A new indole synthesis via [3+3] annulation of 2-(benzotriazol-1-ylmethyl)-pyrroles with α,β-unsaturated aldehydes and ketones

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).

1,2,3-Triazoles

Rapid progress in the synthetic application of benzotriazole derivatives in the last 20 years has resulted in over 1000 scientific papers on the subject. This fact is reflected in Section 5.01.7, which involves almost a half of the volume of this chapter. The section is arranged according to hybridization of the C-α atom and atomic numbers of the atoms in positions β and γ to allow an easy access to the material of interest. Recent discovery of copper catalysis in [3+2] cycloadditions of azides to acetylenes, the so-called ‘click chemistry’, which boosted application of the 1,2,3-triazole derivatives, especially in medicinal chemistry, is presented in Section 5.01.9. From the point of view of practical applications, Section 5.01.11 is organized according to the number, position, and combination of the substituents at the aromatic rings. Another novel feature that has no precedence in the previous editions of Comprehensive Heterocyclic Chemistry is an addition of triazole and benzotriazole complexes with various transitions metals to Section 5.01.4.

Benzotriazole: An Ideal Synthetic Auxiliary

Benzotriazole is a synthetic auxiliary that offers many advantages. It is inexpensive, odorless, and stable. A benzotriazole group is easily introduced, activates molecules towards numerous transformations, and can be removed easily at the end of the reaction sequence. This Concept provides some recent examples of the synthetic application of benzotriazole methodology and is intended to draw attention to the versatile applications of benzotriazole in organic chemistry.

Benzannulation of 3-substituted pyrroles to indoles

In a new general indole synthesis, the anion derived from benzotriazolyl derivative 5b underwent regioselective 1,4-addition to various alpha,beta-unsaturated ketones; subsequent acid-catalyzed cyclization formed the corresponding indoles 1a-f.

General Synthesis of Polysubstituted Benzo[b]furans

2-(Benzotriazol-1-ylmethyl)furans 4a-c, on treatment with n-BuLi followed by various alpha,beta-unsaturated ketones and aldehydes, gave the adducts 6a-f which underwent intramolecular cyclization to form substituted benzo[b]furans 10a-f. Lithiation and subsequent alkylation of 4a-c afforded intermediates 8a-c, which underwent similar transformations with alpha,beta-unsaturated ketones or aldehydes to give the corresponding polysubstituted benzo[b]furans 7a-c. o-Iodophenol (11) with 1-propargylbenzotriazole (1) in the presence of (PPh(3))(2)PdCl(2), CuI, and Et(3)N gave 2-(benzotriazol-1-ylmethyl)benzo[b]furan (12). The benzotriazolyl-CH(2) side chain of 12 was further elaborated by (i) alkylation with trans-cinnamaldehyde to give the adduct 14, followed by intramolecular cyclization to give dibenzofuran 17; (ii) alkylation with benzyl bromide followed by elimination or nucleophilic displacement of the benzotriazolyl moiety with Grignard reagent and subsequent dehydrogenation to give 2-alkenylbenzofurans 15 or 20, respectively; (iii) alkylation with (CH(3))(3)CCHO followed by low-valent titanium-promoted olefination to give olefin 19; and (iv) alkylation with benzaldehyde followed by Lewis acid catalyzed pinacol type reaction to give 18.