An extension of the linear stability analysis for the prediction of multiple necking during dynamic extension of round bar

Linear stability analysis has been widely used in order to describe the evolution of the dominant necking pattern in different configurations. Effects of hardening, strain rate and temperature sensitivity, and effects of configuration geometry and loading, have been established by this mean. However, experimental and numerical observations have demonstrated that a whole distribution of spacing between necks is obtained instead of a unique dominant pattern. In this work, an extension of the classical linear stability analysis applied to the dynamic extension of a round bar case has been developed to take into account the contribution of all perturbation modes on the preliminary evolution of pre-necks. This approach, corresponding physically to the case of thin ring expansion, is able to determine a distribution of pre-neck spacing. This distribution, starting from the initial perturbation pattern, evolves with time so that it is finally centered around a dominant distance determined by the linear stability approach.     

Synthesis of Regioisomeric Functionalized Benzodifurans and Angelicins

Arenofurans have important biological and pharmacological activities. Compared to benzofurans, the reports on the synthesis of benzodifurans are rather limited. Here, we report the synthesis of a linear and an angular 3,3′‐bis(carboxymethyl)substituted benzodifuran and 4′‐carboxymethyl‐substituted angelicins from phloroglucinol, using 4‐halomethyl‐substituted dipyrones as key intermediates in the synthetic route. This strategy shows that the stability of a pyrone ring depends on the type of substituent at C(4) and the conditions used.     

Nucleophilic cleavage of lactones and esters with zinc selenolates prepared from diselenides in the presence of Zn/AlCl3

The utility of zinc selenolates for effecting nucleophilic cleavage of simple lactones and esters has been investigated. When zinc selenolate generated via Zn/AlCl₃-promoted cleavage of diselenides was reacted with simple lactones and esters, efficient nucleophilic alkyl-oxygen bond cleavage proceeded generating the corresponding carboxylic acids in moderate to excellent yields.     

Palladacycle as highly efficient catalyst for ring opening of oxabicyclic alkenes with organozinc halides

Ring opening reaction of oxabicylic alkenes 4 with in situ prepared organozinc halides 5 was catalyzed by palladacycle 3 with high efficiency. Good yields of the corresponding 1,2-dihydronaphth-1-ols (6) were provided when as low as 0.05 mol% of palladacycle 3 was used. ³¹P NMR study showed that the skeleton of 3 remained intact in the reaction, which implied that palladacycle 3 did not serve as a catalyst precursor but a catalyst in the reaction.     

5-Spirocyclopropane Isoxazolidines as Versatile Intermediates in Organic Synthesis

Summary. The review outlines the various different azaheterocycles available from 5-spirocyclopropane isoxazolidines. These compounds can be regarded as versatile direct precursors of tetrahydro-, dihydropyridone and azetidinone derivatives. The current scope and limitations, and the selectivity aspects of the different processes are discussed therein.     

Synthesis and glycosidase inhibitory activity of new penta-substituted C8-glycomimetics

The syntheses of new C8-carbasugars and -aminocyclitols related to miglitol and voglibose are described. The key step involves the ring closing metathesis of 1,9-dienes derived from d-mannitol. Chemical transformations of the newly created double bond of the resulting cyclooctenes involved notably hydroboration and reductive amination. The inhibitory activity of the glycomimetics so-obtained has been evaluated towards 24 commercially available glycosidases.     

Pyrolysis of Sulfones as a Synthetic Method [New synthetic methods (28)]

Cyclic sulfoness containing structural elements such as aromtic rings, heteroatoms. functional groups, and further SO₂-groups as ring members decompose on heating with cleavage of SO₂ and formation of a new C? C bond. In the last decade this “sulfone pyrolysis” has been expanded into a generally applicable method even allowing the synthesis of sterically strianed medium-membered and multi-membered cyclic and polycyclic systems containing aromatic ring. By the pyrolysis of sulfones which are only unilateally activated by benzyl moieties, aromatic systms can be bridged by ? (CH₂)_n chains of any desired length. In addition, ? (CH₂)? chains can be split off together with two SO₂ molecules, with recombination of the remaining centers, resulting in ring contraction by four to n atoms. However, sulfone pyrolysis is of importance not only as a ring-contraction method but as a crucial final step in the synthesis of multi-membered hydrocarbon cycles, e.g. of the phane type.     

The effect of fluorine atoms on gas transport properties of new polynorbornene dicarboximides

The new N-4-trifluoromethylphenyl-norbornene-5,6-dicarboximide (2a) and N-3,5-bis(trifluoromethyl)phenyl-norbornene-5,6-dicarboximide (2b) mixtures of exo and endo monomers were synthesized and polymerized via ring opening metathesis polymerization (ROMP) using bis(tricyclohexylphosphine) benzylidene ruthenium(IV) dichloride (I) and tricyclohexylphosphine [1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene][benzylidene] ruthenium dichloride (II) to produce the corresponding polynorbornene dicarboximides Poly-2a and Poly-2b, respectively. The transport of five gases He, N₂, O₂, CO₂ and CH₄ across membranes prepared from Poly-2a was determined at 35 °C using a constant volume permeation cell. The gas transport properties of the fluorine containing polymer Poly-2a were compared with those found for membranes from non-fluorinated poly(N-phenyl-exo-endo-norbornene-5,6-dicarboximide) (P-PhNDI). Gas permeability, diffusion and solubility coefficients of the fluorine containing polynorbornene Poly-2a were up to an order of magnitude larger than those of the non-fluorinated one. Poly-2a was found to have one of the largest gas transport coefficients reported to date in glassy polynorbornene dicarboximides.     

Ring opening of 2-acylaziridines by acid chlorides

Good nucleophilicity of the ring nitrogen in chiral (2R,1′R)-2-acyl-(1′-phenylethyl)aziridines initiated the reaction with various acid chlorides to form the corresponding acylaziridinium ion intermediates whose rings were opened by the chloride anion to yield the β-amino-α-chlorocarbonyl compounds. The subsequent displacement of the chloride with the internal oxygen nucleophile originated from methylchloroformate, acetyl chloride, and methyl chlorooxoacetate yielded oxazolidin-2-ones, β-amino-α-acetyloxypropionates, and morpholin-2,3-diones, respectively.     

The first ring inversion of pyranoses induced by bulky silyl protections at the 2- and 3-positions

The pyranose rings of the 2,3-bis-O-tert-butyldiphenylsilyl-α- and β-d-glucopyranoses, and of the 2,3-bis-O-tert-butyldimethylsilyl-β-d-glucopyranose were in the ¹C₄ form. These findings indicate that the introduction of bulky silyl protecting groups at the 2- and 3-positions can flip a pyranose ring into the axial-rich chair form. Previous such ring inversions have been carried out by the silyl protections at the 3- and 4-positions.