Alcohols deracemization

Finally in this section on deracemization via cyclic oxidation/reduction methods, there has been some limited work carried out on the deracemization of secondary alcohols. Soda et al. [22] employed lactate oxidase in combination with sodium borohydride to deracemize D/i-lactate (18) via the intermediate pyruvate (19) (Figure 5.12). 

Microbial Deracemization of Secondary Alcohols Using a Single Microorganism... 

It is well known that certain microorganisms are able to effect the deracemization of racemic secondary alcohols with a high yield of enantiomerically enriched compounds. These deracemization processes often involve two different alcohol dehydrogenases with complementary enantiospedficity. In this context Porto ef al. [24] have shown that various fungi, induding Aspergillus terreus CCT 3320 and A. terreus CCT 4083, are able to deracemize ortho- and meta-fluorophenyl-l-ethanol in good... 

Deracemization of Alcohols Using Two Enzyme/Microorganism Systems... 

An alternative approach to the microbial deracemization of secondary alcohols is to use two different microorganisms with complementary stereoselectivity. Fantin et al. studied the stereoinversion of several secondary alcohols using the culture supernatants of two microorganisms, namely Bacillus stearothermophilus and Yarrowia lipolytica (Figure 5.18) [31]. The authors tested three main systems for deracemization. First, they used the supernatant from cultures of B. stearothermophilus, to which they added Y. lipolytica cells and the racemic alcohols. Secondly, they used the culture supernatant of Y. lipolytica and added B. stearothermophilus cells and the racemic alcohols. Finally, they resuspended the cells of both organisms in phosphate buffer and added the racemic alcohols. The best results were obtained in the first system with 6-penten-2-ol (26) (100% ee and 100% yield). The phosphate buffer system gave... 

Figure 5.17 Deracemization of alcohol (24) using Cunninghamella echinulata.

For the deracemization of phenylethanol derivatives using G. candidum under aerobic conditions (Figure 8.41b), the (S)-specific enzyme was reversible and (R) enzyme was irreversible, so (R)-alcohol accumulated when the cell and racemic alcohols were mixed [31b,c]. Para-substituted phenylethanol derivatives gave better results than meta-substituted derivatives. Sphingomonas was used for... 

Biooxidative deracemization of racemic sec-alcohols to single enantiomers [47,48] is complementary to combined metal-assisted lipase-mediated strategies [49,50]. In general, deracemization can be realized by either an enantioconvergent, a dynamic kinetic resolution, or a stereoinversion process. The latter concept is particularly appealing, as only half of the substrate needs to be converted, as the remaining half already represents the product with correct stereochemistry. 

Scheme 9.5 A two step process for the deracemization of sec-alcohols by two dehydrogenases.

Several suitable whole-cell systems have been identified for deracemization biotransformations on a large diversity of substrates, as compiled recently [48]. In particular, heterocyclic alcohols were successfully converted by Sphingomonas [55]. Access to enantiocomplementaiy products was achieved with various strains of Aspergillus [56] or Rhizopus [57]. Biotransformations can even be accomplished with yacon and ginger [58]. Substrate titers were reported up to 8gl for Candida parapsUosis mediated biotransformations [59]. 

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]. 

Deracemization via the biocatalytic stereoinversion is usually achieved by employing whole cells. In the case of secondary alcohols, it is believed that microbial stereoinversion occurs by an oxidation-reduction sequence... 

Scheme 5.12 Deracemization of secondary alcohols via resolution followed by chemical stereoinversion.

Scheme 5.13 Deracemization of secondary alcohols based on biocatalytic stereoinversion [26, 50b].

Scheme 5.14 Chemoenzymatic enantioconvergent deracemization of secondary alcohols via hydrolysis of their sulfate esters.

Figure 14.6 Enzymatic deracemization concepts for production of chiral alcohols, amines and amino acids...

Voss, C.V., Gruber, C.C. and Kroutil, W. (2008) Deracemization of secondary alcohols through a concurrent tandem biocatalytic oxidation and reduction. Angewandte Chemie-International Edition, 47 (4), 741-745. 

