Regioselectivite de la reduction de tropones de la quinoxaline

The reduction of quinoxalinotropones by some borohydrides is studied. We calculate the electronic structures, ab initio STO 3G method, of the various possible complexes and compare to the α-enones. It is demonstrated that the borohydride reduction of these tropones is under orbital frontier control and that the important element is carbonyl complexation by the cation.     

Etude de la stabilite de la conformation de copolypeptides alternes

Conformation stability of alternating copolypeptides with the repeating units $\text{Nε-}\text{carbobenzoxy}\text{-}\text{l}\text{-}\text{lysyl}\text{, β-}\text{benzyl}\text{-}\text{l}\text{-}\text{aspartate}\text{(}\text{CLL}\text{-β}\text{BLA}\text{)}\text{and}\text{Nε-}\text{carbobenzoxy}\text{-}\text{l}\text{-}\text{lysyl}\text{, γ-}\text{benzyl}\text{-}\text{l}\text{-}\text{glutamate}\text{(}\text{CLL}\text{-γ}\text{BLG}\text{)}$ has been studied by NMR 220 MHz and by viscosity in deuterochloroform/trifluoroacetic acid (TFA) pair at 25°. NMR spectra show a helix-coil transition at low TFA contents for both copolypeptides as compared to other types of copolypeptides. In the case of poly (CLL-γBLG), the helix begins to collapse at 4% TFA and the transition is complete at 6% TFA. Poly(CLL-βBLA) helix is still more fragile since the transition is complete at 3% TFA. Viscosity measurements seem to indicate helix coil transition to take place at lower TFA content but the disagreement between the two sets of results is only artificial. This low helix stability of both copolypeptides is discussed according to the Lifson—Roig—Cortijo theory taking into account formation of H-bonding between side-chains having different chemical structures. This low helix stability would be due to a co-operative collapsing effect.     

Developpement de la methode thermique de mesure de la tension superficielle liquide-solide

A thermal method connects the solid-liquid surface tension/s of a substance to its triple point temperature depression. Here a practical process for application of this method is described. A previous determination of the thickness of the condensate layer which does not freeze near the wall of the porous material is no longer necessary. The process is applied to some organic substances and to a metal (indium).

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Zusammenfassung Eine thermische Methode ermöglicht die Oberflächenspannung/s einer Substanz mit der Erniedrigung der Tripelpunkttemperatur in Bezug zu bringen. Eine vorangehende Bestimmung der Dicke der Kondensatschicht, deren Aggregatzustand sich bei Kontakt mit dem porösen Material nicht ändert, ist überflüssig. Die Methode wird auf einige organische Verbindungen und ein Metall (Indium) angewandt.

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Etude de la fragmentation de derives de L'uracile et de la thymine par Spectrometrie de masse

Original mass spectra of uracil and thymine derivatives are presented with the corresponding fragmentation schemes. In the first series of spectra, the fragmentations of thymine derivatives, including ¹⁴C-2-thymine, dimers, and bromo thymines, confirm the basic retro Diels-Alder mechanism. The second series includes dihydro 5,6-derivatives of uracil (hydroxy and bromo substituents). The behaviour of these molecules is quite different; they are more sensitive to the substituents and a part of the fragmentation is often explained by protonated molecular ions.     

Etude theorique de la premiere protonation de la proflavine

A comparison between theory and experiment (the last performed by J. H. Lhoste) gives consitent results pertaining to the difference in acid-base behaviour of proflavin in the ground state and in the excited singlet and triplet states. In particular, several facts emerge. An extraordinary inequality between the different pK′s of acid-base equilibria are indicated: p_triplet^K < p_fondamental^K < p_singulet^K A change occured by protonation in the electronic configuration of the lowest triplet state of that molecule and the variation of solvation energy during the absorption ground state-first excited singlet state plays an important part.     

Etude de la Fragmentation en Spectrographie de Masse de Spiropyrannes Benzodithioliques. Influence de la Position et de la Nature des Substituants

The mass spectra of 12 benzodithiolic spiropyrans are reported. Their fragmentation pathways have been determined by exact mass and metastable transition measurements. The nature of the fragmentation is strongly dependent upon the position and nature of the substituents. A comparison is made with their photo and thermochemical decompositions.     

