Chemoenzymatic synthesis advantage

The second strategy for the chemoenzymatic synthesis of block copolymers from enzymatic macroinitiators employs an individual modification step of the enzymatic block with an initiator for the chemical polymerization (route B in Fig. 4). This strategy has the advantage that it does not depend on a high incorporation rate of the dual initiator. On the other hand, quantitative end-functionalization becomes more... 

Compartmentalization by sequential monomer addition was also successfully demonstrated in the chemoenzymatic synthesis of block copolymers in SCCO2 using lH,lH,2H,2H-perfluorooctyl methacrylate (FOMA) as an ATRP monomer. Detailed analysis of the obtained polymer P(FOMA-i>-PCL) confirmed the presence of predominantly block copolymer structures (16). The clear advantage of the SCCO2 in this approach is that unlike conventional solvents it solubilises the fluorinated monomer. 

Chemoenzymatic carbohydrate synthesis has been emerging as an extremely powerful tool to obtain complex oligosaccharides. Chemoenzymatic synthesis has many potential advantages, of which the most significant ones are probably that it offers perfect stereoselectivity and renders the tedious work of synthesizing various selectively protected monosaccharide units unnecessary. It also makes the most difficult glycosylation reactions seem easy (86). Chemoenzymatic synthesis of oligosaccharides is out of the scope of this chapter, but it was the topic of many excellent reviews (73,87-89). 

Although both chemical and enzymatic synthetic methods have their uniqueness and limitations, a combination of the two approaches in the synthesis of PBS may be advantageous. The area of chemoenzymatic synthesis has been studied only a little, so not enough information is available to discuss its merits. Nevertheless, Rao et al. [62] described that glycerol-linked amino acid fatty acid esters can be obtained by a chemoenzymatic synthesis, as shown schematically in Fig. 2, with conversions of 50-90%. 

Apart from ATRP, the concept of dual initiation was also applied to other (controlled) polymerization techniques. Nitroxide-mediated living free radical polymerization (LFRP) is one example reported by van As et al. and has the advantage that no further metal catalyst is required [43], Employing initiator NMP-1, a PCL macroinitiator was obtained and subsequent polymerization of styrene produced a block copolymer (Scheme 4). With this system, it was for the first time possible to successfully conduct a one-pot chemoenzymatic cascade polymerization from a mixture containing NMP-1, CL, and styrene. Since the activation temperature of NMP is around 100 °C, no radical polymerization will occur at the reaction temperature of the enzymatic ROP. The two reactions could thus be thermally separated by first carrying out the enzymatic polymerization at low temperature and then raising the temperature to around 100 °C to initiate the NMP. Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-MeCL for the synthesis of chiral block copolymers. 

One of the inherent advantages of enzymes is the ability to discriminate between stereoisomers, often generating products with ena-tiomeric excesses (i.e., of over 98%). Judicious application of biocatalysis can also reduce the number of chemical steps needed to synthesize certain drugs, leading to hybrid chemoenzymatic processes with lower costs and less waste. The range of enzymes used in the synthesis of chiral intermediates has expanded beyond esterases and acylases and... 

Chemoenzymatic techniques provide relatively fast routes to complex oligosaccharide structures. Nevertheless, they always combine biochemical and synthetic elements and therefore require the cooperation of both fields. To date, few glycosyltransferases have been cloned, expressed, and produced in quantities sufficient for preparative enzymatic syntheses. Given the advantages of enzymatic synthesis of oligosaccharides over traditional schemes, research into overexpression of glycosyltransferases will continue to flourish and will offer the benefit of preparative enzymology techniques for carbohydrate chemists in the future. 

The mechanism of catalysis of green biocatalysts and metal catalysts is quite different, but if booth are mutually compatible, then it allows the application concurrently in the same reaction system. The combination of these different types of catalysts is also addressed as chemoenzymatic method. Utilizing the advantages of enzymes, the chemoenzymatic method has been developed for the synthesis of various block copolymers, which are otherwise difficult to prepare. 

Several groups in the United Kingdom and two in the United States began to exploit the asymmetric features of these diverse and potentially valuable synthons in synthesis. A look at a general design for entire classes of compounds from the simple, optically pure materials provided by the enzymes make the advantages of a chemoenzymatic approach to the preparation of complex molecules clear. 

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