Viscoelastic properties of polymerlike reverse micelles

A dramatic increase in the viscosity of reverse micellar solutions of lecithin in a variety of organic solvents of up to a factor of 10⁶ upon the addition of a small amount of water can be observed. The formation of viscoelastic solutions can be explained by a water-induced aggregation of lecithin molecules into flexible cylindrical reverse micelles and the subsequent formation of a transient network of entangled micelles. The viscoelastic properties of these solutions are characterized as a function of water content and temperature for different organic solvents by means of dynamic shear viscosity measurements. The results are interpreted by making analogies to the behavior of semidilute polymer solutions and living polymers.Dedicated to Prof. Dr. J. Meissner on the occasion of his 60th birthday.     

Synthesis of 2,3-dihydro-10bh-oxazolo[2,3-a]isoquinolines from ortho-lithiated phenyloxazolinyloxiranes

A general method for the synthesis of 2,3-dihydro-10bH-oxazolo[2,3-a]isoquinolines from the reaction of (R*,R*)-configured ortho-bromophenyloxazolinyloxiranes and organolithiums is described.     

Asymmetric synthesis of α,β-substituted β-aminoalkanamides and stereochemical determination

Highly enantiomerically enriched β-aminoalkanamides 12 and β-phenylaminoalkanamides 13 have been prepared by the addition reaction of α-lithiated 2-alkyl-2-oxazolines 9-Li, derived from optically active oxazolines 9, to N-cumyl nitrones 2. The relative stereochemistry of alkanamides 5 and 6 has been established by 1D-NOESY experiments carried out on the related pyrimidinones 7, whereas the absolute configuration of alkanamides 12 and 13 has been confirmed by an X-ray analysis.     

Modulation of heme redox potential in the cytochrome c6 family

Cytochrome c6A is a unique dithio-cytochrome of green algae and plants. It has a very similar core structure to that of bacterial and algal cytochromes c6 but is unable to fulfill the same function of transferring electrons from cytochrome f to photosystem I. A key feature is that its heme midpoint potential is more than 200 mV below that of cytochrome c6 despite having His and Met as axial heme-iron ligands. To identify the molecular origins of the difference in potential, the structure of cytochrome c6 from the cyanobacterium Phormidium laminosum has been determined by X-ray crystallography and compared with the known structure of cytochrome c6A. One salient difference of the heme pockets is that a highly conserved Gln (Q51) in cytochrome c6 is replaced by Val (V52) in c6A. Using protein film voltammetry, we found that swapping these residues raised the c6A potential by +109 mV and decreased that of c6 by almost the same extent, -100 mV. X-ray crystallography of the V52Q protein showed that the Gln residue adopts the same configuration relative to the heme as in cytochrome c6 and we propose that this stereochemistry destabilizes the oxidized form of the heme. Consequently, replacement of Gln by Val was probably a key step in the evolution of cytochrome c6A from cytochrome c6, inhibiting reduction by the cytochrome b6f complex and facilitating establishment of a new function.     

Coordination polymer gels: synthesis, structure and mechanical properties of amorphous coordination polymers

The synthesis and mechanical properties of a new class of coordination polymer based materials, amorphous coordination polymer sol-gels and xerogels, is presented.     

BH3-promoted stereoselective β-lithiation of N-alkyl-2-phenylaziridines

BH(3) complexes of N-alkyl-2-phenylaziridines have been synthesized and their structure and stereochemistry proved with DFT calculations and NMR experiments. It has been demonstrated that the Lewis acid complexation is able to promote a regioselective β-lithiation in 2-phenylaziridino-borane complexes. The lithiated intermediates were configurationally stable, allowing an enantioselective preparation of cis-2,3-disubstituted aziridines.     

Fluoro-Substituted Methyllithium Chemistry: External Quenching Method Using Flow Microreactors

The external quenching method based on flow microreactors allows the generation and use of short-lived fluoro-substituted methyllithium reagents, such as fluoromethyllithium, fluoroiodomethyllithium, and fluoroiodostannylmethyllithium. Highly chemoselective reactions have been developed, opening new opportunities in the synthesis of fluorinated molecules using fluorinated organometallics.     

Fluoro‐Substituted Methyllithium Chemistry: External Quenching Method Using Flow Microreactors

The external quenching method based on flow microreactors allows the generation and use of short‐lived fluoro‐substituted methyllithium reagents, such as fluoromethyllithium, fluoroiodomethyllithium, and fluoroiodostannylmethyllithium. Highly chemoselective reactions have been developed, opening new opportunities in the synthesis of fluorinated molecules using fluorinated organometallics.     

Harnessing the ortho‐Directing Ability of the Azetidine Ring for the Regioselective and Exhaustive Functionalization of Arenes

This work demonstrates how the directing ability of the azetidine ring could be useful for regioselective ortho‐C?H functionalization of aryl compounds. Robust polar organometallic (lithiated) intermediates are involved in this synthetic strategy. The reagent n‐hexyllithium emerged as a safer, yet still effective, basic reagent for the hydrogen/lithium permutation relative to the widely used reagent nBuLi. Two different reaction protocols were discovered for regioselective lithiation at the ortho positions adjacent to the azetidine ring, which served as a toolbox when other competing directing groups were installed on the aromatic ring. The coordinating ability of the azetidine nitrogen atom, as well as the involvement of dynamic phenomena related to the preferential conformations of 2‐arylazetidine derivatives, were recognized to be responsible for the observed reactivity and regioselectivity. A site‐selective functionalization of the aromatic ring was achieved for aryl azetidines with either coordinatively competent groups (e.g. methoxy) or inductively electron‐withdrawing substituents (e.g. chlorine and fluorine). By fine‐tuning the reaction conditions, regioselective introduction of several substituents on the aromatic ring could be realized. Several substitution patterns were accomplished, which included 1,2,3‐trisubstitution, 1,2,3,4‐tetrasubstitution, and 1,2,3,4,5‐pentasubstitution, up to the exhaustive substitution of the aromatic ring.