Synthesis of Chromeno[2,3-b]indol-11(6H)-one via PhI(OAc)2-Mediated Intramolecular Oxidative C(sp2)–N(H2) Bond Formation

Rearrangement of C2-Spirooxindoles: Conversion to the 2-Hydroxyhemi-Indigo and Chromenoindole

This study demonstrates the rearrangement of C2-spirooxindoles to the 2-hydroxyhemi-indigo and chromenoindole. The N-H-spirooxindole exhibits double proton translocation and its conversion to the (Z)-2-hydroxyhemi-indigo photoswitch with trifluoroacetic acid, while the N-methyl-spirooxindole undergoes structural rearrangement to the chromenoindole. The mechanism of the reactions was proposed, and the structure of the products was confirmed by one-dimensional (1D) and two-dimensional (2D) NMR spectra and X-ray structure analysis. The photoswitching performance of (Z)-2-hydroxyhemi-indigo, which allows the stabilization of the E-switched form by intramolecular hydrogen bonding, has been studied in solvents of different polarities. It was found that in the less polar solvents, the E-switched metastable isomer is characterized by high stability.

PIFA-mediated intramolecular N-arylation of 2-aminoquinoxalines to afford indolo[2,3-b]quinoxaline derivatives

We present the PIFA-mediated intramolecular N-arylation of 2-aminoquinoxalines at room temperature for the first time. This method provides a wide range of indolo[2,3-b]quinoxalines in good to excellent yields within a short time. The C-H bond functionalization occurs without the need for an inert atmosphere or additives. Additionally, a double C-H bond functionalization was observed, where the first reaction forms a C-N bond (N-arylation) and the second forms a C-O bond, yielding an acetal-functionalized product. Mechanistic investigations suggest that the C-H bond functionalization proceeds through an ionic mechanism, whereas acetal functionalization follows a radical pathway. This method extends to the derivation of indoloquinoxalines, including the target compound BIQMCz.

Access to Chromenopyrrole via Tandem [3 + 2] Cycloaddition and Intramolecular C-O Coupling

A mild and concise method for the synthesis of chromenopyrrole from 2'-hydroxychalcone is devised. The reaction proceeds via an initial [3 + 2] cycloaddition on the C═C bond of 2'-hydroxychalcone and 1,3-dipolarophile, generated in situ by the reaction of ethyl isocyanoacetate and AgOAc. This is then followed by an intramolecular C-O bond formation with the -OH group and C5-H of the in situ generated pyrrole, leading to chromenopyrroles.

Copper-assisted synthesis of dihydropyrano[2.3-b]indole-4-ones by domino cascade reaction

A novel copper-catalyzed cascade intermolecular and intramolecular oxidation/cyclization domino one-pot reaction process for the regioselective synthesis of dihydropyrano[2.3-b]indol-4(9H)-ones has been successfully developed. In this methodology, it is proposed for the first time that the 4-benzyloxy group of indole substrates can be used as a guiding group to promote cyclization under mild conditions. Meanwhile, reaction mechanism studies indicate that carbonyl oxygen in pyranoindole-4-ones came from water and the guiding group is critical to the progress of the reaction.

Pd-catalyzed versus uncatalyzed, PhI(OAc)2-mediated cyclization reactions of N6-([1,1'-biaryl]-2-yl)adenine nucleosides

In this work we have assessed reactions of N⁶-([1,1'-biaryl]-2-yl)adenine nucleosides with Pd(OAc)₂ and PhI(OAc)₂, via a Pd^II/Pd^IV redox cycle. The substrates are readily obtained by Pd/Xantphos-catalyzed reaction of adenine nucleosides with 2-bromo-1,1'-biaryls. In PhMe, the N⁶-biarylyl nucleosides gave C6-carbazolyl nucleoside analogues by C-N bond formation with the exocyclic N⁶ nitrogen atom. In the solvent screening for the Pd-catalyzed reactions, an uncatalyzed process was found to be operational. It was observed that the carbazolyl products could also be obtained in the absence of a metal catalyst by reaction with PhI(OAc)₂ in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). Thus, under Pd catalysis and in HFIP, reactions proceed to provide carbazolyl nucleoside analogues, with some differences. If reactions of N⁶-biarylyl nucleoside substrates were conducted in MeCN, formation of aryl benzimidazopurinyl nucleoside derivatives was observed in many cases by C-N bond formation with the N1 ring nitrogen atom of the purine (carbazole and benzimidazole isomers are readily separated by chromatography). Whereas Pd^II/Pd^IV redox is responsible for carbazole formation under the metal-catalyzed conditions, in HFIP and MeCN radical cations and/or nitrenium ions can be intermediates. An extensive set of radical inhibition experiments was conducted and the data are presented.

A practical approach for the synthesis of oxindole and isatin derivatives by Pd-catalyzed intramolecular amination

A highly efficient approach for the selective synthesis of bioactive oxindole/isatin-N-acetamide derivatives via Pd-catalyzed intramolecular amination has been developed.

PhI(OAc)2-mediated 1,2-aminohalogenation of alkynes: a general access to (E)-4-(halomethylene)oxazolidin-2-ones

A new PhI(OAc)₂-mediated 1,2-aminohalogenation of prop-2-yn-1-yl carbamates and halogen sources is presented with excellent selectivity. The reaction is general for the construction of (E)-4-(halomethylene)oxazolidin-2-ones through the three-membered ring or N-radical.

Facile synthesis of functionalized 6-cyano-2-oxa-7-azabicyclo[4.1.0]hept-3-en-1-yl acetates: a catalyst free approach to access the pyran fused 2-acetoxy-NH-aziridines

Novel pyran fused 2-acetoxy-NH-aziridine scaffold was constructed by reacting the enamine fragment of 4-H-pyrans and spiropyrans using iodobenzene diacetate under catalyst free environment.

Synthesis of Functionalized Chromeno[2,3-b]pyrrol-4(1H)-ones by Silver-Catalyzed Cascade Reactions of Chromones/Thiochromones and Isocyanoacetates

A novel and convenient approach to the synthesis of chromeno[2,3-b]pyrrol-4(1H)-ones has been developed. Furthermore, the method involves a facile silver-catalyzed cascade cyclization reaction including an intramolecular C-O bond formation. The silver salt acts as a key promoter.