Ingmar Bauer

Synthesis of Pyrrole and Carbazole Alkaloids

An overview of recent transition metal-catalyzed syntheses of pyrroles and carbazoles is presented. The focus is on methods which have been applied to the preparation of biologically active naturally occurring pyrrole and carbazole alkaloids. For pyrroles, special attention is paid to silver(I)-catalyzed cyclization reactions. For carbazoles, iron(0)-mediated and palladium(0/II)-catalyzed cyclization reactions are highlighted and their broad range of applications is discussed.

Snapshot of the palladium(II)-catalyzed oxidative biaryl bond formation by X-ray analysis of the intermediate diaryl palladium(II) complex.

Pd(II) caught in the act: The diaryl Pd(II) intermediate of a Pd(II)-catalyzed oxidative biaryl bond formation proceeding via a double C-H bond activation has been isolated and fully characterized, including an X-ray crystal structure analysis. Stabilization due to chelation by adjacent pivaloyloxy and acetyl groups has allowed the isolation of this long-sought crucial intermediate. On gentle warming, the complex is transformed into a carbazole product, and the catalytically active Pd(II) species is regenerated by oxidation with Cu(II).

Antioxidant interaction between organic phosphites and hindered amine light stabilizers: effects during photoxidation of polypropylene—II

Abstract The behaviour of mixtures of hindered amine light stabilizers (HALS), their nitroxyl radical, and alkylamine analogues, with aromatic and aliphatic phosphites during photoxidation of polypropylene (PP) has been examined. The photostabilizing performance of the above mixtures of stabilizers was compared with that of the individual antioxidants and with the performance of a number of synthesized bifunctional stabilizers containing aromatic phosphite or phosphonite and either of the above HALS or related functions in the same molecule (referred to as ‘HALS-Phosphites’ or ‘HALS-Phosphonite’). Combinations of the hindered aryl phosphite Irgafos 168, with the hindered amine Tinuvin 770, the tertiary amine Tinuvin 292 and the bis-nitroxyl-analogue of Tinuvin 770, gave antagonistic effects at most molar ratios examined. Furthermore, in all cases, the extent of antagonism was reduced with increasing the amount of Irgafos 168 in the mixtures. Combination of the aliphatic phosphite, trilaurylphosphite, TLP, with Tinuvin 770 in PP, on the other hand, has shown synergistic effect under photoxidative conditions. The HALS-Phosphites and HALS-Phosphonite examined have shown better stabilizing efficiency than the corresponding mixtures of the individual stabilizers.

Antioxidant interaction between organic phosphites and hindered amine light stabilisers during processing and thermoxidation of polypropylene

Abstract Mixtures of hindered amine light stabilisers (HALS) and their radical analogues, with aromatic and aliphatic phosphites have been examined for their effectiveness as melt and thermal antioxidants in polypropylene (PP). Stabilisers containing two different antioxidant functions in the same molecule (aromatic phosphite and either of the above HALS or related functions), referred to as ‘HALS-phosphites’, were also examined and their antioxidant effectiveness compared with the optimum performance of the corresponding combination of mixtures of the individual antioxidants. Binary combinations of the phosphites Irgafos 168 (aromatic) and TLP (aliphatic) with Tinuvin 770, its bis-nitroxyl analogue, or with Tinuvin 292 (tertiary amine) exhibited synergistic effects at different molar ratios; the level of synergism was generally higher during thermal ageing than during melt processing. The optimum combination, leading to the most effective synergism, varied with the chemical nature of the phosphite: under thermoxidative conditions, molar excess of the Irgafos 168 is required whereas molar excess of the HALS is needed for optimum of TLP. HALS-phosphites act autosynergistically and show, in most cases, higher efficiency than their corresponding additive mixtures of stabilisers.

Synthesis and isolation of homeomorphous isomers of P-containing cryptands

The novel isomeric phosphite cryptands 2, 3, and 4 could be synthesized by a simple one-pot tripod capping method starting from bisphenol 1 and PCl3. The assembling of five components led to the formation of a macrobicyclic structure, which probably requires an appropriate preorganization of the reactants. In contrast to the NMR spectra of 2, 3, and 4 in solution, the X-ray structures of 2 and 3 reveal that these molecules have no C3 symmetry in the solid state. In the 31P NMR spectra, both in- and out-P atoms have remarkably different chemical shifts due to a distorted geometry around the in-phosphorus. Phosphorus atoms in the inposition have a decreased reactivity. They are, therefore, more slowly oxidized by cumene hydroperoxide than out-P atoms. A stepwise synthesis was developed for phosphite/phosphatecryptands (5, 7, 9, and 15) via the monoprotected bisphenol 11 and the phosphate 14. In addition, the cylindrical macrotricycle 16 was isolated as a mixture of diastereomers from the crude product of this reaction.

Hydroperoxide decomposing ability and hydrolytic stability of organic phosphites containing hindered amine moieties (HALS-Phosphites)

Abstract The hydroperoxide decomposing ability and the hydrolytic stability of some HALS-Phosphite stabilisers and some of their hydrolytic transformation products have been investigated by means of 31 P NMR spectroscopy. All HALS-Phosphites, including those bearing sterically hindered phenolic substituents proved to be efficient hydroperoxide decomposers. HALS-Phosphites with tertiary HALS moieties are more effective than comparable compounds with secondary HALS groups. The hydrolytic stability of HALS-Phosphites is much higher than those of common phosphites. The path of hydrolysis of HALS-Phosphites was established. In a first step the phenolic moieties are substituted followed by a fast removal of one hindered piperidine group to give the corresponding hydrogen phosphonates. These compounds are hydrolytically stable due to their betaine structure with increased electron density at the phosphorus atom.

