Hydroxy Acids by DKR with a Two-enzyme System

Deracemization of Hydroxy Acids by DKR with a Two-enzyme System 

Deracemization of mandelic add with the combined action of two enzymes has been reported. rac-MandeUc acid is acylated by a Pseudomonas sp. lipase in diisopropyl ether. After solvent removal the mfacture of mandeUc acid enriched in the R-form and the 0-acetyl derivative of the S-configuration are subjected to the mandelate racemase-catalyzed racemization in aqueous buffer. In these conditions only the non-acetylated hydroxy acid is racemized. In order to obtain (S)-0-acetylmandelic acid in an 80% isolated yield and a 98% e.e. the process must be repeated four times [9]. 

This example of a classical cyclic resolution-racemization methodology indicates the limitations of a multistep enzyme-catalyzed reaction when the reaction medium, the reagents, or the reaction conditions are not mutually compatible. 

Mandelate racemase, which is probably the best known of all racemizing enzymes [10], shows an activity limited to compounds structurally related to man-delates [11]. The presence of an aromatic group or an unsaturation in the a-position is essential for activity. 

In order to extend the two-enzyme system to other 2-hydroxy acids, a racemase with a broader activity was found in Lactobacillus paracasei. This was exploited for deracemization of 2-hydroxy-4-phenylbutanoic acid and 3-phenyllactic acid, which are important synthetic intermediates. In addition, in this case the procedure requires a kinetic resolution step and a successive racemization step. O-Acetyl derivatives of the absolute (S)-configuration can be obtained in two successive repeating cycles. Yields are around 60%. Of course the 0-acetyl derivatives of opposite configuration can be obtained when the lipase-catalyzed reaction is apphed in the hydrolysis direction. Obtaining the O-acetyl derivatives of the absolute (R)-configuration requires an additional acetylation step of the initially resolved and racemized (S)-hydroxy acid [12]. 

---