Erythro Forms Research Articles

Analytical characterization and differentiation between threo- and erythro-4-fluoroethylphenidate.

Methylphenidate analogs appeared on the drug market during the last years. Its analogs contain two chiral centers and, thus, have potential varying configurations (i.e., threo and erythro forms). This study presents the analytical characterization of 4-fluoroethylphenidate (4-FEP) and its differentiation between threo- and erythro-4-FEP. Analysis of the samples included high-performance liquid chromatography (HPLC), gas chromatography-electron ionization-mass spectrometry (GC-EI-MS), high-resolution mass spectrometry (HRMS) analyses, nuclear magnetic resonance (NMR) spectroscopy and X-ray crystal structure analysis. NMR spectroscopic investigations confirmed the differences between threo- and erythro-4-FEP, and demonstrated that both isomers could be separated using HPLC and GC methods. Two samples obtained from one vendor in 2019 consisted of threo-4-FEP, whereas the other two samples obtained from a different vendor in 2020 consisted of a mixture of threo- and erythro-4-FEP. Several analytical approaches including HPLC, GC-EI-MS, HRMS analyses, NMR spectroscopy and X-ray crystal structure analysis enabled the unambiguous identification of threo- and erythro-4-FEP. The analytical data presented in this article will be useful for identifying threo- and erythro-4-FEP included in illicit products.

Characterization of some African tropical heartwood lignins by 1D 13C and 1H-NMR: molecular structure and hydroxyl groups’ distribution

In order to understand the potential of some tropical hardwood wastes as source of utilizable lignins, the chemical structure of acetylated dioxane acid lignins from Testulea gabonensis Pellegr (T. gabonensis), Julbernardia pellegriniana (J. pellegriniana), Aucoumea klaineana Pierre (A. klaineana) and Tieghemella africana Pierre (T. africana) widely used in the tropical timber industry of Congo basin was investigated by 1D 1H and 13C-NMR. T. gabonensis and J. pellegriniana were the richest in guaiacyl (G) units, while A. klaineana and T. africana ones exhibited the highest syringyl (S) content. 1H-NMR exhibited an S/G 1. 13C-NMR aliphatic to phenolic hydroxyl group ratios were 1.42; 2.10; 2.0 and 1.41 for T. gabonensis, J. pellegriniana, A. klaineana and T. africana, respectively. The guaiacyl hydroxyl groups of T. gabonensis and J. pellegriniana dioxane acid lignins were more acetylated than A. klaineana and T. africana ones. The lowest acetylation of A. klaineana dioxane acid lignin was reinforced by the weak esterification of its guaiacyl-type hydroxyl groups at the Cα and Cγ position. The same trend was observed for acetylated syringyl-type hydroxyl groups at the Cα position of A. klaineana. Nevertheless, T. gabonensis was the richest in β-O-4 and 5-5′ condensed structures. Those lignins showed a low content on β-5 links for which the weakest signal strength was for T. africana which exhibited, however, the highest content on β–β structures of pinoresinol moieties. The occurrence of guaiacyl-type β-O-4 with Cα=O or β-O-4 threo and erythro forms within the studied hardwood lignins was discussed.

Synthesis and enzymatic hydrolysis of a diaryl benzyl ester model of a lignin-carbohydrate complex (LCC)

Abstract Specific degradation of the bonds between lignin and carbohydrates is an important step towards separating individual lignocellulosic biopolymers for sustainable production of materials and chemicals. One of the most established covalent lignin-carbohydrate (LC) interactions is the ester bond between the α- or γ-hydroxyl group of a lignin phenylpropane unit and a glucuronic acid side chain of xylan. In this work, a model of the LC benzyl ester bond was synthesized in a one-pot reaction from a β-O-4 lignin unit and d-glucuronic acid, both from commercial sources. The resulting lignin-carbohydrate complex (LCC) model was unstable in aqueous solution. However, at pH 4, the rate of spontaneous hydrolysis was sufficiently low to allow for enzymatic splitting experiments. The enzymatic hydrolysis of the LC benzyl ester bond of the LCC model was demonstrated by means of the glucuronoyl esterase StGE2 from Sporotrichum thermophile, which showed a preference for erythro forms of the LCC model.

