Chiral synthesis, using enzymes

Novel chiral. separations using enzymes and chiral surfactants as carriers have been realized using facilitated transport membranes. Japanese workers have reported the synthesis of a novel norbornadiene polymeric membrane with optically active pendent groups that show enantio.selectivity, which has shown promi.se in the. separation of propronalol. 

Chemical reactions enhanced by catalysts or enzymes are an integral part of the manufacturing processes for the majority of chemical products. The total market for catalysts and enzymes amounts to 11.5 billion (2005), of which catalysts account for about 80%. It consists of four main applications environment (e.g., automotive catalysts), 31% polymers (e.g., polyethylene and polypropylene), 24% petroleum processing (e.g., cracking and reforming), 23% and chemicals, 22%. Within the latter, particularly the catalysts and enzymes for chiral synthesis are noteworthy. Within catalysts, BINAPs [i.e., derivatives of 2,2 -bis(diphenylphosphino) -1, l -bis-l,l -binaphthyl) have made a great foray into chiral synthesis. Within enzymes, apart from bread-and-butter products, like lipases, nitrilases, acylases, lactamases, and esterases, there are products tailored for specific processes. These specialty enzymes improve the volumetric productivity 100-fold and more. Fine-chemical companies, which have an important captive use of enzymes, are offering them to third parties. Two examples are described here ... 

Example 1 Synthesis of Chiral Compounds Using Enzymes in Solid/Cas Reactors... 

In synthetic operations, when a symmetrical (achiral) substrate is used, the product is racemic. Diastereomers are separated by physical methods, and the enantiomers of racemic amines are frequently obtained by fractional crystallization of diastereomeric salts formed with chiral acids. Although resolution of racemic amines by fractional crystallization of enantiomeric salts is still an important technique and the laboratory scale resolutions of many racemic amines have been reported , the separation of the enantiomers of chiral amines by chromatography " and their preparation by asymmetric synthesis using enzymes and other asymmetric catalysts have had extensive development during the... 

Asymmetric synthesis using enzymes to produce chiral precursors has been employed to prepare optically pure N-benzoyl-5-C-methyl-L-acosamine (21) by the sequence summarised in Scheme 6. A... 

Consonant with the present interest in chiral synthesis, two additional contributions can be cited. Sih al utilized a combined microbiological and organic chemical sequence in which key chirality establishing steps include the conversion of to chiral, but unstable, 18 by enzymic reduction using the fungus Diplodascus uninucleatus. Lower side-chain synthon was prepared by reduction of achiral with Pencillium decumbens. 

Another approach to the synthesis of chiral non-racemic hydroxyalkyl sulfones used enzyme-catalysed kinetic resolution of racemic substrates. In the first attempt. Porcine pancreas lipase was applied to acylate racemic (3, y and 8-hydroxyalkyl sulfones using trichloroethyl butyrate. Although both enantiomers of the products could be obtained, their enantiomeric excesses were only low to moderate. Recently, we have found that a stereoselective acetylation of racemic p-hydroxyalkyl sulfones can be successfully carried out using several lipases, among which CAL-B and lipase PS (AMANO) proved most efficient. Moreover, application of a dynamic kinetic resolution procedure, in which lipase-promoted kinetic resolution was combined with a concomitant ruthenium-catalysed racem-ization of the substrates, gave the corresponding p-acetoxyalkyl sulfones 8 in yields... 

Enzymes may be used either directly for chiral synthesis of the desired enantiomer of the amino acid itself or of a derivative from which it can readily be prepared, or for kinetic resolution. Resolution of a racemate may remove the unwanted enantiomer, leaving the intended product untouched, or else the reaction may release the desired enantiomer from a racemic precursor. In either case the apparent disadvantage is that the process on its own can only yield up to 50% of the target compound. However, in a number of processes the enzyme-catalyzed kinetic resolution is combined with a second process that re-racemizes the unwanted enantiomer. This may be chemical or enzymatic, and in the latter case, the combination of two simultaneous enzymatic reactions can produce a smooth dynamic kinetic resolution leading to 100% yield. 

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],... 

A similiar approach was performed by van de Velde (1999), using incorporation of vanadate into an acid phosphatase (phytase) to create a semi-synthetic peroxidase similar to the heme-dependent chloroperoxidase. The latter is a useful enzyme for the asymmetric epoxidation of olefins, but less stable due to oxidation of the porphyrin ring and difficult to express outside the native fungal host. The authors exploited the structural similarity of active sites from vanadate-dependent halo-peroxidases and acid phosphatases and have shown the useful application as an enantioselective catalyst for the synthesis of chiral sulfoxides (van de Velde, 1999). 

The preparation of chiral alcohols can be carried out very simply because the regeneration of NADPH is possible by the addition of isopropanol. Unpurified crude extract samples of the ADH from L. kefir were found to be a useful catalyst for the synthesis of (P)-alcohols [160] some examples for the preparation of some chiral alcohols using this enzyme are given in Table 8. Though this ADH becomes unstable to such a degree during the purification process, enough material of the pure enzyme could be prepared to produce polyclonal antibodies and to screen for related (R)-specific enzymes. 

Hydrolases are widely used enzymes in organic synthesis, with most applications concentrating on lipases and esterases. This chapter discloses the possibilities of asymmetric accesses to chiral building blocks for the synthesis of natural products... 

Although biocatalysis is the new kid on the block, more and more companies are using enzymes for chemical manufacture. One reason for this is that biocatalysts give sustainable alternatives to chemical manufacture, and not just for making chiral products. The synthesis of acrylamide via an enzyme-catalyzed water addition to acrylonitrile (2-propenenitrile) is a classic example (Figure 1.15). It uses the Rhodo-coccus enzyme nitrile hydratase. Commercialized in 1985 by Nitto Chemicals in... 

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