Derives 14-hydroxyles de la vincadifformine et de la vincamine

Hydroboration-oxidation of tabersonine 1 yielded as major product 14 β-hydroxy vincadifformine 9b, which was correlated with 14 β-hydroxy N(1)-methyl 2β-H, 16β-H dihydro vincadifformine 6b (previously prepared and characterised), and 14α-hydroxy vincadifformine 9a as minor product. The regio- and stereoselectivity of hydroboration-oxidation were interpreted. 9a and 9b were respectively oxidised and rearranged to the corresponding 14-hydroxy vincamines 13a and 13b. The coupling constants of H(14) on the NMR of the acetyl derivatives of 9a, 9b, 13a and 13b are consistent with an inversion of N(4) during the rearrangement leading from the vincadifformine to the vincamine skeleton.     

Vinylallenes—VI : Synthese de cetones de la serie de la jasmone

Dehydrojasmone 1a and nor-methyl dehydrojasmone 1b have been prepared by action of a peracid on appropriately substituted vinylallenes. Several pathways to these hydrocarbons are described.     

Etude de l'action de la pression sur la pression osmotique des solutions de polymeres

Osmotic pressures of solutions of polymers were measured as a function of pressure. The measurements were made at room temperature at pressures up to 500 bars. The results are as follows:

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-the effect of pressure depends upon the polymer-solvent pair. Depending upon the particular case the effects can be positive, negative or zero. We have given examples for each case.     

Etude de la reaction chlorocarbene-acetals de cetenes : II. Stereoselectivite de la reaction

We have studied the stereoselectivity of the addition reaction of chloro and chloromethyl carbenoids with ketene alkylsilyl acetals. The best stereoselectivity was generally observed with the dimethyl tertiobutylsilyloxy group. With the chlorocarbenoid, using an E ketene acetal we obtained in majority (8?0%) an E α,β-ethylenic ester and using a Z ketene acetal we obtained in majority (7?0%) a Z α,β-ethylenic ester. In the case of the chloromethyl carbenoid the two ketene acetal isomers led to the same E α-substituted α,β-ethylenic ester (8?8% of selectivity). With the chlorophenylcarbenoid, formation of 9?0% of E α phenyl α,β-ethylenic ester is observed.     

Utilisation du calcul des modes normaux de vibration pour la détermination de la conformation de la glycylglycine et de la triglycine en solution aqueuse

The valence force field of α-glycylglycine was applied to different G matrices calculated for several values of the dihedral angles ψ₁, φ₂ and ψ₂. The relations v_i = ?(ψ_i,φ_i) obtained for different isotopic species were compared with the experimental frequencies of the aqueous solution thereby giving the absolute value of the angles. A complementary study by means of depolarised light scattering led to the determination of their sign. These relations were then used in the conformational analysis of triglycine in aqueous solution. The conformation of glycylglycine differs strongly in aqueous solution from that in the solid state, especially concerning the relative positions of the peptide and carboxylic group (φ₂ = ?70° and ?160°). For triglycine, we showed the existence of a chelated ring “C₇” similar to that observed for the acetylated dipeptides.     

Influence des effets de substituants sur la protonation de la pyridine

Standard thermodynamic values of proton ionisation of 3 substituted (Cl?, Br?, I?, C₂H₅? and CH₂CN?) pyridine derivatives are determined at 25°C, in an aqueous medium of ionic strength 0.5 M KNO₃.Free energies are deduced from equilibrium constants log K potentiometrically calculated. Enthalpies are obtained from calorimetric measurements.ΔG^o, ΔH^o and ΔS^o values are discussed in the context of the results of our earlier studies on this subject.The Hammett plot corresponding to the thirteen systems examined gives the proton ionisation constant of a substituted pyridine from the equation log K = 5,48 – 5,94Σσ.Besides, a linear relationship is found between ΔG^o and ΔH^o, which confirms the observation made previously in other series of pyridine derivatives.     

Etude de la nature des especes ioniques intervenant au cours de la polymerisation anionique de la vinyl-4 pyridine

Initiation of anionic polymerization of 4-vinyl-pyridine (4-VP) in tetrahydrofuran solution, using organo-metallic compounds of Na and K, involves the same type of side reactions as the polymerization of 2-vinyl pyridine (2-VP). The living polymers of 2- and 4-VP have similar u.v. spectra. The values of the dissociation constants of ionic species of 2- and 4-VP are similar when the chain [poly(styrene-b vinyl-4 pyridine)] contains one 4-VP unit (K_n ≈ 10^?9). But when the size of 4-VP block increases, the conductivity goes up to the value observed for living polystyrene (K_D ≈ 10^?7). This phenomenon could be due to a solvation of the counter-ion by the poly 4-vinyl pyridine chain, acting as a polydentate complexing agent.