Zur Anionenselektivität von Distannylderivaten in Membranen

Anion-Selectivity of Distannyl Derivatives in Membranes A series of distannyl derivatives (2,2-bis(trimethylstannyl)-1,3-dithiane, 2,2-bis(tributylstannyl)-1,3-dithiane, hexamethyldistannane, hexabutyldistannane, hexaphenyldistannane, bis(triphenylstannyl)sulfide, o-bis(trimethylstannyl)benzene) has been studied in view of their applicability as anion-selective ionophores in solvent polymeric membranes. None of these compounds induces significant changes in the anion-selectivity pattern as compared with the membranes containing no organotin compound. Representatives with tributylstannyl groups, however, undergo chemical reactions leading to highly active anion ionophores of the type Bu₃SnX, several of which (e.g. Bu₃SnCl and Bu₃SnOH) may be present in equilibrium in the membrane phase depending on the measuring conditions.     

Trapping quasiminimizing submanifolds in spaces of negative curvature

LetM be a Hadamard manifold with all sectional curvatures bounded above by some negative constant. A well-known lemma essentially due to M. Morse states that every quasigeodesic segment inM lies within an a priori bounded distance from the geodesic arc connecting its endpoints. In this paper we establish an analogue of this fact for quasiminimizing surfaces in all dimensions and codimensions; the only additional requirement is that the sectional curvatures ofM be bounded from below as well. We apply this estimate to obtain new solutions to the asymptotic Plateau problem in various settings. The second author was supported by the Swiss National Science Foundation and enjoyed the hospitality of the University of Bonn. The collaboration between the authors was facilitated by the program GADGET II.     

Steric vs. electrostatic effects in the nucleophilic addition to a hindered cyclohexanone

The relative importance of steric vs. electrostatic effects in the nucleophilic addition to (4R,6S)-4-(tert-butyldimethylsiloxy)-2,2,6-trimethylcyclohexanone (1), a well-known chiral building block, is investigated.     

Synthese von [2,7-Cystin]-Gramicidin S,einem künstlichen homodet-heterodet-bicyclischen Decapeptid

The first synthesis of a homodetic-heterodetic-bicyclic polypeptide, [2,7-cystine]-gramicidin S, is described. For the protection of the C-terminal carboxyl and the cysteine sulfhydryl functions, the 2-(toluene-p-sulfonyl)-ethyl- (Tsa) and the acetamidomethyl- (Acm) groups, respectively, were used. Stepwise synthesis from the C-terminus, using N^α Boc-amino acids, and selective removal of protecting groups yielded the two pentapeptide derivatives: Boc · Val-Cys-(Acm)-Lem)-Leu-phe-Pro · OH and H · Val-Cys(Acm)-Leu-phe-Pro · OTsa. They were condensed with dicyclohexyl-carbodiimide and 1-hydroxybenzotriazole to give the crystalline decapeptide Boc · [Val-Cys(Acm)-Leu-phe-Pro]₂ · OTsa. Removal of the Tsa group by β-elimination at pH 11.5 yielded the crystalline free acid, which was further converted (by treatment with di-p-nitrophenyl sulfite followed by TFA) to TFA, H · [Val-Cys(Acm)-Leu-phe-Pro]₂ · ONp. Cyclization of the active ester in warm pyridine gave a mixture of (2,7-bis-S-acetamidomethyl-cysteine]-gramicidin S (27% yield) and (2,7-cysteine)-gramicidin S (4%). The former compound was readily converted to the latter by treatment with I₂ in MeOH. In the bicyclic peptide, the decapeptide ring is contained in a β-type secondary structure, identical with that in gramicidin S; the disulfide bridge shows P-helical chirality and gives rise to a negative Cotton effect at 271 nm [3].     

Synthesis of Pure Al35Cl3 and Al37Cl3

Chemically and isotopically pure Al³⁵Cl₃ and Al³⁷Cl₃ are synthesized from Al (s) and HCl (g). The yield is quantitative and no measurable decrease in isotopic content from HCl to AlCl₃ takes place.     

