Current Organic Synthesis - Volume 9, Issue 5, 2012

This review reports on recent developments on the structure, biological activities and synthesis of hyacinthacine alkaloids and related compounds.

2-Naphthol and its analogs are widely used starting materials for a large variety of target molecules which range from natural products to compounds of medicinal importance. Having a cascade of reactions occur in one pot is highly advantageous for green chemistry, as it reduces time, energy and solvent consumption. With 2-naphthol being such a versatile and widely used starting material capable of reaction with a variety of synthons, it is logical that there is interest in making these syntheses greener. The ability to perform onepot multi-component cascade reactions can translate into saved time and reduced solvent use, and lower energy consumption. With movement towards greener chemical syntheses, one-pot cascade reactions are highly desired for their efficiency both in time and in atom economy. This review provides an account of all one-pot cyclization reactions of 2-naphthol to date, along with product yield and mechanistic pathway in an attempt to aid fellow chemists studying 2-naphthol chemistry.

Catecholic motifs are present in numerous natural products and synthetic compounds used in various sectors of the chemical industries such as food, cosmetic, pharmaceutical and polymer industries. The antioxidant activity usually conferred to compounds bearing catecholic motifs is the property on which their application is often based. Although several (bio) chemical methods are available to chemists to produce catechols, the oxygenation of phenols constitutes one of the most practical approaches as long as the orthoselectivity of the process can be controlled. In this context, oxygenating hypervalent iodine(V) reagents offer a convenient metal-free solution. Among these reagents, 2-iodoxybenzoic acid (IBX) or its stabilized (non explosive) SIBX and polymer-supported versions have found the most useful and successful applications in the conversion of phenols into ortho-quinones, followed by reduction into catechols. Examples of oxygenative demethylation of 2-methoxyphenols and ortho-hydroxylation of phenolic compounds for the synthesis of biologically and industrially-relevant catechols are highlighted in this review article, together with some mechanistic discussions on these transformations.

Clays are extraordinary layered materials whose structure is composed of stacked inorganic metal oxide layers built by connected sheets. These layers hold usually charge excess or deficiency which is balanced by the presence of either cations (cationic clays) or anions (anionic clays) at the interlayer spacing. By taking advantage of this relevant property, many works have been dedicated to explore this by introducing cations or anions inside the interlayer spacing of clays. This includes exchange from simple cations to complex structures such as large polyoxoions or even metal complexes. The resulting tuned materials find wide use in several catalytic applications with the advantage that in some cases the heterogeneous process overcomes the homogeneous one. Important chemical reactions such as the Suzuki and Heck coupling reactions or other such as Friedel-Crafts, Michael addition and Diels- Alder reactions can be successfully catalyzed by such materials. Oxidation reactions including epoxidation of olefins and oxidation of alcohols to carbonyl compounds are also important chemical processes where clays can be effective catalysts. From the economical point of view such catalysts feature high synthetic flexibility, tunable acid-base properties and ease of set-up and work-up, among others. These properties lead to the development of more efficient catalytic systems with improved yields and selectivity, which are valuable achievements in the present days by contributing towards the establishment of environmentally friendly technologies.

In continuation to our earlier reviews on organocatalysis, a systematic approach for the synthesis of chiral β-amino acid derivatives in the presence of mainly organocatalysts and transition metal catalysts are described. Asymmetric Mannich reactions catalyzed by proline and bifunctional organocatalysts are explained, whereas metal catalysis based on hydrogenation, Mannich and conjugate additions are elaborated. The resulting β-amino carbonyl moieties are essential structural units of many natural products and are important chiral building blocks for the synthesis of nitrogen-containing bioactive molecules.

Barium sulfate nano-particles (BaSO4 nano-particles) has been prepared form the homogeneous precipitation of BaCl2 with triethylammonium hydrogen sulfate in aqueous media at room temperature. The prepared nano-particles has been used for the preparation of 2,4,5-triaryl and N-aryl(alkyl)-2,4,5-triaryl imidazoles in high yields under thermal conditions with easy catalyst separation and possible reusability of the catalyst.