Synthesis of chiral cyclic nitrones by asymmetric addition of β-ketosulfones to nitroalkenes followed by reductive cyclization

The first organocatalytic addition of β-ketosulfones to nitroalkenes catalyzed by thiourea cinchona alkaloids is presented. The readily obtained addition products were selectively transformed into chiral nitrones by reduction of the nitro group and in situ cyclization (up to 99:1 e.r. and >98:2 d.r.). Moreover, the utility of this method has additionally been demonstrated by the further transformation to functionalized N-hydroxypyrrolidines that possess a quaternary center by addition of trimethylsilyl cyanide (TMSCN) to the C=N bond of the cyclic nitrones in the presence of a Lewis acid.     

Synthesis and Structure of Vitamin B<Subscript>12</Subscript>-Derivatives with a Modified Ribose-Unit

Summary. The acylation at the 5′-OH group of the ribose-unit of vitamin B₁₂ (cyanocobalamin) or of aquocobalamin with two conventional reagents gave mono-acylated B₁₂-derivatives with good to very high selectivity. The site of the modification was deduced from spectral data of the products and was further supported by the crystal structure data of three such modified B₁₂-derivatives. These three B₁₂-derivatives were found to crystallize in the space group P2₁2₁2₁, irrespective of the nature of the appendage. Acylation at 5′-OH has been used to protect (or block) this group in the context of functionalization of 2′-OH or elsewhere in the B₁₂-molecule. Attachment of the bifunctional succinyl-unit has allowed the preparation of further modified derivatives of vitamin B₁₂ and binding of B₁₂-derivatives to biological carriers and other macromolecules. In aqueous solution, 5′-acylcobalamins turned out to be rather susceptible to hydrolytic loss of the acyl-functionality. Bernhard Kr?utler: In memoriam Prof. Karl Schl?gl     

A template-based mnemonic for monoamine oxidase (MAO-N) catalyzed reactions and its application to the chemo-enzymatic deracemisation of the alkaloid (+/-)-crispine A

A template-based mnemonic has been developed for the enzyme monoamine oxidase from Aspergillus niger and has been used to successfully identify the alkaloid (+/-)-crispine A as a target for chemo-enzymatic deracemisation yielding the biologically active (R)-enantiomer in 97% e.e.     

Gelled polymerizable microemulsions. 1. Phase behavior

Microemulsions are gaining increasing importance as templates since a great deal is known about how to tune their structure and the size of the domains. The concept of synthesizing a bicontinuous high surface area polymer is well-known, by "arresting" the oil (water) phase and polymerizing the water (oil) phase. However, a general route for the 1:1 replication of the bicontinuous structure has not been found yet. Our approach to achieving this goal entails arresting the oil phase by gelling it, i.e., by forming an organogel, and polymerizing the aqueous phase. The ternary base system water-n-dodecane-Lutensol AO5 (technical-grade nonionic n-alkyl polyglycol ether with an average molecular structure of C(13/15)E(5)) was chosen, and the organogelator 12-hydroxyoctadecanoic acid (12-HOA) as well as a polymerizable aqueous phase containing the monomer N-isopropylacrylamide (NIPAm) and the cross-linker N,N'-methylenebisacrylamide (BisAm) were added. To understand the influence of adding 12-HOA to the oil and NIPAM + BisAm to the aqueous phase on the phase behavior, phase diagrams were determined after each compositional change. The respective phase diagrams are presented and discussed in terms of their potential use as templates for new high surface area polymers.     

The achiral tetrapeptide Z‐Aib‐Aib‐Aib‐Gly‐OtBu

The title achiral peptide N‐benzyloxycarbonyl‐α‐aminoisobutyryl‐α‐aminoisobutyryl‐α‐aminoisobutyrylglycine tert‐butyl ester or Z‐Aib‐Aib‐Aib‐Gly‐OtBu (Aib is α‐aminoisobutyric acid, Z is benzyloxycarbonyl, Gly is glycine and OtBu indicates the tert‐butyl ester), C₂₆H₄₀N₄O₇, is partly hydrated (0.075H₂O) and has two different conformations which together constitute the asymmetric unit. Both molecules form incipient 3₁₀‐helices. They differ in the relative orientation of the N‐terminal protection group and at the C‐terminus. There are two 4→1 intramolecular hydrogen bonds.     

Poly(ester urethane)s derived from lactide,isosorbide, terephthalic acid,and various diisocyanates

Isosorbide‐initiated oligomerizations of l ‐lactide were preformed in bulk using SnCl₂ as catalyst. The resulting telechelic OH‐terminated oligoesters were in situ subjected to simultaneous polycondensation and polyaddition with mixtures of terephthaloyl chloride and diisocyanates. Most polymerizations were conducted with 4,4′‐diisocyanatodiphenyl methane and 2,4‐diisocyanato toluene. The consequences of excess diisocyanate and four different catalysts were studied. The isosorbide/lactide ratio and the terephthalic acid/diisocyanate ratio were varied. Number average molecular weights up to 15 kDa with polydispersities around 3–5 were obtained. Depending on the chemical structure of the copolyester and on the feed ratio, incorporation of urethane groups may reduce or enhance the glass‐transition temperature, but the thermal stability decreases dramatically regardless of composition. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 867–875     

Penalized marginal likelihood estimation of finite mixtures of Archimedean copulas

This paper proposes finite mixtures of different Archimedean copula families as a flexible tool for modelling the dependence structure in multivariate data. A novel approach to estimating the parameters in this mixture model is presented by maximizing the penalized marginal likelihood via iterative quadratic programming. The motivation for the penalized marginal likelihood stems from an underlying Bayesian model that imposes a prior distribution on the parameter of each Archimedean copula family. An approximative marginal likelihood is obtained by a classical quadrature discretization of the integral w.r.t. each family-specific prior distribution, thus yielding a finite mixture model. Family-specific smoothness penalties are added and the penalized marginal likelihood is maximized using an iterative quadratic programming routine. For comparison purposes, we also present a fully Bayesian approach via simulation-based posterior computation. The performance of the novel estimation approach is evaluated by simulations and two examples involving the modelling of the interdependence of exchange rates and of wind speed measurements, respectively. For these examples, penalized marginal likelihood estimates are compared to the corresponding Bayesian estimates.