Absolute Configuration of β- and α-Asymmetric Carbons within β-O-4-Structures in Hardwood Lignin

In this study, we investigated the proportion of erythro- and threo-forms of β-O-4-ether structures and their enantiomeric compositions in hardwood lignin by applying the ozonation method to birch wood meal. Optical activity was not substantially observed in either the erythronic or threonic acids obtained as the ozonation products of β-O-4-structures in birch wood meal. The proportions of the four stereoisomeric forms {(αS,βR)-erythro, (αR,βR)-threo, (αS,βS)-threo, and (αR,βS)-erythro forms} were estimated to be 37-38%, 13-14%, 12-13%, and 36-37% based on the yields of erythronic and threonic acids, and on their optical activities. The proportions suggest that the entire components of β-O-4-ether structures in birch wood lignin have R- and S-configurations at the β-carbon in approximately the same quantities {(βR)-β-O-4-structure: (βS)-β-O-4-structure = 50–52:48–50}; i.e., that the β-ether structures are essentially racemic. This estimation implies that, during lignin biosynthesis, an equal number of enantiomeric forms of β-O-4-bonded quinone methides were formed by radical coupling reactions.

Mechanism for folate‐independent aldolase reaction catalyzed by serine hydroxymethyltransferase

Serine hydroxymethyltransferase catalyzes the cleavage of β-hydroxyamino acids into glycine and aldehydes in the absence of tetrahydrofolate. The enzyme accepts various β-hydroxyamino acids as the substrate of this reaction. The reaction rate varies depending on the substituent and stereochemistry at the Cβ atom: the erythro forms and the β-phenyl substituent are preferred over the threo forms and the β-methyl substituent, respectively. Although several mechanisms have been proposed, what determines the substrate preference remains unclear. We first performed quantum mechanical calculations to assess the validity of the reaction mechanisms. The results indicate that the retro-aldol mechanism starting with abstraction of the proton from the β-hydroxyl group is plausible. This also suggests that Cα-Cβ bond cleavage is the rate-limiting step. We next measured the dependence of the rate constants on temperature with four representative substrates and calculated the activation energies and pre-exponential factors from the Arrhenius plots. The activation energies of the erythro forms were lower than those of the threo forms. The β-phenyl substituent lowered the activation energy in the threo form, whereas it did not alter the activation energy but increased the pre-exponential factor in the erythro form. We present a unified model to explain the origin of the substituent and stereochemical preferences by combining the theoretical and experimental results. A possible biological role of the tetrahydrofolate-independent activity in thermophiles is also discussed.

Stereoisomers of 1,2-diaryl-1,3-propanediols:erythroforms

Racemic erythro-1,2-diphenyl-1,3-propanediol, C(15)H(16)O(2), is a model compound representative of erythro forms of structural elements of the 1,2-diaryl-1,3-propanediol type in lignins. In the crystal structure, the torsion angle between the bulky phenyl groups is -62.26 (11)degrees. Strong hydrogen bonds take part in a directed co-operative O-H...O-H...O-H...O-H pattern that is assumed to have a decisive influence on the conformation. This is supported by comparisons with the geometries of related compounds.

