Hans-Ulrich Reißig

Substrate Specificity and Regioselectivity of Tryptophan 7‐Halogenase from Pseudomonas fluorescens BL915

Tryptophan 7-halogenase which is involved in pyrrolnitrin biosynthesis is the first halogenating enzyme to be isolated that has substrate specificity and regioselectivity. This FADH2-dependent halogenase catalyzes the chlorination of its natural substrate tryptophan exclusively at the 7-position, a position at which direct chemical chlorination is not possible. Other substrates such as N-Ω-methyltryptamine, 5-methyltryptamine, 5-methylindole, 3-methylindole, or indole-3-acetonitrile are also chlorinated by the enzyme, whereas compounds like 1-methyltryptophan, indole-3-carboxylic acid, indole-3-acetic acid, or indole are not accepted as substrates. In addition, phenylpyrrole derivatives are also chlorinated by the enzyme. However, in contrast to tryptophan, the tryptophan and indole derivatives are chlorinated at positions 2 or/and 3 of the indole ring system and not at the 7-position. Chlorination of the phenylpyrrole derivatives also proceeds without regioselectivity and a mixture of mono- and dichlorinated products is obtained.

On the Stereoselectivity of γ-Lactol Substitutions with Allyl- and Propargylsilanes − Synthesis of Disubstituted Tetrahydrofuran Derivatives

Monosubstituted γ-lactols 1a−1c, 3a−3c and 4a−4c, as well as disubstituted γ-lactol 5 and the γ-hydroxy-substituted γ-lactone 6, were transformed into disubstituted tetrahydrofuran derivatives by treatment with allyl- and propargylsilanes in the presence of Lewis acids. The diastereoselectivities were moderate to excellent and are interpreted by application of the Felkin−Anh model to cyclic oxocarbenium ions. The effects of the equilibria between conformers of the intermediates are discussed. The surprisingly high diastereoselectivity of 4-substituted γ-lactols 3 can be explained on this basis.

Optimisation, Scope and Limitations of Enantioselective Aldol Reactions of an S‐Ketene Silyl Acetal with Aliphatic Aldehydes under (R)‐BINOL‐Titanium(IV) Catalysis Conditions

The Mukaiyama aldol reaction between the functionalised aldehydes 5 and the S-ketene silyl acetal 2 catalysed by 1,1′-binaphthyl-derived chiral titanium(IV) complex 4 afforded the corresponding aldol products 6 in good yields and with good to excellent enantioselectivities. The chemical yield could further be enhanced, without loss of stereoselection, by addition of phenol and/or molecular sieves. The presented aldol reactions with aluminium, boron and ytterbium-BINOL catalysts demonstrate that only low chiral induction can be achieved. Aldol product 6a was converted into an α-lipoic acid precursor 8, thus providing a formal synthesis of this biologically active compound. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Lewis Acid‐Promoted Reactions of γ‐Lactols with Silyl Enol Ethers — Stereoselective Formation of Functionalized Tetrahydrofuran Derivatives

The monosubstituted γ-lactols 1a, 1b, 1c, and 1d and the disubstituted γ-lactol 1e were converted into tetrahydrofuran derivatives by reaction with typical silyl enol ethers in the presence of Lewis acids. Although the most suitable Lewis acid appears to be zinc chloride, BF3·Et2O or diethylaluminium chloride are also suitable under appropriate conditions. The stereoselectivities of these substitution reactions are similar to those observed with other silylated nucleophiles; however, there are several important differences. A comparison of the diastereoselectivities of different γ-lactols and of various silylated nucleophiles and organometallic compounds will also be presented in this paper.

One‐Pot Transformation of Silyl Enol Ethers into 1,3‐Dienes: In situ Generation of Alkenyl Nonaflates and Subsequent Heck Reactions − Scope and Limitations

Palladium-catalysed reactions between methyl acrylate and the isolated alkenyl nonaflates 2a, 2d and 2e proceed without difficulties, furnishing the desired 1,3-dienes 3, 14 and 15 in good yields. The use of other alkenyl nonaflates and olefins in this Heck reaction was also examined. The main purpose of this study was the development of an in situ generation of the required alkenyl nonaflates 2 from the corresponding silyl enol ethers 1 and their one-pot transformation into 1,3-dienes. Thus, the previously described fluoride-promoted exchange of the trimethylsilyl substituent of typical enol ethers 1 for a nonafluorobutylsulfonyl group was directly combined with the palladium-catalysed coupling step. This sequence allowed the efficient transformation of a variety of silyl enol ethers 1 into highly substituted 1,3-dienes in a practical one-pot procedure. The scope and limitations, together with the chemo- and stereoselectivity, of this process are discussed. A particular intriguing example involves a one-pot synthesis of silyl enol ether 1j by means of a Diels−Alder reaction, subsequent nonaflation and a Heck reaction with tert-butyl acrylate, furnishing the highly functionalised 1,3-diene 30 in good overall yield. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

An Expedient and Stereoselective Synthesis of Alkenyl Nonaflates from Silyl Enol Ethers: Optimization, Scope and Limitations

