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PubMed1996

Pyrrolidino enaminones structural related to gyrase inhibitors: synthesis, cyclization and pharmacological activity.

The synthesis and stereochemical characteristics of pyrrolidino enaminones with structural analogy to quinolone antibacterial agents are reported. Retro-aldol reaction of the esters 7 is observed under all conditions of hydrolysis, the enzyme catalyzed reaction also does not yield the corresponding acids. In contrast to clinically used quinolones exemplary tests with monocyclic esters of type 7 and the tricyclic derivative 10 indicate low or no antibacterial activity. Additionally to the lack of the carboxylic acid function the enaminones 7 differ from quinolones in stereochemical demands and the electronic situation of the phenyl substituents of 10, respectively possibly preventing the formation of a co-operative tetrametric system between compounds and DNA bases.

Anti-Bacterial Agents↗
PubMed1999

Rhodium-Promoted Linear Tetramerization and Cyclization of 3,3-Dimethylbut-l-yne.

With tetrahydrofuran as the solvent [Rh(thf)(2)(cod)](+) promotes the selective coupling of tBuC(2)H to form triene-yne 1; at high alkyne concentrations, however, an unusual C-H activation process intervenes, leading to the formation of complex 2, which contains two linked five-membered rings. cod=1,5-cyclooctadiene.

Journal Article↗
PubMed1999

Stereoselective Mukaiyama-Michael/Michael/Aldol Domino Cyclization as the Key Step in the Synthesis of Pentasubstituted Arenes: An Efficient Access to Highly Active Inhibitors of Cholesteryl Ester Transfer Protein (CETP).

Seemingly heartbreaking for a stereochemist, the one-step selective construction of a stereopentad and its prompt destruction by aromatization has been proven to be an efficient strategy for the synthesis of fivefold substituted, pharmacologically highly active arenes (see scheme).

Journal Article↗
PubMed2001

On ring carbomers of cyclobutane, cyclopentane, and cyclodecane and cyclization reactions through bis(alkynyl-propargyl) coupling.

A copper-mediated procedure for terminal alkynyl-propargyl coupling has been applied to "skipped" bis-terminal undecatetrayne and 1,4-bis(pseudo)halobut-2-ynes with the aim of preparing ring carbomers of representative strained and loose cycloalkanes, namely [N]pericyclynes. Two unprecedented, cyclic. "skipped" polyynes with CH2 vertices have been isolated as mixtures of diastereoisomers: an isomer 1b and a dimer 2a of [5]pericyclyne 1a. The isomer 1b is a cyclotetrayne with an exocyclic allene function resulting from a unique formal SN process. Its structure has been established by 1H/13C HMQC and HMBC two-dimensional NMR analysis. According to density functional theory calculations, it is about 6 kcalmol(-1) more stable than [5]pericyclyne (1a). Compound 1b can also be regarded as a C13-relaxed [4]pericyclyne, a long sought "skipped" C12 tetrayne. The dimer 2a is a C30 ring that results from a formal SN process. It is a stable ring carbomer of cyclodecane, that is, a [10]pericyclyne, with four CH2 vertices.

Journal Article↗
PubMed2001

Chemo-, regio-, and stereoselective cyclizations of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with alpha-chloroacetic acid chlorides and alpha-chloroacetic acetals.

Treatment of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with alpha-chlorocarboxylic acid chlorides resulted in chemo- and regioselective formation of 6-chloro3,5-dioxo esters, which were regioselectively converted into functionalised 3(2H)furanones. Chemo- and regioselective condensation of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with alpha-chloroacetic dimethyl acetal afforded 6-chloro-5-methoxy-3-oxo esters, which could be regio- and stereoselectively transformed into 2-alkylidene-4-methoxytetrahydrofurans.

Journal Article↗
PubMed2001

The 12,13-diol cyclization approach for a truly stereocontrolled total synthesis of epothilone B and the synthesis of a conformationally restrained analogue.

A highly convergent and stereocontrolled synthesis of epothilone B (1) has been developed. The epoxide moiety in 1 was generated by regioselective mesylation and base treatment of the 12,13-diol 30 which was formed by a chelate Cram controlled Grignard addition of 14 and methyl ketone 13. Both fragments were synthesized from the chiral carbon pool precursors (S)-citronellol and (S)-lactic acid, respectively. A highly diastereoselective aldol addition of epoxy-aldehyde 7 and the known Southern hemisphere ketone 8 delivered the full carbon skeleton, containing all the stereogenic centers of 1. Functional group manipulation, macrolactonization and removal of two protecting groups then yielded 1. The spatial closeness of the C4-beta-methyl and C6-methyl group in the crystal structure of 1 inspired us to connect them through a methylene bridge to give a cyclohexanone derivative. Thus, the Northern hemisphere aldehyde 7 was added to the enolate of the cyclohexanone 47. Further manipulations and macrolactonization delivered the conformationally restrained epothilone derivative 42.

Antineoplastic Agents↗
PubMed2001

Regio- and diastereoselective cyclization reactions of free and masked 1,3-dicarbonyl dianions with 1,2-dielectrophiles.

Despite their simplicity and synthetic usefulness, cyclisation reactions of 1,3-dicarbonyl dianions with 1,2-dielectrophiles are problematic, since both dianions and 1,2-dielectrophiles are highly reactive compounds (low reactivity matching). In addition, 1,2-dielectrophiles are often rather labile, and reactions with nucleophiles can result in polymerisation, decomposition, formation of open-chained products, elimination or SET-reactions. These intrinsic limitations can be overcome by a proper reactivity tuning and by the use of electroneutral dianion equivalents (masked dianions) in Lewis acid catalysed reactions. The cyclisations reported herein allow for an efficient, regio- and stereoselective one-pot synthesis of biologically relevant ring systems.

Journal Article↗
PubMed2000

Versatile Indole Synthesis by a 5-endo-dig Cyclization Mediated by Potassium or Cesium Bases We thank the DFG (Leibniz-Programm) and the Fonds der Chemischen Industrie for generous financial support. A.L.R. and C. K. thank the Humboldt-Foundation and the BASF AG, respectively, for fellowships. W.D. was supported by the BMBF program (03 D 0056 2). We thank Dr. J. Henkelmann (BASF AG) for helpful discussions and the BASF AG for the generous gift of chemicals.

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Journal Article↗