Practical synthesis of [1-C]- and [6-C]-d-galactose

The chemical synthesis of ¹³C-labeled d-galactose as useful molecular probes for studying the conformation of oligosaccharides attached to proteins was performed. The method for synthesizing the title labeled compounds was newly developed via the corresponding 1-ene and 5-ene compounds derived from 1,2:5,6-di-O-isoproppylidene-α-d-galactofuranose by considering the efficient introduction of the atom. All protons of galactose from H-1 to H-6 were observed by the HMQC-HOHAHA technique using 1:1 mixtures of methyl [1-¹³C]- and [6-¹³C]-β-d-galactopyranoside, which were prepared from the title compounds.     

An asymmetric synthesis of l-[3-C]phenylalanine and l-[3-C]tyrosine from [C]carbon monoxide

-[3-¹³C]Phenylalanine and -[3-¹³C]tyrosine were synthesized. [α-¹³C]Benzyl bromides were prepared from [¹³C]carbon monoxide via the palladium-catalyzed carboalkoxylation of aryl halides. The asymmetric carbon corresponding to the 2-position in phenylalanine was introduced by the diastereoselective alkylation of Dellaria's oxazinone with [α-¹³C]benzyl bromides. Finally, ethanolysis, deprotection, hydrogenolysis and acid hydrolysis of the resulting alkylated oxazinones gave -[3-¹³C]phenylalanine and -[3-¹³C]tyrosine in high optical purity.     

Enantioselective synthesis of d- and l-α-methylcysteine with hydantoinase

A scalable and cost-effective synthesis of d- and l-α-methylcysteine is described. A key step is d-selective cyclization of N-carbamoyl S-tert-butyl-d,l-α-methylcysteine catalyzed by hydantoinase. d-5-tert-Butylthiomethyl-5-methylhydantoin and N-carbamoyl S-tert-butyl-l-α-methylcysteine were obtained with excellent yield and optical purity, and these compounds were easily separated by filtration. After hydrolysis and cleavage of the tert-butyl group, d- and l-α-methylcysteine hydrochloride were obtained.     

Chemoenzymatic synthesis of [3,9-C]-labeled NeuAc and KDN

The chemoenzymatic synthesis of ¹³C-labeled sialic acid (NeuAc) and 3-deoxy-d-glycero-d-galacto-2-nonulosonic acid (KDN) as useful molecular probes for studying the conformation of sialyl or KDN oligosaccharides attached to proteins was performed by using [6-¹³C]-ManNAc, [6-¹³C]-Man and [3-¹³C]-pyruvic acid sodium salt. In the synthesis of the compounds, 5,6-anhydro intermediates were found to easily provide not only 6-¹³C-labeled but also 5-, and 6-modified NeuAc and KDN analogs. Furthermore, it was demonstrated that identical results are obtained by NMR for both [3,9-¹³C]-NeuAc and 1:1 mixtures of [3-¹³C]- and [9-¹³C]-NeuAc.     

Tetrahydrofuran-mediated radical processes: stereoselective synthesis of d,l-hexestrol

The highly stereoselective synthesis of d,l-hexestrol (1), an inhibitor of microtubule assembly, is developed by using, as a key step, an intermolecular coupling of Co₂(CO)₆-complexed propargyl radicals. The latter are generated by novel complementary processes involving an interaction of tetrahydrofuran with Co₂(CO)₆-complexed propargyl alcohols and cations. An isomerically pure d,l-μ-η²-[3,4-di(4-methoxyphenyl)-1,5-hexadiyne]-bis-dicobalthexacarbonyl (d,l-6) is isolated in 69-91% yield with intermolecular coupling reactions exhibiting an excellent chemo- (0.5-7%) and d,l-diastereoselectivity (90-94%). The structure of d,l-6 is determined by X-ray diffraction. The subsequent steps include BBr₃-induced demethylation of 4-methoxyaryl groups, demetalation with cerium(IV) ammonium nitrate, and hydrogenation of acetylenic termini affording d,l-hexestrol (1).     

