An approach to the synthesis of α-(1-6)-C-disaccharides by tandem Tebbe methylenation and Claisen rearrangement

Uronic acids, most efficiently synthesised from the corresponding alcohols by two step Dess-Martin and sodium chlorite mediated oxidation, may be used as coupling partners for esterification with an allo glycal as substrates for the tandem Tebbe/Claisen approach to the synthesis of 1-6 linked C-disaccharides. Whilst esters of glucuronic and mannuronic acids successfully undergo Tebbe methylenation, esters derived from galacturonic acids are unreactive under these conditions. Thermal Claisen rearrangement of vinyl ethers produced by methylenation yields α-C-disaccharides with complete control of anomeric stereochemistry.     

A new finding in selective Baeyer-Villiger oxidation of α-fluorinated ketones; a new and practical route for the synthesis of α-fluorinated esters

α-Fluorinated esters were effectively prepared by the Baeyer-Villiger oxidation of α-fluorinated ketones with m-chloroperbenzoic acid (m-CPBA) under mild conditions. The yield of the esters was influenced by the choice of solvent, base, and substituent on the aryl group of the ketones. 4-Methoxyphenyl substituted fluoroketones were oxidized almost quantitatively with m-CPBA within 10 min to 12 h at room temperature using 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as a cosolvent with CH₂Cl₂ (1:1, v/v) and aqueous buffer (KH₂PO₄-NaOH, pH 7.6) as an additive base. The oxidation reaction rates of α-fluorinated ketones were higher than those of the corresponding non-fluorinated ketones. The fluorine atom at α-position of fluoromethyl aryl ketones enhanced the reactivity in the Baeyer-Villiger oxidation.     

Efficient synthesis of halohydroxypyridines by hydroxydeboronation

This paper describes a general method for the synthesis of halohydroxypyridines from novel halopyridinylboronic acids and esters recently described by some of us. Halopyridinylboronic acids and esters have been efficiently hydroxydeboronated under mild conditions by employing hydrogen peroxide or meta-chloroperbenzoic acid. These hydroxylations take place regioselectively without other oxidation (N-oxide formation).     

Iodine-mediated Z-selective oxidation of ketones to α,β-unsaturated esters: synthesis and mechanistic studies

The Z-selective oxidation of simple acyclic ketones to Z-2,3-trisubstituted α,β-unsaturated esters is described. Enolates generated under the reaction conditions undergo double iodination followed by a Favorskii-related rearrangement to the unsaturated ester. This reaction represents the first stereoselective one-step transformation of ketones to α,β-unsaturated esters. Mechanistic studies suggest that an electrocyclic reaction governs the Favorskii-related rearrangement.     

Sulfonamide synthesis using N-hydroxybenzotriazole sulfonate: an alternative to pentafluorophenyl (PFP) and trichlorophenyl (TCP) esters of sulfonic acids

The N-hydroxybenzotriazole (HOBt) sulfonate esters undergo amidation under ambient conditions in the presence of di-isopropylethyl amine. This method can be applied to varieties of amines including sterically hindered amino acid esters in less time with reasonably good yields.HOBt ester of sulfonic acids can be a replacement for pentafluorophenyl and trichlorophenyl ester as well as chloride moiety of sulfonyl chloride for amidation.     

Facile synthesis of α, β-unsaturated esters through a one-pot copper-catalyzed aerobic oxidation-Wittig reaction

An efficient one-pot synthesis of α, β-unsaturated esters through the aerobic oxidation – Wittig tandem reaction of alcohols and phosphorous ylide is developed. This new method operates under mild reaction conditions, and uses CuI/TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) as co-catalyst and air (O₂) as the oxidant. It tolerates a wide range of functionalized benzylic alcohol and aliphatic alcohols.     

The rhodium-catalyzed selective oxidative Heck reaction of amides with allylic esters

The Rh-catalyzed direct selective oxidation Heck reaction of benzamides with allylic esters has been developed for the synthesis of amides and isoquinolin-1-ones. According to the functional groups on N-atom, benzamides or isoquinolin-1-ones were synthesized under optimal reaction conditions. The ambient catalytic system has good functional group tolerance, afforded corresponding products in moderate to good yields.     

