Epimeric Mixture Research Articles

Guaiane dimers from Xylopia vielana

From the leaves of Xylopia vielana (Annonaceae) two dimeric guaianes named vielanins D and E were isolated and structurally elucidated by mass and NMR spectroscopy. Vielanin D and E consist of bridged ring systems formally representing the Diels–Alder products from the hypothetical guaiane-type monomers. Due to a hemiketal function at C-8′ both compounds occurred as epimeric mixtures.

Isolation and structure determination of lyngbyastatin 3, a lyngbyastatin 1 homologue from the marine cyanobacterium Lyngbya majuscula. Determination of the configuration of the 4-amino-2,2-dimethyl-3-oxopentanoic acid unit in majusculamide C, dolastatin 12, lyngbyastatin 1, and lyngbyastatin 3 from cyanobacteria.

The structure of lyngbyastatin 3 (1), including the configurations of the two unusual amino acid residues, viz., the 3-amino-2-methylhexanoic acid (Amha) and 4-amino-2,2-dimethyl-3-oxopentanoic acid units (Ibu), has been established by chemical degradation. Analysis of the cyanobacterial samples of lyngbyastatin 3 (1), lyngbyastatin 1 (2), and dolastatin 12 (3) demonstrated that they are mixtures of Ibu epimers [R (major) and S (minor)], whereas the structurally related majusculamide C (4) is a single diastereomer having an S-Ibu unit.

Epimer interconversion, isomerization, and hydrolysis of tetrahydrouridine: Implications for cytidine deaminase inhibition

Tetrahydrouridine (THU) is an inhibitor of cytidine deaminase (CDA), the enzyme responsible for the deactivation of ara‐C and other cytidine analogues in vivo, and therefore is capable of improving the therapeutic efficacy of these antitumor agents. In aqueous solution formulations, THU exists as a mixture of epimers differing in stereochemistry of the 4‐OH substituent. The aims of this study were to investigate the interconversion kinetics of the epimers of THU, the CDA inhibitory effects of these epimers, and the stability and degradation mechanisms of THU epimer mixtures in aqueous solution with the ultimate goal of developing optimal conditions for a parenteral formulation of THU. A stability indicating HPLC assay utilizing a derivatized β‐cyclodextrin column was developed to separate the two epimers of THU and to monitor their reversible isomerization to their β‐ribopyranosyl counterparts and their hydrolysis to form N‐glycosidic bond cleavage products. MS and one‐ and two‐dimensional 1H‐ and 13C‐NMR measurements were conducted to identify THU epimers and degradation products and to quantitatively model the degradation kinetics. The interconversion reaction between the two THU epimers is acid catalyzed with a first‐order rate constant for conversion of epimer 11 to epimer 12 of (7.4 ± 0.3) × 10−3 h−1 and an equilibrium constant ([12]/[11] of 1.7 ± 0.1 at pH 7.4 and 25°C. Epimer interconversion was therefore sufficiently slow at pH 7.4 to allow the isolation of each and evaluation of their CDA inhibitory activities utilizing 1% (w/v) mouse kidney homogenates as a source for cytidine deaminase and cytidine as a substrate. Inhibition constants for the two THU epimers (11 and 12) were determined to be 8 ± 1 × 10−7 M and 6.2 ± 0.2 × 10−8 M, respectively. Studies at elevated temperature suggested that THU degradation from epimer mixtures is biphasic with the initial rate of disappearance being acid catalyzed and first order in initial THU concentration, thus ruling out dimerization as a potential reaction mechanism. NMR/MS analyses revealed that the major degradation products included the β‐ribopyranosyl THU isomers (two epimers), the reduced pyrimidinone base (tetrahydrouracil), and various anomers of D‐ribose formed through N‐glycosidic bond cleavage, and the products of subsequent reactions of the base. Kinetic modeling of the data obtained from both HPLC and NMR measurements indicated that in an acidic solution THU β‐ribofuranosyl → β‐ribopyranosyl isomerization is a rapid equilibrium reaction, which proceeds through an intermediate observable in 1H‐NMR, and is followed by slower N‐glycosidic bond hydrolysis. All the reactions between THU, its ribopyranosyl isomers, the intermediate, and the base are acid catalyzed and appear to proceed through the same sugar ring‐opened intermediate (carbinolamine), consistent with previous literature. © 2003 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 92:2027–2039, 2003