Boat-chair conformational preference of exo, exo-9-oxobicyclo[3.3.1]nonane-2,4-dicarboxylic acid

The ¹H and ¹³C nmr spectra of $\text{exo}$, $\text{exo}$-9-oxobicyclo[3.3.1]nonane-2,4-dicarboxylic acid, 3, in DMSO-d₆ or alkaline D₂O, clearly show that it exists in a $\text{boat}$-$\text{chair}$-conformation with $\text{equatorial}$ carboxyl groups, thus being the first case of $\text{boat}$-$\text{chair}$ preference of a bicyclo[3.3.1] nonan-9-one due to the presence of $\text{exo}$,$\text{exo}$-2,4- substituents.     

Phase behavior of biodegradable amphiphilic poly(l,l-lactide)-b-poly(ethylene glycol)-b-poly(l,l-lactide)

This study describes the miscibility phase behavior in two series of biodegradable triblock copolymers, poly(l-lactide)-block-poly(ethylene glycol)-block-poly(l-lactide) (PLLA-PEG-PLLA), prepared from two di-hydroxy-terminated PEG prepolymers (M_n = 4000 or 600 g mol⁻¹) with different lengths of poly(l-lactide) segments (polymerization degree, DP = 1.2-145.6). The prepared block copolymers presented wide range of molecular weights (800-25,000 g mol⁻¹) and compositions (16-80 wt.% of PEG). The copolymer multiphases coexistance and interaction were evaluated by DSC and TGA. The copolymers presented a dual stage thermal degradation and decreased thermal stability compared to PEG homopolymers. In addition, DSC analyses allowed the observation of multiphase separation; the melting temperature, T_m, of PLLA and PEG phases depended on the relative segment lengths and the only observed glass transition temperature (T_g) in copolymers indicated miscibility in the amorphous phase.     

A highly efficient synthesis of [1-C, O]- and [1-C, H2]-glycerol for the elucidation of biosynthetic pathways

Labeled glycerol is a widely used biochemical probe to investigate biosynthetic pathways. A highly efficient synthesis of [1-¹³C, ¹⁸O]- and [1-¹³C, ²H₂]-glycerol is described in which the ¹³C label is introduced using cyanide. The ¹⁸O label was introduced by a Pinner synthesis and reduction of the ester 5 allowed incorporation of the ²H labels.     

Mechanism of complex formation between [AuCl4] and l-methionine

The kinetics of the reaction between the tetrachloroaurate(III) ion and l-methionine (l-Met) (0.1 M HClO₄, pH 1.0–2.5) have been studied spectrophotometrically using a stopped-flow technique at different temperatures. Initially, the fast substitution reaction was ascribed to the formation of the short-lived square-planar Au(III)–(l-Met) that was followed by the replacement of a Cl⁻ ligand and a subsequent, slower reduction to Au(I)–(l-Met). This is an intermolecular process, involving attack on the [AuCl₄]⁻ complex by an outer-sphere l-methionine. The activation parameters (ΔH^≠ and ΔS^≠) for substitution and reduction were determined. IR spectroscopy indicates that l-methionine acts as a bidentate ligand, most likely coordinating via the S and N atoms, while ¹H and ¹³C NMR data indicate methionine sulfoxide as the final product. Finally, the components of the reaction were treated thermally in order to investigate the solid phase synthesis of the resulting complex.     

Synthesis of [1-15N,2-13C]-labeled difloxacin

Abstract  

The synthesis of [1-¹⁵N,2-¹³C]-difloxacin, an arylfluoroquinolone antibacterial agent, is reported. As a crucial initial step, the starting materials ethyl 2,4,5-trifluorobenzoylacetate, [formyl-¹³C]-triethyl orthoformate, and [¹⁵N]-4-fluoroaniline were reacted to ethyl [¹⁵N,3-¹³C]-3-(4-fluoroanilino)-2-(2,4,5-trifluorobenzoyl)acrylate. After cyclization and ester cleavage, the resulting intermediate was reacted with 1-methylpiperazine to [1-¹⁵N,2-¹³C]-1-(4-fluorophenyl)-6-fluoro-7-(4-methyl-1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylate, i.e., [1-¹⁵N,2-¹³C]-difloxacin. The overall yield was 62% based on the non-labeled and 43% based on the labeled starting materials (both used in 1.4 molar excess). The product was identified by ¹H-, ¹³C-, and ¹⁵N-NMR spectroscopy and by cochromatography (TLC, HPLC) with an authentic reference; its purity (HPLC) was above 98%. Prior to synthesis of [1-¹⁵N,2-¹³C]-difloxacin, non-labeled difloxacin was synthesized in order to optimize procedures and to identify and characterize all intermediates.     

