Absolute Rate Constants for the Addition of the 1‐(tert‐Butoxy)carbonylethyl Radical onto Cyclic Alkenes in Solution

Absolute rate constants are reported for the addition of the 1‐[(tert‐butoxy)carbonyl]ethyl (= 2‐(1,1‐dimethylethoxy)‐1‐methyl‐2‐oxoethyl) radical ^.CHMeCO₂(t‐Bu) to several cyclic and monosubstituted alkenes in MeCN as obtained by time‐resolved electron paramagnetic resonance (EPR). The activation energies for the addition of this alkyl radical are mainly governed by the addition enthalpy but are also substantially lowered by the ambiphilic effect and by relief of cyclic strain.     

Synthese und Molekülstruktur neuer Ringsysteme aus 1,1,3,3-Tetrachlor-1,3-diphosphapropan

Synthesis and Molecular Structure of new Ring Systems from 1,1,3,3-Tetrachloro-1,3-diphosphapropane 1,1,3,3-Tetrachloro-1,3-diphosphapropane 1 reacts in two different ways to form new heterocycles. Partial oxidation of 1 with tetrachloroorthobenzoquinone furnishes the methylene-bridged λ³P, λ⁵P species 3 . Subsequent reactions with di- and triethylamine lead to the condensed ring system 6 with the P?C bonds connected to a central four-membered ring. Compound 6 displays crystallographic inversion symmetry, a short transannular P? P distance and an extremely distorted tetrahedral coordination geometry at the four-membered ring phosphorus atoms. 1 reacts with 7 to give the heterocycle 8 with a central eight-membered ring involving four phosphorus atoms. The eight – membered ring shows a ?bent”? crown conformation, the condensed five – membered rings display envelope conformation.     

Fragmentation of 1-aryl-5-(2-dialkylaminovinyl)-1H-tetrazoles upon electron impact

The mass spectrometric behaviour of 1-aryl-5-(2-dialkylaminovinyI)-lH-tetrazoles was studied, especially 1-phenyl-5-(2-dimethylaminovinyl)-l H-tetrazole as a typical example of D- and ¹⁵N-labelled derivatives revealed a rearrangement via a carbodiimide-like intermediate ion.     

Synthese,Struktur und einige Reaktionen [N-(N′,N′,N″,N″-tetramethyl)-guanidinyl]-substituierter Phosphorylverbindungen

Synthesis, Structure, and some Reactions of N-(N′,N′,N″,N″-tetramethyl)guanidinyl-substituted Phosphoryl Compounds The tetramethylguanidinyl-substituted phosphoryl compounds 1 – 10 were prepared in the reaction of the appropriate chlorophosphoryl compounds with either N′,N′,N″,N″-tetramethylguanidine (HTMG) or N-trimethylsilyl-(N′,N′,N″,N″-tetramethyl)guanidine (TMSTMG). With methyl iodide 1 reacted with N-alkylation to give the ammonium salt 11. 1 reacted with BF₃ · Et₂O at both imino nitrogen atoms with formation of the bis-BF₃-adduct 12 . The X-ray structure determination of phenylphosphonic acid-bis(N′,N′,N″,N″-tetramethylguanidinide) 3 shows shortened PN-bonds and widened PNC-angles, consistent with the partial double bond character of the PN-bond.     

Solvent intercalation in cobalt dimethylglyoximato complexes: A miniature host-guest system

When crystallized from appropriate solvents, the complex aqua-bis(dimethylglyoximato)nitrocobalt (III) may incorporate solvent molecules, thus forming a variety of mixed crystals. In the resulting host-guest crystals, the space groupP2_l/m and the packing motif of the pure host compound are retained. Lattice constantsa andb remain essentially unaltered upon intercalation, whereasc and the monoclinic angle depend largely on the clathrated guest. Space filling and intermolecular contacts are discussed.Dedicated to Professor Peter Paetzold at the occasion of his 60th birthday. A preliminary account of this work has been given at the Spring Meeting of the British Crystallographic Association, Newcastle upon Tyne, 1994.     

