A facile stereoselective synthesis of alpha-glycosyl ureas

Alpha-glycosyl ureas can be synthesised directly from tetra-O-benzyl glycosyl azides and isocyanates, using a one-pot procedure that is simple and general in scope. The benzyl protecting groups are easily removed from the urea products by catalytic hydrogenation. The synthesised alpha-glycosyl ureas represent a new class of neo-glycoconjugates with the potential of being resistant towards carbohydrate processing enzymes.     

A short, stereoselective synthesis of neo-inositol

A practical synthesis of neo-inositol is described in which the target is prepared on a multigram scale in six operations from bromobenzene.     

Microbial cells as catalysts for stereoselective red–ox reactions

Enzyme catalyzed reactions are commonly used at laboratory or industrial scale. Contrarily, the whole cell catalyzed reactions are restricted to special cases. The tremendous advances in the last years in Molecular Biology and more specifically in Metabolic Engineering and Directed Enzyme Evolution have opened the door to create tailor-made microorganisms or “designer bugs” for industrial purposes. Whole cell catalysts can be much more readily and inexpensively prepared than purified enzymes and the enzymes – inside the cells – are protected from the external environment and stabilized by the intracellular medium. Three situations have traditionally been considered convenient to select the use of whole cell catalyzed processes against the free enzyme catalyzed process: i) when the enzyme is intracellular; ii) when the enzyme needs a cofactor to carry out the catalytic act and iii) in the development of multienzymatic processes. Red–ox reactions represent the molecular basis for energy generation in the cell. These reactions are catalyzed by intracellular enzymes and are cofactor dependent as red–ox reactions need electron carriers as helpers in reduction reactions (gain of electrons) or oxidation (loss of electrons).In this review we present an overview of the state of the art of red–ox biotransformations catalyzed by whole cells — wild-type or genetically engineered microorganisms. Stereoselective reductions, hydroxylations of arenes and unfunctionalized alkanes, alkene monooxygenation, and Baeyer–Villiger reactions are among the processes described along the text, focusing in their chemo-, regio- and stereoselectivity.     

Novel stereoselective synthesis and chromatographic evaluation of E-guggulsterone

A new stereoselective synthesis of E-guggulsterone is described starting from androsten-3,17-dione. Protection of the ring A enonic system, followed by regioselective Wittig reaction and C-16 oxidation, affords E-guggulsterone in good yields and high stereoselectivity, making this approach easily accessible and scalable. Moreover, an original normal-phase HPLC method enabling the fast quantitation of the guggulsterone isomeric purity, combined with the suitability for sampling procedures, is detailed. The relying upon the cellulose-based Chiralpak IB column and the chloroform as the "non-standard" component of the eluent mixture, allows to get profitably high chromatographic performances.     

A multi-gram-scale stereoselective synthesis of Z-endoxifen

Z-Endoxifen is widely regarded as the most active metabolite of tamoxifen, and has recently demonstrated a 26.3% clinical benefit in a phase I clinical trial to treat metastatic breast cancer after the failure of standard endocrine therapy. Future pharmacological and pre-clinical studies of Z-endoxifen would benefit from reliable and efficient synthetic access to the drug. Here, we describe a short and efficient, stereoselective synthesis of Z-endoxifen capable of delivering multi-gram (37?g) quantities of the drug in?>97% purity with a Z/E ratio?>99% after trituration.     

Attempted stereoselective synthesis of P-chiral analogues of oligodeoxyribonucleotides

Reaction of diastereomeric 5'MMT-nucleoside 3'-O-(4-nitrophenylmethanephosphate) or 3'-O-[O-(4-nitrophenyl)-S-(2-nitrobenzyl)phosphorothioate] with tBuMgCl-activated, 3'-protected nucleoside is stereospecific and leads, after appropriate work-up, to dinucleoside-3',5'-methanephosphonate or dinucleoside-3',5'-phosphorothioate, respectively. This procedure extended to 5'-activated nucleotides, allows to prepare short, stereoregular oligonucleotide analogues bearing at internucleotide positions methanephosphonate or phosphorothioate function of predetermined sense of chirality at P-centers.     

