Enzymatic approach alcohol

Several enzymatic procedures have been developed for the synthesis of carbohydrates from acyclic precursors. Aldolases appear to be useful catalysts for the construction of sugars through asymmeteric C-C bond formation. 2-deoxy-KDO, 2-deoxy-2-fluoro-KDO, 9-0-acetyl sialic acid and several unusual sugars were prepared by a combined chemical and enzymatic approach. Alcohol dehydrogenases and lipases have been used in the preparation of chiral furans, hydroxyaldehydes, and glycerol acetonide which are useful as building blocks in carbohydrate synthesis. 

An enzymatic approach has been advocated for the single-step preparation of small quantities of enantiomerically enriched 2-oxynorbomanes and 2-oxybicyclo[3,2,l]-octanes. Thus, for example, horse fiver alcohol dehydrogenase-catalysed oxido-reduction of ( )-2-norbornanone results in the production of (—)-(lR,4S)-2-nor-bornanone and (-f)-(lS,21, 4R)-endo-2-norbomanol of up to 46 and 66% optical purity respectively. The space-conjugated olefinic double bonds in various nor-bornene and bicyclo[2,2,2]octene derivatives are rapidly reduced by lithium in liquid ammonia benzonorbomadiene, for example, is reduced to benzonorbomane. ... 

The esterification of the free -COOH terminus of pHAs by fatty alcohol enhances their physical properties, namely, the viscosity is lowered and VI increased with low melting temperature characteristics retained (12,24). The formation of an ester bond between a pHA chain and fatty alcohol can occur chemically (12,30) or enzymatically without destruction of the pHA s ester bonds (24). In addition, Hayes and Kleiman successfully esterified di- and poly(hydoxy acid) synthesized chemically to the hydroxyls of 1,10-decanediol via biocatalysis at high yield (24). The resultant materials had melting point temperatures < -17°C and VI values > 150 (24). Also, Gross and co-workers synthesized a star polymer containing a ethyl glucoside core and pCL and poly(lactide) arms via a chemo-enzymatic approach (43). 

Baldessari s group reported that lipase from CCL and CAL catalyzes the removal of acetyl groups from 3p-acetoxypregn-5-en-20-one 137 and 3P-acetoxy-20(5)-hydroxycho-lest-5-en-23-one 138, through a transesterification reaction in organic solvents [29], giving alcohols 87 and 139 (Table 13). It is known that compound 139 could not be obtained by conventional methods. The enzymatic approach allows its preparation in fair yield. 

One approach called enzymatic resolution, involves treating a racemic mixture with an enzyme that catalyzes the reaction of only one of the enantiomers Some of the most commonly used ones are lipases and esterases enzymes that catalyze the hydrol ysis of esters In a typical procedure one enantiomer of the acetate ester of a racemic alcohol undergoes hydrolysis and the other is left unchanged when hydrolyzed m the presence of an esterase from hog liver... 

Stereoinversion Stereoinversion can be achieved either using a chemoenzymatic approach or a purely biocatalytic method. As an example of the former case, deracemization of secondary alcohols via enzymatic hydrolysis of their acetates may be mentioned. Thus, after the first step, kinetic resolution of a racemate, the enantiomeric alcohol resulting from hydrolysis of the fast reacting enantiomer of the substrate is chemically transformed into an activated ester, for example, by mesylation. The mixture of both esters is then subjected to basic hydrolysis. Each hydrolysis proceeds with different stereochemistry - the acetate is hydrolyzed with retention of configuration due to the attack of the hydroxy anion on the carbonyl carbon, and the mesylate - with inversion as a result of the attack of the hydroxy anion on the stereogenic carbon atom. As a result, a single enantiomer of the secondary alcohol is obtained (Scheme 5.12) [8, 50a]. 