Baker's yeast: production of d- and l-3-hydroxy esters

Baker's yeast grown under oxygen limited conditions and used in the reduction of 3-oxo esters results in a shift of the stereoselectivity of the yeast towards d-hydroxy esters as compared with ordinary baker's yeast. The highest degree of stereoselectivity was obtained with growing yeast or yeast harvested while growing. In contrast, the stereoselectivity was shifted towards l-hydroxy esters when the oxo esters were added slowly to ordinary baker's yeast supplied with gluconolactone as co-substrate. The reduction rate with gluconolactone was increased by active aeration. Ethyl l-(S)-3-hydroxybutanoate was afforded in >99% ee. Both enantiomers of ethyl 3-hydroxypentanoate, d-(R) in 96% ee and l-(S) in 93% ee, and of ethyl 4-chloro-3-hydroxybutanoate, d-(S) in 98% ee and l-(R) in 94% ee, were obtained. The results demonstrate that the stereoselectivity of baker's yeast can be controlled to a large extent without the use of inhibitors, heat treatment, etc.     

Asymmetric synthesis of α- and β-amino acids by diastereoselective addition of triorganozincates to N-(tert-butanesulfinyl)imines

The diastereoselective addition of triorganozincates to (R)-N-(tert-butanesulfinyl)imines has been used as a key step to achieve the synthesis of highly enantiomerically enriched N-protected α- and β-amino acids. Desulfinylation of the addition products followed by benzoylation of the nitrogen atom of the obtained primary amines and oxidation of one of the substituents on the carbon atom connected to the nitrogen complete the sequence. Using the same configuration in the sulfinyl chiral auxiliary, α-amino acids with the (R) or the (S) configuration can be prepared by choosing the proper combination of imine and organozincate. α,α-Disubstituted α-amino esters with high enantiomeric purity can also be prepared when α-imino esters are the starting substrates.     

Polylactides with aldaric ester end groups or chain extending groups

The synthesis and characterisation of a series of poly(S,S-lactides) and poly(RR/SS-lactides) end capped or chain extended with aldaric ester groups are described. The aldaric esters are prepared from acetyl protected d-gluconolactone, d-galactonolactone and d-mannonolactone by ring opening with butanol or 1,4-butanediol, under acidic conditions. The aldaric esters are fully characterised, including by NMR spectroscopy and mass spectrometry. They are used as co-initiators, with an ethylzinc complex, to enable the controlled ring opening polymerisation of S,S- and rac-lactide. Thus, a series of polylactides are produced with various carbohydrate chain end or extending groups and are fully characterised, including by NMR spectroscopy and gel permeation chromatography (GPC). The polymerisation kinetics are examined, under pseudo first order conditions, and indicate that chain transfer reactions are occurring more rapidly than propagation reactions. The degradation kinetics of the polylactides are examined under acidic conditions; the aldaric esters increase the degradation rates compared to unfunctionalised end groups.     

Application of the addition of triorganozincates to N-(tert-butanesulfinyl)imines to the enantioselective synthesis of α-amino acids

Highly enantiomerically enriched N-protected α-amino acids can be easily prepared from optically pure N-(tert-butanesulfinyl)imines by a four-step sequence involving: diastereoselective addition of a triorganozincate to the imine, removal of the sulfinyl group, benzoylation of the nitrogen atom of the obtained primary amine and oxidation of one of the substituents on the carbon atom α to the nitrogen. Using the same configuration in the sulfinyl chiral auxiliary, amino acids with the (R) or the (S) configuration can be prepared by choosing the proper combination of imine and organozincate. α,α-Disubstituted α-amino esters with high optical purity can also be prepared by the diastereoselective addition of trialkylzincates to α-imino esters.     

Interaction of biomass of aerobic bacteria and fungi with Pu(IV) at low pH

The aim of this study was to investigate the change of Pu oxidation states due to interaction with aerobic bacteria and fungi at low pH under laboratory conditions. Microorganisms were isolated from samples collected from the low-level radioactive waste repository within the confines of Ignalina NPP. Abilities of the fungi (Absidia spinosa var spinosa Lendn. and Paecilomyces lilacinus Thom Samson) as well as Gram-positive bacteria (Bacillus mycoides and Micrococcus luteus) and Gram-negative bacterium I-m1 to transform the oxidation states of Pu under aerobic conditions were investigated. Oxidized and reduced Pu was tested using two radiochemical procedures. The amount of reduced and oxidized Pu was determined by measuring alpha activity after radiochemical separation. The results have shown that all bacteria and fungi can very slightly alter oxidation states of Pu due to their microbial activity. All the microorganisms tested demonstrate quite a fast process of Pu biosorption under the experimental conditions.     

Asymmetric benzoin condensation promoted by chiral triazolium precatalyst bearing a pyridine moiety

Chiral triazolium salts bearing a pyridine ring were developed as N-heterocyclic carbene precursors. In the presence of the chiral triazolium salt and a base, the catalytic asymmetric benzoin condensation proceeded to afford the product in high level of chemical yield and enantioselectivity. A wide range of aromatic aldehydes were applicable to this reaction.     

