Asymmetric syntheses of dihydroxyhomoprolines via doubly diastereoselective lithium amide conjugate addition reactions

The asymmetric syntheses of novel dihydroxyhomoprolines have been achieved using the doubly diastereoselective conjugate additions of the antipodes of lithium N-benzyl-N-(α-methylbenzyl)amide to a set of four chiral α,β-unsaturated esters (derived from d-pentoses) as one of the key steps. A full account of the diastereoselectivity observed in these conjugate additions is presented and the stereochemical outcomes of these reactions have been established unambiguously via a combination of hydrogenolytic chemical correlation and single crystal X-ray diffraction analyses. A tandem hydrogenolysis/intramolecular reductive amination reaction was then used to create the corresponding enantiopure pyrrolidines, providing access to (2′S,3′S,4′R)-dihydroxyhomoproline and (2′S,3′R,4′S)-dihydroxyhomoproline after deprotection.     

Influence of the photoionization process on the fragmentation of laser desorbed polycyclic aromatic hydrocarbons

Characterization of polycyclic aromatic hydrocarbons (PAHs) samples has been performed by laser desorption combined with multi-photon ionization technique using two different geometries of the ionization laser beam. This comparative study evidences the strong influence of ionization laser fluence on PAH fragmentation. Through a ∼10³ enlargement of the ionization probe volume and 10⁴ reduction of laser fluence over previous studies, fragment free mass spectra are obtained with higher sensitivity and selectivity. The ability to measure fragment free PAH mass spectra is a very important step in the end goal of measuring complex unknown mixtures of PAH desorbed from solid surface such as soot samples.     

Identification of geometric parameters influencing the flow-induced vibration of a two-layer self-oscillating computational vocal fold model

Simplified models have been used to simulate and study the flow-induced vibrations of the human vocal folds. While it is clear that the models' responses are sensitive to geometry, it is not clear how and to what extent specific geometric features influence model motion. In this study geometric features that played significant roles in governing the motion of a two-layer (body-cover), two-dimensional, finite element vocal fold model were identified. The model was defined using a flow solver based on the viscous, unsteady, Navier-Stokes equations and a solid solver that allowed for large strain and deformation. A screening-type design-of-experiments approach was used to identify the relative importance of 13 geometric parameters. Five output measures were analyzed to assess the magnitude of each geometric parameter's effect on the model's motion. The measures related to frequency, glottal width, flow rate, intraglottal angle, and intraglottal phase delay. The most significant geometric parameters were those associated with the cover--primarily the pre-phonatory intraglottal angle--as well as the body inferior angle. Some models exhibited evidence of improved model motion, including mucosal wave-like motion and alternating convergent-divergent glottal profiles, although further improvements are still needed to more closely mimic human vocal fold motion.     

Non-empirical LCAO-MO-SCF calculations of the electronic structure of SiF2

Non-empirical LCAO-MO-SCF calculations on SiF₂ using two Gaussian basis sets are reported. The larger basis set gives a calculated geometry in good agreement with experiment. The effect on the energy and population analysis of optimization of the Si 3d exponent was investigated. 3d orbitals are found to be much less important in the bonding than in the isoelectronic molecule SO₂.     

Electron impact mass spectra of synthetically modified milbemycins

The potent endectocide 23-(O-methyloximino)-F28249α and related compounds were identified and characterized by mass spectrometry. The fragmentation pathway of 23-(O-methyloximino)-F28249α was identified by its high-resolution mass spectrum and the electron impact unit mass spectra of its homologs. This fragmentation pathway is presented and discussed.     

MS3 using the collision cell of a tandem mass spectrometer system

We report the feasibility of multistage fragmentation in combination with a fast background subtraction method, yielding the equivalent of MS3. The first quadrupole selects an ion of interest, and the ion is axially accelerated into Q2 to generate fragment ions. Subsequent stages of mass selection and fragmentation are obtained by quadrupolar resonant excitation within the Q2 collision cell. The fragments are analyzed downstream by either a resolving quadrupole or a time-of-flight (TOF) mass spectrometer, and multistage spectra are obtained by subtraction (MS(n) - MS(n-1)) for n = 3 or 4. We discuss the characterization of this method, including product ion arrival times, fragmentation efficiencies, and ion selectivity. We report accurate TOF mass spectra of background-subtracted MS3 for protonated molecules reserpine (m/z 609), bosentan (m/z 1552), and taxol (m/z 854).