A (1,5)-Vinyl Shift

The structure of the by-product of the pyrolysis of 1 has been proved to be 3 . It is proposed that a (1, 5) Shift of an (alkoxycarbonyl)vinyl group ( 8 ? 10 ) is the step in formation of 3 .     

Orthoesters versus 2-O-acyl glycosides as glycosyl donors: theorectical and experimental studies

n-Pentenyl orthoesters (NPOEs) undergo routine acid catalyzed rearrangement into 2-O-acyl n-pentenyl glycosides (NPGs). The reactant and product can both function as glycosyl donors affording 1,2-trans linked glycosides predominantly. However, both donors differ in their rates of reactions, the yields they produce, and the nature of their byproducts, indicating that the NPOE/NPG pair may not be reacting through the same intermediates. We have therefore applied quantum chemical calculations using DFT methods and MP second order perturbation theory to learn more about orthoesters and their 2-O-acyl glycosidic counterparts. The calculations show that in the case of a manno NPG and NPOE pair, each donor goes initially to a different cationic intermediate. Thus, the former goes to a high-energy oxocarbenium ion before descending to a trioxolenium ion in which the charge is distributed over the pyrano ring oxygen, as well as the carbonyl and ether oxygen atoms of the putative C2 ester. On the other hand, ionization of the NPOE produces a dioxolenium ion lying slightly above the more stable trioxolenium counterpart. For the gluco pair, the NPG also goes to a very high-energy oxocarbenium ion, which also descends to a trioxolenium ion. However, unlike the manno analogue, the gluco NPOE does not give a dioxolenium ion; indeed, the dioxolenium is not energetically distinguishable from the trioxolenium counterpart. The theoretical observations have been tested experimentally. Thus, it was found that with manno derivatives, the orthoester is a more reactive donor than the corresponding NPG donor, whereas, for gluco derivatives, there is no advantage to using one over the other, unless one resorts to carefully selected promoters.     

Synthese von Triafulven-Vorstufen für Retro-Diels-Alder-Reaktionen

Synthesis of Triafulvene Precursors for Retro-Diels-Alder Reactions Triafulvene precursors exo? 15 and endo? 15 have been prepared by addition of dibromocarbene to benzobarrelene 12 followed by a lithium-halogen exchange, methylation, and elimination of HBr ( 12→13→14→15 ), (Scheme 2). Gas-phase pyrolysis of exo/endo-mixtures of 15 above 400° gave minor amounts of naphthalene ( 16 ), traces of a hydrocarbon C₄H₄ identified by MS (presumably triafulvene 1 ) and predominantly (36%) the isomerization product 17 (Scheme 3). In a second synthetic approach the well-known cycloheptatriene-norcaradiene equilibrium of type 26?27 has been utilised to prepare various endo-trans-3-(X-methyl) tricyclo[3.2.2.0^2,4]nona-6,8-dienes 31 (Scheme 5). However, numerous elimination experiments 31→9 failed so far. The structure of two rearrangement products 33 and 34 (Scheme 6) has been elucidated.     

Die «Zip»-Reaktion: Eine neue Ringerweiterungsreaktion. Synthese von 17-, 21- und 25-gliedrigen Polyaminolactamen. 4. Mitteilung über Umamidierungsreaktionen

The Zip-reaction: A New Method for the Synthesis of Macrocyclic Polyaminolactams The 21- and 25-membered aminolactams 11 and 25 were synthesized from the 13-membered lactam 4 . To introduce the ring enlargement unit (a propylamino group) 4 was N-alkylated using acrylonitrile and the resulting product hydrogenated. Repetition of this reaction sequence gave 3 , which was converted in the presence of base in 90% yield to the ring-enlarged macrocyclic base 11 (Scheme 2). In a similar but stepwise synthesis consisting of two separate ring-enlargement reactions 4 was transformed to 11 via 13 (Scheme 4). Introducing three ringenlargement units into 4 the 25-membered aminolactam 25 was synthesized in 84% yield (Scheme 5). The mechanism of the ring-enlargement reaction is given in Scheme 3. In comparison to a zip-fastener or zipper this reaction is called “zipreaction”.