IR, Raman, and UV/Vis Spectra of Corannulene for Use in Possible Interstellar Identification

The spectroscopic characterization of corannulene (C(20)H(10)) is carried out by several techniques. The high purity of the material synthesized for this study was confirmed by gas chromatography-mass spectrometry (GC-MS). During a high-performance liquid chromatography (HPLC) process, the absorption spectrum of corannulene in the ultraviolet (UV) and visible (vis) ranges is obtained. The infrared (IR) absorption spectrum is measured in CsI pellets, and the Raman scattering spectrum is recorded for pure crystal grains. In addition to room temperature measurements, absorption spectroscopy in an argon matrix at 12 K is also performed in the IR and UV/Vis ranges. The experimental spectra are compared with theoretical Raman and IR spectra and with calculated electronic transitions. All calculations are based on the density functional theory (DFT), either normal or time-dependent (TDDFT). Our results are discussed in view of their possible application in the search for corannulene in the interstellar medium.

ON THE RELEVANCE OF POLYYNYL-SUBSTITUTED POLYCYCLIC AROMATIC HYDROCARBONS TO ASTROPHYSICS

We report on the absorption spectra of the polycyclic aromatic hydrocarbon (PAH) molecules anthracene, phenanthrene, and pyrene carrying either an ethynyl (-C2H) or a butadiynyl (-C4H) group. Measurements were carried out in the mid infrared at room temperature on grains embedded in CsI pellets and in the near ultraviolet at cryogenic temperature on molecules isolated in Ne matrices. The infrared measurements show that interstellar populations of polyynyl-substituted PAHs would give rise to collective features in the same way non-substituted PAHs give rise to the aromatic infrared bands. The main features characteristic of the substituted molecules correspond to the acetylenic CH stretching mode near 3.05 mum and to the almost isoenergetic acetylenic CCH in- and out-of-plane bending modes near 15.9 mum. Sub-populations defined by the length of the polyynyl side group cause collective features which correspond to the various acetylenic CC stretching modes. The ultraviolet spectra reveal that the addition of an ethynyl group to a non-substituted PAH molecule results in all its electronic transitions being redshifted. Due to fast internal energy conversion, the bands at shorter wavelengths are significantly broadened. Those at longer wavelengths are only barely affected in this respect. As a consequence, their relative peak absorption increases. The substitution with the longer butadiynyl chain causes the same effects with a larger magnitude, resulting in the spectra to show a prominent if not dominating pi-pi* transition at long wavelength. After discussing the relevance of polyynyl-substituted PAHs to astrophysics, we conclude that this class of highly conjugated, unsaturated molecules are valid candidates for the carriers of the diffuse interstellar bands.We report on the absorption spectra of the polycyclic aromatic hydrocarbon (PAH) molecules anthracene, phenanthrene, and pyrene carrying either an ethynyl (–C2H) or a butadiynyl (–C4H) group. Measurements were carried out in the mid-infrared at room temperature on grains embedded in CsI pellets and in the near-ultraviolet at cryogenic temperature on molecules isolated in Ne matrices. The infrared measurements show that interstellar populations of polyynyl-substituted PAHs would give rise to collective features in the same way as non-substituted PAHs give rise to the aromatic infrared bands. The main features characteristic of the substituted molecules correspond to the acetylenic CH stretching mode near 3.05 μm and to the almost isoenergetic acetylenic CCH in- and out-of-plane bending modes near 15.9 μm. Sub-populations defined by the length of the polyynyl side group cause collective features which correspond to the various acetylenic CC stretching modes. The ultraviolet spectra reveal that the addition of an ethynyl group to a non-substituted PAH molecule results in all of its electronic transitions being redshifted. Due to fast internal energy conversion, the bands at shorter wavelengths are significantly broadened. Those at longer wavelengths are only barely affected in this respect. As a consequence, their relative peak absorption increases. The substitution with the longer butadiynyl chain causes the same effects with a larger magnitude, resulting in the spectra showing a prominent if not dominating π-π* transition at long wavelengths. After discussing the relevance of polyynyl-substituted PAHs to astrophysics, we conclude that this class of highly conjugated, unsaturated molecules represents valid candidates for the carriers of the diffuse interstellar bands.

Phosphite macrocycles of varying size

Abstract The condensation reaction of a dichlorophosphite with different bisphenols afforded macrocyclic phosphites of different sizes in dependence on the geometry of the bisphenol applied. A tri-nuclear flexible bisphenol yielded a dimeric macrocycle in moderate yield. Application of a bi-nuclear bisphenol in this reaction resulted in the formation of both dimeric and trimeric macrocycles in almost equal amounts, whereas the linear 1,1′-biphenyl-4,4′-diol led to a tetrameric, square, macrocyclic phosphite.

Red Algae (Rhodophyta) from the Coast of Madagascar: Preliminary Bioactivity Studies and Isolation of Natural Products

Several species of red algae (Rhodophyta) from the coastal regions of Madagascar have been investigated for their natural products. The most abundant compound was cholesterol (5) in combination with a series of oxidized congeners. The brominated indoles 1–3 along with the sesquiterpene debilone (4) have been isolated from Laurencia complanata. For the first time, debilone (4) has been obtained from a marine plant. From the methanol extract of Calloseris sp., we have achieved the second isolation of the unusual A-ring contracted steroids (−)-2-ethoxycarbonyl-2β-hydroxy-A-nor-cholest-5-en-4-one (9) and phorbasterone B (10). The crude extracts of Laurencia complanata exhibited antimicrobial activity against Bacillus cereus, Staphylococcus aureus, Streptococcus pneumoniae, and Candida albicans.