In a lot of cases active biomolecules are complexes of higher order, thus methods capable of counting the number of building blocks and elucidating their geometric arrangement are needed. Therefore, we experimentally validate here spin-counting via 4-pulse electron-electron double resonance (PELDOR) on well-defined test samples. Two biradicals, a symmetric and an asymmetric triradical, and a tetraradical were synthesized in a convergent reaction scheme via palladium-catalyzed cross-coupling reactions. PELDOR was then used to obtain geometric information and the number of spin centers per molecule in a single experiment. The measurement yielded the expected distances (2.2-3.8 nm) and showed that different spin-spin distances in one molecule can be resolved even if the difference amounts to only 5 A. The number of spins n has been determined to be 2.1 in both biradicals, to 3.1 and 3.0 in the symmetric and asymmetric triradicals, respectively, and to 3.9 in the tetraradical. The overall error of PELDOR spin-counting was found to be 5% for up to four spins. Thus, this method is a valuable tool to determine the number of constituting spin-bearing monomers in biologically relevant homo- and heterooligomers and how their oligomerization state and geometric arrangement changes during function.

[reaction: see text] Enantioselective catalysis of Diels-Alder reactions is mostly achieved by coordinating the dienophile to relatively strong chiral Lewis acids. Here we report on a novel approach employing the hydrogen-bond-mediated association of dienophiles to chiral host molecules. In a reaction forming the steroid skeleton of norgestrel, chiral amidinium ions induce 5:ent-5 ratios of up to 2.5:1. Improved and simplified amidinium catalysts may become interesting candidates to perform stereoselective transformations.

Dedicated to Albert Eschenmoser on the occasion of his 70th birthday (12.VI.95)A concept of first choice for the synthesis of the title compounds had been proposed by Dane in the late 1930s. It was soon turned down, because the opening move -a chirogenic Diels-Alder reaction -did not work. With Lewis acids as mediators, however, a successful start has been achieved now. With Ti complexes of chelating ligands (Seebach 's TADDOLs ( = a,a,a',a'-tetraaryl-1,3-dioxolane-4,5-dimethanols)), enantioselective formation of the desired adducts does occur. Efficient total syntheses of 2 and 3a have been accomplished.

The chiral bisamidine 5 has been prepared in just two steps from malonodinitrile. In the monocationic form this compound adopts a planar conformation with an almost convergent orientation of two N-H groups. Ketones, aldehydes, and nitro compounds are assumed to bind to this strongly polar cleft via hydrogen bonds, resulting in a Lewis-acid-like activation of the carbonyl groups. A broad scope of reactions (Diels-Alder, hetero-Diels-Alder, Friedel-Crafts) can be catalyzed. The observed accelerations surpass the rate effects of neutral hydrogen-bond donors such as thioureas or TADDOLs.