Cyclosulfamide as a chiral auxiliary: application to efficient asymmetric synthesis (alkylation/aldolization)

The chiral cyclosulfamide (S)-2-benzyl-4-isopropyl-1,2,5-thiadiazolidine 1,1-dioxide was designed as a chimera of Evans and Oppolzer chiral auxiliaries. The N-propionyl derivative appeared to be very powerful for the stereocontrolled synthesis of chiral building blocks through asymmetric aldolization and alkylation reactions.     

Separation of free sterols by high temperature liquid chromatography

Increasing the column temperature accelerates markedly elution in HPLC. The separation of five free sterols was studied on three packing materials that can withstand high temperatures. These stationary phases included graphitic carbon, a polymeric C18 silica, and a zirconia-based adsorbent. Measurements of retention data were made at up to 150 degrees C with mobile phases of different compositions. Since the columns tested afford different retention mechanisms, a variety of elution patterns were observed, with some being more advantageous than others for certain sterol separations. Effects observed include some selectivity improvements and some elution order reversals. The separation of free sterols in selected fruit juices is also presented. Albeit at the expense of a longer analysis time, the graphitic carbon column produced the best separation of the sterols in this study.     

Characterizing the Memory Effect on the amylose tris(3,5-dimethylphenyl) carbamate stationary phase

Acid/base modifiers are sometimes used as additives in normal phase elution on columns packed with CHIRALPAK^® AD^®. They affect enantioseparations in ways that are not understood for the lack of systematic studies, which makes the scale-up of preparative separations difficult to predict. Once a column has been exposed to these modifiers, the selectivity of certain pairs of enantiomers may change, for the better or the worse. Numerous molecules that are affected by this phenomenon are listed in the literature. We selected five of them, the selectivity of which changes as more acidic or basic solutions are percolated through the column. The selectivity of ketoprofen, 4-chlorophenylalanine methyl and ethyl esters improves as a solution of Ethanesulfonic Acid is percolated through the column. The selectivity of Propranolol HCl and Tröger’s base increases as a solution of diisopropylamine is percolated through the column. Each one of these five compounds is inversely affected by the percolation of the opposite acid/base solution. We used trans-Stilbene Oxide (tso) as a “standard” to determine the columns stability because no Memory Effect is observed for it (its retention, enantioselectivity, and resolution remain constant). Karl Fisher titrations showed that only slight changes in the water content of the mobile phase occurred, and that a unique water to polymer moiety ratio is important. Analytical studies of the stationary phase suggest that slow protonation/deprotonation of water attached to the carbamate moiety may be responsible for the acid/base Memory Effect. Finally, we showed that the Memory Effect can be minimized by percolating through the column a sufficiently concentrated solution of the appropriate acid or base. Thus, columns that were unreliable for method development, due to the Memory Effect, can now be used. As a result, the need to store several CHIRALPAK AD columns, specific for each condition of the Memory Effect, is eliminated.     

Investigation of the adsorption mechanism of a peptide in reversed phase liquid chromatography,from pH controlled and uncontrolled solutions

The single-component equilibrium adsorption of the tripeptide Leucyl-Leucyl-Leucine (LLL) on a high-efficiency Jupiter Proteo column (C₁₂$C_{12}$) was investigated experimentally and modeled theoretically. The experimental equilibrium isotherms of LLL for adsorption on a C₁₂$C_{12}$ packing material from an aqueous solution of methanol (48%) and trifluoroacetic acid (0.1%) were measured by frontal analysis (FA). The FA measurements were done with two solutions, one in which the pH was controlled, the other in which it was not. Two solutions of LLL in the mobile phase were prepared (4.3 and 5.4 g/L) and their pH measured (2.94 and 2.88), respectively. The first solution was titrated with TFA to match the pH of the mobile phase (2.03), so its pH was controlled. The pH of the other solution was left uncontrolled. In both cases the isotherms could be modeled by a bi-Langmuir equation, a choice consistent with the bimodal affinity energy distribution (AED) obtained for LLL. The isotherm parameters derived from the inverse method (IM) of isotherm determination under controlled pH conditions (by fitting calculated profiles to experimental breakthrough profiles) are in a good agreement with those derived from the FA data. Under uncontrolled pH conditions, the application of IM suggests the coexistence of two different adsorption mechanisms. According to the isotherm parameters found by these three methods (FA, AED and IM), the C₁₂$C_{12}$-bonded silica can adsorb around 500 and 70 g/L of LLL under controlled and uncontrolled pH conditions, respectively. The adsorption of LLL on the C₁₂$C_{12}$ material strongly depends on the pH of the mobile phase and on the quantity of TFA added, which plays the role of an ion-pairing agent.     

