Mini-Reviews in Organic Chemistry - Volume 6, Issue 4, 2009

In this review recent uses of the inexpensive and commercially available bismuth(III) salts in organic chemistry will be highlighted. Their application to the development of new processes or synthetic routes that lead to compounds of pharmaceutical interest will be matter of discussion. It will focus on bismuth(III) salt-mediated reactions involving the preparation of non-heterocyclic compounds such as aliphatics and alicyclics, monocyclic and polycyclic aromatics, amino acids and peptides, terpenes and steroids.

Coordination compounds based on bis-quinolizidine alkaloids sparteine and &-isosparteine as ligands with Zn(II) and Cu(II) salts are described from the viewpoint of their spectroscopic and structural properties. In particular the complexes of LMX2 types where L = alkaloid, X = halides (Cl-, Br-, I-), pseudo-halides (CN-, SCN-) and oxy-anions (C4H5OO-, CH3COO-, NO3-).

Calixarenes, macrocycles containing a preorganized central cavity, are often used to bind molecular and ionic guests. Though these guests are mostly limited to alkali metal or organic cations, and solvents, it is possible to assemble transition metal coordination complexes from calixarene derivatives. This review will illustrate how careful design strategies can be employed to devise target compounds and focus on synthetic approaches to prepare the desired calixarene derivatives.

Industrial biotechnology or white biotechnology is an area of growing academic, public and private interest. The global white (or industrial) biotechnology market of ~50 billion US$ is smaller than the red biotechnology (pharmaceutical) market (>70 billion US$). However, industrial white biotechnology represents a greater long term business potential than red biotechnology. It is estimated that at least 20% of global chemicals (∼2'290 billion US$ today) could be produced by biotechnological means in 2020. The bad news is that this potential is spread over diverse markets and various product/molecule classes. This variety is more difficult to handle on the one hand, but on the other it comes with longer life cycles. Currently there are a number of in vitro and in vivo methods available for the manufacture chemicals on an industrial scale by biotechnological means. These methods include reactions with enzymes in ordinary stirred tank chemical reactors, microbial fermentation, plant cell culture, manufacturing products with plants (pharming) or other methods. This review summarises and discusses the advantages and limitations of the different in vivo and in vitro methods which could potentially be used for the biotechnological production of chemicals.

Microbial transformation of triterpenoids has provided new derivatives that are potentially useful for pharmacological studies. In these biotransformation processes, several reactions that are difficult to achieve by chemical means have been accomplished, such as: introduction of hydroxyl groups into remote positions of the molecules; selective cleavage of the side chains of tetra-cyclic terpenoids to produce C19 steroids; regioselective glycosidic transfer reactions; selective ring cleavage through a Baeyer-Villiger-type oxidation to render seco-triterpenoids; and carbon skeleton rearrangements involving a methyl group migration. These biotransformations have also been used as in vitro models to mimic and predict the mammalian metabolism of biologically active triterpenoids.

This short review describes the recently reported semi- and total syntheses of naturally occurring triterpenoid saponins. Accent is placed on natural monodesmosidic and bidesmosidic saponins of the oleanane- and lupane-type that possess potent pharmacological properties including cytotoxic and antitumor activities.

In recent years, several classes of biologically active molecules containing the γ-lactone ring, pesticides, plant and fungal growth inhibitors, and antibiotics have been found. Thus the synthesis of substituted dihydrofuran-2(3H) ones is a continuously developing area. Few general synthetic approaches to their stereoselective synthesis with broad structural variety are known. This article reviews the latest developments in the synthesis of γ-butyrolactones. We focus on the ring-closing steps and pay special attention to how different authors obtain the correspondent 4-hydroxycarbonyl compound; an acyclic synthon for the γ-lactone ring.

Heteropolyacids (HPAs) and related compounds have received considerable attention as solid acids in recent years. HPAs possess unique physicochemical properties, with their structural mobility and multifunctionality being the most important for catalysis. They possess a very strong Bronsted acidity and appropriate redox properties that make them excellent catalysts in substoichiometric quantities. This review will focus on describing new developments in the application of HPAs in the organic synthesis of heterocycles. Heterocycles that will be covered include coumarins, flavones, chromones, quinazolidones, dihydropyrimidinones, imidazoles, isoxazoles, pyrimidines, tetrazines, pyrazoles, dihydropyridines, and others.

N-Arylindoles are known to be important subunits due to their key role in medically biological activities. This mini-review initially covers the important advances on the N-arylation of indoles with aryl halides by palladium, copper or other transition metalscatalyzed cross-coupling reactions in recent ten years. Subsequently, the synthesis of N-arylindoles from indoles with aryl halides, activated by electron-withdrawing substituents, by aromatic nucleophilic substitution reactions (SNAr) is also surveyed.

Modified nucleosides possessing intrinsically fluorescent heterocycles as base surrogates that are capable of canonical basepairing are becoming important biomolecular tools. These “base-discriminating fluorophores” (BDFs) find use in hybridization-based mismatch detection, elucidation of DNA damage and the study of localized structural phenomena as exemplified in the study of ribozymes and nucleic acid/ligand interactions. In this review, an historical perspective will be given along with some of the more recent highlights in this rapidly developing field.

The combination of Pd catalyst and triethylborane induces allylic alcohols to undergo direct electrophilic allylation of soft nucleophiles. Similar conditions also accelerate nucleophilic allylations of aldehydes and aldimines to provide homoallyl alcohols and homoallylamines, respectively. Moreover, 2-methylenepropane-1,3-diol undergoes a sequential amphiphilic activation to react with aldehydes and aldimines, giving rise to 3-methylenecyclopentanols and 3-methylenepyrrolidines.

Resorcinarenes constitute a very attractive class of macrocycles possessing a cavity capable of complexing small molecules. They can be simply transformed into chiral derivatives by functionalisation of their hydroxy groups as well as the “ortho” position at the resorcinol ring. The appropriate modifications lead to various chiral resorcinarene derivatives. The chirality of these compounds results from their structure (the axial chirality) or from the presence of chiral auxiliaries. Moreover, the synthesis of chiral resorcinarene derivatives from chiral substrates is also possible. In this short review, we wish to present the strategies and methodology of the synthesis of chiral resorcinarenes, treating this article as an introduction to the chirality of resorcinarenes.