Stereospecificity in the biosynthesis of papaverine

The stereochemistry of hydrogen removal from C-3 and C-4 in the aromatization of the heterocyclic ring of papaverine has been determined by incorporation experiments with stereospecifically tritiated norreticulines.     

Stereochemistry of the reactions catalyzed by chicken liver fatty acid synthase

The stereochemistry of the four partial reactions catalyzed by chicken liver fatty acid synthase that lead to the synthesis of palmitic acid has been determined. The reduction of acetoacetyl-CoA to 3-hydroxybutyryl-CoA by NADPH proceeds with the transfer of the pro-4S hydrogen of NADPH to form D-3-hydroxybutyryl-CoA. During the subsequent dehydration of D-3-hydroxybutyryl-CoA the pro-2S hydrogen and the 3-hydroxyl group are removed in a syn elimination to form crotonyl-CoA. Crotonyl-CoA is reduced to butyryl-CoA by NADPH, with the transfer of the pro-4R hydrogen of NADPH to the pro-3R position in butyryl-CoA and the transfer of a solvent hydrogen to the pro-2S position. The occurrence of the syn dehydration, when combined with the results of a previous study [ Sedgwick , B., & Cornforth , J. W. (1977) Eur. J. Biochem. 75, 465-479], implies that the condensation of the enzyme-bound malonyl moiety with the enzyme-bound saturated fatty acid to form a 3-keto intermediate proceeds with inversion at C-2 of the malonyl. The stereochemistry of the hydration was derived from an analysis of the spin-spin coupling constant of 3-hydroxy[2-2H]butyric acid benzylamides obtained from 3-hydroxy[2-2H]butyryl-CoA synthesized by fatty acid synthase. The elucidation of the stereochemistry of the reduction of crotonyl-CoA relied on the previously established stereochemistry of pork liver acyl-CoA dehydrogenase. The source of all 28 prochiral hydrogens of the palmitic acid synthesized by chicken liver fatty acid synthase was inferred from the results of this work.     

3(2H)-Benzofuranones and chromanes from liquid cultures of the mycoparasitic fungus Coniothyrium minitans

Two 3(2H)-benzofuranones and three chromanes were isolated from the mycoparasitic fungus Coniothyrium minitans. Their structures and absolute stereochemistry were determined by spectroscopic methods.     

Stereochemistry of Cervicarcin

The relative stereochemistry of cervicarcin, an antitumor antibiotic, was determined as shown in 1, which represents the absolute stereochemistry also.     

Absolute configuration of a polyol fragment of blasticidin A, a specific inhibitor of aflatoxin production

Blasticidin A (1) and aflastatin A (2), Streptomyces metabolites with similar structures, are specific inhibitors of aflatoxin production by Aspergillus parasiticus. The stereochemistry of the polyol fragment of 1 (3a) containing ten chiral centers was elucidated by applying acetonide and MTPA methods to a variety of acetonide derivatives of 3a, which determined the absolute configuration of 3a. By using the similar methods, the absolute configuration of the polyol fragment of 2 (4a) was determined, which was the same as that elucidated by J-based and other chemical methods previously.     

Stereochemistry of spathulin

The stereochemistry of the pseudoguaianolide spathulin has been determined by NMR spectrometry at 270 mHz.     

Some Properties of a New Flavonoid,Tithymalin, Isolated from the Herbs of Euphorbia Helioscopia Linnaeous

The stereochemistry of fukiic acid (I) was elucidated. The relative configuration was determined to be erythro by IR spectrum and the evidence of acetonide formation. The absolute configuration of fukiic acid was assigned as S-configuration at C–2 and R-configuration at C–3, by Horeau’s method and the identification of ozonolysis product to be hibiscus acid (10 a).     

Stereochemistry of phosphoenolpyruvate carboxylation catalyzed by phosphoenolpyruvate carboxykinase

The stereochemistry of the carboxylation of phosphoenolpyruvate to yield oxalacetate, catalyzed by chicken liver phosphoenolpyruvate carboxykinase and by Ascaris muscle phosphoenolpyruvate carboxykinase, was determined. The substrate (Z)-3-fluorophosphoenolpyruvate was used for the stereochemical analysis. The carboxylation reaction was coupled to malate dehydrogenase to yield 3-fluoromalate, and the stereochemistry of the products was identified by 19F NMR. In separate experiments, the enantiomeric tautomers of 3-fluorooxalacetate were shown to be utilized by malate dehydrogenase to yield (2R,3R)- and (2R,3S)-3-fluoromalate in nearly identical amounts. The products were identified by 19F NMR. When (Z)-3-fluorophosphoenolpyruvate was used as a substrate for phosphoenolpyruvate carboxykinase from avian liver and from Ascaris, and malate dehydrogenase was used to trap the product, only a single diastereomer was observed. This product was shown to be (2R,3R)-3-fluoromalate in each case. The assignments were based on coupling constants taken from Keck et al. [Keck, R., Hess, H., & Rétey, J. (1980) FEBS Lett. 114, 287]. These results indicate that the stereochemistry of carboxylation, catalyzed by chicken phosphoenolpyruvate carboxykinase and by Ascaris phosphoenolpyruvate carboxykinase, is identical and takes place from the si side of the enzyme-bound phosphoenolpyruvate. The carboxylation reaction was run both in H2O and in D2O. No deuterium incorporation into fluoromalate was shown to occur. The product 3-fluorooxalacetate is thus released from phosphoenolpyruvate carboxykinase as the keto form and is reduced more rapidly by reduced nicotinamide adenine dinucleotide with malate dehydrogenase than by the occurrence of tautomerization.     