Two New Epimeric Pairs of Acetogenins Bearing a Carbonyl Group from Annona cherimolia Seeds

Further studies on the seeds of Annona cherimolia have led to the isolation of two pairs of Annonaceous acetogenins, a mixture of epimers of annomolon A (1) and 34-epi-annomolon A (1'), and a mixture of epimers of annomolon B (2) and 34-epi-annomolon B (2'), containing a rare gamma-hydroxymethyl-gamma-lactone moiety and a carbonyl group. Their structures were elucidated on the basis of spectroscopic and chemical methods, and their absolute stereochemistry was solved by preparing their respective per-Mosher ester derivatives. These acetogenins showed substantial cytotoxicity, comparable to that of adriamycin, against a human pancreatic tumor cell line (MIA PaCa-2).

Regioselective and Diastereoselective Allylic Amination Using Chlorosulfonyl Isocyanate. A Novel Asymmetric Synthesis of Unsaturated Aromatic 1,2‐Amino Alcohols.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Diastereoselective Preparation of 2,4,6‐Trisubstituted‐2′‐cyanopiperidines: Application to the Construction of the Carbon Framework of Perhydrohistrionicotoxin.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Cardiobutanolide, a Styryllactone from Goniothalamus cardiopetalus.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stereoselective Epoxidation and Bromoalkoxylation with 3‐Ylidenepyrazine‐2,5‐diones.

3-Ylidenepyrazine-2,5-diones 1 were stereoselectively epoxidsed by dimethyldioxirane giving access to spirooxiranes 2 and diols 3. Bromohydroxylation and bromoalkoxylation of 3-ylidenepyrazine-2,5-diones 1 produced high yields of optically active 3-(1-bromoalkyl)pyrazine-2,5-diones 4 with a 3-hydroxy or 3-alkoxy function, respectively. Whereas direct hydrogenation of epoxides 2 afforded epimeric mixtures of 3-( 1-hydroxyalkyl)pyrazine-2,5-diones 5 and 6, the highly stereoselective transformation into 5 was possible by primary acid cleavage of the oxirane ring followed by hydrogenation of the resulting keto-enol mixtures 7/8.

Regioselective and diastereoselective allylic amination using chlorosulfonyl isocyanate. A novel asymmetric synthesis of unsaturated aromatic 1,2-amino alcohols.

The diastereoselective synthesis of unsaturated aromatic 1,2-amino alcohols can be achieved on an epimeric mixture of optically active allylic ethers having a hydroxyl group attached to an allylic chiral center to the pi-system using chlorosulfonyl isocyanate. These reactions produced the unsaturated anti-1,2-amino alcohols either exclusively or predominantly only for aromatic derivatives. The anti-selectivity may be explained by the Cieplak electronic model during the conversion from ethers to carbamates.

Synthesis of furan 4′-thio- C-nucleosides, their methylsulfonium and sulfoxide derivatives. Evaluation as glycosidase inhibitors

A series of furan C-nucleosides having a sulfur atom in the sugar ring were synthesised. The α and β anomers of 3-ethoxycarbonyl-2-methyl-5-(4′-thio- d-erythrofuranosyl)furans 10 and 11 were obtained by acid treatment of (4′- S-acetyl-4′-thio- d- arabino-tetritol-1-yl)furan 9 . Oxidation of 10 with m-chloroperbenzoic acid gave sulfoxide 12 as one epimer at the sulfur atom whereas 11 was transformed into sulfoxide 13 as an epimeric mixture. S-Methylation of 10 and 11 with methyl triflate led to sulfonium salts 14 and 15 . The prepared compounds were found to be moderate inhibitors of α- l-fucosidase.

