Peloruside A: a potent cytotoxic macrolide isolated from the new zealand marine sponge Mycale sp

A novel, polyoxygenated, pyranose ring containing 16-membered macrolide peloruside A (1) exhibiting cytotoxic activity in the nanomolar range was isolated from the New Zealand marine sponge Mycale sp. The structure of 1 and relative stereochemistry of the 10 stereogenic centers were determined on a 3 mg sample using a variety of spectroscopic methods. Compound 1 was isolated along with the previously reported cytotoxins mycalamide A (2) and pateamine (3) from a single specimen of this sponge.     

The Mukaiyama-Michael reaction of N-acyl-2,3-dihydro-4-pyridones: regio- and stereoselective synthesis of cis-2,6-disubstituted 1,2,5,6-tetrahydropyridines

[formula: see text] A convenient regio- and stereoselective preparation of 1,2,5,6-tetrahydropyridines of the type 1 has been developed, starting from readily available N-acyl-2,3-dihydro-4-pyridones 2.     

The possibility of different time scales in the dynamics of pore imbibition

To model the imbibition of liquids into porous solids, use is often made of the Lucas-Washburn equation, which relates the distance of penetration of a liquid at a given time to the pore radius, the viscosity and surface tension of the liquid, and the effective contact angle between the liquid and the solid. In this paper, we extend previous large-scale molecular dynamics simulations to show how this tool can be used to study the details of liquid imbibition, including the impact of the contact angle on the dynamics of penetration and the evolution of the internal flow field. In particular, we show that the asymptotic behavior of the contact angle versus time for a completely wetting liquid is given by approximately t(-1/4).     

Tuning the acceptors in catalyzed cyclizations initiated by allenes. Silylstannylation/cyclization of allene-aldehydes for synthesis of polyalkylated indolizidines including 223A congeners

Starting from succinamide and 1,2-heptadiene-4-ol, a racemic allene-aldehyde substrate, 20, suitable for R(3)SiSnR'(3)-mediated cyclization was synthesized in six steps and in 21% yield. Stereoselective cyclization (relative cis configuration at the new stereogenic centers of the homoallyl alcohol generated) proceeded smoothly, giving a mixture of indolizidinols bearing five contiguous stereocenters in a combined yield of 80%. Relative configurations of each of the products were unequivocally established by a combination of 2D NMR experiments and single-crystal X-ray analysis. The major indolizidinol obtained in 32% yield was elaborated into indolizidine 5,8-epi-indolizidine 223A via a five-step reaction sequence in 32% overall yield. The second major component (24%) of the key cyclization yielded, in four steps, indolizidine 6,8-epi-223 in 14% yield. Even though revision of the initially postulated structure foiled our original synthetic plans for the natural product, indolizidine 223A, the new stereoselective cyclization strategy and several selective transformations of the indolizidine derivatives reported here may find further applications for the synthesis of highly alkylated indolizidine and other related alkaloids.     

Gas-phase structures and mass spectra of binary pentafluorides

Most known non-radioactive pentafluorides have been examined by molecular-beam mass spectrometry and by the deflection of molecular beams in inhomogeneous electric fields. Extensive association of the vapors occurs for all but the lighter pentafluorides and the interhalogens. The interhalogen pentafluorides are strongly polar, consistent with the accepted C_4v symmetry. The transition-metal and Group V pentafluorides are all non-polar, except VF₅ and CrF₅ for which temperature-dependent polarity is observed. However, uncertainty exists as to whether these observations are applicable to monomeric pentafluorides in all cases. Mass-spectral cracking patterns are presented for all species.     

Investigation on the chemistry of berbans—VII: Synthesis of 10,11-dimethoxy(depyrrolo)raunescine stereoisomers

Acyloxy-ketone rearrangement, proceeding by the action of base, has been observed in the case of ketone 2. By the reduction of ketones 2 and 3 with sodium boron hydride, new 10, 11-dimethoxy(depyrrolo)raunescine stereoisomers (5a–7a) have been prepared. Previously known alcohols with alloberban skeleton (12,13) have been similarly converted into tetracyclic raunescine stereoisomers (8a,9) by the oxido-reductive method. The stereochemistry of these compounds has been investigated by physical (¹H NMR, IR) and by chemical methods.     

Potentiometric studies of dithiooxamide with a sulphide-selective membrane electrode

The sulphide-selective electrode is applicable to the determination of dithiooxamide by titration with silver nitrate. The effect of the alkali content of the solutions on the reaction has been studied. The reaction products are silver sulphide, oxalic acid, and nitric acid. If the alkali concentration of the solution is lower than that equivalent to the acid formed during the titration, the amount of sulphide produced by hydrolysis decreases and the equilibrium potential is established more slowly.The determination is rapid and accurate in the presence of 1 M sodium hydroxide.     

Interaction of adenine and thymine with aqueous sugar solutions

The solubility of the nucleic acid bases, adenine and thymine, in aqueous erythritol, xylose, glucose, and sucrose solutions has been studied. The solubility of adenine increases linearly with glucose and sucrose concentration, whereas with the other reagents a nonlinear increase is observed. Below 1.5M reagent concentration, the solubility of adenine increases in the order erythritol < robose, xylose < glucose < sucrose. The solubility of thymine in these solutions, on the other hand, decreases, increases, or does not change depending upon the reagent. The effect of temperature on the solubility of adenine and thymine in sugar solution indicates that the transfer of these molecules from water to sugar solution is exothermic.Presented in part at the VIIth All-India Symposium in Biophysics held at Visva Bharati University during October 1976.     

Adsorption Peculiarities of Carbon Monoxide on Iridium Electrodeposits

The transients of current and open-circuit potential observed at CO adsorption on Ir/Pt electrodes in solutions of sulfuric and hydrochloric acids are shown to adequately agree with those theoretically expected. From voltammetric curves of electrooxidation of chemisorbed CO, the values of its adsorption are estimated by making a proper double-layer correction. It is concluded that CO is preferentially adsorbed in the bridge form. During CO adsorption from sulfuric acid solutions on an electrode with a preliminarily accumulated monolayer of copper adatoms (Cu_ad), small negative current are observed and Cu_ad are displaced by 50%. To explain these results, it is assumed that CO is reduced by copper adatoms.     

Atomic Absorption Assay of Noble Metals in Ores with the Use of Sample Decomposition by Ultrasound

An atomic absorption assay of gold, silver, and palladium in various types of ores is performed with the use of ultrasound at the fire assay stage. It is shown that ultrasound can reduce the temperature and duration of fusion and increase the recovery of noble metals into the lead button.