Use of naturally occurring chiral compounds as building blocks

1 Use of naturally occurring chiral compounds as building blocks 

Perhaps the most obvious way to obtain a chiral compound is simply to take an appropriate chiral precursor from a natural source and modify the structure in a series of chemical steps to arrive at the desired compound. The steps will generally not involve the stereogenic centre(s) required for the product and indeed care has to be taken that there is no chance of racemisation during the sequence. In some cases a stereogenic centre may undergo reaction as long as this proceeds with retention of chirality, an Sjs 2 substitution being a common example. It may also be necessary to carry out steps which destroy 

Resolution may be considered the classical method of obtaining enantiomerically pure products. The procedure relies on the fact that diastereomers, unlike enantiomers, have different physical properties. If the racemic compound which is to be resolved is derivatised by reaction with a naturally occurring enantiomerically pure compound, then the resulting diastereomeric compounds may be separated, most commonly by crystallisation but also by chromatography, and then separately treated to liberate the two enantiomers. If we represent the 

Note that the resolving agent is recovered unchanged after this procedure and can be reused repeatedly. Because of the need to obtain crystalline adducts which are readily broken down to their components again, the ionic salts formed between amines and acids, either carboxylic or sulphonic, are ideal for resolution. Thus even in the last century very many amines were resolved by formation of salts with, for example, tartaric acid (16) or camphorsulphonic acid (29), while organic acids were resolved with bases such as quinine, cinchonine and the highly toxic alkaloids brucine (36) and strychnine (37). Although reliable resolution methods have now been worked out for 

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