Cardiobutanolide, a styryllactone from Goniothalamus cardiopetalus

A styryllactone namely cardiobutanolide was isolated from the stem bark of Goniothalamus cardiopetalus together with four known styryllactones goniothalamin, goniodiol, goniofufurone, goniofupyrone and known acetogenins squamocin and an epimeric mixture of goniodonin and 34- epi-goniodonin. The structure of the new compound was elucidated on the basis of 1D and 2D NMR experiments and mass spectroscopic techniques.

Diastereoselective preparation of 2,4,6-trisubstituted-2′-cyanopiperidines: application to the construction of the carbon framework of perhydrohistrionicotoxin

The anodic cyanation of methanolic solutions of the 2-alkyl-N-phenylpiperidines 6b-d was performed in a flow cell equipped with a graphite felt anode. The reaction led to the formation of the 2-cyano-6-alkyl-N-phenylpiperidines 2b-d and proceeded with a high degree of regioselectivity. The 1H NMR spectra of the aminonitriles 2b-d showed an epimeric mixture at C-6. The major isomer has a trans configuration in which the cyano group is axial and the alkyl substituent is equatorial. Conversely, electrochemical oxidation of the 4-methyl-6-pentyl-N-phenylpiperidine 6e afforded the trisubstituted aminonitrile 2e as a single diastereomer (> 98% de). The 4-cyanobutyl side chain was incorporated in a two-step procedure to yield dinitrile 4e. This latter compound was directly converted into spiropiperidine 5e by using the Thorpe-Ziegler annulation procedure. The overall sequence (4 steps, 43%) allows the construction of the basic carbon framework of perhydrohistrionicotoxin.

Epimerization of hydroxyl group in the lupan series of triterpenes

Two methods of obtaining of 3 alpha-betulinic acid and related compounds from their 3 beta-epimers were studied: the reaction of bimolecular substitution and the stereoselective reduction of 3-ketoderivatives. The substitution of acyloxy by formyloxy group in 3-O-tosyllupeol or of the betulin hydroxyl by benzoyloxy group resulted only in delta 2, 3-elimination products, with none of the expected products of bimolecular substitution being found. The catalytic hydrogenation of betulonic acid over Raney nickel resulted only in reduction of the isopropenyl double bond, whereas the use of 5% Ru/C gave a 60:40 mixture of epimers of dihydrobetulinic acid. Practically the same mixture of betulinic acid epimers was obtained when reducing betulonic acid with L-Selectride. The cytotoxic activity of 3 alpha-betulinic acid increased toward melanoma Bro cells and decreased toward melanoma MS cells.

The economical synthesis of [2'-(13)C, 1,3-(15)N2]uridine; preliminary conformational studies by solid state NMR.

The synthesis of [2'-(13)C, 1,3-(15)N2]uridine 11 was achieved as follows. An epimeric mixture of D-[1-(13)C]ribose 3 and D-[1-(13)C]arabinose 4 was obtained in excellent yield by condensation of K13CN with D-erythrose 2 using a modification of the Kiliani-Fischer synthesis. Efficient separation of the two aldose epimers was pivotally achieved by a novel ion-exchange (Sm3+) chromatography method. D-[2-(13)C]Ribose 5 was obtained from D-[1-(13)C]arabinose 4 using a Ni(II) diamine complex (nickel chloride plus TEMED). Combination of these procedures in a general cycling manner can lead to the very efficient preparation of specifically labelled 13C-monosaccharides of particular chirality. 15N-labelling was introduced in the preparation of [2'-(13)C, 1,3-(15)N2]uridine 11 via [15N2]urea. Cross polarisation magic angle spinning (CP-MAS) solid-state NMR experiments using rotational echo double resonance (REDOR) were carried out on crystals of the labelled uridine to show that the inter-atomic distance between C-2' and N-1 is closely similar to that calculated from X-ray crystallographic data. The REDOR method will be used now to determine the conformation of bound substrates in the bacterial nucleoside transporters NupC and NupG.

