Scale Cyanohydrin Production

The hydroxynitrile lyase (HNL)-catalyzed addition of HCN to aldehydes is the most important synthesis of non-racemic cyanohydrins. Since now not only (f )-PaHNL from almonds is available in unlimited amounts, but the recombinant (S)-HNLs from cassava (MeHNL) and rubber tree (HbHNL) are also available in giga units, the large-scale productions of non-racemic cyanohydrins have become possible. The synthetic potential of chiral cyanohydrins for the stereoselective preparation of biologically active compounds has been developed during the last 15 years. 

R)- and ( -selective HNLs. A number of recombinant HNLs have also been expressed in E. coli, Saccharomyces cerevisiae, and Pichia pastoris. Recently, protein engineering has been successfully applied to the development of a tailor-made HNL for large-scale production of specific cyanohydrins [69,70]. 

For the production of (5)-m-phenoxybenzaldehyde cyanohydrin (47) DSM established an enzymatic hydrocyanation process on an industrial scale (Scheme 31). An efficient (S)-oxynitrilase biocatalyst has been developed. This enzyme is derived from the plant Hevea brasiliensis, and has been cloned and overexpressed in a microbial host organism [117]. In the presence of this biocatalyst the desired product 47 has been obtained with high enantioselec-tivity. 

Several industrial processes using lyases as catalysts have been reported. Perhaps the most prominent lyase-catalyzed process is the production of acrylamide from acrylnitrile. This process is carried out by the Nitto Chemical Company of Japan at a scale of more than 40,000 tons per year. Another example is the use of a fumarase for the production of (5 )-malic acid from fumaric acid. As shown in Fig. 7, a water molecule is added to the double bond in fumarate by means of an addition reaction. The result is a cleavage of the carbon-carbon double bond, and a formation of a new carbon-oxygen bond. A third example is bio-catalytic production of a cyanohydrin from a ketone. This reaction is catalyzed by a lyase called oxynitrilase. It consists of the cleavage of one carbon-oxygen bond, and the addition of a HCN molecule. The chirality of the product is based on the form of the enzyme used (/ -oxynitrilase or 5-oxynitrilase). ... 

Fig. 8 General production figure for the cyanohydrins on a large-scale.

Chemical or enzymatic hydrolysis of the chiral cyanohydrin gives access to 2-hy-droxycarboxylic acids. The most prominent examples are (S)- and (R)-mandclic acids, which are mainly used for racemate resolution. Other chiral acids, such as (R)-2-chlorom an del i c acid, are used as precursors for pharmaceuticals (see Fig. 11). Scale-up and production of (R)-2-chloromandelic acid was successfully performed with a space-time yield of 250 g/L/d (ee = 95%). 

Both recombinant (R)- and (S)-HNL have been successfully used in the synthesis of chiral cyanohydrins at the plant-scale level. Their availability on a large-scale via fermentation and their striking similarities in reaction technology and chemical behavior have been crucial for the development of robust, cost-effective processes applicable to a wide variety of substrates. Exploitation of the possibilities of HNL technology has just begun. The large number of substrates and follow-up products with applications in fine chemistry reflects the attractiveness of this transformation. 

The major process currently operated on the commercial scale is known as the acetone cyanohydrin (23) (ACN) process [14]. This process uses readily available cheap raw materials (Scheme 2.3). Acetone is produced from propylene and hydrogen cyanide or can be obtained as a byproduct from acrylonitrile production. Acrylonitrile is manufactured via propylene ammoxidation or by catalytic ammoxidation of natural gas. Sulphuric acid is readily available but constitutes the major environmental problem of the acetone cyanohydrin process since a large excess is required to effect the hydrolysis of acetone cyanohydrin to form the methacrylamide sulphate intermediate. 

Cyanohydrins are bifunctional molecules and therefore constitute a particularly useful class of compounds for synthetic purposes. A hydroxyl and a nitrile functional group are available for chemical and enzymatic follow-up reactions (Scheme 25.2 [7, 93-95]), resulting in hydroxy carboxylic acids [96-99], carbamates [KXl], hydroxy-amides [101], primary and secondary hydroxyamines [46,102-104], aziridines [105, 106], aminonitriles [107, 108], diamines [108], azidonitriles [108], a-fluoronitriles [109], hydroxy ketones [110], and many more. The (S)-selective H HNL and the (R)-selective PaHNL, in particular are used on large scale either for the s)mthesis of (S)-3-phenoxybenzaldehyde cyanohydrin, a precursor for p5U ethroids, a class of insecticides (R)- and (S)-mandelonitrile, which can be further converted to man-delic acids (R)-chloromandelonitrile, a precursor for an anticoagulant (R)-2-hydroxy-4-phenylbutyronitrile, which serves as intermediate for the production of angiotensin-converting enzyme inhibitors (ACEi) or (R)-2-amino-l-(2-furyl)ethanol [94]. Several HNLs are commercially available as free or immobilized enzymes. 

Hydroxynitrile lyases (also known as oxynitrilases) are used for the synthesis of chiral cyanohydrins. Because the hydroxynitrile moiety can be easily converted into a wide range of functional groups (Scheme 28.1), the cyanohydrins represent a versatile building block in total synthesis of natural products. Moreover, both R- and S-selective enzymes are available. They are also straightforward to handle and therefore represent catalysts of choice for syntheses on an industrial scale. Already as early as 1908, they had been used in enantioselective synthesis indeed, the first enantio-selective reaction was performed with an HNL. Consequently, many applications for these enzymes have been developed in natural products synthesis. 

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