Asymmetric base-mediated epoxide isomerisation

The base-mediated rearrangement of epoxides into allylic alcohols is a well-known synthetic transformation. The first enantioselective version of the reaction using a chiral base was reported in 1980. Since then, the reaction has received a lot of attention mostly due to the great usefulness of chiral allylic alcohols in organic synthesis. Major breakthroughs in the area were the first report on using a sub-stoichiometric amount of chiral base, and the development of chiral bases for a true catalytic reaction protocol. The present review covers the time from when the first asymmetric epoxide isomerisation reaction was reported (1980) up to now, focusing on the period 1997-2001.     

Synthesis of Cembranoid Analogues through Ring‐Closing Metathesis of Terpenoid Precursors: A Challenge Regarding Ring‐Size Selectivity

A systematic study on ring‐closing metathesis with Grubbs II catalyst to cembranoid macrocycles is described. Acyclic terpenoids with a functional group X in the homoallylic position relative to an RCM active terminus and substituents R, R¹ directly attached to the other terminal double bond were prepared from geraniol derived trienes and fragments that are based on bromoalkenes and dimethyl malonate. Such terpenoids were suitable precursors, despite the presence of competing double bonds in their framework. The size of R and R¹ is crucial for successful macrocyclization. Whereas small alkyl substituents at the double bond directed the RCM towards six‐membered ring formation, cross metathesis leading to dimers dominated for bulkier alkyl groups. A similar result was obtained for precursors without functional group X. In the case of unsymmetrically substituted terpenoid precursor (R=Et, R¹=Me) with homoallylic OTBS or OMe group, the RCM could be controlled towards formation of macrocyclic cembranoids, which were isolated with excellent E‐selectivity. The role of the substituents was further studied by quantum chemical calculations of simplified model substrates. Based on these results a mechanistic rationale is proposed.     

Heterogeneous Platinum‐Catalyzed CH Perfluoroalkylation of Arenes and Heteroarenes

Fluorinated organic compounds are gaining increasing interest for life science applications. The replacement of hydrogen in arenes or heteroarenes by a perfluoroalkyl group has a profound influence on the physical and biological properties of such building blocks. Here, an operationally simple protocol for the direct C? H perfluoroalkylation of (hetero)arenes with R_fI or R_fBr has been developed, using a robust supported platinum catalyst. The ready availability of the starting materials, the excellent substrate tolerance, and the reusability of the catalyst make this method attractive for the synthesis of a variety of perfluoroalkyl‐substituted aromatic compounds. Preliminary mechanistic studies revealed the formation of radicals to be crucial in the reaction system.     

Synthesis and Magnetic Properties of Novel Azamacrocyclic LnIII,CuII, FeIII,and SrII Complexes and Conformational Analysis of the Ligands

Hexaalkoxy‐substituted azamacrocyclic metal complexes 1 · MX_n [MX_n = FeCl₃, CuCl₂, SrCl₂, Ln(NO₃)₃; Ln = La, Pr, Eu, Ho, Er] with various chain lengths were prepared by a convergent approach using 1,2‐bisalkoxy‐4,5‐diamines 3 and 4‐alkoxy‐pyridine‐2,6‐dicarbaldehydes 4 as key building blocks for template‐assisted cyclocondensation. Metal complexes 1 · MX_n were considered as potential metallomesogens. However, differential scanning calorimetry and optical polarizing microscopy of 1 · MX_n did not reveal any mesomorphism. The magnetic susceptibility shows deviation from Curie‐like behavior. Due to ligand field effects the effective magnetic moments are a function of the temperature. In order to obtain structural informations on the free ligand 1 , which is synthetically not accessible, theoretical calculations were carried out. For the free azamacrocycle 2a and the free hexamethoxy‐substituted azamacrocycle 1a the IR spectra were computed at the level of density functional theory. A planar and a saddle‐shaped conformation was considered. The IR spectra and especially the dependence of the C=N vibration on the structural parameters and the charge distribution are discussed.     

