Deracemization of Secondary Alcohols

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. 

Such isolated enzyme approaches for deracemization have a clear disadvantage in that they require two operational manipulations with an intermediate recovery step. A one-pot strategy is offered by employing whole-cell biotransformations with strains containing set(s) of complementary dehydrogenases operating in both biooxidative and bioreductive modes. Trace amounts of the intermediate ketone species can be isolated in several cases. In order to lead to an efficient deracemization 

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]. 

Simultaneous biocatalytic oxidation and reduction was also reported for the resolution of secondary alcohols through combination of molecular oxygen and stereoselective reducing agent (alcohol dehydrogenase) with a cofactor-recycling 

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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... 

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... 

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]. 

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.

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. 

The method is of general applicability in the deracemization of secondary alcohols and amines and consists of a Upase-catalyzed irreversible acylation and in situ racemization of the non-reacted enantiomer catalyzed by a ruthenium catalyst. 

Scheme 11.3 Deracemization of secondary alcohols by a biocatalytic cascade using Alcaligenes faecalis who e cells in the oxidation step and an ADH with opposite stereoselectivity in the reduction step.

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... 

The method of combined enzyme- and transition metal-catalyzed reactions widely applied to the DKR of secondary alcohols has also been applied to the DKR of a-hydroxy acid esters rac-1. The principle is based on the enantioselective acylation catalyzed by Pseudomonas species lipase (PS-C from Amano Ltd) using p-Cl-phenyl acetate as an acyl donor in cyclohexane combined with in situ racemization of the non-acylated enantiomer catalyzed by ruthenium compounds [7]. Under these conditions, various a-hydroxy esters of type 1 were deracemized in moderate to good yields and high enantioselectivity (Scheme 13.2). 

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... 

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. 

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. 

Deracemization by stereoinversion is a process in which one form (S of the racemic starting material (Rf -i- Sf) is enantioselectively transformed into an intermediate (Si) which can in turn react to give the form of opposite configuration (Rf). An example of this method could be the selective oxidation of one enantiomer of a racemic secondary alcohol and the subsequent reduction with a catalyst of opposite stereopreference [2]. 

Microbial stereoinversion consists of a deracemization process in which a single whole cell system is applied for the two-step inversion of the configuration of one enantiomer, usually a compound containing a secondary alcohol group. Examples of a two-enzyme system is known for deracemization of mandelates where a single microorganism is used [18]. 

Deracemization. In this type of process, one enantiomer is converted to the other, so that a racemic mixture is converted to a pure enantiomer, or to a mixture enriched in one enantiomer. This is not quite the same as the methods of resolution previously mentioned, although an outside optically active substance is required. To effect the deracemization two conditions are necessary (7) the enantiomers must complex differently with the optically active substance (2) they must interconvert under the conditions of the experiment. When racemic thioesters were placed in solution with a specific optically active amide for 28 days, the solution contained 89% of one enantiomer and 11 % of the other. In this case, the presence of a base (Et3N) was necessary for the interconversion to take place. Biocatalytic deracemization processes induce deracemization of chiral secondary alcohols. In a specific example, Sphingomonas paucimobilis NCIMB 8195 catalyzes the efficient deracemization of many secondary alcohols in up to 90% yield of the (R)-alcohol. ... 

Deracemization via Biocatalytic Stereoinversion. Racemic secondary alcohols may be converted into a single enantiomer via stereoinversion which proceeds through a two-step redox sequence (Scheme 2.130) [38, 941, 942] In a first step, one enantiomer from the racemic mixture is selectively oxidized to the corresponding ketone while the other enantiomer remains unaffected. Then, the ketone is reduced in a second subsequent step by another redox-enzyme displaying opposite stereochemical preference. Overall, this process constitutes a deracemization technique, which leads to the formation of a single enantiomer in 100% theoretical yield... 

The term deracemization covers reactions in which two enantiomers are inter-converted by a stereoinversion process such that a racemate can be transformed to a non-racemic mixture without any net change in the composition of the molecule. Deracemization reactions usually involve a redox process, for example, the interconversion of chiral secondary alcohols via the ketone or alternatively the interconversion of amino acids/amines via the corresponding imine (Scheme 4.37). 

It is well known that various microbial systems are able to deracemise racemic secondary alcohols via a process that generally involves two different alcohol dehydrogenases with complementary enantiospecificity. For example racemic benzoin may be deracemized using Rhizopus oryzae ATCC 9363 (Scheme 4.38). Interestingly, through control of the pH of the medium, it was possible to control the absolute configuration of the major enantiomer produced at pH 7.5-8.S, the (J )-enantiomer was produced in 75% yield and 97% ee whereas at pH 4-5, the (S)-enantiomer was produced in 71% yield and 85% ee [89]. 

Further investigations allowed the selection of ADHs with sufficiently high cofactor preference for performing the deracemization reactions toward enantio-pure (S)- or (R)-alcohols in cascade systems (Scheme 11.4a) [10]. The feasibility of this simultaneous multienzymatic transformations was demonstrated using a set of 10 different racemic secondary alcohols as substrates. 

The chiral synthesis of allylic alcohols has been the focus of many research works due to the high versatility of these molecules in the preparation of many active com-poimds [58,82], Allen and Williams reported the first example of DKR of allylic alcohols via lipase-palladium catalyst coupling deracemization of cyclic allylic acetates [83]. However, the accumulation of secondary products, as well as the long reaction times required, limited the use of this strategy. 

Allan GR, Camell AJ. Microbial deracemization of 1-aryl and 1-heteroaryl secondary alcohols. J. Org. Chem. 2001 66 6495-6497. 

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