Racemization chemoenzymatic routes

The four-step synthesis of a, -diaminocaprolactam shown in Figure 5.29 is part ofa chemoenzymatic route to (S)-lysine, an essential amino add in our diet [135], The racemic caprolactam (azepan-2-one) product is then hydrolyzed selectively to (S)-lysine, using an immobilized (S)-hydrolase enzyme. 

Subtilisin Carlsberg, a laundry enzyme, is extraordinarily cheap and stable even at extreme concentrations of substrate and salt at elevated temperature. A major disadvantage of this chemoenzymatic route was the fact that no racemization procedure for the unwanted (R)-enantiomer [(R)-3] could be established [40]. 

A big factor for the success of the process was the close similarity of the substrate to the natural substrates of Subtilisin Carlsberg, an enzyme with extraordinary properties. Since cheap commercial sources for this enzyme were available on the market, the enzyme could be discarded. A major disadvantage of the present chemoenzymatic route was the fact that no satisfactory racemization procedure for the unwanted enantiomer (R)-2 could be established due to elimination and/or hydrolysis side reactions. 

Extension of DKR to polymer chemistry would readily result in chiral polyesters, polycarbonates, or polyamides from an optically inactive monomer mixture. Scheme 10 describes three variants of chemoenzymatic catalysis applied in polymer chemistry that recently appeared in the literature. Route A uses AA and BB monomers to prepare chiral polymers from racemic/diasteromeric diols. Route B converts an enantiomer mixture of AB monomers to homochiral polymers. Route C is the enzymatic ring-opening polymerization of co-methylated lactones to homochiral polyesters. Details will be given in Sect. 3.4.2. 

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