Acid hydrolysis kinetics and identification of erythro and threo α-ethyl ether derivatives of non-phenolic arylglycerol-β-syringyl ether lignin model compounds

Abstract Based on their hydrolysis behavior, NMR spectra of the erythro and threo forms of 3-(3,4-dimethoxy-phenyl)-2-(2,6-dimethoxyphenoxy)-3-ethoxypropan-1-ol (named VSE, i.e., α-ethyl ether derivative of veratrylglycerol-β-syringylether) were successfully assigned. A mixture of erythro and threo VSEs was treated at 90°C in pH 1.2 nitric acid solution. In order to assign them, the molar ratios of erythro and threo forms of 1-(3,4-dimethoxyphenyl)-2-(2,6-dimethoxy-phenoxy)propane-1,3-diol (named VS, i.e., veratrylglycerol-β-syringyl) ether given by each VSE were kinetically determined during the treatment. It was found that the inversion ratio at which the first VSE isomer gave erythro-VS was 0.73. Therefore, this isomer was identified as threo-VSE. In addition, from calculations using the least-squares method, the inversion ratio at which the other VSE isomer gave threo-VS was found to be >0.73. These values mean that threo-VSE yields 73% erythro- and 27% threo-VS, while erythro-VSE yields mainly threo-VS.

Gamma-fluorinated analogues of glutamic acid and glutamine.

Gamma-fluorinated analogues of glutamic acid and glutamine are compounds of biological interest. Syntheses of such compounds are extensively reviewed in this article. 4-fluoroglutamic acid was prepared as a mixture of racemic diastereomers by Michael reaction, inverse-Michael reaction or by electrophilic / nucleophilic fluorination. Optically enriched 4-fluoroglutamic acids were obtained by several resolution techniques as well as by asymmetric methodologies using the chiral pool. 4-fluoroglutamine was prepared as a mixture of stereoisomers as well as in racemic erythro and threo forms from the corresponding 4-fluoroglutamic acids using aminolysis and conventional protection and deprotection strategies. Racemic 4,4-difluoroglutamic acid was synthesized by a nitroaldol reaction and its L-enantiomer obtained via three different asymmetric routes. Racemic 4,4-difluoroglutamic acid was converted into the corresponding 4,4-difluoroglutamine using a protection / aminolysis / deprotection sequence while N-Boc-L-4,4-difluoroglutamine was prepared directly from (R)-Garner's aldehyde using a Reformatsky reaction as the key step.

A novel enzyme, D-3-hydroxyaspartate aldolase from Paracoccus denitrificans IFO 13301: purification, characterization, and gene cloning.

A novel enzyme, D-3-hydroxyaspartate aldolase (D-HAA), catalyzing the conversion of D-3-hydroxyaspartate to glyoxylate plus glycine, was purified to homogeneity from Paracoccus denitrificans IFO 13301. D-HAA is strictly D-specific as to the alpha-position, whereas the enzyme does not distinguish between threo and erythro forms at the beta-position. In addition to D-3-hydroxyaspartate, the enzyme also acts on d-threonine, D-3-3,4-dihydroxyphenylserine, D-3-3,4-methylenedioxyphenylserine, and D-3-phenylserine. The D-HAA gene was cloned and sequenced. The gene contains an open reading frame consisting of 1,161 nucleotides corresponding to 387 amino acid residues. The predicted amino acid sequence displayed 35% and 22% identity with that of the D-threonine aldolase of Arthrobacter sp. DK-38 and Alcaligenes xylosoxidan IFO 12669, respectively. This is the first paper reporting both a purified enzyme with D-3-hydroxyaspartate aldolase activity and also its gene cloning.

Guaiacylglycerol-7'-O-methyl 8'-vanillic acid ether and related compounds from Boreava orientalis.

The threo and erythro forms of guaiacylglycerol-7'-O-methyl 8'-vanillic acid ethers, threo and erythro guaiacylglycerol 8'-vanillin ethers, and threo guaiacylglycerol 8'-(4-hydroxymethyl-2-methoxyphenyl) ether have been isolated from fruits of Boreava orientalis. Structural determinations were made on the basis of UV, MS, 1H- and 13C-NMR spectral data, including two-dimensional shift correlation. The relative configurations were assigned on the basis of 1H-NMR chemical shifts.

