Synthesis of 2′-O-α-d-ribofuranosyladenosine, monomeric unit of poly(ADP-ribose)

The first chemical synthesis of 2′-O-α-d-ribofuranosyladenosine, monomeric unit of poly(ADP-ribose), has been achieved starting from 3′,5′-O-bis protected 9-(2-O-α-d-arabinofuranosyl-β-d-ribofuranosyl)-adenine. Configurational inversion of 2′-hydroxyl group of arabinose moiety was performed by oxidation-reduction sequence.     

Rigid bis-zinc(II) salphen building blocks for the formation of template-assisted bidentate ligands and their application in catalysis

The template-induced formation of chelating bidentate ligands by the selective self-assembly of two monodentate pyridyl phosphorus ligands on a rigid bis-zinc(II) salphen template with two identical binding sites was studied. Using UV-vis, NMR-spectroscopy and X-ray analysis the formed structures were unambiguously proven. The application of these templated bidentate ligands in transition metal catalysis showed, in most cases, typical bidentate character. Compared to previous work based on a more flexible bis-zinc(II) porphyrin template, the current catalytic data suggest that the rigidity of the template is not an important factor for the improvement of the regio- and enantioselectivity under the applied reaction conditions.     

Palladium(II)-allyl complexes containing chiral N-donor ferrocenyl ligands

A study of the reactivity of enantiopure ferrocenylimine (S_C)-[FcCHN-CH(Me)(Ph)] {Fc =  (η⁵-C₅H₅)Fe{(η⁵-C₅H₄)-} (1a) with palladium(II)-allyl complexes [Pd(η³-1R¹,3R²-C₃H₃)(μ-Cl)]₂ {R¹ = H and R² = H (2), Ph (3) or R¹ = R² = Ph (4)} is reported. Treatment of 1a with 2 or 3 {in a molar ratio Pd(II):1a = 1} in CH₂Cl₂ at 298 K produced [Pd(η³-3R²-C₃H₄){FcCHN-CH(Me)(Ph)}Cl] {R² = H (5a) or Ph (6a)}. When the reaction was carried out under identical experimental conditions using complex 4 as starting material no evidence for the formation of [Pd(η³-1,3-Ph₂-C₃H₃){FcCHN-CH(Me)(Ph)}Cl] (7a) was found. Additional studies on the reactivity of (S_C)-[FcCHN-CH(R³)(CH₂OH)] {R³ = Me (1b) or CHMe₂ (1c)} with complex 4 showed the importance of the bulk of the substituents on the palladium(II) allyl-complex (2-4) or on the ferrocenylimines (1) in this type of reaction. The crystal structure of 5a showed that: (a) the ferrocenylimine adopts an anti-(E) conformation and behaves as an N-donor ligand, (b) the chloride is in acis-arrangement to the nitrogen and (c) the allyl group binds to the palladium(II) in a η³-fashion. Solution NMR studies of 5a and 6a and [Pd(η³-1,3-Ph₂-C₃H₃){FcCHN-CH(Me)(CH₂OH)}Cl] (7b) revealed the coexistence of several isomers in solution. The stoichiometric reaction between 6a and sodium diethyl 2-methylmalonate reveals that the formation of the achiral linear trans-(E) isomer of Ph-CHCH-CH₂Nu (8) was preferred over the branched derivative (9). A comparative study of the potential utility of ligand 1a, complex 5a and the amine (S_C)-H₂N-CH(Me)(Ph) (11) as catalysts in the allylic alkylation of (E)-3-phenyl-2-propenyl (cinnamyl) acetate with the nucleophile diethyl 2-methylmalonate (Nu⁻) is reported.     