The synthesis of novel hexa-C-labelled glucosinolates from [C6]-d-glucose

An isotopically labelled building block, 2,3,4,6-tetra-O-acetyl-1-thio-β-d-[¹³C₆]glucopyranose (4), is obtained from the commercially available [¹³C₆]-d-glucose. This hexa-¹³C-labelled thioglucose can be employed to make any glucosinolate (8) for use as an internal standard for isotopic dilution LCMS analysis. Herein three typical glucosinolates in their hexa-¹³C-labelled form: [glucose-¹³C₆]gluconasturtiin, [glucose-¹³C₆]sinigrin and [glucose-¹³C₆]glucoerucin are synthesised by coupling the isotopically labelled thioglucose (4) with the corresponding hydroximoyl chlorides followed by sulfation with pyridine sulfur trioxide and deacetylation with a catalytic amount of potassium methoxide, respectively.     

Synthesis of [carbonyl-C]acetophenone via the Stille cross-coupling reaction of [1-C]acetyl chloride with tributylphenylstannane mediated by Pd2(dba)3/P(MeNCH2CH2)3N·HCl

The Stille cross-coupling reaction of [1-¹¹C]acetyl chloride with tributylphenylstannane leading to [carbonyl-¹¹C]acetophenone was studied with the goal of developing a new ¹¹C-labeling method for positron emission tomography tracer synthesis. The coupled product [carbonyl-¹¹C]acetophenone was synthesized using the Pd₂(dba)₃/P(MeNCH₂CH₂)₃N·HCl system with a 60-61% radiochemical conversion from [1-¹¹C]acetyl chloride (decay-corrected, n = 3).     

Synthesis and proposed conformations of l-prolyl-l-leucyl-β-alanine alkylamides

The synthesis of H-Pro-Leu-β-Ala-NH₂, H-Pro-Leu-β-Ala-NHCH₃ and H-Pro-Leu-β-Ala-N(CH₃)₂ is described. On the basis of IR and ¹H NMR spectral data, a 7-membered ring including the NH of β-alanine with the CO of proline should be assigned for the H-Pro-Leu-β-Ala-N(CH₃)₂. Consequently, the plausible conformations for H-Pro-Leu-β-Ala-NH₂ and H-Pro-Leu-β-Ala-NHCH₃ derive from the formation of an 11-membered ring, between the trans amide proton and the CO of Pro, or from the formation of an 8-membered ring, between this carboxamide proton and the CO of Leu, plus the aforementioned 7-membered ring.     

An efficient and novel synthesis of chromonyl chalcones using recyclable Zn(l-proline)2 catalyst in water

A novel approach was adopted for the synthesis of a series of chromonyl chalcones (3a-o) from 3-formylchromones (1a-c) and different cyclic active methyl compounds (2a-e), employing Zn(l-proline)₂ as a recyclable Lewis acid catalyst in water. In each conversion, the catalyst was successfully recovered and reused several times without significant loss in yield and selectivity. All the newly synthesized compounds were characterized using elemental analysis and spectral data (IR, ¹H NMR, ¹³C NMR and mass spectrometry).     

Novel formal synthesis of stereospecifically C-6 deuterated d-glucose employing configurationally stable alkoxymethyllithiums

The synthesis and testing of configurational stability of chirally monodeuterated PMB- and THP-substituted oxymethyllithiums are described. Macroscopically they are configurationally stable up to −35 °C, the limit of their chemical stability, and microscopically even up to 0 °C. Furthermore, THP-protected oxy-[D₁]methyllithium has been applied in the formal synthesis of (6R)-[6-D₁]-d-glucose (four steps, 40% yield), an example of its use as a homochiral hydroxymethyl synthon.     

A flexible approach for the synthesis of selectively labelled l-arginine

A simple and efficient synthesis of l-arginine has been achieved in 12 steps and 24% overall yield. Regioselective reduction and functional group manipulation of the β-side chain of aspartic acid allowed the preparation of an ornithine derivative, which was then guanylated with a bis-protected 1-guanyl-pyrazole and deprotected to give l-arginine. This approach allows the flexible incorporation of stable isotopes and this is demonstrated using potassium ¹³C-cyanide, which has resulted in the preparation of 5-¹³C-l-arginine.     

