Enantiopure, definition

In order to illustrate an example of process design for the manufacture of enantiopure drug substances on an industrial SMB system, consider manufacturing 10 ton/ year of an enantiopure drug. The racemic drug by definition is a 50 50 mixture of each enantiomer (products A and B). The goal is to process enantiopure drug substances in order to obtain 99 % purity for both the extract and the raffinate. 

Unfortunately, perhaps because of a misinterpretation of a clear definition (see Section 1.1.3.4.), the term homochiral, describing a relationship and not a property between molecules (see Section 1.1.3.4.), has been frequently used since 1985 in the sense of the above enantiopure. As was aptly remarked20, this is inappropriate just as one does not call a notepad identical just because it is made up of identical sheets of paper . 

The term chiral is also used to describe a sample of a substance. When used in this sense, it is not necessary that every molecule in the sample has the same handedness see the definitions below for optical purity, enantiopurity, etc. A racemic sample is one containing (statistically) equal numbers of right-handed and left-handed enantiomers, and therefore showing zero optical activity at all wavelengths. A sample can also be chiral, nonracemic i.e., it contains an excess of one enantiomer. 

Asymmetric synthesis, the selective generation of new chirality elements (as one definition goes), has developed from a specialty pursued by outsiders to an art cultured by some learned ones, and now may be considered a standard laboratory methodology for everybody s use. This development has taken place exponentially (explosively ) in the last two decades, triggered by a number of circumstances. Also the practitioners of pharmaceutical, vitamin, and agro synthesis need to produce enantiopure, rather than racemic active compounds (for registration ). 

There is also a definite trend towards chirality or enantiopurity in the flavour/fragrance industry, but the reasons for this are somewhat different than for drugs and agrochemicals. There is consumer demand for natural ingredients , produced by natural processes (including fermentation) and preferably involving zero chemical or synthetic steps. Since natural is synonymous with enantiopure, there is a demand for enantioselective synthesis in this industry. 

Homochiral has been used by some as a synonym for "enantiomerically pure". This is another usage of a term that should be discouraged, as homochiral already had a clear and useful definition, and using the same term to signify two completely different concepts can only lead to confusion. A better term for designating an enantiomerically pure sample is simply enantiopure. 

In the enantioselective addition of diethylzinc to aldehydes catalyzed by nonracemic amino alcohols, the phenomenon named asymmetric amplification has been well recognized as a consequence of an in situ increase in the enantiopurity of the active catalyst, as the racemic ligand is trapped in the more stable, unreactive meso species [3, 11]. Although the reaction will definitely give racemic product if only racemic Hgands are used alone, the addition of an easily accessible alternative... 

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