In another approach, the alcohol moiety, formed by an enzymatic hydrolysis of an ester, can act as a nucleophile. In their synthesis of pityol (8-37a), a pheromone of the elm bark beetle, Faber and coworkers [17] used an enzyme-triggered reaction of the diastereomeric mixture of ( )-epoxy ester 8-35 employing an immobilized enzyme preparation (Novo SP 409) or whole lyophilized cells of Rhodococcus erythro-polis NCIMB 11540 (Scheme 8.9). As an intermediate, the enantiopure alcohol 8-36 is formed via kinetic resolution as a mixture ofdiastereomers, which leads to the diastereomeric THF derivatives pityol (8-37a) and 8-37b as a separable mixture with a... 

A classical approach to driving the unfavorable equilibrium of an enzymatic process is to couple it to another, irreversible enzymatic process. Griengl and coworkers have applied this concept to asymmetric synthesis of 1,2-amino alcohols with a threonine aldolase [24] (Figure 6.7). While the equilibrium in threonine aldolase reactions typically does not favor the synthetic direction, and the bond formation leads to nearly equal amounts of two diastereomers, coupling the aldolase reaction with a selective tyrosine decarboxylase leads to irreversible formation of aryl amino alcohols in reasonable enantiomeric excess via a dynamic kinetic asymmetric transformation. A one-pot, two-enzyme asymmetric synthesis of amino alcohols, including noradrenaline and octopamine, from readily available starting materials was developed [25]. 

The enzymatic resolution of racemic substrates now is a well-established approach for the synthesis of single enantiomers [1, 2]. A representative example is the kinetic resoluhon of secondary alcohols via lipase-catalyzed transesterification for the preparation of enantiomericaUy enriched alcohols and esters [3], The enzymatic resolution in general is straightforward and satisfactory in terms of optical purity, but it has an intrinsic Hmitation in that the theoretical maximum yield of a desirable enantiomer cannot exceed 50%. Accordingly, additional processes such as isolation, racemization and recycling of unwanted isomers are required to obtain the desirable isomer in a higher yield (Scheme 1.1). 

There are basically two approaches to the synthesis of enantiomerically pure alcohols (i) kinetic resolution of the racemic alcohol using a hydrolase (lipase, esterase or protease) or (ii) reduction mediated by a ketoreductase (KRED). Both of these processes can be performed as a cascade process. The first approach can be performed as a dynamic kinetic resolution (DKR) by conducting an enzymatic transesterification in the presence of a redox metal [e.g. a Ru(ll) complex] to catalyze in situ racemization of the unreacted alcohol isomer [11] (Scheme 6.1). We shall not discuss this type of process in any detail here since it forms the subject of Chapter 1. 

P-Adrenergic-blocking agents, such as propranolol, have been synthesized by different chemoenzymatic methods where the key step to introduce the chirality is an enzymatic acylation or a hydrolysis process. The main reason to prepare these amino alcohols in optically pure form is due to the fact that the activity of these pharmaceuticals resides in the (S)-enantiomer. In Scheme 10.1 we have represented a chemoenzymatic approach that has been carried out for the preparation of this dmg where the key step is the resolution of the key intermediate 1-chloro-... 

In work concerning the directed evolution of enantioselective enzymes, there was a need for fast and efficient ways to determine the enantiomeric purity of chiral alcohols, which can be produced enzymatically either by reduction of prochiral ketones (e.g., 26) using reductases or by kinetic resolution of rac-acetates (e.g., 19) by lipases (111). In both systems, the CD approach is theoretically possible. In the former case, an LC column would have to separate the educt 26 from the product (A)/(J )-20, whereas in the latter, (5)/(J )-20 would have to be separated from (S)/(R)-19. 

Prior to the breakthrough by E.Buchner s investigations, Berthelot had tried to demonstrate cell-free (and thus chemical or enzymatic nature of the) conversion of sugar to alcohol in the 1850 s, but indeed his approach could not be successful, as was obvious from Pasteur s clarification of the manifold somces of microbial infection, notably inoculation from the air. It was these findings which were a condition for Buchner s concept to exclude microorganisms (at least to an essential extent, as he was able to show) in his experiments. The progress in understanding and methods thus was necessaiy for the chemical approach, even if Pasteur opposed it himself... 

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