Stereochemical aspects of the hydration of trans-anethole epoxide in the rat

Racemic trans-anethole epoxide [1-(4′-methoxyphenyl)-propane-1,2-oxide] was incubated with water, buffers, and rat liver microsomes and cytosol and the stereochemistry of the diols produced was determined by HPLC as their dicamphanyl esters. The diol metabolites were isolated by HPLC from the urine of rats administered [1′-¹⁴C] trans-anethole and their stereochemistry determined after derivatization to their camphanyl esters. The stereochemical course of the metabolism of trans-anethole by rat liver microsomes and cytosol is discussed. © 1995 Wiley-Liss, Inc.     

24α-Methyl-5α-cholest-7-en-3β-ol from seed oil of Helianthus annuus

24-Methyl-5α-cholest-7-en-3β-ol (24-methyllathosterol) isolated from the seed oil of Helianthus annuus was shown to have 24α-configuration by ¹H NMR spectroscopy. The stereochemistry at C-24 of some other 24-alkylsterols isolated from this plant material also was determined.     

Determination of the absolute stereochemistry of the epoxide group in alpinia epoxide by NMR

The absolute stereochemistry of the epoxide group in alpinia epoxide [14,15-epoxylabda-8(17),12-dien-16-al (E)] has been determined by simultaneous reduction of the aldehyde and epoxide functional groups in this molecule to primary and secondary alcohols, followed by selective protection of the primary alcohol and derivitization of the secondary alcohol with S(+) and R(−) MTPCl as Mosher esters. Changes in ¹H NMR chemical shifts for all positions in these two esters were determined by 2D-NMR and used to infer the absolute stereochemistry of the epoxide group in the natural product alpinia epoxide.     

Enantioselective preparation of (R) and (S)-3-hydroxycyclopentanone by kinetic resolution

A straightforward approach to enantiomerically enriched (R) and (S)-3-hydroxycyclopentanone is described. The key step involves a kinetic resolution of racemic 3-hydroxycyclopentanone using commercial Pseudomonas cepacia lipase immobilized on diatomite (Amano lipase PS-DI). The absolute stereochemistry of the product was determined by derivatization into (R)-3-(benzyloxy)cyclopentanone.     

Synthesis of Two 3-(7′-Theophyllyl)glycals, 3-Deoxy-3-(7′-theophyllyl)-d-xylo-hex-1-enopyranose and Methyl 3-Deoxy-3-(7′-theophyllyl)-d-xylo-hex-1-enopyranuronate,and their Derivatives

Two 3-(7′-theophyllyl)glycals, (IV) and (V), were synthesized by fusion of theophylline and the appropriate glycals in the presence of p-toluenesulfonic acid. The structure and stereochemistry of the glycals were determined mainly from NMR analysis of their dihydro and 1,6-anhydro derivatives.     

Guaiane sesquiterpenes from Amoora rohituka

The petroleum ether extract of the stem bark of Amoora rohituka afforded two novel guaiane-derived sesquiterpenoids, 6beta,7beta-epoxyguai-4-en-3-one (1) and 6beta,7beta-epoxy-4beta,5-dihydroxyguaiane (2). The structures of 1 and 2 were determined by extensive NMR and MS analyses and by comparison of their spectral data with related compounds. The relative stereochemistry of the asymmetric centers in 1 and 2, except at C-5 of 2, were determined by selective 1D-NOESY experiments.     

Isolation and identification of dihydrochrysanolide and its 1-epimer from Chrysanthemum coronarium L

A new sesquiterpene lactone (1) was isolated with known dihydrochrysanolide derivatives (2 and 3) from the flowers of Chrysanthemum coronarium L., and their structures were identified by spectroscopic data. The stereochemistry of the epimers (1 and 2) was determined from NOESY data and an X-ray crystallographic analysis. The isolated compounds (1-3) were examined for their cytotoxic activity against such human cell lines as A549, PC-3 and HCT-15.     

