Gema Domínguez

The Pauson-Khand reaction, a powerful synthetic tool for the synthesis of complex molecules.

There are still some synthetic chemists who hesitate to use metal-mediated or -catalysed reactions. The Pauson-Khand reaction (PKR) is a powerful transformation that has now been sufficiently well developed to be routinely considered when planning a synthesis, especially of polycyclic complex molecules. This tutorial review aims to encourage the use of this process explaining the best ways of performing a PKR both in the stoichiometric and the catalytic version, showing the scope of the process and its limitations. Additionally, asymmetry can be introduced in the reaction using several strategies, which will be discussed. The most recent examples of the synthetic applications of the PKR in natural product synthesis will give the reader an idea of the great usefulness of this reaction.

Chromium arene complexes in organic synthesis

The complexation of an arene to a chromium tricarbonyl unit changes its chemical behavior, giving rise to unprecedented transformations. The electron-withdrawing effect of the unit allows efficient nucleophilic attack (S(N)Ar and dearomatization reactions), stabilizes negative charges in benzylic positions and activates C(Ar)-halogen bonds for cross-coupling reactions. In addition, the Cr(CO)(3) moiety exerts great facial control so it can be used as an auxiliary that can easily be removed. The 1,2- and 1,3-unsymmetrically disubstituted complexes are planar chiral and there are various ways to prepare them in enantiomerically pure form. Planar chiral chromium complexes are becoming useful intermediates and ligands for asymmetric catalysis. This mature field of organometallic chemistry has given rise to several synthetic applications of chromium arene complexes in the synthesis of natural products. This chemistry is overviewed in this tutorial review, giving special attention to the most recent and outstanding contributions in the area.

Alkenes in [2+2+2] Cycloadditions

Participation of alkenes and allenes in [2+2+2] cycloaddition reactions has attracted much attention recently. This version of the well-established alkyne cyclotrimerization renders interesting products, such as cyclohexadienes and other polycycles, through cascade processes. Many mechanistic variations are observed when using certain metal complexes as catalysts. The frequent generation of stereogenic centers has prompted the development of efficient asymmetric versions. This Minireview summarizes the efforts reported to date on the use of double bonds as partners in [2+2+2] cyclotrimerizations.

RCM in indoles. A new entry to the mitosene skeleton

Abstract The RCM metathesis reaction catalyzed by Grubbs’ ruthenium catalyst is used with a series of 1,2-disubstituted indoles leading with excellent yields to tricyclic compounds. When applied to 1-allyl-2-vinylindoles this methodology allows a new entry to the mitosene skeleton.

Tandem RCM-isomerization-cyclopropanation reactions.

A new triple tandem process has been discovered in which simple acyclic substrates can be transformed into bicyclic compounds via RCM-double bond isomerization-cyclopropanation. This process is catalyzed by second generation Grubbs catalyst without the requirement of other reactives or additives. In addition, a one-pot RCM-isomerization reaction followed by cyclopropanation with CHCl3/NaOH allows the synthesis of products related to iNOS (nitric oxide synthase) inhibitors, which are currently under clinical evaluation.

Synthesis and antitumour activity of new dendritic polyamines-(imide-DNA-intercalator) conjugates: potent Lck inhibitors.

A series of dendritic polyamines-(imide-DNA-intercalators) conjugates with different connectivity in their basic chain were synthesised and evaluated as antitumour compounds. Although their antiproliferative activity against HT-29 was not significant, conjugates 13 and 16 showed a promising profile as inhibitors of Lck.

Lanthanide-Chelating Carbohydrate Conjugates Are Useful Tools To Characterize Carbohydrate Conformation in Solution and Sensitive Sensors to Detect Carbohydrate–Protein Interactions

The increasing interest in the functional versatility of glycan epitopes in cellular glycoconjugates calls for developing sensitive methods to define carbohydrate conformation in solution and to characterize protein-carbohydrate interactions. Measurements of pseudocontact shifts in the presence of a paramagnetic cation can provide such information. In this work, the energetically privileged conformation of a disaccharide (lactose as test case) was experimentally inferred by using a synthetic carbohydrate conjugate bearing a lanthanide binding tag. In addition, the binding of lactose to a biomedically relevant receptor (the human adhesion/growth-regulatory lectin galectin-3) and its consequences in structural terms were defined, using Dy(3+), Tb(3+), and Tm(3+). The described approach, complementing the previously tested protein tagging as a way to exploit paramagnetism, enables to detect binding, even weak interactions, and to characterize in detail topological aspects useful for physiological ligands and mimetics in drug design.

Indole and Quinoline Synthesis via Intramolecular Pauson-Khand Reactions of Enamines and Allylamines

Syntheses of indoles and quinolines based upon the Pauson-Khand reaction is described. The starting materials are an aromatic ethynylenamine and ethynylallylamine, which are obtained via Sonogashira coupling. Also, an aromatic ynamine is used for an attempt to synthesize indole.

New protocol for the catalytic Pauson-Khand reaction induced by molecular sieves

Abstract The catalytic Pauson–Khand reaction is favoured in the presence of molecular sieves. This easy protocol probably involves the adsorption of carbon monoxide by the zeolites, making it possible to effect the reaction in the absence of CO atmosphere.

Synthesis of aromatic natural product frameworks using enyne metathesis

Abstract Enynes, easily obtained by the Sonogashira coupling reactions of aromatic iodides, undergo, with good yields, enyne metathesis using the Grubbs catalyst. The resulting dienes are interesting carbo- and heterocycles which can give complex natural frameworks by Diels–Alder reactions. Thus, an estradiol type skeleton is obtained in two steps from the corresponding enynes. An example of a metathesis-Diels–Alder cascade one-pot process is reported.