Multiple ionization of argon in coincidence with projectile ions in 60–120 MeV Si q+-Ar collisions

A projectile ion-recoil ion coincidence technique has been employed to study the multiple ionization and the charge transfer processes in collisions of 60–120 MeV Si ^q+ (q = 4−14) ions with neutral argon atoms. The relative contribution of different ionization channels, namely; direct ionization, electron capture and electron loss leading to the production of slow moving multiply charged argon recoil ions have been investigated. The data reported on the present collision system result from a direct measurement in the considered impact energy for the first time. The total ionization cross-sections for the recoil ions are shown to scale as q ^1.7/E _p ^0.5 , where E _p is the energy in MeV of the projectile and q its charge state. The recoil fractions for the cases of total- and direct ionizations are found to decrease with increasing recoil charge state j. The total ionization fractions of the recoils are seen to depend on q and to show the presence of a ‘shell-effect’ of the target. Further, the fractions are found to vary as 1/j ² upto j = 8+. The average recoil charge state 〈j〉 increases slowly with q and with the number of lost or captured electrons from or into the projectile respectively. The projectile charge changing cross-sections σ _qq′ are found to decrease with increasing q for loss ionization and to increase with q for direct-and capture ionization processes respectively. The physics behind various scaling rules that are found to follow our data for different ionization processes is reviewed and discussed.     

Estimation of parameters of non-linear dynamical systems

A method is proposed to describe non-linear systems without memory, forced by random excitation. The concept of statistical linearization is extended to an approximation with a series of functions. The parameters of these functions describing the non-linearity are estimated in the presence of noise by using correlation techniques. The results apply also if the non-linearity cannot be separated from a linear dynamic model.     

Solution Structure of a RNA Decamer Duplex,Containing 9‐[2‐O‐(β‐D‐ribofuranosyl)‐β‐D‐ribofuranosyl]adenine,a Special Residue in Lower Eukaryotic Initiator tRNAs

The solution structure of the self‐complementary deca‐ribonucleotide 5′‐r(GCGA*AUUCGC)‐3′ containing 9‐[2‐O‐(β‐D ‐ribofuranosyl)‐β‐D ‐ribofuranosyl]adenine (A*), a modified nucleotide that occurs in lower eukaryotic methionine initiator tRNAs (tRNAs_i^Met), was determined by NMR spectroscopy. Unexpectedly, the modification has no effect on the thermal stability of the duplex. However, the extra ribose moiety is in the C(3′)‐endo conformation and takes up a well‐defined position in the minor groove, which is in agreement with its position in tRNAs_i^Met as determined by X‐ray crystallography. Molecular‐dynamics simulations on the RNA duplex in H₂O show that the position of the extra ribofuranose moiety seems to be stabilized by bridged H‐bonds (mediated by two H₂O molecules) to the backbone of the complementary chain.     

The effect of the amido substituent on polymer molecular weight in propene homopolymerisation by titanium cyclopentadienyl‐amide catalysts

In the homopolymerisation of propene by the cyclopentadienyl‐amide titanium catalyst systems [η⁵,η¹‐C₅H₄(CH₂)₂NR]TiCl₂/MAO and [η⁵,η¹‐C₅H₄(CH₂)₂NR]Ti(CH₂Ph)₂/B(C₆F₅)₃ (R = ^tBu, ⁱPr, Me), the catalyst with the smallest substituent (Me) on the amido moiety consistently gives the highest polymer molecular weight. This differs from the trend usually observed in related catalysts with tetramethylcyclopentadienyl‐amide ancillary ligands, where larger amide substituents result in higher molecular weights. Based on the present information a hypothesis is formulated in which an increased cation‐anion interaction for the less sterically hindered catalyst is responsible for disfavouring chain transfer relative to chain growth.     

Mechanistic Implications of the Observation of Kinetic Resolution in a Palladium-Catalyzed Enantioselective Allylic Alkylation

Preferential rotation in substrate—palladium intermediates in a catalyzed asymmetric allylic alkylation is proposed to be responsible for both the observed kinetic resolution of the racemic allylic acetate starting material as well as the high selectivity found in the enantiodiscriminating product-forming step [Eq. (a)].