Phase-switch purification

Figure 8.7 Phase-switch purification for removal of scavengers in a catch and release strategy.

B. Linclau, A. K. Singh, D. P. Curran, Organic-Eluorous Phase Switches A Fluorous Amine Scavenger for Purification in Solution Phase Parallal Synthesis , J. Org. Chem 1999, 64, 2835. 

Tris[(2-perfluorohexyl)ethyl]tin hydride has three perfluorinated segments with ethylene spacers and it partitions primarily (> 98%) into the fluorous phase in a liquid-liquid extraction. This feature not only facilitates the purification of the product from the tin residue but also recovers toxic tin residue for further reuse. Stoichiometric reductive radical reactions with the fluorous tin hydride 3 have been previously reported and a catalytic procedure is also well established. The reduction of adamantyl bromide in BTF (benzotrifluoride) " using 1.2 equiv of the fluorous tin hydride and a catalytic amount of azobisisobutyronitrile (AIBN) was complete in 3 hr (Scheme 1). After the simple liquid-liquid extraction, adamantane was obtained in 90% yield in the organic layer and the fluorous tin bromide was separated from the fluorous phase. The recovered fluorous tin bromide was reduced and reused to give the same results. Phenylselenides, tertiary nitro compounds, and xanthates were also successfully reduced by the fluorous fin hydride. Standard radical additions and cyclizations can also be conducted as shown by the examples in Scheme 1. Hydrostannation reactions are also possible, and these are useful in the techniques of fluorous phase switching. Carbonylations are also possible. Rate constants for the reaction of the fluorous tin hydride with primary radicals and acyl radicals have been measured it is marginally more reactive than tributlytin hydrides. ... 

The technique of CPC was also employed as a key step in the purification of 26 phenolic compounds from the needles of Norway spruce (Picea abies, Pinaceae). An aqueous extract of needles (5.45 g) was separated with the solvent system CHCl3-Me0H-i-Pr0H-H20 (5 6 1 4), initially with the lower phase as mobile phase and then subsequently switching to the upper phase as mobile phase. Final purification of the constituent flavonol glycosides, stilbenes, and catechins was by gel filtration and semipreparative HPLC. °... 

Linclau B, Sing AK, Curran DP. Organic-Fluorous phase switches a fluorous amine scavenger for purification in solution phase parallel synthesis. J Org Chem. 1999 64 2835-2842. Parlow JJ. Polymer-assisted solution-phase chemical library synthesis. Curr. Opin. Drug Discov. Devel. 2005 8 757-775. 

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

The tin azide 11 (Scheme 16) is an example of a fluorous reagent that, unlike the fluorous tin hydride 1, transfers the fluorous tag to the product. This type of phase switch is very powerful only the nitrile that reacts with 11 is switched to the fluorous phase whereas any unreacted nitrile or impurities are left behind in the organic phase. Cycloaddition reaction of 11 with excess nitrile leads to fluorous tetrazoles 12. Again, BTF is a unique reaction solvent that serves to dissolve all reactants. Purification is effected in the usual way via fluorous-organic extraction. Deprotection yields the tetrazoles 13 in good yields and purities. 

Using immobilized -glucuronidase reactors, estriol and estradiol glucuronides have been determined in urine by a column-switching technique (270, 271). Both glucuronides were hydrolyzed by the immobilized enzyme at pH 7. The steroid mixture was subsequently separated by gradient elution on a reversed-phase column, to be finally detected by UV absorbance at 280 nm. In this procedure, the activity of enzyme did not alter even after 150 h continuous run and exposure to a mobile phase containing 10% methanol. When a separate reversed-phase precolumn was inserted in the LC system, additional sample purification and shorter analysis time could be attained (272). 

Liquid chromatography cleanup on a LiChrosorb Diol column has been further proposed for the offline purification of chloramphenicol residues from bovine muscle and eggs (32). An online approach based on reversed-phase principles has also been described for isolation of chloramphenicol residues from swine kidney by an automated column switching system (63). Use of a protein exclusion column (Hisep) has been also suggested in an online trace-enrichment method for the determination of chloramphenicol in animal tissues (52). By employing a column-switching system, all chloramphenicol that eluted from the protein exclusion column was trapped at the entry of a 5 m Supelcosil LC-18 preconcentration column, to be subsequently back-flashed into the analytical column. 

H2O extn, purification by online dialysis and subsequent trace enrichment on reversed phase, 40 m, preconcn column and switching to analytical column... 

Colistin (COL) is a multicomponent antibiotic (polymyxins E) that is produced by strains of inverse Bacillus polymyxa. It consists of a mixture of several closely related decapeptides with a general structure composed of a cyclic heptapeptide moiety and a side chain acetylated at the N-terminus by a fatty acid. Up to 13 different components have been identified. The two main components of colistin are polymyxins El and E2 they include the same amino acids but a different fatty acid (216). A selective and sensitive HPLC method was developed for the determination of COL residues in milk and four bovine tissues (muscle, liver, kidney, and fat). The sample pretreatment consists of protein precipitation with trichloracetic acid (TCA), solid-phase purification on Cl 8 SPE cartridges, and precolumn derivatization of colistin with o-phthalaldehyde and 2-mercaptoethanol in borate buffer (pH 10.5). The last step was performed automatically, and the resulting reaction mixture was injected into a switching HPLC system including a precolumn and the reversed-phase analytical column. Fluorescence detection was used. The structural study of El and E2 derivatives was carried out by HPLC coupled with an electrospray MS. Recoveries from the preseparation procedure were higher than 60%. 

SPOS also benefited from other technical improvements such as the on-resin analytical methods [112], the capture-release purification technique [113], and hydrophobically assisted switching phase [114], Recently, chiral auxiliary-mediated 1,2-n.v-glycosylations were introduced to SPOS, allowing for the efficient and stereoselective assembly of oligosaccharides [115] (see Chapter 7). 

After the reaction two clear liquid phases occur (Figure 3) that can easily be separated. The upper phase is the pure product - no solvent is needed any more - and the lower is the pure IL [2,6]. HMIMCl as an IL has a great advantage over the classical dialkylated systems it can be switched on and off just by protonation and deprotonation. This is crucial when recycling and purification of the ILs are considered. To distinguish the switchable ILs - the HMIM - from the conventional ones, Freeman tie used the term smart ionic liquids in the context of the BAS 1L process [4]. 

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