Structure–Activity Relationships of Acetolactate Synthase Inhibition among New Benzenesulfonylureas in Rice (Oryza sativa) and Barnyardgrass (Echinochloa crus-galli var. oryzicola)

Abstract Structure–activity relationships of 15 new benzenesulfonylurea derivatives were investigated using acetolactate synthase (ALS) assays in vitro and in vivo in rice (Oryza sativa) and barnyardgrass (Echinochloa crus-galli var. oryzicola). In vitro ALS activities of both rice and barnyardgrass were significantly inhibited by the compounds with smaller numbers of carbon atoms and α-fluorine substituents in the R radical of the new benzenesulfonylurea structure. Among the stereoisomers, erythro forms showed 10 times higher ALS inhibition than threo forms. However, the in vitro ALS assay did not clearly account for the selectivity of the compounds tested in rice and barnyardgrass. The selectivity could, however, be explained by the results of the in vivo ALS assay using an inhibitor of ketol-acid reductoisomerase, 1,1-cyclopropanedicarboxylic acid. Among the analogues tested, the compound K-8 was the most potent ALS inhibitor in vitro, and it has a good selectivity in in vivo ALS assays in rice and barnyardgrass. These results should lead to an assay model for structure–activity relationships for the synthesis and development of new ALS-inhibiting herbicides.

Distribution oferythro andthreo forms of different types of β-O-4 structures in aspen lignin by13C NMR using the 2D INADEQUATE experiment

Carbon–carbon connectivity spectra of 13C-enriched aspen lignin recorded using the 2D INADEQUATE experiment revealed cross peaks which can be assigned to four types of arylglycerol β-aryl ethers (β-O-4 structures): erythro forms of arylglycerol β-syringyl ethers, threo forms of arylglycerol β-syringyl ethers, erythro forms arylglycerol β-guaiacyl ethers and threo forms of arylglycerol β-guaiacyl ethers. The intensities of the cross peaks suggest larger amounts of β-syringyl ethers than β-guaiacyl ethers. The erythro isomers dominate among the β-syringyl ethers. Erythro and threo forms of β-guaiacyl ethers are present in similar amounts. © 1998 John Wiley & Sons Ltd.

Distribution of erythro and threo forms of different types of β‐O‐4 structures in aspen lignin by 13C NMR using the 2D INADEQUATE experiment

Carbon–carbon connectivity spectra of 13C-enriched aspen lignin recorded using the 2D INADEQUATE experiment revealed cross peaks which can be assigned to four types of arylglycerol β-aryl ethers (β-O-4 structures): erythro forms of arylglycerol β-syringyl ethers, threo forms of arylglycerol β-syringyl ethers, erythro forms arylglycerol β-guaiacyl ethers and threo forms of arylglycerol β-guaiacyl ethers. The intensities of the cross peaks suggest larger amounts of β-syringyl ethers than β-guaiacyl ethers. The erythro isomers dominate among the β-syringyl ethers. Erythro and threo forms of β-guaiacyl ethers are present in similar amounts. © 1998 John Wiley & Sons Ltd.

Low-specificity l -threonine aldolase of Pseudomonas sp. NCIMB 10558: purification, characterization and its application to β-hydroxy-α-amino acid synthesis

Low-specificity l-threonine aldolase, catalyzing the reversible cleavage/condensation reaction between l-threonine/l-allo-threonine and glycine plus acetaldehyde, was purified to homogeneity from Pseudomonas sp. NCIMB 10558. The enzyme has an apparent molecular mass of approximately 145 kDa and consists of four identical subunits with a molecular mass of 38 kDa. The enzyme, requiring pyridoxal- 5′-phosphate as a coenzyme, is strictly l-specific at the α position, whereas it can not distinguish between threo and erythro forms at the β position. Besides the reversible cleavage/condensation of threonine, the enzyme also catalyzes the reversible interconversion between glycine plus various aldehydes and l-β-hydroxy-α-amino acids, including l-β-(3,4-dihydroxyphenyl)serine, l-β-(3,4-met‐hylenedioxyphenyl)serine and l-β-phenylserine, providing a new route for the industrial production of these important amino acids.

