The Alcohol Moiety of Synthetic Pyrethroids

The alcohol residue of the commercial pyrethroids shows a considerable diversity. Just a few of the alcohols are Hsted here, the discussion of which focuses only on the chiral compounds. 

Markedly elegant are the enzymatic kinetic resolution and in situ racemisation of the cyanohydrin (dynamic kinetic resolution). [109, 110] The propensity of cyanohydrins for racemisation allows to esterify stereoselectively Just the (S)-enantiomer with lipases. In a subsequent step, the ester is then enzymatically hydrolysed. 

Addition of hydrogen cyanide to aldehydes by means of oxynitrilases gives cyanohydrins in high yields and high optical purity. It is often unnecessary to work with free hydrogen cyanide instead, the cyanohydrin of acetone can be used to form hydrogen cyanide in situ, which is then reacted with the aldehyde. The oxynitrilases are compatible with organic solvents or multiphase solvent systems. [Ill, 112] 

DSM developed a recombinant oxynitrilase from the rubber tree (Hevea brasiliensis), with which it is possible to produce the cyanohydrin of (S)-3-phenoxy-benzaldehyde with an enantiomeric excess of 98.5 % and a space/time-yield of 1,000 g/l/day. [113] 

One of the most interesting developments in recent years is the non-enzy-matic, enantioselective, organocatalytic addition of hydrogen cyanide to aldehydes. This reaction is, for example, very efficiently catalysed by optically pure diketopiperazines, such as cydo-(f )-Phe-(i )-His), which are readily accessible from the corresponding amino acids. [114-116] 

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