Synthesis of 3-Phenoxy Benzaldehyde Cyanhydrine

The approved way of preparing the cyanohydrine 299 by the addition of prussic acid is no problem in the case of 3-phenoxybenzaldehyde 280, The use of acetone cyanhydrine as a source of prussic acid [629] seems to be advantageous. In order to circumvent the aldehyde 280, an alternative route from the trimethyl-3-phenoxy benzylammonium salt 296 and the corresponding 3-phenoxybenzyl nitrile 297 [630] was proposed, in which oxalic ester condensation and subsequent brominative degradation of this whole group was applied (Reaction scheme 211). This procedure supposedly yields a particularly pure a-bromobenzyl cyanide 298 [631]. 

Direct reaction of the bisulphite adduct, 3-phenoxy-a-hydroxybenzylsulfonate, with sodium cyanide also gives the desired cyanohydrine [632]. 

Insecticidal activity of 3-phenoxy-a-cyanopyrethroid esters are restricted to esters with absolute configuration (S) at the a-carbon. It was therefore a challenge to provide the more or less optically pure S-cyanohydrine of 299, in spite of the known lability of such compounds. 

The amide of a-S-3-phenoxy mandelic acid (obtained after resolution of the racemate with (—)phenethyl amine), was esterified with 1-R-cis-deltamethric acid, then dehydrated to give deltamethrin [633]. 

The free optically active cyanohydrine was prepared after chromatographic separation or fractionated crystallization of the ketal 300 with 1-R-cis-caronaldehyde hemiketal 155 [634,635,636] and subsequent Uberation by acidic treatment (Reaction scheme 212) preferentially with BCI3 at —30 °C [637]. It is not stable under weakly alkaline conditions. The (S)-4-fluoro-analogue 301 was similarly prepared [638]. 

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