Chiral pool definition

The third approach is the main topic of this volume. According to the definition given above it involves enantiomerically pure starting materials which at some point must be provided by resolution or ex-chiral-pool synthesis. It is more or less equivalent to the term asymmetric synthesis defined by Marckwald in 19047 as follows Asymmetric syntheses are those reactions which produce optically active substances from symmetrically constituted compounds with the intermediate use of optically active materials but with the exclusion of all analytical processes . In today s language, this would mean that asymmetric syntheses are those reactions, or sequences of reactions, which produce chiral nonracemic substances from achiral compounds with the intermediate use of chiral nonracemic materials, but excluding a separation operation. 

Although these definitions are vague, it appears that partial synthesis is restricted to those modifications which do not change the basic chemical structure, whereas ex-chiral-pool synthesis generally leads to a new type of compound. For instance, the introduction of new substituents into a steroid nucleus is partial synthesis , whereas the conversion of D-glucose into the pheromone e.vo-brevicomin is ex-chiral-pool synthesis . 

In its strictest definition ex-chiral-pool synthesis" requires that... 

There are two drawbacks in employing the chiral pool approach. First, a natural product usually exists as a single enantiomer, and therefore only a single enantiomer of a pheromone can be prepared easily. Secondly, a natural product possesses a definite structure, which is often quite different from that of a pheromone. Accordingly, the synthesis may become lengthy to remove the undesired structural features in the starting natural product. 

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