Synthesis of 6,19-sulfamidate bridged pregnanes

Conformationally restrained substituted pregnane-20-one derivatives were obtained by an intramolecular nitrene addition onto a C-5/C-6 double bond involving a tethered C-19 sulfamoyl moiety. The resulting aziridine underwent regioselective nucleophilic ring opening at C-5 at room temperature with cyanide, fluoride, and acetate. In the isolated case of acetate, a reversal of regioselectivity was observed at higher temperatures, a result attributed to a rearrangement process involving aziridine ring opening at the C-5 position and subsequent migration of the acetyl moiety to C-6.     

New evidence of confinement effects in mesoporous materials and the definition of confined Pitzer acentric factors

In the present work, the corresponding states principle is proposed as a new approach to clarify the comparison between adsorption-desorption isotherms obtained on porous solids. The applicability of this principle at the capillary critical point in adsorption-desorption isotherms is demonstrated. Deviations of the Clausius-Clapeyron curves in reduced coordinates are interpreted in terms of polarizability, and those are perfectly correlated using analogous Pitzer acentric factors in confined environments. New acentric factors are proposed to take confinement effects in mesoporous materials into account. Enthalpic excesses during capillary condensation could be predicted, and those are supported by calorimetric experiments.     

Synthesis of chiral α-substituted N-[((2S)-2-hydroxy-2-phenyl)-ethyl]-2-phenylglycine derivatives by diastereocontrolled alkylation of (6 R)-2,3,5,6-tetrahydro-3,6-diaryl-N-[(2′R)-(2′-methyl)phenyl-methyl]-4H-1,4-oxazin-2-ones

The synthesis of α-substituted N-[((2S)-2-hydroxy-2-phenyl)-ethyl]-2-phenylglycine derivatives is reported. The key step of the sequence is the highly diastereoselective alkylation of (6R)-2,3,5,6-tetrahydro-3,6-diaryl-N-[(2′R)-(2′-methyl)phenylmethyl]-4H-1,4-oxazin-2-ones after deprotonation with t-BuOK. Opening of the resulting oxazinone with ethanolic KOH, followed by hydrogenolysis of the corresponding N-[(2R)-(2-methyl)phenylmethyl] compound to furnish the expected 2-phenylglycine derivative, is also described.     

Reactions CO/H2 en phase liquide. Synthese de precurseurs de l'ethanol par homologation du methanol.: I. Influences de la temperature et de la pression

The CO/H₂ homologation of methanol to acetaldehyde and subsequently to its dimethyl acetal in the presence of cobalt acetate promoted by iodine was examined under various conditions. Temperature and pressure were found as critical parameters. High pressures (140 MPa) and low temperatures (160–170°C) give optimal yields and selectivity to acetaldehyde. According to pressure, temperature, contact time, gas ratio and ligand/catalyst ratio, the reaction is oriented towards acetaldehyde, its dimethyl acetal or methyl acetate.     

A new GROMOS force field for hexopyranose-based carbohydrates

A new parameter set (referred to as 45A4) is developed for the explicit-solvent simulation of hexopyranose-based carbohydrates. This set is compatible with the most recent version of the GROMOS force field for proteins, nucleic acids, and lipids, and the SPC water model. The parametrization procedure relies on: (1) reassigning the atomic partial charges based on a fit to the quantum-mechanical electrostatic potential around a trisaccharide; (2) refining the torsional potential parameters associated with the rotations of the hydroxymethyl, hydroxyl, and anomeric alkoxy groups by fitting to corresponding quantum-mechanical profiles for hexopyranosides; (3) adapting the torsional potential parameters determining the ring conformation so as to stabilize the (experimentally predominant) (4)C(1) chair conformation. The other (van der Waals and nontorsional covalent) parameters and the rules for third and excluded neighbors are taken directly from the most recent version of the GROMOS force field (except for one additional exclusion). The new set is general enough to define parameters for any (unbranched) hexopyranose-based mono-, di-, oligo- or polysaccharide. In the present article, this force field is validated for a limited set of monosaccharides (alpha- and beta-D-glucose, alpha- and beta-D-galactose) and disaccharides (trehalose, maltose, and cellobiose) in solution, by comparing the results of simulations to available experimental data. More extensive validation will be the scope of a forthcoming article. (c) 2005 Wiley Periodicals, Inc. J Comput Chem 26: 1400-1412, 2005.     

Synergistic effects of ion-pairing in the enantiomeric separation of basic compounds with cyclodextrin derivatives in nonaqueous capillary electrophoresis

The enantiomeric separation of various kinds of basic pharmaceuticals has been investigated in nonaqueous capillary electrophoresis (NACE) systems using an ion-pairing reagent in combination with cyclodextrins (CDs). The simultaneous addition to the methanolic background electrolyte (BGE) of (+)-S-camphorsulfonate or alkanesulfonates and an anionic beta-cyclodextrin derivative, heptakis(2,3-dimethyl-6-sulfato)-beta-cyclodextrin (HDMS-beta-CD), led to partial or complete enantioresolution in most cases. In the absence of ion-pairing reagent, the enantiomeric resolution obtained with this CD derivative was most often completely lost or strongly reduced, indicating the important role of ion-pairing in the chiral recognition mechanism in these NACE systems. The influence of the nature and concentration of the counterion and the anionic CD derivative on the enantioseparation of basic compounds was studied. Synergistic effects between these two kinds of charged additives were clearly observed.     

