Macrolactonization enzymatic

Various a-methylenemacrolides were enzymatically polymerized to polyesters having polymerizable methacrylic methylene groups in the main chain (Fig. 3, left). The free-radical polymerization of these materials produced crosslinked polymer gels [10, 12]. A different chemoenzymatic approach to crosslinked polymers was recently introduced by van der Meulen et al. for novel biomedical materials [11]. Unsaturated macrolactones like globalide and ambrettolide were polymerized by enzymatic ROP. The clear advantage of the enzymatic process is that polymerizations of macrolactones occur very fast as compared to the chemically catalyzed reactions [13]. Thermal crosslinking of the unsaturated polymers in the melt yielded insoluble and fully amorphous materials (Fig. 3, right). 

Fig. 3 Chemoenzymatic routes to crosslinked macrolactones. Left enzymatic polymerization of a-methylenemacrolides and subsequent vinyl polymerization [10]. Right Enzymatically synthesized highly crystalline poly(ambrettolide) before and after thermal crosslinking [11]...

The isolated TE domain from the tyrocidine (tyc) NRPS has recently been shown to catalyze the macrocyclization of unnatural substrates to generate a variety of cyclic peptides. In conjunction with standard solid-phase peptide synthesis, Walsh and coworkers demonstrated a broad substrate tolerance for peptidyl-N-acetylcysteamine thioesters by the tyrocidine TE [41,42], Cyclization of peptide analogs, where individual amino acids were replaced with ethylene glycol units, was observed with high efficiency. In addition, hydroxyacid starter units were readily cyclized by the isolated TE domain to form nonribosomal peptide-derived macrolactones. More recently, Walsh and coworkers have demonstrated effective cyclization of PEGA resin-bound peptide/polyketide hybrids by the tyrocidine TE domain [43], Utilization of a pantetheine mimic for covalent attachment of small molecules to the resin, serves as an appropriate recognition domain for the enzyme. As peptide macrocyclizations remain challenging in the absence of enzymatic assistance, this approach promises facile construction of previously unattainable structures. 

Heise and co-workers [45] studied the thiol-ene functionalisation of a polyester synthesised by the enzymatic ring-opening polymerisation (ROP) of globalide (an unsaturated macrolactone prepared from hydroxyl fatty acids) (Scheme 6.15). Authors claimed that this was the first example of the thiol-ene fimctionalisation of a fatty acid-derived polymer. 

The acidic form of classical sophorolipid (a deacetylated compound) produced by alkaline hydrolysis of the native lactonic form (Fig. 2) was successfully converted by enzymatic biotransformation (glycosidase catalysis) to an acidic glucose lipid [44]. Another modification was performed with alkyl-amines leading to alkylamides of the acidic sophorolipid [123]. Bisht et al. [128] reported an efficient chemoenzymatic route that led to an 6-O-acryloyl-sophorolipid macrolactone analog. The homopolymerization of this monomer as well as its copolymerization with acrylic acid and acrylamide led in maximum to a molecular weight of 4.2 x 10" Da. The first total synthesis of a major component of the microbial classical lactonic sophorolipid was described by Fiirstner et al. [129]. 

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