Uri Shmueli

Latrunculin, a new 2-thiazolidinone macrolide from the marine sponge latrunculia magnifica

Abstract Three new toxins were isolated from the Red Sea sponge Latrunculia magnifica A full structure determination of these toxins, by spectroscopic methods, was aided by an X-ray diffraction analysis of a crystalline derivative ( 1 b) of latrunculin-A. The Latrunculins exhibit a new class of 14 and 16 membered macrolides to which the rare 2-thiazolidinone moiety is attached. A biogenesis of these biologically active compounds is put forward.

Chemistry of bridged carboxylate ruthenium complexes. Crystal structures of [Ru(PhCOO)(CO)2(PhCOOH)]2, [Ru(4-FC6H4COO)2(CO)5(H2O)]2•0.5 C6H6, and [Ru2(s-BuCO2)2(CO)4(s-BuCO2H)]2

Abstract Aliphatic and aromatic carboxylic acids with Ru 3 (CO) 12 to give bridging carboxylate complexes containing the structural core unit [Ru(μ 3 -RCOO)(CO) 2 ] 2 . The presence of an excess of acid results in side ligation of two carboxylic acid molecules, which are also hydrogen-bonded to the bridging carboxylate ligand; this structure was confirmed by X-ray crystallography. Most of the complexes readily form mon-aquato compounds one of which was investigated by X-ray crystallography. In the case of 2-methylbutyric acid a Ru tetrameter complex, in which two core units are bound by two RuORu bridges, was isolated; we suggest that this structural element constitutes the back-bone of the ruthenium carboxylate polymers that were obtained under the appropriate reaction conditions from the most of the acids. The intial poroduct of the reaction between Ru 3 (CO) 12 and a carboxylic acid is [Ru(RCOO)(CO) 3 ] 2 , which then reacts further. Its reactivity varies on going from straight chain to α-carbon branched chain aliphatic acids and to aromatic acids. Spectral and chemical evidence suggests an associative type mechanism for displacement of CO by other ligands in the primary product. The distribution and rates of formation of the products depend on the nature of the carboxylate ligand and the molar ratio of dodecacarbonyltriruthenium to acid and on whether the reaction is carried out in an open or a closed vessel.

The mean-square Friedel intensity difference in P1 with a centrosymmetric substructure.

For non-centrosymmetric structures in space group P1 containing a centrosymmetric substructure, analytical expressions have been obtained for various functions of the diffraction intensity of Friedel opposites. These functions are the average intensity of Friedel opposites, the mean difference in intensity of Friedel opposites and the mean-square difference in intensity of Friedel opposites. A Bijvoet intensity ratio is defined for the evaluation of resonant-scattering effects in non-centrosymmetric and pseudo-centrosymmetric structures. Analysis of these expressions confirms that both resonant and non-resonant atoms are necessary to produce differences in intensity between Friedel opposites and also shows that in some circumstances atoms may lie on a centrosymmetric substructure without diminishing the Bijvoet intensity ratio. The effects of the real component of resonant scattering, of the variation of the scattering factors with sin theta/lambda, of isotropic atomic displacement parameters, of a crystal twinned by inversion, of atoms in special positions and of weak reflections are considered. Software is available for the evaluation of the Bijvoet intensity ratio.

Sipholenol and sipholenone, two new triterpenes from the marine sponge (levi).

Abstract Several squalene-derived triterpenes possessing a hitherto unknown ring system were isolated from the Red Sea Sponge, Siphonochalina siphonella . The structure of one of the new compounds, sipholenol ( 1 a), was determined by spectroscopic methods including a full x-ray diffraction analysis.

