Aqueous Titanium Trichloride Promoted Reductive Cyclization ofo-Nitrostyrenes to Indoles: Development and Application to the Synthesis of Rizatriptan and Aspidospermidine

Synthesis of indole derivatives as prevalent moieties present in selected alkaloids

Indoles are a significant heterocyclic system in natural products and drugs. They are important types of molecules and natural products and play a main role in cell biology. The application of indole derivatives as biologically active compounds for the treatment of cancer cells, microbes, and different types of disorders in the human body has attracted increasing attention in recent years. Indoles, both natural and synthetic, show various biologically vital properties. Owing to the importance of this significant ring system, the investigation of novel methods of synthesis have attracted the attention of the chemical community. In this review, we aim to highlight the construction of indoles as a moiety in selected alkaloids.

Mo-Catalyzed One-Pot Synthesis of N-Polyheterocycles from Nitroarenes and Glycols with Recycling of the Waste Reduction Byproduct. Substituent-Tuned Photophysical Properties

A catalytic domino reduction-imine formation-intramolecular cyclization-oxidation for the general synthesis of a wide variety of biologically relevant N-polyheterocycles, such as quinoxaline- and quinoline-fused derivatives, and phenanthridines, is reported. A simple, easily available, and environmentally friendly dioxomolybdenum(VI) complex has proven to be a highly efficient and versatile catalyst for transforming a broad range of starting nitroarenes involving several redox processes. Not only is this a sustainable, step-economical as well as air- and moisture-tolerant method, but also it is worth highlighting that the waste byproduct generated in the first step of the sequence is recycled and incorporated in the final target molecule, improving the overall synthetic efficiency. Moreover, selected indoloquinoxalines have been photophysically characterized in cyclohexane and toluene with exceptional fluorescence quantum yields above 0.7 for the alkyl derivatives.

Reductive C-N Coupling of Nitroarenes: Heterogenization of MoO3 Catalyst by Confinement in Silica

The construction of C-N bonds with nitroaromatics and boronic acids using highly efficient and recyclable catalysts remains a challenge. In this study, nanoporous MoO₃ confined in silica serves as an efficient heterogeneous catalyst for C-N cross-coupling of nitroaromatics with aryl or alkyl boronic acids to deliver N-arylamines and with desirable multiple reusability. Experimental results suggest that silica not only heterogenizes the Mo species in the confined mesoporous microenvironment but also significantly reduces the reaction induction period and regulates the chemical efficiency of the targeted product. The well-shaped MoO₃ @m-SiO₂ catalyst exhibits improved catalytic performance both in yield and turnover number, in contrast with homogeneous Mo catalysts, commercial Pd/C, or MoO₃ nanoparticles. This approach offers a new avenue for the heterogeneous catalytic synthesis of valuable bioactive molecules.

Enantioselective Total Synthesis of (+)-Alstilobanine C, (+)-Undulifoline, and (-)-Alpneumine H

We report herein the enantioselective total synthesis of three monoterpene indole alkaloids, namely, (+)-alstilobanine C, (+)-undulifoline, and (-)-alpneumine H. The key features of our synthesis include: a) introduction of chirality via enantioselective deprotonation of a prochiral 4-substituted cyclohexanone; b) use of methoxymethyl (MOM) ether as both a hydroxyl protective group and a latent oxonium species for the formation of bridged oxepane and c) domino double reductive cyclization to build both the indole and the piperidine ring at the end of the synthesis. The synthesis confirmed the absolute configuration of these natural products assigned based on the biogenetic hypothesis.

Cation Triggered Domino Aza-Piancatelli Rearrangement/Friedel-Crafts Alkylation of Indole-Tethered Furfuyl Alcohols to Access Cycloocta[b]indole Core of Alkaloids

A domino approach to bridged cycloocta[b]indolone through a cascade of aza-Piancatelli rearrangement/Friedel-Crafts alkylation is developed. This transformation has been realized by reaction of an indole-tethered 2-furylcarbinol and substituted aniline in the presence of a Lewis acid to initiate aza-Piancatelli rearrangement followed by an in situ intramolecular Friedel-Crafts alkylation to access bridged tetracyclic frameworks in one pot.

Crystal structure of 1-(5-bromo-2-(4-methoxyphenyl)-1H-indol-7-yl)ethan-1-ol, C17H14BrNO2

C17H14BrNO2, monoclinic, C2/c (no. 15), a = 31.437(3) Å, b = 5.5224(5) Å, c = 16.6545(16) Å, β = 94.703(4)°, V = 2881.6(5) Å3, Z = 8, R gt(F) = 0.0251, wR ref(F 2) = 0.067, T = 173(2) K.

Direct Reductive N-Functionalization of Aliphatic Nitro Compounds

The first general protocol for the direct reductive N-functionalization of aliphatic nitro compounds is presented. The nitro group is partially reduced to a nitrenoid, with a mild and readily available combination of B₂ pin₂ and zinc organyls. Thereby, the formation of an unstable nitroso intermediate is avoided, which has so far severely limited reductive transformations of aliphatic nitro compounds. The reaction is concluded by an electrophilic amination of zinc organyls.