Ulrich Groth

Tuning of chemo- and regioselectivities in multicomponent condensations of 5-aminopyrazoles, dimedone, and aldehydes.

Regio- and chemoselective multicomponent protocols for the synthesis of 1,4,6,7,8,9-hexahydro-1H-pyrazolo[3,4-b]quinolin-5-ones, 5,6,7,9-tetrahydropyrazolo[5,1-b]quinazolin-8-ones, and 5a-hydroxy-4,5,5a,6,7,8-hexahydropyrazolo[4,3-c]quinolizin-9-ones starting from 5-amino-3-phenylpyrazole, cyclic 1,3-dicarbonyl compounds and aromatic aldehydes are described. Whereas the three-component coupling in ethanol under reflux conditions provides mixtures of pyrazoloquinolinones and pyrazoloquinazolinones, the condensation can be successfully tuned toward the formation pyrazoloquinolinones (Hantzsch-type dihydropyridines) by performing the reaction at 150 degrees C in the presence of triethylamine base applying sealed vessel microwave or conventional heating. On the other hand, using sonication at room temperature under neutral conditions favors the formation of the isomeric pyrazoloquinazolinones (Biginelli-type dihydropyrimidines). These products are also obtained when the three-component condensation is executed in the presence of trimethylsilylchloride as reaction mediator at high temperatures. A third reaction pathway leading to pyrazoloquinolizinones in a ring-opening/recyclization sequence can be accessed by switching from triethylamine to a more nucleophilic base such as sodium ethoxide or potassium tert-butoxide. The reaction mechanism and intermediates leading to these three distinct tricyclic condensation products are discussed.

Charge transport characteristics of diarylethene photoswitching single-molecule junctions.

We report on the experimental analysis of the charge transport through single-molecule junctions of the open and closed isomers of photoswitching molecules. Sulfur-free diarylethene molecules are developed and studied via electrical and optical measurements as well as density functional theory calculations. The single-molecule conductance and the current-voltage characteristics are measured in a mechanically controlled break-junction system at low temperatures. Comparing the results with the single-level transport model, we find an unexpected behavior of the current-dominating molecular orbital upon isomerization. We show that both the side chains and end groups of the molecules are crucial to understand the charge transport mechanism of photoswitching molecular junctions.

Cytotoxic Titanium Salan Complexes: Surprising Interaction of Salan and Alkoxy Ligands

The synthesis, biochemical evaluation, and hydrolysis studies of a wide selection of alkyl- and halogen-substituted titanium salan alkoxides are presented herein. A systematic change in the employed alkoxides revealed that both the bulk of the salan ligands and the steric demand of the labile ligands are of great importance for the obtained biological activity. Surprisingly, these two factors are not independent from each other; lowering the steric demand of the alkoxide of a hitherto nontoxic complex renders it cytotoxic. Therefore, our data suggest that the overall size of the complex exerts a strong influence on its biological activity. To decide whether the correlation between the cytotoxicity and the steric demand of the whole complex is merely based on an altered hydrolysis or on the interaction with biomolecules, the behavior of selected complexes under hydrolytic conditions and the influence of transferrin were investigated. Complexes differing only in their labile alkoxy ligands gave the same hydrolysis products with similar hydrolysis rates but displayed cytotoxicities that differed in the range of one order of magnitude. Thus, it seems that the hydrolysis product is not the active species but rather that the unhydrolysed complex is important for the first interaction with a biomolecule. This promoted the idea of hydrolysis being a detoxification pathway. In accordance with the above conclusion, chloro-substituted complex [Ti(Ph(Cl)N(Me))(2)(O(iPr))(2)] displayed a high cytotoxicity (IC(50) approximately 5 microM) and surprisingly high hydrolytic stability (t(1/2)=108 h). These findings, together with the observed cytotoxicity in a cisplatin-resistant cell line, make halo-substituted salan complexes an interesting target for further studies.