Exact random-walk models in crystallographic statistics. I. Space groups P and P1

Probability density functions that are exact solutions to classical random-walk problems have been adapted to represent distributions of the magnitude of the normalized structure factor, for the space groups P\bar and P1. The functions are given by readily summable Fourier and Fourier-Bessel series, and account explicitly for the atomic composition of the asymmetric unit. These new probability density functions have been extensively tested by comparison with simulated histograms of |E|, for a wide range of atomic compositions. The most heterogeneous compositions examined are C14U and C29U, for P\bar 1 and P1, respectively. Very good agreement between the simulated and theoretical distributions has been obtained in all these tests, over the entire (useful) range 0 < |E| < 3. A distribution of |E| values, recalculated from published data on a triclinic platinum complex with chloroorganic ligands, has also been compared with the new probability functions and excellent agreement with the (expected) P\bar 1 theoretical distribution has been obtained. The discrepancy between the recalculated distribution and the P1 theoretical rules out the latter space group both by visual comparison and quantitative discrepancy criteria. It is concluded that probability density functions are definitely preferable to moments in attempting to resolve a space-group ambiguity. Measures of discrepancy to be used in such statistical tests are proposed and discussed in some detail.

Reactions of coordinated cyclic polyolefins. A novel perispecific Diels-Alder reaction of tricarbonyl(η4-cycloheptatriene)iron and (carbomethoxy)maleic anhydride controlled by secondary orbital overlap☆

Abstract The cycloaddition reactions of (carbomethoxy)maleic anhydride (CMA) with (η 4 -cycloheptatriene)Fe(CO) 3 and with uncoordinated cycloheptatriene were investigated. Both reactions are highly stereospecific and give single Diels-Alder adducts. The structure of the metal-bonded adduct X was determined by single-crystal X-ray diffraction. A full analysis of the 1 H and 13 C NMR spectra of X is described. The corresponding uncomplexed adduct XI was obtained by Diels-Alder reaction of CMA with norcaradiene. The unusual Diels-Alder periselectivity and the stereochemical consequences of the 4 + 2 cycloaddition to X are best rationalized in terms of a concerted reaction controlled by secondary orbital interactions in the transition state.

Trans-1,4,5,8-tetracarboethoxy- and 1,4,5,8-tetramethyl- 1,4,5,8-tetraazadecalin (tad) ☆ ☆☆

Abstract Chemical correlation between the title compounds ( 7 and 8 ) was performed. X-ray crystallographic analysis of the novel tetraurethane ( 7 ) proved the trans configuration of ( 7 ) and indirectly that of tetramethyl TAD ( 8 ). The latters chemical and conformational behavior is examined and discussed.

Chemistry and structure of [1]benzothieno[2,3-b][1]benzothiophene

Abstract The reaction of 1,1-diphenylethylene with sulphur yields [1]benzothieno[2,3-b][1]benzothiopene (I), together with 3-phenylbenzothiophene. Oxidation of I with aqueous peracetic acid gives the corresponding monosulphone (III) and disulphone (IV). The structures of I and its derivatives were established by chemical reactions and electronic and NMR spectra. The structure of the parent compound I was determined by X-ray analysis. The aromatic character of the condensed thiophene system (I) was demonstrated by chemical reactions as compared with those of the corresponding sulphones, and from the observed bond lengths obtained from the X-ray analysis of I.

Intensity statistics of Friedel opposites.

A previous analysis of the average intensity and mean-square intensity difference of Friedel opposites, confined to the space group P1 [Flack & Shmueli (2007). Acta Cryst. A63, 257-265], is here extended to all the non-centrosymmetric space groups. The present analysis presumes purely non-centrosymmetric content of the unit cell. An important result of this study is that the average intensity and mean-square intensity difference of Friedel opposites have the same values for all the non-centrosymmetric space groups as those previously obtained for the triclinic space group P1. The ratios of average intensity and root-mean-square intensity difference to their triclinic equivalents were derived and exemplified for general as well as for special reflections. For the latter, enhancements were obtained which are shown to be due to those of average intensity and not to a mechanism related to Friedel opposites being explicitly considered.

Simple and efficient approach to preparation of molecular drawings

Abstract A convenient approach to the preparation of good quality molecular drawings is described. The method is based on transforming the atomic coordinates provided by a conventional crystallographic structure analysis or deduced from a model of a particular molecule, to a suitably weighted molecular inertia system and forming a projection of the molecule down the axis that corresponds to its highest moment of inertia. When implemented in a routine which prepares the input to the program ORTEP, the new method very nearly eliminates trial-and-error procedures from the preparation of a molecular diagram with suitable orientation of the molecule.