Recent advances in the stereoselective synthesis of isoprostanes and neuroprostanes

This review delineates several reported methods for the synthesis of isoprostanes and neuroprostanes with particular emphasis on the stereocontrolled construction of a suitably functionalized cyclopentane core. The alpha- and omega-side chains of these PG-like molecules are typically assembled by Wittig-type olefination reactions, standard transformations in the PG synthesis. The synthetic strategies include free radical cyclizations, a palladium-promoted coupling of three different components, an intramolecular cyclopropanation reaction-ring-opening sequence, a [2+2] photocycloaddition-ring-opening metathesis approach, and an intramolecular cross-coupling reaction of an alkyl iodide and a tethered alkenylsiloxane.     

A fast, efficient and stereoselective synthesis of hydroxy-pyrrolidines

A five-step, protecting group free synthesis of 2,3-cis substituted hydroxy-pyrrolidines is presented. Key steps in the synthesis are the chemoselective formation of a primary amine via a Vasella reductive amination using ammonia as the nitrogen source, and the stereoselective formation of a cyclic carbamate from an alkenylamine. Improvement of the reductive amination, by way of the use of α-picoline borane as a more environmentally benign reducing agent, is also presented.     

Samarium(II) promoted stereoselective synthesis of antiproliferative cis-beta-alkoxy-gamma-alkyl-gamma-lactones

Samarium(II) iodide promotes the stereoselective synthesis of cis-beta-alkoxy-gamma-alkyl-gamma-lactones under mild conditions starting from linear precursors. The in vitro antiproliferative activities were examined in the human solid tumor cell lines from diverse origin A2780, SW1573, and WiDr. From the growth inhibition data a structure-activity relationship was obtained. Overall the results point to the relevant role of cis-beta-alkoxy-gamma-alkyl-gamma-lactones as novel scaffolds for the development of new anticancer drugs.     

Continuous coenzyme dependent stereoselective synthesis of sulcatol by alcohol dehydrogenase

Summary 6-methyl-5-hepten-2-one was reduced to sulcatol ((+)-6-methyl-5-hepten-2-ol) by using alcohol dehydrogenase fromThermoanaerobium brockii in a continuous process. The cofactor NADP(H) was retained by a charged UF-membrane and regenerated by oxidation of isopropanol to acetone. Use of native NADP in a charged UF-membrane reactor proved to be superior to use of PEG coupled NADP in a uncharged UF-membrane reactor.     

Highly stereoselective synthesis and structural characterization of new amino sugar derivatives

2-C-Nitroalkyl-1,4:3,6-dianhydromannitols were synthesized via a Henry reaction of nitroalkyls with 1,4:3,6-dianhydrofructose. Catalytic hydrogenation then afforded the corresponding vicinal amino alcohols. Oximation of 1,4:3,6-dianhydrofructose with hydroxylamine, followed by hydrogenation, gave 2-amino-1,4:3,6-dianhydro-2-deoxymannitol. All compounds were elucidated by their HRMS, 1H NMR, 13C NMR, and IR spectra. The absolute configurations of the amino sugar derivatives were confirmed by single-crystal X-ray analysis or NOESY spectral studies. The possible mechanism for hydrogenation of the nitro 2-C-nitroalkyl sugar is proposed. The conformations of the fused furan rings of nitro and amino sugar derivatives are presented.     

Performance of an immobilized fuculose-1-phosphate aldolase for stereoselective synthesis

A derivative of fuculose-1-phosphate aldolase, immobilized with high loading on glyoxal–agarose gels, has been characterized and evaluated as a biocatalyst for an aldol addition reaction. The reaction of the solid biocatalyst was diffusion-controlled for conversion of its natural substrate. Nevertheless, when catalyzing the synthesis of a biologically active aminopolyol, the lower reaction rate with non-natural substrates led to a process controlled by the intrinsic enzyme kinetics. The resulting biocatalyst has high synthetic specific activity and has been successfully used in batch synthesis reactions with high conversion. In addition, the immobilized aldolase has been employed in fed-batch synthesis, increasing the selectivity of the reaction and obtaining high conversion (88%).     

