1,2-Shift of a Carboxyl Group in a Wagner-Meerwein Rearrangement

On treatment with HSO₃F in SO₂C1F at 0°, 3-hydroxy-2,2-dimethyl-3-phenyl-propionic acid ( 1a ) is transformed into 2-phenyl-3-methyl-2-butenoic acid ( 2a ) (isolated yield: 40–44%). Using monolabelled [3-¹³C]- 1a ( 1a *) and doubly labelled [1,3-¹³C₂]- 1a ( 1a **), the migration of HOOC (or a mechanistically equivalent group) was proved; a cross experiment established the intramolecular character of the rearrangement. By following the reaction at low temperature in an NMR. spectrometer, the formation of intermediates and side products was demonstrated.     

Synthesen von Moniliformin,einem Mycotoxin mit Cyclobutendion-Struktur

Syntheses of Moniliformin, a Mycotoxine with a Cyclobutenedione Structure Six different routes to 3-hydroxy-3-cyclobutene-1, 2-dione ( 4 ), the free acid of the mycotoxine Moniliformin (=alkali salt of 4 ) are described. A common feature of all pathways is the synthesis of cyclobutanes having the oxidation level 6. Moniliformin precursors which are easily transformed to 4 by acid catalysed hydrolysis include [2+2]-cycloadducts of ketene with tetraalkoxy-olefins, 3,4,4-trialkoxycyclobutenes, derivatives of polyfluorinated cyclobutenes, the brominated [2+2]-cycloadduct of ethyl vinyl ether and dichloroketene, tetrabromocyclobutanone, [2+2]-photocycloadducts of dichlorovinylenecarbonate with dichloroethylenes, and the dimer of chloroketene. The most convenient synthesis via the [2+2]-cycloadduct of tetraethoxyethylene and ketene ( 14b ) is reported in detail and produces 4 in four simple steps in 57% overall yield. In addition, two new syntheses of squaric acid ( 56 ) are described.     

Metal Nanoparticles on Stainless Steel Surfaces as Novel Heterogeneous Catalysts

The goal of this work was the development of a novel type of heterogeneous catalyst, consisting of bare metal nanoparticles on stainless steel foils, which can be shaped to any kind of architecture and, if necessary, heated electrically. Solutions of pre-prepared, ligand protected and monodispersed gold, palladium, platinum and rhodium nanoparticles were sprayed onto stainless steel foils, followed by the careful removal of the ligand molecules by an oxygen plasma treatment. Due to this, bare particles become irreversibly fixed on the steel support. It could be shown that the original particle sizes do not change during the plasma treatment. Foils, densely coated with the nanoparticles, were used for gas phase catalyses in a self-made reactor at room temperature or at 60 °C. Hydrogenation of 1,3-butadiene at 15 nm Pd and 2 nm Pt, CO oxidation at 16 nm, 8 nm and 1.4 nm gold and NO reduction with NH₃ at 2 nm Rh particles were performed, indicating that the novel catalysts might in principle be applicable in technical processes if the experimental conditions like form and temperature would be optimized. Dedicated to Professor Dieter Fenske on the occassion of his 65th birthday.     

The separation of hypericin's enantiomers and their photophysics in chiral environments

We report the first separation of the enantiomers of hypericin. Their steady-state optical spectra and ultrafast primary photoprocesses are investigated in chiral environments. Within experimental error, there is no difference between the two enantiomers in any of the systems considered. This is consistent with the emerging picture that the rich and extended absorption spectrum of hypericin is not a result of ground-state heterogeneity. It is also consistent with the observation that the spectra and photophysics of hypericin are generally insensitive to environments in which it does not aggregate.     

3-(Dimethylamino)-2,2-dimethyl-2H-azirin als α-Aminoisobuttersäure(Aib)-Äquivalent: Cyclische Depsipeptide durch direkte Amid-Cyclisierung

3-(Dimethylamino)-2,2-dimethyl-2H,-azirine as an α-Aminoisobutyric-Acid (Aib) Equivalent: Cyclic Depsipeptides via Direct Amid Cyclization In MeCN at room temperature, 3-(dimethylamino)-2,2-dimethyl-2H-azirine ( 1 ) and α-hydroxycarboxylic acids react to give diamides of type 8 (Scheme 3). Selective cleavage of the terminal N,N-dimethylcarboxamide group in MeCN/H₂O leads to the corresponding carboxylic acids 13 (Scheme 4). In toluene/Ph SH , phenyl thioesters of type 11 are formed (see also Scheme 5). Starting with diamides 8 , the formation of morpholin-2,5- diones 10 has been achieved either by direct amide cyclization via intermediate 1,3-oxazol-5(4H)-ones 9 or via base-catalyzed cyclization of the phenyl thioesters 11 (Scheme 3). Reaction of carboxylic acids with 1 , followed by selective amide hydrolysis, has been used for the construction of peptides from α-hydroxy carboxylic acids and repetitive α-aminoisobutyric-acid (Aib) units (Scheme 4). Cyclization of 14a, 17a , and 20a with HCI in toluene at 100° gave the 9-, 12-, and 15-membered cyclic depsipeptides 15, 18 , and 21 , respectively.     

