Transition metal complexes IX. Nickel complexes with a chiral azaphosphole ligand

The reaction of (−)-(R)-myrtenal and (+)-(R)-phenylethylamine gave a Schiff base 1 which was reacted with MePBr₂ in the presence of a base to give under dehydrohalogenation of an intermediate McCormack product a salt 2. Treatment of 2 with sodium led to the formation of the azaphosphole 4. η³-C₃H₅NiCl and 4 gave a 1:1 adduct 5 and nickel(0) gave a 1:4 complex 6. Compounds 4–6 were characterized by NMR spectroscopy as well as by single crystal X-ray structure determination.     

Effects of osmotic pretreatments on oxidative stress, antioxidant profiles and cryopreservation of olive somatic embryos

A three-day pretreatment of olive somatic embryos (SE) with 0.75 M sucrose, combined with cryoprotection (0.5 M DMSO, 1 M sucrose, 0.5 M glycerol and 0.009 M proline) and controlled rate cooling, supported regrowth (as 34.6% fresh weight gain) and resumption of embryo development after cryopreservation. Pretreatment with mannitol or sorbitol did not support regrowth. Profiles of sugars, proline, antioxidant enzymes, Reactive oxygen species (ROS), secondary oxidation products and ethylene were constructed for the most successful (0.75 M) pretreatment series. Sucrose was the optimal pretreatment for supporting recovery, it also elevated glutathione reductase (GR) activity compared to controls, whereas superoxide dismutase (SOD), catalase and guaiacol peroxidase activities remained relatively unchanged. Superoxide dismutase activity was higher in SE pretreated with sucrose, compared with those pretreated with polyols; H₂O₂ was enhanced in SE pretreated with sorbitol and sucrose compared to mannitol. The overall trend for ethylene and OH production revealed their levels were highest in SE pretreated with polyols albeit, for individual treatments this was not always the case. Generally, pretreatments did not significantly change embryo secondary oxidation profiles of ThioBarbituric Acid Reactive Substances (TBARS) and Schiff's bases. In combination these studies suggest oxidative processes may influence regrowth of cryopreserved olive SE and that optimal pretreatments could, in part, increase tolerance by an overall enhancement of endogenous antioxidants (particularly GR), proline and sugars.     

Tuning one dimensional chiral channel interior in mixed ligand Cu(II) complexes of l-histidine derivative and substituted pyridine

Four pentacoordinated square-pyramidal Cu(II) complexes with the general formula [Cu(L)(X)], where L is a l-histidine derived tetradentate ligand and X is either 3-hydroxypyridine or 2-methylpyridine, has been synthesized. Structural analysis showed that the presence of water filled one dimensional chiral channel in the lattices. The interiors of the channels were varied using aromatic ring substitution on the ligand as well as on the monodentate ligand. The dimensions of the channels range from ∼7 to 9 Å.     

Synthesis of novel benzimidazole functionalized chiral thioureas and evaluation of their antibacterial and anticancer activities

A small library of benzimidazole functionalized chiral thioureas was prepared starting from natural amino acids (S)-alanine, (S)-phenylalanine, (S)-valine and (S)-leucine and also their (R)-isomers and studied their antimicrobial activity against a various Gram-positive and Gram-negative bacterial strains. In this study, compounds 5g and 5j were found to exhibit good antibacterial activity against both Gram-positive and Gram-negative bacterial strains such as Staphylococcus aureus, Bacillus subtilis, Micrococcus luteus, Klebsiella planticola, Escherichia coli and Pseudomonas aeruginosa. In the cytotoxicity study, thioureas derived from non-natural amino acids 5a–l showed good activity against human cancer cell lines A549, MCF7, DU145, HeLa, and no cytotoxicity was observed with their antipodes 6a–l.     

Spiroleptosphol isolated from Leptosphaeria doliolum

A novel γ-methylidene-spirobutanolide spirolephtoshol (1) was isolated from ascomycetous fungus Leptosphaeria doliolum as a cytotoxic compound. The relative structure was established by the NMR analysis involving the NOE experiments. Absolute structure of the bicyclic moiety was determined by chemical derivation followed by the CD analysis. The relative and absolute stereochemistry of the side chain was established by comparison of the ¹H NMR spectra and the chiral GC chromatograms of the degradation product with the synthetic samples.     

Stereoselectivity of the demethylation of nicotine piperidine homologues by Nicotiana plumbaginifolia cell suspension cultures

The metabolism of (R,S)-N-methylanabasine and (R,S)-N-methylanatabine has been studied in a cell suspension culture of Nicotiana plumbaginifolia. Both substrates are effectively demethylated, anabasine or anatabine, respectively, accumulating in the medium. Similarly, there is strong stereoselectivity for the (R)-isomers of both substrates. The kinetics of metabolism of (R,S)-N-methylanabasine differ significantly from those of nicotine in that no further degradation of the initial demethylation product occurs. (R,S)-N-Methylanatabine, however, shows kinetics closer to those of nicotine, with loss of alkaloid from the system. Further more, (R,S)-N-methylanabasine does not diminish (S)-nicotine demethylation, indicating a lack of competition. However, the metabolism of (S)-nicotine is affected by the presence of (R,S)-N-methylanabasine. Hence, the demethylation of the piperidine homologues of nicotine is seen to be similar but not identical to that of the pyridine analogues. The implications of these different metabolic profiles in relation to the demethylation activity are discussed.     

