Phase-switch approach

Figure 1. The basic strategy for phase-switching approaches to iterative biopolymer synthesis for simplified purification of intermediates.

Transition matrix estimators have received less attention than the multicanonical and Wang-Landau methods, but have been applied to a small collection of informative examples. Smith and Bruce [111, 112] applied the transition probability approach to the determination of solid-solid phase coexistence in a square-well model of colloids. Erring ton and coworkers [113, 114] have also used the method to determine liquid-vapor and solid-liquid [115] equilibria in the Lennard-Jones system. Transition matrices have also been used to generate high-quality data for the evaluation of surface tension [114, 116] and for the estimation of order parameter weights in phase-switch simulations [117]. 

Phase-switching strategy was applied to the multistep synthesis of isoxazolines 45-47. The approach exploits the boronic acid functionality as a phase-tag which can be easily immobilized on diethanolaminomethyl polystyrene (DEAM-PS) resin and then released by treatment with wet THF. Each reaction step was carried out in solution and the products were purified by addition of the scavenging resin, followed by filtration, washing of the supported material, hydrolytic release and evaporation. Finally, the boronic acid could be used to introduce additional substituents on the molecule. For example, 46 was converted into the biaryl-containing isoxazoline 47 by a Suzuki cross-coupling reaction <07JCO193>. 

A somewhat different approach was used by Vincent in phase-switching reactions using pyridyl-labelled substrates and products. The pyridyl-containing tag is here thought of as a masked phase tag, which allows for phase switching with the help of a heavy fluorous copper(ll)-carboxylate complex. Comparison with a non-fluorous system indicated that a problem of release of the strongly coordinating pyridine linker was avoided in the fluorous approach. ... 

A quantitative bioassay for erythromycin 2 -ethylsuccinate (EM-ES, M, 861 Da), a prodrug of the macrolide antibiotic erythromycin, using Cf-FAB LC-MS was described by Kokkonen et al. [53-54]. Reversed-phase LC of extracted plasma samples was performed at a flow-rate of 1 ml/min. In order to meet the flow-rate requirements of the Cf-FAB interface, i.e., 15 pl/min, without splitting, the phase-system switching approach [53] was used. After post-column dilution of the column effluent with water, the eluent fraction of interest was enriched on a short precolumn, from which the compound of interest was desoibed and transferred to the Cf-FAB interface probe. A [ Hj]-analogue was used as internal standard. Good linearity was observed in the range of 0.1 to 10 pg/ml EM-ES in plasma. The within-ran precision was ca. 6%. The accuracy and inter-day precision, determined at 1.05 pg/ml in plasma, were 0.93 0.11 pg/ml and 12%, respectively (n=6). The determination limit was 0.1 pg/ml [54]. 

Figure 4. Schematic diagram of the phase-system switching approach applied in the moving belt LC/MS analysis of metropolol enantiomers. AC=analytical column, UV=UV-detector, ACN=desorbing eluent (see text), and TC=trapping column.

For the excitation of DQC under MAS, the interference with the sample rotation must be considered, as was the case for the CRAMPS experiment discussed in the previous section. In particular, if a sequence designed for a static MQ experiment is used without modification, the excitation (and reconversion) time is limited to ttr/2, since the rotor modulation causes the action of the pulse sequence in the second half of the rotor period to be the time reversal of that which occurred in the first half of the rotor period. Starting with the suggestion of Meier and Earl,89,90 who simply proposed the phase switching of the static sequences used by Baum et al.,47,48 every half rotor period to prevent the process of self-time-reversal, many different approaches have been presented that allow excitation (and reconversion) times of one or more rotor periods. Such pulse sequences that counteract the effect of MAS are referred to as recoupling methods,1112 examples that have been used in homonuclear DQ MAS NMR spectroscopy include BABA,91 C7,92 DRAMA,93 DRAWS,94 and HORROR.95 We note that Levitt and co-workers have recently introduced a very helpful classification system, based on symmetry principles, which covers such sequences.96,97... 

Phase Switching-Bipyridyl Tagging Approaches to Scavengers... 

Figure 11.3 Typical configuration for the on-line coupling of an achiral and chiral cliro-matograpliic system by means of a switching valve. The non-enantio-resolved solute is isolated on the achiral phase and then stereochemically separated on the chiral phase. Reprinted from G. Subramanian, A Practical Approach to Chiral Separation by Liquid Chromatography, 1994, pp. 357-396, with permission from Wiley-VCH.

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