Total Synthesis of Tiacumicin B: Study of the Challenging β-Selective Glycosylations**

We give a full account of the total synthesis of tiacumicin B (Tcn-B), a natural glycosylated macrolide with remarkable antibiotic properties. Our strategy is based on our experience with the synthesis of the tiacumicin B aglycone and on unique 1,2-cis-glycosylation steps. We used sulfoxide anomeric leaving-groups in combination with a remote 3-O-picoloyl group on the donors that allowed highly β-selective rhamnosylation and noviosylation that rely on H-bond-mediated aglycone delivery. The rhamnosylated C1–C3 fragment was anchored to the C4–C19 aglycone fragment by a Suzuki–Miyaura cross-coupling. Ring-size-selective Shiina macrolactonization provided a semiglycosylated aglycone that was engaged directly in the noviolysation step with a virtually total β-selectivity. Finally, a novel deprotection method was devised for the removal of a 2-naphthylmethyl ether on a phenol, and efficient removal of all the protecting groups provided synthetic tiacumicin B.     

Potential energy surfaces of an adenine-thymine base pair and its methylated analogue in the presence of one and two water molecules: molecular mechanics and correlated ab initio study

Potential energy surfaces of monohydrated and dihydrated adenine-thymine and 9-methyladenine-1-methylthymine base pairs were examined by the molecular dynamics/quenching technique using the Cornell et al. force field (J. Am. Chem. Soc. 1995, 117, 5179). Long runs of molecular dynamics/quenching calculations allowed us to evaluate the free energy surface. The most stable and populated structures found were fully reoptimized at the correlated ab initio level employing the resolution of identity M?ller-Plesset method. A systematic study of the base pairs' microhydration using both the empirical and the high-level correlated ab initio approaches is presented for the first time. We show that the occurrence of water molecules and their gradually increasing number as well as the methylation of the bases favor stacked structures over the planar hydrogen-bonded ones. These results based on the correlated ab initio calculations are in the excellent agreement with data obtained from our previous empirical potential molecular dynamics study (Kabelác et al. Chem.-Eur. J. 2001, 7, 2067).     

Mixed oxides obtained from Co and Mn containing layered double hydroxides: Preparation, characterization, and catalytic properties

Co-Mn-Al layered double hydroxides (LDHs) with various Co:Mn:Al molar ratios (4:2:0, 4:1.5:0.5, 4:1:1, 4:0.5:1.5, and 4:0:2) were prepared and characterized. Magnesium containing LDHs Co-Mg-Mn (2:2:2), Co-Mg-Mn-Al (2:2:1:1), and Co-Mg-Al (2:2:2) were also studied. Thermal decomposition of prepared LDHs and formation of related mixed oxides were studied using high-temperature X-ray powder diffraction and thermal analysis. The thermal decomposition of Mg-free LDHs starts by their partial dehydration accompanied by shrinkage of the lattice parameter c from ca. 0.76 to 0.66 nm. The dehydration temperature of the Co-Mn-Al LDHs decreases with increasing Mn content from 180 °C in Co-Al sample to 120 °C in sample with Co:Mn:Al molar ratio of 4:1.5:0.5. A subsequent step is a complete decomposition of the layered structure to nanocrystalline spinel, the complete dehydration, and finally decarbonation of the mixed oxide phase. Spinel-type oxides were the primary crystallization products. Mg-containing primary spinels had practically empty tetrahedral cationic sites. A dramatic increase of the spinel cell size upon heating and analysis by Raman spectroscopy revealed a segregation of Co-rich spinel in Co-Mn and Co-Mn-Al specimens. In calcination products obtained at 500 °C, the spinel mean coherence length was 5-10 nm, and the total content of the X-ray diffraction crystalline portion was 50-90%. These calcination products were tested as catalysts in the total oxidation of ethanol and decomposition of N₂O. The catalytic activity in ethanol combustion was enhanced by increasing (Co+Mn) content while an optimum content of reducible components was necessary for high activity in N₂O decomposition, where the highest conversions were found for calcined Co-Mn-Al sample with Co:Mn:Al molar ratio of 4:1:1.     