Achiral-chiral LC/LC-MS/MS coupling for determination of chiral discrimination effects in phenprocoumon metabolism

Many physiological processes show a high degree of stereoselectivity, including the metabolism of xenobiotics as catalyzed by cytochrome P450 enzymes. An analysis of these chiral discrimination effects in drug metabolism is essential for an in-depth understanding of metabolic pathways that differ between enantiomers of a given chiral drug or metabolite thereof. Achiral chromatographic separation and structural identification followed by chiral analysis of metabolites from blood specimens usually requires a time-consuming multistage analytical technique. In an effort to optimize such a complicated analytical scheme, a novel two-dimensional online achiral-chiral liquid chromatography-tandem mass spectrometry (LC/LC-MS/MS) coupling method was developed by using a peak parking technique in combination with a makeup flow system. Metabolites were separated in the first dimension using a C18 reversed-phase system. A makeup eluent of water/methanol (95/5) was split into the flow before storing the metabolites separately on chiral cartridges. Subsequently, the metabolite enantiomers were eluted backward onto the analytical chiral column and separated, and the ratio of enantiomers was determined. The method was successfully validated with respect to limit of detection, linearity, intra- and interday accuracy, and precision. In the course of a human volunteer study investigating the influence of CYP (cytochrome) 2C9 genetic polymorphism on phenprocoumon (PPC) metabolism, we used this new two-dimensional online analytical technique for the analysis of PPC metabolites in plasma. The enantiomeric forms of 4'-, 6-, and 7-hydroxy-PPC metabolites as well as two novel metabolites were identified, and the ratio of the enantiomers was calculated. We found that the enantiomeric ratio for the different metabolites in the plasma sample of each measured individual differs markedly from a nearly 100% chiral discrimination for the two new putative metabolites. This new analytical coupling method possesses general utility in the analysis of chiral discrimination effects, particularly as it relates to pharmacokinetics and dynamics, a scientific field that is rapidly becoming an area of concern and interest.     

Engineering the phenylacetaldehyde reductase mutant for improved substrate conversion in the presence of concentrated 2-propanol

Phenylacetaldehyde reductase (PAR) from Rhodococcus sp. ST-10 is useful for chiral alcohol production because of its broad substrate specificity and high stereoselectivity. The conversion of ketones into alcohols by PAR requires the coenzyme NADH. PAR can regenerate NADH by oxidizing additional alcohols, especially 2-propanol. However, substrate conversion by wild-type PAR is suppressed in concentrated 2-propanol. Previously, we developed the Sar268 mutant of PAR, which can convert several substrates in the presence of concentrated 2-propanol. In this paper, further mutational engineering of Sar268 was performed to achieve higher process yield. Each of nine amino acid positions that had been examined for generating Sar268 was subjected to saturation mutagenesis. Two novel substitutions at the 42nd amino acid position increased m-chlorophenacyl chloride (m-CPC) conversion. Moreover, several nucleotide substitutions identified from libraries of random mutations around the start codon also improved the PAR activity. E. coli cells harboring plasmid pHAR1, which has the integrated sequence of the top clones from the above selections, provided greater conversion of m-CPC and ethyl 4-chloro-3-oxobutanoate than the Sar268 mutant, with very high optical purity of products. This mutant is a promising novel biocatalyst for efficient chiral alcohol production.     

Asymmetric hydrogenation of dehydrodipeptide esters bearing different protective groups

Summary N-[(Z)-N-Benzoyl- orN-Boc-(2-fluorophenyl)dehydroalanyl]-(R)-or (S)-phenylalanine esters were synthesized and hydrogenated to give the corresponding dipeptide derivatives with optical yields in the range of 53–87%de using the cationic rhodium complexes of PROPRAPHOS and BPPM. The efficiency of chiral diphosphine ligands as well the effect of the chiral center in the substrate on the catalytic asymmetric induction was studied.Dedicated to Professor Günther Oehme on the occasion of his 60th birthday     

Synthesis and use of <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-di-Boc-glutamate γ-semialdehydes and related aldehydes

Summary.  This review article focuses on the synthesis and reactions of N,N-di-Boc glutamate and aspartate semialdehydes as well as related aldehydes. These building blocks are prepared according to various strategies from glutamic and aspartic acids and find interesting synthetic applications. In the first part, the methods for the synthesis of N,N-di-Boc-amino aldehydes are summarized. The applications of these chiral synthons for the synthesis of unnatural amino acids and other bioactive compounds are discussed in the second section. Received April 24, 2002 Accepted August 13, 2002 Published online January 30, 2003 Authors' address: Prof. Violetta Constantinou-Kokotou, Chemical Laboratories, Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece, E-mail: vikon@aua.gr Abbreviations: AcNH-TEMPO, 4-acetamido-2,2,6,6-tetramethyl-1-piperidinyloxy free radical; AIBN, 2,2′-azobis(2-methylpropionitrile); Aliquat, methyltrioctylammonium chloride; Bn, benzyl; Boc, tert-butoxycarbonyl; Bu^t, tert-butyl; m-CPBA, 3-chloroperoxybenzoic acid; DAST, diethylaminosulfur trifluoride; DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene; (R,R)-(+)-DET, (R,R)-(+)-diethyltartrate; DIBALH, diisobutyl aluminium hydride; DMAP, 4-dimethylaminopyridine; DMF, dimethylformamide; Et₃N, triethylamine; KHMDS, potassium bis(trimethylsilyl)amide; (S)-LLB, lanthanium-lithium-bis-metallic binaphthol catalyst; MsCl, methanesulfonyl chloride; NEM, N-ethylmorpholine; NMO, 4-methylmorpholine N-oxide; PPA, propylphosphonic acid anhydride; TBHP, tert-butyl hydroperoxide; TFA, trifluoroacetic acid; THF, tetrahydrofuran; TMSI, 1-(trimethylsilyl)imidazole; Trt, trityl.