Isolation and characterization of D-threonine aldolase, a pyridoxal-5'-phosphate-dependent enzyme from Arthrobacter sp. DK-38.

D-Threonine aldolase is an enzyme that catalyzes the cleavage of D-threonine into glycine and acetaldehyde. Its activity was found in several genera of bacteria such as Arthrobacter, Alcaligenes, Xanthomonas, and Pseudomonas, but not in yeasts or fungi. The enzyme was purified to homogeneity from one strain, Arthrobacter sp. DK-38. The enzyme appeared to consist of a single polypeptide chain with an apparent molecular mass of 51 kDa. This enzyme, as well as L-threonine aldolase, requires pyridoxal 5'-phosphate (pyridoxal-P) as a coenzyme. Unlike other pyridoxal-P enzymes, D-threonine aldolase also requires a divalent cation such as Co2+, Ni2+, Mn2+, or Mg2+ for its catalytic activity. The enzyme completely lost its activity in the absence of either pyridoxal-P or a divalent cation. A divalent cation was also essential for the thermal stability of the enzyme. The metal-free enzyme tends to become thermally unstable, resulting in the irreversible loss of its catalytic activity. The enzyme is strictly D-specific for the alpha-position, whereas it cannot distinguish between threo and erythro forms at the beta-position. Thus, D-threonine and D-allothreonine act as substrates of the enzyme, but their kinetic parameters are different; the Km and Vmax values are 3.81 mM and 38.8 micromol x min(-1) x mg(-1) toward D-threonine, and 14.0 mM and 102 micromol x min(-1) x mg(-1) toward D-allothreonine. respectively. The aldolase reaction is reversible, and the enzyme is therefore able to produce nearly equimolar amounts of D-threonine and D-allothreonine through C-C bond formation between glycine and acetaldehyde. The enzyme also acts, in the same manner, on several other D-beta-hydroxy-alpha-amino acids, including D-beta-phenylserine, D-beta-hydroxy-alpha-aminovaleric acid, D-beta-3,4-dihydroxyphenylserine, and D-beta-3,4-methylenedioxyphenylserine.

On the distribution of the diastereomers of the structural elements in lignins: the steric course of reactions mimicking lignin biosynthesis

Stereochemical studies on the formation of the diastereomers of arylglycerol-β-aryl ether structures during lignin biosynthesis have been carried out with model compounds. The addition of water to quinone methides of the β-syringyl ether type gives arylglycerol β-syringyl ethers with a predominance of the erythro isomer when the pH of the medium is low. Since erythro forms of arylglycerol β-syringyl ethers are prevalent in hardwood lignins, this indicates that the pH of the medium in which lignin biosynthesis occurs is lower than has been assumed until now. Equilibration studies with non-phenolic model compounds of the arylglycerolβ-guaiacyl ether and β-syringyl ether types under acidolysis conditions indicate that the erythro predominance observed in the syringyl ethers in lignins does not correspond to equilibrium conditions. A remarkable resistance to acidolysis is observed in the model compounds of etherified syringylglycerol β-syringyl ether type.

Einfluß der Enolisierung auf die Reduktion von β Ketocarbonsäure-Derivaten mit Zinkborhydrid / Influence of the Enol Formation on the Reduction of β-Keto Carboxylic Acid Derivatives by Zinc Borohydride

Depending on the keto-enol equilibrium 1 ⇄ 2 reduction of the esters and the amides 1/2 a—g with Zn(BH4)2/ZnCl2 attacks either at the CO or at the CO2R group. A high diastereoselectivity (98:2 or more) is observed for the formation of the β-hydroxy compounds 3a,b,c,e,f in favour of the erythro forms. As to the structure determination of 3, different steric effects and different intramolecular hydrogen bonding have to be considered, leading to different rotameric populations.