Theoretical advantages and drawbacks of on-line,multidimensional liquid chromatography using multiple columns operated in parallel

The theoretical advantages and drawbacks of using a multiple-, parallel column approach in on-line multidimensional liquid chromatography systems were investigated. Much time or peak capacity can be gained with the use of multiple parallel columns at the second-dimension while the aggregate time of separation increases only by the increment of the gradient time of the second-dimension. Multidimensional chromatographic systems are now used to perform many tasks ranging from routine, fast analyses to specialized, arduous separations. In this work, we focus on the advantages of a multiple, parallel columns approach to on-line multidimensional liquid chromatography systems. Calculations of the achievable peak capacities were made as functions of the number of columns operated in parallel. Increasing the number of second-dimension columns from one to two or three causes the largest increase in peak capacity with only a slight increase of aggregate time. We also present some practical aspects to consider when attempting multidimensional separations with multiple columns operated in parallel.     

Electronic Properties and Photovoltaic Performances of a Series of Oligothiophene Copolymers Incorporating Both Thieno[3,2‐b]thiophene and 2,1,3‐Benzothiadiazole Moieties

A series of donor‐acceptor alternated conjugated copolymers, composed of thiophene, bithiophene, thieno[3,2‐b]thiophene, and 2,1,3‐benzothiadiazole units and differing from each other by the nature and the number of 3‐alkylthiophene in the backbone, have been synthesized by Stille cross‐coupling polymerization. The material's optical and electrochemical properties, in solution and in thin films, have been investigated using UV‐Visible absorption and cyclic voltammetry. Bulk heterojunction solar cells using blends of the newly synthesized copolymers, as electron donor, and C60‐PCBM or C70‐PCBM, as electron transporting material, have been elaborated. A maximum power conversion efficiency of 1.8% is achieved with a 1:4 PPBzT²‐C12:C70‐PCBM weight ratio.

     

Nd:GdVO4 as a three-level laser at 879 nm

We present what we believe to be the first true three-level laser based on a Nd-doped crystal. From the 4F3/24I9/2 laser transition, the lower laser level being the ground state, emission at 879 nm in NdGdVO4 has been obtained with diode pumping. Up to 0.8 W of power has been achieved in cw operation and 24 microJ per pulse (35 ns) in the Q-switched regime. Intracavity second-harmonic generation in the pulsed regime is also demonstrated with 17 8 mW of average output power at 439.5 nm, corresponding to an energy of 17microJ per pulse.     

The alpha-gamma transition of cerium is entropy driven

We emphasize, on the basis of experimental data and theoretical calculations, that the entropic stabilization of the gamma phase is the main driving force of the alpha-gamma transition of cerium in a wide temperature range below the critical point. Using a formulation of the total energy as a functional of the local density and of the f-orbital local Green's functions, we perform dynamical mean-field theory calculations within a new implementation based on the multiple linear muffin tin orbital (LMTO) method, which allows us to include semicore states. Our results are consistent with the experimental energy differences and with the qualitative picture of an entropy-driven transition, while also confirming the appearance of a stabilization energy of the alpha phase as the quasiparticle Kondo resonance develops.     

Design of a Linear Poly(3‐hexylthiophene)/Fullerene‐Based Donor‐Acceptor Rod‐Coil Block Copolymer

Novel fullerene‐grafted poly(3‐hexylthiophene) (P3HT)‐based rod‐coil block copolymers have been synthesized. The regioregular P3HT rod block has been synthesized by a modified Grignard metathesis reaction (GRIM). An original in situ end‐capping reaction has been developed in order to convert the P3HT block into an efficient macro‐initiator for the nitroxide‐mediated radical polymerization (NMRP) of the coil block. Controlled radical polymerization of the second poly(butylacrylate‐stat‐chloromethylstyrene) [P(BA‐stat‐CMS)] block has been done through various conditions leading to different coil block lengths. The final electron donor‐acceptor block copolymer has been obtained after C₆₀ grafting in soft conditions. Copolymers have been characterized by ¹H NMR and size exclusion chromatography. Optical characterizations, before and after C₆₀ grafting, are reported.

     

Wigner-Type Theorems for Projections

The Wigner theorem, in its Uhlhorn’s formulation, states that a bijective transformation of the set of all one-dimensional linear subspaces of a complex Hilbert space which preserves orthogonality is induced by either a unitary or an antiunitary operator. There exist in the literature many Wigner-type theorems and the purpose of this paper is to prove in an algebraic setting a very general Wigner-type theorem for projections (idempotent linear mappings). As corollaries, Wigner-type theorems for projections in real locally convex spaces, infinite dimensional complex normed spaces and Hilbert spaces are obtained.