Stereoselective Epoxidation and Bromoalkoxylation with 3-Ylidenepyrazine-2,5-diones

3-Ylidenepyrazine-2,5-diones 1 were stereoselectively epoxidsed by dimethyldioxirane giving access to spirooxiranes 2 and diols 3. Bromohydroxylation and bromoalkoxylation of 3-ylidenepyrazine-2,5-diones 1 produced high yields of optically active 3-(1-bromoalkyl)pyrazine-2,5-diones 4 with a 3-hydroxy or 3-alkoxy function, respectively. Whereas direct hydrogenation of epoxides 2 afforded epimeric mixtures of 3-( 1-hydroxyalkyl)pyrazine-2,5-diones 5 and 6, the highly stereoselective transformation into 5 was possible by primary acid cleavage of the oxirane ring followed by hydrogenation of the resulting keto-enol mixtures 7/8.

A new simple preparation of d-alloisoleucine suitable for large-scale manufacture

d-Alloisoleucine ( d-aIle) was obtained by the resolution of the epimer mixture of l-isoleucine ( l-Ile) and d-aIle, which was formed by epimerization of l-Ile, with a resolving agent such as (2 S,3 S)-dibenzoyltartaric acid ((2 S,3 S)-DBTA) or (2 S,3 S)-di-4-toluoyl-tartaric acid ((2 S,3 S)-DTTA).

Stereoselective Synthesis of 7α- and 7β-Aminocholesterol as Δ8-Δ7 Sterol Isomerase Inhibitors, with Fungicidal Activities towards Resistant Strains

A mixture of epimeric 7α- and 7β-aminocholesterol was shown to be a stronger inhibitor of yeast cell growth than morpholine inhibitors. In fact, this epimeric mixture inhibits Δ8-Δ7-sterol reductase. This epimeric mixture is fungicidal and active against Saccharomyces cerevisiae resistant strains. Therefore, 7α- and 7β-aminocholesterol were selectively synthesized. According to in vitro bioassay studies on resistant strains such as Candida tropicalis [Amphotericin B resistant; minimum inhibitory concentration (MIC) of Amp B: 12.5 μg/mL], the MIC values for 7β-aminocholesterol and 7α-aminocholesterol were found to be 0.8 μg/mL and 1.5 μg/mL, respectively. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2002)

Diastereoselective cycloreductions and cycloadditions catalyzed by Co(dpm)(2)-silane (dpm = 2,2,6,6-tetramethylheptane-3,5-dionate): mechanism and partitioning of hydrometallative versus anion radical pathways.

In the presence of phenylsilane and 5 mol % cobalt(II) bis(2,2,6,6-tetramethylheptane-3,5-dionate), aryl-substituted monoenone monoaldehydes and bis(enones) undergo reductive cyclization to afford syn-aldol and anti-Michael products, respectively. For both aldol and Michael cycloreductions, five- and six-membered ring formation occurs in good yield with high levels of diastereoselectivity. Cycloreduction of monoenone monoaldehyde 1a in the presence of d(3)-phenylsilane reveals incorporation of a single deuterium at the enone beta-position as an equimolar mixture of epimers, inferring rapid isomerization of the kinetically formed cobalt enolate prior to cyclization. The deuterated product was characterized by single-crystal neutron diffraction analysis. For bis(enone) substrates, modulation of the silane source enables partitioning of the competitive Michael cycloreduction and [2 + 2] cycloaddition manifolds. A study of para-substituted acetophenone-derived bis(enones) reveals that substrate electronic features also direct partitioning of cycloreduction and cycloaddition manifolds. Further mechanistic insight is obtained through examination of the effects of enone geometry on product stereochemistry and electrochemical studies involving cathodic reduction of bis(enone) substrates. The collective experiments reveal competitive enone reduction pathways. Enone hydrometalation produces metallo-enolates en route to aldol and Michael cycloreduction products, that is, products derived from coupling at the alpha-position of the enone. Electron-transfer-mediated enone reduction produces metallo-oxy-pi-allyls en route to [2 + 2] cycloadducts and, under Ni catalysis, homoaldol cycloreduction products, that is, products derived from coupling at the beta-position of the enone. The convergent outcome of the metal-catalyzed and electrochemically induced transformations suggests the proposed oxy-pi-allyl intermediates embody character consistent with the mesomeric metal-complexed anion radicals.

