CeCl3·7H2O/IBX-promoted oxidation of 3-alkylindoles to 3-hydroxyoxindoles

3-Alkylindoles undergo smooth oxidation with IBX in the presence of CeCl₃·7H₂O in aqueous acetonitrile at ambient temperature to afford the corresponding 3-hydroxyoxindoles in high yields. This method is very useful for the direct preparation of 3-hydroxyoxindoles from 3-alkylindoles. Operational simplicity, mild and neutral reaction conditions in aqueous solution are the key features of this protocol.     

CeCl3·7H2O/AcCl-catalyzed Prins-Ritter reaction sequence: a novel synthesis of 4-amido tetrahydropyran derivatives

Homoallylic alcohols, carbonyl compounds and nitriles undergo a smooth tandem Prins-Ritter type cyclization in the presence of CeCl₃·7H₂O/AcCl at ambient temperature to produce 4-amido tetrahydropyrans in high yields with all cis-selectivity. Spirocyclic 4-amido tetrahydropyrans are obtained in the case of cyclic ketones.     

Carbohydrate-derived spiroketals: stereoselective synthesis of di-d-fructose dianhydrides

A one-pot synthesis of di-d-fructose dianhydrides (DFAs) having the 1,6,9,13-tetraoxadispiro[4.2.4.2]tetradecane and 1,7,10,15-tetraoxadispiro[5.2.5.2]hexadecane skeleton has been accomplished. The methodology relies on the ability of per-O-protected 1,2-O-isopropylidene β-d-fructofuranose and β-d-fructopyranose derivatives to undergo a tandem acetal cleavage-intermolecular glycosylation-intramolecular spiroketalization process by reaction with suitable acid promoters, such as boron trifluoride etherate or trifluoromethanesulfonic acid, in apolar organic solvents. Spirocyclization proceeds then under irreversible reaction conditions to give binary mixtures of di-d-fructofuranose (α,α and α,β diastereomers) or di-d-fructopyranose 1,2′:2,1′ dianhydrides (β,β and α,β), respectively, the stereochemical outcome being dependent on the non-participating or participating character of the protecting groups. Thus, benzylated and allylated derivatives afford, preferentially, the non-symmetric DFAs (α,β), with diastereomeric excess up to 92%. In contrast, the use of participating benzoyl groups favours the C₂-symmetric diastereomer in both series.     

Sc(OTf)3-catalyzed sugar based tandem ene-Prins cyclization: a novel synthesis of hexahydro-2H-furo[3,2-b]pyranopyran scaffolds

Ttandem ene-Prins cyclization between an aldehyde and an olefin tethered sugar aldehyde has been achieved using a catalytic amount of scandium triflate (10 mol %) at ambient temperature to give a novel series of sugar annulated pyranopyran derivatives in good yields with high selectivity. This is the first report on sugar based ene-Prins cyclization between an aldehyde and O-prenyl derivative of a sugar aldehyde.     

A convenient synthesis of l-ribose from d-fructose

An efficient method for the stereoselective synthesis of l-ribose was accomplished starting from commercially inexpensive d-fructose. The intermediates in the process can serve as versatile precursors for the preparation of l-nucleoside analogues.     

Chemo-enzymatic preparation of chiral 3-aminopyrrolidine derivatives

A new simple method for the enantioselective enzymatic hydrolysis of N-protected D-asparagine esters suitable for the use on the preparative scale is presented. Due to major obstacles observed under conventional reaction conditions—racemization of the retained ester and a strong enzyme inactivation—a comparatively low pH together with an organic co-solvent had to be employed. Under these conditions, nearly all tested proteases demonstrated good activity and excellent enantioselectivity giving access to the corresponding d-esters and L-asparagines in high optical purities (>95% ee) and good chemical yields (>40%). From the unnatural d-asparagine derivative, sequential cyclization, selective deprotection and reduction yielded efficiently benzyl protected (R)-3-aminopyrrolidine, a homo-chiral building block utilized in numerous drug candidates.     

