Chromatographic purification, proposal

Molina and co-workers employed iminophosphoranes derived from a-azido-ketones in their one-step syntheses of oxazole alkaloids (Scheme 1.75). Reaction of 4-methoxyphenacyl azide with nicotinoyl chloride in the presence of triphenylphos-phine gave O-methylhalfordinol 280 directly in 56% yield. Similarly, reaction of 4-methoxyphenacyl azide with 3,4-dimethoxycinnamoyl chloride gave annuloline 281 in comparable yield after chromatographic purification. The authors proposed initial formation of an iminophosphorane, which was acylated with elimination of triphenylphosphine oxide to afford an imidoyl chloride, which then cychzed to the oxazole. 

It should be possible to use the special properties of chiral structures for particular separation problems. According to Belinski and Tencer, one possible way in which nature solved the ribose problem could have involved an enantioselective and diastereoselective purification process acting on a mixture of biomolecules, which left ribose as the only molecule available for further reactions. The authors propose a theoretical mechanism in which a type of chromatographic process occurs at chiral mineral surfaces. This paper is likely to stimulate new experiments as well as the quest for as yet unknown surfaces which can separate racemic carbohydrate mixtures. The question arises, however, as to whether there were minerals present on the young Earth which are now unknown, as they no longer exist on the Earth of today (Belinski and Tencer, 2007). 

Very similar results were presented by Coffman et al. [2]. They introduced a yet different approach to the prediction of the efficiency of large molecule (proteins) separations on very short columns. Their approach is based on the fact that since short columns yield non-Gaussian effluent distributions, measuring the degree of binary separation using conventional chromatographic resolution is inadequate. Instead, they proposed the fractional purification P, of component z, defined as ... 

Since interferences could not be adequately eliminated with the alumina column, Gissel and Schaal (611) proposed an alternative procedure for extract purification involving use of Kieselgel thin-layer chromatographic plates, elution with trichloroethane/ethanol (8 2), and detection at 254 nm. The detection limit for three thiouracil residues in thyroid glands was in the range 0.5-1.0 ppm. 

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