Current Organic Chemistry - Volume 15, Issue 23, 2011

Its outstanding ability to generate singlet oxygen or reactive oxygen species upon irradiation with light makes hypericin one of the most powerful photosensitizers to be found in Nature. The most important fields of application of hypericin seem presently to be its use as a photosensitizing antiviral agent to destroy virus particles in blood preparations, to sensitize the destruction of superficial tumors, and its use as a diagnostic tool for fluorescence detection of malignant tissue. Despite all of its benefits and its strong photosensitizing ability, the application of hypericin in photodynamic therapy (PDT) suffers from three main disadvantages: cost intensive isolation from natural sources, limited solubility under physiological conditions, and long wavelength absorption maximum outside the optimum wavelength range for a PDT photosensitizer. Thus, over the last years several efforts have been undertaken to synthesize new hypericin derivatives with increased solubility, a bathochromically shifted long wavelength absorption band, enhanced photosensitizing potential, and applicability to a more widespread field (second generation hypericin based photosensitizers). The objective of this review is to discuss the investigations undertaken in the development of second generation hypericin based photosensitizers. The focus will be on the synthesis of such derivatives as well as on their photochemical properties, their advantages, and disadvantages. Several of these novel synthetic hypericin derivatives have already been tested in vitro and/or in vivo. Accordingly, a summary of the corresponding results will be presented.

Combined reactions through organoboron intermediates belong to the class of sequential reactions whereby the advantages are clear in terms of efficiency and selectivity. The goal to prepare target molecules from simple substrates requires strategic steps and very selective formation of new bonds. The addition of boryl units to unsaturated substrates can be performed chemo-, regio- and stereoselectively, therefore the organoboron intermediates might be easily transformed into the functionalized molecules, with total retention of configuration. Taking advantage of these protocols, here we describe the most recent advances in combined reactions through B chemistry.

The methods to prepare Nafion materials with significantly improved specific activities, their characterization and applications as useful and efficient catalysts are summarized. The sol-gel technique used most widely, covalent anchoring of the sulfonic acid group to the surface via a perfluoroalkane tether via post-synthesis modification, impregnation of high-porosity spinodal silica and mesoporous silica materials, and mechanochemistry all allow the formation of Nafion materials with enhanced accessibility of the active sites and, consequently, improved catalytic properties can be experienced. The review includes and discusses the use of related immobilized fluoroalkanesulfonic acid preparations. The transformations induced by these solid acids including Friedel-Crafts and related reactions, various transformations of alkenes, alcohols, and carbonyl compounds, protective group chemistry, chemistry of carbohydrates and heterocycles, oxidations, and a few examples of non-catalytic applications are treated in detail.

This review highlights the application of sulfated, molybdated and tungstated zirconia solid acid catalysts, and their modified forms for variety of organic synthesis and transformation reactions in the liquid phase. Most of these catalysts offer significant improvements in various organic reactions with regard to the yield of products, simplicity in the operation, reusability of the catalysts and green features by avoiding toxic conventional catalysts. Preparation of various zirconia-based solid acid catalysts has been briefly described. Characterization of these catalysts by different techniques has also been presented. Most of these catalysts are highly promising for numerous organic reactions in the liquid phase which include condensation, isomerization, esterification and transesterification, muticomponent reactions and so on.

Triphenylphosphine (TPP), dialkyl acetylenedicarboxylates (DAAD), and acids such as phenols, imides, amides, enols, oximes and alcohols react with each other via a multicomponent reaction to produce stabilized phosphorus ylides. The reactions take place easily, through formation of intermediate formed by the Michael addition of the triphenylphosphine to dialkyl acetylenedicarboxylates and concomitant protonation of the intermediate by an acid leads to vinyltriphenylphosphonium salts. The salts are unstable intermediates and undergo a subsequent Michael addition leads to stabilized phosphorus ylides. In some cases ylide products are stable, but in other cases they cannot be isolated and appear to occur as intermediates on the pathway to an observed product. The stabilized phosphorus ylides are able to take part in the normal intramolecular Wittig reactions and produce a variety of heterocyclic or carbocyclic compounds, but they are not able to participate in the intermolecular Wittig reactions.