Chirons, chiral building blocks

Enantiopure epoxides and vicinal diols are important versatile chiral building blocks for pharmaceuticals (Hanson, 1991). Their preparation has much in common and they may also be converted into one another. These chirons may be obtained both by asymmetric synthesis and resolution of racemic mixtures. When planning a synthetic strategy both enzymic and non-enzymic methods have to be taken into account. In recent years there has been considerable advance in non-enzymic methods as mentioned in part 2.1.1. Formation of epoxides and vicinal diols from aromatics is important for the break down of benzene compounds in nature (See part 2.6.5). 

As a Source of Other Chiral Building Blocks. The reagent is readily elaborated into several other key chirons, most notably the corresponding protected glycerol, (4/J)-2,2-dimethyl-l,3-dioxolane-4-methanol (1) obtained by sodium borohydride reduction of aqueous solutions of the reagent. ... 

These results have led to an interesting industrial apphcation for the synthesis of the j5-blockers Metoprolol and Atenolol. Thus, epoxidation of the prochiral allyl ethers by several bacteria, including the P. oleovorans strain mentioned above, led to the corresponding (S)-epoxides which showed excellent enantiomeric purities (Fig. 3). Further on, these chirons (i.e. chiral building blocks) were transformed into the corresponding (S)-enantiomers of the drugs developed by the Shell and Gist-Brocades companies [44]. Refinement of this approach... 

We conclude this section with two examples of the synthesis of chiral products by starting from an appropriate chiral building block. This has been named the chiron approach by Hanessian, and is described in great detail in his book.t l The distinctive feature of this type of reaction is that no new stereogenic units are formed and all the stereogenic units of the product are directly derived from the starting material. For this reason these cannot be considered as asymmetric synthesis but methods of this type are of some practical importance in areas where suitable chiral starting materials are readily available. 

Since biomass is chiral, these saturated molecules could also be chiral and could be useful chiral synthetic building blocks, so-called chirons .3 In addition to possibly producing useful chemicals, the transfer of hydrogen to the biomass material could also lead to a higher quality, more hydrogen-rich liquid fuel. 

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