A short synthesis of koniamborine, a naturally occurring pyrano[3,2-b]indole

Selective Synthesis of Pyrano[3,2-b]indoles or Cyclopenta[b]indoles Tethered with Medium-Sized Rings via Cascade C-C σ-Bond Cleavage and C-H Functionalization

Highly atom-economical tandem reactions have been developed for the synthesis of pyrano[3,2-b]indoles or cyclopenta[b]indoles tethered with 7-, 8-, or 9-membered rings. These reactions first undergo a carbon-carbon σ-bond cleavage reaction of cyclic β-ketoesters. Next, in the presence of CuCl₂ and Ag₂CO₃, intramolecular O-H/C-H coupling occurs to give pyrano[3,2-b]indoles. This is the first example for capture of the enoloxyl radical of the intramolecular C-O bond formation reaction, whereas C3 nucleophilic addition afforded cyclopenta[b]indoles using TsOH·H₂O.

Simple, novel, and efficient synthesis of ethyl 2-(2-(1H-indol-3-yl)benzo[d]thiazol-3(2H)-yl)-2-cyanoacetate or acetate via a three-component reaction

Ethyl 2-(2-(1 H-indol-3-yl)benzo[ d]thiazol-3(2 H)-yl)-2-cyanoacetate or acetate 6a–f as a series of novel compounds were synthesized from the reactions of benzothiazole, 2-methylbenzothiazole, or 2,5-dimethylbenzothiazole and ethyl bromocyanoacetate with indole derivatives. The reactions proceeded in acetone as a solvent under reflux for 5 h in good yields. The structures of new compounds were confirmed by 1H nuclear magnetic resonance, 13C nuclear magnetic resonance, infrared, elemental analyses, and mass spectroscopy.

2,3-Dimethoxyindolines: a latent electrophile for SNAr reactions triggered by indium catalysts

An unprecedented utilization of 2,3-dimethoxyindolines (DiMeOINs) as a latent electrophile in regioselective In-catalyzed aromatic substitutions has been reported.

Biphilic Organophosphorus-Catalyzed Intramolecular Csp2-H Amination: Evidence for a Nitrenoid in Catalytic Cadogan Cyclizations

A small-ring phosphacycloalkane (1,2,2,3,4,4-hexamethylphosphetane, 3) catalyzes intramolecular C-N bond forming heterocyclization of o-nitrobiaryl and -styrenyl derivatives in the presence of a hydrosilane terminal reductant. The method provides scalable access to diverse carbazole and indole compounds under operationally trivial homogeneous organocatalytic conditions, as demonstrated by 17 examples conducted on 1 g scale. In situ NMR reaction monitoring studies support a mechanism involving catalytic P^III/P^V═O cycling, where tricoordinate phosphorus compound 3 represents the catalytic resting state. For the catalytic conversion of o-nitrobiphenyl to carbazole, the kinetic reaction order was determined for phosphetane catalyst 3 (first order), substrate (first order), and phenylsilane (zeroth order). For differentially 5-substituted 2-nitrobiphenyls, the transformation is accelerated by electron-withdrawing substituents (Hammett factor ρ = +1.5), consistent with the accrual of negative charge on the nitro substrate in the rate-determining step. DFT modeling of the turnover-limiting deoxygenation event implicates a rate-determining (3 + 1) cheletropic addition between the phosphetane catalyst 3 and 2-nitrobiphenyl substrate to form an unobserved pentacoordinate spiro-bicyclic dioxazaphosphetane, which decomposes via (2 + 2) cycloreversion giving 1 equiv of phosphetane P-oxide 3·[O] and 2-nitrosobiphenyl. Experimental and computational investigations into the C-N bond forming event suggest the involvement of an oxazaphosphirane (2 + 1) adduct between 3 and 2-nitrosobiphenyl, which evolves through loss of phosphetane P-oxide 3·[O] to give the observed carbazole product via C-H insertion in a nitrene-like fashion.

Complete analysis of the 1H and 13C NMR spectra of diastereomeric mixtures of (R,S- and S,S-)-3,6-dimethoxy-2,5-dihydropyrazine-substituted indoles and their conformational preference in solution

Complete analysis of the (1)H and (13)C NMR spectra obtained with and without a chemical shift reagent (Eu(fod)(3)), of bis-lactim ether 1 (Schöllkopf auxiliary) and monosubstituted 3- or 2-{(2R,5S or 2S,5S)-5-isopropyl-3,6-dimethoxy-2,5-dihydropyrazin-2-yl]methyl}-1H-indoles is presented using gradient-selected one-dimensional (1D) and two-dimensional NMR techniques, such as 1D TOCSY, 1D NOESY (DPFGSE NOE), gCOSY, NOESY, ROESY gHETCOR, gHSQC and gHMBC. The contour plot of the gCOSY spectrum of 1-10 revealed cross peaks arising from the five-bond coupling between the H2 and H5 resonances of the dihydropyrazine ring for syn- ((5)J(H2, H5) = 4-5.7 Hz) and for anti-isomers ((5)J(H2, H5) = 3.4-3.8 Hz). The magnitude of the coupling constant was utilized to distinguish between the syn- and the anti-isomers (diastereomers). The precise values of (n)J(HH) (n = 3, 4, 5, 6) coupling constants for the indole and 2,5-dihydropyrazine moieties deduced from the calculated NMR spectra were supported by 1D TOCSY and gCOSY experiments and gauge invariant atomic orbital (GIAO) calculations. The magnitude of the coupling constants ((5)J(H2, H5)) indicates that the dihydropyrazine ring exists in a boat conformation. In both isomers, the indole group adopts a 'folded' conformation in which one diastereotopic face is effectively shielded by the aromatic benzene ring of the indole. This is supported by gradient-selected 1D NOESY and 2D NOESY experiments. Theoretical calculations of the conformation were performed to support the through-space shielding effect of the aromatic indole moiety based on the DFT/GIAO calculated (1)H NMR data (chemical shifts and coupling constants) for 2-syn- and 2-anti-diastereomers in CDCl(3).