A case study in conformation design: learning by doing

Efforts are described to design simple, fully flexible but conformationally preorganised omega-hydroxy-nonanoic acids that could serve as the conformation controlling unit in analogues of the potent protein-kinase C activator aplysiatoxin. Such analogues are macrodilactones incorporating the designed omega-hydroxy-nonanoic acid and 3,4-dihydroxy-pentanoic acid, which contains the pharmacophoric groups. The design process (replacement of CH(2) groups by an oxygen atom, annelation of a six-membered ring and placement of alkyl substituents) of the omega-hydroxy-nonanoic acids was monitored by force-field calculations. In the end of this process simple analogues of aplysiatoxin are proposed in which the proper disposition of the pharmacophoric groups is secured by a conformationally flexible but preorganised template structure as part of the macrodilactone ring.     

Toward the synthesis of reidispongiolide a: stereocontrolled synthesis of the C17-C22 and C23-C35 degradation fragments

[structure: see text] By relying on the asymmetric aldol reactions of chiral ketones, a highly stereocontrolled synthesis of each of the C(17)-C(22) and C(23)-C(35) degradation fragments of reidispongiolide A has been achieved. This permits a configurational assignment of the complete C(17)-C(36) region of this antimitotic macrolide, along with providing advanced intermediates for a projected total synthesis.     

Stable Mesoionic N-Heterocyclic Olefins (mNHOs)

We report a new class of stable mesoionic N-heterocyclic olefins, featuring a highly polarized (strongly ylidic) double bond. The ground-state structure cannot be described through an uncharged mesomeric Lewis-structure, thereby structurally distinguishing them from traditional N-heterocyclic olefins (NHOs). mNHOs can easily be obtained through deprotonation of the corresponding methylated N,N′-diaryl-1,2,3-triazolium and N,N′-diaryl-imidazolium salts, respectively. In their reactivity, they represent strong σ-donor ligands as shown by their coordination complexes of rhodium and boron. Their calculated proton affinities, their experimentally derived basicities (competition experiments), as well as donor abilities (Tolman electronic parameter; TEP) exceed the so far reported class of NHOs.     

Effect of the homogeneity of the column set on the performance of a simulated moving bed unit. II. Experimental study

Previous studies of the simulated moving bed (SMB) process assume identical characteristics of all the columns incorporated in a given unit. Due to the practical impossibility to manufacture identical columns, numerical applications of the theory to modeling and optimization use for each of the needed column parameter the average value for the entire column set. In this study, the effects of these simplifications on the actual productivity of the SMB process are evaluated by making exact calculations, i.e., by taking the differences in the porosity values into account. We apply a revised set of separation conditions previously introduced and derived from the equilibrium theory. Earlier theoretical results are compared to experimental results obtained in the study of the enantiomeric separation of Tr?ger's base on Chiralpak AD. Due to the nonLangmuirian character of the adsorption isotherms of these two compounds on the packing material used, the separation area cannot be determined analytically. As an alternative, a reliable numerical algorithm was used to scan a wider region and to define the separation area. The form of this area depends on the applied porosity values. A UV detector and a laser polarimeter located at one node of the SMB monitor on-line the internal concentration profiles. Excellent agreement between the calculated and the experimentally determined concentration profiles was obtained under nonlinear conditions. The influence of column-to-column variations on the performance of the SMB process was found to be more significant under nonlinear than under linear conditions.     

Ancillary ligand effect on single-site styrene polymerization: isospecificity of group 4 metal bis(phenolate) catalysts

Styrene is polymerized isospecifically by group 4 metal complexes that contain a C2-symmetrically coordinated 1,4-dithiabutane-linked bis(phenolato) ligand activated by methylaluminoxane.     

Multichannel effective range approach to low energy nucleon-antinucleon interactions

MultichannelM-matrix method is applied to an N¯N-system with taking into account elasticp¯ p-scattering, charge-exchangep¯ p → n¯ n reaction andp¯ p annihilation. The isospin breaking in theM-matrix is discussed. We describe the differential and total cross-sections forp¯ p → p¯ p andp¯ p → n¯ n processes, cross-section σ_tot(p¯ p) and the real-to-imaginary ratio of the forward amplitude of elasticp¯ p scattering. The fitting leads to bound state in thep-wave near thep¯ p-threshold, which elucidates a significant role of this wave at small energies.