Almond meal

Partially Defatted Nuts. There is considerable demand for nuts and nut products of reduced fat content. Almond meal and peanut meal are examples of products having low fat content achieved by pressing oil from the nuts and by grinding the cake. Much of the flavor is in the oil defatted nuts are thus less tasty. 

Synthesis of optically active cyanohydrins using almond meal... 

In the first step, (R)-2-chlorobenzaldehyde cyanohydrin is prepared by the almond meal-catalyzed addition of HCN to 2-chlorobenzaldehyde. The cyanohydrin is then transformed into the corresponding 2-hydroxy ester by Pinner reaction. Subsequently, the hydroxy group is activated by sulfonylafion and reacted with tetrahydrothieno[3.2-c]pyridine to give, under complete inversion of configuration, the (5 ) — a-amino acid derivative CLOPIDOGREL. ... 

S. Singh, M. Scigelova, and D. H. G. Crout, Glycosidase-catalysed synthesis of mannobioses by the reverse hydrolysis activity of alpha-mannosidase Partial purification of alpha-mannosidases from almond meal, limpets and Aspergillus niger, Tetrahedron Asymmetry, 11 (2000) 223-229. 

Indeed the only conversion where biocatalysis should be seriously considered is the transformation of aldehydes into optically active cyanohydrins1 2. For example, the conversion of aryl aldehydes into the appropriate (R)-cyanohydrins using almond meal may be accomplished in quantitative yield and gives products... 

The almond meal PaHnl can be obtained from commercial sources or alternatively can be prepared [42,43] by grinding almonds and defatting the powder three times with ethyl acetate. These procedures makes this Hnl an attractive enzyme to use on a multigram scale and hence it has been widely applied in organic synthesis (as outlined in Sect. 3.3.1). 

Crystallisation studies using the PaHnl have also been reported [50]. Four isomeric forms of this Hnl were isolated from the aforementioned defatted almond meal and subsequently separated. Three of these isoenzymes were successfully crystallised and shown to belong to the monoclinic space group. Though by this means additional data were included, the clearest picture for the 3D structure of a hydroxynitrile lyase still remains with the HbHnl. 

Results are expressed as fig of p-nitrophenol liberated per g of seed from 6 mM p-nitrophenyl a-D-mannoside in 1 hr at 37° and pH 5. Coarsely ground beans. Defatted sweet-almond meal. A commercial /3-D-glucosidase preparation purified from emulsin. a-D-Mannosidase activity is given per g of purified material. 

The simplest and cheapest way to produce supported PaHNL is to use almond meal itself. The step of purification of the enzyme is avoided [43], but low loading of enzyme is the main disadvantage. Other HNLs have also been employed using this crude type of enzyme preparation, such as apple, apricot, cherry, plum [44, 45], peach and loquat [46] meal. However, these catalysts were applied to a minor extent compared with almond meal. 

Iin and co-workers [40] have developed a so-called micro-aqueous organic reaction system. In contrast to former preparations of almond meal, the almond kernels are soaked in water prior to grinding. After the defatting step, the meal contains 8-10% water (w/w), making it unnecessary to add the amount of water needed for enzyme activity. The reactions are carried out in buffer-saturated organic solvents to avoid a possible drying effect of the solvent on the biocatalyst. Further addition of water to the reaction results in lower conversions and values,... 

The Gotor group succeeded in using almond meal for the enantioselective cleavage of ketone cyanohydrins, which served additionally as the cyanide source in the subsequent asymmetric addition of HCN to co-bromoaldehydes in one pot (Figure 9.4) [53]. 

The conversion of co-hydroxyalkanals to the corresponding cyanohydrins in moderate enantioselectivities could also be accomplished by transhydrocyanation with acetone cyanohydrin as the cyanide source. These substrates are considered difficult because of their high solubility in water. Through the employment of an almond meal preparation in a micro-aqueous organic reaction system, the ee-values could be significantly improved [54]. 

As well as almond meal, Sorghum bicolor shoots have also found application in the synthesis of aromatic cyanohydrins [55]. The enantiomeric purity obtained in transhydrocyanation experiments with acetone cyanohydrin as the cyanide source suffers from the high water content (>14% v/v) necessary for the decomposition of acetone cyanohydrin. In contrast, the application of HCN allows the use of low amounts of water (2% v/v), leading to yields and optical purities comparable with those obtained by the isolated enzymes. 

Initial preparative work with oxynitrilases in neutral aqueous solution [517, 518] was hampered by the fact that under these reaction conditions the enzymatic addition has to compete with a spontaneous chemical reaction which limits enantioselectivity. Major improvements in optical purity of cyanohydrins were achieved by conducting the addition under acidic conditions to suppress the uncatalyzed side reaction [519], or by switching to a water immiscible organic solvent as the reaction medium [520], preferably diisopropyl ether. For the latter case, the enzymes are readily immobilized by physical adsorption onto cellulose. A continuous process has been developed for chiral cyanohydrin synthesis using an enzyme membrane reactor [61]. Acetone cyanhydrin can replace the highly toxic hydrocyanic acid as the cyanide source [521], Inexpensive defatted almond meal has been found to be a convenient substitute for the purified (R)-oxynitrilase without sacrificing enantioselectivity [522-524], Similarly, lyophilized and powered Sorghum bicolor shoots have been successfully tested as an alternative source for the purified (S)-oxynitrilase [525],... 

The k-selective Prunus amygdalus HNL is readily available from almonds. Approximately 5 g of pure enzyme can be isolated from 1 kg of almonds, alternatively crude defatted almond meal has also been used with great success. This enzyme has already been used for almost 100 years and it has successfully been employed for the synthesis of both aromatic and aliphatic R-cyanohydrins (Scheme 5.2) [11]. More recently it has been cloned into Pichea pastoris, guaranteeing unlimited access to it and enabling genetic modifications of this versatile enzyme [12]. 

A Avicel, ethyl acetate/HOAc-buffer (pH 5.4), HCN, rt. B almond meal, ethyl acetate/citrate buffer (pH 5.5), HCN, 4°C. C EtOH/HOAc-buffer (pH 5.4), HCN, 0°C. D Avicel, diisopropyl ether/HOAc-buffer (pH 3.3), HCN, RT. E almond meal, diisopropyl ether/citrate buffer (pH 5.5), acetone cyanohydrin, RT. F Et,O/HOAc-buffer (pH 5.0), acetone cyanohydrin, RT. G almond meal, methyl f-butyl ether/citrate buffer (pH 5.5), HCN, 5 °C. 

The commercial process run by Solvay Duphar, based on the work of van der Gen,41 uses an aqueous extract of almond meal for (R)-126 and the sorghum enzyme for (S)-126. Again a two-phase system (H20/Et20) is used and the rather unstable cyanohydrins, that may racemise by reversible loss of HCN, are stabilised by conversion to silyl ethers such as 129-131. The last example shows that direct is preferred to conjugate addition. 

In 1908 Rosenthaler [124] reported the enantioselective synthesis of (R)-man-delonitrile catalysed by an enzyme preparation from almond meal called emul-sin. This remarkable result did not attract any scientific attention for a considerable time [125-127]. In the early 1960s the investigation of this interesting... 

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