Enantioselective synthesis of chiral porphyrin macrocyclic hosts and kinetic enantiorecognition of viologen guests

The construction of macromolecular hosts that are able to thread chiral guests in a stereoselective fashion is a big challenge. We herein describe the asymmetric synthesis of two enantiomeric C₂-symmetric porphyrin macrocyclic hosts that thread and bind different viologen guests. Time-resolved fluorescence studies show that these hosts display a factor 3 kinetic preference (ΔΔG^‡_on = 3 kJ mol⁻¹) for threading onto the different enantiomers of a viologen guest appended with bulky chiral 1-phenylethoxy termini. A smaller kinetic selectivity (ΔΔG^‡_on = 1 kJ mol⁻¹) is observed for viologens equipped with small chiral sec-butoxy termini. Kinetic selectivity is absent when the C₂-symmetric hosts are threaded onto chiral viologens appended with chiral tails in which the chiral moieties are located in the centers of the chains, rather than at the chain termini. The reason is that the termini of the latter guests, which engage in the initial stages of the threading process (entron effect), cannot discriminate because they are achiral, in contrast to the chiral termini of the former guests. Finally, our experiments show that the threading and de-threading rates are balanced in such a way that the observed binding constants are highly similar for all the investigated host–guest complexes, i.e. there is no thermodynamic selectivity.

Chiral guests display kinetic stereoselective threading through chiral porphyrin cages if their chirality is located at the chain ends and not in the centers, supporting the previously reported entron effect of threading.     

Synthesis and self-assembly of giant porphyrin discs

A giant porphyrin disc (M(w)= 15 kDa) has been synthesized and its self-assembly behaviour at an interface studied by liquid STM which reveals the presence of huge domains (>400 x 400 nm2) of very well ordered and molecularly resolved columnar stacks.     

Oxygen binding and activation by the complexes of PY2- and TPA-appended diphenylglycoluril receptors with copper and other metals

The copper(I) complexes of diphenylglycoluril basket receptors and , appended with bis(2-ethylpyridine)amine (PY2) and tris(2-methylpyridine)amine (TPA), respectively, and their dioxygen adducts were studied with low-temperature UV-vis and X-ray absorption spectroscopy (XAS). The copper(I) complex of, [.Cu(I)2] or, forms a micro-eta2:eta2 dioxygen complex, whereas the copper(I) complex of, [.Cu(I)2] or, does not form a well defined dioxygen complex, but is oxidized to Cu(II). Dioxygen is bound irreversibly to and the formed complex is stable over time. The coordination geometries of the above complexes were determined by XAS, which revealed that pyridyl groups and amine N-donors participate in the coordination to Cu(I) ions in the complexes of both receptors. The catalytic activities of various metal complexes of and , that were designed as mimics of dinuclear copper enzymes that can activate dioxygen, were investigated. Phenolic substrates that were expected to undergo aromatic hydroxylation, showed oxidative polymerization without insertion of oxygen. The mechanism of this polymerization turns out to be a radical coupling reaction as was established by experiments with the model substrate 2,4-di-tert-butylphenol. In addition to Cu(II), the Mn(III) complex of and the Fe(II) complex of were tested as oxidation catalysts. Oxidation of catechol was observed for the Cu(II) complex of receptor but the other metal complexes did not lead to oxidation.     

Ein neues 3-Amino-2H-azirin als Aib-Pro-Baustein: Synthese des C-terminalen Nonapeptids von Trichovirin I 1B

A New 3-Amino-2H-azirine as an Aib-Pro Synthon: Synthesis of the C-Terminal Nonapeptide of Trichovirin I 1B The synthesis of methyl N-(2,2-dimethyl-2H-azirin-3-yl)-L -prolinate ( 3 ), a novel 3-amino-2H-azirine, is described (Scheme 2). It is shown that the reaction of COCl₂ with thioamide 5 is remarkably faster than with the corresponding amide 4 , and the yield of 3 is much better in the synthesis starting with 5 . The 3-amino-2H-azirine 3 has been used as a building block of the dipeptide moieties Aib-Pro in the synthesis of nonapeptide 17 (Schemes 4 and 5), the C-terminal 6–14 segment of the peptaibole trichovirin I 1B. The structure of 17 was established by single-crystal X-ray crystallography (Figs.1 and 2).     

Conformational comparison of five follicular fluid meiosis-activating sterol-related active and inactive compounds

The crystal structures of five follicular fluid meiosis-activating sterol-related Δ^8,14-sterol compounds are presented. These are 4,4-di­methyl-23-phenyl-24-nor-5α-chola-8,14-dien-3β-ol, C₃₁H₄₄O, 4,4-di­methyl-22-phenyl-23,24-dinor-5α-chola-8,14-dien-3β-ol, C₃₀H₄₂O, (20R)-4,4-di­methyl-22-oxa-5α,20-chol­es­ta-8,14,24-trien-3β-ol, C₂₈H₄₄O₂, 4,4-di­methyl-23-phenyl-22-oxa-24-nor-5α-chola-8,14-dien-3β-ol–water (4/1), 4C₃₀H₄₂O₂·H₂O, and 4,4-di­methyl-5α-cholesta-8,14-dien-3-one, C₂₉H₄₆O. Two of the derivatives are inactive and three are active as agonists. Preliminary structure–activity relationship studies showed that the positions of the double bonds in the skeleton and the structures of the side chains are important determinants for activity. The conformations of the skeletons were compared with double-bond isomers retrieved from the Cambridge Structural Database [Allen & Kennard (1993). Chem. Des. Autom. News, 8 , 1, 31–37]; no significant differences were found. Thus, conformational changes induced by the double bonds are not discriminative with respect to the activity of the compounds. Comparisons of the side-chain conformations of active and inactive structures revealed that the crystal structures were not conclusive as far as correlation of conformation and activity of the side chains were concerned.     

Biocompatible polystyrenes containing pendant tetra(ethylene glycol) and phosphorylcholine groups

The synthesis and characterization of styrene‐based polymers and copolymers containing pendant tetra(ethylene glycol) and phosphorylcholine groups is reported. These polymers are obtained via radical polymerization reactions using α,α′‐azobis(isobutyronitrile) as the initiator, and are developed as protective biocompatible coatings for implantable biosensors. Cell morphology studies show that none of the synthesized polymers and copolymers are toxic, and that the rate of cell growth can be tuned by changing the monomer composition. The presence of tetra(ethylene glycol) groups in the coatings lowers the protein adsorption, thereby influencing the rate of cell growth. An equally profound effect is observed when a low percentage of phosphorylcholine groups is present in the polymers. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 468–474, 2001