Complete assignment of the 1h- and 13c-nmr spectra of [1]benzothieno[2,3-c]naphtho[1,2-h]quinoline and [1]benzothieno[2,3-c]naphtho[1,2-h][1,2,4]triazolo[4,3-a]quinoline. Concerted use of two-dimensional nmr techniques

The ¹H- and ¹³C-nmr spectra of [1]benzothieno[2,3-c]naphtho[1,2-h]quinoline and [1]benzothieno[2,3-c]-naphtho[1,2-h][1,2,4]triazolo[4,3-a]quinoline were totally assigned using a combination of two-dimensional nmr techniques. After correlation of the proton signals by a COSY spectrum and one-bond heteronuclear correlation, complete assignment of the ¹H- and ¹³C-nmr spectra of the novel heterocyclic compounds required the application of long-range CH coupling information particularly for quarternary resonance assignments and the orientations of individual spin systems relative to one another.     

Synthesis of a heterodinuclear ruthenium(II)–platinum(II) complex linked by l-cysteine methyl ester

A potential anticancer heterodinuclear ruthenium(II)–platinum(II) complex, [ruthenium(II)(4,4′-dimethyl-2,2′-bipyridine)₂(5-(l-cysteine-methyl ester)-1,10-phenanthroline)-trans-chlorodiammineplatinum(II)] chloride, [Ru(Me₂bipy)₂(5-(l-cysteine-Me)-phen)-trans-Pt(NH₃)₂Cl]Cl₃, was synthesised. l-Cysteine methyl ester was used to link the two metal centres, as more conventional straight chain diaminoalkanes and 2-mercaptoethylamine failed to couple to the phenanthroline ligand. From the precursor mononuclear ruthenium(II) complexes, which were separated into their Δ- and Λ-isomers by column chromatography, the dinuclear complex was synthesised and characterised by ¹H and ¹³C NMR, UV–Vis, circular dichroism, fluorescence and electrospray ionisation mass spectrometry.     

l-Lactide homopolymerization and l-lactide-ε-caprolactone block copolymerization by lanthanide tris(2,4,6-trimethylphenolate)s

The l-lactide (LLA) homopolymerization and copolymerization with ε-caprolactone (CL) in solution initiated by lanthanide tris(2,4,6-trimethylphenolate)s (Ln(OTMP)₃) are systematically investigated. The results indicate that La(OTMP)₃ is quite effective for the LLA polymerization. The ¹H NMR spectrum suggests the homopolymerization proceeds through an acyl-oxygen bond cleavage. Thermal analysis of homopolymers by DSC shows typical features of optically pure PLLA. The copolymerization of LLA with CL can only be achieved when CL is first polymerized followed by LLA. Feeding the two monomers simultaneously, however, only results in the formation of LLA homopolymers. The structure of the copolymers has been characterized by ¹H NMR and ¹³C NMR and the thermal behavior has also been evaluated. All these measurements demonstrate the pure diblock copolymer has been synthesized successfully.     

High selectivity for l-cysteine residue at axial solvated site of trigonal-bipyramidal palladium(II) complex with tripodal tetradentate phosphine

The five-coordinate trigonal-bipyramidal palladium(II) and platinum(II) complexes with sulfur-coordinated glutathione at the axial position, [Pd(gluta)(pp₃)](BF₄) and [Pt(gluta)(pp₃)](PF₆) (gluta = glutathionate, pp₃ = tris[2-(diphenylphosphino)ethyl]phosphine), were prepared and characterized by ³¹P NMR spectroscopy. The dimeric square-planar platinum(II) complex [Pt(pp₃)]₂(PF₆)₄ gave the monomeric five-coordinate solvated complex, [Pt(pp₃)(CH₃CN)]²⁺, in acetonitrile. Extraction experiments for amino acids from the aqueous solution to the chloroform layer were carried out by using [Pd(pp₃)(CH₃CN)]²⁺, [Pt(pp₃)(CH₃CN)]²⁺, and [Pd(p₃)(CH₃CN)]²⁺ (p₃ = bis[2-(diphenylphosphino)ethyl]phenylphosphine) as extractants. High selectivity for the thiolate sulfur atom in l-cysteinate was observed at the solvated coordination site in [Pd(pp₃)(CH₃CN)]²⁺. The selectivity was applied to extraction of l-cysteinate from a mixture of some amino acids and, further, the reduced form of glutathionate from a mixture of the reduced and oxidized forms of glutathione.