Biocatalytic reversible control of the stiffness of DNA-modified responsive hydrogels: applications in shape-memory,self-healing and autonomous controlled release of insulin

The enzymes glucose oxidase (GOx), acetylcholine esterase (AchE) and urease that drive biocatalytic transformations to alter pH, are integrated into pH-responsive DNA-based hydrogels. A two-enzyme-loaded hydrogel composed of GOx/urease or AchE/urease and a three-enzyme-loaded hydrogel composed of GOx/AchE/urease are presented. The biocatalytic transformations within the hydrogels lead to the dictated reconfiguration of nucleic acid bridges and the switchable control over the stiffness of the respective hydrogels. The switchable stiffness features are used to develop biocatalytically guided shape-memory and self-healing matrices. In addition, loading of GOx/insulin in a pH-responsive DNA-based hydrogel yields a glucose-triggered matrix for the controlled release of insulin, acting as an artificial pancreas. The release of insulin is controlled by the concentrations of glucose, hence, the biocatalytic insulin-loaded hydrogel provides an interesting sense-and-treat carrier for controlling diabetes.

Biocatalytic control over the stiffness of pH-responsive hydrogels is applied to develop shape-memory, self-healing and controlled release matrices.     

Copolymerization of 1,6-anhydro-ß-D-galactopyranose and 1,6-anhydro-ß-D-mannopyranose derivatives

1,6-Anhydro-2,3,4-tri-O-(p-methylbenzyl)-ß-D -galactopyranose (TXGal,M₁) has been copolymerized with 1,6-anhydro-2,3,4-tri-O-benzyl-ß-D -mannopyranose (TBMan,M₂), the products characterized by NMR, specific rotation, and viscosity, and the reactivity ratios calculated. The reactivity ratios r₁ = 0.37 ± 0.15 and r₂ = 38 ± 4 indicate that the anhydromannose derivative is about 100 times as reactive as that of anhydrogalactose. A comparison of glucose, mannose, and galactose copolymerizations suggests that the reactivity differences of the three propagating cations are comparatively small and the reactivity differences of the monomers large. This result is consistent with a mechanism proposed earlier. Methyl substitution on the aromatic rings of the p-xylyl groups inhibits the initiation process significantly relative to benzyl, but propagation only slightly.     

The Enol of Acetone during Photochemical Reactions of 3-Hydroxy-3-methyl-2-butanone and of Acetone

The enol of acetone, formed by disproportionation reactions of 1-hydroxy-1-methylethyl radicals, is detected by NMR, spectroscopy during photoreactions of 3-hydroxy-3-methyl-2-butanone in acetonitrile and of acetone in 2-propanol and slowly tautomerizes to acetone. The photolysis of 3-hydroxy-3-methyl-2-butanone is shown to proceed via Type I cleavage, predominantly from an excited triplet state.     

High performance liquid chromatographic separation of dipeptide and tripeptide enantiomers using a chiral crown ether stationary phase

A chiral stationary phase (CSP) based on (-)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was evaluated for the direct resolution of the enantiomers of dipeptides and tripeptides. The type and concentration of the acid and the methanol content were optimized with regard to retention time and resolution using Ala-Phe as model peptide. A mobile phase consisting of 10 mM sulfuric acid in 70% aqueous methanol was applied to the separation of a set of 16 structurally diverse dipeptides and tripeptides. Generally, the configuration of the amino acid at the N-terminus determined the enantiomer elution order. With a few exceptions the LL- and LD-enantiomers interacted stronger with the CSP compared to the corresponding DD- or DL-enantiomers. The experimental conditions also allowed the simultaneous separation of all four stereoisomers of Ala-Phe. Addition of ammonium sulfate generally reduced retention times and enantiomer resolution. Addition of triethylamine as modifier led to an overall increase of the retention times while the resolution did not show a general trend, increasing in the case of Ala-Ala but decreasing in the case of Ala-Phe.