Stereoselective synthesis of (2R,3S,4S)-3-hydroxy-4-methyl-2-tetradecyl-4-butanolide starting from 2,5-anhydro-d-mannitol

A novel approach to natural β-hydroxy-γ-lactone 2 from 2,5-anhydro-d-mannitol (1) is described. The key reactions in this synthesis include stereoselective methylation of aldehyde 3 with lithium dimethylcuprate, an intramolecular radical cyclization of seleno carbonate 11 and an intermolecular cross-metathesis of 3-allyl-4-hydroxy-γ-lactone 16 with 1-tridecene.     

A stereoselective approach to the synthesis of pseudodistomin D

A stereoselective total synthesis of the core unit of pseudodistomin D, starting from l-serine, is described. The key step is a stereoselective intramolecular Michael reaction in which the piperidine ring is formed with the obtention of 1.     

Concise and practical synthesis of (2S,3R,4R,5R) and (2S,3R,4R,5S)-1,6-dideoxy-1,6-iminosugars

The syntheses of (2S,3R,4R,5R) and (2S,3R,4R,5S)-1,6-dideoxy-1,6 iminosugars 1a and 1b, respectively, from d-glucose are described. The key transformations in this reaction sequence include regio-selective epoxide ring opening with N-benzylamine followed by intramolecular reductive amination of amino-aldehyde.     

Improved synthesis of 6-amino-6-deoxy-d-galactono-1,6-lactam and d-mannono-1,6-lactam from corresponding unprotected d-hexono-1,4-lactones

Regioselective bromination of unprotected d-galactono-1,4-lactone and d-mannono-1,4-lactone with PPh₃/CBr₄ led to 6-bromo-6-deoxy derivatives. These intermediates were treated with LiN₃ and hydrogenated to give 6-amino-6-deoxy-d-galactono-1,6-lactam (8) and 6-amino-6-deoxy-d-mannono-1,6-lactam (13) in 74 and 67% overall yield, respectively.     

Selection of synthetic receptors capable of sensing the difference in binding of d-Ala-d-Ala or d-Ala-d-Lac ligands by screening of a combinatorial CTV-based library

Screening of a combinatorial CTV-based artificial, synthetic receptor library 1 {1-13, 1-13, 1-13} for binding of a variety d-Ala-d-Ala and d-Ala-d-Lac containing ligands (6-11) was carried out in phosphate buffer (0.1 N, pH=7.0). After screening and Edman sequencing, synthetic receptors were found containing amino acid sequences, which are either characteristic for binding dye labeled d-Ala-d-Ala or d-Ala-d-Lac containing ligands. For example, receptors capable of binding d-Ala-d-Ala containing ligands 6, 7, 9 and 11 contained—almost in all cases—at least one basic amino acid residue—predominantly Lys—in their arms. This was really a striking difference with the arms of the receptors capable of binding d-Ala-d-Lac containing ligands 8 and 10, which usually contained a significant number of polar amino acids (Gln and Ser), especially in ligand 8, but hardly any basic amino acids. Use of different (fluorescent) dye labels showed that the label has a profound, albeit not decisive, influence on the binding by the receptor. A hit from the screening of the CTV-library with FITC-peptidoglycan (6) was selected for resynthesis and validation.     

Novel practical synthesis of d-cycloserine

The synthesis of d-cycloserine has been successfully accomplished from the readily available d-serine through three simple and efficient routes. In each synthetic strategy, cyclization reactions are involved as the key step, and one-pot processes are employed. The simple treatment and mild reaction conditions are attractive features in this methodology.     

Low cost, surfactant-less, one pot synthesis of Cu2O nano-octahedra at room temperature

Cu₂O octahedra were successfully synthesized via a novel wet-chemical method using d-glucose and hydrazine as reducing agent at room temperature without the presence of any other surfactant. Presence of d-glucose was important for the stabilization of the evolved copper octahedra and also for facilitating the reduction of the Cu(II) ions. The existence of glucose moieties on the surface as capping agent was confirmed by the FT-IR spectra while there was presence of excess oxygen atoms on the surface leading to the formation of a thin CuO layer at the octahedra surface, as confirmed by the XPS study, probably promoted by the capping glucose. Effect of NaOH concentration on the reaction and the formation of octahedra was also studied. The formation mechanism of obtained Cu₂O octahedra has been discussed. These octahedra were then studied for their photocatalytic properties in degradation of organic dyes, rhodamine B and methyl orange.     

