Benzaldehyde from mandelonitrile

It is important to mix the mandelonitrile with hydrochloric acid immediately it has been separated from the water. Standing results in rapid conversion to the acetal of benzaldehyde and mandelonitrile C(H,CH[OCH(CN)C H,] and/or the iso-nitrile the yield of mandelic acid will, in consequence, be reduced. 

Benzaldehyde cyanohydrin (mandelonitrile) provides an interesting example of a chemical defense mechanism in the biological world. This substance is synthesized by millipedes (Apheloria corrugata) and stored in special glands. When a millipede is threatened, the cyanohydrin is released from the storage gland and undergoes enzyme-catalyzed reversal of cyanohydrin formation to produce HCN, which is then... 

Figure 11.13 The synthesis of enantiomer-ically pure (S)-mandelic acid from ben-zaldehyde in the presence of a triple CLEA [73]. Conversion benzaldehyde )), (S)-mandelonitrile ( ), (S)-mandelic acid (O , enantiomeric excess), (S)-mandelic amide...

It is commercially prepared from benzaldehyde and hydrogen cyanide. Mandelonitrile is used by certain insects (tiger beedes, an African millipede) as a defense duid (38). After expelling the duid an enzyme catalyzes the conversion of mandelonitrile to benzaldehyde and HCN, which is usually fatal to the insect s enemy. 

Mandelic acid is best prepared by the hydrolysis of mandeloni-trile with hydrochloric acid. The mandelonitrile has been prepared from amygdalin, by the action of hydrocyanic acid on benzaldehyde, and by the action of sodium or potassium cyanide on the sodium bisulfite addition product of benzaldehyde. ... 

As early as 1908, Rosenthaler found in the ferment mixture of emulsin a u-oxynitrilase , which directed the addition of hydrocyanic acid (hydrogen cyanide) to benzaldehyde asymmetrically to give x-hydroxybenzeneacetonitrilc (mandelonitrile)9. This result was confirmed1 °, however, it was not until 1963 that Pfeil ct al. first isolated and characterized the enzyme (R)-oxyni-trilase [EC 4.1.2.101 from bitter almonds (Prunus amygdalus)1 12. The yellow-colored enzyme contains a flavin-adenine dinucleotide (FAD)11 and loses its activity by splitting off this prosthet-... 

Starting from enantiomerically pure 4-methylsulfanyl-mandelonitrile, thiamphenicol and florfenicol have been enantioselectively synthesized (Figure 5.14). The enantiomerically pure 4-methylsulfanyl-mandelonitrile was obtained by hydrocyanation reaction of 4-methy lsulfany 1-benzaldehyde catalyzed by (M)-hydroxynitrile lyase of Badamu (almond from Xinjiang, China) (Prunus communis L. var. dulcis Borkh), which, after an extensive screening, was found to be a highly effective bio-catalyst for this reaction [85]. 

Figure 8.12 Conversion of benzaldehyde into enantiomerically pure (S)-mandelic acid by the sequential addition of HCN catalyzed by the (.S )-selective oxynitrilase from Manihot esculenta (MeHnL), and subsequent hydrolysis of the resultant (5)-mandelonitrile by the nitrilase from Pseudomonas fluorescens ECB 191 (PfNLase)...

Further evidence for the formation of intermediate compounds in catalytic reactions is afforded by the observation (a) that optically active camphor is formed from optically inactive (racemic) camphor carboxylic acid in the presence of the d- or /-forms of quinine, quinidine or nicotine and (6) that optically active bases, e.g., quinidine, catalyze the synthesis of optically active mandelonitrile from benzaldehyde and hydrocyanic acid.10 These results hardly admit of any other interpretation than the intermittent production of a catalyst-reactant compound. 

There are a few data in the literature to suggest that the hydrolysis of aliphatic nitriles occurs in mammals, but only as a minor or even undetectable pathway in competition with oxidative denitrilation. For example, benzyl cyanide (11.80, Fig. 11.12) undergoes cytochrome P450 catalyzed hydroxy-lation to mandelonitrile (11.81), from which cyanide and benzaldehyde are produced, the latter being oxidized to benzoic acid (11.83) [118]. However, a careful metabolic study of mandelonitrile has shown that, in the rat, this pathway accounts for ca. 90% and not 100% of the dose [122], Only ca. 10% of orally administered benzyl cyanide was converted to mandelic acid (11.82, Fig. 11.12) by hydrolysis of the CN group. 

