Zur Stereochemie der Carbonsäuredianion-Aldehyd-Addition unter kinetisch und thermodynamisch kontrollierten Bedingungen — Reindarstellung und Konfigurationszuordnung von 2,3-disubstituiertenthreo- underythro-3-Hydroxycarbonsäuren

Diastereoselective Reduction of Selected α-substituted β-keto Esters and the Assignment of the Relative Configuration by 1H-NMR Spectroscopy

Background::

Chiral β-hydroxy esters and α-substituted β-hydroxy esters represent versatile building blocks for pheromones, β-lactam antibiotics and 1,2- or 1,3-aminoalcohols.

Objective::

Synthesis of versatile α-substituted β-keto esters and their diastereoselective reduction to the corresponding syn- or anti-α-substituted β-hydroxy esters. Assignment of the relative configuration by NMR-spectroscopy after a CURTIUS rearrangement of α-substituted β-keto esters to 4-substituted 5-methyloxazolidin-2-ones.

Method::

Diastereoselective reduction was achieved by using different LEWIS acids (zinc, titanium and cerium) in combination with complex borohydrides as reducing agents. Assignment of the relative configuration was verified by 1H-NMR spectroscopy after CURTIUS-rearrangement of α-substituted β-hydroxy esters to 4-substituted 5-methyloxazolidin-2-ones.

Results::

For the syn-selective reduction, titanium tetrachloride (TiCl4) in combination with a pyridine-borane complex (py BH3) led to diastereoselectivities up to 99% dr. High anti-selective reduction was achieved by using cerium trichloride (CeCl3) and steric hindered reducing agents such as lithium triethylborohydride (LiEt3BH). After CURTIUS-rearrangement of each α-substituted β-hydroxy ester to the corresponding 4-substituted 5-methyloxazolidin-2-one, the relative configuration was confirmed by 1H NMR-spectroscopy.

Conclusion::

We have expanded the procedure of LEWIS acid-mediated diastereoselective reduction to bulky α-substituents such as the isopropyl group and the electron withdrawing phenyl ring.

γ-Silylated α,β-unsaturated amides — Preparation by [1,5]-sigmatropic rearrangement and use as masked dienolate equivalents in carbonyl condensations

The reaction of lithium dienolates derived from N,N-dialkylsenecioamides (1a–1c) with triorganosilyl electrophiles occurs initially at the oxygen atom predominantly, and is followed by an O → C silicon migration to afford the γ-silylated senecioamides (4a–4h). The γ-silylated senecioamide Z-4a undergoes fluoride-ion-mediated condensations with aromatic aldehydes to give kinetic α-(6) and thermodynamic γ-(5) condensation product patterns comparable to lithium dienolates. The TiCl4-mediated reactions with aldehydes gives α-products (6) in a highly syn-selective manner. Possible transition-state models for the syn-selective condensations are discussed and a chair-like transition state featuring bidentate coordination to titanium (11) is proposed.

Practical, catalytic, asymmetric synthesis of beta-lactones via a sequential ketene dimerization/hydrogenation process: inhibitors of the thioesterase domain of fatty acid synthase

The recent finding that the FDA-approved antiobesity agent orlistat (tetrahydrolipstatin, Xenical) is a potent inhibitor of the thioesterase domain of fatty acid synthase (FAS) led us to develop a concise and practical asymmetric route to pseudosymmetric 3,4-dialkyl-cis-beta-lactones. The well-documented up-regulation of FAS in cancer cells makes this enzyme complex an interesting therapeutic target for cancer. The described route to 3,4-dialkyl-beta-lactones is based on a two-step process involving Calter's catalytic, asymmetric ketene dimerization of acid chlorides followed by a facial-selective hydrogenation leading to cis-substituted-beta-lactones. Importantly, the ketene dimer intermediates were found to be stable to flash chromatography, enabling opportunities for subsequent transformations of these optically active, reactive intermediates. Subsequent alpha-epimerization and alpha-alkylation or acylation led to trans-beta-lactones and beta-lactones bearing alpha-quaternary carbons, respectively. Several of the ketene dimers and beta-lactones displayed antagonistic activity (apparent Ki in the low micromolar range) in competition with a fluorogenic substrate toward a recombinant form of the thioesterase domain of fatty acid synthase. The best antagonist, a simple phenyl-substituted cis-beta-lactone 3d, displayed an apparent Ki (2.5 +/- 0.5 microM) of only approximately 10-fold lower than that of orlistat (0.28 +/- 0.06 microM). In addition, mechanistic studies of the ketene dimerization process by ReactionView infrared spectroscopy support previous findings that ketene formation is rate determining.

Synthesis and fungicidal activity of 1-(alpha-tert-butylcinnamoyl)imidazoles

Several 1-(alpha-tert-butylcinnamoyl)imidazoles were prepared to examine their fungicidal activity. The (Z)-4-chlorocinnamoyl derivative was prepared from (anti)-2-tert-butyl-3-(4-chlorophenyl)-3-hydroxypropanoic acid by treating with 1,1'-carbonyldiimidazole and a subsequent beta-elimination reaction at an elevated temperature. The (Z)-isomer of the 4-chlorocinnamoyl derivative showed good fungicidal activity against Erysiphe graminis and Botrytis cinerea in pot tests, whereas the corresponding (E)-isomer derived from the (Z)-isomer through photoisomerization was much less active.