Structural elucidation and antigenicity of a novel phenolic glycolipid antigen from Mycobacterium haemophilum

The structure of a novel antigenic glycolipid that distinguishes the opportunistic pathogen Mycobacterium haemophilum from all other mycobacteria was established by a series of degradation reactions leading to products that were analyzed by gas/liquid chromatography-mass spectrometry. The complete structure of the oligosaccharide unit was determined as 2,3-di-O-CH3-alpha-L-Rhap(1----2)3-O-CH3-alpha-L-Rhap(1----4 )-2,3-di-O-CH3-alpha-L-Rhap(1----. The lipid portion of the phenolic glycolipid was composed of two component phenolphthiocerols differing by two methylene groups, as determined by analysis of their per-O-trideuteriomethylated derivatives. The diol unit of the phenolphthiocerols has a threo relative configuration. The absolute stereochemistry of the asymmetric centers of the phenolphthiocerols is uncertain, but the centers are probably 3R, 4S, 9R, and 11R as found for phthiocerol A from Mycobacterium tuberculosis. The hydroxyl functions of the branched glycolic chain are esterified to a complex mixture of multi-methyl branched mycocerosic acids, C27, C30, C32, C34, and C37 with molecular weights (as methyl esters) of 424, 466, 494, 522, and 564, respectively. The stereochemistry of the methyl branches of the mycocerosates have R absolute configuration. The glycolipid is highly antigenic and appears to be specific for M. haemophilum. There are intriguing similarities between the product from M. haemophilum and the well-known phenolic glycolipid I of Mycobacterium leprae, a matter that is discussed.     

Natural dolapyrrolidone: Isolation and absolute stereochemistry of a substructure of bioactive peptides

During the course of our chemical analysis of the hydrophilic fractions from marine cyanobacterium Moorena producens, we have isolated natural dolapyrrolidone (Dpy, 1 ), a natural pyrrolidone derived from phenylalanine, for the first time as a single compound. Compound 1 , with an (S)-l absolute stereochemistry, was previously identified as a substructure that is common among several bioactive natural peptides. Surprisingly, the absolute stereochemistry of the isolated natural 1 , determined through total synthesis, was (R)-d . This result was unambiguously determined by HPLC analysis using a chiral stationary column by comparing the retention times of the natural 1 and authentic samples of synthetic enantiomers. To verify the unexpected result, the absolute stereochemistry of the natural 1 was confirmed by X-ray crystallographic analysis of Pt-complex derivative using the synthetic enantiomer.     

(1′R,2′S,5′R)-Menthyl-(5R)-acetoxy-1,3-oxathiolan-(2R)-carboxylate: Synthesis and isomeric purity determination by HPLC

Chemoselective reduction of one isomer of the 1-menthylester of 1,3-oxathiolan-5-one-2-carboxylic acid produces a mixture of four lactol diastereomers from which the title compound was isolated after acylation. The isomeric purity and absolute stereochemistry were determined by spectroscopic methods, chiral HPLC techniques, and conversion to (?)-2′-deoxy-3′-thiacytidine (Lamivudine, 3TC^TM). © 1994 Wiley-Liss, Inc.     

A proton magnetic resonance investigation of the glycosyl torsion angle of uracil nucleosides and nucleotides.

The use of line-shape decomposition techniques permitted the small 5-bond (5-J51') and 4-bond (4-J61') proton-proton coupling constants of a series of uracil nucleosides and nucleotides to be determined accurately. From an analysis of these coupling constants we have determined that the uracil base is in a predominantly anti conformation in aqueous solution and the mean position is not substantially altered by phosphate substitution at the 2', 3', or 5' positions, by changing the furanose stereochemistry from a ribose to a deoxyribose or an arabinose, or by an increase in temperature of 43 degree C.     

Stereochemistry of the hydrogen transfer to NAD catalyzed by (S)alanine dehydrogenase from Bacillus subtilis.

The stereochemistry of the hydrogen transfer to NAD catalyzed by (S)alanine dehydrogenase [ (S)alanine: NAD oxidoreductase (EC 1.4.1.1) ] from B. subtilis was investigated. The label at C-2 of (S) [2,3--3H] alanine was enzymatically transferred to NAD, and the [4--3H]NADH produced isolated and the stereochemistry at C-4 investigated. It was found that the label was exclusively located at the (R) position which indicates that (S)alanine dehydrogenase is an A-type enzyme. This result was confirmed in an alternate way by reducing enzymatically [4--3H]NAD with non labeled (S)alanine and (S)alanine dehydrogenase and investigating the stereochemistry of the ]4--3H]NADH produced. As expected, the label was now exclusively located at the (S) position. This proves that (S)alanine dehydrogenase isolated from B. subtilis should be classified as an A-enzyme with regard to the stereochemistry of the hydrogen transfer to NAD.