Relatively little attention has been paid to the conversion of racemic compounds into their enantiomerically pure versions in a single process, in other words a deracemization. For certain classes of chiral compounds such as secondary alcohols, this approach should provide many benefits, particularly to the pharmaceutical industry. Existing routes to high value intermediates in their racemic form may be modified to provide the equivalent homochiral product, thus reducing the extent of development chemistry required. In addition, the... 

Medici et al. have used a combined sequential oxidation-reduction to access a range of imsaturated secondary alcohols from their racemates [7] (Scheme 1). Here the S-alcohol 2 is oxidized by B. stereothermophilus which is displaying Prelog specificity leaving the l -enantiomer untouched. The other microorganism, Y. lipolytica contains an anti-Prelog dehydrogenase which is therefore able to reduce the ketone 1 to the l -alcohol 2. Thus the combination of the two steps effects a net deracemization of substrate 2. 

Nakamura et al. have used another strain of this fungus. Geotrichum candidum IFO 5767, to deracemize various arylethanols to give the (i )-alcohol 13 in high yield and enantiomeric excess [161 (Scheme 7). 

In a time course study on the conversion of ( )-l-phenylethanol 13 (X=H), formation of acetophenone was observed to a maximum of around 20% during the conversion of (S)- to (R)- alcohol which occurred over 24 h to give (R)-13 in 96 % yield, 99% e.e. The effect of ring substitution on the efficiency of the dera-cemization was notable. While para substituents (Cl, OMe, Me) gave good results, ortho derivatives could not be deracemized and the biocatalyst showed little activity towards meta substituted compounds. On addition of allyl alcohol, improvements in e.e. were obtained, particularly for the conversion of l-(m-methylphenyl)ethanol (from 21 to 94% e.e.). However these improvements did appear to be at the expense of yield (89% diminished to 55%). The authors sug-... 

Cell cultures of Catharanthus roseus entrapped in calcium alginate have been employed by Takemoto and Achiwa to deracemize pyridyl alcohols such as 15 and 16 [17] (Scheme 8). 

Kroutil et al. have recently reported [18] an elegant one-pot oxidation/reduction sequence for the deracemization of a chiral secondary alcohol using a single biocatalyst. LyophiUzed cells of a Rhodococcus sp. CBS IVJ.Ti converted racemic 2-decanol into the (S)-enantiomer in 82% yield and 92% enantiomeric excess (e.e.). via a non-specific oxidation followed sequentially by an (S)-selective reduction (Scheme 6.5). Acetone was used as the hydrogen acceptor in the first step and isopropanol as the hydrogen donor in the second step. 

Oxidoreductases are, after lipases, the second most-used kinds of biocatalysts in organic synthesis. Two main processes have been reported using this type of enzymes-bioreduction of carbonyl groups [39] and biohydroxylation of non-activated substrates [40]. However, in recent few years other processes such as deracemization of amines or alcohols [41] and enzymatic Baeyer-Villiger reactions of ketones and aldehydes [42] are being used with great utility in asymmetric synthesis. 

Substituted acrylates (which reseitible the enamide substrates employed 1n asymmetric hydrogenation) may be deracemized by reduction with an optically active catalyst, especially DIPAMPRh . Selectivity ratios of 12 1 to 22 1 have been obtained for a variety of reactants with compounds of reasonable volatility, separation of starting material and product may be effected by preparative GLC. Recovered starting material can then be reduced with an achiral catalyst to give the optically pure anti product. Examples of kinetic resolutions by this method are given in Table II. More recently very successful kinetic resolutions of allylic alcohols have been carried out with Ru(BINAP) catalysts. 

The first example of chemoenzymatic DKR of allylic alcohol derivatives was reported by Williams et al. [37]. Cyclic allylic acetates were deracemized by combining a lipase-catalyzed hydrolysis with a racemization via transposition of the acetate group, catalyzed by a Pd(II) complex. Despite a limitation of the process, i.e. long reaction times (19 days), this work was a significant step forward in the combination of enzymes and metals in one pot Some years later, Kim et al. considerably improved the DKR of allylic acetates using a Pd(0) complex for the racemization, which occurs through Tt-allyl(palladium) intermediates. The transesterification is catalyzed by a lipase (Candida antarctica lipase B, CALB) using isopropanol as acyl acceptor (Scheme 5.19) [38]. 

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