Zur Darstellung von Oxiden cyclischer,sekundärer Arsine

Hydrolysis of 1-chloro-arsolane, C₄H₈AsCl, and 1-chloro-arsenane, C₅H₁₀AsCl, leads to oxides C₈H₁₀As₂O and C₁₀H₂₀As₂O, respectively. On direct alkylation of arsenic trioxide with 1.4-butane- and 1.5-pentane-bis-magnesium bromide these oxides are formed only in trifling amounts. Better results are gained by alkylation of N,N-dimethylaminodichloroarsane into amino-arsolane and amino-arsenane, which are also transformed into these oxides by hydrolysis. By hydrolysis of 1.4-dichloro-diarsenane only a small amount of 1.4-epoxo-arsenane is formed, whereas mainly a dimer of this oxide results. Solvolysis of 2.6-dichloro-diarsa-[3.3.0]-bicyclooctane with water, hydrogen sulphide or methylamine leads to 7-oxa-, 7-thia- and 7-methylaza-2.5-diarsa-noradamantane, respectively.     

A reinvestigation of asymmetric induction in diels-alder reactions to chiral acrylates.

The chiral induction in the Diels-Alder addition $\text{1}$ → $\text{2}$, assessed reliably by ¹⁹F-NMR-spectroscopy of the endo-esters $\text{4}$, varied between 47 - 93% in favor of the 2-(R)-adducts $\text{2}$ depending on the auxiliary chiral group and the Lewis-acid catalyst.     

1,6-dithia-spiro[4.4]nonadiene aus 2-thiazolin-5-thionen

4,4-Dimethyl-2-phenyl-2-thiazolin-5-thione ($\text{4}$) reacts with 2,3-diphenylcyclopropenone ($\text{2a}$) at 145°C and with benzonitrilio-2-propanide ($\text{6}$) at room temperature to yield the 1,6-dithia-spiro[4.4]nonadienes $\text{5}$ and $\text{7}$, respectively.     

Hexaruthenium carbonyl cluster complexes with basal edge-bridged square pyramidal metallic skeleton: efficient synthesis of 2-imidopyridine derivatives and determination of their reactive sites in carbonyl substitution reactions

The reactions of [Ru(3)(CO)(12)] with half equivalent of 2-amino-6-methylpyridine (H(2)ampy) or 2-aminopyridine (H(2)apy) in refluxing xylene give the hexanuclear products [Ru(6)(mu(3)-H)(2)(mu(5)-eta(2)-L)(mu-CO)(2)(CO)(14)] (L = ampy, 1; apy, 2). These reactions represent the first high-yield syntheses of hexanuclear complexes with a basal edge-bridged square pyramidal metallic skeleton. Five metal atoms of these complexes are bridged by the N-donor ligand in such a way that the edge-bridging metal atom is attached to the pyridine nitrogen, while the basal atoms of the square pyramid are capped by an imido fragment that arises from the activation of both N-H bonds of the NH(2) group. The reactive sites of these complexes in CO substitution reactions have been determined by studying the reactivity of 1 with triphenylphosphine. Two kinetically controlled monosubstitutions take place on the edge-bridging metal atom in positions cis to the pyridine nitrogen, leading to a mixture of two isomers of formula [Ru(6)(mu(3)-H)(2)(mu(5)-eta(2)-ampy)(mu-CO)(2)(CO)(13)(PPh(3))] (3 and 4). On heating at 80 degrees C, these monosubstituted isomers are transformed, via a dissociative pathway, into the product of thermodynamic control (5), which has the PPh(3) ligand on the apical Ru atom. The di- and trisubstituted derivatives [Ru(6)(mu(3)-H)(2)(mu(5)-eta(2)-ampy)(mu-CO)(2)(CO)(12)(PPh(3))(2)] (6) and [Ru(6)(mu(3)-H)(2)(mu(5)-eta(2)-ampy)(mu-CO)(2)(CO)(11)(PPh(3))(3)] (7) are stepwise formed from 3-5 and PPh(3). Compound 6 has the PPh(3) ligands on the edge-bridging and apical Ru atoms, and compound 7 has an additional PPh(3) ligand on an unbridged basal Ru atom. The compound [Ru(6)(mu(3)-H)(2)(mu(5)-eta(2)-ampy)(mu-CO)(2)(CO)(12)(mu-dppm)] (8), in which a basal and the apical Ru atoms are spanned by the dppm ligand, has been isolated from the reaction of 1 with bis(diphenylphosphino)methane.