Evaluation of 3-hydroxy-3-methylglutaryl-COA-reductase, cholesterol-7alpha-hydroxylase and acyl-COA:cholesterol acyltransferase activities: alternative chromatographic methods to separate metabolites

Alternative HPLC and solid-phase extraction column methods were developed to separate metabolites of enzymes involved in cholesterol metabolism in rabbit liver microsomes: hydroxyl-methylglutaryl-CoA reductase, cholesterol-7alpha-hydroxylase and acyl-CoA:cholesterol acyltransferase. A comparison method of thin-layer chromatography and solid-phase extraction column were assayed to separate substrate and metabolite of hydroxy-methylglutaryl-CoA reductase, whereas for cholesterol-7alpha-hydroxylase and acyl-CoA:cholesterol acyltransferase, this comparison was done between thin layer chromatography and HPLC. The results obtained by the new analytical chromatographic methods are not significantly different than those observed in literature. Moreover a larger percentage recovery was obtained for analysed metabolites. Our results demonstrate the reliability of these alternative chromatographic techniques and showed that they are valuable tools to precisely and rapidly measure the activity of those enzymes.     

Supramolecular Organization of Dye Molecules in Zeolite L Channels: Synthesis,Properties, and Composite Materials

Sequential insertion of different dyes into the 1D channels of zeolite L (ZL) leads to supramolecular sandwich structures and allows the formation of sophisticated antenna composites for light harvesting, transport, and trapping. The synthesis and properties of dye molecules, host materials, composites, and composites embedded in polymer matrices, including two‐ and three‐color antenna systems, are described. Perylene diimide (PDI) dyes are an important class of chromophores and are of great interest for the synthesis of artificial antenna systems. They are especially well suited to advancing our understanding of the structure–transport relationship in ZL because their core fits tightly through the 12‐ring channel opening. The substituents at both ends of the PDIs can be varied to a large extent without influencing their electronic absorption and fluorescence spectra. The intercalation/insertion of 17 PDIs, 2 terrylenes, and 1 quaterrylene into ZL are compared and their interactions with the inner surface of the ZL nanochannels discussed. ZL crystals of about 500 nm in size have been used because they meet the criteria that must be respected for the preparation of antenna composites for light harvesting, transport, and trapping. The photostability of dyes is considerably improved by inserting them into the ZL channels because the guests are protected by being confined. Plugging the channel entrances, so that the guests cannot escape into the environment is a prerequisite for achieving long‐term stability of composites embedded in an organic matrix. Successful methods to achieve this goal are described. Finally, the embedding of dye–ZL composites in polymer matrices, while maintaining optical transparency, is reported. These results facilitate the rational design of advanced dye–zeolite composite materials and provide powerful tools for further developing and understanding artificial antenna systems, which are among the most fascinating subjects of current photochemistry and photophysics.     

Ruthenium tetrazene complexes bearing glucose moieties on their periphery: Synthesis,characterization, and in vitro cytotoxicity

Ruthenium tetrazene complexes with general formula [Cp*RuCl(1,4-R₂N₄)] (Cp* = η⁵-C₅Me₅), where R = benzyl ( 1 ), 2-fluorobenzyl ( 2 ), β-d -glucopyranosyl-unprotected ( 3a ) and acyl-protected ( 3b – d ), 2-acetamido-β-d -glucopyranosyl-unprotected ( 4a ) and acyl-protected ( 4b – d ), propyl-β-d -glucopyranoside-unprotected ( 5a ), and O-acetylated ( 5b ), were synthesized and characterized using nuclear magnetic resonance and electrospray ionization–mass spectrometry. In addition, the molecular structure of 3b was determined using X-ray crystallography. The cytotoxicity of complexes against ovarian (A2780, SK-OV-3) and breast (MDA-MB-231) cancer cell lines and noncancerous cell line HEK-293 was evaluated and compared to cisplatin activity. The carbohydrate-modified complexes bearing acyl-protecting groups exhibited higher efficacy (in low micromolar range) than unprotected ones, where the most active 4d was superior to cisplatin up to five times against all investigated cancer cell lines; however, no significant selectivity was achieved. The complex induced apoptotic cell death at low micromolar concentrations (0.5 μM for A2780 and HEK293; 2 μM for SK-OV-3 and MDA-MB-231).     

Relationship between electronic structures and antiplasmodial activities of xanthone derivatives: a 2D-QSAR approach

Structural Chemistry - Malaria is an important disease causing many death in several countries of Africa and Asia. In these continents, some plants such as Garcinia cola are used to fight against...