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Four polymorphs of 2-(perfluorophenyl)-2-(phenylamino)acetonitrile have been crystallized and structurally analyzed: in addition to three racemic crystals, a rare kryptoracemate has been obtained. The central single bonds allow for conformational flexibility: the kryptoracemate as well as two of the remaining polymorphs contain several independent molecules with different conformation. In contrast to these uncommon packing modes, the fourth phase is unexceptional and crystallizes with a single molecule in the asymmetric unit. Individual crystallization batches may contain several crystal forms concomitantly. [ABSTRACT FROM AUTHOR]

The terpenols L(-)-borneol, (1S2S3S5R)-3-pinanol, (-)-menthol, and (-)-myrtenol are easily available chiral alcohols for the preparation of enantiomerically pure catechol esters 1-4-H2. Those ligands are used for the hierarchical assembly of triple lithium-bridged dinuclear titanium(IV) triscatecholate helicates Li[Li3(1-4)6Ti2]. In solution, the dimeric species are in a solvent dependent equilibrium with the monomer Li2[(1-4)3Ti]. The equilibrium is studied by ¹H NMR. CD spectroscopy indicates that the configuration at the complex units of the enantiomerically pure dimeric α-chiral derivatives Li[Li3(1-3)6Ti2] is opposite to the configuration of the monomers Li2[(1-3)3Ti]. For the γ-chiral complex Li2[(4)3Ti] only a de of 25% is observed and in this case no interpretation of the mechanism of stereocontrol is possible. [ABSTRACT FROM AUTHOR]

Five derivatives of 2,6,9,10-tetraoxatricyclo[3.3.1.13,8]decane were synthesized and their molecular structures in the solid state were determined by means of X-ray diffraction analysis. In addition, the structure of all the molecules were optimized at different levels of computational quantum chemistry (HF/6-31G*, B3LYP/6-31G*). Experimentally determined bond lengths were compared with their calculated counterparts, and striking differences between the Hartree-Fock (HF) results and the experimental data could be traced back to the lack of correlation energy in the geometry optimizations. Two of the compounds under consideration are diastereomers and their relative stability was not only calculated with a conventional ab inito method (ZPE+MP2/6-31G*//HF/6-31G*), but also within the framework of density functional theory (B3LYP/6-31G*). In some of the calculations the influence of the solvent was included in single point calculations by means of an electrostatic model. Moreover, combining experimental as well as computational results, the exclusive axial orientation of a Cl atom in one of the compounds could be explained by an interplay of kinetic and energetic effects.

Meta-analysis has been used increasingly in reviewing and summarising epidemiological studies. Reviews incorporating meta-analyses have appeared in medical journals in increasing numbers. Although there are several methodology papers on meta-analysis, most of these papers have been written primarily for discussion among epidemiologists. The present paper considers some of the basic methodological issues, the more practical aspects of meta-analysis, and targets an audience of mainly non-epidemiologists. Thus, the main objective of this paper is to provide some basic guidelines for non-epidemiologists to evaluate meta-analysis in occupational cohort studies. In this methodology paper, the limitations and problems of traditional qualitative reviews are pointed out. Some of these problems can be dealt with by quantitative meta-analysis. The potential limitations and benefits of quantitative meta-analysis are discussed. Rather than replacing traditional qualitative review, quantitative meta-analysis should be made part of the overall assessment. The term "meta-review" is proposed to emphasise the importance of both qualitative and quantitative components in a comprehensive review process. The basic steps in a meta-review are outlined, with a discussion on how to recognise and avoid some of the problems which are likely to occur at each step. A meta-review is useful in selecting studies, and in organising, presenting, and summarising results from individual studies. A meta-review can also be used to detect heterogeneity among studies. Major benefits of conducting a meta-analysis (the quantitative component in a meta-review) include the increase in statistical power and the estimate of a properly weighted summary risk estimate. [ABSTRACT FROM AUTHOR]