(S)-Camphorsulfonic acid catalyzed highly stereoselective synthesis of pseudoglycosides

A mild and efficient synthesis of pseudoglycosides has been developed using metal free (S)-camphorsulfonic acid. (S)-CSA acts as an excellent catalyst for conversion of 2,4,6-tri-O-acetyl-d-glucal to 2,3-unsaturated O-glycosides. A wide range of biologically active natural products, alcohols and thiols could be coupled with glucal to give the desired pseudoglycosides in good to excellent yields with exclusive α-stereoselectivity.     

Synthesis of enantiopure 2-(aminoalkyl)phenol derivatives and their application as catalysts in stereoselective reactions

Enantiopure 2-(aminoalkyl)phenol derivatives are an interesting class of compounds widely used in homogeneous ligand accelerated catalysis. A series of practical and convenient methods available for their preparation are revised, together with the methodologies for the determination of their configuration. The uses of these compounds in metal catalysed asymmetric reactions in the addition of dialkyl zinc reagents to aldehydes and in the reduction of ketones with borane are described. Moreover 2-(aminoalkyl)phenol derivatives have found use also as chiral shift reagents for carboxylic acids.     

Fungal laccases as green catalysts for dye synthesis

Laccases from different sources catalyse oxidation of various phenolic and aromatic compounds to products that very often are colourful and may be used as dyes, especially in the textile industry. They catalyse not only catabolic processes such as depolymerisation and degradation but can also carry out various dimerization, oligomerization, and polymerization reactions of some hundred aromatic substrates that synthesize new molecules with valuable functions. Because of their versatile biochemical properties, high protein stability, breadth of substrate spectrum, laccases are the key enzymes having applications in biotechnological processes as eco-friendly biocatalyst. This review refers to the natural abilities of laccases to synthesize colour products with respect to the type of the enzymatic reaction, catalyst characterization and possible use of these colour products as dyestuffs.     

4-Thiofuranoid glycals: versatile synthons for stereoselective synthesis of 4'-thionucleosides

Beta-anomers of 4'-thionucleosides have been synthesized stereoselectively, through PhSeCl- or N-iodosuccimide (NIS)-initiated electrophilic glycosidation to 3,5-O-(di-t-butylsilylene)-4-thiofuranoid glycal (1). This synthetic method has been applied to the synthesis of those analogues branched at the anomeric position using 1-C-carbon-substituted 3,5-O-(tetraisopropyldisiloxane-1,3-diyl)-4-thiofuranoid glycals (11-14) prepared based on lithiation of 10.     

The stereoselective enzymatic synthesis of 9-beta-D-2'-deoxyribofuranosyl 1-deazapurine.

The transfer of 2-deoxyribose from thymidine to 1-deazapurine which is catalysed by N-deoxyribosyl transferases from Lactobacillus leichmanii occurs in high yield. This is a very stereoselective process and only one product, 9-beta-D-2'-deoxyribofuranosyl 1-deazapurine, is formed. Nmr spectroscopy, and in particular, nuclear Overhauser enhancement experiments, confirm that the 2-deoxyribose moiety is bound to N-9 rather than N-7 and that the glycosidic link has the beta-configuration.     

The stereoselective synthesis of (E)-alkene dipeptide isosteres

A diastereo- and enantio-selective synthesis of the (E)-alkene dipeptide isostere of -Ala--Ala from -alanine has been developed which proceeds via stereocontrolled addition of a (Z)-vinyllithium reagent followed by a [2,3]-Wittig rearrangement. The synthesis proceeds in seven steps overall from -alanine methyl ester. It is believed that this approach will provide a fairly general and convenient route to isosteres of a number of different dipeptides.