Chromatographic methods schemes

The aldol reaction between a chiral a-amino aldehyde 16 and an acetate derived enolate 17 creates a new stereogenic center and two possible diastereomers. Several different methods for the synthesis of statine derivatives following an aldol reaction have been reported most of them lead to a mixture of the (35,45)- and (3/ ,45)-diastereomers 18 (Scheme 3), which have to be separated by laborious chromatographic methods.[17 211 Two distinct approaches for stereochemical control have been used substrate control and reagent control. 

An alternative strategy towards benzimidazole synthesis relies on the palladium-catalysed cyclisation of (2-bromophenyl)amidines. This chemistry has been reported to take place under aqueous reaction conditions, in the presence of sodium hydroxide in sealed microwave vials. The products were isolated by a catch and release method using a strongly acidic ion exchange resin, thereby avoiding conventional chromatographic purification (Scheme 3.14)23. Selectively, N-functionalised benzimidazoles were conveniently prepared by this method. 

Liquid chromatographic methods may be classified according to either the mechanism by which analytes are retained on the column or the mechanism by which they are removed from it. The mechanism of retention classification is the most popular scheme, and five major retention mechanisms have been identified ... 

Section 2.2.1 briefly addresses the possibility to incorporate a scheme for the selection of chromatographic methods in a computer program, a so-called expert system. This is a relatively recent proposition, and progress may be expected in this area. 

Figure 10.4 Scheme of the three main approaches for the analyses of sterols and steryl conjugates using chromatographic methods. 

In order to solve analytical problems for a particular compound or class of compounds, as well as to save time and resources, it is essential that systematic method development concepts are applied. Such methods then enable a successful scaling up to preparative purifications as well as the design and application of MD-HPLC purification schemes. Moreover, if used in conjunction with dereplication procedures, these advances in high-resolution chromatographic methods may lead to new discoveries that can be used to advance science or medicine and at the same time respect the environment through reduced solvent and reagent usage. 

Chemometrics even allow the examination of photoreactions with unknown intermediates without combination of different spectroscopic or chromatographic methods under certain conditions. As an example, the photoreaction of the Z-fulgide to a ring compound via the -derivative [195] is given, using the following scheme of reaction ... 

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