Uli Kazmaier

Chelated Enolates of Amino Acid Esters—Efficient Nucleophiles in Palladium-Catalyzed Allylic Substitutions

Complementary to the Claisen rearrangement, the title reaction-which when carried out with chelated enolates of amino acid esters as nucleophiles gives rise to unsaturated amino acids-preferentially provides anti instead of syn products. This palladium-catalyzed reaction proceeds under very mild conditions and is suitable for the synthesis of enantiomerically pure amino acids (see reaction scheme). Tfa=trifluoroacetyl.

Novel imidazol-1-ylmethyl substituted 1,2,5,6-tetrahydropyrrolo[3,2,1-ij]quinolin-4-ones as potent and selective CYP11B1 inhibitors for the treatment of Cushing's syndrome.

CYP11B1 inhibition is a promising therapy for Cushings syndrome. Starting from etomidate, references I and II, the title compounds were designed and synthesized. Cyclopropyl analogue 4 was identified as a CYP11B1 inhibitor more potent (IC(50) = 2.2 nM) than leads and more selective (SF = 11) than I and metyrapone. Since it also showed potent inhibition of rat CYP11B1 and good selectivity over human CYP17 and CYP19, it is a promising candidate for further development.

Asymmetric chelated Claisen rearrangements in the presence of chiral ligands--scope and limitations.

Claisen rearrangements of glycine crotyl ester enolates in the presence of chelating metal salts and chiral ligands provide ,-unsaturated amino acids in a highly stereoselective fashion. Best results are obtained with electron withdrawing protecting groups, isopropylates of aluminum and magnesium, and the cinchona alkaloids as chiral ligands. While the use of quinine gives rise to the (2R)-configured amino acids, quinidine provides the opposite enantiomer. The different enantiomers can also be obtained by using only one of the chiral ligands by simply changing the reaction conditions. A mechanistic rational for the stereochemical outcome of the reaction is given, which is supported by several experiments.

Pretubulysin, a Potent and Chemically Accessible Tubulysin Precursor from Angiococcus disciformis

Simplify, simplify, simplify! Pretubulysin (structure without the green substituents), a simplified tubulysin was prepared in the laboratory and also found in a natural myxobacterial source. This biosynthetic precursor of the tubulysins is not as active as tubulysins A and D but is still effective in picomolar concentrations against cancer cell lines.

Via Ugi reactions to conformationally fixed cyclic peptidesDedicated to Professor Dr D. Seebach on the occasion of his 65th birthday.

A simple approach to several cyclopeptidmimetics containing an N-alkylated amino acid was found via a multicomponent reaction followed by a ring-closing metathesis starting from readily available precursors. The combinatorial technique has the advantage that different polar, hydrophilic or hydrophobic moieties can be placed at any position in the cycles and unnatural amino acids can also be incorporated.

Synthesis of amino acid derivatives via enantio- and diastereoselective Pd-catalyzed allylic substitutions with a non-stabilized enolate as nucleophileThis work is dedicated to Professor Walter Siebert on the occasion of his 65th birthday.

Diastereomer ratios of up to 95:5 and enantiomeric excesses of up to 95% were achieved in Pd-catalyzed asymmetric alyllic substitutions with zinc enolates of glycine esters as nucleophiles; a remarkable effect of the ligand on the diastereoselectivity of the substitution was found.

Discovery of 23 Natural Tubulysins from Angiococcus disciformis An d48 and Cystobacter SBCb004

The tubulysins are a family of complex peptides with promising cytotoxic activity against multi-drug-resistant tumors. To date, ten tubulysins have been described from the myxobacterial strains Angiococcus disciformis An d48 and Archangium gephyra Ar 315. We report here a third producing strain, Cystobacter sp. SBCb004. Comparison of the tubulysin biosynthetic gene clusters in SBCb004 and An d48 reveals a conserved architecture, allowing the assignment of cluster boundaries. A SBCb004 strain containing a mutant in the putative cyclodeaminase gene tubZ accumulates pretubulysin A, the proposed first enzyme-free intermediate in the pathway, whose structure we confirm by NMR. We further show, using a combination of feeding studies and structure elucidation by NMR and high-resolution tandem mass spectrometry, that SBCb004 and An d48 together biosynthesize 22 additional tubulysin derivatives. These data reveal the inherently diversity-oriented nature of the tubulysin biosynthetic pathway.

Direct Michael, Aldol, and Mannich Additions Catalyzed by Alkaline Earth Metals

Alkaline earth metals are vastly abundant, inexpensive and commercially available, and also relatively nontoxic. Surprisingly, only a few examples of alkali earth metal catalyzed processes have been reported so far. Evans and Nelson were the first, reporting an application of magnesium bisulfonamide complexes as catalysts for the enantioselective amination of N-acyloxazolidinones. This approach provided an interesting stereoselective approach towards arylglycines. Alkali earth metals show both Lewis acidic as well as Bronsted basic properties, which makes them ideal candidates as catalysts for enolate additions. Kumaraswamy et al. reported on asymmetric Michael additions of various malonates and b-ketoesters to a,b-unsaturated ketones in the presence of binol–calcium complexes (Scheme 1; binol= 1,1’2-binaphthol). The best results were obtained with octahydrobinol complexes A.

