Current Organic Chemistry - Volume 16, Issue 3, 2012

The enantiomeric differentiation ability of chiral sugar-based crown ethers can be utilized for the determination of the enantiomeric composition of chiral amino compounds and for the separation of racemic amino compounds. Chiral crown ethers incorporating various carbohydrate units in the macrocyclic structure showing enantiomeric recognition towards chiral primary amines, amino acids and amino acid esters are reviewed here. The chiral recognition of organic ammonium salts by crown ethers may be tested with various methods. In this review, practical applications are also summarized, where the enantiomers of racemic amino compounds and their derivatives are separated by carbohydrate-based crown ethers immobilized on a stationary phase.

Relationship between enantioselectivity of asymmetric reactions and Cn symmetry of chiral metal complexes, as catalysts, has been analyzed in the case of monofunctionalized (one-point binding) substrates. It has been found that effective reactions (ee up to >99%) go through nonsymmetric (asymmetric) intermediate complex responsible for asymmetric induction. If important intermediate retains the C2 or C3 symmetry of initial catalytic complex, enantioselectivity (ee of the product) should be relatively small. Enantioselectivities of the catalytic reactions which deal with three- or two-point binding substrates are independent of C1 or C2 symmetry of initial catalytic complex. These reactions will proceed with high enantioselectivities (up to >99% ee) in both cases. The reasons for all phenomena are discussed.

The Nicholas reaction involves the treatment of a cobalt carbonyl-stabilized propargylic cation with a variety of nucleophiles to form a new carbon-carbon or carbon-heteroatom bond. Several features of this reaction make it especially well suited for applications in natural product synthesis, in particular when cyclic and polycyclic target molecules are involved. This review summarizes the results in this area from the last ten years, including approaches towards terpenes, alkaloids and marine natural products.

Tamoxifen (TAM) comprises a triphenylethylene derivative with pronounced activity as Selective Estrogen Receptor Modulator (SERM) that acts on the Estrogen Receptor (ER)-positive breast cancer cells preventing the tumorigenesis in high risk women. TAM constitutes the most prescribed chemotherapeutic drug for the treatment of breast cancer, acting through its in vivo metabolites (Z)- 4-hydroxytamoxifen (Z-4-HTAM) and 4-hydroxy-N-desmethyl-tamoxifen (endoxifen). Literature search has indicated that the efficient construction of TAM backbone is achieved through three major routes. One concerns the formation of the appropriately phenylsubstituted double bond of TAM, a second includes the reductive coupling of two aromatic ketones and the third refers to the coupling of olefins with metallated aromatics in the presence of transition metal catalysts. A deconstruction of these pathways and a comprehensive picture of the synthetic strategies towards the efficient approach of TAM are presented in this review. Furthermore, novel synthetic routes that differentiate or act complementary to these general routes are also considered. Issues such as the reactions efficiency and stereoselectivity have also been addressed.

Spirostanes, family of steroidal sapogenins continue to attract attention of organic chemists due to their easily occurring cleavage reactions in F ring and because of the renaissance of many natural structures containing steroid spiroketal moiety characterized by interesting biological activity. This article reviews progress in the steroid spirostanes chemistry that was published between 1992 and 2010.

In this account we present the rapidly expanding bibliography of the published researches concerning the progress in the area of pyrazolones chemistry including synthetic strategies and their applications

The present review focuses on the latest development in the synthesis of pyrazolo[3,4-b]pyridines with special accent placed on ring closing methodologies and analysis of diversity of synthons. It also overviews the current green chemistry and multi-component reaction approaches. The major methods and modifications are analyzed.