Cyanohydrins enzymatic preparation

For practical applications of HNLs as catalysts for the preparation of chiral cyanohydrins, three objectives have been achieved first, to get high enantioselec-tivity it is decisive to suppress the non-enzymatic addition of HCN to the substrate ... 

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],... 

Relatively few of the enzymatic methods applicable to the preparation of secondary cyanohydrins have been adapted successfully to the synthesis of optically pure tertiary cyanohydrins [1,3,4]. Similarly, progress in asymmetric hydro-cyanation of ketones with synthetic catalysts lagged far behind advances in aldehyde cyanation. This situation has changed fairly dramatically over the past... 

Cyanohydrins are versatile building blocks that are used in both the pharmaceutical and agrochemical industries [2-9]. Consequently their enantioselective synthesis has attracted considerable attention (Scheme 5.1). Their preparation by the addition of HCN to an aldehyde or a ketone is 100% atom efficient. It is, however, an equilibrium reaction. The racemic addition of HCN is base-catalyzed, thus the enantioselective, enzymatic cyanide addition should be performed under mildly acidic conditions to suppress the undesired background reaction. While the formation of cyanohydrins from aldehydes proceeds readily, the equilibrium for ketones lies on the side of the starting materials. The latter reaction can therefore only be performed successfully by either bio- or chemo-cat-... 

Enzymatic processes are now being applied to a wide range of pharmaceutical product syntheses (46). Examples are given for the preparation of cyanohydrins, which can then be used to prepare a-hydroxy acids and a-amino acids. 

The first total synthesis of amiclenomycin, an inhibitor of biotin biosynthesis, was completed by A. Marquet and co-workers. In order to prove its structure unambiguously, both the cis and trans isomers were prepared. The L-amino acid functionality was installed by a Strecker reaction using TMSCN in the presence of catalytic amounts of Znla. The resulting O-TMS protected cyanohydrin was exposed to saturated methanolic ammonia solution, which gave rise to the corresponding a-amino nitrile. Enzymatic hydrolysis with immobilized pronase afforded the desired L-amino acid. 

Other possibilities to prepare chiral cyanohydrins are the enzyme catalysed kinetic resolution of racemic cyanohydrins or cyanohydrin esters [107 and references therein], the stereospecific enzymatic esterification with vinyl acetate [108-111] (Scheme 2) and transesterification reactions with long chain alcohols [107,112]. Many reports describe the use of fipases in this area. Although the action of whole microorganisms in cyanohydrin resolution has been described [110-116],better results can be obtained by the use of isolated enzymes. Lipases from Pseudomonas sp. [107,117-119], Bacillus coagulans [110, 111], Candida cylindracea [112,119,120] as well as lipase AY [120], Lipase PS [120] and the mammalian porcine pancreatic lipase [112, 120] are known to catalyse such resolution reactions. 

However, the most advantageous preparations of optically active cyanohydrins, with respect to the obtained enantioselectivities, are the enzymatically controlled approaches discussed in the present chapter. Two common enzyme systems are described and reviewed11 161 firstly, esterases or lipases, which have been employed... 

A series of cyanohydrin acetates with an e.e. up to 98% has been prepared by enzymatic hydrolysis of their racemic acetates in the presence of an esterase from Pseudomonas spJ137]. Lipoprotein lipase from Pseudomonas sp. catalysed irreversible transesterification using enol esters was applied to the resolution of different aromatic cyanohydrins[138> 139). 

The addition reaction of carbon-11 labelled cyanide ion to the bisulphite addition adduct of an aldehyde has been extended to prepare carbon-11 labelled amines. Maeda and coworkers prepared both p- and m-octopamine [2-(p-and m-hydroxyphenyl)-2-hydroxyethyl-amine] from the corresponding benzaldehyde by reducing the cyanohydrin formed in the reaction between the appropriate benzaldehyde and cyanide ion both under enzymatic conditions and by the basic modification of the Bucherer-Strecker synthesis, with borane-THF. The synthesis of / -octopamine is presented in equation 64. 

The vindoline synthesis required prior preparation of the amine coupling partner, the 2,4-dinitrobenzenesulfonamide 713, which was prepared from the pent-anal 710, as shown in Scheme 43. A notable feature of this route was the enzyme-mediated resolution of the cyanohydrin acetate 711, via enzymatic hydrolysis to selectively afford a diastereomeric mixture of only the (5)-cyanohydrins 712. 

P-Cyclodextrin is also an enantioselective catalyst for HCN-additions on some aromatic aldehydes, but not in the case of 3-phenoxy benzaldehyde [651]. Recently the enzymatic enantioselective cleavage of S-cyanohydrine acetate by lipases from bacteria or Candida cylindrica [652] has [653] been claimed in a patent. Similarly, the optically active S-3-phenoxy-4-fluorobenzaldehyde cyanohydrine 301 can also be prepared by these esteratic methods, preferentially at a pn between 3.5 and 6.0 [654, 655]. Optically active cyanohydrines must be stabilized e.g. by alkanephosphonic acids [646], to inhibit slow racemization, even in the absence of bases. 

Many other hydroxy compounds bearing a secondaiy hydroxy group can be enan-tioselectively acylated. In the case of a few cyanohydrins, optically active acetates are prepared in a one-pot procedure from the aldehyde and acetone cyanohydrin, followed by a lipase-catalyzed transesterification [196]. Also, 2-hydroxy acids and esters [197-201] can be enzymatically resolved, as shown in Scheme 43. 

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