A direct synthesis of 1-aryl- and 1-alkenylcyclopropylamines from aryl and alkenyl nitriles

The reaction of various aromatic nitriles with 1.1 equiv of Ti(Oi-Pr)(4) and 2.2 equiv of EtMgBr followed by addition of a Lewis acid gave 1-aryl cyclopropylamines in 43-76% yields. Under similar conditions, conjugated alkenenitriles afford 1-alkenylcyclopropylamines (42-65%).     

Rh(I) coordination chemistry of chiral alpha-aminophosphine(eta6-arene)chromium tricarbonyl ligands

The diastereoselective addition of Ph(2)PH to the chiral ortho-substituted eta(6)-benzaldimine complexes (eta(6)-o-X-C(6)H(4)CH=NAr)Cr(CO)(3) (1, X = MeO, Ar = p-C(6)H(4)OMe; 2, X = Cl, Ar = Ph) leads to the formation of the corresponding chiral aminophosphines (alpha-P,N) Ph(2)P-CH(Ar(1))-NHAr(2) (3, Ar(1) = o-C(6)H(4)(OCH(3))[Cr(CO)(3)], Ar(2) = p-C(6)H(4)OCH(3); 4, Ar(1) = o-C(6)H(4)Cl[Cr(CO)(3)], Ar(2) = Ph) in equilibrium with the starting materials. The uncomplexed benzaldimine (o-ClC(6)H(4)CH=NPh), 2', analogously produces an equilibrium amount of the corresponding aminophosphine Ph(2)P-CH(Ar(1))-NHAr(2) (4', Ar(1) = o-C(6)H(4)Cl, Ar(2) = Ph). Depending on the equilibrium constant, the subsequent addition of (1)/(2) equiv of [RhCl(COD)](2) (COD = 1,5-cyclooctadiene) leads to either Ph(2)PH oxidative addition in the case of 3 or to the corresponding [RhCl(COD)(alpha-P,N)] complexes [RhCl(COD)(Ph(2)P-CH[o-C(6)H(4)Cl[Cr(CO)(3)]]-NHPh)] (5) and [RhCl(COD)(Ph(2)P-CH(o-C(6)H(4)Cl)-NHPh)] (5') in the cases of the aminophosphines 4 and 4'. The addition of the latter ligands, as racemic mixtures, to (1)/(4) equiv of [Rh(CO)(2)Cl](2) leads to the [RhCl(CO)(alpha-P,N)(2)] complexes [RhCO(Ph(2)P-CH[o-C(6)H(4)Cl[Cr(CO)(3)]]-NHPh)(2)Cl] (7) or [RhCO(Ph(2)P-CH(o-C(6)H(4)Cl)-NHPh)(2)Cl] (7') as mixtures of (R(C),S(C))/(S(C),R(C)) and (R(C),R(C))/(S(C),S(C)) diastereomers. The rhodium complexes 5 and 7' have been fully characterized by IR and (31)P NMR spectroscopies and X-ray crystallography. These compounds exhibit intramolecular Rh-Cl.H-N interactions in the solid state and in solution. The stability of the new rhodium complexes has been studied under different CO pressures. Under 1 atm of CO, 5 is converted to an unstable complex [RhCl(CO)(2)(alpha-P,N)], 6, which undergoes ligand redistribution leading to 7 plus an unidentified complex. This reaction is inhibited under higher CO or syngas pressure, as confirmed by the observation of the same catalytic activity in hydroformylation when styrene was added to a catalytic mixture that was either freshly prepared or left standing for 20 h under high CO pressure.     

A computational study of radical haloacetal cyclizations controlled by the acetal center

The stereochemical outcome of the radical haloacetal cyclization reaction (Ueno-Stork reaction) has been examined by ab initio and other molecular orbital techniques. It was found that the stereochemistry of 5-exo- and 6-exo trig cyclizations can be accurately predicted from calculations using moderate levels of theory (UHF/6-311G** or B3LYP/6-311G**). A simplified computational procedure, easily run on a standard desktop computer, has been developed that provides excellent predictive ability for the stereochemical outcome for the reactions in question. Interestingly, a novel twist transition state has been identified for the first time in 5-exo-trig radical cyclization reactions.     

Direct evidence for highly organized networks of circular surface micelles of surfactant at the air-water interface

We report the first direct evidence for the formation of circular surface micelles (hemimicelles) on the surface of water. These highly monodisperse 30 nm hemimicelles, made from a semifluorinated alkane deposited as a Langmuir monolayer, form organized hexagonal arrays as determined by small-angle X-ray diffraction conducted directly on the water surface at grazing incidence.