Die Umlagerung substituierter Aminoacrylderivate. Präparative anwendungsbreite

The addition of carboxylic acids to dimethylamino-propinal ( 1a ) and 4-dimethyl-amino-but-3-in-2-on ( 1b ) gives, after rearrangement of the very instable primary adducts ( 2 ), Z-3-acetoxy-N,N-dimethylacrylamides and -crotonamides 3 to 8 in excellent yields and in a stereospecific manner. Similarly, the adducts of HCl and HBr to the alkynes 1a and 1b may be rearranged at low temperature by traces of acid to cis/trans equilibria of 3-halo-acrylamides and -crotonamides 9 and 10 . - On the other hand, treatment of 3-alkoxy-3-dimethylaminoacrolein with traces of acid yields alkylesters of E-3-dimethylaminoacrylic acid ( 12 , X = OR). - The preparative aspects of the rearrangement are discussed, and a brief outline of the spectroscopic properties of the compounds 3 to 8 is given.     

Gezielte Synthese von Kohlenwasserstoff-verbrückten Komplexen. Nucleophile addition von Pentacarbonylrhenat und -manganat an π-gebundene Dien-, Cyclodien-, Trimethylenmethan-, Cycloheptatrienyl- und Alkin-Liganden

Pentacarbonyl-rhenate and -manganate react with the cationic complexes [cpMo(CO)₂(diene)]⁺, [cpMo(CO)₂(cyclopentadiene]⁺, [cpMo(CO)₂(cyclohexadiene)]⁺, [cpMo(CO)₂(trimethylenemethane]⁺, [(OC)₃Mo(η⁷-C₇H₇)]⁺, [cp(OC)-(Ph₃P)Mo(alkyne)]⁺ to give the corresponding heteronuclear hydrocarbon-bridged complexes.     

Übergangsmetallkomplexe mit Anionen von 1-substituierten 1,2,3,4-Tetrazolin-5-onen und -thionen und von 1,3-Thiazolin-2-thion

Transition Metal Complexes with Anions of 1-substituted Tetrazolinones and -thiones and of 1,3-Thiazolin-2-thione The preparation of a series of anionic and phosphine containing complexes with the ambivalent anions of 1-R-tetrazolin-5-ones and -5-thiones, and of 1,3-thiazolin-2-thione is reported. According to the i.r. and ¹H n.m.r. spectra the S-heterocycles are bonded through the exocyclic sulfur atom to the metal; in the oxotetrazolinate compounds coordination occurs via the N² or N⁴ ring atoms. The copper and silver complexes [(Ph₃P)MX]₂ (X = anionic heterocycle) have been found to be dimeric.     

Energetic and stereochemical effects of the protein environment on substrate: a theoretical study of methylmalonyl-CoA mutase

QM/MM methods were used to study the isomerization step from (2R)-methylmalonyl-CoA to succinyl-CoA. A pathway via a "fragmentation-recombination" mechanism is ruled out on energetic grounds. For the other radicalic pathway, involving an addition recombination step, geometries and vibrational contributions have been determined, and a barrier height of 11.70 kcal/mol was found. The effect of adjacent hydrogen-donating groups was found to reduce the energy barrier by 1-2 kcal/mol each and thus to provide a significant catalytic effect for this reaction. By means of molecular dynamics studies, the stereochemistry of the methylmalonyl-CoA mutase catalyzed reaction was examined. It is shown that TYR89 is essential for maintaining stereoselectivity of the abstraction of a hydrogen in the backreaction. The subsequent selective formation of one isomer of methylmalonyl-CoA is probably due to the presence of a bulky side chain.     

Double proton transfer in the isolated and DNA-embedded guanine-cytosine base pair

The energetics and dynamics of double proton transfer (DPT) is investigated theoretically for the Watson-Crick conformation of the guanine-cytosine (GC) base pair. Using semiempirical density functional theory the isolated and DNA-embedded GC pair is considered. Differences in the energetics and dynamics of DPT thus addresses the question of how relevant studies of isolated base pairs are for the understanding of processes occurring in DNA. Two-dimensional potential energy surfaces involving the transferring hydrogen atoms and the proton donors and acceptors are presented for both systems. The DPT reaction is accompanied by a contraction of the distance between the two bases with virtually identical energetic barriers being 18.8 and 18.7 kcal/mol for the isolated and DNA-embedded system, respectively. However, the transition state for DPT in the DNA-embedded GC pair is offset by 0.1 A to larger N-H separation compared to the isolated GC pair. Using activated ab initio molecular dynamics, DPT is readily observed for the isolated base pair with a minimal amount of 21.4 kcal/mol of initial average kinetic energy along the DPT normal mode vector. On a time scale of approximately 100 fs DPT has occurred and the excess energy is redistributed. For the DNA-embedded GC pair considerably more kinetic energy is required (30.0 kcal/mol) for DPT and the process is completed within one hydrogen vibration. The relevance of studies of isolated base pairs and base pair analogs in regard of reactions or properties involving DNA is discussed.     

