In this paper structural and microbiological studies on the ionophorous antibiotic monensin A and its derivatives have been collected. Monensin A is an ionophore which selectively complexes and transports sodium cation across lipid membranes, and therefore it shows a variety of biological properties. This antibiotic is commonly used as coccidiostat and nonhormonal growth promoter. The paper focuses on both the latest and earlier achievements concerning monensin A antimicrobial activity. The activities of monensin derivatives, including modifications of hydroxyl groups and carboxyl group, are also presented.

New Monensin A acid complexes with water molecule, sodium chloride and sodium perchlorate were obtained and studied by X-ray and (1)H, (13)C NMR and FT-IR methods as well as ab initio calculations. The crystal structure of the complexes indicates the complexation of the water molecule and Na(+) cation in the pseudo-cycle conformation of the Monensin acid molecule stabilised by intramolecular hydrogen bonds. Important for stabilisation of this structure is also the intermolecular hydrogen bonds with water molecule or the coordination bonds with Na(+) cation. It is demonstrated that the counterions forming intermolecular hydrogen bonds with OH groups influence the strength of the intramolecular hydrogen bonds, but they have no influence on the formation of pseudo-cyclic structure. Spectroscopic studies of the complexes in dichloromethane solution have shown that the pseudo-cyclic structure of the compounds is conserved. As follows from the ab initio calculations, the interactions between the Na(+) cation and the electronegative oxygen atoms of Monensin acid totally change the molecular electrostatic potential around the supramolecular Monensin acid-Na(+) cationic complex relative to that of the neutral Monensin acid molecule.

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A highly efficient asymmetric hydrosilylation (AHS) of a wide variety of prochiral aryl ketones catalyzed by zinc acetate with TPS-he-pybox (tert-butyldiphenylsilyl hydroxyethyl pybox) ligand has been successfully developed. Cheap and readily available chiral Lewis acids based on zinc salts have been used as promising catalyst for the reduction of aryl ketones under mild conditions at room temperature leading to chiral alcohols in excellent yields and good to high enantioselectivities (up to 85% ee).Keywords: asymmetric synthesis; chiral alcohols; metal complexes; reduction; zinc Asymmetric hydrosilylation (AHS) of prochiral ketones represents one of the most efficient methods for the preparation of chiral secondary alcohols. Although the asymmetric hydrosilylation using catalysts based on expensive precious metals such as ruthenium, rhodium or iridium is well established, [1] an efficient means to access chiral alcohols by using cheaper and more environmentally friendly metals still remains a significant challenge. Noteworthy recent contributions in this field have emerged by utilizing chiral iron [2] and copper [3] complexes. Despite some advances, the demand for less expensive, environmentally more benign zinc-based catalytic systems for the asymmetric hydrosilylation of ketones is still an urgent need and of great significance because of the broad availability and biomimetic nature of zinc. Surprisingly, only few studies have been presented in this field, mostly utilizing zinc catalysts made of unstable and hazardous dialkylzinc compounds.[4] Synthesis and application of these organometallic species is dangerous and inconvenient, especially for industrial or large-scale use. Therefore, considerable efforts should be directed towards the development of cheap, reliable and safe zinc complexes for asymmetric hydrosilylation.For the first time an inorganic zinc salt was applied for asymmetric hydrosilylation by Nishiyama et al. in 2009.[5] These authors demonstrated that ketones can undergo reduction in an asymmetric manner with ees ranging at about 80% (one example with 92% ee) by using zinc acetate complexes with a ligand based on chiral diaminocyclohexane (DACH). Recently Lai [6] applied a similar zinc-based catalyst with chiral DACH ligands containing furan rings.In 2012 Beller and co-workers [7] used chiral i-Pr-(1) and Ph-pybox (2) ligands (Scheme 1) with either zinc acetate or diethylzinc. These authors showed that both of these catalysts can induce stereoselectivity in the reduction of acetophenone, but the reactions catalyzed by ZnA C H T U N G T R E N N U N G (OAc) 2 -pybox led to poor ees and converScheme 1. Structures of pybox-type ligands used in this work: i-Pr-pybox, Ph-pybox and he-pyboxes.