Reversed Stereochemical Control in the Presence of CeCl(3) and TiCl(4) in the Lewis Acid Mediated Reduction of alpha-Alkyl-beta-keto Esters by Metal Hydrides. A General Methodology for the Diastereoselective Synthesis of syn- and anti-alpha-Alkyl-beta-hydroxy Esters

The Lewis acid-mediated reduction of alpha-alkyl-beta-keto esters has been shown to proceed by different stereochemical control depending on the nature of the metal atom. Strongly chelating TiCl(4) led to the syn isomer in high diastereomeric excess in noncoordinating solvents (CH(2)Cl(2)) at -78 degrees C with BH(3).py as reducing agent, while nonchelating CeCl(3) gave a high excess of the anti isomer in coordinating solvents (THF) at the same temperature with lithium triethylborohydride (LiEt(3)BH) as reducing agent. The methodology has been successfully utilized for obtaining important syn- and anti-alpha-alkyl-beta-hydroxy esters with high diastereoselectivity.

Rh(II)-Catalyzed Reaction Of Some alpha',alpha'- and beta'-Branched-O-Alkyl alpha-(Alkoxycarbonyl)-alpha-diazoacetates

alpha',alpha'- and beta'- Branched-O-alkyl alpha-(alkoxycarbonyl)-alpha-diazoacetates 1a,b and 2a,b were prepared. The regio- and chemoselectivities in the Rh(II)-catalyzed reaction of these compounds were studied. The model systems 1a,b showed that the reactivity of the ester-substituted Rh(II) carbenoid is responsive to the nature of the Rh(II) catalyst; rhodium(II) acetamide favored insertion into the tertiary C-H bond and rhodium(II) perfluorobutyrate favored insertion into the secondary C-H bond. We also found that the nature of the O-alkyl group in the ester moiety had no influence on the reactivity of the Rh(II) carbenoid. In more complex systems such as compounds 2, it was found that rhodium(II) acetate promoted insertion into tertiary C-H bonds. Intramolecular cyclopropanation, to give 3-oxabicyclo[5.1.0]octane and 3-oxabicyclo[6.1.0]nonane products, was found to be competitive with tertiary C-H insertion; it is favored when rhodium(II) perfluorobutyrate was used as the catalyst. The Rh(II)-catalyzed reaction of 3 ("carbon" analog of 2a) resulted mainly in the cyclopentanone derivative, and no cyclopropanation product was detected. This result indicates that the ester oxygen in compounds of type 2 plays an important role in influencing the regio- and chemoselectivities of their reaction. Overall the results indicate that for compounds of a particular type of structure the regio- and chemoselectivities can be controlled via the judicious choice of the Rh(II) catalyst.

Ability of the phototrophic bacterium Rhodospirillum rubrum to produce various poly (beta-hydroxyalkanoates): potential sources for biodegradable polyesters

Studies have been carried out in order to optimize growth and culture conditions for the intracellular formation of poly(beta-hydroxyalkanoates) (PHA) in the phototrophic, purple, non-sulphur bacterium Rhodospirilum rubrum. Its potential to produce novel copolymers was investigated. Recently, it has become of industrial interest to evaluate these polyesters as potentially biodegradable plastics for a wide range of possible applications. On an industrial scale, the use of photosynthetic bacteria could harness sunlight as an energy source for the production of these materials. R. rubrum was grown anaerobically in the light on different linear and branched beta-hydroxycarboxylic acids and various n-alkanoic acids. Under nitrogen-limiting conditions a PHA content of up to 45% of cellular dry weight was detected. When R. rubrum was grown on different concentrations of various n-alkanoic acids, intracellular PHA production was detected on all acids used. In most of the cases, the storage polymer contained beta-hydroxybutyrate (HB) and beta-hydroxyvalerate (HV) monomer units. Grown on n-alkanoic acids with a chain length of four carbon atoms and more, R. rubrum produced a copolymer containing the beta-hydroxyhexanoate (HC) repeating unit in addition to the HB and HV monomer. Using beta-hydroxyheptanoic acid as the carbon source, a polyester which contained HB, HV, HC, and beta-hydroxyheptanoate was formed. These copolyesters represent a novel class of biodegradable thermoplastics. The results demonstrate the metabolic flexibility of R. rubrum to form many different types of polyesters which might substitute plastics synthesized from petrochemicals.

Pseudomonas oleovorans as a Source of Poly(β-Hydroxyalkanoates) for Potential Applications as Biodegradable Polyesters

Pseudomonas oleovorans was grown in homogeneous media containing n -alkanoic acids, from formate to decanoate, as the sole carbon sources. Formation of intracellular poly(β-hydroxyalkanoates) was observed only for hexanoate and the higher n -alkanoic acids. The maximum isolated polymer yields were approximately 30% of the cellular dry weight with growth on either octanoate or nonanoate. In most cases, the major repeating unit in the polymer had the same chain length as the n -alkanoic acid used for growth, but units with two carbon atoms less or more than the acid used as a carbon source were also generally present in the polyesters formed. Indeed, copolymers containing as many as six different types of β-hydroxyalkanoate units were formed. The weight average molecular weights of the poly(β-hydroxyalkanoate) copolymers produced by P. oleovorans ranged from 90,000 to 370,000. In spite of the higher cell yields obtained with octanoate and nonanoate, the use of hexanoate and heptanoate yielded higher-molecular-weight polymers. These copolyesters represent an entirely new class of biodegradable thermoplastics.