Ripened Lfme fruits P. mume) (obtained from local fruit market in June and stored at 4 °C until use, 1 kg or more) potassium cyanide benzaldehyde R)- and (5)-mandelonitrile (NH4)2S04 citrate buffer (pH 4.0) potassium phosphate buffer (pH 6.0) hexane isopropanol... 

The enzyme activity was assayed by measuring the production of optically active mandelonitrile synthesized from benzaldehyde and cyanide. The standard assay solution contained 300 gmo citrate buffer (pH 3.5-6.0), 50 /rmol of benzaldehyde, 100 /rmol potassium cyanide and 100 jA of the enzyme in a final volume of 1.0 mL. The reaction was started by an addition of 100 fx of the enzyme solution and incubated at 25 °C for 1-120 min. Aliquots (100 jiY) were withdrawn at various reaction times and the reaction was stopped by the addition of 0.9 mL of organic solvent (9 1 hexane iso-propanol by volume). The mandelonitrile formed was extracted and the supernatant, obtained by centrifugation (15,000 x g, 1.0 min at 4 °C), was assayed by HPLC. A blank reaction was also performed without enzyme and the amount of mandelonitrile obtained was deducted from the biocatalyzed reaction product. One unit of the enzyme is defined as the amount of the enzyme that produces 1 /imol of (R)-mandelonitrile under reaction conditions in 1 min. 

Salicin is an (9-glycoside of a phenol, namely salicyl alcohol. Salicin is a natural antipyretic and analgesic found in willow bark, and is the template from which aspirin (acetylsalicylic acid, see Box 7.13) was developed. Prunasin from cherry laurel is an example of a cyanogenic glycoside, hydrolysis of which leads to release of toxic HCN (see Box 7.7). It is the (9-glucoside of the alcohol mandelonitrile, the trivial name for the cyanohydrin of benzaldehyde. It is the further hydrolysis of mandelonitrile that liberates HCN. 

Another application of HNLs can be found in the chemoenzymatic synthesis of the broadband antibiotic thiamphenicol and its fluorinated derivative florfenicol [203]. The conversion of 4-methylsulfanyl-benzaldehyde to the mandelonitrile was catalyzed by a novel enzyme from a Chinese almond (Prunus communis L. var. dulcis Borkh). A concentrated powder from the kernels was prepared and a mixture of the cmde meal, aldehyde, and HCN was stirred in isopropyl ether at room temperature for 12 h, yielding the cyanohydrin with 99% ee after recrystallization. The building block formed was then successfully applied in the total synthesis of thiamphenicol and florfenicol (Fig. 41). 

The millipede Apheloria corrugata secretes a mixture of HCN and benzaldehyde to prevent other animals from eating it. The millipede stores mandelonitrile (benzaldehyde cyanohydrin) in a reservoir. When attacked, it discharges mandelonitrile through a reaction chamber containing enzymes that catalyze the conversion of the cyanohydrin to benzaldehyde and HCN. 

The reaction of S,S,S-(136) with tris-dimethylaminophosphine/PCl3 in CH3CN at 0°C gave the chiral azaphosphatrane (137) in overall 56% yield. Unfortunately (137) did not induce asymmetry in mandelonitrile formed from the catalyzed reaction of Me3SiCN with PhCHO. It was also inefficient in catalyzing the addition of alkyl cyanide to benzaldehyde, and was not sufficiently basic to effect rearrangement of cyclohexene oxide to 2-cyclohexenol. Further experiments with analogues of (137) are promised for future publications. 

Another nucleophilic addition reaction—this time in reverse—is involved in the chemical defense mechanism by which the millipede Aphe-loria corrugata protects itself from predators. When attacked by ants, it secretes the cyanohydrin mandelonitrile and an enzyme that catalyzes the decomposition of mandelonitrile into benzaldehyde and HCN. The millipede actually protects itself by discharging poisonous HCN at its attackers. 

Bredig, in a pioneering investigation in 1908, was able to prepare mandelonitrile 1 from benzaldehyde and HCN in the presence of an alkaloid (quinine or quini-dine) as catalyst (Scheme 1) [10]. The enantioselectivities were less than 10%, however this work was conceptually important, though it did not lead to developments in other laboratories. It was only in 1955 that Prelog and Wilhelm reinvestigated this system and proposed a mechanistic picture [11]. 

Spiegel169 recommended the use of potassium cyanide and hydrogen chloride for preparation of mandelonitrile somewhat more than 1 mole of pure potassium cyanide is moistened with a little water, covered with a layer of 1 mole of benzaldehyde, and treated dropwise, with repeated shaking and with cooling, with 1 mole of hydrogen chloride in the form of fuming hydrochloric acid. The resulting brown oil is then decanted from the slurry of salts. 

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