The solid-phase Nenitzescu indole synthesis

One-pot Three-component Access to 5-Hydroxyindoles Based on Oxidative Dearomatization Strategy

One-pot three-component method for the synthesis of 5-hydroxyindoles based on oxidation of non-toxic hydroquinone, which involves multi-unit reactions. The reactions for the construction of a new C-C bond and two...

Applications of alkyl orthoesters as valuable substrates in organic transformations, focusing on reaction media

In this review we focus on applications of alkyl orthoesters as valuable and efficient substrates to perform various classes of two-component and multi-component organic reactions. The article has classified them according to two aspects, which are: (i) a focus on the reaction medium (solvent-free conditions, aqueous media, and organic solvents); and (ii) an examination of product structures. Reaction accomplishment under solvent-free conditions is an eco-friendly process with the absence of volatile toxic solvents, which puts it in line with green chemistry goals. Water is an interesting choice in organic transformations due to its inexpensiveness and safety. The authors hope their assessment will help chemists to attain new approaches for utilizing alkyl orthoesters in various organic synthetic methods. The review covers the corresponding literature up to the beginning of 2020.

A chemoselective route to β-enamino esters and thioesters

Conditions were developed for syntheses of β-enamino esters, thioesters, and amides. These reactions involve hydroxybenzotriazole derivatives in buffered media. Illustrative syntheses of some heterocyclic systems are given, including some related to protein–protein interface mimics.

Indole molecules as inhibitors of tubulin polymerization: potential new anticancer agents

Agents that interfere with tubulin function have a broad anti-tumor spectrum and they represent one of the most significant classes of anticancer agents. In the past few years, several small synthetic molecules that have an indole nucleus as a core structure have been identified as tubulin inhibitors. Among these, several aroylindoles, arylthioindoles, diarylindoles and indolylglyoxyamides have shown good inhibition towards the tubulin polymerization. This article reviews the synthesis, biological activities and SARs of these main classes of indoles. Brief mention has also been made about the fused indole analogs as tubulin inhibitors.

Synthesis of diverse indole libraries on polystyrene resin - Scope and limitations of an organometallic reaction on solid supports

The synthesis of diverse substituted indole structures on solid supports is described. The immobilization of nitrobenzoic acid onto Merrifield resin and the subsequent treatment with alkenyl Grignard reagents delivered indole carboxylates bound to solid supports. In contrast to results in the liquid phase, ortho,ortho-unsubstituted nitroarenes also delivered indole moieties in good yields. Subsequent palladium-catalyzed reactions (Suzuki, Heck, Sonogashira, Stille) delivered, after cleavage, the desired molecules in moderate to good yields over four steps. The scope and limitations are presented.

Bartoli indole synthesis on solid supports

[reaction: see text] Bartoli indole synthesis has been performed for the first time on solid supports. Starting from Merrifield resin, immobilization of five nitro benzoic acids was performed. Addition of four different alkenyl Grignard reagents and basic cleavage leads to substituted methyl indole carboxylates in excellent purities. Features of this reaction are the stability of halide groups, ester moieties, and tolerance of o,o'-unsubstituted nitro resins. Heck and Sonogashira reactions are also possible with immobilized indoles.

Novel solid-phase synthesis of 1,2-dialkoxyindoles

[reaction: see text] A novel solid-phase synthesis of 1,2-dialkoxyindoles on SynPhase lanterns is described. A unique C-C bond formation involving a nucleophilic displacement of a solid-bound aryl fluorine by dimethyl malonate afforded the arylnitro methyl ester, which upon treatment with tin(II) chloride dihydrate gave the N-hydroxyindolone. Alkylation of the N-hydroxyindolones afforded the corresponding N-hydroxy-2-alkoxyindoles, which were further alkylated to give the 1,2-dialkoxyindole. A library of 64 (8R(1) x 8R(2)) discrete 1,2-dialkoxyindoles was prepared using a color encoding technique on SynPhase A-series lanterns.

Solid-phase synthesis of indolecarboxylates using palladium-catalyzed reactions

Polymer-supported, palladium-catalyzed cyclization reactions effectively synthesized indolecarboxylates. Palladium-catalyzed carbon-carbon bond-forming reactions of immobilized enaminoesters followed by transesterification yielded indole 2- or 3-carboxylates with various functional groups on the benzene ring. Indolecarboxylates were efficiently cyclized via an intramolecular palladium-catalyzed amination reaction of immobilized N-substituted dehydrohalophenylalanines, and immobilized N-acetyl-dehydroalanines were efficiently converted into indolecarboxylates via tandem Heck-amination reactions.

Practical synthesis and elaboration of methyl 7-chloroindole-4-carboxylate

A synthesis of a previously unknown indole derivative is presented. The route reported herein allows for the preparation of multihundred gram quantities of material without any chromatographic purification. Conditions are presented for the Pd-catalyzed elaboration of one of the "diversity generating elements" of this important pharmacophore.

Synthesis of 2-substituted benzofurans and indoles using functionalized titanium benzylidene reagents on solid phase

Titanium(IV) benzylidenes bearing a masked oxygen or nitrogen nucleophile in the ortho position were generated from thioacetals, using low-valent titanocene complex, Cp2Ti[P(OEt)3]2. Methylene acetal, alkyl ether, silyl ether, fluoro, tertiary amino, and N-alkyl, N-benzyl, N-prenyl, and N-silyl tert-butyl carbamate groups were tolerated in the titanium alkylidene reagents (Schrock carbenes). Aryl-chlorine bonds were stable to the titanium benzylidene functionality, but there was poor chemoselectivity for the reduction of the thioacetal in the presence of an aryl chloride. The titanium benzylidenes converted Merrifield and Wang resin-bound esters into enol ethers. The oxygen nucleophile was masked as a TMS ether, and when the resin-bound enol ethers bearing this ortho substituent were treated with 1% TFA in dichloromethane, benzofurans were released from resin in high yields. The chameleon catch strategy ensured excellent purity. In a similar way, N-alkylated and N-silylated tert-butyl carbamates were used for the synthesis of N-alkyl and N-Boc indoles, respectively. These traceless solid-phase syntheses of heterocycles are believed to involve postcleavage modification rather than cyclative termination.

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.

Formats for combinatorial synthesis: solid-phase, liquid-phase and surface

Methods for combinatorial and parallel synthesis continue to evolve in order to meet the demands of modern synthetic organic chemistry. The nature of the support, while typically overlooked, is a key consideration for successful combinatorial organic synthesis. Developments in combinatorial synthesis technologies such as the 'lab-on-a-chip' concept and 96-well-plate-compatible resin plugs have been reported, which should contribute to meeting the increasing challenges of this field.

Novel functionalized titanium(IV) benzylidenes for the traceless solid-phase synthesis of indoles

[reaction: see text] Titanium(IV) benzylidenes bearing a masked nitrogen nucleophile in the ortho position converted Merrifield resin-bound esters into enol ethers. An unusual nitrogen protecting group, N-silylated tert-butyl carbamate, was employed. One percent TFA released N-Boc indoles in high yield and purity. N-Methyl indoles were also prepared. Cyclative termination was not required to release the chameleon catch. The first example of a carbonyl group within a titanium alkylidene reagent is reported.

Traceless solid-phase synthesis of 2,4-diaminoquinazolines

[reaction: see text] The solid-phase synthesis of 2,4-diaminoquinazolines is presented. The chemistry involves the sequential condensation of 2-aminobenzonitriles and amines starting from an acyl isothiocyanate resin via a traceless cleavage and cyclization. The alpha-1 antagonist prazosin was synthesized, as well as several other examples, in good yields and purity.