Asymmetric syntheses via heterocyclic intermediates. XXII: Enantioselective synthesis of α-alkenyl glycine methyl esters and α-alkenyl glycines (β-, γ-unsaturated amino acids)

Abstract Enantioselective syntheses of α-alkenyl glycines of type 10 and of type 23 are described that provide these uncommon amino acids with predictable configuration and with ee-values of >95%. Both approaches are based on the bislactim ether method developed by Schollkopf . As for 10 : The lithiated bis-lactim ether 6 of cyclo (L-val-gly) is reacted with 2-[(dimethyl t-butyl)silyl]alkanals 2 to give the addition products 7 with de>95%. These on acid hydrolysis afford L-valinate 8 and the methyl (2 R )-2-amino-4-(dimethyl t-butyl)silyl-3-hydroxyalkanoate 9 which are convertible into the ( R )-α-alkenyl glycines of type 10 . The scope of this synthesis is limited by the fact that the compounds 9 are thermolabile when disubstitued at C-4. As for 23 : The lithiated bis-lactim ether 6 is reacted with thioketones 14 to give the addition products 15 with de>95% The S -methyl compounds 16 undergo elimination to give regioselectively the olefins 18 when treated with Raney-Ni Alternatively, the olefins 18 are obtained from the sulfonium salts 24 by dimethyl sulfide elimination, although this route is less regiospecific. The compounds 18 are cleaved by dilute hydrochloric acid, liberating L-valinate 8 and ( R )-α-alkenyl glycine methyl esters 21 , which on further hydrolysis yield ( R )-α-alkenyl glycines 23 . This synthesis is limited only by the availability of thioketones 14

Diastereoselective Ce(OiPr)(3)-Catalyzed Pinacol Couplings of Aldehydes.

Aliphatic aldehydes as well as aromatic aldehydes can be converted by means of cerium-catalyzed pinacol coupling [Eq. (1)]. The coupling reaction products can be isolated with good to very good diastereoselectivities in favor of rac isomers (rac:meso>88:12). R=alkyl, aryl.

Cytotoxic dinuclear titanium-salan complexes: Structural and biological characterization

Controlled hydrolysis of donor-substituted titanium-salan complexes led to the formation of well-defined dinuclear complexes. Structure determination by means of X-ray and NMR-studies revealed the presence of a single μ-oxo bridge and one labile alkoxide ligand per titanium center. Concomitant cytotoxicity assays of the isolated dinuclear complexes showed cytotoxicities in the low micro-molar region, surpassing in this respect even their monomeric ancestors, thus making them possible highly active metabolites of titanium-salan anti-cancer drugs.

Titanium Salan Complexes Displays Strong Antitumor Properties In Vitro and In Vivo in Mice

The anticancer activity of titanium complexes has been known since the groundbreaking studies of Köpf and Köpf-Maier on titanocen dichloride. Unfortunately, possibly due to their fast hydrolysis, derivatives of titanocen dichloride failed in clinical studies. Recently, the new family of titanium salan complexes containing tetradentate ONNO ligands with anti-cancer properties has been discovered. These salan complexes are much more stabile in aqueous media. In this study we describe the biological activity of two titanium salan complexes in a mouse model of cervical cancer. High efficiency of this promising complex family was demonstrated for the first time in vivo. From these data we conclude that titanium salan complexes display very strong antitumor properties exhibiting only minor side effects. Our results may influence the chemotherapy with metallo therapeutics in the future.

Synthesis and Photoswitching Studies of Difurylperfluorocyclopentenes with Extended π‐Systems

In an attempt to design molecular optoelectronic switches functioning in molecular junctions between two metal tips, we synthesized a set of photochromic compounds by extending the π-system of 1,2-bis-(2-methyl-5-formylfuran-3-yl)perfluorocyclopentene through suitable coupling reactions involving the formyl functions, thereby also introducing terminal groups with a binding capacity to gold. Avoiding the presence of gold-binding sulphur atoms in the photoreactive centre, as they are present in the frequently used analogous thienyl compounds, the newly synthesized compounds should be more suitable for the purpose indicated. The kinetics of reversible photoswitching of the new compounds by UV and visible light was quantitatively investigated in solution. The role of conformational flexibility of the π-system for the width of the UV/Vis spectra was clarified by using quantum chemical calculations with time-dependent (TD)-DFT. As a preliminary test of the potential of the new compounds to serve as optoelectronic molecular switches, monolayer formation and photochemical switching on gold surfaces was observed by using surface plasmon resonance.

Highly Selective Apoptotic Cell Death Induced by Halo-Salane Titanium Complexes

Metal‐based antitumor agents: Halogen‐substituted titanium salane complexes showed IC50 values comparable to cisplatin. In contrast to their alkyl‐substituted congeners, they almost exclusively induced apoptotic cell death. This unique combination of very low IC50 values and pronounced preference for apoptosis makes them promising therapeutic agents.

Titanocene Difluorides with Improved Cytotoxic Activity

Titanocene difluorides can be obtained by halide metathesis of the respective titanocene dichlorides with trimethyltin fluoride (Me(3)SnF), giving access to a new class of cytotoxic active substances. Furthermore, an improved method for the synthesis of diaryl-substituted titanocene dichlorides is presented.