Dehalogenase, resolution

The reversibility of halohydrin dehalogenase-catalyzed reactions has been used for the regioselective epoxide-opening with nonnatural nucleophiles (an example is given in Scheme 10.34) [133]. The stereoselectivity of the enzyme results in the resolution of the racemic substrate. At the same time, the regioselectivity imposed by the active site geometry yields the anti-Markovnikov product. [128]... 

Majeric Elenkov, M., Tang, L., Hauer, B. and Janssen, D.B., Sequential kinetic resolution catalyzed by halohydrin dehalogenase. Org. Lett., 2006, 8, 4227. 

Whole cells are used in stirred tanks with pH control, producing fS )-2-chloropropanoic acid in 50% yield from the racemate (0.3 M) with an enantiomeric excess of over 95%. This approach was selected in preference to other methods of resolution such as acylation of the racemate and then stereoselective hydrolysis. The dehalogenase enzyme is specific for substrates with a carboxyl group and a 2-chloro or bromo substituent. No cofactor or metal ion is required and reaction involves an inversion of configuration. 

A number of enzymatic resolutions have been investigated to the chiral halo ester, including esterification, transesterification, and aminolysis.69246 250 Zeneca (ICI) has commercialized the approach that relies on a dehalogenase (Scheme 31.25).69,245,248 249... 

Another chiral small molecule, (S)-2-chloropropionic acid (S-CPA), is a key intermediate in the synthesis of the 2-phe-noxypropionic acid class of herbicides, including Fusilade (Avecia) and Mecoprop (BASF). Kinetic resolution of racemic CPA is performed with a whole cell biocatalyst expressing a dehalogenase enzyme selective for the (R)-enantiomer and producing (S)-lactic acid, itself a useful byproduct. The (S)-chloropropionic acid is extracted with a... 

Ridder, IS., Rozeboom, H.J., and Dijkstra, B.W. (1999) Haloalkane dehalogenase from Xanthobacter autotrophicus GJIO refined at 1.15 A resolution, Acta Cryst. D55, 1273-1290. 

Enzymes are often used in kinetic resolutions, when one isomer reacts much faster than the other.25 This can be done on an industrial scale. Zeneca uses a dehalogenase on racemic 2-chloropropionic acid to obtain about 2000 ton/yr of the L-isomer for use in making a herbicide.26 Enzymatic resolutions may involve ring opening of epoxides,27 formation or hydrolysis of esters and amides, reduction of carbonyl compounds, and so on. Lipases are often used for this purpose.28 A typical example is reaction 10.16.29... 

Benning, M.M., Taylor, K.L., Liu, R.-Q., Yang, G, Xiang, H., Wesenbeig, G, Dunaway-Mariano, D. Holden, H.M. (1996) Biochemistry, 35, 8103-8109, Structure of 4-chlorobenzoyl coenzyme A dehalogenase determined to 1.8 A resolution an enzyme eatalyst generated via adaptive mutation. 

In this enzymatic resolution (Section 7.15), the dehalogenase enzyme catalyzes the hydrolysis of the / -enantiomer of 2-chloropropanoic acid to (S)-lactic acid. The desired (S)-2-chloropropanoic acid is unaffected and recovered in a nearly enantiomerically pure state. 

Scheme 2.234 Resolution of 2-chloropropanoic acid derivatives by 2-haloacid dehalogenase...

Scheme 2.237 Kinetic resolution of halohydrins using halohydrin dehalogenase...

Previous studies on other bacterial hydrolytic dehalogenases have revealed different mechanisms and structures. High resolution X-ray structures of haloalkane dehalogenase(iO), 4-chlorobenzoyl CoA dehalogenase 31) and L-2-haloacid dehalogenase (52) revealed that these enzymes use an active site aspartate in nucleophilic displacement of the chlorine substituent as chloride anion. The enzyme-substrate ester intermediate is subsequently hydrolyzed by... 

Dehydro- halogenation Halohydrin Epoxide Synthesis of epoxides resolution of halohydrins and epoxides Halohydrin dehalogenase... 

Epoxides can be produced by elimination of HCl from vidnal-halohydrins by halohydrin dehalogenase enzymes. These enzymes can be used for the resolution of racemic halohydrins to chiral halohydrins that can be converted easily to chiral epoxides, an important building block for synthesis of variety of compounds (Figure 1.14). 

Dynamic kinetic resolution (DKR) is a method that allows for conversion of the racemic mixture into the desired enantiomer with up to 100% of the theoretical yield. " DKR is a powerful approach to asymmetric synthesis and can be achieved by the application of transition-metal catalysts, Lewis acids, organocatalysts, or enzymes (e.g., hydrolases, dehydrogenases, haloalcohol dehalogenases, and transaminases). 

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