Conformational analysis of self-organized monolayers with scanning tunneling microscopy at near-atomic resolution

We describe the synthesis and a novel approach to the conformational analysis of 2,2'-bipyridines (bpy) bearing aromatic rich Frechet-type dendritic wedges of the first and second generation as substituents. The evaporation of solutions of these new ligands on graphite surfaces under ambient conditions results in the formation of self-organized monolayers. Scanning tunneling microscopy (STM) investigations of the monolayers under ambient conditions (air, 298 K) gave images at submolecular and near-atomic resolution. The analysis of the STM images includes the following processes: (i) identification and reproduction of potential homoconformational domains, (ii) exclusion of improper data using quality criteria for drift and feedback artifacts, (iii) compilation of running averages and checking for averaging artifacts, (iv) analysis of three-dimensional and contour plots, (v) calculation of the HOMO properties of the free molecules, and (vi) final conformational assignment based on all accessible information. Following this procedure, two different conformations could be assigned to domains observed in the monolayers of the first-generation (G1) and second-generation (G2) dendritic compounds. Homoconformational domains are observed side-by-side. The different conformations arise from syn or anti arrangements at the ether substituents. An additional conformational effect is found upon treating the G1 domains with HCl gas, when a partial rearrangement of the bpy from trans to cis occurs, concomitant with protonation.     

Cover Picture

The cover picture shows a model of the "progenitor-TADDOLs" in front of a panorama of the Fünffingersstocks and the Wenden Glacier in the Titlisgebiet (Switzerland). The model is flanked by a stylized view of the general mechanistic model for the preferred stereochemical pathway of the Ti-TADDOLate-catalyzed reaction of chelating substrates (left) and a blackboard (right) showing formulae of compounds that can be prepared with high enantioselectivity by nucleophilic addition in the presence of titanium TADDOLates. From TADDOL, introduced in 1982, a genuine auxiliary system for the "introduction of chirality" (not only by reaction) has been developed, which, in the final sprint against the competition has a good chance of securing a place on the winning podium. The photograph of the mountain landscape was taken by Dr. J. Frackenpohl, the cover picture was generated by one of the authors (A.H.) of the comprehensive review of TADDOLs in this issue (p. 92 ff.) with the help of the program Povray. Supporting information for this article is available on the WWW under http://www.angewandte.com or from the author.     

Structure and properties of the dendron-encapsulated polyoxometalate (C(52)H(60)NO(12))(12)[(Mn(H(2)O))(3)(SbW(9)O(33))(2)], a first generation dendrizyme

Combining analytical and theoretical methods, we present a detailed study of a heteropolytungstate cluster encapsulated in a shell of dendritically branching surfactants, namely (C(52)H(60)NO(12))(12)[(Mn(H(2)O))(3)(SbW(9)O(33))(2)], 3. This novel surfactant-encapsulated cluster (SEC) self-assembles spontaneously from polyoxometalate-containing solutions treated with a stoichiometric amount of dendrons. Compound 3 exhibits a discrete supramolecular architecture in which a single polyoxometalate anion resides in a compact shell of dendrons. Our approach attempts to combine the catalytic activity of polyoxometalates with the steric properties of tailored dendritic surfactants into size-selective catalytic systems. The structural characterization of the SEC is based on analytical ultracentrifugation (AUC) and small-angle neutron scattering (SANS). The packing arrangement of dendrons at the cluster surface is gleaned from molecular dynamics (MD) simulations, which suggests a highly porous shell structure due to the dynamic formation of internal clefts and cavities. From analysis of the MD trajectory of 3, a theoretical neutron-scattering function is derived that is in good agreement with experimental SANS data. Force field parameters used in MD simulations are partially derived from a quantum mechanical geometry optimization of [(Zn(H(2)O))(3)(SbW(9)O(33))(2)](12)(-), 2b, at the density functional theory (DFT) level. DFT calculations are corroborated by X-ray structure analysis of Na(6)K(6)[(Zn(H(2)O))(3)(SbW(9)O(33))(2)].23H(2)O, which is isostructural with the catalytically active Mn derivative 2a. The combined use of theoretical and analytical methods aims at rapidly prototyping smart catalysts ("dendrizymes"), which are structurally related to naturally occurring metalloproteins.     

Oxydation von Allylalkoholen mit Blei(IV)-acetat

Lead tetraacetate (LTA) oxidation of the allylic alcohols 1, 10, 14 and 19 leads to the formation of the epoxides 2, 11, 15 and 20 , products of a novel internal addition reaction of the electron deficient alcohol oxygen to the allylic double bond. In some cases ( 10, 14 ) the formation of a new type of acetoxylated enolethers ( 12, 16 ) is observed. The LTA oxidation of the allylic dienols 21 and 29 gives rise to the formation of the epoxyacetates 25 and 33 , products of a similar internal addition reaction. Furthermore, a variety of cyclization products ( 22, 23, 24, 26, 30, 31, 32 and 34 ) has been isolated whose formation requires an isomerisation of the allylic trans double bond to a cis one.