Chelatisierte Aminosäureesterenolate – effiziente Nucleophile für Palladium‐katalysierte allylische Alkylierungen

Komplementar zur Claisen-Umlagerung ist die Titelreaktion, die mit chelatverbruckten Aminosaureesterenolaten als Nucleophilen ungesattigte Aminosauren liefert, da hier bevorzugt anti- anstelle der syn-Produkte gebildet werden. Diese Pd-katalysierte Reaktion verlauft unter sehr milden Bedingungen und last sich bei Verwendung chiraler Allylsubstrate auch enantioselektiv durchfuhren (siehe Schema). Tfa = Trifluoracetyl.

Biosynthesis of 2‐Alkyl‐4(1H)‐Quinolones in Pseudomonas aeruginosa: Potential for Therapeutic Interference with Pathogenicity

Pseudomonas aeruginosa is a ubiquitous Gram-negative bacterium capable of surviving in a broad range of natural environments and known to be involved in infectious diseases of various hosts. In humans, the opportunistic pathogen is one of the leading causes for nosocomial infections in immuno-compromised patients and is responsible for chronic lung infections in the majority of cystic fibrosis patients. The ability of P. aeruginosa to adapt to different environments and lifestyles is closely related to its ability to coordinate the survival strategy of a population by so-called quorum-sensing (QS) systems. QS is based on the production and release of small signaling molecules, called autoinducers, that increase in concentration as a function of cell density and activate corresponding transcriptional regulators after a threshold concentration has been reached. Three different QS systems are known from P. aeruginosa. The las and rhl systems use acyl-homoserine lactone (AHL) autoinducers and belong to the LuxI/LuxR-type systems that are widespread among Gram-negative bacteria. The third QS system is rather unique and restricted to particular Pseudomonas and Burkholderia strains. Therein 2-alkyl-4(1H)quinolones (AQ) autoinducers such as 2-heptyl-3-hydroxy4(1H)-quinolone (the Pseudomonas quinolone signal : PQS) and its direct precursor 2-heptyl-4(1H)-quinolone (HHQ) are used (see Scheme 1). The pqs system is involved in the regulation of P. aeruginosa virulence such as pyocyanin biosynthesis, biofilm formation and maturation, the production of exoproducts like elastase, alkaline proteases, rhamnolipids, and hydrogen cyanide, and the expression of efflux pumps. Further, PQS itself can down-regulate the host-innate immune response. The sum of these effects makes the pqs system a highly attractive target for drug development to interfere with P. aeruginosa pathogenicity and biofilm formation. The general validity of this approach is supported by the results of several infection models in which PQS-deficient mutants show a reduced pathogenicity compared to P. aeruginosa wild type. 10] A reduced pathogenicity was also observed in a mouse infection model when animals infected with the wild-type strain were treated with halogenated anthranilic acid derivatives that inhibit PQS biosynthesis. This treatment led to a significant increase in survival in comparison to the control group. To further explore this target it is necessary to understand the details of PQS formation in the pathogen. It is known that HHQ biosynthesis absolutely requires the genes pqsA–D, encoding an anthranilate:coenzyme A (CoA) ligase (pqsA) and three b-ketoacyl-acyl carrier protein synthase III (KAS III) homologues. An additional gene (pqsH), located apart from pqsA– D, is responsible for the hydroxylation of HHQ to form PQS. Feeding studies in vivo have demonstrated that HHQ most likely arises from “head-to-head” condensation of an anthraniloyl precursor and a b-keto fatty acid derivative (Scheme 1). However, the details of the enzymatic mechanism of this reaction and the nature of the b-keto fatty acid remained elusive. Furthermore, it has been shown in vitro that PqsA and PqsD catalyze the formation of 2,4-dihydroxyquinoline (DHQ), another secondary metabolite of P. aeruginosa. In DHQ biosynthesis PqsA activates anthranilic acid to anthraniloyl-CoA which is loaded to the active-site cysteine (C112) of PqsD, which itself catalyzes the decarboxylative Claisen condensation with malonyl-CoA. Based on the structural similarity between DHQ and HHQ, we reasoned that PqsD might be involved in a similar condensation reaction in HHQ biosynthesis. To prove this hypothesis, we heterologously expressed PqsD from strain PA14 in Escherichia coli and purified the enzyme for biochemical characterization in vitro. Anthraniloyl-CoA and three potential b-keto acid derivatives were chemically synthesized as substrates, including b-ketodecanoic acid (1), b-ketodecanoyl-CoA (2), and b-ketodecanoyl-N-acetylcysteamine thioester (3) as mimics of the hypothetical ACP-bound substrate (for details see the Supporting Information). The in vitro reaction contained recombinant PqsD, anthraniloyl-CoA, and one of the Scheme 1. Biosynthetic pathways to DHQ, HHQ, and PQS. The nature of the accepted b-ketodecanoyl moiety is unknown to date. Possible substrates are b-ketodecanoic acid (1), b-ketodecanoyl-CoA (2) and b-ketodecanoyl-ACP.