One-pot inversion of d-mannono-1,4-lactone for the practical synthesis of l-ribose

l-Ribose was synthesized in a concise manner from d-mannono-1,4-lactone using one-pot inversion conditions. Treatment of d-mannono-1,4-lactone with piperidine, followed by mesylation-induced S_N2-type O-alkylation, afforded the desired one-pot inversion in an optimum yield, and the following straightforward transformations provided l-ribose in good yields.     

Robust synthesis of enantiopure cyclohexenyl analogues of 2/3-deoxyribose sugars as carbocyclic nucleoside precursors

An expedient synthesis of 2-deoxy (10) and 3-deoxy (11) cyclohexenyl analogues of 2-deoxy and 3-deoxy-d-ribose sugar from commercially available starting materials is reported. Highly efficient enzymatic resolution of the key compound 10 is described using lipase under hydrolytic conditions. The robust methodology applied here will be useful to synthesize cyclohexenyl nucleosides, which possess potent antiviral activity and are capable of gene silencing via RNAi or antisense applications.     

Total synthesis of deoxymannojirimycin and d-mannolactam via carbonylation of 5-vinyloxazolidin-2-ones

The stereoselective synthesis of piperidine alkaloids deoxymannojirimycin and d-mannolactam from d-serine has been achieved. The key step involves palladium-catalysed decarboxylative carbonylation of a serine-derived 5-vinyloxazolidin-2-one to give 6-(tert-butyldimethylsilyloxymethyl)-3,6-dihydro-1H-pyridin-2-one which was subsequently converted into the title compounds.     

A new, improved synthesis of the trisaccharide repeating unit of the O-antigen from Xanthomonas campestris pv. campestris 8004

Recent studies have revealed that lipid-A and core fragments of the lipopolysaccharide from Xanthomonas campestris pv. campestris 8004 (Xcc), a phytopathogenic Gram-negative bacterium, are able to elicit plant immunity with two independent mechanisms. To date, nothing is known about the effect of the O-antigen portion. Since its separation from the core region by selective chemical degradation is very difficult, the chemical synthesis of related oligosaccharides is strictly necessary. In this paper a new, improved synthesis of the O-antigen repeating unit is presented. The main improvements in the synthesis are: (1) a shorter, high-yielding preparation of an efficient glycosyl donor of the rare sugar 3-acetamido-3,6-dideoxy-d-galactopyranose (3-acetamido-d-fucose, d-Fucp3NAc); (2) a new protecting group pattern, which is demonstrated to open a path to the future synthesis of higher oligomers.     

Electronic structure and reactivity of Mg+(H2O)n cluster ions

Electronically excited states of magnesium-water cluster ions, Mg⁺(H₂O) _n ,n=1–5, are studied by photodissociation after mass selection. The observed photodissociation spectra are assigned to the²P–²S type transitions localized on the Mg⁺ ion with the aid of ab initio CI calculations. In addition to evaporation of water molecules, photoinduced intracluster reaction to produce MgOH⁺(H₂O) _n is found to occur efficiently, with pronounced size dependence. The intriguing features observed in the mass spectrum of nascent cluster ions are discussed in relation to the stepwise solvation of this reaction.     

Synthesis, heat capacity and enthalpy of formation of [Ho2(l-Glu)2(H2O)8](ClO4)4·H2O

A complex of holmium perchlorate coordinated with l-glutamic acid, [Ho₂(l-Glu)₂(H₂O)₈](ClO₄)₄·H₂O, was prepared with a purity of 98.96%. The compound was characterized by chemical, elemental and thermal analysis. Heat capacities of the compound were determined by automated adiabatic calorimetry from 78 to 370 K. The dehydration temperature is 350 K. The dehydration enthalpy and entropy are 16.34 kJ mol⁻¹ and 16.67 J K⁻¹ mol⁻¹, respectively. The standard enthalpy of formation is −6474.6 kJ mol⁻¹ from reaction calorimetry at 298.15 K.     

An efficient synthesis of selectively functionalized d-rhamnose derivatives

d-Rhamnose is an important component of bacterial lipopolysaccharides. This paper describes a short and highly efficient synthesis of d-rhamnose from d-mannose. The synthesis of selectively C-4 modified d-rhamnosides and 6-deoxy-d-talosides as potential building blocks for complex oligosaccharide synthesis is also discussed.