Silylation partielle et réduction de composés gem-polychlorés fonctionnels

The trimethylchlorosilane/magnesium/hexamethylphosphoric triamide (HMPT) system reacts with some functional polychlorinated compounds (chloral, hexachloroacetone, acetone chloroform) affording a partial reduction accompanied by silylation and leads to new functional silylated derivatives.These reactions exhibit reductive properties of magnesium, in HMPT, towards the CCl bond.     

Spontaneous ring transformation of a 5-azido-1,2,3-thiadiazole

The reaction of 4-carbethoxy-5-chloro-1,2,3-thiadiazole ($\text{1}$) with sodium azide results in the formation of ethyl α-thiatriazolyldiazoacetate ($\text{3}$) instead of the corresponding azide ($\text{2}$). Two plausible mechanisms for this new rearrangement are formulated.     

Crystallographic evidence of hexacoordination at phosphorus via intramolecular coordination of donor groups on phosphane and phosphane sulphide

The X-ray structure analysis of bis(8-dimethylamino-l-naphthyl)phenylphosphane (3) and of the corresponding sulphide 4 has revealed hexacoordination at phosphorus in both cases, the N … P separations being less than the sum of the van der Waal radii. Furthermore, in both cases the overall geometry corresponds to a distorted bicapped tetrahedron. The optimum geometry calculated for 4 via the program developed by Autodesk (MM + method) suggests that the structure of the molecule is a function not only of steric requirements but also of electronic effects.     

Synthesis, Conformational Studies, X-ray Structures, and Complexation Properties of Semirigid Macrocyclic Phosphonamides

The semirigid phosphonamide ligands 1-5 have been synthesized from the macrocyclic precursors 6-9 by reaction with 1,3-propanediol ditosylate or 1,2-dichloroethane. For the thiophosphoryl compounds 1 and 2, and the phosphoryl derivative 5, the reactions were carried out in biphasic aqueous NaOH solutions. The phosphoryl derivatives 3 and 4 were better obtained from NaH in anhydrous tetrahydrofuran. The conformations of the hosts in solution were deduced from low-temperature NMR and NOE difference experiments. Conformational equilibria between exo and endo forms are observed for the 18-membered macrocycles 1 and 3. The exo conformer predominates in solution for the 21-membered macrocycle 2, whereas 4 exists as rapidly exchanging conformers. The X-ray crystal structures of macrocycles 1, 2, and 5 have been determined as well as the complexes 1.Hg(SCN)(2) and 5.LiNO(3). In the Hg(2+) complex the metal ion is located out of the macrocyclic cavity and is coordinated to the thiophosphoryl unit. In 5.LiNO(3)()()the Li(+) cation is located inside the macrocyclic cavity and is coordinated to a tetrahedral array of oxygen donors. Free energies of complexation (DeltaG degrees ) of the phosphorylated ligands 3-5 with alkali metal and ammonium cations were determined in CHCl(3) saturated with H(2)O by picrate extraction experiments. The -DeltaG degrees values are greatest for 4 complexing K(+) and NH(4)(+) (7.3 and 8.0 kcal/mol, respectively). The relationships between structure and binding are discussed.     

Dirichlet Series and Curious infinite Products

By number-theoretical methods we give a result generalizingthose of J. O. Shallit, one particular case of which reads asfollows: ...=2^–;, where a(n) is equal to 1 if the sum of the digits of n in basetwo is even, and – 1 if this sum is odd. Moreover we provea conjecture of Shallit concerning a way of approximating 2^–     

Reductive cleavage of axially disymmetric tertiary amines and quaternary ammonium salts by lithium aluminium hydride. Synthesis of new 1,1′-binaphthyl substituted amines

Axially disymmetric tertiary amines or quaternary ammonium salts $\text{A}$ are synthesized by double alkylation of primary or secondary amines with racemic or optically pure 2,2′-bis (bromomethyl)-1,1′-binaphthyl. Their reductive cleavage by lithium aluminium hydride in refluxing THF leads to chiral secondary or tertiary amines $\text{B}$, substituted by a binaphthyl unit, with high yields and absence of racemization.     

Facile Preparation of (±)-12-Epiprostaglandins from 7-Oxabicyclo[2.2.1]hept-5-en-2-one via an all-cis-formyllactone related to Corey lactone

The bicyclic monoselenoacetal 7 , easily obtained from (±)-7-oxabicyclo[2.2.1]hept-5-en-2-one ( 6 ) via a radical addition-acyl migration sequence, was converted to racemic 12-epiprostaglandins 3 and 4 . The key intermediate was the all-cis-formyllactone 2b related to Corey lactone (see 12 ; Scheme 1). The presence of a (tert-butyl)-dimethylsilyl protective group for the 11-OH substituent (prostaglandin numbering) was found to be crucial in avoidingβ -elimination and epimerization during the Wittig-Horner reaction (Scheme 2). Epimerization at C(12) at the formyllactone stage (see 2b ) was also possible and gave the known precursor 1b of naturally occurring prostaglandins and analogs.     

Synthesis of 15N-enriched pseudouridine derivatives

[reaction: see text]. A procedure for the chemical synthesis of [3-15N]-labeled pseudouridine and a methylated derivative was developed. A suitably protected pseudouridine precursor was nitrated at N3 followed by treatment with 15NH4Cl to afford the 15N-labeled product in six steps with a 20% yield. This methodology will allow for the production of RNAs with [3-15N]pseudouridine and [3-15N-methyl]pseudouridine at specific locations.