Extending Pummerer reaction chemistry: application to the assembly of the pentacyclic core of dibromopalau'amine

Room Temperature Diels-Alder Reactions of 4-Vinylimidazoles

In the course of studying Diels-Alder reactions of 4-vinylimidazoles with N-phenylmaleimide, it was discovered that they engage in cycloaddition at room temperature to give high yields of the initial cycloadduct as a single stereoisomer. In certain cases, the product precipitated out of the reaction mixture and could be isolated by simple filtration, thereby avoiding issues with aromatization observed during chromatographic purification. Given these results, intramolecular variants using doubly activated dienophiles were also investigated at room temperature. Amides underwent cycloaddition at room temperature in modest yields, but the initial adducts were not isolable with Nimid-benzyl-protected systems. Attempts to extend these results to the corresponding esters and hydroxamate were less successful with these substrates only undergoing cycloaddition at elevated temperatures in lower yields. Density functional theory calculations were performed to evaluate the putative transition states for both the inter- and intramolecular variants to rationalize experimental observations.

A Pummerer Reaction-Enabled Modular Synthesis of Alkyl Quinoline-3-carboxylates and 3-Arylquinolines from Amino Acids

Concise synthesis of functionalized quinolines has received continuous research attention owing to the biological importance and synthetic potential of bicyclic N-heterocycles. However, synthetic routes to the 2,4-unsubstituted alkyl quinoline-3-carboxylate scaffold, which is an important motif in drug design, remain surprisingly limited, with modular protocols that proceed from readily available materials being even more so. We herein report an acidic I₂-DMSO system that converts readily available aspartates and anilines into alkyl quinoline-3-carboxylate. This method can be extended to a straightforward synthesis of 3-arylquinolines by simply replacing the aspartates with phenylalanines. Mechanistic studies revealed that DMSO was activated by HI via a Pummerer reaction to provide the C1 synthon, while the amino acid catabolized to the C2 synthon through I₂-mediated Strecker degradation. A formal [3 + 2 + 1] annulation of these two concurrently generated synthons with aniline was responsible for the selective formation of the quinoline core. The synthetic utility of this protocol was illustrated by the efficient synthesis of human 5-HT4 receptor ligand. Moreover, an unprecedented chemoselective synthesis of 2-deuterated, 3-substituted quinoline, featuring this reaction, has been established.

Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Electrophilic Cyclization of Phenylalkynediols to Naphthyl(aryl)iodonium Triflates with Chelating Hydroxyls: Preparation and X-ray Analyses

Alkynediols containing one propargylic alcohol as well as a second alcohol, which is propargylic or homopropargylic, react with PhI⁺CN^-OTf (Stang's reagent) or 3,5-(CF₃)₂C₆H₃I⁺CN^-OTf to afford naphthyl(aryl)iodonium triflates. The reaction occurs at room temperature over the course of 6-12 h and provides 36-82% yields of microcrystalline solids. Slow diffusion of Et₂O into CH₃CN solutions of the salts afforded X-ray quality crystals of five compounds with hydroxyl groups forming five- and six-membered chelation complexes with the iodine atom. Crystallizations from larger scale reactions (≥∼0.25 mmol) were generally facile from CH₂Cl₂.

Syntheses of Cyclic Guanidine-Containing Natural Products

Naturally occurring guanidine derivatives frequently display medicinally useful properties. Among them, the higher order pyrrole-imidazole alkaloids, the dragmacidins, the crambescidins/batzelladines, and the saxitoxins/tetradotoxins have stimulated the development of many new synthetic methods over the past decades. We provide here an overview of the syntheses of these cyclic guanidine-containing natural products.

Dimeric pyrrole-imidazole alkaloids: synthetic approaches and biosynthetic hypotheses

The pyrrole-imidazole alkaloids are a group of structurally unique and biologically interesting marine sponge metabolites. Among them, the cyclic dimers have caught synthetic chemists' attention particularly. Numerous synthetic strategies have been developed and various biosynthetic hypotheses have been proposed for these fascinating natural products. We discuss herein the synthetic approaches and the biosynthetic insights obtained from these studies.

Pyrrole aminoimidazole alkaloid metabiosynthesis with marine sponges Agelas conifera and Stylissa caribica

Game-SET-match: Pyrrole aminoimidazole alkaloids (PAIs) are metabiosynthesized from chlorinated analogues of oroidin by cell-free enzyme preparations from PAI-producing sponges. Evidence and implications for the biosynthesis of PAIs include putative single-electron transfers (SETs) that promote C-C bond-forming reactions of precursors.

Asymmetric synthesis of ageliferin

We describe herein an asymmetric synthesis of ageliferin. A Mn(III)-mediated oxidative radical cyclization reaction was used as the key step to construct the core skeleton of this pyrrole-imidazole dimer. This approach resembles the biogenic [4 + 2] dimerization in an intramolecular fashion.

Biosynthesis, asymmetric synthesis, and pharmacology, including cellular targets, of the pyrrole-2-aminoimidazole marine alkaloids

The pyrrole-2-aminoimidazole (P-2-AI) alkaloids are a growing family of marine alkaloids, now numbering well over 150 members, with high topographical and biological information content. Their intriguing structural complexity, rich and compact stereochemical content, high N to C ratio (~1 : 2), and increasingly studied biological activities are attracting a growing number of researchers from numerous disciplines world-wide. This review surveys advances in this area with a focus on the structural diversity, biosynthetic hypotheses with increasing, but still rare, verifying experimental studies, asymmetric syntheses, and biological studies, including cellular target receptor isolation studies, of this stimulating and exciting alkaloid family.

Extending Pummerer reaction chemistry: studies in the palau'amine synthesis area

Exploratory oxidative cyclization studies on cyclopentanelated and cyclohexenelated oroidin derivatives utilized Pummerer chemistry to generate pentacyclic structures related to the palau'amine family of sponge metabolites. Stereochemical issues were paramount, and appropriate choice of annelated ring size led to formation of the pentacyclic framework with complete diastereoselectivity for all of the core bonds.

Diversity-oriented approach to pyrrole-imidazole alkaloid frameworks

An exploration of imidazolylpropargyl amides as linchpin synthons for the construction of a diverse array of heterocyclic frameworks, many of which are related to those found in the oroidin derived alkaloids, is described. One such intermediate has been used in a formal total synthesis of cyclooroidin.

Synthetic Studies on Palau'amine. Construction of the Cyclopentane Core via an Asymmetric 1,3-Dipolar Cycloaddition

The cyclopentane core of palau'amine has been constructed in optically pure form through the use of an asymmetric azomethine ylid [1,3]-dipolar cycloaddition reaction.