Elaboration of the ether cleaving ability and selectivity of the classical Pearlman's catalyst [Pd(OH)2/C]: concise synthesis of a precursor for a myo-inositol pyrophosphate

The cleavage of propargyl, allyl, benzyl, and PMB ethers by Pd(OH)₂/C can be tuned in that order, by varying the reaction conditions. Other moieties such as C-C double bonds, esters, trityl ether, p-bromo and p-nitrobenzyl ethers are stable to these reaction conditions. Cleavage of allyl ethers can be made catalytic by using 1:1 mixture of Pd(OH)₂/C and Pd/C. The synthetic potential of the selective ether cleaving ability of Pd(OH)₂/C, essentially under neutral conditions, has been demonstrated by an efficient synthesis of a precursor for the preparation of an inositol pyrophosphate derivative.     

The cyanide-catalyzed conversion of β-formylmuconates into triglochinates: A useful biomimetic synthesis

Upon treatment with cyanide under slightly basic conditions, β-formyl-cis,cis-muconic acid and esters, prepared from vanillin, undergo a sequence of prototropic shifts equivalent to an internal redox process in which 1,4-reduction of the diene system is blanaced by oxidation of the formyl group.     

Selective deprotection and amidation of 2-pyridyl esters via N-methylation

The 2-pyridyl residue serves as a protecting group for various carboxylic acids. The protecting group is selectively cleaved under mild conditions via N-methylation of the pyridyl group. During the deprotection process, the various functional groups as well as the other ester moieties remain intact. The N-methylated active esters can be subsequently transformed into amides.     

Gas‐Phase Oxidation of Methyl‐10‐undecenoate in a Jet‐Stirred Reactor

To better understand the chemistry of biodiesel surrogates, the gas‐phase oxidation of a C₁₂ unsaturated methyl ester, methyl‐10‐undecenoate, has been studied in a jet‐stirred reactor in the temperature range 500–1100 K. These experiments were performed using neat fuel synthesized in the laboratory, with an initial fuel mole fraction set as 0.0021, at quasi‐atmospheric pressure (1.07 bar), at a residence time of 1.5 s with dilute mixtures in helium of equivalence ratios of 0.5, 1.0, and 2.0. The maximum obtained conversion was shown to be more than twice lower than that of methyl decanoate under the same conditions. This difference cannot be reproduced by the only published model for an unsaturated ester with a close number of carbon atoms (methyl‐9‐decenoate). A large range of products was quantified in addition to common oxidation products: saturated and unsaturated aldehydes, saturated and unsaturated methyl esters with a second carbonyl function, C₂–C₁₀ alkenes, C₄–C₁₀ dienes, C₄–C₁₀ unsaturated methyl esters, C₈–C₉ saturated methyl esters, and saturated, unsaturated, and hydroxyl methyl esters involving a cyclic ether. Pathways of formation for the products specific to unsaturated ester oxidation were proposed, and possible model improvements were discussed.     

A new synthesis of β-amino acids by use of ketene diethyl acetal as enolate equivalent

Reaction of phenylsulfonyl formamides, readily available in a single step from an aromatic aldehyde and sodium phenylsulfinate, with ketene diethyl acetal under basic conditions gives N-formyl β-amino acid esters in good yield, which can be kinetically resolved with a lipase.     

Benzylic bromination catalyzed by triphenylphosphine selenide via Lewis basic activation

The synthesis of benzyl bromides was achieved under mild reaction conditions using N-bromosuccinimide as a bromine source and triphenylphosphine selenide as a catalyst. These conditions were compatible with a variety of substrates including boronic esters, protected amines to give imines, and carboxylic acids (to give phthalides when ortho to the benzylic center). A preliminary mechanistic investigation utilizing calculated bond dissociation energies (BDE) is employed to rationalize how a Lewis base can activate a radical mechanism.     

Concise and diastereoselective approach to syn- and anti-N-tosyl-α-hydroxy β-amino acid derivatives

The methyl diazoacetate and aryl (N-tosyl)imines can be transformed into syn or anti α-hydroxy β-amino esters with high diastereoselectivities in three steps: the base promoted nucleophilic condensation of the methyl diazoacetate and aryl (N-tosyl)imines to give β-(N-tosyl)amino α-diazoesters, followed by oxidation with Oxone^® to generate α-oxo esters, which were reduced with NaBH₄ to yield the anti-N-tosyl-α-hydroxy β-amino ester, or hydrogenated with Pd/C (10%) as the catalyst to yield corresponding syn isomer, both in high diastereoselectivity.