Chiral Diselenides in the Total Synthesis of (+)-Samin.

Chiral selenium compounds are applied to stoichiometric as well as to catalytic reactions in the synthesis of substituted tetrahydrofuran derivatives: The selenium compound 1 was used in catalytic amounts for a rapid access to chiral diselenide 3. The efficient stereoselective addition to alkene 5 yields product 8 with a selenium functionality as a precursor for an intramolecular radical cyclization. In this way a short total synthesis of (+)-samin (11), a naturally occurring furofuran lignan, was achieved.

Chiral boron-bridged bisoxazoline (borabox) ligands: structures and reactivities of Pd and Cu complexes.

Anionic boron-bridged bisoxazolines (borabox ligands) have been synthesized and characterized in their protonated forms. The ligands are tuneable over a wide range, allowing either alkyl or aryl substituents at the oxazoline rings and the central bridging boron atom. The structural parameters of this new ligand type have been investigated by X-ray analyses of palladium and copper complexes. Electronic properties have been studied by (13)C NMR spectroscopy and by DFT calculations on palladium allyl complexes and compared to those of analogous bisoxazoline (box) complexes. Borabox complexes are more electron-rich at the metal center than their neutral box congeners, and as a consequence of the longer bonds between the bridging atom and the oxazoline rings, their bite angles are larger. Palladium(II) complexes bearing an unsubstituted allyl ligand and homoleptic copper(II) complexes each possess an almost flat chelate ring. NMR analysis of a (1,3-diphenylallyl)(borabox)palladium complex showed a 92:8 mixture of (syn,syn) and (anti,syn) allyl isomers, in contrast with a previously reported box analogue that existed exclusively in the (syn,syn) form. Comparison of the corresponding crystal structures revealed that the distance between the bisoxazoline and the allyl ligand in the borabox complex is shorter. In the copper-catalyzed allylic oxidation of cyclohexene and cyclopentene with tert-butyl perbenzoate, borabox ligands gave results similar-and in some cases superior-to those obtained with analogous box ligands.

When electron exchange is chemical exchange-assignment of 1H NMR spectra of paramagnetic cobalt(II)-2,2':6'2''-terpyridine complexes.

Electron exchange between [Co(terpy)2]3+ and [Co(terpy)2]2+ can be monitored by 1H NMR exchange spectroscopy and allows the cobalt(II) spectra to be fully assigned.

Flexible but with a defined turn-influence of the template on the binding properties of two-armed receptors.

Combinatorial binding studies revealed that the di(trans-4-aminoproline)diketopiperazine is an ideal template for two-armed receptors with highly selective binding properties towards peptides. It is not only superior to structurally very different diamines but also to the diastereomeric di(cis-4-aminoproline)diketopiperazine. These empiric results are rationalized by the analysis of the conformation of the diastereomeric diketopiperazines in the solid state, by X-ray crystal structure analysis, as well as by NMR studies in solution: to observe highly selective binding, the template needs to be not only conformationally rigid but it must have a specific turn geometry. The combination of combinatorial binding studies, X-ray crystal structure analysis, and NMR spectroscopy gave insight into why the trans,trans-diketopiperazine is a superior template compared to other diamines. Additionally, the results provide a guide for the rational design of two-armed receptors with good binding properties towards peptidic guests.