Efficient synthesis of 3-substituted 2-arylindoles via Suzuki coupling reactions on the solid phase

Palladium-Catalyzed Synthesis of Indoles and Isoquinolines with in Situ Generated Phosphinimine

A palladium-catalyzed synthesis of polysubstituted indoles and isoquinolines through the coupling of aryl bromides with 2-alkynyl arylazides or 2-alkynyl benzylazides has been developed. This method provides straightforward access to indoles and isoquinolines with high efficiency and excellent functional group compatibility. In this transformation, the iminophosphorane in situ generated from azides is served as the nucleophile that attacks the alkyne moiety in the cyclization process.

Rh-Catalyzed oxidative C–H activation/annulation: converting anilines to indoles using molecular oxygen as the sole oxidant

A practical and efficient Rh(iii)-catalyzed oxidative C–H annulation using molecular oxygen as the sole oxidant has been developed for the facile synthesis of a broad range of indoles from simple anilines and alkynes.

Microwave irradiated synthesis of 2-bromo(chloro)indoles via intramolecular cyclization of 2-(gem-dibromo(chloro)vinyl)anilines in the presence of TBAF under metal-free conditions

2-Bromo(chloro)indoles were readily prepared through TBAF-promoted intramolecular cyclization of 2-(gem-dibromo(chloro)vinyl)anilines in excellent yields under metal-free and microwave irradiation conditions.

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).

Arylboronic Acids and Arylpinacolboronate Esters in Suzuki Coupling Reactions Involving Indoles. Partner Role Swapping and Heterocycle Protection

Yields of Suzuki couplings involving indoles depended upon (i) whether arylboronic acids or arylpinacolboronate esters were used, (ii) whether the heterocycle was the aryl halide or the arylboron coupling partner, and (iii) whether the heterocycle was protected or not. Highest yields, which were unaffected by incorporating Boc or Tos protection at the heterocyclic nitrogen, were obtained when indole bromides were reacted with phenylboronic acids. When indolylboronic acids were reacted with phenyl bromides, yields were somewhat lower and depended on the nitrogen substituent, being highest in the absence of protection, lower in the presence of the Boc group, and lowest of all with the Tos group. Arylpinacolboronate esters were less reactive than arylboronic acids. They required considerably longer reaction times and furnished generally lower yields of biaryl. Furthermore, irrespective of whether the heterocycle was the aryl bromide or the arylpinacolboronate ester, these yields were highest when it was protected with the Tos group. Yields were lower with the Boc group, and unprotected heterocycles gave only traces of biaryl. Careful selection of arylboron reagent, of coupling partner roles, and of protecting groups are essential to ensuring optimum results in these Suzuki couplings. These results may also be relevant to couplings involving other substrates.

The recent impact of solid-phase synthesis on medicinally relevant benzoannelated nitrogen heterocycles

Benzoannelated heterocycles such as benzodiazepines and indoles can be prepared efficiently through cyclization on solid supports, although no single approach is currently universal for the preparation of all benzoannelated N-heterocycle chemistries. In this review, a number of synthetic strategies for the generation of benzoannelated nitrogen heterocycles using resin-bound substrates have been described. Classical heterocycle forming reactions such as the Fischer indole, the Bischler-Napieralski tetrahydroisoquinoline, the Pictet-Spengler tetrahydro-beta-carboline, the Tsuge, the Nenitzescu and the Richter cinnoline reaction are presented. In addition, the Heck, Sonogashira, Wittig, Diels-Alder, and olefin metathesis reactions have been also used. Multicomponent reactions such as the Grieco three-component assembly have been exploited for the synthesis of heterocycles. Cyclative cleavage from the solid support is particularly suitable for the synthesis of heterocycles while particular emphasis has been focused on the synthesis of libraries and the use of combinatorial chemistry techniques. In addition, the most relevant pharmacological properties of benzoannelated nitrogen heterocycles are included.