the second one, configuration of the obtained diastereomer formally corresponded to the syn-endo approach of the azomethine ylide in the case of cycloaddition to the ethylenic component.

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].