Resolution of β-hydroxy-α-amino acids by the action of proteases on their N-acyl methyl esters

Methyl N-acetyl phenylserinates (1, 2), methyl N-acetyl nitrophenylserinates (3, 4), and methyl N-benzoyl threoninates (5, 6) were resolved conveniently into the enantiomers with high optical purities by enzymatic hydrolysis in the presence of α-chymotrypsin, subtilisin, or bromelain. The threo and erythro forms were treated separately and the latter form was usually more reactive. Chymotrypsin and subtilisin are highly specific for the enantiomer having the same configuration on carbon-2 (S) as the natural standard amino acids. In contrast, bromelain shows the reverse specificity on phenylserine derivatives. This is a notable exception to the usual S-stereospecificity pattern of proteases. Keywords: enzymatic hydrolysis, resolution of amino acids.

An analysis of the inhibition of replication of HOV and MULV by some 3′-blocked pyrimidine analogs

Some 3′-blocked pyrimidine analogs were synthesized and tested as inhibitors of replication of human immunodeficiency virus (HTV) and Moloney-murine leukemia virus (MuLV). The analogs were of 3 kinds: (1) analogs of 3′-azido-3′-deoxythymidine (AZT) in which the C-5 CH 3 of the base was exchanged for H (AZU) or C 2H 5 (AZEU); (2) 3′-fluoro-3′-deoxythymidine (FLT) and analogs thereof, in which the C-5 CH 3 of the base was exchanged for H (FLU), C 2H 5 (FLEU) or nC 3H 7 (FLPU); (3) the threo analogs of AZT (AZT↑) and AZU (AZU↑). All analogs were less active inhibitors of HTV replication than AZT, except FLT, which was as active as AZT. The 3′-fluoro analogs and AZEU did not inhibit MuLV replication at non-cytotoxic concentrations. Oral administration of FLT to MuLV-infected mice result in antiviral effects only at toxic drug levels. AZU and FLU were less potent inhibitors of HIV replication than AZT or FLT, but the 2′-deoxy uridine analogs were also less cytotoxic to human embryonic fibroblasts than the thymidine analogs. The 5′-triphosphates of AZU, AZT, AZEU, FLT and FLEU were tested as inhibitors of the HIV-and MuLV-reverse transcriptases. Ranking of the K i K m values for HIV-RT resulted in the following order of potency of the 5′-triphosphates AZT = FLT > AZU > AZEU > FLEU. The 5′-triphosphates of AZEU, FLT and FLEU did not inhibit the MuLV-RT, which explains, in part, the lack of effect of these analogs against MuLV replication. The threo forms (azido “up”) of AZU and AZT were less active inhibitors of HIV replication than the erythro forms (azido “down”). A 15N-NMR and 1H-NMR study showed that the furanose moieties of analogs with the azido function “up” assume a conformation distinct from that of the analogs with azido “down”. This is due to intramolecular stabilisation of the “N” conformer in the threo (“up”) diastereomer, due to interaction of the azido functions with the nucleobase and possibly the OH group of C-5′ of the furanose. As discussed, this conformation might explain the decreased biological activity of threo forms compared with the erythro forms.

REACTIONS OF PHOSPHINES WITH ACETYLENES. PART 19.1CONFIGURATIONAL AND CONFORMATIONAL ANALYSIS OF 1,2-DIMETHOXYCARBONYL-2-TRIS(CYANOETHYL)PHOSPHONIUM-ETHANE SULPHONATES

Abstract 1,2-Dimethoxycarbonyl-2-tris(cyanoethyl)phosphonium-ethane sulphonates are present in both threo and erythro forms in TFA (trifluoroacetic acid) and DMSO (dimethylsulphoxide). The relative proportions change from a 5 : 1 ratio for a solution in DMSO to a 1 : 1 ratio for a solution in TFA. The predominant conformation in the threo betaine also depends on the nature of the medium-the conformer with the vicinal protons trans predominates in DMSO while that with the ester groups trans predominates in TFA. In contrast the erythro form adopts essentially the same conformation in both solvents.