BIOCATALYTIC STRATEGIES FOR SELECTIVE ORGANIC SYNTHESIS

Proteins are macromolecules constituted by a network of chiral (L)-alpha-amino acids linked by amide bonds that, thanks to their extensive chirality and 3D-folding, can catalyze highly selective transformations. Thus, enzymes can discriminate between two enantiomers (ndr kinetic resolution), interact with a pro-chiral group leading to the stereoselective formation of a new stereocenter (ndr asymmetric synthesis) or can guide the chemical bonds formation or breaking in a specific direction as well as toward a specific functional group (ndr regio- and chemo-selective processes). In this research work, different enzymatic activities have been exploited to promote selective transformations to assess synthetic problems in organic chemistry. The regioselective lipase-mediated trans-acylation reactions, discussed in a review article published by our research group in 2015, have been exploited to selectivity produce aloin conjugates) to be tested for their ability as self-assembly inducers. The outstanding regioselectivity of an immobilized Candida antarctica (Novozym435) and lipase PS allowed to produce key synthons of the target compounds via the selective acylation of the aglycone primary hydroxyl group. A subclass of novel thermostable aminotransferases (B3, It6 and Is3; TM up to 80 °C) discovered from hot-spring metagenomes, was investigated for their potential synthetic exploitation to prepare enantiomerically enriched amines. In this work, more than 10 amino donors and acceptors were screened as substrates to produce (S)-primary amines by direct asymmetric synthesis from the correspondent pro-chiral ketones. A sustainable, convenient, scalable, one-pot, two-enzyme method for the glucosylation of arylalkyl alcohols was developed. The reaction scheme was based on a transrutinosylation catalyzed by a rutinosidase from A. niger using the cheap commercially available natural flavonoid rutin as glycosyl donor, followed by selective ‘trimming’ of the rutinoside unit catalyzed by a rhamnosidase from A. terreus. The process was validated with the syntheses of several natural bioactive glucosides, which could be isolated in up to 80% yield without silica-gel chromatography. Unspecific peroxygenases (UPOs, EC 1.11.2.1) represent an emerging class of biocatalysts with a wide range of potential applications in different oxyfunctionalization reactions. They are extracellular heme-thiolate enzymes, a class of hemoproteins in which a thiolate group, typically from a cysteine residue, is the axial ligand of heme iron. The best characterized UPO (AaeUPO) has been identified in 2004 from the edible mushroom Agrocybe aegerita. Only a few preliminary data were available so far about the exploitation of UPOs in the (enantioselective) oxyfunctionalization of sulfides to sulfoxides. Thus, the AaeUPO-catalyzed enantioselective sulfoxidation of arylalkyl sulphides to the correspondent sulphoxides was studied to produce valuable chiral synthons containing a stereogenic sulfur atom, key compounds in the development of different class of ligands or catalysts. Starting from a small library of arylalkyl sulphides, the correspondent sulphoxides were obtained as (R)-enantiomers with ee and conversion up to the 99%. As a last example of selective biocatalyzed processes, laccases, selective activators of Csp2-H bonds, were exploited to produce (±)-trans-2,3-dihydrodimers by the oxidative coupling of phenolic substrates. In a first study, the oxidation of glycosylated lignols was used to easily produce complex structural derivatives of bioactive natural compounds. In a second study, the laccase-mediated chemo- and regio-selective oxidative coupling of a small library of (4)-E-styrylphenols allowed the production of a family of potential allosteric modulators of the90 KDa heat shock protein 90 (Hsp90). As the modulation of Hsp90 conformational dynamics can occur by targeting their ATPase cycle, the use small-molecules modulators acting as activators of this pivotal hydrolytic process is a well reported strategy. In this field, both the synthesis and the in vitro evaluation of a family of substituted (±)-trans-2,3-dihydrobezofurans as potential allosteric modulators of the Hsp90’s ATPase were investigated exploiting the oxidative coupling of E-stilbenes as a convenient and easy entry to this family of scaffolds. All the synthetized compounds were in vitro tested for their ability of modulating the chaperone ATPase cycle of Hsp90, computational studies, to confirm and explain at the molecular level the results obtained in the in vitro testing were also conducted.