Selective conjugate addition of nitromethane to enoates derived from d-mannitol and l-tartaric acid

The conjugate addition of nitromethane to enoates prepared from d-(+)-mannitol, substituted at the α-position by a methyl or a benzyl group, was investigated. While excellent syn-selectivity (d.e. >90%) was obtained from α-benzyl enoates (used as a mixture of epimers, E/ Z=1.8:1), for α-methyl enoates the selectivity depended on the stereochemistry of the double bond in the acceptor (d.e. >90% for the ( Z)-enoate and 50% for the ( E)-enoate). In all cases, a mixture of epimers was formed at the newly generated stereocenter at the α-position. The epimeric syn-adducts were transformed into the corresponding pure α,β,γ-trisubstituted γ-butyrolactones by cyclization in acid medium followed by epimerization of the stereocenter at the α-position in DBU/CH 2Cl 2. When enoates derived from l-tartaric acid were used as acceptors, syn-selective conjugate additions were also observed (d.e. >90% for the ( Z)-isomer and 50% for the ( E)-isomer). The configuration at the newly generated stereogenic centers were assigned based on X-ray analyses, 1H– 1H coupling constants and NOE experiments in NMR spectroscopy.

Synthesis of chiral organotin reagents: synthesis of enantiomerically enriched bicyclo[2.2.1]hept-2-yl tin hydrides from camphor. X-Ray crystal structures of (dimethyl)[(1R,2S,4R)-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]tin chloride and methyl(phenyl)bis[(1R,2S,4R)-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]stannane

2-Iodo-1,7,7-trimethylbicyclo[2.2.1]hept-2-ene 27 was prepared in two steps from camphor 23. Halogen–metal exchange using butyllithium followed by addition of the appropriate tin halide gave the corresponding bicyclo[2.2.1]hept-2-en-2-ylstannanes 26, 35–38 and the (diphenyl)bis[1,7,7-trimethylbicyclo[2.2.1]hept-2-en-2-yl]stannane 48. Reduction of the 1,7,7-trimethylbicyclo[2.2.1]hept-2-en-2-ylstannanes 35–38 using diimide took place predominantly from the exo-face to give the endo-1,7,7-trimethylbicyclo[2.2.1]hept-2-ylstannanes 18, 43–45, endo–exo = ca. 80 ∶ 20 in all cases. The methyl(phenyl)bis[endo-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]stannane 51 was prepared from the diphenyl(methyl)[endo-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]stannane 40 by selective removal of one of the phenyl groups using iodine to give the dialkyl(phenyl)tin iodide 49 which was treated with the alkenyllithium reagent generated from the vinyl iodide 27 to give the bicyclo[2.2.1]hept-2-en-2-yl(dialkyl)phenylstannane 50, as a mixture of epimers at the tin. Reduction using diimide then gave the methyl(phenyl)bis[endo-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]stannane 51 whose structure was established by X-ray crystallography. The major (trimethyl)[1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]stannane 39 was shown to be the endo-isomer by an X-ray crystal structure determination of the tin chloride 46 prepared by treatment of the trimethylstannane 39 with tin tetrachloride. The configurations of the other stannanes 40–42 were established by analogy and by comparison of their 1H NMR spectra with those of 39. The dimethyl[1-dimethylaminomethyl-7,7-dimethylbicyclo[2.2.1]hept-2-enyl](phenyl)stannane 56 was similarly prepared from the parent ketone 52. The stannanes 41/44 and 51 were converted into the tin hydrides 59 and 61, but these gave only very modest enantiomeric excesses when used to reduce the bromoketone 62.