Accurate Modelling of Group Electrostatic Potential and Distributed Polarizability in Dipeptides

Within the scope of accurate structure-property correlations in biomolecules, this work investigates how conformations and electronic configurations of biologically relevant macromolecules affect their intermolecular potentials. With the purpose of testing the suitability of a simple and universal model, the dipeptides are made from the assembly of their building blocks, namely the amino acid residuals or, more finely tuned, the individual functional groups. The model makes use of functional-group electrostatic potentials (GEP) and distributed polarizabilities (GDP), which enable an in depth analysis of the correlation between structural features and property build-up. GEPs and GDPs are calculated for various conformers and protonation states of L-alanyl-L-alanine, glycyl-L-alanine, L-alanylglycine, and glycylglycine, which are prototypic molecules to model the pertinent functional groups. The model provides GEPs that reproduce the exact potential to an average accuracy of ca. 0.05 au. The good agreement between the properties estimated with the simple model and those calculated with state-of-the-art quantum chemical methods encourages further testing of the predictive power of this model, simulating for example interaction energies and optoelectronic properties.     

Fabrication of nanoporous templates from diblock copolymer thin films on alkylchlorosilane-neutralized surfaces

The fabrication of nanoporous templates from poly(styrene)-b-poly(methyl methacrylate) diblock copolymer thin films (PS-b-PMMA, volume ratio 70:30) on silicon requires precise control of interfacial energies to achieve a perpendicular orientation of the PMMA cylindrical microdomains relative to the substrate. To provide a simple, rapid, yet tunable approach for surface neutralization, we investigated the self-assembled ordering of PS-b-PMMA diblock copolymer thin films on silicon substrates modified with a partial monolayer of octadecyldimethyl chlorosilane (ODMS), i.e., a layer of ODMS with a grafting density less than the maximum possible monolayer surface coverage. We demonstrate herein the fabrication of nanoporous PS templates from annealed PS-b-PMMA diblock copolymer thin films on these partial ODMS SAMs.     

Novel fluoro-substituted benzo- and benzothieno fused pyrano[2,3-c]pyrazol-4(1H)-ones

A straightforward, two-step synthesis of fluoro substituted chromeno[2,3-c]pyrazol- and [1]benzothieno[2′,3′:5,6]pyrano[2,3-c]pyrazol-4(1H)-ones, respectively, is presented. Hence, treatment of 1-substituted or 1,3-disubstituted 2-pyrazolin-5-ones with fluoro substituted 2-fluorobenzoyl chlorides or 3-chloro-6-fluoro-1-benzothiophene-2-carbonyl chloride using calcium hydroxide in refluxing 1,4-dioxane gave the corresponding 4-aroylpyrazol-5-ols, which were cyclized into the fused ring systems. 5-Fluorochromeno[2,3-c]pyrazol-4(1H)-one was obtained upon treatment of the 1-(4-methoxybenzyl) protected congener with trifluoroacetic acid. Treatment of 5-fluorochromeno[2,3-c]pyrazol-4(1H)-ones with methylhydrazine afforded novel tetracyclic ring systems such as 2-methyl-7-phenyl-2,7-dihydropyrazolo[4′,3′:5,6]pyrano[4,3,2-cd]indazole. Detailed NMR spectroscopic investigations (¹H, ¹³C, ¹⁵N, ¹⁹F) with the obtained compounds were undertaken.     

Macroporous polymers obtained in highly concentrated emulsions stabilized solely with magnetic nanoparticles

Magnetic macroporous polymers have been successfully prepared using Pickering high internal phase ratio emulsions (HIPEs) as templates. To stabilize the HIPEs, two types of oleic acid-modified iron oxide nanoparticles (NPs) were used as emulsifiers. The results revealed that partially hydrophobic NPs could stabilize W/O HIPEs with an internal phase above 90%. Depending upon the oleic acid content, the nanoparticles showed either an arrangement at the oil-water interface or a partial dispersion into the oil phase. Such different abilities to migrate to the interface had significant effects on the maximum internal phase fraction achievable and the droplet size distribution of the emulsions. Highly macroporous composite polymers were obtained by polymerization in the external phase of these emulsions. The density, porosity, pore morphology and magnetic properties were characterized as a function of the oleic acid content, concentration of NPs, and internal phase volume of the initial HIPEs. SEM imaging indicated that a close-cell structure was obtained. Furthermore, the composite materials showed superparamagnetic behavior and a relatively high magnetic moment.