The fluoride-catalysed reaction between silyl enol ethers 1 and nonafluorobutanesulfonyl fluoride (NfF) has been optimized, resulting in an expedient synthesis of the corresponding alkenyl nonaflates 3. Tetra-n-butylammonium fluoride, dried either with molecular sieves or with potassium fluoride, and potassium fluoride in the presence of dibenzo-18-crown-6 were the best and most practical catalysts for this process. The reaction allows the synthesis of a wide variety of cyclic and acyclic alkenyl nonaflates 3 in good to excellent yields. For E/Z isomeric alkenes the configuration of the double bond is essentially retained. Remarkably, enolates derived from methyl ketones also provide C-sulfonylation products 4 as a side product; the desired alkenyl nonaflates 3l and 3m could, however, be prepared in good yields by further optimization. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Synthesis and Heck Reactions of Ethenyl‐ and (Z)‐Butadien‐1‐yl Nonaflate Obtained by the Fragmentation of Furan Derivatives

The nonaflation of lithium enolates or of silyl enol ethers, formally derived from acetaldehyde or crotonaldehyde, with nonafluorobutanesulfonyl fluoride gave ethenyl nonaflate (1b) and (Z)-buta-1,3-dien-1-yl nonaflate (2) in good yields. The required enolates were obtained by aldehyde-free routes by the lithiation of tetrahydrofuran or 2,5-dihydrofuran followed by the cyclofragmentation of the metallated heterocycles. The application of this approach to the synthesis of allenyl nonaflate 3 failed, presumably due to the intrinsic instability of this allene derivative. The nonaflates 1b and 2 were also prepared by the fluoride-catalysed reaction of the corresponding silyl enol ethers 5 and 7 with nonafluorobutanesulfonyl fluoride; however, the overall yields are slightly lower for these two-step pathways. The cyclofragmentation of lithiated 2,2-dimethyl-4-methylene-[1,3]dioxolane allowed the easy preparation of trimethylsiloxyallene (10) in moderate yield. The nonaflates 1b and 2 reacted smoothly with monosubstituted alkenes in the presence of a catalytic amount of palladium(II) acetate to give the anticipated Heck coupling products in good to moderate yields and with high stereoselectivities.

Stereodivergent De Novo Synthesis of Branched Amino Sugars by Lewis Acid Promoted Rearrangement of 1,2‐Oxazines

Well concealed: 1,2-oxazines such as 1 rearrange under Lewis acidic conditions to bicyclic products of type 2, which can be incorporated into oligosaccharides as protected amino sugar equivalents. Subsequent reductive steps provide unusual oligosaccharides 3 having C2-branched 4-amino sugar units. Most of the reactions proceed with excellent stereocontrol and allow the synthesis of a collection of stereoisomers.

Synthesis of Enantioenriched 2‐Substituted 4‐Phenylbutylamines by Hydrogenolysis of Optically Pure 6‐Alkoxy‐5,6‐dihydro‐4H‐1,2‐oxazines

Lewis acid promoted exchange of the 6-ethoxy group of 6H-1,2-oxazines 1−3 with (−)-menthol furnished the optically active heterocycles 4−6. Diastereomers 4a and 4b, which could be separated efficiently by chromatography, were excellent substrates for highly diastereoselective conjugate additions of phenyllithium and n-butyllithium, thus providing the enantiopure trans-substituted 1,2-oxazines 7a, 7b, 8a, and 8b in good yields. Exhaustive hydrogenolysis of 7a afforded the primary amine 9 with an enantiomeric excess of 80%, whereas hydrogenolysis of 8a and 8b gave the corresponding amines (R)-11 and (S)-11, respectively, with an ee of more than 90%. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Human polymerase α inhibitors for skin tumors. Part 2. Modeling, synthesis and influence on normal and transformed keratinocytes of new thymidine and purine derivatives

Recently, the three-dimensional structure of the active site of human DNA polymerase α (pol α) was proposed based on the application of molecular modeling methods and molecular dynamic simulations. The modeled structure of the enzyme was used for docking selective inhibitors (nucleotide analogs and the non-nucleoside inhibitor aphidicolin) in its active site in order to design new drugs for actinic keratosis and squamous cell carcinoma (SCC). The resulting complexes explained the geometrical and physicochemical interactions of the inhibitors with the amino acid residues involved in binding to the catalytic site, and offered insight into the experimentally derived binding data. The proposed structures were synthesized and tested in vitro for their influence on human keratinocytes and relevant tumor cell lines. Effects were compared to aphidicolin which inhibits pol α in a non-competitive manner, as well as to diclofenac and 5-fluorouracil, both approved for therapy of actinic keratosis. Here we describe three new nucleoside analogs inhibiting keratinocyte proliferation by inhibiting DNA synthesis and inducing apoptosis and necrosis. Thus, the combination of modeling studies and in vitro tests should allow the derivation of new drug candidates for the therapy of skin tumors, given that the agents are not relevant substrates of nucleotide transporters expressed by skin cancer cells. Kinases for nucleoside activation were detected, too, corresponding with the observed effects of nucleoside analogs.